| 1. |
- Amiri, Kaveh, et al.
(författare)
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Experimental Investigation of the Interblade Flow in a Kaplan Runner at Several Operating Points Using Laser Doppler Anemometry
- 2016
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Ingår i: Journal of Fluids Engineering. - : ASME International. - 0098-2202 .- 1528-901X. ; 138:2
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents laser Doppler anemometry (LDA) measurements within the runner blade channels and at the runner outlet of a Kaplan turbine model. The model was investigated at six operating points located on two propeller curves of the turbine to study the flow condition during on-cam and off-cam operations. Main and secondary flows within and after the runner were analyzed, and the effects of the hub and tip clearances on the velocity fields within and after the runner were evaluated. Operation of the turbine at flow rates that are lower than the designed rate for the corresponding propeller curve resulted in vortex breakdown and the formation of a rotating vortex rope (RVR). The RVR formation produced an asymmetrical velocity distribution within and after the runner. The results demonstrated the occurrence of an oscillating flow with the same frequency as the vortex rope within the blade channels located upstream of the RVR. This results in an asymmetric flow through the runner and oscillating forces on the runner blades. The measured velocities indicated that the geometrical asymmetries in the runner manufacturing process resulted in various flow asymmetries at the measured sections. The asymmetries were up to 3% within the runner and 7% at the runner outlet
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| 2. |
- Andersson, Niklas, 1976, et al.
(författare)
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Numerical Simulation of Stirling Engines Using an Unsteady Quasi-One-Dimensional Approach
- 2015
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Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 137:5, s. Art. no. 051104-
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Tidskriftsartikel (refereegranskat)abstract
- An existing computer code for solving the quasi-one-dimensional flow equations governing unsteady compressible flow in tubes with smoothly varying cross-section areas, has been adapted to the simulation of the oscillatory flow in Stirling engines for engine design purposes. By utilizing an efficient smoothing algorithm for the area function that preserves the total volume of the tube, it has been possible to achieve a highly accurate and fully conservative numerical scheme. Sub-models for wall friction and heat transfer have been added, enabling the simulation of gas heaters, gas coolers, and regenerators. The code has been used for the modeling of an alpha-type Stirling engine and validated for a range of operating conditions with good results.
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| 3. |
- Andreasson, Patrik, et al.
(författare)
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A note on a generalized eddy-viscosity hypothesis
- 1992
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Ingår i: Journal of Fluids Engineering. - : ASME International. - 0098-2202 .- 1528-901X. ; 114:3, s. 463-466
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Tidskriftsartikel (refereegranskat)abstract
- The standard eddy-viscosity concept postulates that zero velocity gradient is accompanied by zero shear stress. This is not true for many boundary layer flows: wall jets, asymmetric channel flows, countercurrent flows, for example. The generalized eddy-viscosity hypothesis presented in this paper, relaxes this limitation by recognizing the influence of gradients in the turbulent length scale and the shear. With this new eddy-viscosity concept, implemented into the standard k - ε model, predictions of some boundary layer flows are made. The modelling results agree well with measurements, where predictions with the standard eddy-viscosity concept are known to fail.
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| 4. |
- Angele, Kristian, et al.
(författare)
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Instantaneous behavior of streamwise vortices for turbulent boundary layer separation control
- 2007
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Ingår i: Journal of Fluids Engineering. - : ASME International. - 0098-2202 .- 1528-901X. ; 129:2, s. 226-235
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Tidskriftsartikel (refereegranskat)abstract
- The present study investigates turbulent boundary layer separation control by means of streamwise vortices with focus on the instantaneous vortex behavior. A turbulent boundary layer is exposed to a pressure gradient that generates a separation bubble with substantial backflow. The separation bubble is controlled by conventional passive vortex generators creating pairs of counterrotating vortices. Quantitative information is achieved by applying Particle Image Velocimetry (PIV) to the cross-stream plane of the vortices. The characteristics of a pair of counter-rotating vortices shed from a vortex generator is investigated in the near-field downstream of the vortex generator. The vortices were found to grow with the boundary layer in the downstream direction, and the maximum vorticity decreases as the circulation is conserved. The vortices are nonstationary, and the movements in the spanwise direction are larger than those in the wall-normal direction, due to the presence of the wall. The vortices fluctuate substantially and move over a spanwise distance, which is approximately equal to their size. The most probable instantaneous separation between the two vortices shed from one vortex generator equals the difference between their mean positions. The unsteadiness of the vortices contributes to the observed maxima in the Reynolds stresses around the mean vortex centers. The instantaneous vortex size and the instantaneous maximum vorticity are also fluctuating properties, and the instantaneous vortex is generally smaller and stronger than the mean vortex. A correlation was found between a large instantaneous vortex size and a low instantaneous maximum vorticity (and vice versa), suggesting that the vortices are subjected to vortex stretching.
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| 5. |
- Arabnejad Khanouki, Mohammad Hossein, 1988, et al.
(författare)
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Numerical Assessment of Cavitation Erosion Risk in a Commercial Water-Jet Pump
- 2022
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Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 144:5
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, the risk of cavitation erosion is assessed in a commercial water-jet pump using a recently developed numerical erosion assessment method by Arabnejad et al. [1]. This assessment is performed for two flow conditions with different cavitation erosion risk according to the experimental paint tests and the high erosion risk areas identified by the method are compared with the experimental results. This comparison shows that the applied method is capable of both identifying the regions of high erosion risk and also capturing the difference between the cavitation erosion risk in the two studied conditions. The latter capability of the numerical assessment method, which has not been reported in the literature for other published methods, is one step forward toward the application of the method in the design process of hydraulic machines. Furthermore, the numerical results are analysed to explain the reasons for different erosion risk in the two conditions. This analysis reveals that this difference is mostly related to the stronger flow non-uniformities entering the rotor in the most erosive condition. Using the numerical results, one reason behind these stronger nonuniformities is identified to be the stronger bursting of vortices shed from the shaft in the most erosive condition.
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| 6. |
- Ask, Jonas, 1970, et al.
(författare)
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A Numerical Investigation of the Flow Past a Generic Mirror and its Impact on Sound Generation
- 2009
-
Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 131:061102, s. 0621011-06110212
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Konferensbidrag (refereegranskat)abstract
- The case investigated is the flow past a generic side mirror mounted on a flat plate at the Reynolds number of Re(D)=5.2x10(5) based on the mirror diameter. The present work studies both flow and acoustic sources by evaluating two second-order advection schemes, different levels of turbulence modeling, and three different grids. The advection schemes discussed in the present study are a second-order upwind scheme and a monotonic central scheme. The turbulence models investigated cover three levels of modeling. These are the original formulation of the detached eddy simulation (DES) model, the Smagorinsky-Lilly sub-grid scale (SGS) model with near-wall damping, and a dynamic Smagorinsky model. The different grids are as follows: a primary grid where all parameter studies are conducted and a second grid with significantly higher wake resolution and to some extent also increased plate resolution, while maintaining the resolution at the front side of the mirror. The final grid uses a significantly higher plate resolution and a wake resolution similar to that of grid two, but a comparably lower mirror front side resolution as compared with the two other grids. The general outcome of this work is that the estimation of the grid cutoff frequency through a relation of the velocity fluctuation and the grid size matches both the experimental results and trend lines perfectly. Findings from the flow field show that the horseshoe vortex in front of the mirror causes pressure fluctuations with a magnitude exceeding the maximum levels at the rear side of the mirror. Its location and unsteady properties are perfectly captured in the final simulation as compared with the experiments conducted by Daimler-Chrysler. A laminar separation at the front side of the mirror is more or less found for all wall resolved cases except the DES simulation. The third grid fails to predict this flow feature, but it is shown that this effect has no significant effect on either the static pressure sensors at the mirror surface or at the dynamic sensors located downstream of the mirror. The simulation also supports the fundamental frequency based on the eddy convection in the mirror shear layer, which is shown to be twice as high as the frequency peak found in the lateral force spectra.
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| 7. |
- Ask, J., et al.
(författare)
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Flow and Dipole Source Evaluation of a Generic SUV
- 2010
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Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 132:5, s. 0511111-05111112
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Tidskriftsartikel (refereegranskat)abstract
- Accurately predicting both average flow quantities and acoustic sources at the front window of today's ground vehicles are still a considerable challenge to automotive companies worldwide. One of the most important aspects in terms of obtaining not only trustworthy results but also the most tedious one and therefore perhaps overlooked, is the control and outcome of the mesh generation process. Generating unstructured volume meshes suitable for large eddy simulations with high level representation of geometrical details is both a time consuming and an extremely computer demanding activity. This work investigates two different mesh generation processes with its main aim to evaluate their outcome with respect to the prediction of the two dominating dipole sources in a temporal form of the Curle's equation. Only a handful of papers exists that report a high level representation of the vehicle geometry and the aim of predicting the fluctuating exterior noise sources. To the author's knowledge no studies have been conducted in which both these source terms are evaluated quantitatively against measurements. The current paper investigates the degree to which the amplitude of these two source terms can be predicted by using the traditional law-of-the-wall and hex-dominant meshes with isotropic resolution boxes for a detailed ground vehicle geometry. For this purpose, the unstructured segregated commercial FLUENT finite volume method code is used. The flow field is treated as incompressible and the Smagorinsky-Lilly model is used to compute the subgrid stresses. Mean flow quantities are measured with a 14 hole probe for 14 rakes downstream of the side mirror. The dynamic pressure sensors are distributed at 16 different positions over the side window to capture the fluctuating pressure signals. All measurements in this work were conducted at Ford's acoustic wind tunnel in Cologne. All three simulations accurately predict the velocity magnitude closest to the window and downstream of the mirror head recirculation zone. Some variations in the size and shape of this recirculation zone are found between the different meshes, most probably caused by differences in the detachment of the mirror head boundary layer. The Strouhal number of the shortest simulation was computed from the fundamental frequency of the drag force coefficient. The computed Strouhal number agrees well with the corresponding results from similar objects and gives an indication of an acceptable simulation time. The dynamic pressure sensors at 16 different locations at the vehicle side window were also used to capture the levels of the two dipole source terms. These results are compared with the three simulations. With the exception of three positions, at least one of the three simulations accurately captures the levels of both source terms up to about 1000 Hz. The three positions with less agreement as compared with measurements were found to be in regions sensitive to small changes in the local flow direction. [DOI: 10.1115/1.4001340]
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| 8. |
- Baidar, Binaya, et al.
(författare)
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Numerical study of the Winter-Kennedy method : a sensitivity analysis
- 2018
-
Ingår i: Journal of Fluids Engineering. - : The American Society of Mechanical Engineers (ASME). - 0098-2202 .- 1528-901X. ; 140:5
-
Tidskriftsartikel (refereegranskat)abstract
- The Winter-Kennedy (WK) method is commonly used in relative discharge measurement and to quantify efficiency step-up in hydropower refurbishment projects. The method utilizes the differential pressure between two taps located at a radial section of a spiral case, which is related to the discharge with the help of a coefficient and an exponent. Nearly a century old and widely used, the method has shown some discrepancies when the same coefficient is used after a plant upgrade. The reasons are often attributed to local flow changes. To study the change in flow behavior and its impact on the coefficient, a numerical model of a semi-spiral case (SC) has been developed and the numerical results are compared with experimental results. The simulations of the SC have been performed with different inlet boundary conditions. Comparison between an analytical formulation with the computational fluid dynamics (CFD) results shows that the flow inside an SC is highly three-dimensional (3D). The magnitude of the secondary flow is a function of the inlet boundary conditions. The secondary flow affects the vortex flow distribution and hence the coefficients. For the SC considered in this study, the most stable WK configurations are located toward the bottom from θ =30deg to 45deg after the curve of the SC begins, and on the top between two stay vanes.
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| 9. |
- Bensow, Rickard, 1972, et al.
(författare)
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Implicit LES Predictions of the Cavitating Flow on a Propeller
- 2010
-
Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 132:4, s. 0413021-04130210
-
Tidskriftsartikel (refereegranskat)abstract
- We describe an approach to simulate dynamic cavitation behavior based on large eddy simulation of the governing flow, using an implicit approach for the subgrid terms together with a wall model and a single fluid, two-phase mixture description of the cavitation combined with a finite rate mass transfer model. The pressure-velocity coupling is handled using a PISO algorithm with a modified pressure equation for improved stability when the mass transfer terms are active. The computational model is first applied to a propeller flow in homogeneous inflow in both wetted and cavitating conditions and then tested in an artificial wake condition yielding a dynamic cavitation behavior. Although the predicted cavity extent shows discrepancy with the experimental data, the most important cavitation mechanisms are present in the simulation, including internal jets and leading edge desinence. Based on the ability of the model to predict these mechanisms, we believe that numerical assessment of the risk of cavitation nuisance, such as erosion or noise, is tangible in the near future.
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| 10. |
- Bonitz, Sabine, 1985, et al.
(författare)
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Numerical investigation of crossflow separation on the a-pillar of a passenger car
- 2018
-
Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 140:11
-
Tidskriftsartikel (refereegranskat)abstract
- The flow around passenger cars is characterized by many different separation structures, typically leading to vortices and areas of reversed flow. The flow phenomena in these patches show a strong interaction and the evolution of flow structures is difficult to understand from a physical point of view. Analyzing surface properties, such as pressure, vorticity, or shear stress, helps to identify different phenomena, but still it is not well understood how these are created. This paper investigates the crossflow separation (CFS) on the A-pillar of a passenger car using numerical simulations. It is discussed how the CFS and the resulting A-pillar vortex can be identified as well as how it is created. Additionally, the vortex strength is determined by its circulation to understand and discuss how the vortex preserves until it merges with the rear wake of the vehicle.
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| 11. |
- Bredberg, Jonas, 1971, et al.
(författare)
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Low-Reynolds Number Turbulence Models:An Approach for Reducing Mesh Sensitivity
- 2004
-
Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 126:1, s. 14-21
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Tidskriftsartikel (refereegranskat)abstract
- This study presents a new near-wall treatment for low-Reynolds number (LRN) turbulence models that maintains accuracy in 'coarse' mesh predictions. The method is based on a thorough examination of approximations made when integrating the discretized equations in the near-wall region. A number of modifications are proposed that counteract errors introduced when an LRN-model is used on meshes for which the first interior node is located at y+≈5. Here the methodology is applied to the k-ω turbulence model by Bredberg et al., although similar corrections are relevant for all LRN models. The modified model gives asymptotically, in the sense of mesh refinement, identical results to the baseline model. For coarser meshes (y+≤10), the present method improves numerical stability with less mesh-dependency than the non-modified model. Results are included for fully developed channel flow, a backward-facing step flow and heat transfer in a periodic rib-roughened channel.
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| 12. |
- Carlsson, Allan, 1979-, et al.
(författare)
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Fibre orientation control related to papermaking
- 2007
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Ingår i: Journal of Fluids Engineering. - : ASME International. - 0098-2202 .- 1528-901X. ; 129:4, s. 457-465
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Tidskriftsartikel (refereegranskat)abstract
- The orientation of fibers suspended in a shear flow flowing over a solid wall has been studied experimentally. The possibility to control this orientation with physical surface modifications, ridges, has also been studied. The fiber suspension was driven by gravity down a slightly inclined glass plate and a CCD-camera was used to capture images of the fibers in the flow. Image analysis based on the concept of steerable filters extracted the position and orientation of the fibers in the plane of the image. From these data, the velocity of the fibers was determined. When viewing the flow from the side, the velocity of the fibers at different heights was measured and found to agree with the theoretical solution for Newtonian flow down an inclined plate. Moving the camera so that the flow was filmed from below, the orientation and velocity of fibers in the plane parallel to the solid surface was determined. The known relationship between the velocity and the wall normal position of the fibers made it possible to determine the height above the plate for each identified fiber. Far away from the wall, the fibers were aligned with the flow direction in both cases. In a region close to the smooth plate surface the fibers oriented themselves perpendicular to the flow direction. This change in orientation did not occur when the surface structure was modified with ridges.
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| 13. |
- Cervantes, Michel, et al.
(författare)
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Factorial design applied to CFD
- 2004
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Ingår i: Journal of Fluids Engineering. - : ASME International. - 0098-2202 .- 1528-901X. ; 126:5, s. 791-798
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Tidskriftsartikel (refereegranskat)abstract
- Factorial design, a statistical method widely used for experiments, and its application to CFD are discussed. The aim is to propose a systematic, objective, and quantitative method for engineers to design a set of simulations in order to evaluate main and joint effects of input parameters on the numerical solution. The input parameters may be experimental uncertainty on boundary conditions, unknown boundary conditions, grid, differencing schemes, and turbulence models. The complex flow of the Turbine-99 test case, a hydropower draft tube flow, is used to illustrate the method, where four factors are chosen to perform a 24 factorial design. The radial velocity at the inlet (not measured) is shown to have an important influence on the pressure recovery (7%) and the energy loss factor (49%).
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| 14. |
- Cervantes, Michel, et al.
(författare)
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On the use of the Squire-Long equation to estimate radial velocities in swirling flows
- 2007
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Ingår i: Journal of Fluids Engineering. - : ASME International. - 0098-2202 .- 1528-901X. ; 129:2, s. 209-217
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Tidskriftsartikel (refereegranskat)abstract
- A method to estimate the radial velocity in swirling flows from experimental values of the axial and tangential velocities is presented. The study is motivated by the experimental difficulties to obtain this component in a draft tube model as evidenced in the Turbine-99 IAHR/ERCOFTAC Workshop. The method uses a two-dimensional nonviscous description of the flow. Such a flow is described by the Squire-Long equation for the stream function, which depends on the boundary conditions. Experimental values of the axial velocities at the inlet and outlet of the domain are used to obtain the boundary conditions on the bounded domain. The method consists of obtaining the equation related to the domain with an iterative process. The radial velocity profile is then obtained. The method may be applied to flows with a swirl number up to about Sw=0.25. The critical value of the swirl number depends on the velocity profiles and the geometry of the domain. The applicability of the methodology is first performed on a swirling flow in a diffuser with a half angle of 3 deg at various swirl numbers, where three-dimensional (3D) laser Doppler velocimeter (LDV) velocity measurements are available. The method is then applied to the Turbine-99 test case, which consists in a model draft tube flow where the radial inlet velocity was undetermined. The swirl number is equal to Sw=0.21. The stability and the convergence of the approach is investigated in this case. The results of the pressure recovery are then compared to the experiments for validation.
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| 15. |
- Dobriyal, Ritvik, et al.
(författare)
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Effects of Streamlining a Bluff Body in the Laminar Vortex Shedding Regime
- 2020
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Ingår i: Journal of Fluids Engineering. - : ASME. - 0098-2202 .- 1528-901X. ; 142:2
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Tidskriftsartikel (refereegranskat)abstract
- Two-dimensional flow over bluff bodies is studied in the unsteady laminar flow regime using numerical simulations. In previous investigations, lift and drag forces have been studied over different cross-sectional shapes like circles, squares, and ellipses. We aim to extend the previous research by studying the variation of hydrodynamic forces as the shape of the body changes from a circular cylinder to a more streamlined or a bluffer body. The different body shapes are created by modifying the downstream circular arc of a circular cylinder into an ellipse, hence elongating or compressing the rear part of the body. The precise geometry of the body is quantified by defining a shape factor. Two distinct ranges of shape factors with fundamentally different behavior of lift and drag are identified. The geometry constituting the limit is where the rear part ellipse has a semi-minor axis of half the radius of the original circle, independent of the Reynolds number. On the other hand, the vortex shedding frequency decreases linearly over the whole range of shape factors. Furthermore, the variation of the forces and frequency with Reynolds number, and how the relations vary with the shape factor are reported.
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| 16. |
- Ekman, Petter, 1988-, et al.
(författare)
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Importance of Sub-Grid Scale Modeling for Accurate Aerodynamic Simulations
- 2021
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Ingår i: Journal of Fluids Engineering. - : ASME. - 0098-2202 .- 1528-901X. ; 143:1
-
Tidskriftsartikel (refereegranskat)abstract
- The Ahmed body is one of the most well-investigated vehicle bodies for aerodynamic purposes. Despite its simple geometry, the flow around the body, especially at the rear, is very complex as it is dominated by a large wake with strong interaction between vortical structures. In this study, the flow around the 25 deg Ahmed body has been investigated using large eddy simulations and compared to high-resolution particle image velocimetry (PIV) measurements. Special emphasis was put on studying three commonly used sub-grid scale (SGS) models and their ability to capture vortical structures around the Ahmed body. The ability of the SGS models to capture the near-wall behavior and small-scale dissipation is crucial for capturing the correct flow field. Very good agreement between simulations and PIV measurements were seen when using the dynamic Smagorinsky-Lilly and the wall-adopting local eddy-viscosity SGS models, respectively. However, the standard Smagorinsky-Lilly model was not able to capture the flow patterns when compared to the PIV measurements due to shortcomings in the near-wall modeling in the standard Smagorinsky-Lilly model, resulting in overpredicted separation.
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| 17. |
- Feymark, Andreas, 1984, et al.
(författare)
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Numerical Simulation of an Oscillating Cylinder Using Large Eddy Simulation and Implicit Large Eddy Simulation
- 2012
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Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 134:3
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we use large eddy simulation (LES) to study the influence of grid and subgrid model on the lift and drag force predictions of a fixed cylinder undergoing streamwise sinusoidal oscillations in a steady flow, resulting in a varying Reynolds number, Re, within the range 405
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| 18. |
- Gao, Xiufang, et al.
(författare)
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Effects of inclination angle of ribs on the flow behavior in rectangular ducts
- 2004
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Ingår i: Journal of Fluids Engineering. - : ASME International. - 0098-2202 .- 1528-901X. ; 126:4, s. 692-699
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Tidskriftsartikel (refereegranskat)abstract
- The flow behavior in rib-roughened ducts is influenced by the inclination of ribs and the effect is investigated in the present study by Particle Image Velocimetry (PIV). The local flow structures between two adjacent ribs were measured. The Reynolds number was fixed at 5800. The flow field description was based on the PIV results in planes both parallel and perpendicular to the ribbed walls at various locations. The rib angle to the main flow direction was varied as 30 deg, 45 deg, 60 deg and 90 deg. The ribs induce three dimensional flow fields. The flow separation and reattachment between adjacent ribs are clearly observed. In addition, the inclined ribs are found to alter the spanwise distribution of the streamwise velocity component. The streamwise velocity component has its highest values at the upstream end of the ribs, and decreases continuously to its lowest values at the downstream end. Strong secondary flow motion occurs over the entire duct cross section for the inclined ribs. The flow structures between two consecutive ribs show that the fluid flows along the ribs from one end of the ribs to the other end, and then turns back at the transverse center Downwash and upwash flows are observed at the upstream end and downstream end of the ribs, respectively.
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| 19. |
- Ghadimi, P., et al.
(författare)
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Application of an iterative high order difference scheme along with an explicit system solver for solution of stream function-vorticity form of navier-stokes equations
- 2013
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Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X. ; 135:4
-
Tidskriftsartikel (refereegranskat)abstract
- This paper describes the general convection-diffusion equation in 2D domain based on a particular fourth order finite difference method. The current fourth-order compact formulation is implemented for the first time, which offers a semi-explicit method of solution for the resulting equations. A nine point finite difference scheme with uniform grid spacing is also put into action for discretization purpose. The proposed numerical model is based on the Navier-Stokes equations in a stream function-vorticity formulation. The fast convergence characteristic can be mentioned as an advantage of this scheme. It combines the enhanced Fournié's fourth order scheme and the expanded fourth order boundary conditions, while offering a semi-explicit formulation. To accomplish this, some coefficients which do not influence the solutions are also omitted from Fournié's formulation. Consequently, very accurate results can be acquired with a relatively coarse mesh in a short time. The robustness and accuracy of the proposed scheme is proved using the benchmark problems of flow in a driven square cavity at medium and relatively high Reynolds numbers, flow over a backward-facing step, and flow in an L-shaped cavity.
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| 20. |
- Ghahremanian, Shahriar, et al.
(författare)
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Evaluation of RANS models in predicting low reynolds, free, turbulent round jet
- 2014
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Ingår i: Journal of Fluids Engineering. - : ASME International. - 0098-2202 .- 1528-901X. ; 136:1
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Tidskriftsartikel (refereegranskat)abstract
- In order to study the flow behavior of multiple jets, numerical prediction of the three-dimensional domain of round jets from the nozzle edge up to the turbulent region is essential. The previous numerical studies on the round jet are limited to either two-dimensional investigation with Reynolds-averaged Navier-Stokes (RANS) models or three-dimensional prediction with higher turbulence models such as large eddy simulation (LES) or direct numerical simulation (DNS). The present study tries to evaluate different RANS turbulence models in the three-dimensional simulation of the whole domain of an isothermal, low Re (Re = 2125, 3461, and 4555), free, turbulent round jet. For this evaluation the simulation results from two two-equation (low Re k-ε and low Re shear stress transport (SST) k-ω), a transition three-equation (k-kl-ω), and a transition four-equation (SST) eddy-viscosity turbulence models are compared with hot-wire anemometry measurements. Due to the importance of providing correct inlet boundary conditions, the inlet velocity profile, the turbulent kinetic energy (k), and its specific dissipation rate (ω) at the nozzle exit have been employed from an earlier verified numerical simulation. Two-equation RANS models with low Reynolds correction can predict the whole domain (initial, transition, and fully developed regions) of the round jet with prescribed inlet boundary conditions. The transition models could only reach to a good agreement with the measured mean axial velocities and its rms in the initial region. It worth mentioning that the round jet anomaly is still present in the turbulent region of the round jet predicted by the low Re k-ε. By comparing the k and the ω predicted by different turbulence models, the blending functions in the cross-diffusion term is found one of the reasons behind the more consistent prediction by the low Re SST k-ω.
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| 21. |
- Ghahremanian, Shahriar, et al.
(författare)
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Investigation in the near-field of a row of interacting jets
- 2015
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Ingår i: Journal of Fluids Engineering. - : ASME International. - 0098-2202 .- 1528-901X. ; 137:12
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Tidskriftsartikel (refereegranskat)abstract
- Multiple interacting jets (confluent jets) are employed in many engineering applications, and the significant design factors must be investigated. Computational fluid dynamics (CFD) is used to numerically predict the flow field in the proximal region of a single row of round jets. The numerical results that are obtained when using the low Reynolds k — ε are validated with the experimental data that is acquired by particle image velocimetry (PIV). The low Reynolds k — ε underpredicts the streamwise velocity in the onset of the jets’ decay. The characteristic points are determined for various regions between two neighboring jets. The comparison of the merging point and the combined point computed from measurements and simulations shows good agreement in the different regions between the jets. In this study, a computational parametric study is also conducted to determine the main effects of three design factors and the interactions between them on the flow field development using response surface methodology. The influences of the inlet velocity, the spacing between the nozzles and the diameter of the nozzles on the locations of the characteristic points are presented in the form of correlations (regression equations). CFD is used to numerically predict the characteristic points for a set of required studies, for which the design values of the simulation cases are determined by the Box-Behnken method. The results indicate that the spacing between the nozzles has a major impact on the flow characteristics in the near-field region of multiple interacting jets. The response surface methodology shows that the inlet velocity has a marginal effect on the merging and combined points.
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| 22. |
- Ghosh, Debarshee, 1995, et al.
(författare)
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Aerodynamic Analysis of Low-Pressure Axial Fans Installed in Parallel
- 2024
-
Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - 1528-901X .- 0098-2202. ; 146:5
-
Tidskriftsartikel (refereegranskat)abstract
- Ducted rotor-only low-pressure axial fans play an integral role in automotive thermal management. The tightly packed under-hood region and down-stream heat-exchanger shape limit the fan diameter. In order to circumvent this limitation, multiple cooling fans of small diameters are tightly packaged and placed in parallel. Currently, there is limited scientific work, that study the aerodynamics of low-pressure axial fans when installed in parallel. This work aims to quantify the aerodynamic performance and the flow-field as a result of installing low-pressure axial fans in parallel through computational fluid dynamics (CFD). Publicly available experimental data from Friedrich-Alexander University is used for the validation of the numerical setup. Three-dimensional, full-annulus, unsteady Reynolds-averaged Navier‐Stokes (URANS) analysis has been performed for both a single-fan and two-fans installed in parallel and their respective aerodynamic performance has been compared for the operation condition identified as the best efficiency point in experiments. Only small differences are observed in the overall aerodynamic performance of the two-fans in parallel compared to a single-fan. A circumferential nonuniformity in the form of a local high-pressure zone at the inlet of the fan is observed when the two-fans are placed in parallel. The resulting circumferential nonuniformity is quantified, both in space and time. A strong correlation is found between the pressure fields of the two-fans installed in parallel.
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| 23. |
- Goyal, Rahul, et al.
(författare)
-
A Study of the Velocity Field During Mitigation of Vortex Breakdown in Model Francis Turbine at High Load
- 2023
-
Ingår i: Journal of Fluids Engineering. - : Asme. - 0098-2202 .- 1528-901X. ; 145:4
-
Tidskriftsartikel (refereegranskat)abstract
- Off-design operation leads to the development of flow instabilities like vortex breakdown phenomenon which manifests as an enlarged vortex core in the draft tube at high load operating conditions. These flow instabilities are known to potentially have detrimental effects on turbine performance necessitating investigations on their formative and mitigation mechanisms. This work clarifies the evolving velocity maps characterizing vortex breakdown seen in a model Francis turbine draft tube during the transition from high load to best efficiency point. Velocity measurements have been performed inside a draft tube cone using a 2D particle image velocimetry system. Results show a wake-like velocity profile characterizing the vortex core in the draft tube cone at high load condition. The vortex core is a centrally located flow feature embodying a quasi-stagnant flow with recirculation regions. Surrounding the core, an axial outflow is seen with shear layers arising at the interface of core and outflow due to a substantial velocity gradient. Mitigation of this vortex core through a load rejection operation was further investigated. It is seen that as the flowrate approaches the best efficiency point, the shear layers between the outflow and central stagnation region break. The breakup leads to an axially dominated and streamlined flow. This is enabled by the reduction of the swirl until no central flow separation at the stagnation point occurs. The flow at the best efficiency point is thus devoid of the vortex core due to the absence of flow stagnation, the primary instability causing the core development.
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| 24. |
- Goyal, Rahul, et al.
(författare)
-
Vortex Rope Formation in a High Head Model Francis Turbine
- 2017
-
Ingår i: Journal of Fluids Engineering. - : American Society for Mechanical Engineers (ASME). - 0098-2202 .- 1528-901X. ; 139:4
-
Tidskriftsartikel (refereegranskat)abstract
- Francis turbine working at off-design operating condition experiences high swirling flow at the runner outlet. In the present study, a high head model Francis turbine was experimentally investigated during load rejection, i.e., best efficiency point (BEP) to part load (PL), to detect the physical mechanism that lies in the formation of vortex rope. For that, a complete measurement system of dynamic pressure, head, flow, guide vanes (GVs) angular position, and runner shaft torque was setup with corresponding sensors at selected locations of the turbine. The measurements were synchronized with the twodimensional (2D) particle image velocimetry (PIV) measurements of the draft tube. The study comprised an efficiency measurement and maximum hydraulic efficiency of 92.4±0.15% was observed at BEP condition of turbine. The severe pressure fluctuations corresponding to rotor-stator interaction (RSI), standing waves, and rotating vortex rope (RVR) have been observed in the draft tube and vaneless space of the turbine. Moreover, RVR in the draft tube has been decomposed into two different modes; rotating and plunging modes. The time of occurrence of both modes was investigated in pressure and velocity data and results showed that the plunging mode appears 0.8 s before the rotating mode. In the vaneless space, the plunging mode was captured before it appears in the draft tube. The physical mechanism behind the vortex rope formation was analyzed from the instantaneous PIV velocity vector field. The development of stagnation region at the draft tube center and high axial velocity gradients along the draft tube centerline could possibly cause the formation of vortex rope
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| 25. |
- Hadi Jafari, Pantea, et al.
(författare)
-
Modeling of Particle-Laden Cold Flow in a Cyclone Gasifier
- 2018
-
Ingår i: Journal of Fluids Engineering. - : The American Society of Mechanical Engineers (ASME). - 0098-2202 .- 1528-901X. ; 141:2
-
Tidskriftsartikel (refereegranskat)abstract
- Isothermal transient Eulerian–Lagrangian simulation of the turbulent gas–solid flow in a cyclone gasifier with two inlet tubes at 890 °C has been performed. The single-phase gas flow is modeled using SSG Reynolds stress turbulence model. Ten thousand representative solid particles of different sizes are injected from each inlet continuously at every second of simulation time. Particles are finally stopped as soon as they arrive at the outlet or reach the bottom plate of the gasifier. The effect of particle-to-gas coupling on the pressure and velocity of the flow and particles motion inside the gasifier is studied. The numerical approach can reasonably predict the impact of particle load on the gas flow as presented in the experimental results. Single particles are traveled throughout the transient gas flow field by using Lagrangian approach. High temperature of the gas flow inside the gasifier has significant effects on the swirl intensity reduction, damping the turbulence in the core region, pressure, and particle behaviors. However, the presence of solid particles does not have a notable influence on the swirl intensity and turbulence.
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| 26. |
- Han, Xingsi, 1981, et al.
(författare)
-
Very Large Eddy Simulation of Passive Drag Control for a D-Shaped Cylinder
- 2013
-
Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 135:10
-
Tidskriftsartikel (refereegranskat)abstract
- The numerical study reported here deals with the passive flow control around a two-dimensional D-shaped bluff body at a Reynolds number of Re=3.6×104. A small circular control cylinder located in the near wake behind the main bluff body is employed as a local disturbance of the shear layer and the wake. 3D simulations are carried out using a newly developed very large eddy simulation (VLES) method, based on the standard k − ε turbulence model. The aim of this study is to validate the performance of this method for the complex flow control problem. Numerical results are compared with available experimental data, including global flow parameters and velocity profiles. Good agreements are observed. Numerical results suggest that the bubble recirculation length is increased by about 36% by the local disturbance of the small cylinder, which compares well to the experimental observations in which the length is increased by about 38%. A drag reduction of about 18% is observed in the VLES simulation, which is quite close to the experimental value of 17.5%. It is found that the VLES method is able to predict the flow control problem quite well.
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| 27. |
- HELGASON, EYSTEINN, 1983, et al.
(författare)
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Optimization using Arbitrary Lagrangian-Eulerian formulation of the Navier-Stokes equations
- 2015
-
Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 137:6, s. Art. no. 061202-
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper we present a new shape optimizationmethod by using sensitivities obtained from the ArbitraryLagrangian-Eulerian (ALE) form of the Navier-Stokesequations. In the ALE description the nodes of thecomputational domain may be moved with the fluid asin the Lagrangian description, held fixed in space as inthe Eulerian description or moved in some arbitrary wayin between. Applying the adjoint method with respectto mesh motion allows the whole sensitivity field for theshape changes to be calculated using only two solvercalls, a primal solver call and an adjoint solver call. Weshow that the sensitivities with respect to the meshmotion can be calculated in a post processing step to theprimal and adjoint flow simulations. The resulting ALEsensitivities are compared to sensitivities obtained usinga finite difference approach. Finally, the sensitivities arecoupled to a mesh motion smoothing algorithm, and aduct is optimized with respect to the total pressure dropusing the proposed method.
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| 28. |
- Hemida, Hassan, 1972, et al.
(författare)
-
Transient Simulation of the Aerodynamic Response of a Double-Deck Bus in Gusty Winds
- 2008
-
Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 131:3, s. 0311011-03110110
-
Tidskriftsartikel (refereegranskat)abstract
- The purpose of the research reported in this paper was to investigate the aerodynamic response of a double-deck bus in gusty winds using a Detached-Eddy Simulation (DES). The bus was subjected to three different scenarios of wind gusts: gust in a wind tunnel, gust in a natural wind and gust behind the exit of a tunnel. The proposed scenarios of gusts are in the time domain and take into accountthe dynamic behavior of natural winds. The Reynolds number of the flow, based on the time-averaged speed of the side wind and a reference length of squre root of 0.1 [m], was 1300000. Detailed transient responses of the aerodynamic coefficients and flow structures were investigated. Good agreement was found between the DES results and the available experimental data. A comparison between the influence of the different gust scenarios on the aerodynamic coefficients shows that the gust behind the exit from a tunnel has a stronger influence on the aerodynamics than the other gust scenarios. Moreover, the influence of the gusts on the time history of aerodynamics coefficients is found to be limited to the period of the gust.
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| 29. |
- Holmstedt, Elise, et al.
(författare)
-
Modeling Transport and Deposition Efficiency of Oblate and Prolate Nano- and Micro-particles in a Virtual Model of the Human Airway
- 2016
-
Ingår i: Journal of Fluids Engineering. - : ASME International. - 0098-2202 .- 1528-901X. ; 138:8
-
Tidskriftsartikel (refereegranskat)abstract
- A model for the motion and deposition of oblate and prolate spheroids in the nano- and microscale was developed. The aim was to mimic the environment of the human lung, but the model is general and can be applied for different flows and geometries for small nonspherical particle Stokes and Reynolds numbers. A study of the motion and orientation of a single oblate and prolate particle has been done yielding that Brownian motion disturbs the Jeffery orbits for small particles. Prolate microparticles still display distinguishable orbits while oblate particles of the same size do not. A statistical study was done comparing the deposition efficiencies of oblate and prolate spheroids of different size and aspect ratio observing that smaller particles have higher deposition rate for lower aspect ratio while larger particles have higher deposition rates for large aspect ratio.
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| 30. |
- Hosseini, Seyed Mohammad, et al.
(författare)
-
Experimental Investigation of Flow Field Structure in Mixing Tee
- 2009
-
Ingår i: Journal Of Fluids Engineering-Transactions Of The Asme. - : ASME International. - 0098-2202. ; 131:5
-
Konferensbidrag (refereegranskat)abstract
- T-junction is one of the familiar components in the cooling system of power plants with enormous capability of high-cycle thermal fatigue. This research investigates the structure and mixing mechanism of turbulent flow in a T-junction area with a 90 deg bend upstream. According to the wide distribution of turbulent jets in the T-junction, a re-attached jet was selected previously as the best representative condition with the highest velocity fluctuation and the most complex structure. For considering the mixing mechanism of re-attached jet, T-junction is subdivided into few lateral and longitudinal sections, and each section is visualized separately by particle image velocimetry technique. Corresponding to the experimental data, the branch flow acts as a finite turbulent jet, develops the alternative type of eddies, and causes the high velocity fluctuation near the main pipe wall. Three regions are mainly subject to maximum velocity fluctuation: the region close to the jet boundaries (fluctuation mostly is caused by Kelvin-Helmholtz instability), the region above the jet and along the main flow (fluctuation mostly is caused by Karman vortex), and the re-attached area (fluctuation mostly is caused by changing the pressure gradient in the wake area above the jet). Finally, the re-attached area (near the downstream of wake area above the jet) is introduced as a region with strongest possibility to high-cycle thermal fatigue with most effective velocity fluctuation on the main pipe wall above the branch nozzle. [DOI: 10.1115/1.3112383]
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| 31. |
- Högberg, Sofie M., et al.
(författare)
-
Time-dependent deposition of micro- and nanofibers in straight model airways
- 2012
-
Ingår i: Journal of Fluids Engineering. - : ASME International. - 0098-2202 .- 1528-901X. ; 134:5, s. 051208-1
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, we increase the understanding of the influence of the breathing pattern on the fate of inhaled non-spherical micro and nanoparticles and examine the accuracy of replacing the cyclic flow field with a quasi-steady flow. This is done with new analysis and numerical simulations on straight model airways using a previously developed discrete model for fiber motion. For the conditions studied, maximum deposition is obtained when fibers are released at the start of the inspiratory cycle, and minimum is received at the peak of inhalation. A quasi-steady solution generally provides a relatively good approximation to cyclic flow if an average velocity over one residence time of the particles moving with the mean fluid velocity is used. For a batch type, supply of particles deposition is favored in light activity breathing as compared to heavy breathing and the inclusion of a short pause after the inhalation results in an increased deposition in the terminal bronchiole. During zero-flow over the time of a breathing pause, spherical 10 nm particles experience considerable deposition in the distal airways, whereas only a few percent of larger and/ or fibrous nanoparticles were deposited. Hence, size and shape are crucial variables for deposition for no flow conditions. Common for all breathing parameters examined was that minimum deposition was obtained for the spherical 1 µm-particles and the fibrous 100 nm-particles. The former is expected from studies on spherical particles, and the latter is in agreement with results from a recent publication on steady inspiration.
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| 32. |
- Jansson, Marcus, et al.
(författare)
-
Experimental Assessment of Water Hammer-Induced Column Separation in Oil-Hydraulic Pipe Flow
- 2019
-
Ingår i: Journal of Fluids Engineering. - : ASME. - 0098-2202 .- 1528-901X. ; 141:10
-
Tidskriftsartikel (refereegranskat)abstract
- Cavitation erosion through water hammer and column separation is a major concern in hydraulic applications such as percussive rock drilling. Cavitation aspects must be considered both in early and late design stages, which require deep knowledge and tools for prediction. In this study, an oil-hydraulic test equipment for water hammer assessment was designed using state-of-the-art simulation tools. Several tests were performed, with and without column separation, showing good repeatability on measured pressures. At higher flow rates, column separation was the dominating feature and several high-ressure peaks with subsequent time delay reduction could be observed. These patterns were affected by the oil temperature, with most substantial changes at lower temperature ranges (amp;lt; 32 degrees C). Standard transmission line simulations managed to predict the water hammer, but as expected not the column separation, which is the theme of future work using this setup.
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| 33. |
- Javadi, Ardalan, 1984, et al.
(författare)
-
Experimental and numerical investigation of the precessing helical vortex in a conical diffuser, with rotor-stator interaction
- 2016
-
Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 138:8, s. Art. no. 081106-
-
Tidskriftsartikel (refereegranskat)abstract
- The flow unsteadiness generated in a swirling apparatus is investigated experimentally and numerically. The swirl apparatus has two parts: a swirl generator and a test section. The swirl generator includes two blade rows, one stationary and one rotating, is designed such that the emanating flow resembles that of a Francis hydro turbine operated at partial discharge. The test section consists of a conical diffuser similar to the draft tube cone of a Francis turbine. A new control method based on a magneto rheological brake is employed in the rotating section, runner, in order to produce several swirling flow regimes. The LDV measurements are performed along three survey axes in the test section. The measured mean velocity components and its fluctuating parts are used to validate the results of unsteady numerical simulations, conducted using the FOAM-extend-3.0 CFD code. A dynamic mesh is used together with the sliding General Grid Interfaces (GGI) to mimic the effect of the rotating runner. The delayed detached eddy simulation method, conjugated with the Spalart-Allmaras turbulence model (DDES-SA), is applied to achieve a deep insight about the ability of this advanced modeling technique and the physics of the flow. The RNG k-epsilon model is also used to represent state-of-the art of industrial turbulence modeling. Both models predict the mean velocity reasonably well while DDES-SA presents more realistic flow features at the highest and lowest rotational speeds. The highest level of turbulence occurs at the highest and lowest rotational speeds which DDES-SA is able to predict well in the conical diffuser. The special shape of the blade plays more prominent role at lower rotational speeds and creates coherent structures with opposite sign of vorticity. The vortex rope is captured by both turbulence models while DDES-SA presents more realistic one at higher rotational speeds.
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| 34. |
- Jin, Zhi-jiang, et al.
(författare)
-
A parametric study of hydrodynamic cavitation inside globe valves
- 2018
-
Ingår i: Journal of Fluids Engineering. - : ASME International. - 0098-2202 .- 1528-901X. ; 140:3
-
Tidskriftsartikel (refereegranskat)abstract
- Hydrodynamic cavitation that occurs inside valves not only increases the energy consumption burden of the whole piping system but also leads to severe damages to the valve body and the piping system with a large economic loss. In this paper, in order to reduce the hydrodynamic cavitation inside globe valves, effects of valve body geometrical parameters including bending radius, deviation distance, and arc curvature linked to in/ export parts on hydrodynamic cavitation are investigated by using a cavitation model. To begin with, the numerical model is compared with similar works to check its accuracy. Then, the cavitation index and the total vapor volume are predicted. The results show that vapor primarily appears around the valve seat and connecting downstream pipes. The hydrodynamic cavitation does not occur under a small inlet velocity, a large bending radius, and a large deviation distance. Cavitation intensity decreases with the increase of the bending radius, the deviation distance, and the arc curvature linked to in/export parts. This indicates that valve geometrical parameters should be chosen as large as possible, while the maximal fluid velocity should be limited. This work is of significance for hydrodynamic cavitation or globe valve design.
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| 35. |
- Jin, Zhi-jiang, et al.
(författare)
-
Pressure Drop Analysis of Pilot-Control Globe Valve With Different Structural Parameters
- 2017
-
Ingår i: Journal of Fluids Engineering. - : ASME International. - 0098-2202 .- 1528-901X. ; 139:9
-
Tidskriftsartikel (refereegranskat)abstract
- Pilot-control globe valve (PCGV) can use the pressure drop caused by fluid flowing through the orifice located at valve core bottom to open or close the main valve using a small pilot valve. In this paper, computational fluid dynamics (CFD) method is adopted to analyze the pressure drop before and after valve core of PCGV and minor loss of orifice under different structural parameters and inlet velocities, and the simulation results show a good agreement with the experimental results. It turns out that the valve diameters, orifice diameters, and pilot pipe diameters have great influences on the pressure drop and the loss coefficient. Moreover, an expression is proposed which can be used to calculate minor loss coefficient, then to estimate the pressure drop and driving force of a PCGV within limited conditions. This paper can be referenced as guidance for deciding the dimension of structural parameters and spring stiffness during design process of a PCGV.
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| 36. |
- Josefsson, Erik, 1995, et al.
(författare)
-
Investigation of Tyre Pattern Effect on the Aerodynamics of a Passenger Vehicle
- 2022
-
Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 144:11
-
Tidskriftsartikel (refereegranskat)abstract
- The wheels of a passenger vehicle are one of the major contributors to the total aerodynamic drag, making their aerodynamic performance a considerable factor for the overall energy efficiency of the vehicle. Previous studies have shown that the complex flow field created by the wheels is sensitive to small geometrical variations of the tyre and that features such as shoulder profile and tread pattern can have a significant impact on drag and lift. In this study, the DrivAer model is used to evaluate the flow fields and aerodynamics of four tyre tread patterns with two rim designs. Full-scale wind tunnel tests were conducted where forces, surface pressures and flow fields were measured. Numerical simulations were also performed to aid the analysis. Using a slick tyre as the reference, it was found that rain grooves typically reduced the drag, whereas the effect of lateral grooves was dependent on the rim configuration. For the lift forces, the largest lift variations were obtained for the front lift which, in general, was reduced by rain grooves and increased by lateral grooves, most notably for the closed rim. The importance of considering the parasitic lift force acting on the wheel drive units when comparing experiments and simulations was demonstrated.
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| 37. |
- Jouybari, Nima, et al.
(författare)
-
A General Macroscopic Model for Turbulent Flow in Porous Media
- 2018
-
Ingår i: Journal of Fluids Engineering. - : The American Society of Mechanical Engineers (ASME). - 0098-2202 .- 1528-901X. ; 140:1
-
Tidskriftsartikel (refereegranskat)abstract
- The present study deals with the generalization of a macroscopic turbulence model in porous media using a capillary model. The additional source terms associated with the production and dissipation of turbulent kinetic energy due to the presence of solid matrix are calculated using the capillary model. The present model does not require any prior pore scale simulation of turbulent flow in a specific porous geometry in order to close the macroscopic turbulence equations. Validation of the results in packed beds, periodic arrangement of square cylinders, synthetic foams and longitudinal flows such as pipes, channels and rod bundles against available data in the literature reveals the ability of the present model in predicting turbulent flow characteristics in different types of porous media. Transition to the fully turbulent regime in porous media and different approaches to treat this phenomenon are also discussed in the present study. Finally, the general model is modified so that it can be applied to lower Reynolds numbers below the range of fully turbulent regime in porous media.
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| 38. |
- Kalantar Neyestanaki, Mehrdad, 1989-, et al.
(författare)
-
Experimental Study of The Pressure-Time Method With Potential Application for Low-Head Hydropower
- 2023
-
Ingår i: Journal of Fluids Engineering. - : American Society of Mechanical Engineers (ASME). - 0098-2202 .- 1528-901X. ; 145:7
-
Tidskriftsartikel (refereegranskat)abstract
- The pressure-time method allows measuring the flowrate in hydraulic turbines, according to IEC 60041 standard. According to this standard, the applicability of the pressure-time method is limited to a straight pipe with a constant cross section with specific limits for length and velocity. However, low-head hydropower plants usually have an intake with a variable cross section and short length making this method difficult to be applied. In this paper, a test rig has been developed to extend the method's applicability. The test rig is designed for developing flows condition and small measurement lengths for pipes with constant and variable cross section, which could be similar to low-head turbine conditions. For pipe with constant cross section, three measuring lengths of 0.5, 1, and 1.5 m can be obtained on this test rig to study the pressure-time method. For pipe with variable cross section, the method is applied across a concentric reducer with a length of 255 mm, i.e., a 9.46 deg reducing angle. Different flow assumptions for estimation of the head loss and dynamic pressure variation are considered to compare the accuracy of discharge measurement. The results showed that the quasi-steady assumption for friction factor and kinetic energy, along with friction factor correction, increases the method accuracy up to 0.4% compared with the current standard recommendations. Moreover, the Monte Carlo method (MCM) is applied to estimate the transient measurement uncertainty and compared with the Taylor series method (TSM).
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| 39. |
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| 40. |
- Khullar, Subodh, et al.
(författare)
-
Numerical Analysis of Water Jet Injection in the Draft Tube of a Francis Turbine at Part Load Operations
- 2022
-
Ingår i: Journal of Fluids Engineering. - : ASME Press. - 0098-2202 .- 1528-901X. ; 144:11
-
Tidskriftsartikel (refereegranskat)abstract
- The off-design operation of Francis turbines results in the onset of flow instabilities. These instabilities lead to severe pressure pulsations, power swings, fatigue damage, and torque fluctuations in the turbine unit. Axial water jet injection in the draft tube is a relatively recent method proposed to reduce the detrimental effects of flow instabilities on turbine performance. However, its efficacy at different operating points needs to be ascertained before implementing in actual prototype turbines. This work reports the findings of numerical investigations performed with water injection at three different part-load conditions. These operating points represent distinct flow regimes in the draft tube. The effect of water injection on the velocity and pressure fields in the draft tube is investigated. The results indicate that the water jet strongly influences the turbine performance at part-loads involving a precessing vortex rope. However, little influence of water jet is observed at deep part-load operation. The interaction of the jet with the draft tube bend is also investigated. The results show that the amount of water jet needs to be cautiously controlled as higher water jet injection impacting the bend may deteriorate the performance. The influence of water jet injection on the pressure recovery, power output, and efficiency of the turbine unit is also reported.
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| 41. |
- Krajnovic, Sinisa, 1970, et al.
(författare)
-
Comparison of Partially Averaged Navier–Stokes and Large-Eddy Simulations of the Flow Around a Cuboid Influenced by Crosswind
- 2012
-
Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 134:101202
-
Tidskriftsartikel (refereegranskat)abstract
- The paper presents a partially averaged Navier–Stokes (PANS) simulation of the flow around a cuboid influenced by crosswind. The results of the PANS prediction are validated against experimental data and results of a large-eddy simulation (LES) made using the same numerical conditions as PANS. The PANS shows good agreement with the experimental data. The prediction of PANS was found to be better than that of the LES in flow regions where simulations suffered from poor near-wall resolution.
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| 42. |
- Krajnovic, Sinisa, 1970, et al.
(författare)
-
Flow Around a Simplified Car, Part 1: Large Eddy Simulation
- 2005
-
Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 127:5, s. 907-918
-
Tidskriftsartikel (refereegranskat)abstract
- Large eddy simulations (LES) were made of flows around a generic ground vehicle with sharp edges at the rear end (an Ahmed body with a 25° angle of the rear slanted surface). Separation of the flow at the rear results in large regions with recirculating flow. As the separation is determined by the geometry, the Reynolds number effects are minimized. Resolution requirements of this recirculating flow are smaller than those in LES of wall attached flows. These two consequences of the geometry of the body are used to predict the experimental flow at relatively high Reynolds number. Recommendations are pre-sented for the preparation and realization of LES for vehicle flows. Comparison of the LES results with the experimental data shows good agreement. Copyright © 2005 by ASME.
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| 43. |
- Krajnovic, Sinisa, 1970, et al.
(författare)
-
Flow Around a Simplified Car, Part 2: Understanding the Flow
- 2005
-
Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 127:5, s. 919-928
-
Tidskriftsartikel (refereegranskat)abstract
- Results of a large eddy simulation (LES) are used to explore the flow around a generic car model. A new, refined picture of this flow is established. Many parts and aspects of this flow are studied and explained. The development of the instantaneous flow and its resulting time-averaged flow are depicted. Large differences are found between the instantaneous and the time-averaged flows. Special attention is given to the flow above the rear slanted surface. The origin, the development, and the interactions of the instantaneous vortices in this part of the flow are presented for the first time. This instantaneous flow is shown to be very unsteady and to contain a large number of different vortices that range in size from those of the size of the body over the intermediate hairpin-like vortices to very small coherent structures. Besides the variety in the length scales, the flow covers a wide spectrum of the time scales from the relatively steady motion of the cone-like trailing vortices on the slanted edges to highly frequent collisions of the hairpin-like vortices in the region of the attachment on the rear slanted surface. Copyright © 2005 by ASME.
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| 44. |
- Krajnovic, Sinisa, 1970
(författare)
-
Large Eddy Simulation Exploration of Passive Flow Control Around an Ahmed Body
- 2014
-
Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 136:12
-
Tidskriftsartikel (refereegranskat)abstract
- Large eddy simulations (LES) are used to study passive flow control for drag reduction in a simplified ground vehicle. Add-on devices in the form of short cylinders are used for the formation of streaks in the streamwise direction that lead to the separation delay. The results of the present numerical simulations are compared with the experimental data and show good agreement. The two-stage flow control mechanism is analyzed from the LES results. It was found to be in agreement with the previous experimental observations that the counter-rotating vortices behind the impinging devices influence the separation only indirectly through the longitudinal vortices further downstream.
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| 45. |
- Krajnovic, Sinisa, 1970, et al.
(författare)
-
Large Eddy Simulation Investigation of the Hysteresis Effects in the Flow Around an Oscillating Ground Vehicle
- 2011
-
Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 133:12, s. 121103-
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents large eddy simulations (LES) of flow around a simplified vehicle model oscillating around its vertical axis. The frequency of the Strouhal number St = 0.068 and a relatively small amplitude of the oscillation are chosen to be representative for the crosswind conditions of vehicles on the road. The results were found to agree well with data from previous experimental investigations. Furthermore, the differences in LES flows between quasi-steady and dynamic flow conditions are presented and underlying flow mechanisms are explored. The cause of the phenomena of hysteresis and phase shift was found in the inertia of the flow to adjust to sudden changes in the direction of the oscillation of the body.
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| 46. |
- Krajnovic, Sinisa, 1970, et al.
(författare)
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Numerical Investigation of the Flow Around a Simplified Wheel in a Wheelhouse
- 2011
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Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 133:11, s. 111001-
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Tidskriftsartikel (refereegranskat)abstract
- The flow around generic wheels in wheel housings used in previous experimental investigations is studied using large eddy simulations (LES). A comparison is given here of the results of the simulations with existing experimental data and previous results of RANS simulations. Both instantaneous and time-averaged flows are described, showing agreement with previous knowledge and adding new insight in flow physics. Two different widths of the wheel housing are used in the simulations, and their influence on the flows is studied. The present work shows that the width of the wheel housing has an influence on flows on both the inside and the outside of the wheelhouse.
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| 47. |
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| 48. |
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| 49. |
- Kranenbarg, Jelle, et al.
(författare)
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Mitigation of the Pressure Pulsations in an Axial Turbine at Speed-No-Load With Independent Guide Vanes Opening
- 2023
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Ingår i: Journal of Fluids Engineering. - : American Society of Mechanical Engineers (ASME). - 0098-2202 .- 1528-901X. ; 45:11
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Tidskriftsartikel (refereegranskat)abstract
- Hydraulic turbines are operated more frequently at no-load conditions, also known as speed-no-load (SNL), to provide a spinning reserve that can rapidly connect to the electrical grid. As intermittent energy sources gain popularity, turbines will be required to provide spinning reserves more frequently. Previous studies show vortical flow structures in the vaneless space and the draft tube and rotating stall between the runner blades of certain axial turbines operating at SNL conditions. These flow phenomena are associated with pressure pulsations and torque fluctuations which put high stress on the turbine. The origin of the instabilities is not fully understood and not extensively studied. Moreover, mitigation techniques for SNL must be designed and explored to ensure the safe operation of the turbines at off-design conditions. This study presents a mitigation technique with independent control of each guide vane. The idea is to open some of the guide vanes to the best efficiency point (BEP) angle while keeping the remaining ones closed, aiming to reduce the swirl and thus avoid the instability to develop. The restriction is to have zero net torque on the shaft. Results show that the flow structures in the vaneless space can be broken down, which decreases pressure and velocity fluctuations. Furthermore, the rotating stall between the runner blades is reduced. The time-averaged flow upstream of the runner is changed while the flow below the runner remains mainly unchanged.
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| 50. |
- Larsson, Sofia, et al.
(författare)
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Secondary flow in semi-circular ducts
- 2011
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Ingår i: Journal of Fluids Engineering. - : ASME International. - 0098-2202 .- 1528-901X. ; 133:10
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Tidskriftsartikel (refereegranskat)abstract
- Turbulent secondary flows are motions in the transverse plane, perpendicular to a main, axial flow. They are encountered in non-circular ducts and can, although the velocity is only of the order of 1–3% of the streamwise bulk velocity, affect the characteristics of the mean flow and the turbulent structure. In this work, the focus is on secondary flow in semi-circular ducts which has previously not been reported. Both numerical and experimental analyses are carried out with high accuracy. It is found that the secondary flow in semi-circular ducts consists of two pairs of counter rotating corner vortices, with a velocity in the range reported previously for related configurations. Agreement between simulation and experimental results are excellent when using a second moment closure turbulence model, and when taking the experimental and numerical uncertainty into account. New and unique results of the secondary flow in semi-circular ducts have been derived from verified simulations and validating laser-based experiments.
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