| 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
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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
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Ingår i: Journal of Fluids Engineering. - : The American Society of Mechanical Engineers (ASME). - 0098-2202 .- 1528-901X. ; 140:5
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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
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Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 132:4, s. 0413021-04130210
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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
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Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 140:11
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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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