| 1. |
- Alin, Niklas, 1963, et al.
(författare)
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3D Unsteady Computations for Submarine-Like Bodies
- 2005
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Ingår i: 43rd AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nevada, Jan. 10-13, 2005. ; , s. 353-369
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Konferensbidrag (refereegranskat)abstract
- Results from a computational study using Unsteady Reynolds Averaged Navier Stokes (URANS) models and Large Eddy Simulation (LES) of flows past submarine-like bodies are here presented. The aims are to evaluate URANS and LES for high-Re number hydrodynamic flows, to investigate the influence of the turbulence and subgrid turbulence modeling, and to discuss some features of submarine hydrodynamics. For this purpose we have chosen to examine the flow past a prolate spheroid at 10° and 20° angle of attack at a body length Re number of 4-106, and the flow past the DARPA-2 Suboff bare hull and fully appended hull configurations at a body length Re number of 12-106. For both cases experimental data is available for comparison. One finite element and one finite volume flow solver has been used - both with the capability of employing a range of turbulence models and with the capacity of using unstructured and hybrid grids. Better agreement between predictions and experimental data is obtained with LES than with the URANS models, but at a considerably higher price, due to the finer grids and finer temporal resolution in LES.
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| 2. |
- Alin, Niklas, 1963, et al.
(författare)
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Current Capabilities of DES and LES for Submarines at Straight Course
- 2010
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Ingår i: Journal of Ship Research. - 1542-0604 .- 0022-4502. ; 54:3, s. 184-196
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Tidskriftsartikel (refereegranskat)abstract
- The flow around an axisymmetric hull, with and without appendages, is investigated using large eddy simulation (LES), detached eddy simulation (DES), and Reynolds averaged Navier Stokes (RANS) models. The main objectives of the study is to investigate the effect of the different simulation methods and to demonstrate the feasibility of using DES and LES on relatively coarse grids for submarine flows, but also to discuss some generic features of submarine hydrodynamics. For this purpose the DARPA Suboff configurations AFF1 (bare hull) and AFF8 (fully appended model) are used. The AFF1 case is interesting because it is highly demanding, in particular for LES and DES, due to the long midship section on which the boundary layer is developed. The AFF8 case represents the complex flow around a fully appended submarine with sail and aft rudders. An actuator disc model is used to emulate some of the effects of the propulsor for one of the AFF8 cases studied. Results for the AFF8 model are thus presented for both “towed” and “self-propelled” conditions, whereas for the bare hull, only a “towed” condition is considered. For the AFF1 and the “towed” AFF8 cases experimental data are available for comparison, and the results from both configurations show that all methods give good results for first-order statistical moments although LES gives a better representation of structures and second-order statistical moments in the complex flow in the AFF8 case.
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| 3. |
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| 4. |
- 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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| 5. |
- Arabnejad Khanouki, Mohammad Hossein, 1988, et al.
(författare)
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Numerical assessment of cavitation erosion risk using incompressible simulation of cavitating flows
- 2021
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Ingår i: Wear. - : Elsevier BV. - 0043-1648. ; 464-465
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, a numerical method to assess the risk of cavitation erosion is proposed, which can be applied to incompressible simulation approaches. The method is based on the energy description of cavitation erosion, which considers an energy transfer between the collapsing cavities and the eroded surface. The proposed framework provides two improvements compared with other published methods. First, it is based on the kinetic energy in the surrounding liquid during the collapse instead of the potential energy of collapsing cavities, which avoids the uncertainty regarding the calculation of the collapse driving pressure in the potential energy equation. Secondly, the approach considers both micro-jets and shock-waves as the mechanisms for cavitation erosion, while previous methods have taken into account only one of these erosion mechanisms. For validation, the proposed method is applied to the cavitating axisymmetric nozzle flow of Franc et al. (2011), and the predicted risk of cavitation erosion is compared with the experimental erosion pattern. This comparison shows that the areas predicted with high erosion risk agree qualitatively well with the experimental erosion pattern. Furthermore, as the current method can be used to study the relationship between the cavity dynamics and the risk of cavitation erosion, the hydrodynamic mechanism responsible for the high risk of cavitation erosion at the inception region of the sheet cavity is investigated in detail. It is shown for the first time that the risk of cavitation erosion in this region is closely tied to the separation of the flow entering the nozzle.
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| 6. |
- Arabnejad Khanouki, Mohammad Hossein, 1988, et al.
(författare)
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Scale resolving simulations of the non-cavitating and cavitating flows in an axial water jet pump
- 2020
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Ingår i: 33rd Symposium on Naval Hydrodynamics.
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Konferensbidrag (refereegranskat)abstract
- In this paper, the non-cavitating and cavitating flows in the AxWJ-2 axial water jet pump of Johns Hopkins University are simulated using a Large Eddy Simulation(LES) approach. The non-cavitating simulations are performed on grids with different mesh resolutions and the effect of mesh resolution on predicting the performance and capturing the structure of Tip Leakage Vortex (TLV) is investigated. Based on this investigation, it can be concluded that the main feature of the non-cavitating TLV can be well captured compared with the experiment provided that a refinement region with at least 40 cells in the tip gap is used. The cavitating simulation using the same grid refinement also shows that the cavitating structures described in the experiment are predicted by the simulation. Furthermore, the structure of non-cavitating and cavitating TLVs are compared and the effects of cavitation on the structure of TLV are analyzed using the simulation results.
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| 7. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
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Analysis of tip vortex inception prediction methods
- 2018
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Ingår i: Ocean Engineering. - : Elsevier BV. - 0029-8018. ; 167, s. 187-203
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Tidskriftsartikel (refereegranskat)abstract
- The current study investigates different cavitation inception prediction methods to characterize tip vortex flows around an elliptical foil, and a high skewed low-noise propeller. Adapted inception models cover different levels of complexity including wetted flow, Eulerian cavitation simulations, and Rayleigh-Plesset bubble dynamics models. The tip vortex flows are simulated by Implicit Large Eddy Simulation on appropriate grid resolutions for tip vortex propagation, at least 32 cells per vortex diameter according to previous studies guidelines. The results indicate that the cavitation inception predictions by the minimum pressure criterion of the wetted flow analysis are similar to weak water inception measurements. In the wetted flow analysis, the proposed energy criterion is noted to provide reasonably accurate inception predictions, similar to the predictions by Eulerian cavitation simulations with much lower computational costs. Comparison between high speed videos and numerical results of the propeller shows the capability of the numerical methodology in predicting tip vortex structures in different conditions. The interaction between vortices and their impact on the pressure field and the cavitation inception are also highlighted. The strong dependency of the inception on the initial nuclei sizes are demonstrated, and it is shown that for weaker tip vortices this dependency becomes more significant.
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| 8. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
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COMPARATIVE ANALYSIS OF TIP VORTEX FLOW USING RANS AND LES
- 2017
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Ingår i: VII International Conference on Computational Methods in Marine Engineering. ; 2017-May
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Konferensbidrag (refereegranskat)abstract
- The current study focuses on the numerical analysis of tip vortex flows, with the emphasis on the investigation of turbulence modelling effects on tip vortex prediction. The analysis includes comparison of RANS and LES methods at two different mesh resolutions. Implicit LES, ILES, modelling is employed here to mimic the turbulent viscosity. In RANS, the two equation k-ω SST model is adopted. In order to also address possible benefits of using streamline curvature variations in RANS, two curvature correction methods proposed for k-ω SST are tested, and compared. ILES results show very good agreement with the experimental observations. The predicted vortex in ILES is also stronger than RANS predictions. ILES has predicted accelerated vortex core axial velocity very well, while tested RANS models under predict the axial velocity. Adoption of curvature correction has not improved the tip vortex prediction, even though it has reduced the turbulent viscosity at the vortex core.
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| 9. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
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Evaluation of Curvature Correction Methods for Tip Vortex Prediction in SST kOmega Turbulence Model Framework
- 2019
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Ingår i: International Journal of Heat and Fluid Flow. - : Elsevier BV. - 0142-727X. ; 75, s. 135-152
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents and studies effects of curvature correction (CC) methods to improve two equation RANS simulations of tip vortex flows, exemplified using the SST kOmega turbulence model. Performance of the CC models is first evaluated in the classical Rankine vortex flow field, and then extended into the study of tip vortex flows over an elliptical foil. The results have been compared with experimental measurements in terms of the vortex strength and velocity field, and the importance of the turbulence closure in tip vortex simulations is highlighted. Contribution of the CC models in different terms of the turbulent kinetic energy and specific dissipation transport equations are described, and it is discussed why a CC model may have mesh resolution dependent results. By considering the distribution of the CC function, it is shown that although some of the models can predict the location of the tip vortex core accurately, they still do not significantly improve the vortex prediction as the impact on the turbulent viscosity is wrong or not enough. It is further noted that as some of these models have been calibrated on specific vortex flows, they may not be completely applicable for other cases without recalibration. It is shown that some CC models provide accurate tip vortex predictions, primarily the ones based on the sensitization of the turbulent viscosity. Further, it is noteworthy that the successful models are active not only around the vortex, but also change the boundary layer characteristics on the foil, and the boundary layer separation lines, which consequently can provide the required momentum for the vortex core accelerated axial velocity.
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| 10. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
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Implicit Large Eddy Simulation of Tip Vortex on an Elliptical Foil
- 2017
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Ingår i: Fifth International Symposium on Marine Propulsion.
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Konferensbidrag (refereegranskat)abstract
- In this study, Implicit Large Eddy Simulation (ILES) in OpenFOAM has been employed to study tip vortex flow on an elliptical foil. This type of foils has similar tip vortex behaviour as a propeller, making it a suitable benchmark for both numerical and experimental investigations of tip vortex flows in cavitating and non-cavitating conditions. The study includes investigation of the impact of streamwise and inplane mesh resolutions in tip vortex roll- up and its transportation. Vortex properties such as trajectory, axial and inplane velocity distributions, and also vortex core pressure distributions are computed for each mesh resolution and compared with available experimental data. Comparisons show that at least 16 cells per vortex diameter in inplane section is required to predict the tip vortex in the near field region. Results of varying foil angle of attack show the capability of the current numerical approach in ranking tip vortex properties. Employed numerical approach is fully capable of capturing the accelerated axial velocity at the vortex core for different operating conditions, and shows very good agreement with the experimental observations.
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