| 1. |
- 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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| 2. |
- 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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| 3. |
- 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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| 4. |
- 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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| 5. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
-
Numerical and experimental analysis of cavitation inception behaviour for high-skewed low-noise propellers
- 2018
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Ingår i: Applied Ocean Research. - : Elsevier BV. - 0141-1187. ; 79, s. 197-214
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Tidskriftsartikel (refereegranskat)abstract
- Numerical and experimental investigations of two highly skewed low-noise propellers with a slight alteration in the tip geometry are conducted in both wetted and cavitating flows. The performance of the propellers is measured in open water and inclined setups at different operating conditions and high speed video recordings are used to determine tip vortex cavitation inception behaviour. The numerical simulations are conducted by employing the Implicit Large Eddy Simulation on appropriate grid resolutions for tip vortex propagation, at least 32 cells per vortex diameter according to our previous studies. In order to investigate cavitation inception characteristics of the propellers, different inception prediction methods are employed and evaluated at different advance ratios. It is shown that in addition to the well-captured difference in e.g. the amount of cavitation, the simulations are capable of correctly predicting the small but crucial differences in flow features and cavitation inception characteristics of the two propeller designs. Numerical predictions of the cavitation inception charts are also compared successfully with the measured data where three different types of cavitation patterns are investigated in details. Supported by the experimental videos, the interaction between the tip vortex and trailing vortices and their impact on the pressure field and the cavitation inception are analyzed. It is shown that the numerical simulation can provide further details about the vortical flow structures, and their contributions to cavitation, and is a powerful tool in advanced propeller design stages.
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| 6. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
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Propeller tip vortex cavitation mitigation using roughness
- 2019
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Ingår i: 8th International Conference on Computational Methods in Marine Engineering, MARINE 2019. - 9788494919435 ; , s. 383-392
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Konferensbidrag (refereegranskat)abstract
- This paper presents an investigation of roughness application on marine propellers in order to alter their tip vortex properties, and consequently mitigate tip vortex cavitation. SST kOmega model along with a curvature correction is employed to simulate the flow on an appropriate grid resolution for tip vortex propagation, at least 32 cells per vortex diameter. The roughness is modelled by using a rough wall function to increase the turbulent properties in roughed areas. In one case, roughness geometry is included as a part of the blade geometry, and the flow around them are resolved. To minimize the negative effects of the roughness on the propeller performance, the roughness area is optimized by simultaneous consideration of the tip vortex mitigation and performance degradation. For the considered operating condition, it is found that having roughness on the tip region of suction side can reduce the cavitation inception by 18 % while keeping the performance degradation in a reasonable range, less than 2%.
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| 7. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
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Roughness Effects on the Tip Vortex Strength and Cavitation Inception
- 2019
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Ingår i: Proceedings of the ... International Symposium on Marine Propulsors. - 2414-6129. - 9788876170478 ; 1, s. 267-273
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Konferensbidrag (refereegranskat)abstract
- The possibility and effectiveness of roughness application to mitigate tip vortex flows are evaluated by numerical simulations of an elliptical foil. The analysis includes investigation of the roughness size and area covered by the roughness, as well as the impact on the cavitation inception. Implicit Large Eddy Simulation (ILES) in OpenFOAM has been employed along with a wall-function incorporating the roughness effects to conduct the simulation on a proper grid resolution having the tip vortex spatial resolution as fine as 0.062 mm. The impact of the roughness size on the tip vortex is noted, and it is observed that for the studied condition, the roughness size of 250 micor m is sufficient. The negative effects of roughness on the forces are also observed where application of roughness leads to lower lift and higher drag forces. To minimize the negative effects of the roughness on the performance, the roughness area optimization is conducted and it is found that the application of roughness on the leading edge and trailing edge of the suction side are acceptable to mitigate the tip vortex and also to limit the performance degradation. This is regarded to be in close relation with the way that the tip vortex forms in the studied operating condition. The study shows while the inception occurs at the cavitation number equal to 6.35 in the smooth condition, application of the roughness on the optimum area will delay the inception to the cavitation number equal to 4.1 while only increasing the drag coefficient 1.7 %. This is found to be in a good agreement with the experimental measurements.
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| 8. |
- Ge, Muye, 1991, et al.
(författare)
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Investigation on Numerical Prediction of Propeller Induced Hull Pressure Pulses
- 2018
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Ingår i: Proceedings of the 10th International Symposium on Cavitation (CAV2018). - : ASME Press. - 9780791861851
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Konferensbidrag (refereegranskat)abstract
- Simulation of a cavitating propeller in behind conditions and analysis of induced hull pressure fluctuations are presented. All the simulations were performed using RANS method in the commercial package Star-CCM+. Cavitation patterns show good agreement with experimental measurements, especially the blade tip refined meshes which captured the dynamic behaviour of tip vortex cavitation. The predicted pressure pulse amplitudes agree reasonably well with experimental measurements up to 3rd to 4th order of blade passing frequency.
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| 9. |
- Ge, Muye, 1991, et al.
(författare)
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Numerical investigation of pressure pulse predictions for propellers mounted on an inclined shaft
- 2019
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Ingår i: Proceedings of the Sixth International Symposium on Marine Propulsors. - 2414-6129. - 9788876170478 ; 1, s. 284-292
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Konferensbidrag (refereegranskat)abstract
- In the presented study, two high-skew model scale marine propellers were tested in the cavitation tunnel and the induced pressure pulses were measured during the test. Propeller shaft was inclined about 10 degrees to create blade load variations. The cavitation pattern were recorded using high speed videos. The open-source package openFOAM and commercial package Star-ccm+ are used as simulation tools to predict pressure pulses numerically. By using the fully turbulent SST k − ω model, the predicted wetted flow pressure pulse levels agreed well compared to experimental measurements, but together with Schnerr-Sauer cavitation mass transfer model, massive cavitation was predicted which lead to inaccurate pressure pulse predictions. The transition sensitive turbulence model γ − Re θ model is used to study the cases, and simulation results reveal the existence of laminar-transition zone and vortex structures on the propeller blades. Attempts are made to linking correlation-based separation region from the transition model and the cavitation model, and good predictions of cavitation pattern are achieved but the predicted pressure pulses levels are merely improved.
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