| 1. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
-
Computational Analysis of Cavitating Marine Propeller Performance using OpenFOAM
- 2015
-
Ingår i: 2nd International Workshop on Cavitating Propeller Performance. ; , s. 8-
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In this paper, numerical results, based on implicit LES andtransport equation mass transfer modelling approach, submitted to the Second International Workshop onCavitating Propeller Performance at the Fourth InternationalSymposium on Marine Propulsors (SMP’15) are presented.The numerical simulations are performed usingOpenFOAM. The present work is focused on the secondtask of the workshop, Propeller in oblique flow insidetunnel in wetted and cavitating conditions. We summariseresults from the three different operating conditions given in the task, where the experimental data of one condition isrevealed, and the two other conditions are put forward asblind tests to workshop participants. For the conditionwhere experimental data is known, we see good agreementfor the forces in wetted conditions that slightly deteriorate in cavitating conditions. Cavitation extent is over predicted, where mid-span bubbly cavitation in the experiments is predicted as a sheet cavity; cavitation in the tip region does however seem reasonable. This is also likely the reason for the larger error in force prediction. A limited mesh resolution study has been performed.
|
|
| 2. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
-
Effect of Turbulence Closure on the Simulation of the Cavitating Flow on the Delft Twist11 Foil
- 2013
-
Ingår i: Numerical Towing Tank Symposium. ; 2013
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In this paper, we continue to test and evaluate different approaches of turbulence closures for the simulation of unsteady sheet cavitation. At the workshop on cavitating flows in connection with the 2nd International Symposium on Marine Propulsion, Bensow (2011) showed results for RANS, DES, and implicit LES, and some peculiarities regarding the behavior of the RANS and DES models were detected, especially in combination with the ad hoc correction by Reboud (3rd International Symposium on Cavitation, 1998) which is frequently used in connection with RANS modeling of cavitating flows. Following up on this, testing of another set of turbulence closures is in progress, as the Spalart-Almaras RANS model used for the RANS simulation in (Bensow, 2011) has also in other papers, e.g. Eskilsson and Bensow (2012), not giving as good results as has been published using other models. Here we present intermediate results using the k-Omega SST RANS model with and without Reboud correction.
|
|
| 3. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
-
Improvement of cavitation mass transfer modeling based on local flow properties
- 2017
-
Ingår i: International Journal of Multiphase Flow. - : Elsevier BV. - 0301-9322. ; 93, s. 142-157
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents and studies the effect of two modifications to improve cavitation mass transfer source term modeling for transport equation based models by considering local flow properties. The first improvement is by creating an analogy between the phase change time scale and turbulence time scale, and have the model to automatically adjust mass transfer rate based on the flow. This will alleviate the manual calibration of model parameter that is often necessary in presently used models. The second modification introduces an influence of shear stress on the liquid rupture in flows relevant for hydromachinery. This relates to that the pressure threshold, which represents the criteria of when phase change occurs, is normally taken as the value relevant for a fluid at rest, but is in reality affected by the flow conditions.To demonstrate the effect of the model modifications, the three-dimensional, fully turbulent, cavitating flow around the Delft Twist11 foil is simulated. The suggested modifications are implemented in and evaluated using the Sauer mass transfer model, with simulations performed with an incompressible implicit LES flow model. The pressure distribution across different sections of the foil, lift force, and cavitation behavior, such as generation, separation, and collapse processes, are studied and compared with the experimental data. The comparison shows the capability of the presented model to improve the prediction of the complex physics of the cavitation around the Twist11 foil, compared with using only the original Sauer mass transfer model.
|
|
| 4. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
-
Numerical Analysis of Tip Vortex Flow
- 2016
-
Ingår i: 19th "Numerical Towing Tank Symposium" (NuTTS'16).
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)
|
|
| 5. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
-
Numerical investigation of the impact of computational resolution on shedding cavity structures
- 2018
-
Ingår i: International Journal of Multiphase Flow. - : Elsevier BV. - 0301-9322. ; 107, s. 33-50
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, three-dimensional cavitation is analysed, and in particular how its dynamics is influenced by computational resolution. Cavitation-vorticity interactions are investigated in several spatial resolutions to address impact of the mesh resolution on cavitating vortical flow predictions. Three-dimensional, fully turbulent, cavitating flow around the Delft Twist11 foil is simulated and investigated numerically with particular focus on cavitation structures, shedding dynamics, and vorticity distribution. Implicit Large Eddy Simulation is employed along with the iso-thermal homogeneous two-phase mixture model to conduct the cavitation simulation in OpenFOAM. The paper clarifies the importance of capturing small scales of the flow, and their impact on the cavitation shedding behaviour, especially for flows where the shed cavity is affected significantly by flow vortices. It is shown that the mesh requirement for vortical cavitating flows is governed by the minimum spatial resolution needed to transport vorticity in the flow rather than the resolution needed to simulate the phase change. Better predictions of the mechanisms driving the shed cavity, and its collapse location, will provide a better control over nuisance behaviour in cavitating flows, which is a critical key for design improvements.
|
|
| 6. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
-
Shear Stress Effects in Cavitating Flows
- 2014
-
Ingår i: Numerical Towing Tank Symposium. ; , s. 6-
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The current study focuses on the improvement of phase change pressure threshold estimation in the cavitatingflows and its effects on the cavity characteristics. Effects of viscous shear stresses have been considered in thecalculation of the tensile strength, and the liquid pocket rupturing. This method is used in the numericalsimulation of the cavitating flows around the 3D Twisted Delft foil. Obtained numerical results are comparedwith experimental data. It is concluded that the using this medication will improve the prediction of the cavitysize even in medium speed flows around foils, and propellers.
|
|
| 7. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
-
Analysis of tip vortex inception prediction methods
- 2018
-
Ingår i: Ocean Engineering. - : Elsevier BV. - 0029-8018. ; 167, s. 187-203
-
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.
|
|
| 8. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
-
COMPARATIVE ANALYSIS OF TIP VORTEX FLOW USING RANS AND LES
- 2017
-
Ingår i: VII International Conference on Computational Methods in Marine Engineering. ; 2017-May
-
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.
|
|
| 9. |
- Asnaghi, Abolfazl, 1984
(författare)
-
Computational Modelling for Cavitation and Tip Vortex Flows
- 2018
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cavitation often brings negative effects, such as performance degradation, noise, vibration, and material damage, to marine propulsion systems, but for optimum performance, cavitation is almost inevitable. Therefore, it is necessary to better understand cavitation in order to maximize the performance without encounter- ing severe problems. Experimental tests can only provide limited information about this complex phenomenon. This thesis deals with improving computational methodologies that can offer a more complete picture of the cavitation process, making it possible to investigate the flow in more details with a higher level of confidence, which eventually enables an improved design. The study describes cavitation behaviour in the early stage of the formation, i.e. cavitation inception and its interaction with tip vortex structures, as well as in the developed form, i.e. sheet and cloud cavitation. The analysis of the tip vortex flows is associated with the spatial mesh resolution, the sub-grid scale and the turbulence modelling, as well as the cavitation-vortex interaction. For inception prediction, different inception methods are investigated to char- acterize tip vortex flows around an elliptical foil and high skewed low noise pro- pellers. The adopted inception models cover different levels of complexity in- cluding wetted flow analysis, Eulerian cavitation simulations, and simplified La- grangian Rayleigh-Plesset bubble dynamics models. For simulations of developed sheet/cloud cavitating flows, a homogeneous two-phase mixture method is adopted along with the Schnerr-Sauer mass transfer model. A manual calibration of the mass transfer model coefficients may signifi- cantly affect both accuracy and stability of the numerical predictions. In order to avoid this issue, an approach is suggested and tested to compute the mass transfer rate based on the flow local time scale during the solution procedure. Comparison between high speed videos and numerical results clearly shows the capability of the developed method in predicting the cavitating structures. It is shown that in addition to the well-captured difference in e.g. the amount of cav- itation, the simulation is capable of correctly predicting the small though crucial differences in flow features and cavitation inception characteristics of different propellers designs. The strong dependency of the inception on the initial nuclei sizes are demonstrated, and it is shown that for weaker propeller tip vortices this dependency becomes even more significant.
|
|
| 10. |
- Asnaghi, Abolfazl, 1984
(författare)
-
Developing Computational Methods for Detailed Assessment of Cavitation on Marine Propellers
- 2015
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cavitation often brings negative effects, such as performance degradation, noise, vibration, and material damage, to a marine propulsion systems, but for optimum performance, cavitation is almost inevitable. Therefore, it is necessary to improve the understanding of cavitation in order to maximize the performance without encountering severe problems. Experimental tests can only provide limited information about this complex phenomenon. This thesis deals with improving numerical simulations methodologies that can offer a more complete picture of the cavitation process, making it possible to investigate the flow in more detail with some confidence, thus enabling an improved design.Numerical simulations of non-cavitating and cavitating flows are conducted using OpenFOAM. The flow is modelled using Implicit Large Eddy Simulation and considering the two phases, i.e. vapour and liquid, as a homogeneous mixture through a volume fraction transport equation method along with the Schnerr-Sauer mass transfer model.To avoid manual calibration of the mass transfer model coefficients, which may significantly affect both the accuracy and stability of the numerical predictions, an approach is suggested and tested to compute the mass transfer rate based on the flow local time scale during the solution procedure. Moreover, the saturation pressure is modified in order to take into account the shear stress effects on the liquid rupturing.To test the proposed modifications, several test cases consisting of 2D and 3D hydrofoils and model scale propellers are simulated and the results are compared with experimental data. Integral quantities, local pressure data, and cavitation extent are studied for both the non-cavitating and the cavitating flows. Furthermore, the computational set-up is tested by varying domain size, mesh type and resolution, numerical schemes, and mass transfer model coefficients.The overall results compare well with the available experimental data, provided the mesh resolution is sufficient. The proposed mass transfer model modifications give a considerably improved prediction of pressure distribution and cavity extent. Some results yield overpredicted cavitation, indicating discrepancies between the modelling approach and model scale experimental techniques.
|
|
