| 1. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
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Computational Analysis of Cavitating Marine Propeller Performance using OpenFOAM
- 2015
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Ingår i: 2nd International Workshop on Cavitating Propeller Performance. ; , s. 8-
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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.
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| 2. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
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Effect of Turbulence Closure on the Simulation of the Cavitating Flow on the Delft Twist11 Foil
- 2013
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Ingår i: Numerical Towing Tank Symposium. ; 2013
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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.
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| 3. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
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Improvement of cavitation mass transfer modeling based on local flow properties
- 2017
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Ingår i: International Journal of Multiphase Flow. - : Elsevier BV. - 0301-9322. ; 93, s. 142-157
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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.
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| 4. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
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Numerical Analysis of Tip Vortex Flow
- 2016
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Ingår i: 19th "Numerical Towing Tank Symposium" (NuTTS'16).
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 5. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
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Numerical investigation of the impact of computational resolution on shedding cavity structures
- 2018
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Ingår i: International Journal of Multiphase Flow. - : Elsevier BV. - 0301-9322. ; 107, s. 33-50
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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.
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| 6. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
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Shear Stress Effects in Cavitating Flows
- 2014
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Ingår i: Numerical Towing Tank Symposium. ; , s. 6-
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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.
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| 7. |
- Cesur, Alper, et al.
(författare)
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Analysis of the wake dynamics of stiff and flexible cantilever beams using POD and DMD
- 2014
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Ingår i: Computers and Fluids. - : Elsevier BV. - 0045-7930. ; 101, s. 27-41
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Tidskriftsartikel (refereegranskat)abstract
- The wake flow behind a cantilever beam of quadratic cross-section at a Reynolds number of 50,000 is investigated using detailed simulations. Two cases are considered, the first one using a stiff beam and the second one with a beam allowing for elastic deformation due to the hydrodynamic forces. The flow is simulated using an implicit large eddy simulation (ILES) approach in OpenFOAM and the structural deformation of the beam is found from a non-linear finite element approach using OOFEM. The motion of the fluid mesh due to the structural deformation is handled by an ALE method. The wake structures are investigated using proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) of the flow field. The results show that apart from the wake structures originating from the vortex shedding there is also a low frequency mode, which is an oscillatory motion in the stream-wise direction, present.
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| 8. |
- Cesur, Alper, 1980, et al.
(författare)
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Fluid-Structure Interaction of the Transient Evolution of a Cantilever Beam
- 2012
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Ingår i: Proceedings of the 25th Nordic Seminar on Computational Mechanics.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- This paper comprises the study of transient dynamics of a cantilever beam in a fluid-structure interaction context. The investigation is focused on the dynamical response of a cantilever exposed to fluid flow at Re=50000, where the implicitly coupling method serves as the key function in the study.
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| 9. |
- Feymark, Andreas, 1984
(författare)
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A Large Eddy Simulation Based Fluid-Structure Interaction Methodology with Application in Hydroelasticity
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The phenomenon of hydroelasticity is a subarea of Fluid-Structure Interaction (FSI) and of major importance in many engineering applications related to hydrodynamics and naval architecture e.g. wave-induced vibrations, such as springing, whipping and slamming, propeller singing, composite propellers or turbines, acoustic signatures from naval vessels, highly loaded thin propeller blades, and cavitation erosion. Some of these phenomena can be assessed with reasonable reliability, but in cases where medium- to small-scale flow features are important the computational models need to be further developed to improve predictive capability and enable new conceptual designs.The work presented in this thesis has this kind of development as objective and a method capable of providing hydroelasticity predictions based on LES is presented and validated. The problem is particularly challenging as the densities of the fluid and the structure are comparable and an implicit coupling is thus needed to ensure a stable solution procedure. Furthermore, LES is not well established in the FSI context and especially not within the area of hydroelasticity. High resolution of the computation is necessary and the algorithm needs to run efficiently on large parallel computer systems. Reliable results also include predicting the correct separation pattern, in general on smoothly curved geometries. To address this a validation of LES in terms of predicting the correct separation pattern was performed and presented here, including also the development and validation of a LES turbulence trip model.The results presented can be divided into three parts, firstly the prediction and validation of open separation phenomena around a prolate spheroid, secondly the prediction and validation of the flow around an oscillating cylinder and thirdly the development of a fluid-structure interaction methodology for hydrodynamic applications and corresponding prediction and validation of the deformation of a flexible hydrofoil. The results all show a good agreement with experimental data, thus supporting the validity of the fluid-structure interaction methodology for hydroelastic applications presented within the scope of this thesis. Finally, the parallel performance of the implementation is analyzed through both weak and strong scaling and found to be satisfactory.
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| 10. |
- Feymark, Andreas, 1984, et al.
(författare)
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Fluid-Structure Interaction Using Open Source Software
- 2011
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Ingår i: 14th Numerical Towing Tank Symposium.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In the on going work we aim at developing a parallelized computational method, capable of producing accu- rate and reliable predictions in order to investigate FSI related problems within the area of Shipping and Marine Technology. This will be done creating an interface between two different open source codes OpenFOAM for fluid dynamics and OOFEM used for structural analysis.
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