The prediction of coherent vortices with standard RANS solvers suffers especially from discretisation and modelling errors which both introduce numerical diffusion. The adaptive Vorticity Confinement (VC) method targe...The prediction of coherent vortices with standard RANS solvers suffers especially from discretisation and modelling errors which both introduce numerical diffusion. The adaptive Vorticity Confinement (VC) method targets to counteract one part of the discretisation error: the one due to the discretisation of the convection term. This method is applied in conjunction with a hybrid RANS-LES turbulence model to overcome the overprediction of turbulence intensity inside vortex cores which is a typical deficiency of common RANS solvers. The third main source for numerical diffusion originates from the spatial discretisation of the solution domain in the vicinity of the vortex core. The corresponding error is analysed within a grid convergence study. A modification of the adaptive VC method used in conjunction with a high-order discretisation of the convection term is presented and proves to be superior. The simulations of a wing tip vortex flow are validated in terms of vortex velocity profiles using the results of a wind tunnel experiment performed by Devenport and colleagues (1996). Besides, the results are compared with another numerical study by Wells (2009) who uses a Reynolds Stress turbulence model. It turns out that the application of the modified adaptive VC method on the one hand reinforces the tip vortex, and on the other hand accelerates the axial flow which leads to a slight degradation compared to the experimental results. The result of Wells is more accurate close to the wing, but the result obtained here is superior further downstream as no excessive diffusion of the tip vortex occurs.展开更多
A hybrid RANS-LES approach is used to resolve the Fore-body Side Vortex (FSV) separating from the KVLCC2 hull at 30° drift angle and Reynolds number ReLoa ≈ 2.56e6. The performance of the DES approach is evaluat...A hybrid RANS-LES approach is used to resolve the Fore-body Side Vortex (FSV) separating from the KVLCC2 hull at 30° drift angle and Reynolds number ReLoa ≈ 2.56e6. The performance of the DES approach is evaluated using a proper grid study. Besides, the following aspects of the CFD results are investigated: the resolution of turbulent energy, the prediction of instantaneous and time-averaged vortical structures, local flow features, the limiting streamlines and the evolution of the vortex core flow. New PIV data from wind tunnel experiments is compared to the latter. The results form a basis for future investigations in particular on the vortex interaction further downstream and the applicability of different kinds of turbulence models to trailing vortices like the FSV. Turbulence modelling is realised with the k-ω-SST-IDDES model presented in [1], the grids’ cell count is 6.4 M, 10.5 M and 17.5 M. Grid convergence of the time-averaged vortex core flow is observed. OpenFOAM version 1806 is used to carry out the simulations and snappyHexMesh to build the mesh.展开更多
In the hybrid RANS-LES simulations,proper turbulent fluctuations should be added at the RANS-to-LES interface to drive the numerical solution restoring to a physically resolved turbulence as rapidly as possible.Such t...In the hybrid RANS-LES simulations,proper turbulent fluctuations should be added at the RANS-to-LES interface to drive the numerical solution restoring to a physically resolved turbulence as rapidly as possible.Such turbulence generation methods mostly need to know the distribution of the characteristic length scale of the background RANS model,which is important for the recovery process.The approximation of the length scale for the Spalart-Allmaras(S-A)model is not a trivial issue since the model’s one-equation nature.As a direct analogy,the approximations could be obtained from the definition of the Prandtl’s mixing length.Moreover,this paper proposes a new algebraic expression to approximate the intrinsic length scale of the S-A model.The underlying transportation mechanism of S-A model are largely exploited in the derivation of this new expression.The new proposed expression is employed in the generation of synthetic turbulence to perform the hybrid RANS-LES simulation of canonical wall-bounded turbulent flows.The comparisons demonstrated the feasibility and improved performance of the new length scale on generating synthetic turbulence at the LES inlet.展开更多
Following the idea of Speziale＇s Very Large Eddy Simulation （VLES） method, a new unified hybrid simulation approach was proposed which can change seamlessly from RANS （Reynolds-Averaged Navier-Stokes） to LES （La...Following the idea of Speziale＇s Very Large Eddy Simulation （VLES） method, a new unified hybrid simulation approach was proposed which can change seamlessly from RANS （Reynolds-Averaged Navier-Stokes） to LES （Large Eddy Simulation） method depending on the numerical resolution. The model constants were calibrated in accordance with other hybrid methods. Besides being able to approach the two limits of RANS and LES, the new model also provides a proper VLES mode between the two limits, and thus can be used for a wide range of mesh resolutions. Also RANS simulation can be recovered near the wall which is similar to the Detached Eddy Simulation （DES） concept. This new methodology was implemented into Wilcox＇s κ- ω model and applications were conducted for fully developed turbulent channel flow at ReT = 395 and turbulent flow past a square cylinder at Re = 22000. Results were compared with LES predictions and other studies. The new method is found to be quite efficient in resolving large flow structures, and can predict satisfactory results on relative coarse mesh.展开更多
Hydrodynamic cavitating flows usually consist of 3-D intense vortical flows that are detached from solid boundaries.Detached vortical flows normally generate heaps of cavitating flow structures,which,in turn,govern th...Hydrodynamic cavitating flows usually consist of 3-D intense vortical flows that are detached from solid boundaries.Detached vortical flows normally generate heaps of cavitating flow structures,which,in turn,govern the location of cavitation erosion before collapse.Thus,this study introduces a new numerical approach based on the improved delayed detached eddy simulation(IDDES)turbulence modeling for predicting cavitating flows.Then,the solution of compressible Eulerian-Eulerian two-phase flow and the IDDES turbulence model was linked to the microjet hypothesis and unsteady behavior of pressure and vapor volume to predict the corresponding erosion of cavitating flows.The method for cavitation erosion prediction,a modified version taken from previous studies,was applied as a post-processing tool.The validation of cavitating flow predictions was performed for the first time on the Grenoble axisymmetric nozzle by comparing them with 21 photos of cavitation from the previous experimental study.The results showed that the present numerical approach estimated various features of hydrodynamic cavitation well,including shedding processes and the length,shape,and collapse of cavitating structures.Using the numerical analysis,three main stages were detected for the present cavitating flow,and the vorticity-cavitation interactions were investigated by the vorticity transport equation.The streak-like and tube-like cavitating(STLIC and TULIC)structures were introduced in the second stage,initiated by flow instability,and entirely governed by corresponding turbulent flow structures.The collapse of these cavitating structures is one of the primary sources of cavitation erosion on lower and upper walls.The results of the numerical erosion predictions were compared with those of the previous erosion tests on the Grenoble axisymmetric nozzle.Satisfactory numerical performance was achieved in predicting the location and intensity of cavitation erosion.展开更多
This paper presents an extensive review of existing techniques used in estimating design wind pressures considering Reynolds number and turbulence effects,as well as a case study of a reference building investigated e...This paper presents an extensive review of existing techniques used in estimating design wind pressures considering Reynolds number and turbulence effects,as well as a case study of a reference building investigated experimentally.We shed light on the limitations of current aerodynamic testing techniques,provisions in design standards,and computational fluid dynamics(CFD)methods to predict wind-induced pressures.The paper highlights the reasons for obstructing the standardization of the wind tunnel method.Moreover,we introduce improved experimental and CFD techniques to tackle the identified challenges.CFD provides superior and efficient performance by employing wall-modeled large-eddy simulation(WMLES)and hybrid RANS-LES models.In addition,we tested a large-scale building model and compared the results with published small-scale data.The findings reinforce our hypothesis concerning the scaling issues and Reynolds number effects in aerodynamic testing.展开更多
文摘The prediction of coherent vortices with standard RANS solvers suffers especially from discretisation and modelling errors which both introduce numerical diffusion. The adaptive Vorticity Confinement (VC) method targets to counteract one part of the discretisation error: the one due to the discretisation of the convection term. This method is applied in conjunction with a hybrid RANS-LES turbulence model to overcome the overprediction of turbulence intensity inside vortex cores which is a typical deficiency of common RANS solvers. The third main source for numerical diffusion originates from the spatial discretisation of the solution domain in the vicinity of the vortex core. The corresponding error is analysed within a grid convergence study. A modification of the adaptive VC method used in conjunction with a high-order discretisation of the convection term is presented and proves to be superior. The simulations of a wing tip vortex flow are validated in terms of vortex velocity profiles using the results of a wind tunnel experiment performed by Devenport and colleagues (1996). Besides, the results are compared with another numerical study by Wells (2009) who uses a Reynolds Stress turbulence model. It turns out that the application of the modified adaptive VC method on the one hand reinforces the tip vortex, and on the other hand accelerates the axial flow which leads to a slight degradation compared to the experimental results. The result of Wells is more accurate close to the wing, but the result obtained here is superior further downstream as no excessive diffusion of the tip vortex occurs.
文摘A hybrid RANS-LES approach is used to resolve the Fore-body Side Vortex (FSV) separating from the KVLCC2 hull at 30° drift angle and Reynolds number ReLoa ≈ 2.56e6. The performance of the DES approach is evaluated using a proper grid study. Besides, the following aspects of the CFD results are investigated: the resolution of turbulent energy, the prediction of instantaneous and time-averaged vortical structures, local flow features, the limiting streamlines and the evolution of the vortex core flow. New PIV data from wind tunnel experiments is compared to the latter. The results form a basis for future investigations in particular on the vortex interaction further downstream and the applicability of different kinds of turbulence models to trailing vortices like the FSV. Turbulence modelling is realised with the k-ω-SST-IDDES model presented in [1], the grids’ cell count is 6.4 M, 10.5 M and 17.5 M. Grid convergence of the time-averaged vortex core flow is observed. OpenFOAM version 1806 is used to carry out the simulations and snappyHexMesh to build the mesh.
基金supported by National Key Research and Development Program of China(No.2019YFA0405201)National Natural Science Foundation of China(Nos.12002360 and 92052301)National Numerical Windtunnel project。
文摘In the hybrid RANS-LES simulations,proper turbulent fluctuations should be added at the RANS-to-LES interface to drive the numerical solution restoring to a physically resolved turbulence as rapidly as possible.Such turbulence generation methods mostly need to know the distribution of the characteristic length scale of the background RANS model,which is important for the recovery process.The approximation of the length scale for the Spalart-Allmaras(S-A)model is not a trivial issue since the model’s one-equation nature.As a direct analogy,the approximations could be obtained from the definition of the Prandtl’s mixing length.Moreover,this paper proposes a new algebraic expression to approximate the intrinsic length scale of the S-A model.The underlying transportation mechanism of S-A model are largely exploited in the derivation of this new expression.The new proposed expression is employed in the generation of synthetic turbulence to perform the hybrid RANS-LES simulation of canonical wall-bounded turbulent flows.The comparisons demonstrated the feasibility and improved performance of the new length scale on generating synthetic turbulence at the LES inlet.
基金supported by the National Natural Science Foundation of China (Grant No. 50936005)the National Basic Research Program of China (Grant No. 2010CB227302)
文摘Following the idea of Speziale＇s Very Large Eddy Simulation （VLES） method, a new unified hybrid simulation approach was proposed which can change seamlessly from RANS （Reynolds-Averaged Navier-Stokes） to LES （Large Eddy Simulation） method depending on the numerical resolution. The model constants were calibrated in accordance with other hybrid methods. Besides being able to approach the two limits of RANS and LES, the new model also provides a proper VLES mode between the two limits, and thus can be used for a wide range of mesh resolutions. Also RANS simulation can be recovered near the wall which is similar to the Detached Eddy Simulation （DES） concept. This new methodology was implemented into Wilcox＇s κ- ω model and applications were conducted for fully developed turbulent channel flow at ReT = 395 and turbulent flow past a square cylinder at Re = 22000. Results were compared with LES predictions and other studies. The new method is found to be quite efficient in resolving large flow structures, and can predict satisfactory results on relative coarse mesh.
文摘Hydrodynamic cavitating flows usually consist of 3-D intense vortical flows that are detached from solid boundaries.Detached vortical flows normally generate heaps of cavitating flow structures,which,in turn,govern the location of cavitation erosion before collapse.Thus,this study introduces a new numerical approach based on the improved delayed detached eddy simulation(IDDES)turbulence modeling for predicting cavitating flows.Then,the solution of compressible Eulerian-Eulerian two-phase flow and the IDDES turbulence model was linked to the microjet hypothesis and unsteady behavior of pressure and vapor volume to predict the corresponding erosion of cavitating flows.The method for cavitation erosion prediction,a modified version taken from previous studies,was applied as a post-processing tool.The validation of cavitating flow predictions was performed for the first time on the Grenoble axisymmetric nozzle by comparing them with 21 photos of cavitation from the previous experimental study.The results showed that the present numerical approach estimated various features of hydrodynamic cavitation well,including shedding processes and the length,shape,and collapse of cavitating structures.Using the numerical analysis,three main stages were detected for the present cavitating flow,and the vorticity-cavitation interactions were investigated by the vorticity transport equation.The streak-like and tube-like cavitating(STLIC and TULIC)structures were introduced in the second stage,initiated by flow instability,and entirely governed by corresponding turbulent flow structures.The collapse of these cavitating structures is one of the primary sources of cavitation erosion on lower and upper walls.The results of the numerical erosion predictions were compared with those of the previous erosion tests on the Grenoble axisymmetric nozzle.Satisfactory numerical performance was achieved in predicting the location and intensity of cavitation erosion.
基金The second author(A.M.Aly)received financial support from the Louisiana Board of Regents(RCS,LEQSF(2021-22)-RD-A-30)Also,the second author received funds from the NSF I-Corps program at Louisiana State University.The findings are those of the authors and do not necessarily reflect the position of the funding sponsors.
文摘This paper presents an extensive review of existing techniques used in estimating design wind pressures considering Reynolds number and turbulence effects,as well as a case study of a reference building investigated experimentally.We shed light on the limitations of current aerodynamic testing techniques,provisions in design standards,and computational fluid dynamics(CFD)methods to predict wind-induced pressures.The paper highlights the reasons for obstructing the standardization of the wind tunnel method.Moreover,we introduce improved experimental and CFD techniques to tackle the identified challenges.CFD provides superior and efficient performance by employing wall-modeled large-eddy simulation(WMLES)and hybrid RANS-LES models.In addition,we tested a large-scale building model and compared the results with published small-scale data.The findings reinforce our hypothesis concerning the scaling issues and Reynolds number effects in aerodynamic testing.
