In this paper a series of numerical simulations are performed to investigate the vortex shedding mechanism for a solitary wave propagating over a submerged breakwater by use of Reynolds averaged Navier-Stokes （RAINS...In this paper a series of numerical simulations are performed to investigate the vortex shedding mechanism for a solitary wave propagating over a submerged breakwater by use of Reynolds averaged Navier-Stokes （RAINS） model combined with a k-ε model. Flows of different Reynolds numbers up to Re = 1.4 × 10^5 corresponding to varying incident wave heights are considered in which the characteristic fluid velocity is represented by the maximum horizontal velocity above the submerged breakwater. For the verification of the accuracy of the numerical model, the incident waves and the velocity field in the vicinity of the breakwater are compared with experimental data. The result shows that the model is capable of describing vortex shedding for a solitary wave propagating over a rectangular submerged breakwater. Key features of vortex generation, evolution and dissipation are investigated. It is found that the vortex shedding and their evolution due to separated boundary layer over the breakwater are strongly related to the Reynolds number. A considerable number of vortices and complicated vortex pattern are observed as the Reynolds number increases.展开更多
We present a machine learning based method for RANS modeling in the rotating frame of reference(RFR).The extended intrinsic mean spin tensor(EIMST)is adopted in a novel expansion of the evolution algorithm,named multi...We present a machine learning based method for RANS modeling in the rotating frame of reference(RFR).The extended intrinsic mean spin tensor(EIMST)is adopted in a novel expansion of the evolution algorithm,named multi-dimensional gene expression programming(MGEP).Based on DNS data,a constrain free model for Reynolds stress is created by considering system rotating.The anisotropy behavior of Reynolds stress is considered in the model,which is then for the first time applied for modeling turbulent flow inside a rotating channel.Compared with the traditional RANS model,the new model can predict the non-symmetric profile of Reynolds stress.Meanwhile,the Taylor-Gortler vortex is captured in our simulations with the new model.It is demonstrated that the application of EIMST in MGEP can be successfully adopted for RANS modeling in the RFR.展开更多
The predictive capability of Reynolds-averaged numerical simulation （RANS） models is investigated by simulating the flow in meandering open channel flumes and comparing the obtained results with the measured data. T...The predictive capability of Reynolds-averaged numerical simulation （RANS） models is investigated by simulating the flow in meandering open channel flumes and comparing the obtained results with the measured data. The flow structures of the two experiments are much different in order to get better insights. Two eddy viscosity turbulence models and different wall treatment methods are tested. Comparisons show that no essential difference exists among the predictions. The difference of turbulence models has a limited effect, and the near wall refinement improves the predictions slightly. Results show that, while the longitudinal velo- cities are generally well predicted, the predictive capability of the secondary flow is largely determined by the complexity of the flow structure. In Case 1 of a simple flow structure, the secondary flow velocity is reasonably predicted. In Case 2, consisting of sharp curved consecutive reverse bends, the flow structure becomes complex after the first bend, and the complex flow structure leads to the poor prediction of the secondary flow. The analysis shows that the high level of turbulence anisotropy is related with the boundary layer separation, but not with the flow structure complexity in the central area which definitely causes the poor prediction of RANS models. The turbulence model modifications and the wall treatment methods barely improve the predictive capability of RANS models in simulating complex flow structures.展开更多
Cloud cavitating flow is highly turbulent and dominated by coherent large-scale anisotropic vortical structures. For the numer- ical investigation of such a class of flow, large eddy simulation （LES） is a reliable m...Cloud cavitating flow is highly turbulent and dominated by coherent large-scale anisotropic vortical structures. For the numer- ical investigation of such a class of flow, large eddy simulation （LES） is a reliable method but it is computationally extremely costly in engineering applications. An efficient approach to reduce the computational cost is to combine Reynolds-averaged Navier-Stokes （RANS） equations with LES used only in the parts of interest, such as massively separated flow regions. A new hybrid RANS/LES model, the modified filter-based method （FBM）, is proposed in the present study which can perform RANS or LES depending on the numerical resolution. Compared to the original FBM, the new method has three modifications： the state-of-the-art shear stress transport （SST） model replaces the k-c model as a baseline RANS model. A shielding function is introduced to obviate the switch from RANS to LES occurring inside the boundary layer. An appropriate threshold controlling the switch from RANS to LES is added to achieve an optimal predictive accuracy. The new model is assessed for its predictive capability of highly unsteady cavitating flows in a typical case of cloud cavitation around a NACA66 hydrofoil. The new mod- el results are compared with data obtained from the Smagorinsky LES and SST model based on the same homogeneous Zwart cavitation model. It is found that the modified FBM method has significant advantages over SST model in all aspects of pre- dicted instantaneous and mean flow field, and its predictive accuracy is comparable to the Smagorinsky LES model even using a much coarser grid in the simulations.展开更多
This paper presents a CFD study about the effect of the V and U grooves in the flow over four cylinders in diamond shape configuration at subcritical flow conditions（Re= 41 000）. The k- ε Realizable turbulence mode...This paper presents a CFD study about the effect of the V and U grooves in the flow over four cylinders in diamond shape configuration at subcritical flow conditions（Re= 41 000）. The k- ε Realizable turbulence model was implemented to fully structured hexahedral grids with near-wall refinements. Results showed that the numerical model was able to reproduce the impinging flow pattern and the repulsive forces present in the lateral cylinders of the smooth cylinder array. As a consequence of the flow alignment induced by the grooves, a jet-flow is formed between the lateral cylinders, which could cause an important vortex induced vibration effect especially in the rear cylinder. The magnitudes of the shear stresses at the valleys and peaks for the V grooved cylinders were lower than those of the U grooved cylinders, but the separation points were delayed due the U grooves presence. It is discussed the presence of a blowing effect caused by counter-rotating eddies located near the grooves peaks that cause a decrease of the shear stresses in the valleys, and promote them at the peaks.展开更多
Different turbulence closures were used to predict the flow interaction between the wakes created by compressor outlet guide vanes(OGVs) and a downstream annular pre-diffuser.Two statistical turbulence models were tes...Different turbulence closures were used to predict the flow interaction between the wakes created by compressor outlet guide vanes(OGVs) and a downstream annular pre-diffuser.Two statistical turbulence models were tested based on the classical Reynolds-averaged Navier-Stokes(RANS) approach.Both high-Re and low-Re(Launder-Sharma) versions of the k-ε model were applied to a selected test problem for OGV wake/diffuser flows.The test problem was specifically chosen because experimentally determined inlet conditions and both profile and performance data were available to validate predictions.A preliminary study was also reported of the more advanced large eddy simulation(LES) approach.The LES sub-grid-scale(SGS) model was the basic Smagorinsky eddy viscosity assumption,with a Van-Driest damping function for improved capture of near-wall viscous behaviour.Comparison between the two RANS models showed little difference in terms of velocity contours at OGV trailing edge and diffuser exit.In terms of overall diffuser performance(static pressure recovery and total pressure loss coefficients),the high-Re model was shown to agree well with experimental data.The preliminary LES study indicates the highly unsteady character of the OGV wake flow,but requires improved treatment of inlet conditions.展开更多
基金supported by the Science Council and Top University of NCKU(Grant Nos .NSC96-2221-E-127-006-MY3 and A0162)
文摘In this paper a series of numerical simulations are performed to investigate the vortex shedding mechanism for a solitary wave propagating over a submerged breakwater by use of Reynolds averaged Navier-Stokes （RAINS） model combined with a k-ε model. Flows of different Reynolds numbers up to Re = 1.4 × 10^5 corresponding to varying incident wave heights are considered in which the characteristic fluid velocity is represented by the maximum horizontal velocity above the submerged breakwater. For the verification of the accuracy of the numerical model, the incident waves and the velocity field in the vicinity of the breakwater are compared with experimental data. The result shows that the model is capable of describing vortex shedding for a solitary wave propagating over a rectangular submerged breakwater. Key features of vortex generation, evolution and dissipation are investigated. It is found that the vortex shedding and their evolution due to separated boundary layer over the breakwater are strongly related to the Reynolds number. A considerable number of vortices and complicated vortex pattern are observed as the Reynolds number increases.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.91852117,91852106),the MOE Key Laboratory of Hydrodynamics,Shanghai Jiao Tong University.
文摘We present a machine learning based method for RANS modeling in the rotating frame of reference(RFR).The extended intrinsic mean spin tensor(EIMST)is adopted in a novel expansion of the evolution algorithm,named multi-dimensional gene expression programming(MGEP).Based on DNS data,a constrain free model for Reynolds stress is created by considering system rotating.The anisotropy behavior of Reynolds stress is considered in the model,which is then for the first time applied for modeling turbulent flow inside a rotating channel.Compared with the traditional RANS model,the new model can predict the non-symmetric profile of Reynolds stress.Meanwhile,the Taylor-Gortler vortex is captured in our simulations with the new model.It is demonstrated that the application of EIMST in MGEP can be successfully adopted for RANS modeling in the RFR.
基金Project supported by the National Basic Research Deve-lopment Program of China(973 Program,Grant No.2012CB417002)the National Science-Technology Support Plan Projects(Grant Nos.2012BAB05B01,2012BAB04B03)+1 种基金the State Key Program of National Natural Science of China(Grant No.51039004)the National Natural Science Foundation of China(Grant No.51579151)
文摘The predictive capability of Reynolds-averaged numerical simulation （RANS） models is investigated by simulating the flow in meandering open channel flumes and comparing the obtained results with the measured data. The flow structures of the two experiments are much different in order to get better insights. Two eddy viscosity turbulence models and different wall treatment methods are tested. Comparisons show that no essential difference exists among the predictions. The difference of turbulence models has a limited effect, and the near wall refinement improves the predictions slightly. Results show that, while the longitudinal velo- cities are generally well predicted, the predictive capability of the secondary flow is largely determined by the complexity of the flow structure. In Case 1 of a simple flow structure, the secondary flow velocity is reasonably predicted. In Case 2, consisting of sharp curved consecutive reverse bends, the flow structure becomes complex after the first bend, and the complex flow structure leads to the poor prediction of the secondary flow. The analysis shows that the high level of turbulence anisotropy is related with the boundary layer separation, but not with the flow structure complexity in the central area which definitely causes the poor prediction of RANS models. The turbulence model modifications and the wall treatment methods barely improve the predictive capability of RANS models in simulating complex flow structures.
基金supported by the National Natural Science Foundation of China(Grant No.51579118)
文摘Cloud cavitating flow is highly turbulent and dominated by coherent large-scale anisotropic vortical structures. For the numer- ical investigation of such a class of flow, large eddy simulation （LES） is a reliable method but it is computationally extremely costly in engineering applications. An efficient approach to reduce the computational cost is to combine Reynolds-averaged Navier-Stokes （RANS） equations with LES used only in the parts of interest, such as massively separated flow regions. A new hybrid RANS/LES model, the modified filter-based method （FBM）, is proposed in the present study which can perform RANS or LES depending on the numerical resolution. Compared to the original FBM, the new method has three modifications： the state-of-the-art shear stress transport （SST） model replaces the k-c model as a baseline RANS model. A shielding function is introduced to obviate the switch from RANS to LES occurring inside the boundary layer. An appropriate threshold controlling the switch from RANS to LES is added to achieve an optimal predictive accuracy. The new model is assessed for its predictive capability of highly unsteady cavitating flows in a typical case of cloud cavitation around a NACA66 hydrofoil. The new mod- el results are compared with data obtained from the Smagorinsky LES and SST model based on the same homogeneous Zwart cavitation model. It is found that the modified FBM method has significant advantages over SST model in all aspects of pre- dicted instantaneous and mean flow field, and its predictive accuracy is comparable to the Smagorinsky LES model even using a much coarser grid in the simulations.
文摘This paper presents a CFD study about the effect of the V and U grooves in the flow over four cylinders in diamond shape configuration at subcritical flow conditions（Re= 41 000）. The k- ε Realizable turbulence model was implemented to fully structured hexahedral grids with near-wall refinements. Results showed that the numerical model was able to reproduce the impinging flow pattern and the repulsive forces present in the lateral cylinders of the smooth cylinder array. As a consequence of the flow alignment induced by the grooves, a jet-flow is formed between the lateral cylinders, which could cause an important vortex induced vibration effect especially in the rear cylinder. The magnitudes of the shear stresses at the valleys and peaks for the V grooved cylinders were lower than those of the U grooved cylinders, but the separation points were delayed due the U grooves presence. It is discussed the presence of a blowing effect caused by counter-rotating eddies located near the grooves peaks that cause a decrease of the shear stresses in the valleys, and promote them at the peaks.
文摘Different turbulence closures were used to predict the flow interaction between the wakes created by compressor outlet guide vanes(OGVs) and a downstream annular pre-diffuser.Two statistical turbulence models were tested based on the classical Reynolds-averaged Navier-Stokes(RANS) approach.Both high-Re and low-Re(Launder-Sharma) versions of the k-ε model were applied to a selected test problem for OGV wake/diffuser flows.The test problem was specifically chosen because experimentally determined inlet conditions and both profile and performance data were available to validate predictions.A preliminary study was also reported of the more advanced large eddy simulation(LES) approach.The LES sub-grid-scale(SGS) model was the basic Smagorinsky eddy viscosity assumption,with a Van-Driest damping function for improved capture of near-wall viscous behaviour.Comparison between the two RANS models showed little difference in terms of velocity contours at OGV trailing edge and diffuser exit.In terms of overall diffuser performance(static pressure recovery and total pressure loss coefficients),the high-Re model was shown to agree well with experimental data.The preliminary LES study indicates the highly unsteady character of the OGV wake flow,but requires improved treatment of inlet conditions.
