This paper presents hybrid Reynolds-averaged Navier-Stokes （RANS） and large-eddy-simulation （LES） methods for the separated flows at high angles of attack around a 6：1 prolate spheroid. The RANS/LES hybrid meth- ...This paper presents hybrid Reynolds-averaged Navier-Stokes （RANS） and large-eddy-simulation （LES） methods for the separated flows at high angles of attack around a 6：1 prolate spheroid. The RANS/LES hybrid meth- ods studied in this work include the detached eddy simulation （DES） based on Spalart-Allmaras （S-A）, Menter＇s k-ω shear-stress-transport （SST） and k-o9 with weakly nonlinear eddy viscosity formulation （Wilcox-Durbin＋, WD＋） models and the zonalANS/LES methods based on the SST and WD＋ models. The switch from RANS near the wall to LES in the core flow region is smooth through the implementation of a flow-dependent blending function for the zonal hybrid method. All the hybrid methods are designed to have a RANS mode for the attached flows and have a LES behavior for the separated flows. The main objective of this paper is to apply the hybrid methods for the high Reynolds number separated flows around prolate spheroid at high-incidences. A fourth-order central scheme with fourth-order artificial viscosity is applied for spatial differencing. The fully implicit lower-upper symmetric-Gauss-Seidel with pseudo time sub-iteration is taken as the temporal differentiation. Comparisons with available measurements are carried out for pressure distribution, skin friction, and profiles of velocity, etc. Reasonable agreement with the experiments, accounting for the effect on grids and fundamental turbulence models, is obtained for the separation flows.展开更多
Accurate predictions of Shock Waves and Boundary Layer Interaction(SWBLI)and strong Shock Waves and Wake Vortices Interaction(SWWVI)in a highly-loaded turbine propose challenges to the currently widely used Reynolds-A...Accurate predictions of Shock Waves and Boundary Layer Interaction(SWBLI)and strong Shock Waves and Wake Vortices Interaction(SWWVI)in a highly-loaded turbine propose challenges to the currently widely used Reynolds-Averaged Navier-Stokes(RANS)model.In this work,the SWBLI and the SWWVI in a highly-loaded Nozzle Guide Vane(NGV)are studied using a hybrid RANS/LES strategy.The Turbulence Kinetic Energy(TKE)budget and the Proper Orthogonal Decomposition(POD)method are used to analyze flow mechanisms.Results show that this hybrid RANS/LES method can obtain detailed flow structures for flow mechanisms analysis.Strong shock waves induce boundary layer separation,while the presence of a separation bubble can in turn lead to a Mach reflection phenomenon.The shock waves cause trailing-edge vortices to break clearly,and the wakes,in turn,can change the shocks intensity and direction.Furthermore,the Entropy Generation Rate(EGR)is used to analyze the irreversible loss.It turns out that the SWWVI can reduce the flow field loss.There are several weak shock waves in the NGV flow field,which can increase the irreversible loss.This work offers flow mechanisms analysis and presents the EGR distribution in SWBLI and SWWVI areas in a transonic turbine blade.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
The massively separated flow past triple cylin- ders （TriC） in tandem arrangement is simulated using the improved delayed detached-eddy simulation （IDDES） method based on the shear stress transport （SST） model, ...The massively separated flow past triple cylin- ders （TriC） in tandem arrangement is simulated using the improved delayed detached-eddy simulation （IDDES） method based on the shear stress transport （SST） model, coupled with the high order adaptive dissipation scheme. The spacing between adjacent cylinders is sub-critical （1.435D）. IDDES prediction of two cylinders （TC） with the same spacing is compared to experimental data for validation, and the numerical results agree well with the available measurements, except for the asymmetry in the gap region. The flow past TriC is investigated using the same method. Generally, the mean flow quantities past TriC, such as the velocity, pressure, and vorticity, are similar to the corresponding components of TC. However, the pressure fluctuations on the TriC surface are uniformly larger than those on TC. Meanwhile, the instantaneous flows past TriC are much more complex. The periodical blockage in the first gap region is found in the TriC case and leads to the up-and-down movement of shear layer in the second gap region.展开更多
To understand the mechanism of premixed flame quenching by porous media,a zonal hybrid RANS/LES model was employed,in which the LES flow solver was used to resolve the large turbulent structures within the non-porous ...To understand the mechanism of premixed flame quenching by porous media,a zonal hybrid RANS/LES model was employed,in which the LES flow solver was used to resolve the large turbulent structures within the non-porous region,while RANS was applied to porous media zone.The predicted results were compared with previous experimental data.And it was evident that the premixed flame propagation rates and structure as well as quenching behaviors were reproduced by this numerical approach with a better accuracy.Due to the inherently higher heat transfer coefficients between the solid and gas phases in porous media,the gas phase temperature has been decreased rapidly.However,upstream obstacles can cause the flame propagating faster and thus reduce the axial gas temperature gradients,resulting in the invalidity of the operation of premixed flame quenching.By comparison with the case without upstream obstacle,the values of reaction rate attained in the case with three pairs of obstacles are higher,which makes a positive impact on the flame passing through the porous medium.In addition,the porous media with greater pore density has an excellent flame quenching property mainly owing to both the larger volumetric heat transfer and higher quenching temperature.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
A concept of entropy increment ratio （ s- ） is introduced for compressible turbulence simulation through a series of direct nu- merical simulations （DNS）. s- represents the dissipation rate per unit mechanical ene...A concept of entropy increment ratio （ s- ） is introduced for compressible turbulence simulation through a series of direct nu- merical simulations （DNS）. s- represents the dissipation rate per unit mechanical energy with the benefit of independence of freestream Mach numbers. Based on this feature, we construct the shielding function f, to describe the boundary layer region and propose an entropy-based detached-eddy simulation method （SDES）. This approach follows the spirit of delayed de- tached-eddy simulation （DDES） proposed by Spalart et al. in 2005, but it exhibits much better behavior after their performanc- es are compared in the following flows, namely, pure attached flow with thick boundary layer （a supersonic fiat-plate flow with high Reynolds number）, fully separated flow （the supersonic base flow）, and separated-reattached flow （the supersonic cavity-ramp flow）. The Reynolds-averaged Navier-Stokes （RANS） resolved region is reliably preserved and the modeled stress depletion （MSD） phenomenon which is inherent in DES and DDES is partly alleviated. Moreover, this new hybrid strategy is simple and general, making it applicable to other models related to the boundary layer predictions.展开更多
A sonic under-expanded transverse jet injection into a Ma 1.6 supersonic crossflow is investigated numerically using our hybrid RANS/LES (Reynolds-averaged Navier-Stokes/large eddy simulation) method. First, a calcula...A sonic under-expanded transverse jet injection into a Ma 1.6 supersonic crossflow is investigated numerically using our hybrid RANS/LES (Reynolds-averaged Navier-Stokes/large eddy simulation) method. First, a calculation is carried out to validate the code, where both the instantaneous and statistical results show good agreement with the existing experimental data. Then the jet-mixing characteristics are analyzed. It is observed that the large-scale vortex on the windward portion of the jet boundary is formed mainly by the intermittent impingement of the incoming high-speed fluid on the relatively low-speed region of the upstream jet boundary, where the interaction between the upstream separated region and the jet supplies a favorable pressure condition for the sustaining acceleration of the high-speed fluid during the vortex forming, associated with which the incoming fluid is entrained into the jet boundary and large-scale mixing occurs. Meanwhile, the secondary recirculation zone between the upstream separated region and the jet is observed to develop evidently during the vortex forming, inducing the entrainment of jet fluid into the upstream separated region. Moreover, effects of the incoming boundary layer on the jet mixing are addressed.展开更多
The paper presents 1) the numerical results of RANS (Reynolds Averaging Navier-Stokes) simulations for two versions of the premixed combustion GE10 burners: the old one with non-premixed and modified one with swirled ...The paper presents 1) the numerical results of RANS (Reynolds Averaging Navier-Stokes) simulations for two versions of the premixed combustion GE10 burners: the old one with non-premixed and modified one with swirled premixed pilot flames;and 2) the numerical results of joint RANS/LES (Large Eddy Simulation) modelling of the ONERA model burner and a simplified GE10 combustor. The original joint RANS/LES approach is based on using the Kolmogorov theory for modelling sub-grid turbulence and combustion intensity and using RANS numerical results for closure the LES model equations. The main conclusion is that developed joint RANS/LES approch is the efficient timesaving tool for simulations both the average and instantaneous fields of parameters in gas turbine and boiler burners with premixed combustion.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
We present our timesaving joint RANS/LES approach (we originally developed it for numerical simulations of turbulent premixed combustion) to simulate flameless combustion with separate injection of gas fuel and strong...We present our timesaving joint RANS/LES approach (we originally developed it for numerical simulations of turbulent premixed combustion) to simulate flameless combustion with separate injection of gas fuel and strong exhaust gas recirculation. It is based on successive RANS/LES numerical modeling where part of the information (stationary average fields) is achieved by RANS simulations and part (instantaneous nonstationary image of the process) by LES. The latter is performed using the RANS field of mean dissipation rate to model the sub-grid turbulent viscosity in the context of the Kolmogorov theory of small-scale turbulence. We analyze flameless combustion in the FLOX? combustor where we also simulate non-premixed flame combustion used for preliminary heating of the combustor. Different regimes take place using different systems of air injection. We applied for both regimes the simple assumption of “mixed is burnt”. The main results are the following: 1) RANS simulations demonstrate for used two injection systems respectively more compact flame and distributed flameless combustion. 2)There is agreement between RANS and corresponding LES results: RANS and averaged LES profiles of the velocity and temperature are in reasonable agreement. 3) LES modeling with Kolmogorov independent on time sub-grid viscosity reproduce instantaneous image of the process including the vortex structures. Probably due to using an annular injector system for air the instantaneous field of the temperature demonstrate significant irregularity in the beginning of the burner, which in an animation looks like moving coherent structures. 4) In the joint RANS/LES approach the computer time of the LES sub-problems is much shorter than classic LES modeling due to using time independent subgrid transport coefficients and avoiding long-continued simulations, which are necessary for averaging of instantaneous LES fields. Practically in our simulations time consuming of the LES sub-problem was only several times lager then the RANS one and it 展开更多
A nonqinear eddy viscosity model （NLEVM） and a scalable hybrid Reynolds averaged Navier-Stokes/large eddy simula- tion （RANS/LES） strategy are developed to improve the capability of the eddy viscosity model （EVM...A nonqinear eddy viscosity model （NLEVM） and a scalable hybrid Reynolds averaged Navier-Stokes/large eddy simula- tion （RANS/LES） strategy are developed to improve the capability of the eddy viscosity model （EVM） to simulate complex flows featuring separations and unsteady motions. To study the performance of the NLEVM, numerical simulations around S809 airfoil are carried out and the results show that the NLEVM performs much better when a large separation occurs. Calculated results of the flow around a triangular cylinder show that the NLEVM can improve the precision of the flow fields to some extents, but the error is still considerable, and the small turbulence structures can not be clearly captured as the EVM. Whereas the scalable hybrid RANS/LES model based on the NLEVM is fairy effective on resolving the turbulent structures and can give more satisfactory predictions of the flow fields.展开更多
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
基金The project supported by the National Natural Science Foundation of China (10502030 and 90505005)
文摘This paper presents hybrid Reynolds-averaged Navier-Stokes （RANS） and large-eddy-simulation （LES） methods for the separated flows at high angles of attack around a 6：1 prolate spheroid. The RANS/LES hybrid meth- ods studied in this work include the detached eddy simulation （DES） based on Spalart-Allmaras （S-A）, Menter＇s k-ω shear-stress-transport （SST） and k-o9 with weakly nonlinear eddy viscosity formulation （Wilcox-Durbin＋, WD＋） models and the zonalANS/LES methods based on the SST and WD＋ models. The switch from RANS near the wall to LES in the core flow region is smooth through the implementation of a flow-dependent blending function for the zonal hybrid method. All the hybrid methods are designed to have a RANS mode for the attached flows and have a LES behavior for the separated flows. The main objective of this paper is to apply the hybrid methods for the high Reynolds number separated flows around prolate spheroid at high-incidences. A fourth-order central scheme with fourth-order artificial viscosity is applied for spatial differencing. The fully implicit lower-upper symmetric-Gauss-Seidel with pseudo time sub-iteration is taken as the temporal differentiation. Comparisons with available measurements are carried out for pressure distribution, skin friction, and profiles of velocity, etc. Reasonable agreement with the experiments, accounting for the effect on grids and fundamental turbulence models, is obtained for the separation flows.
基金supported by the National Natural Science Foundation of China(No.51876098).
文摘Accurate predictions of Shock Waves and Boundary Layer Interaction(SWBLI)and strong Shock Waves and Wake Vortices Interaction(SWWVI)in a highly-loaded turbine propose challenges to the currently widely used Reynolds-Averaged Navier-Stokes(RANS)model.In this work,the SWBLI and the SWWVI in a highly-loaded Nozzle Guide Vane(NGV)are studied using a hybrid RANS/LES strategy.The Turbulence Kinetic Energy(TKE)budget and the Proper Orthogonal Decomposition(POD)method are used to analyze flow mechanisms.Results show that this hybrid RANS/LES method can obtain detailed flow structures for flow mechanisms analysis.Strong shock waves induce boundary layer separation,while the presence of a separation bubble can in turn lead to a Mach reflection phenomenon.The shock waves cause trailing-edge vortices to break clearly,and the wakes,in turn,can change the shocks intensity and direction.Furthermore,the Entropy Generation Rate(EGR)is used to analyze the irreversible loss.It turns out that the SWWVI can reduce the flow field loss.There are several weak shock waves in the NGV flow field,which can increase the irreversible loss.This work offers flow mechanisms analysis and presents the EGR distribution in SWBLI and SWWVI areas in a transonic turbine blade.
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.
基金supported by the National Natural Science Foundation of China (Grant 11372159)
文摘The massively separated flow past triple cylin- ders （TriC） in tandem arrangement is simulated using the improved delayed detached-eddy simulation （IDDES） method based on the shear stress transport （SST） model, coupled with the high order adaptive dissipation scheme. The spacing between adjacent cylinders is sub-critical （1.435D）. IDDES prediction of two cylinders （TC） with the same spacing is compared to experimental data for validation, and the numerical results agree well with the available measurements, except for the asymmetry in the gap region. The flow past TriC is investigated using the same method. Generally, the mean flow quantities past TriC, such as the velocity, pressure, and vorticity, are similar to the corresponding components of TC. However, the pressure fluctuations on the TriC surface are uniformly larger than those on TC. Meanwhile, the instantaneous flows past TriC are much more complex. The periodical blockage in the first gap region is found in the TriC case and leads to the up-and-down movement of shear layer in the second gap region.
基金supported by the National Natural Science Foundation of China(Nos.51174115,51604095)the Research Foundation of Education Bureau of Henan Province of China(No.14A410007)the research fund of Henan Key Laboratory for Green and Efficient Mining&Comprehensive Utilization of Mineral Resources
文摘To understand the mechanism of premixed flame quenching by porous media,a zonal hybrid RANS/LES model was employed,in which the LES flow solver was used to resolve the large turbulent structures within the non-porous region,while RANS was applied to porous media zone.The predicted results were compared with previous experimental data.And it was evident that the premixed flame propagation rates and structure as well as quenching behaviors were reproduced by this numerical approach with a better accuracy.Due to the inherently higher heat transfer coefficients between the solid and gas phases in porous media,the gas phase temperature has been decreased rapidly.However,upstream obstacles can cause the flame propagating faster and thus reduce the axial gas temperature gradients,resulting in the invalidity of the operation of premixed flame quenching.By comparison with the case without upstream obstacle,the values of reaction rate attained in the case with three pairs of obstacles are higher,which makes a positive impact on the flame passing through the porous medium.In addition,the porous media with greater pore density has an excellent flame quenching property mainly owing to both the larger volumetric heat transfer and higher quenching temperature.
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.
基金supported by the National Basic Research Program of China(Grant No.2009CB724104)the Innovation Foundation of BUAA for PhD Graduates and the Academic New Artist Award of BUAA for PhD Graduates
文摘A concept of entropy increment ratio （ s- ） is introduced for compressible turbulence simulation through a series of direct nu- merical simulations （DNS）. s- represents the dissipation rate per unit mechanical energy with the benefit of independence of freestream Mach numbers. Based on this feature, we construct the shielding function f, to describe the boundary layer region and propose an entropy-based detached-eddy simulation method （SDES）. This approach follows the spirit of delayed de- tached-eddy simulation （DDES） proposed by Spalart et al. in 2005, but it exhibits much better behavior after their performanc- es are compared in the following flows, namely, pure attached flow with thick boundary layer （a supersonic fiat-plate flow with high Reynolds number）, fully separated flow （the supersonic base flow）, and separated-reattached flow （the supersonic cavity-ramp flow）. The Reynolds-averaged Navier-Stokes （RANS） resolved region is reliably preserved and the modeled stress depletion （MSD） phenomenon which is inherent in DES and DDES is partly alleviated. Moreover, this new hybrid strategy is simple and general, making it applicable to other models related to the boundary layer predictions.
基金supported by the National Natural Science Foundation of China (Grant Nos. 50906098 and 91016028)Fok Ying Tung Education Foundation (Grant No. 131055)
文摘A sonic under-expanded transverse jet injection into a Ma 1.6 supersonic crossflow is investigated numerically using our hybrid RANS/LES (Reynolds-averaged Navier-Stokes/large eddy simulation) method. First, a calculation is carried out to validate the code, where both the instantaneous and statistical results show good agreement with the existing experimental data. Then the jet-mixing characteristics are analyzed. It is observed that the large-scale vortex on the windward portion of the jet boundary is formed mainly by the intermittent impingement of the incoming high-speed fluid on the relatively low-speed region of the upstream jet boundary, where the interaction between the upstream separated region and the jet supplies a favorable pressure condition for the sustaining acceleration of the high-speed fluid during the vortex forming, associated with which the incoming fluid is entrained into the jet boundary and large-scale mixing occurs. Meanwhile, the secondary recirculation zone between the upstream separated region and the jet is observed to develop evidently during the vortex forming, inducing the entrainment of jet fluid into the upstream separated region. Moreover, effects of the incoming boundary layer on the jet mixing are addressed.
文摘The paper presents 1) the numerical results of RANS (Reynolds Averaging Navier-Stokes) simulations for two versions of the premixed combustion GE10 burners: the old one with non-premixed and modified one with swirled premixed pilot flames;and 2) the numerical results of joint RANS/LES (Large Eddy Simulation) modelling of the ONERA model burner and a simplified GE10 combustor. The original joint RANS/LES approach is based on using the Kolmogorov theory for modelling sub-grid turbulence and combustion intensity and using RANS numerical results for closure the LES model equations. The main conclusion is that developed joint RANS/LES approch is the efficient timesaving tool for simulations both the average and instantaneous fields of parameters in gas turbine and boiler burners with premixed combustion.
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.
文摘We present our timesaving joint RANS/LES approach (we originally developed it for numerical simulations of turbulent premixed combustion) to simulate flameless combustion with separate injection of gas fuel and strong exhaust gas recirculation. It is based on successive RANS/LES numerical modeling where part of the information (stationary average fields) is achieved by RANS simulations and part (instantaneous nonstationary image of the process) by LES. The latter is performed using the RANS field of mean dissipation rate to model the sub-grid turbulent viscosity in the context of the Kolmogorov theory of small-scale turbulence. We analyze flameless combustion in the FLOX? combustor where we also simulate non-premixed flame combustion used for preliminary heating of the combustor. Different regimes take place using different systems of air injection. We applied for both regimes the simple assumption of “mixed is burnt”. The main results are the following: 1) RANS simulations demonstrate for used two injection systems respectively more compact flame and distributed flameless combustion. 2)There is agreement between RANS and corresponding LES results: RANS and averaged LES profiles of the velocity and temperature are in reasonable agreement. 3) LES modeling with Kolmogorov independent on time sub-grid viscosity reproduce instantaneous image of the process including the vortex structures. Probably due to using an annular injector system for air the instantaneous field of the temperature demonstrate significant irregularity in the beginning of the burner, which in an animation looks like moving coherent structures. 4) In the joint RANS/LES approach the computer time of the LES sub-problems is much shorter than classic LES modeling due to using time independent subgrid transport coefficients and avoiding long-continued simulations, which are necessary for averaging of instantaneous LES fields. Practically in our simulations time consuming of the LES sub-problem was only several times lager then the RANS one and it
基金Project supported by the National Natural Science Foun-dation of China(Grant Nos.51179100,51279184)
文摘A nonqinear eddy viscosity model （NLEVM） and a scalable hybrid Reynolds averaged Navier-Stokes/large eddy simula- tion （RANS/LES） strategy are developed to improve the capability of the eddy viscosity model （EVM） to simulate complex flows featuring separations and unsteady motions. To study the performance of the NLEVM, numerical simulations around S809 airfoil are carried out and the results show that the NLEVM performs much better when a large separation occurs. Calculated results of the flow around a triangular cylinder show that the NLEVM can improve the precision of the flow fields to some extents, but the error is still considerable, and the small turbulence structures can not be clearly captured as the EVM. Whereas the scalable hybrid RANS/LES model based on the NLEVM is fairy effective on resolving the turbulent structures and can give more satisfactory predictions of the flow fields.
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.
