A novel method is proposed to combine the wall-modeled large-eddy simulation(LES) with the diffuse-interface direct-forcing immersed boundary(IB) method.The new developments in this method include:(i) the momentum equ...A novel method is proposed to combine the wall-modeled large-eddy simulation(LES) with the diffuse-interface direct-forcing immersed boundary(IB) method.The new developments in this method include:(i) the momentum equation is integrated along the wall-normal direction to link the tangential component of the effective body force for the IB method to the wall shear stress predicted by the wall model;(ii) a set of Lagrangian points near the wall are introduced to compute the normal component of the effective body force for the IB method by reconstructing the normal component of the velocity. This novel method will be a classical direct-forcing IB method if the grid is fine enough to resolve the flow near the wall. The method is used to simulate the flows around the DARPA SUBOFF model. The results obtained are well comparable to the measured experimental data and wall-resolved LES results.展开更多
We analyze the error of large-eddy simulation(LES)in wall pressure fluctuation of a turbulent channel flow.To separate different sources of the error,we conduct both direct numerical simulations(DNS)and LES,and apply ...We analyze the error of large-eddy simulation(LES)in wall pressure fluctuation of a turbulent channel flow.To separate different sources of the error,we conduct both direct numerical simulations(DNS)and LES,and apply an explicit filter on DNS data to obtain filtered DNS(FDNS)data.The error of LES is consequently decomposed into two parts:The first part is the error of FDNS with respect to DNS,which quantifies the influence of the filter operation.The second part is the difference between LES and FDNS induced by the error of LES in velocity field.By comparing the root-mean-square value and the wavenumber-frequency spectrum of the wall pressure fluctuation,it is found that the inaccuracy of the velocity fluctuations is the dominant source that induces the error of LES in the wall pressure fluctuation.The present study provides a basis on future LES studies of the wall pressure fluctuation.展开更多
Physics-informed neural networks are a useful machine learning method for solving differential equations,but encounter challenges in effectively learning thin boundary layers within singular perturbation problems.To r...Physics-informed neural networks are a useful machine learning method for solving differential equations,but encounter challenges in effectively learning thin boundary layers within singular perturbation problems.To resolve this issue,multi-scale-matching neural networks are proposed to solve the singular perturbation problems.Inspired by matched asymptotic expansions,the solution is decomposed into inner solutions for small scales and outer solutions for large scales,corresponding to boundary layers and outer regions,respectively.Moreover,to conform neural networks,we introduce exponential stretched variables in the boundary layers to avoid semiinfinite region problems.Numerical results for the thin plate problem validate the proposed method.展开更多
A physics-informed neural network(PINN)is a powerful tool for solving differential equations in solid and fluid mechanics.However,it suffers from singularly perturbed boundary-layer problems in which there exist sharp...A physics-informed neural network(PINN)is a powerful tool for solving differential equations in solid and fluid mechanics.However,it suffers from singularly perturbed boundary-layer problems in which there exist sharp changes caused by a small perturbation parameter multiplying the highest-order derivatives.In this paper,we introduce Chien's composite expansion method into PINNs,and propose a novel architecture for the PINNs,namely,the Chien-PINN(C-PINN)method.This novel PINN method is validated by singularly perturbed differential equations,and successfully solves the wellknown thin plate bending problems.In particular,no cumbersome matching conditions are needed for the C-PINN method,compared with the previous studies based on matched asymptotic expansions.展开更多
Flapping-powered propulsion is used by many animals to locomote through air or water. Here we review recent experimental and numerical studies on self-propelled mechanical systems powered by a flapping motion. These s...Flapping-powered propulsion is used by many animals to locomote through air or water. Here we review recent experimental and numerical studies on self-propelled mechanical systems powered by a flapping motion. These studies improve our understanding of the mutual interaction between actively flapping bodies and surrounding fluids. The results obtained in these works provide not only new insights into biolocomotion but also useful information for the biomimetic design of artificial flyers and swimmers.展开更多
A large eddy simulation （LES） of the flows around an underwater vehicle model at intermediate Reynolds numbers is performed. The underwater vehicle model is taken as the DARPA SUBOFF with full appendages, where the ...A large eddy simulation （LES） of the flows around an underwater vehicle model at intermediate Reynolds numbers is performed. The underwater vehicle model is taken as the DARPA SUBOFF with full appendages, where the Reynolds number based on the hull length is 1.0x 105, An immersed boundary method based on the moving-least-squares reconstruction is used to handle the complex geometric boundaries. The adaptive mesh refinement is utilized to resolve the flows near the hull, The parallel scalabilities of the flow solver are tested on meshes with the number of cells varying from 50 million to 3.2 billion, The parallel solver reaches nearly linear scalability for the flows around the underwater vehicle model, The present simulation captures the essential features of the vortex structures near the hull and in the wake, Both of the time-averaged pressure coefficients and srreamwise velocity profiles obtained from the LES are consistent with the characteristics of the flows pass an appended axisymmetric body. The code efficiency and its correct predictions on flow features allow us to perform the full-scale simulations on tens of thousands of cores with billions of grid points for higher-Reynolds-number flows around the underwater vehicles.展开更多
The flows past a number 3900 are simulated circular cylinder at Reynolds using large-eddy simulation （LES） and the far-field sound is calculated from the LES results. A low dissipation energy-conserving finite volum...The flows past a number 3900 are simulated circular cylinder at Reynolds using large-eddy simulation （LES） and the far-field sound is calculated from the LES results. A low dissipation energy-conserving finite volume scheme is used to discretize the incompressible Navier- Stokes equations. The dynamic global coefficient version of the Vreman＇s subgrid scale （SGS） model is used to com- pute the sub-grid stresses. Curie＇s integral of Lighthill＇s acoustic analogy is used to extract the sound radiated from the cylinder. The profiles of mean velocity and turbulent fluctua- tions obtained are consistent with the previous experimental and computational results. The sound radiation at far field exhibits the characteristic of a dipole and directivity. The sound spectra display the -5/3 power law. It is shown that Vreman＇s SGS model in company with dynamic procedure is suitable for LES of turbulence generated noise.展开更多
Turbulent flows over rough surfaces widely exist in nature and industry.Investigating its mechanism is of theoretical and practical significance.In this work we simulate the turbulent channel flow with rough walls usi...Turbulent flows over rough surfaces widely exist in nature and industry.Investigating its mechanism is of theoretical and practical significance.In this work we simulate the turbulent channel flow with rough walls using large-eddy simulation with rough elements resolved using the curvilinear immersed boundary method and compare the results obtained in this work with those in the paper by Yuan and Piomelli(J.Fluid Mech.,vol.760,pp.R1,2014),where the volume of fluid method was employed for modeling rough elements.The mean streamwise velocity profiles predicted by the two methods agree well with each other.Differences in Reynolds stresses and dispersive stresses are observed,which are attributed to the different approaches in dealing with the complex geometry of the rough surface.展开更多
The application of large-eddy simulation （LES） to particle-laden turbulence raises such a fundamental question as whether the LES with a subgrid scale （SGS） model can correctly predict Lagrangian time correlations...The application of large-eddy simulation （LES） to particle-laden turbulence raises such a fundamental question as whether the LES with a subgrid scale （SGS） model can correctly predict Lagrangian time correlations （LTCs）. Most of the currently existing SGS models are constructed based on the energy budget equations. Therefore, they are able to correctly predict energy spectra, but they may not ensure the correct prediction on the LTCs. Previous researches investigated the effect of the SGS modeling on the Eulerian time correlations. This paper is devoted to study the LTCs in LES. A direct numerical simulation （DNS） and the LES with a spectral eddy viscosity model are performed for isotropic turbulence and the LTCs are calculated using the passive vector method. Both a priori and a posteriori tests are carried out. It is observed that the subgrid;scale contributions to the LTCs cannot be simply ignored and the LES overpredicts the LTCs than the DNS. It is concluded from the straining hypothesis that an accurate prediction of enstrophy spectra is most critical to the prediction of the LTCs.展开更多
Flow around a real-life underwater vehicle often happens at a high Reynolds number with flow structures at different scales from the boundary layer around a blade to that around the hull. This poses a great challenge ...Flow around a real-life underwater vehicle often happens at a high Reynolds number with flow structures at different scales from the boundary layer around a blade to that around the hull. This poses a great challenge for large-eddy simulation of an underwater vehicle aiming at resolving all relevant flow scales. In this work, we propose to model the hull with appendages using the immersed boundary method, and model the propeller using the actuator disk model without resolving the geometry of the blade. The proposed method is then applied to simulate the flow around Defense Advanced Research Projects Agency(DARPA) suboff. An overall acceptable agreement is obtained for the pressure and friction coefficients. Complex flow features are observed in the near wake of suboff. In the far wake, the core region is featured by a jet because of the actuator disk, surrounded by an annular region with velocity deficit due to the body of suboff.展开更多
This letter describes numerical simulation of the unsteady flow over a slow-flying bat by using the immersed boundary method based on the measured bat wing geometry and kinematics. The main vortical structures around ...This letter describes numerical simulation of the unsteady flow over a slow-flying bat by using the immersed boundary method based on the measured bat wing geometry and kinematics. The main vortical structures around the bat flapping wings are identified, illuminating the lift-generating role of the leading- edge vortices generated mainly in the downstroke. Furthermore, the lift decomposition indicates that the vortex lift has the dominant contribution to the time-averaged lift and the lift associated with the fluid acceleration has the relatively moderate effect.展开更多
The lattice Boltzmann method （LBM） is coupled with the multiple-relaxation- time （MRT） collision model and the three-dimensional 19-discrete-velocity （D3Q19） model to resolve intermittent behaviors on small scal...The lattice Boltzmann method （LBM） is coupled with the multiple-relaxation- time （MRT） collision model and the three-dimensional 19-discrete-velocity （D3Q19） model to resolve intermittent behaviors on small scales in isotropic turbulent flows. The high- order scaling exponents of the velocity structure functions, the probability distribution functions of Lagrangian accelerations, and the local energy dissipation rates are investi- gated. The self-similarity of the space-time velocity structure functions is explored using the extended self-similarity （ESS） method, which was originally developed for velocity spatial structure functions. The scaling exponents of spatial structure functions at up to ten orders are consistent with the experimental measurements and theoretical results, implying that the LBM can accurately resolve the intermittent behaviors. This valida~ tion provides a solid basis for using the LBM to study more complex processes that are sensitive to small scales in turbulent flows, such as the relative dispersion of pollutants and mesoscale structures of preferential concentration of heavy particles suspended in turbulent flows.展开更多
We find an asymptotic expression for the characteristic timescales of decorrelation processes in weakly compressible and isothermal turbulence. This result is used in the Eddy-Damped Quasi-Normal Markovian equation to...We find an asymptotic expression for the characteristic timescales of decorrelation processes in weakly compressible and isothermal turbulence. This result is used in the Eddy-Damped Quasi-Normal Markovian equation to derive the scalings of compressible energy spectra: (1) if the acoustic waves are dominant, the compressible energy spectra exhibit \(-7/3\) scaling; (2) if local eddy straining is dominant, the compressible energy spectra are scaled as \(-3\). Meanwhile, the energy spectra of incompressible components display the same scaling of \(-5/3\) as those in incompressible turbulence. The direct numerical simulations of weakly compressible turbulence are used to examine the scaling.展开更多
Independent component analysis(ICA)is used to study the multiscale localised modes of streamwise velocity fluctuations in turbulent channel flows.ICA aims to decompose signals into independent modes,which may induce s...Independent component analysis(ICA)is used to study the multiscale localised modes of streamwise velocity fluctuations in turbulent channel flows.ICA aims to decompose signals into independent modes,which may induce spatially localised objects.The height and size are defined to quantify the spatial position and extension of these ICA modes,respectively.In contrast to spatially extended proper orthogonal decomposition(POD)modes,ICA modes are typically localised in space,and the energy of some modes is distributed across the near-wall region.The sizes of ICA modes are multiscale and are approximately proportional to their heights.ICA modes can also help to reconstruct the statistics of turbulence,particularly the third-order moment of velocity fluctuations,which is related to the strongest Reynolds shear-stressproducing events.The results reported in this paper indicate that the ICA method may connect statistical descriptions and structural descriptions of turbulence.展开更多
We consider the two-point,two-time(space-time)correlation of passive scalar R(r,τ)in the Kraichnan model under the assumption of homogeneity and isotropy.Using the fine-gird PDF method,we find that R(r,τ)satisfies a...We consider the two-point,two-time(space-time)correlation of passive scalar R(r,τ)in the Kraichnan model under the assumption of homogeneity and isotropy.Using the fine-gird PDF method,we find that R(r,τ)satisfies a diffusion equation with constant diffusion coefficient determined by velocity variance and molecular diffusion.Itssolution can be expressed in terms of the two-point,one time correlation of passive scalar,i.e.,R(r,0).Moreover,the decorrelation o R(k,τ),which is the Fourier transform of R(r,τ),is determined byR(k,0)and a diffusion kernal.展开更多
The large-eddy simulation(LES)with the dynamic Smagorinsky model is used to predict the interior sound of an idealized vehicle cabin under the excitation of the wall pressures from turbulent channel flows.In compariso...The large-eddy simulation(LES)with the dynamic Smagorinsky model is used to predict the interior sound of an idealized vehicle cabin under the excitation of the wall pressures from turbulent channel flows.In comparison with direct numerical simulation(DNS),the LES results overpredict the sound pressure level(SPL)at low frequencies and underpredict the SPL at high frequencies.The incorrect predictions result from the incorrect prediction of LES on surface pressures,where the LES over-estimates the wavenumber and frequencies spectra of surface pressures at small wavenumbers and frequencies and under-estimates the spectra at large wavenumbers and frequencies.However,the LES results are close to the filtered-DNS results,implying that the unresolved scales are also important to surface pressures and interior sound.The Euler-Bernoulli beam under the excitation of exterior pressures,which serves as a simple model for aero-vibro-acoustics in the case of hydrodynamical fast,is used to explain the observed predictions and show that the Corcos model cannot represent the variation of turbulence pressure spectra at wavenumbers and frequencies.Therefore,the new requirement for the LES method,when applied to fluid-structural-acoustic interaction problems at high Reynolds numbers,is the correct prediction of wavenumber and frequency spectra of turbulence wall pressure.展开更多
Due to the recent system developments for the electromagnetic characterization of the subsurface, fast and easy acquisition is made feasible due to the fast measurement speed, easy coupling with GPS systems, and the a...Due to the recent system developments for the electromagnetic characterization of the subsurface, fast and easy acquisition is made feasible due to the fast measurement speed, easy coupling with GPS systems, and the availability of multi-channel electromagnetic induction（EMI） and ground penetrating radar（GPR） systems. Moreover, the increasing computer power enables the use of accurate forward modeling programs in advanced inversion algorithms where no approximations are used and the full information content of the measured data can be exploited. Here, recent developments of large-scale quantitative EMI inversion and full-waveform GPR inversion are discussed that yield higher resolution of quantitative medium properties compared to conventional approaches. In both cases a detailed forward model is used in the inversion procedure that is based on Maxwell＇s equations. The multi-channel EMI data that have different sensing depths for the different source-receiver offset are calibrated using a short electrical resistivity tomography（ERT） calibration line which makes it possible to invert for electrical conductivity changes with depth over large areas. The crosshole GPR full-waveform inversion yields significant higher resolution of the permittivity and conductivity images compared to ray-based inversion results.展开更多
The swimming of a 3Dfish-like bodywith finlets is numerically investigated at Re=1000(the Reynolds number is based on the uniform upstream flow and the length of the fish-like body).The finlets are simply modeled as t...The swimming of a 3Dfish-like bodywith finlets is numerically investigated at Re=1000(the Reynolds number is based on the uniform upstream flow and the length of the fish-like body).The finlets are simply modeled as thin rigid rectangular plates that undulate with the body.The wake structures and the flow around the caudal peduncle are studied.The finlets redirect the local flow across the caudal peduncle but the vortical structures in the wake are almost not affected by the finlets.Improvement of hydrodynamic performance has not been found in the simulation based on this simple model.The present numerical result is in agreement with that of the work of Nauen and Lauder[J.Exp.Biol.,204(2001),pp.2251-2263]and partially supports the hypothesis ofWebb[Bull.Fish.Res.Bd.Can.,190(1975),pp.1-159].展开更多
We present quantum numerical methods for the typical initial boundary value problems(IBVPs)of convection-diffusion equations in fluid dynamics.The IBVP is discretized into a series of linear systems via finite differe...We present quantum numerical methods for the typical initial boundary value problems(IBVPs)of convection-diffusion equations in fluid dynamics.The IBVP is discretized into a series of linear systems via finite difference methods and explicit time marching schemes.To solve these discrete systems in quantum computers,we design a series of quantum circuits,including four stages of encoding,amplification,adding source terms,and incorporating boundary conditions.In the encoding stage,the initial condition is encoded in the amplitudes of quantum registers as a state vector to take advantage of quantum algorithms in space complexity.In the following three stages,the discrete differential operators in classical computing are converted into unitary evolutions to satisfy the postulate in quantum systems.The related arithmetic calculations in quantum amplitudes are also realized to sum up the increments from these stages.The proposed quantum algorithm is implemented within the open-source quantum computing framework Qiskit[2].By simulating one-dimensional transient problems,including the Helmholtz equation,the Burgers’equation,and Navier-Stokes equations,we demonstrate the capability of quantum computers in fluid dynamics.展开更多
基金Project supported by the National Natural Science Foundation of China(Nos.91752118,11672305,11232011,and 11572331)the Strategic Priority Research Program(No.XDB22040104)the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences(No.QYZDJ-SSWSYS002)
文摘A novel method is proposed to combine the wall-modeled large-eddy simulation(LES) with the diffuse-interface direct-forcing immersed boundary(IB) method.The new developments in this method include:(i) the momentum equation is integrated along the wall-normal direction to link the tangential component of the effective body force for the IB method to the wall shear stress predicted by the wall model;(ii) a set of Lagrangian points near the wall are introduced to compute the normal component of the effective body force for the IB method by reconstructing the normal component of the velocity. This novel method will be a classical direct-forcing IB method if the grid is fine enough to resolve the flow near the wall. The method is used to simulate the flows around the DARPA SUBOFF model. The results obtained are well comparable to the measured experimental data and wall-resolved LES results.
基金This research is supported by the National Natural Science Foundation of China(NFSC)Basic Science Center Program for“Multiscale Problems in Nonlinear Mechanics”(Grant 11988102)the National Key Project(Grant GJXM92579)Shizhao Wang acknowledges the support from the National Natural Science Foundation of China(Grant 11922214).
文摘We analyze the error of large-eddy simulation(LES)in wall pressure fluctuation of a turbulent channel flow.To separate different sources of the error,we conduct both direct numerical simulations(DNS)and LES,and apply an explicit filter on DNS data to obtain filtered DNS(FDNS)data.The error of LES is consequently decomposed into two parts:The first part is the error of FDNS with respect to DNS,which quantifies the influence of the filter operation.The second part is the difference between LES and FDNS induced by the error of LES in velocity field.By comparing the root-mean-square value and the wavenumber-frequency spectrum of the wall pressure fluctuation,it is found that the inaccuracy of the velocity fluctuations is the dominant source that induces the error of LES in the wall pressure fluctuation.The present study provides a basis on future LES studies of the wall pressure fluctuation.
基金supported by the National Natural Science Foun-dation of China (NSFC) Basic Science Center Program for"Multiscale Problems in Nonlinear Mechanics"(Grant No. 11988102)supported by the National Natural Science Foundation of China (NSFC)(Grant No. 12202451)
文摘Physics-informed neural networks are a useful machine learning method for solving differential equations,but encounter challenges in effectively learning thin boundary layers within singular perturbation problems.To resolve this issue,multi-scale-matching neural networks are proposed to solve the singular perturbation problems.Inspired by matched asymptotic expansions,the solution is decomposed into inner solutions for small scales and outer solutions for large scales,corresponding to boundary layers and outer regions,respectively.Moreover,to conform neural networks,we introduce exponential stretched variables in the boundary layers to avoid semiinfinite region problems.Numerical results for the thin plate problem validate the proposed method.
基金Project supported by the National Natural Science Foundation of China Basic Science Center Program for“Multiscale Problems in Nonlinear Mechanics”(No.11988102)the National Natural Science Foundation of China(No.12202451)。
文摘A physics-informed neural network(PINN)is a powerful tool for solving differential equations in solid and fluid mechanics.However,it suffers from singularly perturbed boundary-layer problems in which there exist sharp changes caused by a small perturbation parameter multiplying the highest-order derivatives.In this paper,we introduce Chien's composite expansion method into PINNs,and propose a novel architecture for the PINNs,namely,the Chien-PINN(C-PINN)method.This novel PINN method is validated by singularly perturbed differential equations,and successfully solves the wellknown thin plate bending problems.In particular,no cumbersome matching conditions are needed for the C-PINN method,compared with the previous studies based on matched asymptotic expansions.
基金supported by the Chinese Academy of Sciences (Grants KJCX-SW-L08, KJCX3-SYW-S01)the National Natural Science Foundation of China (Grants 11021262, 11023001, 11232011, 11372331)
文摘Flapping-powered propulsion is used by many animals to locomote through air or water. Here we review recent experimental and numerical studies on self-propelled mechanical systems powered by a flapping motion. These studies improve our understanding of the mutual interaction between actively flapping bodies and surrounding fluids. The results obtained in these works provide not only new insights into biolocomotion but also useful information for the biomimetic design of artificial flyers and swimmers.
基金supported by the National Natural Science Foundation of China (11302238, 11232011. and 11572331)support from the Strategic Priority Research Program (XDB22040104)+1 种基金the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (QYZDJ-SSW-SYS002)the National Basic Research Program of China (973 Program 2013CB834100: Nonlinear science)
文摘A large eddy simulation （LES） of the flows around an underwater vehicle model at intermediate Reynolds numbers is performed. The underwater vehicle model is taken as the DARPA SUBOFF with full appendages, where the Reynolds number based on the hull length is 1.0x 105, An immersed boundary method based on the moving-least-squares reconstruction is used to handle the complex geometric boundaries. The adaptive mesh refinement is utilized to resolve the flows near the hull, The parallel scalabilities of the flow solver are tested on meshes with the number of cells varying from 50 million to 3.2 billion, The parallel solver reaches nearly linear scalability for the flows around the underwater vehicle model, The present simulation captures the essential features of the vortex structures near the hull and in the wake, Both of the time-averaged pressure coefficients and srreamwise velocity profiles obtained from the LES are consistent with the characteristics of the flows pass an appended axisymmetric body. The code efficiency and its correct predictions on flow features allow us to perform the full-scale simulations on tens of thousands of cores with billions of grid points for higher-Reynolds-number flows around the underwater vehicles.
基金supported by the National Natural Science Foundation of China(Grant 11232011)
文摘The flows past a number 3900 are simulated circular cylinder at Reynolds using large-eddy simulation （LES） and the far-field sound is calculated from the LES results. A low dissipation energy-conserving finite volume scheme is used to discretize the incompressible Navier- Stokes equations. The dynamic global coefficient version of the Vreman＇s subgrid scale （SGS） model is used to com- pute the sub-grid stresses. Curie＇s integral of Lighthill＇s acoustic analogy is used to extract the sound radiated from the cylinder. The profiles of mean velocity and turbulent fluctua- tions obtained are consistent with the previous experimental and computational results. The sound radiation at far field exhibits the characteristic of a dipole and directivity. The sound spectra display the -5/3 power law. It is shown that Vreman＇s SGS model in company with dynamic procedure is suitable for LES of turbulence generated noise.
基金supported by the National Natural Science Foundation of China (NSFC) Basic Science Center Program for “Multiscale Problems in Nonlinear Mechanics” (Grant No. 11988102)the NSFC Program (Grant No. 11772337)+3 种基金the Science Challenge Program (Grant No. TZ2016001)the Strategic Priority Research Program,Chinese Academy of Sciences (CAS) (Grant No. XDB22040104)the Key Research Program of Frontier Sciences, CAS (Grant No. QYZDJ-SSW-SYS002)the CAS Center for Excellence in Complex System Mechanics
文摘Turbulent flows over rough surfaces widely exist in nature and industry.Investigating its mechanism is of theoretical and practical significance.In this work we simulate the turbulent channel flow with rough walls using large-eddy simulation with rough elements resolved using the curvilinear immersed boundary method and compare the results obtained in this work with those in the paper by Yuan and Piomelli(J.Fluid Mech.,vol.760,pp.R1,2014),where the volume of fluid method was employed for modeling rough elements.The mean streamwise velocity profiles predicted by the two methods agree well with each other.Differences in Reynolds stresses and dispersive stresses are observed,which are attributed to the different approaches in dealing with the complex geometry of the rough surface.
基金the Chinese Academy of Sciences under the Innovative Project"Multi-scale modeling and simulation in complex Systems" (KJCX-SW-L08)the National Basic Research Program of China (973 Program) (2007CB814800) the National Natural Science Foundation of China (10325211, 10628206,10732090 and 10672012)
文摘The application of large-eddy simulation （LES） to particle-laden turbulence raises such a fundamental question as whether the LES with a subgrid scale （SGS） model can correctly predict Lagrangian time correlations （LTCs）. Most of the currently existing SGS models are constructed based on the energy budget equations. Therefore, they are able to correctly predict energy spectra, but they may not ensure the correct prediction on the LTCs. Previous researches investigated the effect of the SGS modeling on the Eulerian time correlations. This paper is devoted to study the LTCs in LES. A direct numerical simulation （DNS） and the LES with a spectral eddy viscosity model are performed for isotropic turbulence and the LTCs are calculated using the passive vector method. Both a priori and a posteriori tests are carried out. It is observed that the subgrid;scale contributions to the LTCs cannot be simply ignored and the LES overpredicts the LTCs than the DNS. It is concluded from the straining hypothesis that an accurate prediction of enstrophy spectra is most critical to the prediction of the LTCs.
基金supported by the National Natural Science Foundation of China(NSFC)Basic Science Center Program for“Multiscale Problems in Nonlinear Mechanics”(No.11988102)NSFC(No.12002345)China Postdoctoral Science Foundation(No.2020M680027)。
文摘Flow around a real-life underwater vehicle often happens at a high Reynolds number with flow structures at different scales from the boundary layer around a blade to that around the hull. This poses a great challenge for large-eddy simulation of an underwater vehicle aiming at resolving all relevant flow scales. In this work, we propose to model the hull with appendages using the immersed boundary method, and model the propeller using the actuator disk model without resolving the geometry of the blade. The proposed method is then applied to simulate the flow around Defense Advanced Research Projects Agency(DARPA) suboff. An overall acceptable agreement is obtained for the pressure and friction coefficients. Complex flow features are observed in the near wake of suboff. In the far wake, the core region is featured by a jet because of the actuator disk, surrounded by an annular region with velocity deficit due to the body of suboff.
基金supported by the National Natural Science Foundation of China(10872201,11232011,11302238,and 11372331)the National Basic Research Program of China(973 Program)(2013CB834100)(Nonlinear science)support from the National Supercomputer Center in Tianjin
文摘This letter describes numerical simulation of the unsteady flow over a slow-flying bat by using the immersed boundary method based on the measured bat wing geometry and kinematics. The main vortical structures around the bat flapping wings are identified, illuminating the lift-generating role of the leading- edge vortices generated mainly in the downstroke. Furthermore, the lift decomposition indicates that the vortex lift has the dominant contribution to the time-averaged lift and the lift associated with the fluid acceleration has the relatively moderate effect.
基金Project supported by the Science Challenge Program(No.TZ2016001)the National Natural Science Foundation of China(Nos.11472277,11572331,11232011,and 11772337)+2 种基金the Strategic Priority Research Program,Chinese Academy of Sciences(CAS)(No.XDB22040104)the Key Research Program of Frontier Sciences,CAS(No.QYZDJ-SSW-SYS002)the National Basic Research Program of China(973 Program)(No.2013CB834100)
文摘The lattice Boltzmann method （LBM） is coupled with the multiple-relaxation- time （MRT） collision model and the three-dimensional 19-discrete-velocity （D3Q19） model to resolve intermittent behaviors on small scales in isotropic turbulent flows. The high- order scaling exponents of the velocity structure functions, the probability distribution functions of Lagrangian accelerations, and the local energy dissipation rates are investi- gated. The self-similarity of the space-time velocity structure functions is explored using the extended self-similarity （ESS） method, which was originally developed for velocity spatial structure functions. The scaling exponents of spatial structure functions at up to ten orders are consistent with the experimental measurements and theoretical results, implying that the LBM can accurately resolve the intermittent behaviors. This valida~ tion provides a solid basis for using the LBM to study more complex processes that are sensitive to small scales in turbulent flows, such as the relative dispersion of pollutants and mesoscale structures of preferential concentration of heavy particles suspended in turbulent flows.
基金supported by the National Natural Science Foundation of China (Grants 11302238, 11232011, 11572331, and 11490551)the support from the Strategic Priority Research Program (Grant XDB22040104)the Key Research Program of Frontier Sciences of Chinese Academy of Sciences and the National Basic Research Program of China (973 Program) (Grant 2013CB834100: Nonlinear science)
文摘We find an asymptotic expression for the characteristic timescales of decorrelation processes in weakly compressible and isothermal turbulence. This result is used in the Eddy-Damped Quasi-Normal Markovian equation to derive the scalings of compressible energy spectra: (1) if the acoustic waves are dominant, the compressible energy spectra exhibit \(-7/3\) scaling; (2) if local eddy straining is dominant, the compressible energy spectra are scaled as \(-3\). Meanwhile, the energy spectra of incompressible components display the same scaling of \(-5/3\) as those in incompressible turbulence. The direct numerical simulations of weakly compressible turbulence are used to examine the scaling.
基金supported by NSFC Basic Science Center Program for“Multiscale Problems in Nonlinear Mechanics”(No.11988102)National Natural Science Foundation of China(Nos.12002344,11232011 and 11572331)+2 种基金The authors would like to acknowledge the support from China Postdoctoral Science Foundation(No.2020M670478)the Strategic Priority Research Program(No.XDB22040104)the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences(No.QYZDJ-SSW-SYS002).
文摘Independent component analysis(ICA)is used to study the multiscale localised modes of streamwise velocity fluctuations in turbulent channel flows.ICA aims to decompose signals into independent modes,which may induce spatially localised objects.The height and size are defined to quantify the spatial position and extension of these ICA modes,respectively.In contrast to spatially extended proper orthogonal decomposition(POD)modes,ICA modes are typically localised in space,and the energy of some modes is distributed across the near-wall region.The sizes of ICA modes are multiscale and are approximately proportional to their heights.ICA modes can also help to reconstruct the statistics of turbulence,particularly the third-order moment of velocity fluctuations,which is related to the strongest Reynolds shear-stressproducing events.The results reported in this paper indicate that the ICA method may connect statistical descriptions and structural descriptions of turbulence.
基金supported by the National Natural Science Foun-dation of China(NSFC)Basic Science Center Program for“Multiscale Problems in Nonlinear Mechanics”(Grant No.11988102).
文摘We consider the two-point,two-time(space-time)correlation of passive scalar R(r,τ)in the Kraichnan model under the assumption of homogeneity and isotropy.Using the fine-gird PDF method,we find that R(r,τ)satisfies a diffusion equation with constant diffusion coefficient determined by velocity variance and molecular diffusion.Itssolution can be expressed in terms of the two-point,one time correlation of passive scalar,i.e.,R(r,0).Moreover,the decorrelation o R(k,τ),which is the Fourier transform of R(r,τ),is determined byR(k,0)and a diffusion kernal.
基金Basic Science Center Program of the National Natural Science Foundation of China for“Multi-scale Problems in Nonlinear Mechanics”(Grant No.11988102)National Key Project(Grant No.GJXM92579).
文摘The large-eddy simulation(LES)with the dynamic Smagorinsky model is used to predict the interior sound of an idealized vehicle cabin under the excitation of the wall pressures from turbulent channel flows.In comparison with direct numerical simulation(DNS),the LES results overpredict the sound pressure level(SPL)at low frequencies and underpredict the SPL at high frequencies.The incorrect predictions result from the incorrect prediction of LES on surface pressures,where the LES over-estimates the wavenumber and frequencies spectra of surface pressures at small wavenumbers and frequencies and under-estimates the spectra at large wavenumbers and frequencies.However,the LES results are close to the filtered-DNS results,implying that the unresolved scales are also important to surface pressures and interior sound.The Euler-Bernoulli beam under the excitation of exterior pressures,which serves as a simple model for aero-vibro-acoustics in the case of hydrodynamical fast,is used to explain the observed predictions and show that the Corcos model cannot represent the variation of turbulence pressure spectra at wavenumbers and frequencies.Therefore,the new requirement for the LES method,when applied to fluid-structural-acoustic interaction problems at high Reynolds numbers,is the correct prediction of wavenumber and frequency spectra of turbulence wall pressure.
文摘Due to the recent system developments for the electromagnetic characterization of the subsurface, fast and easy acquisition is made feasible due to the fast measurement speed, easy coupling with GPS systems, and the availability of multi-channel electromagnetic induction（EMI） and ground penetrating radar（GPR） systems. Moreover, the increasing computer power enables the use of accurate forward modeling programs in advanced inversion algorithms where no approximations are used and the full information content of the measured data can be exploited. Here, recent developments of large-scale quantitative EMI inversion and full-waveform GPR inversion are discussed that yield higher resolution of quantitative medium properties compared to conventional approaches. In both cases a detailed forward model is used in the inversion procedure that is based on Maxwell＇s equations. The multi-channel EMI data that have different sensing depths for the different source-receiver offset are calibrated using a short electrical resistivity tomography（ERT） calibration line which makes it possible to invert for electrical conductivity changes with depth over large areas. The crosshole GPR full-waveform inversion yields significant higher resolution of the permittivity and conductivity images compared to ray-based inversion results.
基金This work was supported by Chinese Academy of Sciences under the Innovative Project’Multi-scale modeling and simulation in complex systems’(KJCX-SW-L08),’Mathematical modeling of complex system’(KJCX3-SYW-S01)National Basic Research Program of China(973 Program)under Project No.2007CB814800National Natural Science Foundation of China under Project Nos.10702074 and 10872201.The computations were supported by the Supercomputing Center of ChineseAcademy of Sciences and the Shanghai Supercomputer Center.
文摘The swimming of a 3Dfish-like bodywith finlets is numerically investigated at Re=1000(the Reynolds number is based on the uniform upstream flow and the length of the fish-like body).The finlets are simply modeled as thin rigid rectangular plates that undulate with the body.The wake structures and the flow around the caudal peduncle are studied.The finlets redirect the local flow across the caudal peduncle but the vortical structures in the wake are almost not affected by the finlets.Improvement of hydrodynamic performance has not been found in the simulation based on this simple model.The present numerical result is in agreement with that of the work of Nauen and Lauder[J.Exp.Biol.,204(2001),pp.2251-2263]and partially supports the hypothesis ofWebb[Bull.Fish.Res.Bd.Can.,190(1975),pp.1-159].
基金NSFC Basic Science Center Program for”Multiscale Problems in Nonlinear Mechanics”(Grant No.11988102)National Natural Science Foundation of China(Grant No.12202454).
文摘We present quantum numerical methods for the typical initial boundary value problems(IBVPs)of convection-diffusion equations in fluid dynamics.The IBVP is discretized into a series of linear systems via finite difference methods and explicit time marching schemes.To solve these discrete systems in quantum computers,we design a series of quantum circuits,including four stages of encoding,amplification,adding source terms,and incorporating boundary conditions.In the encoding stage,the initial condition is encoded in the amplitudes of quantum registers as a state vector to take advantage of quantum algorithms in space complexity.In the following three stages,the discrete differential operators in classical computing are converted into unitary evolutions to satisfy the postulate in quantum systems.The related arithmetic calculations in quantum amplitudes are also realized to sum up the increments from these stages.The proposed quantum algorithm is implemented within the open-source quantum computing framework Qiskit[2].By simulating one-dimensional transient problems,including the Helmholtz equation,the Burgers’equation,and Navier-Stokes equations,we demonstrate the capability of quantum computers in fluid dynamics.
