Numerical simulations were conducted on a 10-blade Sevik rotor ingesting wake downstream of two turbulence-generating grids.These simulations were based on implicit large-eddy simulation(ILES)and the boundary data imm...Numerical simulations were conducted on a 10-blade Sevik rotor ingesting wake downstream of two turbulence-generating grids.These simulations were based on implicit large-eddy simulation(ILES)and the boundary data immersion method(BDIM)for compressible flows,which were solved using a fully self-programmed Fortran code.Results show that the predicted thrust spectrum aligns closely with the experimental measurements.In addition,it captures the thrust dipole directivity of the noise around the rotating propeller due to random pressure pulsations on the blades,as well as the flow structures simultaneously.Furthermore,the differences in the statistical characteristics,flow structures,and low-frequency broadband thrust spectra due to different turbulence levels were investigated.This analysis indicates that the interaction between the upstream,which is characterized by a lower turbulence level and a higher turbulent length of scale,and the rotating propeller results in a lower amplitude in force spectra and a slight increase in the scale of tip vortices.展开更多
A facile ammonium-dichromate solution immersion method was introduced to synthesize the copperwettable Cr3C2 coating on and inside the carbon-carbon （C/C） preform. The formation mechanism and the microstructures of ...A facile ammonium-dichromate solution immersion method was introduced to synthesize the copperwettable Cr3C2 coating on and inside the carbon-carbon （C/C） preform. The formation mechanism and the microstructures of the Cr3C2 coatings were studied. The contact angle between molten copper and the C/C decreased from 140°to 60°, demonstrating the significant improvement in the wettability. The Cr3C2- coated C/C-Cu composite with only 4.2% porosity and 3.69 gcm^-3 density was manufactured through copper infiltration. As a result, the thermal and electrical conductivity of the modified C/C-Cu increased significantly due to the infiltrated copper. Also the mechanical properties of the composites including both the flexural and compressive strengths were enhanced by over 100%. The modified C/C-Cu composite exhibited lower friction coefficients and wear rates for different load levels than those of the commercial C/Cu composite. These results demonstrate the potential of the modified C/C-Cu material for use in electrical contacts.展开更多
The diffuse-interface immersed boundary method(IBM)possesses excellent capabilities for simulating flows around complex geometries and moving boundaries.In this method,the flow field is solved on a fixed Cartesian mes...The diffuse-interface immersed boundary method(IBM)possesses excellent capabilities for simulating flows around complex geometries and moving boundaries.In this method,the flow field is solved on a fixed Cartesian mesh,while the solid boundary is discretized into a series of Lagrangian points immersed in the flow field.The boundary condition is implemented by introducing a force term into the momentum equation,and the interaction between the immersed boundary and the fluid domain is achieved via an interpolation process.Over the past decades,the diffuse-interface IBM has gained popularity and spawned many variants,effectively handling a wide range of flow problems from isothermal to thermal flows,from laminar to turbulent flows,and from complex geometries to fluidstructure interaction scenarios.This paper first outlines the basic principles of the diffuse-interface IBM,then highlights recent advancements achieved by the authors’research group,and finally shows the method’s excellent numerical performance and wide applicability through several case studies involving complex moving boundary problems.展开更多
A hybrid approach based on the immersed boundary method(IBM)is developed for computation of flow-induced sound around moving bodies.In this method,a high-fidelity direct numerical simulation(DNS)solver is used to simu...A hybrid approach based on the immersed boundary method(IBM)is developed for computation of flow-induced sound around moving bodies.In this method,a high-fidelity direct numerical simulation(DNS)solver is used to simulate the incompressible flow field.The sound field is predicted by discretizing acoustic perturbation equations(APEs)with dispersionrelation-preserving space scheme and low-dispersion and low-dissipation Runge-Kutta time integration.A sharp-interface IBM based on ghost-cell is implemented for present two-step DNS-APE approach to deal with complex moving bodies with Cartesian grids.The present method is validated through simulations of sound generation caused by flow past a rotating cylinder,an oscillating cylinder,and tandem oscillating and stationary cylinders.The sound generated by typical kinds of complicated bio-inspired locomotions,i.e.,flapping flight by wings of varied shapes and collective undulatory swimming in tandem,are investigated using present method.The results demonstrate potential of the hybrid approach in addressing flowinduced sound generation and propagation with complex moving boundaries in a fluid medium,especially for the sound characteristics of bio-mimetic flows,which might shed lights on investigations on bio-acoustics,ethology of complex animal system,and related bio-mimetic design for quietness.展开更多
Applying numerical simulation technology to investigate fluid-solid interaction involving complex curved bound-aries is vital in aircraft design,ocean,and construction engineering.However,current methods such as Latti...Applying numerical simulation technology to investigate fluid-solid interaction involving complex curved bound-aries is vital in aircraft design,ocean,and construction engineering.However,current methods such as Lattice Boltzmann(LBM)and the immersion boundary method based on solid ratio(IMB)have limitations in identifying custom curved boundaries.Meanwhile,IBM based on velocity correction(IBM-VC)suffers from inaccuracies and numerical instability.Therefore,this study introduces a high-accuracy curve boundary recognition method(IMB-CB),which identifies boundary nodes by moving the search box,and corrects the weighting function in LBM by calculating the solid ratio of the boundary nodes,achieving accurate recognition of custom curve boundaries.In addition,curve boundary image and dot methods are utilized to verify IMB-CB.The findings revealed that IMB-CB can accurately identify the boundary,showing an error of less than 1.8%with 500 lattices.Also,the flow in the custom curve boundary and aerodynamic characteristics of the NACA0012 airfoil are calculated and compared to IBM-VC.Results showed that IMB-CB yields lower lift and drag coefficient errors than IBM-VC,with a 1.45%drag coefficient error.In addition,the characteristic curve of IMB-CB is very stable,whereas that of IBM-VC is not.For the moving boundary problem,LBM-IMB-CB with discrete element method(DEM)is capable of accurately simulating the physical phenomena of multi-moving particle flow in complex curved pipelines.This research proposes a new curve boundary recognition method,which can significantly promote the stability and accuracy of fluid-solid interaction simulations and thus has huge applications in engineering.展开更多
The lattice Boltzmann method （LBM） and the immersed boundary method （IBM） are alternative, com- putational techniques for solving complex fluid dynamics systems, and can take the place of the Navier-Stokes（N- S）...The lattice Boltzmann method （LBM） and the immersed boundary method （IBM） are alternative, com- putational techniques for solving complex fluid dynamics systems, and can take the place of the Navier-Stokes（N- S） equation. This paper proposes a novel immersed boundary-lattice Boltzmann method （IB-LBM） based on the feedback law. The method uses the immersed boundary concept in the LBM framework to capture the coupling between a body with complex geometry and a uniform fluid, Then, the flows around a stationary circular cylinder and two circular cylinders in a side by side arrangement are simulated by using the method. Results are agreed well with the benchmark data, so, the capability of the method for complex geometry is demonstrated. Different from the conventional IB-LBM, which uses the Hook＇s law or the direct forcing method to compute the interae- tion force, the method uses the feedback law--the feedback of velocity field and displacement information to calculate the force, thus ensuring the method has advantages of easy implementation and full parallelism.展开更多
The wing of a butterfly consists of partially overlapping forewing and hindwing,and forewing sweeping can dynamically change the shape of the whole wing.In this work,the effect of forewing sweeping on aerodynamic perf...The wing of a butterfly consists of partially overlapping forewing and hindwing,and forewing sweeping can dynamically change the shape of the whole wing.In this work,the effect of forewing sweeping on aerodynamic performance of a butterfly like model is studied using a solver based on immersed boundary method and adaptive mesh.For aerodynamic performance,adding a"forward-backward-forward"sweeping motion to the forewing makes it more suitable for fast cruising flight,and compared to the situation without forewing sweeping,the drag is reduced by 46%and the lift to drag ratio is increased by 45%.On the contrary,adding a"backward-for ward-backward"sweeping motion to the forewing increases lift and makes it more suitable for climb flight.For downstroke and middle to late upstroke,the forewing sweeping affects the Leading-Edge Vortex(LEV)through two factors:sweeping velocity and forward sweeping angle,and their effects are coupled.A large forward sweeping velocity can enhance the strength of LEV,while a large forward sweeping angle can weaken it.For early upstroke,the forewing sweeping can affect the wake capture mechanism,sweeping backward can enhance it while sweeping forward can weaken it.The findings in this work provide insight into the design of butterfly like Micro AirVehicles(MAVs).展开更多
The role of hydrodynamic effect in the meeting of multiple fish is a fascinating topic.The interactions of two self-propelled flexi-ble plates swimming in opposite directions horizontally and maintaining a certain lat...The role of hydrodynamic effect in the meeting of multiple fish is a fascinating topic.The interactions of two self-propelled flexi-ble plates swimming in opposite directions horizontally and maintaining a certain lateral distance are numerically simulated using a penalty-immersed boundary method.The effects of the flapping phase and lateral distance on the propulsive performance of two fish meetings are analyzed.Results show that,when two plates meet,if their leading edges diverge laterally,the individual plate can efficiently and rapidly move apart from the other horizontally.If their leading edges converge laterally,the plate motion can be retarded,leading to high energy consumption.Moreover,an increasing lateral distance between two plates significantly weakens the fluid-structure interactions,resulting in an exponential decline in mean cruising speed.A quantitative force analysis based on vortex dynamic theory is performed to gain physics insight into the hydrodynamic interaction mechanism.It is found that lateral separation between the two leading edges enhances the vorticity generation and boundary vorticity flux on the surface of the plate,subsequently reinforcing the thrust effect and increasing horizontal velocity.This study offers insight into the hydro-dynamic mechanisms of the fluid-structure interactions among fish moving toward each other and suggests potential strategies for enhancing the maneuverability of robotic fish in complex environment.展开更多
To explore the relationship between dynamic characteristics and wake patterns,numerical simulations were conducted on three equal-diameter cylinders arranged in an equilateral triangle.The simulations varied reduced v...To explore the relationship between dynamic characteristics and wake patterns,numerical simulations were conducted on three equal-diameter cylinders arranged in an equilateral triangle.The simulations varied reduced velocities and gap spacing to observe flow-induced vibrations(FIVs).The immersed boundary–lattice Boltzmann flux solver(IB–LBFS)was applied as a numerical solution method,allowing for straightforward application on a simple Cartesian mesh.The accuracy and rationality of this method have been verified through comparisons with previous numerical results,including studies on flow past three stationary circular cylinders arranged in a similar pattern and vortex-induced vibrations of a single cylinder across different reduced velocities.When examining the FIVs of three cylinders,numerical simulations were carried out across a range of reduced velocities(3.0≤Ur≤13.0)and gap spacing(L=3D,4D,and 5D).The observed vibration response included several regimes:the desynchronization regime,the initial branch,and the lower branch.Notably,the transverse amplitude peaked,and a double vortex street formed in the wake when the reduced velocity reached the lower branch.This arrangement of three cylinders proved advantageous for energy capture as the upstream cylinder’s vibration response mirrored that of an isolated cylinder,while the response of each downstream cylinder was significantly enhanced.Compared to a single cylinder,the vibration and flow characteristics of this system are markedly more complex.The maximum transverse amplitudes of the downstream cylinders are nearly identical and exceed those observed in a single-cylinder set-up.Depending on the gap spacing,the flow pattern varied:it was in-phase for L=3D,antiphase for L=4D,and exhibited vortex shedding for L=5D.The wake configuration mainly featured double vortex streets for L=3D and evolved into two pairs of double vortex streets for L=5D.Consequently,it well illustrates the coupling mechanism that dynamics characteristics and wake vortex change with gap spacing and reduced velocities.展开更多
The motion of an elliptical rigid particle in a lid-driven cavity flow was numerically simulated using the immersed boundary lattice Boltzmann method(IB-LBM).The effects of the particle's initial orientation angle...The motion of an elliptical rigid particle in a lid-driven cavity flow was numerically simulated using the immersed boundary lattice Boltzmann method(IB-LBM).The effects of the particle's initial orientation angle,initial position,aspect ratio,and size on the motion characteristics were investigated.The computational results indicate that the particle's motion undergoes two distinct stages:a starting stage that involves moving from the release position to a limit cycle,and a periodic stage that involves moving on the limit cycle.The initial orientation of the particle has a minimal impact on both stages of motion.In contrast,the time it takes for the particle to reach the limit cycle may vary depending on the release position.Furthermore,particles with a larger aspect ratio exhibit a greater maximum velocity magnitude;an increase in particle size causes the particle trajectory to contract more toward the center of the cavity,decreasing the centrifugal force experienced by the particle.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 aim of this study is to develop a model of fluid and heat transfer in a biological tissue taking into account the exact structure of the related microvascular network,and to analyze the influence of structural cha...The aim of this study is to develop a model of fluid and heat transfer in a biological tissue taking into account the exact structure of the related microvascular network,and to analyze the influence of structural changes of such a network induced by diabetes.A cubic region representing local skin tissue is selected as the computational domain,which in turn includes two intravascular and extravascular sub-domains.To save computational resources,the capillary network is reduced to a 1D pipeline model and embedded into the extravascular region.On the basis of the immersed boundary method(IBM)strategy,fluid and heat fluxes across a capillary wall are distributed to the surrounding tissue nodes by a delta function.We consider both steady and periodic blood pressure conditions at the entrances of the capillary network.Under steady blood pressure conditions,both the interstitial fluid pressure and tissue temperature around the capillary network are larger than those in other places.When the periodic blood pressure condition is considered,tissue temperature tends to fluctuate with the same frequency of the forcing,but the related waveform displays a smaller amplitude and a certain time(phase)delay.When the connectivity of capillary network is diminished,the capacity of blood redistribution through the capillary network becomes weaker and a subset of the vessel branches lose blood flow,which further aggravates the amplitude attenuation and time delay of the skin temperature fluctuation.展开更多
This article introduces a numerical scheme on the basis of semi-implicit method for pressure-linked equations （SIMPLE） algorithm to simulate incompressible unsteady flows with fluid-structure interaction. The Navier...This article introduces a numerical scheme on the basis of semi-implicit method for pressure-linked equations （SIMPLE） algorithm to simulate incompressible unsteady flows with fluid-structure interaction. The Navier-Stokes equation is discretized spatially with collocated finite volume method and Eulerian implicit method in time domain. The hybrid method that combines immersed boundary method （IBM） and volume of fluid （VOF） method is used to deal with rigid body motion in fluid domain. The details of movement of immersed boundary （IB） and calculation of VOF are also described. This method can be easily applied to any existing finite-volume-based computational fluid dynamics （CFD） solver without complex operation, with which fluid flow interaction of arbitrarily complex geometry can be realized on a fixed mesh. The method is verified by low Reynolds number flows passing both stationary and oscillating cylinders. The drag and lift coefficients acquired by the study well accord with other published results, which indicate the reasonability of the proposed method.展开更多
A numerical study of vortex-induced rotations(VIRs) of an equivalent triangular cylinder, which is free to rotate in the azimuthal direction in a uniform flow, is presented. Based on an immersed boundary method, the n...A numerical study of vortex-induced rotations(VIRs) of an equivalent triangular cylinder, which is free to rotate in the azimuthal direction in a uniform flow, is presented. Based on an immersed boundary method, the numerical model is established, and is verified through the benchmark problem of flow past a freely rotating rectangular body.The computation is performed for a fixed reduced mass of m~*=2.0 and the structural stiffness and damping ratio are set to zero. The effects of Reynolds number(Re=25-180) on the characteristics of VIR are studied. It is found that the dynamic response of the triangular cylinder exhibits four distinct modes with increasing Re: a rest position,periodic rotational oscillation, random rotation and autorotation. For the rotational oscillation mode, the cylinder undergoes a periodic vibration around an equilibrium position with one side facing the incoming flow. Since the rotation effect, the outset of vortex shedding from cylinder shifts to a much lower Reynolds number. Further increase in Re leads to 2 P and P+S vortex shedding modes besides the typical 2 S pattern. Our simulation results also elucidate that the free rotation significantly changes the drag and lift forces. Inspired by these facts, the effect of free rotation on flow-induced vibration of a triangular cylinder in the in-line and transverse directions is investigated. The results show that when the translational vibration is coupled with rotation, the triangular cylinder presents a galloping response instead of vortex-induced vibration(VIV).展开更多
Simulating unsteady turbulent flow in turbomachines is still challenging due to the complexity of blade geometry and relative motion between rotor and stator.This study presents an Immersed Boundary Method(IBM)for hig...Simulating unsteady turbulent flow in turbomachines is still challenging due to the complexity of blade geometry and relative motion between rotor and stator.This study presents an Immersed Boundary Method(IBM)for high-Reynolds turbomachinery internal flows,and shows the advantage of the automatic grid generation techniques and flexible moving boundary treatments.The wall functions are used in the present method to alleviate the wall resolution restriction of turbulence simulation.The Two-Dimensional(2-D)IBM solver,which was previously developed and tested for a low-speed compressor,is further validated for a well-documented Low-Pressure Turbine(LPT)cascade.Both the blade loading and the total pressure losses in the wake are well captured by the present 2-D solver.The complex Three-Dimensional(3-D)effects in turbomachines motivate the further development of an extended 3-D IBM solver by using a curvilinear-coordinate system that facilitates the hub and casing boundary treatment.The good performance of the 3-D solver is demonstrated through comparison with CFX solver solutions for the rotor configuration of Advanced Noise Control Fan(ANCF).Further effects of the grid resolution on capturing the blade wake are discussed.The results indicate that the present 3-D solver is capable of reproducing the evolution of the blade wake with suitable computational grid.展开更多
This paper presents an improved unstructured grid immersed boundary method.The advantages of both immersed boundary method and body fitted grids which are generated by unstructured grid technology are used to enhance ...This paper presents an improved unstructured grid immersed boundary method.The advantages of both immersed boundary method and body fitted grids which are generated by unstructured grid technology are used to enhance the computation efficiency of fluid structure interaction in complex domain.The Navier-Stokes equation was discretized spacially with collocated finite volume method and Euler implicit method in time domain.The rigid body motion was simulated by immersed boundary method in which the fluid and rigid body interface interaction was dealt with VOS(volume of solid) method.A new VOS calculation method based on graph was presented in which both immersed boundary points and cross points were collected in arbitrary order to form a graph.The method is verified with flow past oscillating cylinder.展开更多
This paper presents the extension of the global description approach of a discontinuous function, which is proposed in the previous paper, to a spectral domain decomposition method. This multi-domain spectral immersed...This paper presents the extension of the global description approach of a discontinuous function, which is proposed in the previous paper, to a spectral domain decomposition method. This multi-domain spectral immersed interlace method（IIM） divides the whole computation domain into the smooth and discontinuous parts. Fewer points on the smooth domains are used via taking advantage of the high accuracy property of the spectral method, but more points on the discontinuous domains are employed to enhance the resolution of the calculation. Two that the domain decomposition technique can placed around the discontinuity. The present reached, in spite of the enlarged computational discontinuous problems are tested to verify the present method. The results show reduce the error of the spectral IIM, especially when more collocation points are method is t：avorable for the reason that the same level of the accuracy can be domain.展开更多
In this paper,a class of new immersed interface finite element methods (IIFEM) is developed to solve elasticity interface problems with homogeneous and non-homogeneous jump conditions in two dimensions.Simple non-body...In this paper,a class of new immersed interface finite element methods (IIFEM) is developed to solve elasticity interface problems with homogeneous and non-homogeneous jump conditions in two dimensions.Simple non-body-fitted meshes are used.For homogeneous jump conditions,both non-conforming and conforming basis functions are constructed in such a way that they satisfy the natural jump conditions. For non-homogeneous jump conditions,a pair of functions that satisfy the same non-homogeneous jump conditions are constructed using a level-set representation of the interface.With such a pair of functions,the discontinuities across the interface in the solution and flux are removed;and an equivalent elasticity interface problem with homogeneous jump conditions is formulated.Numerical examples are presented to demonstrate that such methods have second order convergence.展开更多
The immersed boundary method is an effective technique for modeling and simulating fluid-structure interactions especially in the area of biomechanics.This paper analyzes the accuracy of the immersed boundary method.T...The immersed boundary method is an effective technique for modeling and simulating fluid-structure interactions especially in the area of biomechanics.This paper analyzes the accuracy of the immersed boundary method.The procedure contains two parts,i.e.,the code verification and the accuracy analysis.The code verification provides the confidence that the code used is free of mistakes,and the accuracy analysis gives the order of accuracy of the immersed boundary method.The method of manufactured solutions is taken as a means for both parts.In the first part,the numerical code employs a second-order discretization scheme,i.e.,it has second-order accuracy in theory.It matches the calculated order of accuracy obtained in the numerical calculation for all variables.This means that the code contains no mistake,which is a premise of the subsequent work.The second part introduces a jump in the manufactured solution for the pressure and adds the corresponding singular forcing terms in the momentum equations.By analyzing the discretization errors,the accuracy of the immersed boundary method is proven to be first order even though the discretization scheme is second order.It has been found that the coarser mesh may not be sensitive enough to capture the influence of the immersed boundary,and the refinement on the Lagrangian markers barely has any effect on the numerical calculation.展开更多
Currently, many studies on the local discontinuous Galerkin method focus on the Cartesian grid with low computational e ciency and poor adaptability to complex shapes. A new immersed boundary method is presented, and ...Currently, many studies on the local discontinuous Galerkin method focus on the Cartesian grid with low computational e ciency and poor adaptability to complex shapes. A new immersed boundary method is presented, and this method employs the adaptive Cartesian grid to improve the adaptability to complex shapes and the immersed boundary to increase computational e ciency. The new immersed boundary method employs different boundary cells(the physical cell and ghost cell) to impose the boundary condition and the reconstruction algorithm of the ghost cell is the key for this method. The classical model elliptic equation is used to test the method. This method is tested and analyzed from the viewpoints of boundary cell type, error distribution and accuracy. The numerical result shows that the presented method has low error and a good rate of the convergence and works well in complex geometries. The method has good prospect for practical application research of the numerical calculation research.展开更多
基金Supported by the National Key R&D Program of China(2022YFB3303500).
文摘Numerical simulations were conducted on a 10-blade Sevik rotor ingesting wake downstream of two turbulence-generating grids.These simulations were based on implicit large-eddy simulation(ILES)and the boundary data immersion method(BDIM)for compressible flows,which were solved using a fully self-programmed Fortran code.Results show that the predicted thrust spectrum aligns closely with the experimental measurements.In addition,it captures the thrust dipole directivity of the noise around the rotating propeller due to random pressure pulsations on the blades,as well as the flow structures simultaneously.Furthermore,the differences in the statistical characteristics,flow structures,and low-frequency broadband thrust spectra due to different turbulence levels were investigated.This analysis indicates that the interaction between the upstream,which is characterized by a lower turbulence level and a higher turbulent length of scale,and the rotating propeller results in a lower amplitude in force spectra and a slight increase in the scale of tip vortices.
基金the financial support from of the National Basic Research Program of China (Nos. 2012CB619600 and 2011CB012803)
文摘A facile ammonium-dichromate solution immersion method was introduced to synthesize the copperwettable Cr3C2 coating on and inside the carbon-carbon （C/C） preform. The formation mechanism and the microstructures of the Cr3C2 coatings were studied. The contact angle between molten copper and the C/C decreased from 140°to 60°, demonstrating the significant improvement in the wettability. The Cr3C2- coated C/C-Cu composite with only 4.2% porosity and 3.69 gcm^-3 density was manufactured through copper infiltration. As a result, the thermal and electrical conductivity of the modified C/C-Cu increased significantly due to the infiltrated copper. Also the mechanical properties of the composites including both the flexural and compressive strengths were enhanced by over 100%. The modified C/C-Cu composite exhibited lower friction coefficients and wear rates for different load levels than those of the commercial C/Cu composite. These results demonstrate the potential of the modified C/C-Cu material for use in electrical contacts.
基金partially supported by the National Natural Science Foundation of China(Nos.92271103,12202191)。
文摘The diffuse-interface immersed boundary method(IBM)possesses excellent capabilities for simulating flows around complex geometries and moving boundaries.In this method,the flow field is solved on a fixed Cartesian mesh,while the solid boundary is discretized into a series of Lagrangian points immersed in the flow field.The boundary condition is implemented by introducing a force term into the momentum equation,and the interaction between the immersed boundary and the fluid domain is achieved via an interpolation process.Over the past decades,the diffuse-interface IBM has gained popularity and spawned many variants,effectively handling a wide range of flow problems from isothermal to thermal flows,from laminar to turbulent flows,and from complex geometries to fluidstructure interaction scenarios.This paper first outlines the basic principles of the diffuse-interface IBM,then highlights recent advancements achieved by the authors’research group,and finally shows the method’s excellent numerical performance and wide applicability through several case studies involving complex moving boundary problems.
基金supported by the National Natural Science Foundation of China(Grant Nos.52301380 and 11602277).
文摘A hybrid approach based on the immersed boundary method(IBM)is developed for computation of flow-induced sound around moving bodies.In this method,a high-fidelity direct numerical simulation(DNS)solver is used to simulate the incompressible flow field.The sound field is predicted by discretizing acoustic perturbation equations(APEs)with dispersionrelation-preserving space scheme and low-dispersion and low-dissipation Runge-Kutta time integration.A sharp-interface IBM based on ghost-cell is implemented for present two-step DNS-APE approach to deal with complex moving bodies with Cartesian grids.The present method is validated through simulations of sound generation caused by flow past a rotating cylinder,an oscillating cylinder,and tandem oscillating and stationary cylinders.The sound generated by typical kinds of complicated bio-inspired locomotions,i.e.,flapping flight by wings of varied shapes and collective undulatory swimming in tandem,are investigated using present method.The results demonstrate potential of the hybrid approach in addressing flowinduced sound generation and propagation with complex moving boundaries in a fluid medium,especially for the sound characteristics of bio-mimetic flows,which might shed lights on investigations on bio-acoustics,ethology of complex animal system,and related bio-mimetic design for quietness.
基金WJD,JYZ,CLC,ZX,and ZGY were supported by the National Natural Science Foundation of China(Grant Number 51705143)the Education Department of Hunan Province(Grant Number 22B0464)the Postgraduate Scientific Research Innovation Project of Hunan Province(Grant Number QL20230249).
文摘Applying numerical simulation technology to investigate fluid-solid interaction involving complex curved bound-aries is vital in aircraft design,ocean,and construction engineering.However,current methods such as Lattice Boltzmann(LBM)and the immersion boundary method based on solid ratio(IMB)have limitations in identifying custom curved boundaries.Meanwhile,IBM based on velocity correction(IBM-VC)suffers from inaccuracies and numerical instability.Therefore,this study introduces a high-accuracy curve boundary recognition method(IMB-CB),which identifies boundary nodes by moving the search box,and corrects the weighting function in LBM by calculating the solid ratio of the boundary nodes,achieving accurate recognition of custom curve boundaries.In addition,curve boundary image and dot methods are utilized to verify IMB-CB.The findings revealed that IMB-CB can accurately identify the boundary,showing an error of less than 1.8%with 500 lattices.Also,the flow in the custom curve boundary and aerodynamic characteristics of the NACA0012 airfoil are calculated and compared to IBM-VC.Results showed that IMB-CB yields lower lift and drag coefficient errors than IBM-VC,with a 1.45%drag coefficient error.In addition,the characteristic curve of IMB-CB is very stable,whereas that of IBM-VC is not.For the moving boundary problem,LBM-IMB-CB with discrete element method(DEM)is capable of accurately simulating the physical phenomena of multi-moving particle flow in complex curved pipelines.This research proposes a new curve boundary recognition method,which can significantly promote the stability and accuracy of fluid-solid interaction simulations and thus has huge applications in engineering.
基金Supported by the Aeronautical Science Foundation of China(20111453012)the National Defense Pre-Research Foundation of China(9140A13040111HK0329)~~
文摘The lattice Boltzmann method （LBM） and the immersed boundary method （IBM） are alternative, com- putational techniques for solving complex fluid dynamics systems, and can take the place of the Navier-Stokes（N- S） equation. This paper proposes a novel immersed boundary-lattice Boltzmann method （IB-LBM） based on the feedback law. The method uses the immersed boundary concept in the LBM framework to capture the coupling between a body with complex geometry and a uniform fluid, Then, the flows around a stationary circular cylinder and two circular cylinders in a side by side arrangement are simulated by using the method. Results are agreed well with the benchmark data, so, the capability of the method for complex geometry is demonstrated. Different from the conventional IB-LBM, which uses the Hook＇s law or the direct forcing method to compute the interae- tion force, the method uses the feedback law--the feedback of velocity field and displacement information to calculate the force, thus ensuring the method has advantages of easy implementation and full parallelism.
基金supported by the National Natural Science Foundation of China(No.52175277)The computations were conducted in Computing Center in Xi'an.The authors thankfully acknowledge these institutions.
文摘The wing of a butterfly consists of partially overlapping forewing and hindwing,and forewing sweeping can dynamically change the shape of the whole wing.In this work,the effect of forewing sweeping on aerodynamic performance of a butterfly like model is studied using a solver based on immersed boundary method and adaptive mesh.For aerodynamic performance,adding a"forward-backward-forward"sweeping motion to the forewing makes it more suitable for fast cruising flight,and compared to the situation without forewing sweeping,the drag is reduced by 46%and the lift to drag ratio is increased by 45%.On the contrary,adding a"backward-for ward-backward"sweeping motion to the forewing increases lift and makes it more suitable for climb flight.For downstroke and middle to late upstroke,the forewing sweeping affects the Leading-Edge Vortex(LEV)through two factors:sweeping velocity and forward sweeping angle,and their effects are coupled.A large forward sweeping velocity can enhance the strength of LEV,while a large forward sweeping angle can weaken it.For early upstroke,the forewing sweeping can affect the wake capture mechanism,sweeping backward can enhance it while sweeping forward can weaken it.The findings in this work provide insight into the design of butterfly like Micro AirVehicles(MAVs).
基金joined PI of Westlake University(Grant Nos.041030150118 and 103110556022101)Scientific Research Funding Project of Westlake University(Grant No.2021WUFP017).
文摘The role of hydrodynamic effect in the meeting of multiple fish is a fascinating topic.The interactions of two self-propelled flexi-ble plates swimming in opposite directions horizontally and maintaining a certain lateral distance are numerically simulated using a penalty-immersed boundary method.The effects of the flapping phase and lateral distance on the propulsive performance of two fish meetings are analyzed.Results show that,when two plates meet,if their leading edges diverge laterally,the individual plate can efficiently and rapidly move apart from the other horizontally.If their leading edges converge laterally,the plate motion can be retarded,leading to high energy consumption.Moreover,an increasing lateral distance between two plates significantly weakens the fluid-structure interactions,resulting in an exponential decline in mean cruising speed.A quantitative force analysis based on vortex dynamic theory is performed to gain physics insight into the hydrodynamic interaction mechanism.It is found that lateral separation between the two leading edges enhances the vorticity generation and boundary vorticity flux on the surface of the plate,subsequently reinforcing the thrust effect and increasing horizontal velocity.This study offers insight into the hydro-dynamic mechanisms of the fluid-structure interactions among fish moving toward each other and suggests potential strategies for enhancing the maneuverability of robotic fish in complex environment.
基金Supported by the National Natural Science Foundation of China(52201350,52201394,and 52271301)the Innovation Group Project of Southern Marine Science and Engineering Guangdong Laboratory(Zhuhai)(Grant No.SML2022008).
文摘To explore the relationship between dynamic characteristics and wake patterns,numerical simulations were conducted on three equal-diameter cylinders arranged in an equilateral triangle.The simulations varied reduced velocities and gap spacing to observe flow-induced vibrations(FIVs).The immersed boundary–lattice Boltzmann flux solver(IB–LBFS)was applied as a numerical solution method,allowing for straightforward application on a simple Cartesian mesh.The accuracy and rationality of this method have been verified through comparisons with previous numerical results,including studies on flow past three stationary circular cylinders arranged in a similar pattern and vortex-induced vibrations of a single cylinder across different reduced velocities.When examining the FIVs of three cylinders,numerical simulations were carried out across a range of reduced velocities(3.0≤Ur≤13.0)and gap spacing(L=3D,4D,and 5D).The observed vibration response included several regimes:the desynchronization regime,the initial branch,and the lower branch.Notably,the transverse amplitude peaked,and a double vortex street formed in the wake when the reduced velocity reached the lower branch.This arrangement of three cylinders proved advantageous for energy capture as the upstream cylinder’s vibration response mirrored that of an isolated cylinder,while the response of each downstream cylinder was significantly enhanced.Compared to a single cylinder,the vibration and flow characteristics of this system are markedly more complex.The maximum transverse amplitudes of the downstream cylinders are nearly identical and exceed those observed in a single-cylinder set-up.Depending on the gap spacing,the flow pattern varied:it was in-phase for L=3D,antiphase for L=4D,and exhibited vortex shedding for L=5D.The wake configuration mainly featured double vortex streets for L=3D and evolved into two pairs of double vortex streets for L=5D.Consequently,it well illustrates the coupling mechanism that dynamics characteristics and wake vortex change with gap spacing and reduced velocities.
文摘The motion of an elliptical rigid particle in a lid-driven cavity flow was numerically simulated using the immersed boundary lattice Boltzmann method(IB-LBM).The effects of the particle's initial orientation angle,initial position,aspect ratio,and size on the motion characteristics were investigated.The computational results indicate that the particle's motion undergoes two distinct stages:a starting stage that involves moving from the release position to a limit cycle,and a periodic stage that involves moving on the limit cycle.The initial orientation of the particle has a minimal impact on both stages of motion.In contrast,the time it takes for the particle to reach the limit cycle may vary depending on the release position.Furthermore,particles with a larger aspect ratio exhibit a greater maximum velocity magnitude;an increase in particle size causes the particle trajectory to contract more toward the center of the cavity,decreasing the centrifugal force experienced by the particle.
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This study was supported by National Natural Science Foundation of China(NSFC No.51576033)Dalian Innovative Funding of Science and Technology(2018J12SN076)NSFC No 11602053.
文摘The aim of this study is to develop a model of fluid and heat transfer in a biological tissue taking into account the exact structure of the related microvascular network,and to analyze the influence of structural changes of such a network induced by diabetes.A cubic region representing local skin tissue is selected as the computational domain,which in turn includes two intravascular and extravascular sub-domains.To save computational resources,the capillary network is reduced to a 1D pipeline model and embedded into the extravascular region.On the basis of the immersed boundary method(IBM)strategy,fluid and heat fluxes across a capillary wall are distributed to the surrounding tissue nodes by a delta function.We consider both steady and periodic blood pressure conditions at the entrances of the capillary network.Under steady blood pressure conditions,both the interstitial fluid pressure and tissue temperature around the capillary network are larger than those in other places.When the periodic blood pressure condition is considered,tissue temperature tends to fluctuate with the same frequency of the forcing,but the related waveform displays a smaller amplitude and a certain time(phase)delay.When the connectivity of capillary network is diminished,the capacity of blood redistribution through the capillary network becomes weaker and a subset of the vessel branches lose blood flow,which further aggravates the amplitude attenuation and time delay of the skin temperature fluctuation.
文摘This article introduces a numerical scheme on the basis of semi-implicit method for pressure-linked equations （SIMPLE） algorithm to simulate incompressible unsteady flows with fluid-structure interaction. The Navier-Stokes equation is discretized spatially with collocated finite volume method and Eulerian implicit method in time domain. The hybrid method that combines immersed boundary method （IBM） and volume of fluid （VOF） method is used to deal with rigid body motion in fluid domain. The details of movement of immersed boundary （IB） and calculation of VOF are also described. This method can be easily applied to any existing finite-volume-based computational fluid dynamics （CFD） solver without complex operation, with which fluid flow interaction of arbitrarily complex geometry can be realized on a fixed mesh. The method is verified by low Reynolds number flows passing both stationary and oscillating cylinders. The drag and lift coefficients acquired by the study well accord with other published results, which indicate the reasonability of the proposed method.
基金financially supported by the Fundamental Research Funds for the Central Universities(Grant Nos.2018B56414 and2019B12014)the National Natural Science Foundation of China(Grant No.51609077)
文摘A numerical study of vortex-induced rotations(VIRs) of an equivalent triangular cylinder, which is free to rotate in the azimuthal direction in a uniform flow, is presented. Based on an immersed boundary method, the numerical model is established, and is verified through the benchmark problem of flow past a freely rotating rectangular body.The computation is performed for a fixed reduced mass of m~*=2.0 and the structural stiffness and damping ratio are set to zero. The effects of Reynolds number(Re=25-180) on the characteristics of VIR are studied. It is found that the dynamic response of the triangular cylinder exhibits four distinct modes with increasing Re: a rest position,periodic rotational oscillation, random rotation and autorotation. For the rotational oscillation mode, the cylinder undergoes a periodic vibration around an equilibrium position with one side facing the incoming flow. Since the rotation effect, the outset of vortex shedding from cylinder shifts to a much lower Reynolds number. Further increase in Re leads to 2 P and P+S vortex shedding modes besides the typical 2 S pattern. Our simulation results also elucidate that the free rotation significantly changes the drag and lift forces. Inspired by these facts, the effect of free rotation on flow-induced vibration of a triangular cylinder in the in-line and transverse directions is investigated. The results show that when the translational vibration is coupled with rotation, the triangular cylinder presents a galloping response instead of vortex-induced vibration(VIV).
基金co-supported by the National Natural Science Foundation of China(No.52022009)the Science Center for Gas Turbine Project of China(No.P2022-A-II-003-001)the Key Laboratory Foundation,China(No.2021-JCJQ-LB-062-0102).
文摘Simulating unsteady turbulent flow in turbomachines is still challenging due to the complexity of blade geometry and relative motion between rotor and stator.This study presents an Immersed Boundary Method(IBM)for high-Reynolds turbomachinery internal flows,and shows the advantage of the automatic grid generation techniques and flexible moving boundary treatments.The wall functions are used in the present method to alleviate the wall resolution restriction of turbulence simulation.The Two-Dimensional(2-D)IBM solver,which was previously developed and tested for a low-speed compressor,is further validated for a well-documented Low-Pressure Turbine(LPT)cascade.Both the blade loading and the total pressure losses in the wake are well captured by the present 2-D solver.The complex Three-Dimensional(3-D)effects in turbomachines motivate the further development of an extended 3-D IBM solver by using a curvilinear-coordinate system that facilitates the hub and casing boundary treatment.The good performance of the 3-D solver is demonstrated through comparison with CFX solver solutions for the rotor configuration of Advanced Noise Control Fan(ANCF).Further effects of the grid resolution on capturing the blade wake are discussed.The results indicate that the present 3-D solver is capable of reproducing the evolution of the blade wake with suitable computational grid.
文摘This paper presents an improved unstructured grid immersed boundary method.The advantages of both immersed boundary method and body fitted grids which are generated by unstructured grid technology are used to enhance the computation efficiency of fluid structure interaction in complex domain.The Navier-Stokes equation was discretized spacially with collocated finite volume method and Euler implicit method in time domain.The rigid body motion was simulated by immersed boundary method in which the fluid and rigid body interface interaction was dealt with VOS(volume of solid) method.A new VOS calculation method based on graph was presented in which both immersed boundary points and cross points were collected in arbitrary order to form a graph.The method is verified with flow past oscillating cylinder.
基金National Natural Science Foundation of China(51076006)
文摘This paper presents the extension of the global description approach of a discontinuous function, which is proposed in the previous paper, to a spectral domain decomposition method. This multi-domain spectral immersed interlace method（IIM） divides the whole computation domain into the smooth and discontinuous parts. Fewer points on the smooth domains are used via taking advantage of the high accuracy property of the spectral method, but more points on the discontinuous domains are employed to enhance the resolution of the calculation. Two that the domain decomposition technique can placed around the discontinuity. The present reached, in spite of the enlarged computational discontinuous problems are tested to verify the present method. The results show reduce the error of the spectral IIM, especially when more collocation points are method is t：avorable for the reason that the same level of the accuracy can be domain.
基金supported by the US ARO grants 49308-MA and 56349-MAthe US AFSOR grant FA9550-06-1-024+1 种基金he US NSF grant DMS-0911434the State Key Laboratory of Scientific and Engineering Computing of Chinese Academy of Sciences during a visit by Z.Li between July-August,2008.
文摘In this paper,a class of new immersed interface finite element methods (IIFEM) is developed to solve elasticity interface problems with homogeneous and non-homogeneous jump conditions in two dimensions.Simple non-body-fitted meshes are used.For homogeneous jump conditions,both non-conforming and conforming basis functions are constructed in such a way that they satisfy the natural jump conditions. For non-homogeneous jump conditions,a pair of functions that satisfy the same non-homogeneous jump conditions are constructed using a level-set representation of the interface.With such a pair of functions,the discontinuities across the interface in the solution and flux are removed;and an equivalent elasticity interface problem with homogeneous jump conditions is formulated.Numerical examples are presented to demonstrate that such methods have second order convergence.
基金supported by the National Natural Science Foundation of China (No 10472070)
文摘The immersed boundary method is an effective technique for modeling and simulating fluid-structure interactions especially in the area of biomechanics.This paper analyzes the accuracy of the immersed boundary method.The procedure contains two parts,i.e.,the code verification and the accuracy analysis.The code verification provides the confidence that the code used is free of mistakes,and the accuracy analysis gives the order of accuracy of the immersed boundary method.The method of manufactured solutions is taken as a means for both parts.In the first part,the numerical code employs a second-order discretization scheme,i.e.,it has second-order accuracy in theory.It matches the calculated order of accuracy obtained in the numerical calculation for all variables.This means that the code contains no mistake,which is a premise of the subsequent work.The second part introduces a jump in the manufactured solution for the pressure and adds the corresponding singular forcing terms in the momentum equations.By analyzing the discretization errors,the accuracy of the immersed boundary method is proven to be first order even though the discretization scheme is second order.It has been found that the coarser mesh may not be sensitive enough to capture the influence of the immersed boundary,and the refinement on the Lagrangian markers barely has any effect on the numerical calculation.
基金Supported by National Natural Science Foundation of China(Grant No.51405375)National Key Basic Research and Development Program of China(973 Program,Grant No.2011CB706606)
文摘Currently, many studies on the local discontinuous Galerkin method focus on the Cartesian grid with low computational e ciency and poor adaptability to complex shapes. A new immersed boundary method is presented, and this method employs the adaptive Cartesian grid to improve the adaptability to complex shapes and the immersed boundary to increase computational e ciency. The new immersed boundary method employs different boundary cells(the physical cell and ghost cell) to impose the boundary condition and the reconstruction algorithm of the ghost cell is the key for this method. The classical model elliptic equation is used to test the method. This method is tested and analyzed from the viewpoints of boundary cell type, error distribution and accuracy. The numerical result shows that the presented method has low error and a good rate of the convergence and works well in complex geometries. The method has good prospect for practical application research of the numerical calculation research.
