HYBRID CARTESIAN GRID/GRIDLESS METHOD FOR CALCULATING VISCOUS FLOWS OVER MULTI-ELEMENT AIRFOILS

A hybrid Cartesian grid/gridless method is developed for calculating viscous flows over multi-element airfoils. The method adopts an unstructured Cartesian grid to cover most areas of the computational domain and leaves only small region adjacent to the aerodynamic bodies to be filled with the cloud of points used in the gridless methods, which results in a better combination of the computational efficiency of the Cartesian grid and the flexi- bility of the gridless method in handling complex geometries. The clouds of points in the local gridless region are implemented in an anisotropic way according to the features of the thin boundary layer of the viscous flows over the airfoils, and the clouds of points at the vicinity of the interface between the grid and the gridless regions are also controlled by using an adaptive refinement technique during the generation of the unstructured Cartesian grid. An implementation of the resulting hybrid method is presented for solving two-dimensional compressible Navier-Stokes （NS） equations. The simulations of the viscous flows over a RAE2822 airfoil or a two-element airfoil are success- fully carried out, and the obtained results agree well with the available experimental data.     

ADAPTIVE HYBRID CARTESIAN GRID METHOD FOR VORTEX-DOMINATED FLOWS

An efficient compressible Euler equation solver for vortex-dominated flows is presented based on the a- daptive hybrid Cartesian mesh and vortex identifying method. For most traditional grid-based Euler solvers, the excessive numerical dissipation is the great obstruction for vortex capturing or tracking problems. A vortex identif- ying method based on the curl of velocity is used to identify the vortex in flow field. Moreover, a dynamic adaptive mesh refinement （DAMR） process for hybrid Cartesian gird system is employed to track and preserve vortex. To validate the proposed method, a single compressible vortex convection flow is involved to test the accuracy and effi- ciency of DAMR process. Additionally, the vortex-dominated flow is investigated by the method. The obtained re- sults are shown as a good agreement with the previous published data.     

GHOST FLUID METHOD FOR RICHTMYER-MESHKOV INSTABILITY SIMULATION

INTRODUCTIONThe energy of the Richtmyer- Meshkov in-stability is excited by the interaction between ashock wave and a contact interface across thatfluids are different.Itis very important thatcap-ture the interface in multi- material flows exact-ly.Here we will apply the ghost fluid method tosimulate two dimensional Richtmyer- Meshkovinstability.The mathematical equation used to simulatethe Ritchmyer- Meshkov instability is the com-pressible Euler equation for two ideal gases.ρρuρvE…     

NUMERICAL STUDY OF TWO INTERFACE METHODS FOR COMPRESSIBLE MULTI-FLUID FLOWS

INTRODUCTIONThe direct numerical simulation of com-pressible multi- fluid flows is an important re-search topic with various key applications.Therelatively dominant difficulty of simulation lies intreatment of the moving material interface. Ingeneral,there are two basic approaches to treatcontact discontinuities.The first,front trackingmethod[1,2 ] ,explicitly characterize the interfacemotion by the motion of individual points on theinterface. The other approach is front capturingmethod[3…     

CALCULATING METHOD OF AERODYNAMIC HEATING FOR HYPERSONIC AIRCRAFTS

A new calculating method of aerodynamic heating for unsteady hypersonic aircrafts with complex config- uration is presented. This method, which considers the effects of high temperature chemical non-equilibrium and the heat transfer process in thermal protection structure, is based on the combination of the inviscid outerflow solu- tion and the engineering method, where the Euler solver provides the flow parameters on boundary layer edge for engineering method in aerodynamic heating calculation. A high efficient interpolation technique, which can be ap- plied to the fast computation of longtime aerodynamic heating for hypersonic aircraft, is developed for flying trajec- tory. In this paper, three hypersonic test cases are calculated, and the heat flux and temperature distribution of thermo-protection system are shown. The numerical results show the high efficiency of the developed method and the validation of thermal characteristics analysis on hypersonic aerodynamic heating.     

EFFECT OF CONTACT ANGLE HYSTERESIS ON DROPLET BEHAVIORS： TWO-PHASE LATTICE BOLTZMANN SIMULATION

An approach of dealing with contact angle hysteresis in lattice Boltzmann method is introduced in detail. The approach is also used to investigate droplet behaviors on surfaces of chemical inhomogeneities or roughness （non-ideal surfaces）. Droplet slipping on surfaces under gravity or in shear flows, and droplet impacting on sur- faces are numerically simulated. It is found that the present approach is suitable to model droplet motions on non- ideal surfaces and the contact angle hysteresis has an obvious effect on the motion of droplets.     

WEIGHTED COMPACT SCHEME FOR SHOCK CAPTURING

INTRODUCTIONRecently compact schemes have been widelyused in the simulation of complex flows,espe-cially in the directnumerical simulation of turbu-lent flows[1～ 2 ] . Standard finite differenceschemes have explicit forms and need to be atleastone pointwider than the desired approxima-tion order. It is also difficult to find suitable andstable boundary closure for high order schemes.Compared to the standard finite difference ap-proximations,the compact schemes can achievehigher order acc…     

SOLUTION OF 3-D TURBULENCE NAVIER-STOKES EQUATIONS USING HYBRID GRIDS

INTRODUCTIONWith the development of computer,consid-erable process has been made in the developmentof numerical methods for the solution of Navier-Stokes equations.However,most of these me-thods are impractical for complicated flows in adesign environment.The primary reason is thatwe can′t generate a suitable mesh easily for acomplicated configuration.Although there aremany techniques,such as structured grid or un-structured grid,which have been successfullyused for numerical simulation…     

DEVELOPMENT OF HYBRID RANS / LES CODE AND ITS VALIDATION

A computational code based on the hybrid RANSLES approach is developed. The hybrid approach com- bines the delayed detached-eddy simulation（DDES） with an improved RANS-LES hybrid model aiming at wall modeling in LES（WMLES）. In the code, the convective flux is solved using the fourth-order skew-symmetric scheme so as to diminish the negative effect of numerical dissipation. The Spalart-Allmaras（S-A） model is applied as a suhgrid scale（SGS） model. To validate the developed code, homogeneous isotropic turbulence and turbulent channel flow are simulated and the results are compared with experimental data and DNS results. The results of the isotropic turbulence show that the fourth-order skew-symmetric scheme is adequate enough and the model works well coupling with the convective scheme. The results of the turbulent channel flow agree well with the DNS data, the predicted velocity profiles at Reynolds number from 178 to 2 700 match well with the Reichardt＇s law, and the organized vortical structures are well captured.     

NUMERICAL SIMULATION ON MULTI-FLUID INTERFACES

INTRODUCTIONThe study on phenomenon of multi- fluid in-terface driven by shock waves include:( 1 ) totrack the motion of multimaterial interfaces;( 2 )to simulate the chemical reaction on the inter-faces. As the pioneering work,chemical reactionis neglected in this paper. The objective is totrack the interfaces. Of course,the fluids aretreated as compressible medium under such ahigh impact.Although Eulerian schemes workwell for most gas flows,they have been shownto be too dissipative nea…     

NUMERICAL STUDY ON EFFECT OF OPERATING TABLE PROTECTED BY HORIZONTAL LAMINAR FLOW SCREEN

Transmission of airborne bacteria is the main factor causing surgical site infection （SSI）, which is harm- ful to patients＇ health and even lives. Numerical study is conducted on the effect of the operating table protected by horizontal laminar flow screen. Discrete phase model （DPM） is used. Numerical simulation is carried out to evalu- ate particle trajectories with the Lagrange approach. As a result, the protecting effect of horizontal laminar flow screen is established, and the protecting parameters of the air velocity supplied by the screen and the protecting dis- tance are optimized. The optimized air velocity supplied by the screen should be at 0.4--0.6 m/s. And the protec- ting distance should be less than 1.3 m. This work provides references for the study on the depuration of operating table or room.     

SIMULATION OF BLEED AIR BEHAVIOR DURING AIRCRAFT IN FLIGHT BASED ON FLOWMASTER

Bleed air system is one of the most important components of air management system(AMS).It acts as transfer pipes responsible for air supply at high temperature and pressure.The thermal and flow performance of the bleed air system is a key issue for the design of AMS since the characteristics of air source have a great influence on the anti-ice system,the environmental control system and other downstream system in need of high temperature pressurized air.Based on the one-dimensional lumped parameter technology,a computer analysis model of bleed air system is developed in order to analyze the thermal and flow behaviors of the nodal points in the pipeline network.The simulation are performed with a given flight assignment using the analysis model,and the results verify that the system meets the design requirements.     

COMPUTATION OF UNSTEADY AERODYNAMIC FORCES ON WINGS WITH NAVIER-STOCKS EQUATIONS

INTRODUCTIONFor the requirement of instantaneity of un-steady flows,the minimal time step is needed,which leads to a requirement for a large numberof time steps.To an explicit method,it marchesvery slowly and many of the acceleration tech-niques used for steady flows cannot be used be-cause they destroy the time accuracy.The“dual-time” method allows an implicit discretisation tobe used in real time,but at each real time stepmarches the solution in a pseudo time to steadystate,through an …     

Aero-thermo-elastic Numerical Calculation and Analysis of Spinning Rocket

The application of computational fluid dynamics/computational solid method （CFD/CSM） on solving the aero-thermo-elastic problem of spinning rocket is introduced. Firstly, the aerodynamic coefficients of a rocket are calculated, and the results are compared with the available experimental data, which verified the accuracy of the CFD output. Then, analysis is carried using ANSYS Workbench multi-physics coupling platform, which includes fluid, thermal, and structural solvers. The results show that spinning causes a significant effect on the de formations and stresses. Furthermore, thermal stresses due to high temperature at the rocket warhead and tail edges have a dominated effect, even more than those produced by aerodynamic forces. Consequently, this important outcome should be taken into consideration during the rocket design stages.     

Ejection Separation Characteristic Analysis of Parachute Container Cover from Return Capsule for Lunar Exploration

The parachute container cover ejection separation is the first and foremost motion for the return capsule recovery system, which is related to the success of a recovery system. Adopting the computational fluid dynamics （CFD） simulation and flight dynamics coupling method, the parachute container cover ejection separation is simu lated. The rationality of the ejection separation speed and dynamic characteristics of the separation process is ana- lyzed. Meanwhile, the influences of angle of attack, Mach number and ejection separation speed on the parachute container cover ejection are also investigated. Results show that the ejection separation speed design is reasonable. It has a certain design margin for parachute container cover to escape from the wake region, and to pull out the drag parachute completely. The results may provide a theoretical basis for recovery system engineering design of the lunar exploration project.     

Optimal Control Strategy for Buck Converter Under Successive Load Current Change

A new control algorithm is presented for digitally controlled dc-dc converters to achieve a fast response under a successive load-change. Under the steady-state condition, the tight voltage regulation is processed by the conventional digital PID compensator. If the load disturbance is significant, the controller switches to an optimal control scheme. With the integration of the capacitor current, the proposed algorithm predicts the optimal switch over time based on the charge balance control, and the minimal voltage derivation and recovery time are thus achieved when the load current has a successive load-change. The method for calculating the optimal switch over time is described, and the implementation of the proposed algorithm with a digital controller is treated in detail. Furthermore, the simulation and experiment results are provided to validate the effectiveness of the approaches.     

Critical Length of Double-Walled Carbon Nanotubes Based Oscillators

The critical lengths of an oscillator based on double-walled carbon nanotubes （DWCNTs） are studied by energy minimization and molecular dynamics simulation. Van der Waals （vdW） potential energy in DWCNTs is shown to be changed periodically with the lattice matching of the inner and outer tubes by using atomistic models with energy minimization method. If the coincidence length between the inner and outer tubes is long enough, the restoring force cannot drive the DWCNT to slide over the vdW potential barrier to assure the DWCNT acts as an oscillator. The critical coincidence lengths of the oscillators are predicted by a very simple equation and then con- firmed with energy minimization method for both the zigzag/zigzag system and the armchair/armchair system. The critical length of the armchair/armchair system is much larger than that of the zigzag/zigzag system. The vdW po- tential energy fluctuation of the armchair/armchair system is weaker than that of the zigzag/zigzag system. So it is easier to slide over the barrier for the armchair/armchair system. The critical lengths of zigzag/zigzag DWCNT- based oscillator are found increasing along with temperature, by molecular dynamics simulations.     

PARAMETER ANALYSIS ON CIRCULATION CONTROLLED CIRCULAR CYLINDER FOR NOTARTM TAIL BOOM

Lift on circulation control （CC） circular cylinder is calculated via numerical simulations based on 2D real- izable k-e epsilon viscous model and compared with experimental data. The simulation result shows an acceptable agreement with tested data. With the proved grid and simulation method, series of simulations are conducted to study the effect of parameters on lift. Single slotted tail booms under different clown wash velocities are optimized with the principle of generating maximum total moment around the main rotor shaft with same total power con- sumption. The results show that larger jet flow velocity, or smaller blow angle, or larger diameter of the cross section can help generating larger lift while enhancing the attachment of both the jet flow and down wash flow. Multiple slotted tail boom is better because it increases lift with same total slot width, and can increase lift by in- creasing total slot width without causing separation, also it helps generating high steady lift at a big rank of slot at- tack angles. To mount a guide vane （GV） at the exit of the slot, or shape the upper slot wall like a smooth-GV, or design the slot with an edge fillet is not recommended because it reduces the velocity of both the jet flow and the upstream of the attached downwash flow. Compared with other shapes of the slots, arcs-profiled slot performs better because of larger jet flow velocity and smaller blow angle. In order to generate the largest moment with same total power consumed by the entire NOTARTM system, total width of the slots and slot attack angle should be optimized according to velocity of down wash flow.     

Hypersonic Shock Wave/Boundary Layer Interactions by a Third-Order Optimized Symmetric WENO Scheme

A novel third-order optimized symmetric weighted essentially non-oscillatory(WENO-OS3)scheme is used to simulate the hypersonic shock wave/boundary layer interactions.Firstly,the scheme is presented with the achievement of low dissipation in smooth region and robust shock-capturing capabilities in discontinuities.The Maxwell slip boundary conditions are employed to consider the rarefied effect near the surface.Secondly,several validating tests are given to show the good resolution of the WENO-OS3 scheme and the feasibility of the Maxwell slip boundary conditions.Finally,hypersonic flows around the hollow cylinder truncated flare(HCTF)and the25°/55°sharp double cone are studied.Discussions are made on the characteristics of the hypersonic shock wave/boundary layer interactions with and without the consideration of the slip effect.The results indicate that the scheme has a good capability in predicting heat transfer with a high resolution for describing fluid structures.With the slip boundary conditions,the separation region at the corner is smaller and the prediction is more accurate than that with no-slip boundary conditions.     

OSCILLATION PHENOMENA IN FAR FIELD REGION OF PLANE JETS AT LOW REYNOLDS NUMBERS

Oscillation phenomena in far field region of plane jets are studied by lattice Boltzmann method over a range of Reynolds numbers （Re） from 16 to 65. Numerical results show that the instantaneous centerline velocities show periodic oscillation behavior in far field region when Re〉38. In contrast, the periodic behavior is invisible in corresponding flow field when Re≤38. For the cases of Re≤38, the exchange of momentum due to straining mo- tion gradually dominates the downstream flow filed, which qualitatively suggests the possibility of iet instability.