Influence of ground effect on flow field structure and aerodynamic noise of high-speed trains

The simulation of the ground effect has always been a technical difficulty in wind tunnel tests of high-speed trains.In this paper,large eddy simulation and the curl acoustic integral equation were used to simulate the flow-acoustic field results of high-speed trains under four ground simulation systems(GSSs):“moving ground+rotating wheel”,“stationary ground+rotating wheel”,“moving ground+stationary wheel”,and“stationary ground+stationary wheel”.By comparing the fluid-acoustic field results of the four GSSs,the influence laws of different GSSs on the flow field structure,aero-acoustic source,and far-field radiation noise characteristics were investigated,providing guidance for the acoustic wind tunnel testing of high-speed trains.The calculation results of the aerodynamic noise of a 350 km/h high-speed train show that the moving ground and rotating wheel affect mainly the aero-acoustic performance under the train bottom.The influence of the rotating wheel on the equivalent sound source power of the whole vehicle was not more than 5%,but that of the moving ground slip was more than 15%.The average influence of the rotating wheel on the sound pressure level radiated by the whole vehicle was 0.3 dBA,while that of the moving ground was 1.8 dBA.

Effects of Pilot Fuel Ratio on Combustion Process:Flow Field Structure and Pollutant Emissions

As the only controllable means of a micro gas turbine(MGT)combustor during unit operation,pilot fuel ratio(PFR)is the key to achieving stable combustion and low pollutant emission.This paper discusses the influence of PFR on the inner flow field structure and pollutant emissions.The steady-state three-dimensional RANS method with a 40-step reduced methane-air kinetics mechanism is used to study the reaction flow field and species field with PFR of 9.0%,12.7%,15.2%and 17.6%.Results show that,with the decrease in PFR,the axial velocity and temperature near the central axis of the combustion chamber show a tendency to decrease.A similar separation phenomenon occurred in the core pyrolysis reaction zone(measured by HCO)and oxidation zone(measured by OH),which is more conducive to promoting the oxidation of CO.The quantitative effect of the pilot flame on nitrogen oxides(NO_(x))was separated by using inert gas instead of nitrogen in combustion air.It was found that the NOx produced by the pilot flame under the operation condition with a PFR of 9.0%was 3.2×10^(−6),accounting for 17.4%of the total NOx emission,which was twice that of PFR.

Detached eddy simulation on the structure of swirling jet flow field

The improved delayed detached eddy simulation method with shear stress transport model was used to analyze the evolution of vortex structure,velocity and pressure fields of swirling jet.The influence of nozzle pressure drop on vortex structure development and turbulence pulsation was investigated.The development of vortex structure could be divided into three stages:Kelvin-Helmholtz(K-H)instability,transition stage and swirling flow instability.Swirling flow could significantly enhance radial turbulence pulsation and increase diffusion angle.At the downstream of the jet flow,turbulence pulsation dissipation was the main reason for jet velocity attenuation.With the increase of pressure drop,the jet velocity,pulsation amplitude and the symmetry of velocity distribution increased correspondingly.Meanwhile the pressure pulsation along with the axis and vortex transport intensity also increased significantly.When the jet distance exceeded about 9 times the dimensionless jet distance,the impact distance of swirling jet could not be improved effectively by increasing the pressure drop.However,it could effectively increase the swirl intensity and jet diffusion angle.The swirling jet is more suitable for radial horizontal drilling with large hole size,coalbed methane horizontal well cavity completion and roadway drilling and pressure relief,etc.

Numerical study on three-dimensional flow field of continuously rotating detonation in a toroidal chamber

Gaseous detonation propagating in a toroidal chamber was numerically studied for hydrogen/oxygen/nitrogen mixtures. The numerical method used is based on the three-dimensional Euler equations with detailed finiterate chemistry. The results show that the calculated streak picture is in qualitative agreement with the picture recorded by a high speed streak camera from published literature. The three-dimensional flow field induced by a continuously rotating detonation was visualized and distinctive features of the rotating detonations were clearly depicted. Owing to the unconfined character of detonation wavelet, a deficit of detonation parameters was observed. Due to the effects of wall geometries, the strength of the outside detonation front is stronger than that of the inside portion. The detonation thus propagates with a constant circular velocity. Numerical simulation also shows three-dimensional rotating detonation structures, which display specific feature of the detonation- shock combined wave. Discrete burning gas pockets are formed due to instability of the discontinuity. It is believed that the present study could give an insight into the interest- ing properties of the continuously rotating detonation, and is thus beneficial to the design of continuous detonation propulsion systems.

Wake Vortex Structure Characteristics of a Flexible Oscillating Fin

We compute the wake of a two-dimensional and three-dimensional flexible fin in an unsteady flow field with heaving and pitching motions using FLUENT. Deflexion mode is used for a non-uniform cantilever beam with non-uniformly distributed load. The effect of chordwise deflexion length on the characteristics of propulsion is discussed for two-dimensional flexible fin. The thrust coefficient decreases, propulsive efficiency increases and the intensity of turbulence attenuates gradually as the deflexion length increases. For a three-dimensional flexible fin, the intensity of the vortex in the plane of symmetry is higher than that in the plane at 3/4 span length of the caudal fin. But the propulsive perform.ance achieved is not what we expected with the given deflexion mode.

Simulation of 3D parachute fluid–structure interaction based on nonlinear finite element method and preconditioning finite volume method

A fluid–structure interaction method combining a nonlinear finite element algorithm with a preconditioning finite volume method is proposed in this paper to simulate parachute transient dynamics. This method uses a three-dimensional membrane–cable fabric model to represent a parachute system at a highly folded configuration. The large shape change during parachute inflation is computed by the nonlinear Newton–Raphson iteration and the linear system equation is solved by the generalized minimal residual（GMRES） method. A membrane wrinkling algorithm is also utilized to evaluate the special uniaxial tension state of membrane elements on the parachute canopy. In order to avoid large time expenses during structural nonlinear iteration, the implicit Hilber–Hughes–Taylor（HHT） time integration method is employed. For the fluid dynamic simulations, the Roe and HLLC（Harten–Lax–van Leer contact） scheme has been modified and extended to compute flow problems at all speeds. The lower–upper symmetric Gauss–Seidel（LUSGS） approximate factorization is applied to accelerate the numerical convergence speed. Finally,the test model of a highly folded C-9 parachute is simulated at a prescribed speed and the results show similar characteristics compared with experimental results and previous literature.

Unsteady cooperative flow in compression system

When there are several bodies with relative motion in a flow field,such as the flow in the compression system of modern aero-engine,the flow field will have certain special features,one of which is that the time-space structure of such multi-bodies unsteady vorticity flow field would be either of unsteady natural flow(UNF)pattern or of unsteady cooperative flow(UCF)pattern.If we further examine the aerodynamic design system of aero-engine,there is no mechanism for the unsteady cooperative flow to occur,in other words the flow field must be of the unsteady natural flow type.If certain technical measures can be adopted to transform UNF into UCF,the aerodynamic performances will surely be improved.This is the main task the author and their colleague have been devoted to and the results are reviewed in the present paper with emphases laid on basic ideas,technical approaches and experimental verifications.