An Investigation into the Effects of the Reynolds Number on High-Speed Trains Using a Low Temperature Wind Tunnel Test Facility

A series of tests have been conducted using a Cryogenic Wind Tunnel to study the effect of Reynolds number(Re)on the aerodynamic force and surface pressure experienced by a high speed train.The test Reynolds number has been varied from 1 million to 10 million,which is the highest Reynolds number a wind tunnel has ever achieved for a train test.According to our results,the drag coefficient of the leading car decreases with higher Reynolds number for yaw angles up to 30º.The drag force coefficient drops about 0.06 when Re is raised from 1 million to 10 million.The side force is caused by the high pressure at the windward side and the low pressure generated by the vortex at the lee side.Both pressure distributions are not appreciably affected by Reynolds number changes at yaw angles up to 30°.The lift force coefficient increases with higher Re,though the change is small.At a yaw angle of zero the down force coefficient is reduced by a scale factor of about 0.03 when the Reynolds number is raised over the considered range.At higher yaw angles the lift force coefficient is reduced about 0.1.Similar to the side force coefficient,the rolling moment coefficient does not change much with Re.The magnitude of the pitching moment coefficient increases with higher Re.This indicates that the load on the front bogie is higher at higher Reynolds numbers.The yawing moment coefficient increases with Re.This effect is more evident at higher yaw angles.The yawing moment coefficient increases by about 6%when Re is raised from 1 million to 10 million.The influence of Re on the rolling moment coefficient around the leeward rail is relatively smaller.It increases by about 2%over the considered range of Re.

REYNOLDS NUMBER EFFECTS IN TALL BUILDING WIND TUNNEL TESTS

In wind tunnel tests of tall building models comprised of rounded or streamlined surfaces, Reynolds Number Effects (RNE) may be remarkable. In most researches and projects, owing to the presence of sharp edged corners at model, RNE was often treated as negligible, and data colleCted in subcritical flow regime were applied directly to designs which were sure to be supercritical without any modification. But it was proved necessary to take RNE into consideration when a model had predominant rounded smooth surface(s). Some research activities were devoted to the mentioned condition and two experimental techniques accompanied by two wind tunnel model tests were introduced in the article. The authors also presented some amusing phenomenon such as extremely low pressure coefficients caused by the separating bubbles when now past sharp edged corners, unsymmetrical pressure coefficient distributions commenced by biased gap f'low (seen Figs. 3, 4) while models were placed symmetrically in the approaching flow.

Effects of the Reynolds number on a scale-similarity model of Lagrangian velocity correlations in isotropic turbulent flows

A scale-similarity model of a two-point two-time Lagrangian velocity correlation(LVC) was originally developed for the relative dispersion of tracer particles in isotropic turbulent flows(HE, G. W., JIN, G. D., and ZHAO, X. Scale-similarity model for Lagrangian velocity correlations in isotropic and stationary turbulence. Physical Review E, 80, 066313(2009)). The model can be expressed as a two-point Eulerian space correlation and the dispersion velocity V. The dispersion velocity denotes the rate at which one moving particle departs from another fixed particle. This paper numerically validates the robustness of the scale-similarity model at high Taylor micro-scale Reynolds numbers up to 373, which are much higher than the original values(R_λ = 66, 102). The effect of the Reynolds number on the dispersion velocity in the scale-similarity model is carefully investigated. The results show that the scale-similarity model is more accurate at higher Reynolds numbers because the two-point Lagrangian velocity correlations with different initial spatial separations collapse into a universal form compared with a combination of the initial separation and the temporal separation via the dispersion velocity.Moreover, the dispersion velocity V normalized by the Kolmogorov velocity V_η ≡ η/τ_η in which η and τ_η are the Kolmogorov space and time scales, respectively, scales with the Reynolds number R_λ as V/V_η ∝ R_λ^(1.39) obtained from the numerical data.

NUMERICAL SIMULATION OF REYNOLDS NUMBER EFFECT ON EFFETIVE WAKE FIELD AND HYDRODYNAMIC PERFORMANCE OF PROPELLER BEHIND SHIP

The Reynolds number effect of the interaction between the three-dimen-sional ship stern with multi-propellers and its corresponding hydrodynamic performance under four different orders of the Reynolds number (including the full scale Reynolds number 1. 67×109 and the model scale Reynolds number 1. 67×106) are numerically investigated in this paper. This approach to propeller-hull interaction is based on three-di-mensional unsteady RANS equations coupled with a propeller performance program in an interactive and iterative manner to predict the combined flow field and hydrodynamic per- formance of propeller. By comparing the computed effective wake distribution and fraction under four different orders of the Reynolds number, the scale effect of nominal and effec-tive wake distribution is examined and discussed.

Periodical streaming potential and electro-viscous effects in microchannel flow

This paper presents an analytical solution to periodical streaming potential, flow-induced electric field and velocity of periodical pressure-driven flows in twodimensional uniform microchannel based on the Poisson-Boltzmann equations for electric double layer and Navier-Stokes equation for liquid flow. Dimensional analysis indicates that electric-viscous force depends on three factors： （1） Electric-viscous number representing a ratio between maximum of electric-viscous force and pressure gradient in a steady state, （2） profile function describing the distribution profile of electro-viscous force in channel section, and （3） coupling coefficient reflecting behavior of arnplitude damping and phase offset of electro-viscous force. Analytical results indicate that flow-induced electric field and flow velocity depend on frequency Reynolds number （Re = wh^2/v）. Flow-induced electric field varies very slowly with Re when Re 〈 1, and rapidly decreases when Re 〉 1. Electro-viscous effect on flow-induced electric field and flow velocity are very significant when the rate of the channel width to the thickness of electric double layer is small.

Full-Scale Numerical Simulation of the Local Scour Under Combined Current and Wave Conditions Based on Field Data

The monopile is the most common foundation to support offshore wind turbines.In the marine environment,local scour due to combined currents and waves is a significant issue that must be considered in the design of wind turbine foundations.In this paper,a full-scale numerical model was developed and validated based on field data from Rudong,China.The scour development around monopiles was investigated,and the effects of waves and the Reynolds number Re were analyzed.Several formulas for predicting the scour depth in the literature have been evaluated.It is found that waves can accelerate scour development even if the KC number is small(0.78<KC<1.57).The formula obtained from small-scale model tests may be unsafe or wasteful when it is applied in practical design due to the scale effect.A new equation for predicting the scour depth based on the average pile Reynolds number(Rea)is proposed and validated with field data.The equilibrium scour depth predicted using the proposed equation is evaluated and compared with those from nine equations in the literature.It is demonstrated that the values predicted from the proposed equation and from the S/M(Sheppard/Melville)equation are closer to the field data.

Recent progress on the study of asymmetric vortex flow over slender bodies

The investigations of forebody vortex flow and its flow control have great importance in both academic field and engineering application areas. A large number of papers and many review papers have been published. However in this research field of forebody asymmetric vortices, three problems such as tip perturbation effect, Reynolds number effect and flow instability are less studied and thus not understood completely. So many researches are still working on the issues in recent years. The present paper attempts to provide a review of recent research progress on first two problems. The first problem is mainly concerned with how the vortex flow evolves after tip perturbation; how to solve the problem of repeatability and reproducibility of wind tunnel testing data; how to develop a conception of active flow control technique with tip perturbation based on the study of vortex flow response to tip perturbation. For the second problem one is mainly concerned that how the asymmetric vortices are developed with the increase of Reynolds number; how to classify the vortex flow patterns in different Reynolds number regimes; how to develop an appropriate boundary layer transition technique to simulate flows at high Reynolds number in the convention wind tunnels. Finally, some important ques- tions that deserve answers are proposed in the concluding remarks.

FORCE REDUCTION OF FLOW AROUND A SINUSOIDAL WAVY CYLINDER

A large eddy simulation of cross-flow around a sinusoidal wavy cylinder at Re = 3000 was performed and the load cell measurement was introduced for the validation test. The mean flow field and the near wake flow structures were presented and compared with those for a circular cylinder at the same Reynolds number. The mean drag coefficient for the wavy cylinder is smaller than that for a corresponding circular cylinder due to the formation of a longer wake vortex generated by the wavy cylinder. The fluctuating lift coefficient of the wavy cylinder is also greatly reduced. This kind of wavy surface leads to the formation of 3-D free shear layers which are more stable than purely 2-D free shear layers. Such free shear layers only roll up into mature vortices at further downstream position and significantly modify the near wake structures and the pressure distributions around the wavy cylinder. Moreover, the simulations in laminar flow condition were also performed to investigate the effect of Reynolds number on force reduction control.

Statistics of Particle Suspensions in Turbulent Channel Flow

Particle dynamics in a turbulent channel flow is considered.The effects of particle concentration and Reynolds number on the particle velocity statistics are investigated.Four different particle response times,τ^(+)=1,5,30 and 100,are examined for three different Reynolds numbers,Re_(∗)=200,360 and 790(based on channel height and friction velocity).The particle concentration evolves with time and statistics obtained during three different sampling periods might be distinctly different.Themean and fluctuating particle velocities are substantially affected both by the particle response time and by the Reynolds number of the flow.

雷诺数和施密特数对激波-气泡相互作用中变密度混合的影响

惯性约束聚变、水动力空化侵蚀和微观尺度激波加速流动中的激波-气泡相互作用(SBI)构型处于高黏度(低雷诺数)或低扩散率(高施密特数)环境中,其混合性能尚不清楚.通过测定宽范围内的粘度和扩散系数,采用高分辨率数值模拟研究了雷诺数Re和施密特数Sc对SBI流动结构和混合行为的影响.次级斜压涡度(SBV)增长明显受雷诺数的影响,表现为斜压涡和黏性耗散之间的生长抑制涡度动力学平衡.混合速率受施密特数的影响,标度为χ^(∗)=β·Sc^(−α),其中系数β∼Re−0.2和标度指数α∼Re−0.385.混合标度表明,较高雷诺数下的SBV拉伸增强导致混合速率对施密特数的依赖性减弱,这可能为实际应用中的混合增强设计开辟新的途径.