Numerical Investigation On The Boundary-layer And Wake Flows Of A High-speed Train

For a high speed train(HST)running,there happen complex aerodynamic effects prominently,in which the aerodynamic drag and turbulent wake are important issues concerning energy consumption and environmental friendliness.And the part of skin friction drag of the aerodynamic drag is firmly related to turbulent boundary layers on the surfaces of a train body,and some aerodynamic phenomena in the wake region are dominated by the large-scale vortex structure.Hence,the knowledges about the turbulent boundary layer and wake flows need to be enriched and deepened theoretically for the high-speed train aerodynamics.Thus,in the respects of reducing aerodynamic drag and noise and lowing energy consumption of HSTs,as well as strengthening safe and protection engineering,ideas and references could be obtained for suitable technologies and measurements benefiting to sound development of the high-speed railways.So to speak,the present investigational work is of importance in the fields of study and engineering.Flow fields around single HSTs in the open air with no wind have been calculated numerically by the improved delayed detached eddy simulation,where the scaled and simplified models with different lengths have the same aerodynamic shape based on the CRH380 A HST.According to the calculated results,the present investigation has focused on the boundary layers on the wall surfaces(roof and side walls)of a train body and the turbulent wake behind a trailing car.With regard to the wall-normal velocity,thicknesses and skin friction drag of the boundary layers on the roof and side-wall surfaces,comparative analyses have been made at length.According to the calculated results,the boundary flow on the train roof has similar features to the turbulent flat-plate flow.But the boundary layers on the side walls are sensitive to the altitude off the ground due to the so-called ‘ground effect’.So the equations for the side boundary-layer flows have been derived with the dimensional analysis method and numerical results.As a result,a fact can be revealed that the modulating roles of the ground effect result from the mean-flow convection and turbulent fluctuating momentum transport along the vertical direction(the height of a train body).And this mechanism is also responsible for the relatively low coefficient of local skin friction drag on the side walls.Besides,according to the variations of aerodynamic drag of the train models,it is suggested that a long train with a high seating capacity can help to lowering energy consumption per capita when enhancing passenger capacity.And a preliminary study has been made on the drag reduction by the wall surfaces with dimples.The results indicate the skin friction drag can be effectively reduced on the non-smooth walls of a train model.Therefore,it is believed that the pertinent drag-reducing technologies are worth further investigations.In the near wake region,there are resemblance and difference between the numerical and the experimental curves concerning the velocity profile of slipstream.And the correlation between the slipstream issue and the vortex shedding phenomenon has been paid attention to.Therefore,the pertinent discussions indicate that the turbulent flow and aerodynamic characteristics in the wake region are closely related to the wake vortex structure of a HST.Furthermore,analyses have been made on the profiles of turbulent kinetic energy and the influences of shears on Reynolds stresses and turbulence production in the near wake region.And thus the significance of the shears in the wake flow of HST can be revealed.On this basis,the definition and physical meaning of the Omega vortex identification method have been employed.And thus it can be believed that the strong shears depend significantly on the boundary layers separated from the tail in the local flow.When progressing away from the tail,however,the vortical vorticity is dominant.Meanwhile,greatly due to the interaction between the vortices and the ground,the weak shears are believed to be maintained,and make a great contribution to self-sustainment of turbulent eddies.Last but not least,based on the turbulent energy budgets of the mean-flow transport and the turbulent motions(production and transport)in the near wake region,discussions and analyses have been carried out on the dynamics of turbulence governed by the large wake vortices.And there are three main regions of energy relationship proposed for the relatively stable wake structure.