Experimental Study On Flow Characteristics Around A Triangular Prism

The present thesis work is an experimental investigation on the near wake flow characteristics of a triangular prism with different aspect ratios of cross section.The research was focused on the influence of Reynolds number and aspect ratio H on the coefficient of drag,Strouhal number,vortex transport rate and other important wake parameters of the triangular prism's wake flow.Data measurement was carried out in the water towing tank with circulating function in the FMPL laboratory of Xiamen University.Hydrogen bubble technique was employed for the qualitative measurement,which reveals the vortex shedding scenarios as well as the location of separation point.The result indicates that in the range of experimental Reynolds number,separation point is located at the vertex of the maximum lateral width(here called "horizontal vertex"),that is,the separation point is relatively fixed.The quantitative flow information of near wake flow field was acquired by the Particle Image Velocimetry(PIV).Analysis of coherent structures of the flow field using Proper Orthogonal Decomposition method indicates that the last two order modes of flow structure are on behalf of the Karmen vortex structure,the third order mode structure could represent the free shear layer morphology which is sensitive to the Reynolds number.The free shear layer exhibits unsteady phenomenon with the condition of Reynolds number around 400,which retards the exchange of fluid between the free flow and the wake region,leading to a weakened fluid recovery coefficient and a sudden increase in the drag coefficient.The influence of the aspect ratio H on the wake field which obtained under the Reynolds number of 500 shows that the turbulent kinetic energy generation rate is proportional to the aspect ratio H,however,the steady-state transport zone and the vortex streamwise transport rate are in direct proportion to the aspect ratio H,as a result,the turbulent kinetic energy generation rate increases along the spreading distribution,causing the drag coefficient Cd to increase.