Investigation On Performance And Mechanism Of Steady Suction Control And Combined Aerodynamic Control On Flows Around Elliptical Cylinders

The focus of this research is on the flow characteristics and suppression of vortex shedding of flows past an elliptical cylinder,so as to improve its aerodynamic performance.The aerodynamic problems are mostly occurred due to the shedding of alternating vortices,i.e.,on one side first then the other.As a result,alternating low pressure zones are formed on the downstream side of the building and a fluctuating transverse load is created.This causes the building to move towards the low pressure zone.Consequently,if the structural natural frequency coincides with the frequency of the vortex shedding,then large amplitude vibration may occur which will result in catastrophic disasters.Therefore,the motivation of this research is to suppress the vortex shedding so as to solve the aerodynamic problems in the wind-resistance design of civil engineering structures.The experimental studies,including the pressure measurement and PIV(particle image velocimetry)measurement,on the aerodynamic performance and flow field characteristics have been conducted for four elliptical cylinder models(marked as E1,E2,E3 and E4 models respectively)at Re = 6.90 × 104.For the E1,E2,and E3 models,the major axis is parallel to the flow direction,but their suction holes are located at different radial angles,i.e.,at the radial angles of 90° and 270° for the E1 model,at the radial angles of 130° and 230° for the E2 model,and at the radial angles of 50° and 310° for the E3 model respectively.While,for the E4 model,the minor axis is parallel to the flow direction,and the two suction holes are located at 90° and 270°.In addition,three aerodynamic control measures used to suppress the vortex shedding behind the elliptical cylinders,including the air suction control,passive aerodynamic control(surface roughness)and combined aerodynamic control(air suction + surface roughness),are comprehensively discussed in this study.The pressure coefficient distribution,aerodynamic force coefficients and the corresponding power spectra for the test models with or without aerodynamic control are analyzed and compared to discuss the aerodynamic performance of the test models.Moreover,in order to determine the flow field characteristics around different elliptical cylinders,the time-averaged and instantaneous flow streamlines,normalized turbulent kinetic energy(NTKE)and swirling strength are analyzed based on the PIV measurement results.The main contents are shown in four aspects as followings:1.The air suction control has been used to reduce the wind loads and suppress the vortex shedding for the flows around the elliptical cylinder.The effects of the azimuthal location of the suction holes(i.e.,E1-E3 models),suction flux coefficient(i.e.,CQ from 0 to 0.05)and placement of the model(i.e.,E1 and E4 models)on the characteristics of wake flows and aerodynamic forces are assessed comprehensively.The results show that when the suction flux coefficient is increased to a relative small quantity,the mean drag coefficient has been significantly decreased at most cases when compared to the uncontrolled models(i.e.,from 0.689 to 0.420 for the E1 model at CQ= 0.01,with a reduction of 39.0%;from 0.668 to 0.109 for the E2 model at CQ= 0.02,with a reduction of 83.7%;from 1.563 to 0.696 for the E4 model at CQ= 0.01,with a reduction of 55.5%).Moreover,when the suction holes are moved towards the leeward side,i.e.,from 50°(the E3 model)to 90°(the E1 model),and then to 130°(the E2 model),the mean drag coefficient of the suction-controlled models can also be dramatically reduced,and the mean drag coefficient for the E2 model at CQ= 0.02 is 72.5% smaller than that of the E1 model and 76.3% smaller than that of the E3 model.Finally,the flow field visualization of the above models is presented to explain the difference of the resultant aerodynamic performance.2.For the passive aerodynamic control,three kinds of surface roughness(i.e.,ks/D =0,5.3×10-4 and 1.15×10-3)have been used and the effect of different surface roughness on the elliptical cylinder has been experimentally studied.The results show that,the increase in the surface roughness has a significant effect on the reduction in aerodynamic forces,with a maximum reduction in the mean drag coefficient up to 32.9%,fluctuating drag coefficient up to 26.2% and fluctuating lift coefficient up to 19.8% when compared to the smooth elliptical cylinder(i.e.,ks/D =0).Moreover,the corresponding PIV results also verify the reduction in vortex shedding and turbulent kinetic energy.3.For the combined aerodynamic control(i.e.,surface roughness + air suction),the effect of different surface roughness(i.e.,ks/D =0,5.3×10-4 and 1.15×10-3),location of the suction hole(i.e.,E1 to E3)and suction flux coefficient(i.e.,CQ from 0 to 0.05)on the aerodynamic performance of and the flow field around the elliptical cylinder have been studied.The results show that the maximum reduction in the mean drag for the E1 model is 86.6%,for the E2 model the maximum reduction in the mean drag is almost 100% and for the E3 model is 30%,which suggest that combined aerodynamic control has a more significant effect on the reduction in wind loads than the suction control or surface roughness measures.4.The air suction control around an elliptical cylinder has been numerically studied by using large eddy simulation(LES),and the effect of location of the suction holes(the suction holes are located from 70°(290°)to 110°(250°)with an interval of 10°,and the elliptical cylinders are marked as the H1,H2,H3,H4 and H5 models respectively)on the aerodynamic performance and flow field characteristics have been analyzed.Moreover,the LES method has been validated and verified by the experimental results.The LES results show that the suction holes on the leeward side of the model(e.g.,the H5 model)has more reduction in the aerodynamic forces as compared to the model having suction holes on the windward side(e.g.,the H1 model),and the mean drag coefficient and fluctuating lift coefficient of the H5 model are only 12.7% and 56.0% of those of the H1 model,which proves the significant control effect caused by the location of suction holes.