Experimental Investigation In The Three-dimensional Wake Of A Circular Cylinder Based On PIV Measurement

The flow around a bluff body is not only of great significance in the fundamental investigation of fluid mechanics but also a very common flow phenomenon in the engineering field.As a simplified model,the flow around a cylinder has become a typical object in the investigation of flow around a bluff body.At present,the investigation of cylinder wake mainly focuses on Kármán vortex but little on rib vortex,it's also not clear whether the criterion of the origin of the secondary vortex is still applicable at high Reynolds number,and the PIV(Particle Image Velocimetry)measurements of average energy dissipation rate is not accurate enough due to the low resolution.Therefore,an indepth investigation of flow around a cylinder will be instrumental in further explaining the flow phenomenon and solving engineering problems.In this paper,PIV measurements are used to collect the flow field data on(x,y)plane,(x,z)plane and(y,z)plane of a cylinder wake(See Section 2.3 for details on the direction of x,y,z),and the coherent structures in three directions are extracted by proper orthogonal decomposition(POD)method,then evaluating the vortex extraction effect of POD method based on PIV measurement(PIV-POD).Large-scale Kármán vortices on(x,y)plane are extracted by the first and second mode.On(x,z)plane,two kinds of vortex are identified.One is the vortex structure with positive and negative alternation along the y-direction,the other is rib structure,but the frequency of the first type of vortex can not be distinguished,and the corresponding modes of rib vortex are not very clear like Kármán vortices on(x,y)plane.At the same time,the extraction of ribs on(y,z)plane is better than that on(x,z)plane.The POD analysis of hotwire measurements is also carried out,then the PIV-POD method and the hotwire-POD method are compared on Kármán vortices and two components of rib vortices,respectively.PIV-POD method can extract large-scale coherent structures on the whole flow plane,the extraction ability of coherent structures in the flow field is related to energy proportion and energy distribution.PIVPOD method has a good extraction for large-scale vortex structures with high modal energy proportion and obvious distribution,but the ability to extract small scale vortex with low modal energy and unclear distribution is weakened.Hotwire-POD method has good extraction for both Kármán vortices and rib vortices.It has the same effect as the PIV-POD method for extracting Kármán vortices,but the hotwire-POD method is superior to the PIV-POD method in the extraction of ribs.Furthermore,the distribution of centerline velocity(U)and the density of the dipole moment(M)in cylinder wake is also discussed.The behaviors of both U and M are significantly different between low and high Re(Reynolds number)at x > 10 d.In low Re(Re=140?600),local minimum exists at the origin of secondary vortices.While in high Re(Re?2 000),the centerline velocity increases monotonously,and the density of the dipole moment remains unchanged.This implies that it is not appropriate to use U and M to determine the origin of secondary vortex street at high Re.The accuracy of PIV measurements of the average energy dissipation rate is also analyzed in this paper.The PIV measurements are often lower than Hotwire measurements and DNS results,the accuracy of PIV measurements is improved by using the correction method.The corrected results are virtually identical to Hotwire measurements and DNS results in the cylinder wake.The correction method is also extended to PIV measurements of the average energy dissipation rate in the boundary layer,and the corrected results are consistent with DNS results.