Very-large-scale Structure In A Turbulent Cylinder Wake

Blunt body flow is widely existed in social life and engineering practice,including land transportation,land construction,Marine construction and other fields.Therefore,it is of great significance to study the properties and flow field structure of wake flow around blunt body.Cylinder flow is one of the important objects in the study of blunt body flow.When the fluid flows around the cylinder,different scale structures are formed behind the cylinder.It is obvious that studying these structures can better understand the flow field characteristics in the wake.In this paper,very-large-scale structure in a turbulent cylinder wake is studied.The experiment was made in a wind tunnel with a working section 1.2 m×0.8 m and 2.0m working length.The Reynolds number based on the free stream velocity U₁=3m/s and the diameter d=12.7mm is 2.5×10³.A probe consisting of four X-wires and four cold wires were used in this experiment to measure the three-dimensional velocity vector and temperature fluctuation in the downstream of the cylinder at x/d=10,x/d=20 and x/d=40.The energy spectrum has been used to analyze the three-dimensional velocity fluctuations and temperature fluctuation.The premultiplied spectra of streamwise velocity fluctuation and temperature fluctuation indicated a spectral bump corresponding to the dimensionless frequency f^*=0.0508 is obviously observed,especially streamwise velocity fluctuation and temperature fluctuation at x/d=40,which predicted very-large-scale structure about4 times wavelength of the karman vortex exists in turbulent cylinder wake.Following the premultiplied spectra of streamwise velocity fluctuation and temperature fluctuation from x/d=10 to x/d=40,it is found that very-large-scale structure will become more and more obvious as x/d increases.On this basis,we try to calculate energy and Reynolds shear stress distributions of very-large-scale structure.The results show that very-large-scale structure,the dimensionless frequency less than f^*=0.0770,makes greater energy contribution to streamwise velocity fluctuation and temperature fluctuation than lateral velocity fluctuation and spanwise velocity fluctuation.Very-large-scale structure makes the same amount(about 20%)cumulative energy contribution to lateral velocity fluctuation and spanwise velocity fluctuation in the x/d=40 region,but it makes 45%and 50%cumulative contribution to streamwise velocity fluctuation and temperature fluctuation in the x/d=40 region,respectively.In addition,cumulative Reynolds shear stress distributions of very-large-scale structure is about 20%in the x/d=40region.POD method is also used to analyze the correlation between vorticity and temperature,which indicates vorticity?_zand temperature fluctuation are significantly greater than that between vorticity?_x,?_y and temperature fluctuation.