MODELLING OF SHEAR AND SHEARLESS FLOW WITH PERIODIC VELOCITY NONSTATIONARITY

Abstract

The results of experimental modeling of shear and shearless flow with periodic velocity nonstationarity, organized using a generator of periodic wakes such as the "squirrel" cage, are presented. The purpose of this paper is to compare the structure of the flow behind the "squirrel" cages, as well as the analysis of the characteristics of the transition boundary layer for two different ways of locating the working surfaces: in the zone of the shearless core and shear periphery zone. The physical modelling of turbulized flow with velocity periodic nonstationarity is carried out in two experimental installations. It is shown that behind rotating “squirrel” cages there are two regions in the distributions of mean time velocities: the shearless flow core located in the center of “squirrel” cage and peripheral shear part. The aim of this paper is to compare the flow structure behind “squirrel” cages as well as to analyze the features of transient boundary layer for two different installations of working surfaces. The latter were flat plates installed on the different distances from the center of the “squirrel” cages: in the shearless flow core and in shear zone. Total longitudinal fluctuations are characterized by peaks reason of which is intersections of wakes. Behind the “squirrel” cages the levels of fluctuations decrease along the plates at x~100-600 mm from ~12 to 4,5% (II) and from ~6 to 3,5% (I). Despite the development of boundary layer happens under different external conditions (in uniform (I) and shear (II) flows), wake-induced transition takes place in both installations. Transformation of velocity profiles from pseudolaminar to turbulent is similar to one taking place in bypass transition. Distributions of total longitudinal fluctuations across the boundary layer differ by quantity of peaks and their intensity.  Today the physical modeling is one of the most perspective methods for studying transport processes under complex conditions. That is why the experimental investigations of periodic external flow structure are necessary for the further optimization of different equipment and their reliability enhancement.