Effects of external turbulence upon the flow past cylinders

Abstract

Measurements of time-mean flow and of turbulence characteristics in the wake of two-dimensional cylinders show that the process of mixing of the wake flow with the ambient flow can be enhanced significantly when the wake flow with the ambient flow can be enhanced significantly when the external flow is turbulent. For equal turbulence intensities of the external flow of about 3.5 % the effectiveness of a grid-generated external turbulent flow in influencing the development of the wake depends to a large extent upon the ratio of the length scale of the external turbulence with respect to that of the wake. This ratio must be substantially greater than unity for, when the length scales differ only slightly no influence could be observed.

The action of the external turbulence produces both a higher level of turbulence intensity and of shear stresses within the wake, but it does not present the wake from attaining a moving equilibrium between the mean flow and the turbulent motion even closer to the wake origin than is the case for ordinary wakes. Prelimi nary measurements indicate that under the action of external turbulence the large eddies of conventional wake flows are broken down into more intense smaller eddies.