Control of Free-Stream Turbulence by Means of Honeycombs: A Balance Between Suppression and Generation

Abstract

The effects of honeycomb of different lengths on free-stream turbulence are studied in air with hot-wire anemometry and in water using hydrogen bubbles visualization. The honeycombs are viewed as operators which suppress the level of the incoming turbulence and generate, primarily through documented instabilities, new turbulence with scales characteristic of the shear layers present in their near wake. The suppression of the incoming turbulence appears to be mostly due to the inhibition of lateral components of the fluctuating velocity. The level, structure and decay of the generated turbulence depends, in part, on the instabilities of the shear layers and therefore can be modified by passive devices acting on the flow field immediately downstream of the honeycomb. Thus, fixing a fine mesh screen to the back of the honeycombs leads to substantially different characteristics of the generated turbulence. While in all cases a net suppression of the free-stream turbulence was achieved by the honeycombs, the positioning of the chopper screen in the near wake of the honeycomb led to a much reduced turbulence level far downstream.