Grid-generated turbulence revisited

Abstract

In this study we characterize the turbulence, by means of energy spectra, characteristic turbulence length scales, energy dissipation, kinetic energy decay rate etc., behind a set of grids with the feature of having roughly the same solidity but different mesh and bar widths. This is one way of being able to vary the turbulence characteristic length scales while keeping the same turbulence intensity, which is usually a difficult task for experimentalists. Measurements are performed by using, on the one hand, traditional hot-wire x-probes oriented in both directions giving information about all three directional velocity components and, on the other hand, small single-wire probes for faster frequency response. Independent procedures to calculate some quantities are summarized and performed in the present paper and compared with correlation functions based on homogeneous isotropic turbulence as well as semi-empirical relations. For grid-generated turbulence, which often erroneously is described as isotropic (actually a rare condition), relations derived based on isotropic turbulence are frequently used. Here, we show that dissipation rates and length scales may be inaccurate by as much as 50% or more when compared with valid anisotropic relations. The paper ends with a comparison of the turbulence characteristics between the zero pressure gradient case and a favorable pressure gradient case with a small degree of cross flow. With the pressure gradient, a reduction of the integral and Taylor length scales of about 20% and 30%, respectively, is reported for a large mesh width, whereas no change is observed for a small one.