Thomas Kurian

Grid-generated turbulence revisited

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.

Boundary layer receptivity to free-stream turbulence and surface roughness over a swept flat plate

An experimental study of the receptivity of disturbances and their subsequent development into a three-dimensional boundary layer has been carried out. The three-dimensional boundary layer was set ...

The diagnostic plot : a new way to appraise turbulent boundary-layer data

During the last decade there has been a renewed interest in how averaged quantities of the turbulent boundary layer vary in the direction normal to the wall, especially with regard to the mean and fluctuation velocity distributions (see e.g. [1, 2]). Comparing data from different facilities and/or different techniques are, however, often inconclusive since the inaccuracies of the measured quantities may be larger than the trends one wants to investigate.

Evolution Of Traveling Crossflow Modes Over A Swept Flat Plate

An experimental investigation has been carried out to examine the growth of traveling crossflow instabilities over a swept flat plate mimicking the Falkner Skan-Cooke boundary layer. Different turbulence generating grids were placed upstream of the leading edge to vary incoming parameters. Hot-wire measurements were taken for one component of velocity and compared with linear PSE analysis. These showed a decrease in the growth rate for increasing turbulence intensity, which was most likely cause by nonlinear effects. Streamwise correlation measurements were also taken. All the cases except one triggered the same spanwise integral length scale inside the boundary layer. Receptivity coefficients are needed to do the PSE calculations and to see the need for nonlinear PSE.

New results on grid-generated turbulence

To experimentally study the receptivity of laminar boundary layers to free stream turbulence, the use of grids is the simplest and most effective way to generate turbulence. To investigate the receptivity it is essential to be able to vary the parameters of the free stream turbulence such as turbulence level and length scales. By altering the distance of different grids from the receptivity region, these parameters can be adjusted independently.