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ON THE HOMOGENEITY OF TURBULENCE GENERATED BY STATIC GRIDS

Ozgur Ertunc
LSTM-Erlangen, Institute of Fluid Mechanics, Friedrich-Alexander-Universitat Erlangen-Nurnberg, Cauerstrasse 4, D-91058 Erlangen, Germany

Hermann Lienhart
Institute of Fluid Mechanics (LSTM) Friedrich-Alexander University Erlangen-Nuremberg, Cauerstr. 4 D-91058 Erlangen-Nuremberg, Germany

Franz Durst
FMP TECHNOLOGY GMBH, Am Weichselgarten 34, 91058 Erlangen, Germany

Resumo

The homogeneities of unstrained and strained turbulence, which was generated by static grids, were investigated with the help of hot-wire measurements. Two different facilities, one closed loop and one open loop wind tunnel were employed for the investigations. It is shown that the mean streamwise velocity becomes rapidly homogeneous downstream of the grids which had porosities of 64% and 67%. In contrast to the mean streamwise velocity, Reynolds stresses and their anisotropy stay inhomogeneous for a long stretch downstream of the grid, although the porosities of the grids were higher than the the lowest 57% suggested in the literature as the lowest porosity to realize homogeneous turbulence downstream of a grid. Furthermore, independent of the mesh Reynolds number, inhomogeneities of the turbulence quantities persist at the same spatial locations. When grid-generated turbulence is exposed to positive axisymmetric strain through a nozzle, it was observed that the inhomogeneous regions disappear just after turbulence becomes isotropic. The constants in the power decay laws of turbulence quantities were evaluated in the inhomogeneous grid-generated turbulence field and in the turbulence field homogenized by a nozzle with a slight contraction. It is demonstrated that constants of decay laws scatter more in the inhomogeneous field than in the homogeneous field.