Abstract
The decay characteristics of grid turbulence were investigated by means of laboratory experiments conducted in a wind tunnel. A turbulence-generating grid was installed at the entrance of the test section for generating nearly isotropic turbulence. Five grids(square bars of mesh size M= 15, 25 and 50 mm and cylindrical bars of mesh size M= 10 and 25mm) were used. The solidity of all grids is σ=0.36. The instantaneous streamwise and vertical velocities were measured by hot wire anemometry. The mesh Reynolds numbers were adjusted to Re_M = 6,700, 9,600, 16,000 and 33,000. In each case, the result shows that the decay exponent of turbulent intensity is close to the theoretical value of -6/5 for Saffman turbulence. Furthermore, each case shows that streamwise variations in the integral length scales L_<uu> and L_<vv> and the Taylor microscale X grow according to L_<uu>〜2L_<vv>∝(x/M-x_0/M)^<2/5> and λ∝(x/M-x_0/M)<1/2>, respectively, at x/M > 40-60(depending on the experimental conditions, including grid geometry), where x is the streamwise distance from the grid and x_0 is the virtual origin. We demonstrated that in the decay region of grid turbulence, u_<r.m.s>^2L_<uu>^3 and v_<r.m.s>^2L_<vv>^3., which correspond to Saffman's integral, are constant for all grids and examined Re_M values. Consequently, we conclude that grid turbulence is a type of Saffman turbulence for the examined Re_M range of 6,700-33,000 regardless of grid geometry.
