On Invariants in Energy Decay Region in Grid Turbulence

概要

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Decaying characteristics of grid turbulence are investigated by means of the laboratory experiments using a wind tunnel. Turbulence-generating grids are installed at the entrance to the test section to generate nearly isotropic turbulence. Four grids (mesh size M = 10, 15, 25 and 50 mm) are used. Instantaneous streamwise velocity is measured by a hot wire anemometer. The mesh Reynolds numbers are adjusted to ReM = 6,700, 9,600, 16,000 and 33,000. In this study, decaying characteristics and invariants of grid turbulence for four grid sizes and ReM are mainly investigated. The result for each case shows that the decay exponent of turbulent intensity is close to a theoretical value of -6/5 for Saffman turbulence. Each case also shows that streamwise variation of integral length scale Luu and Taylor microscale λ grow as Luu ∝ (x/M - x0/M)2/5 and λ ∝(x/M - x0/M)1/2 at (x/M - x0/M) > 50, where x0 is the virtual origin. Consequently, it is concluded that in the decaying region of grid turbulence, u2r.m.sL3uu, which corresponds to Saffman's integral, are constant for all grids and ReM examined. However, u2r.m.sL5uu, which corresponds to Loitsianskii's integral, is not constant.