On temperature spectra in grid turbulence

This paper reports wind tunnel measurements of passive temperature spectra in decaying grid generated turbulence both with and without a mean transverse temperature gradient. The measurements cover a turbulence Reynolds number range 60<Rel<1200. It is shown that for a linear temperature profile the temperature spectrum has a scaling exponent close to −5/3 and that its dilation is consistent with Kolmogorov–Obukhov–Corrsin (KOC) scaling, i.e., the width of the scaling region increases approximately as Re3/4l. The remarkably low Reynolds number onset (Rel∼70) of Kolmogorov–Obukhov–Corrsin scaling in isotropic grid turbulence is contrasted to the case of scalars in (anisotropic) shear flows where KOC scaling only appears at very high-Reynolds numbers (Rel∼105). It is also shown that when the temperature fluctuations are inserted very close to the grid in the absence of a gradient (by means of a mandoline), the temperature spectrum behaves in a similar way to the linear gradient case, i.e., a spectrum with a scaling exponent close to −5/3 is observed, a result noted earlier in heated grid experiments. However, when the scalar is inserted farther downstream of the grid (in the fully developed turbulence), the spectrum has a scaling region of −1.3 and its dilation with Re is less well defined than for the other cases. The velocity spectrum is also shown to have a scaling region, of slope −1.3, and its onset occurs at higher Reynolds number than for the case of the scalar experiments that exhibit the KOC scaling.