ON TEMPERATURE SPECTRA IN GRID TURBULENCE

This paper reports wind tunnel measurements of passive temperature spe ctra in decaying grid generated turbulence both with and without a mea n transverse temperature gradient. The measurements cover a turbulence Reynolds number range 60 < Re(l) < 1200. It is shown that for a linea r temperature profile the temperature spectrum has a scaling exponent close to -5/3 and that its dilation is consistent with Kolmogorov-Obuk hov-Corrsin (KOC) scaling, i.e., the width of the scaling region incre ases approximately as Re(l)3/4. The remarkably low Reynolds number ons et (Re(l) is similar to 70) of Kolmogorov-Obukhov-Corrsin scaling in i sotropic grid turbulence is contrasted to the case of scalars in (anis otropic) shear flows where KOC scaling only appears at very high-Reyno lds numbers (Re(l) is similar to 10(5)). It is also shown that when th e temperature fluctuations are inserted very close to the grid in the absence of a gradient (by means of a mandoline), the temperature spect rum behaves in a similar way to the linear gradient case, i.e., a spec trum with a scaling exponent close to -5/3 is observed, a result noted earlier in heated grid experiments. However, when the scalar is inser ted farther downstream of the grid (in the fully developed turbulence) , the spectrum has a scaling region of -1.3 and its dilation with Re i s 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 occ urs at higher Reynolds number than for the case of the scalar experime nts that exhibit the KOC scaling.