TEMPERATURE SPECTRA IN SHEAR-FLOW AND THERMAL-CONVECTION

We show that the P-u(omega) proportional to omega(-7/3) shear velocity power spectrum gives rise to a P-theta(omega) proportional to omega(- 4/3) power spectrum for a passively advected scalar, as measured in ex periment [K. Sreenivasan, Proc. R. Soc. A 434 (1991) 165]. Applying ou r argument to high Rayleigh number Rayleigh-Benard flow, we can accoun t for the measured scaling exponents equally well as the Bolgiano-Obuk hov theory (BO59). Yet, of the two explanations, only the shear approa ch might be able to explain why no classical scaling range is seen in between the shear (or BO59) range and the viscous subrange of the expe rimental temperature spectrum [I. Procaccia et al., Phys. Rev. A 44 (1 991) 8091].