Spectrum of a passive scalar in the inertial-convective subrange of an anisotropic turbulent flow

The spectral density of temperature fluctuations in an anisotropic heated turbulent channel flow is studied experimentally using a new optical technique. The spectrum in the direction transverse to the flow shows an equilibrium subrange behavior characteristic of isotropic flows with a wavenumber dependence of ${\mathrm{\Phi }}_{\mathrm{\theta }}$(${\mathrm{\kappa }}_{\mathit{y}}$)∼${\mathrm{\kappa }}_{\mathit{y}}^{\mathrm{-}11/3}$, whereas in the flow direction, ${\mathrm{\Phi }}_{\mathrm{\theta }}$(${\mathrm{\kappa }}_{\mathit{x}}$)∼${\mathrm{\kappa }}_{\mathit{x}}^{\mathrm{-}9/3}$. This difference is shown to result from the dominance of the mean strain rate over the turbulent strain rate field at low wave numbers, and the observed slopes are explained on dimensional grounds.