Limiting behavior of turbulent scaler transport close to a wall

A direct numerical simulation of fully developed turbulent flow in a channe l is used to study passive scalar transport in the immediate vicinity of a wall. The Reynolds number, based on the channel half-height and friction ve locity, is 150 and the Prandtl number is varied from 1 to 10. DNS results a nd experimental measurements of mass transfer rates at high Schmidt numbers are used to investigate the effect of Schmidt or Prandtl number. The waven umber spectra for temperature fluctuations show a damping of the contributi ons of large wavenumbers with increasing Schmidt or Prandtl number. This re sult suggests that the analogy between momentum and scalar transport cannot be used to define the limiting behavior of turbulent diffusivity for y --> 0. Furthermore, this limiting relation cannot be used to calculate the con centration or temperature profile since it is applicable only in the conduc tive sublayer, where turbulent transport is not important. (C) 2000 Elsevie r Science Ltd. All rights reserved.