| This dissertation considers problems concerning (1) mixing and chemical reaction and (2) heat transfer, in a decaying, homogeneous turbulent flow. Direct numerical simulations have been used to compute the flows numerically in order to measure quantities that are difficult to measure in the laboratory and to test some statistical theories of turbulence by examination of the dynamical variables and their statistics. These simulations involve solution of the unsteady Navier-Stokes and mass conservation equations by a pseudo-spectral method in 64;In Part I numerical simulations were used to study chemical selectivity in the parallel-consecutive reaction scheme(UNFORMATTED TABLE OR EQUATION FOLLOWS);Part II deals with heat transfer in a homogenous turbulent flow. The evolution of a passive scalar and its transport by a uniform mean scalar gradient was studied using direct numerical simulations and compared with predictions of a two-point statistical theory (direct interaction approximation of Kraichnan, 1964) and with the laboratory experiments of Sirivat and Warhaft (1983). The experiments that were simulated correspond to the "mandoline" (heated wires to introduce the temperature fluctuations) and "toaster" (uniform temperature gradient existing upstream of the turbulence-generating grid) cases. Although the evolution of the scalar field was affected by initial conditions, the asymptotic state appears to be relatively insensitive to the initial conditions. In the absence of a scalar gradient, however, the asymptotic state of the system depends on the initial conditions. |
