Decay of a coherent scalar disturbance in a turbulent flow
Abstract
The time evolution of an initially coherent, sinusoidal passivescalar disturbance is considered when the wavelength q is less than the length scale of the surrounding isotropic turbulent flow. In 64 sup 3 direct numerical simulations a Gaussian prescription for the average scalar amplitude breaks down after a timescale associated with the wavenumber of the disturbance and there is a transition to a new characteristic decay. The Gaussian prescription is given by exp((1/2) qsquared w(t)), where a form for w(t), the Lagrangian mean square displacement of a single fluid particle, is proposed. After the transition the decay is given by exp(t/tau), where tau is the new characteristic timescale. If q k(sub K), then 1/tau = 1/tau(sub D) + 1/tau(sub K), where k(sub K) is the Kolmogorov wavenumber, tau(sub D) is the diffusive timescale and tau(sub K) is the Kolmogorov timescale. An experiment originally proposed by de Gennesis considered in which the evolution of a coherent laserinduced pattern is read by a diffracting laser. The theory of this experiment involves the dispersion of particle pairs, but it is shown that in a certain limit it reduces to the single Fouriermode problem and can be described in terms of single particle diffusion. The decay of a single mode after the transition in the simulation best describes the experiment.
 Publication:

NASA STI/Recon Technical Report N
 Pub Date:
 June 1985
 Bibcode:
 1985STIN...8530244K
 Keywords:

 Computational Fluid Dynamics;
 Isotropic Turbulence;
 Normal Density Functions;
 Simulation;
 Turbulent Flow;
 Flow Characteristics;
 Kolmogoroff Theory;
 Laser Applications;
 Mean Square Values;
 NavierStokes Equation;
 Reynolds Number;
 Scalars;
 Turbulent Diffusion;
 Fluid Mechanics and Heat Transfer