Acceleration, pressure and related quantities in the proximity of the turbulent/non-turbulent interface

This paper presents an analysis of flow properties in the proximity of the turbulent/non-turbulent interface (TNTI), with particular focus on the acceleration of fluid particles, pressure and related small scale quantities such as enstrophy, omega(2)-omega(i)omega(i), and strain, s(2) = s(ij)s(ij). The emphasis is on the qualitative differences between turbulent, intermediate and non-turbulent flow regions, emanating from the solenoidal nature of the turbulent region, the irrotational character of the non-turbulent region and the mixed nature of the intermediate region in between. The results are obtained from a particle tracking experiment and direct numerical simulations (DNS) of a temporally developing flow without mean shear. The analysis reveals that turbulence influences its neighbouring ambient flow in three different ways depending on the distance to the TNTI: (i) pressure has the longest range of influence into the ambient region and in the far region non-local effects dominate. This is felt on the level of velocity as irrotational fluctuations, on the level of acceleration as local change of velocity due to pressure gradients, Du/Dt similar or equal to partial derivative u/partial derivative t similar or equal to-del p/rho, and, finally, on the level of strain due to pressure Hessian/strain interaction, (D/Dt)(s(2)/2)similar or equal to(partial derivative/partial derivative t)(s(2)/2)similar or equal to s(ij)p,(ij) > 0; (ii) at intermediate distances convective terms (both for acceleration and strain) as well as strain production -s(ij)s(jk)s(ki) > 0 start to set in. Comparison of the results at Taylor-based Reynolds numbers Re-lambda = 50 and Re-lambda = 110 suggests that the distances to the far or intermediate regions scale with the Taylor microscale lambda or the Kolmogorov length scale eta of the flow, rather than with an integral length scale; (iii) in the close proximity of the TNTI the velocity field loses its purely irrotational character as viscous effects lead to a sharp increase of enstrophy and enstrophy-related terms. Convective terms show a positive peak reflecting previous findings that in the laboratory frame of reference the interface moves locally with a velocity comparable to the fluid velocity fluctuations.