Past Event: CSEM Student Forum

A near-wall model for large eddy simulations of turbulent flows

Prakash Mohan, ICES, University of Texas at Austin

Abstract

Turbulence is a complex fluid phenomenon that is ubiquitous in high Reynolds number flows. One of the biggest challenges in understanding turbulence lies in computational modeling. This challenge stems from the fact that turbulence is comprised of a wide range of scales that interact with each other. The problem is particularly difficult in the case of wall bounded turbulence because the presence of a wall introduces new length scales. Large Eddy Simulations (LES) directly represent large scale turbulent motions and model the effects of small scale motions. However, in the near-wall region the large dynamically important eddies scale in viscous wall units, which makes resolving them very expensive. This motivates a wall-modeled LES approach where the near-wall region is modeled. This approach will be very useful in engineering design, for instance in computing flow over an aircraft, or simulating a reacting flow inside a jet engine. This approach can also be applied to modeling atmospheric flow and provide a basis for better weather prediction simulations. To aid in the development of new wall models, we pursue an asymptotic analysis of the filtered Navier-Stokes equations, in the limit in which the horizontal filter scale is large compared to the thickness of the wall layer. We show that in this limit the filtered velocities in the near-wall layer are determined to zeroth order by filtered velocities at the boundary of the wall layer. Further, the asymptotics suggest that there is a scaled universal velocity profile f in the near-wall region. The profile f is evaluated through analysis of DNS data from channel flow at Reτ = 5200. We use the resulting profile f to formulate a predictive near-wall model. The model depends only on the filtered velocities at the boundary of the near-wall layer and can supply boundary conditions for a wall-modeled LES.