Multi-scale Predictive Simulation Methods for Turbulent Flow

California Institute Of Technology, Pasadena CA

Investigators

Abstract

CBET- 0651754 Dale I. Pullin This study will develop our capability for the predictive, numerical simulation of turbulent flows in several new directions. The goal is to both extend the present paradigm of conventional large-eddy simulation (LES) to span length/time scales from the energy-containing range to the Kolmogorov and Batchelor scales, for some turbulence statistics, and also to tackle the formidable near-wall problem in sub-grid-scale modeling. The work to be performed will extend the stretched-vortex (SV), subgrid-scale (SGS) model to multi-scale LES (MSLES) across almost the full-range of active length and time scales in turbulent flow, for some transport statistics. The aim is to move present LES-like methodology substantially closer to the ideal of DNS (direct numerical simulation). The study will develop algorithms that couple stretched-vortex solutions of the Navier-Stokes equations both as a basis for LES, and as a comprehensive model of turbulent fine scales, to produce an MSLES capability. This coupling should enable continuation of the statistics of several turbulent mixing flows from resolved to subgrid scales. Applications will comprise several canonical turbulent flows including decaying turbulence, passive-scalar mixing in the presence of a mean density gradient and variable-density turbulent mixing. For this last class of flows, direct-numerical simulation (DNS) runs will also be performed for validation of the MSLES. While not expecting true point- and time-wise DNS resolution, the MSLES methodology can provide a substantial improvement in the prediction of many turbulent statistics, particularly for turbulent mixing flows, over conventional LES techniques. The study will also further develop, test and apply a subgrid-scale approach designed specifically for the near-wall region of wall-bounded turbulent flows, a major roadblock to our capability for predictive simulation of turbulent flows. Two new concepts have been formulated. The first consists of a special near-wall, subgrid-scale element that is based on the idea of local inner scaling combined with wall-normal averaging and wall-parallel filtering. The second is an extension of the stretched-vortex, SGS-flux formalism, to model the principal dynamical behavior of near-wall, longitudinal vortices in their wall-normal transport of streamwise momentum. The method will implement boundary conditions at a virtual wall positioned at the lower limit of the log-law region. The viscous sublayer is not resolved. Intellectual merit: This study will provide a physically-based, analytical framework for the extension of conventional LES to both multiscale LES, and to the solution of the near-wall, subgrid-scale modeling problem. The paradigm of large-scale fluid-dynamical simulation has had an enormous impact on many diverse areas of science and engineering applications over a large range of Reynolds numbers. Broader impact: This research will extend capability to perform multi-dimensional, numerical simulations of complex fluid behavior. MSLES will provide some measure of DNS-like fidelity in numerical simulation but at a small fraction of the DNS cost in computational resources. With the continued explosion in the availability of desktop computing power, we expect this style of simulation to become readily available at the engineering application level within a decade.