FLOW-3D News
Summer 2003Application Note:
Turbulent Relationships
LANS-Alpha
That Renaissance genius, Leonardo da Vinci, was the first to analyze turbulence. He observed that the circulating energy of water entering a pool becomes a series of increasingly tighter, more rapidly turning flow lines swept along by larger motions.
The increasingly smaller eddies Leonardo observed are cause by nonlinear convection. Energy input, once distributed, "cascades" faster and faster into circulating motions of smaller and smaller sizes. The characteristics of turbulence-eddy size, shape, speed, vorticity, viscosity, circulation, and nonlinear convection-can be captured using the exact Navier-Stokes equations. But the cascade to smaller and smaller scales accelerates geometrically until it outstrips the numerical resolution capabilities of even the largest computer. In order to perform turbulence simulations, the solution to date has been to cut off the calculations at a scale below which eddies are dissipated computationally as heat, losing the effects of the small scales on the larger motion that sweeps the eddies along.
The Los Alamos National Laboratory (LANL) has been leading an international group of scientists in the development of a new turbulence model it calls the "LANS-alpha Turbo-Simulator." According to the lead developer, Dr Darryl Holm, the model is fast, accurate, and cost-effective and its predictions agree accurately with classic turbulence experiments.
LANS-alpha uses Lagrangian averaging to compute eddies and circulations at the resolved scales (larger than alpha, specified by the user) while averaging the smaller eddies and circulations by modifying the nonlinearity of Navier-Stokes equations so that the flow's kinetic energy at scales smaller than alpha does not continue to cascade. Instead of dissipating the kinetic energy of the smaller eddies into heat, the alpha models' modified nonlinearity converts it into the energy of swirling associated with eddies (circulation energy).
The corresponding circulation resulting from the smaller scales is preserved and is swept along with the larger scales like a weather balloon being carried along by larger weather patterns. Because of this approach, it is able to preserve the essential properties of convection and circulation in numerical calculations of turbulent flow.
By cutting off the increasingly faster cascade of power to the smallest scales, without destroying its circulation, circulation properties are preserved, rendering more-accurate results.
Flow Science is working with LANL on incorporating the LANS-alpha Turbo-Simulator into FLOW-3D and expects to have it included in a release next year. LANS-alpha will become an additional module that can be integrated into FLOW-3D.