On the Decay of Freestream Turbulence Predicted by Two-equation Eddy-viscosity Models
Author Lopes, R., Eça, L. and Vaz, G.
Title On the Decay of Freestream Turbulence Predicted by Two-equation Eddy-viscosity Models
Conference/Journal 20th Numerical Towing Tank Symposium (NuTTS), Wageningen, The Netherlands
Month October
Year 2017

Abstract
As reported before by Spalart and Rumsey, the decay of freestream turbulence in two-equation eddy-viscosity turbulence models is overestimated. In simulations of external flows at high Reynolds numbers that assume "fully turbulent" flow, this excessive decay has a negligible impact on the solution. The laminar regime is confined to a very small region near the leading edge of the body and so the correct prediction of transition is not relevant. On the other hand, in model scale experiments it is common practice to force transition through the use of trip wires or roughness. Therefore, the incorrect prediction of the transition location by two-equation eddy-viscosity models becomes an asset instead of a problem.
This known shortcoming of the most common turbulence models led to the development of transition models, which supplement the two-equation eddy-viscosity SST model. Its local formulation and capability to account for several transition mechanisms such as natural transition, bypass transition and separation-induced transition make it an attractive option. The onset of transition is strongly dependent on the level of the free-stream turbulence intensity, which makes the excessive decay of turbulence predicted by the SST model troublesome.
In this paper we present the first step of this development that is performed for the flow over a flat plate, for which there are experimental data available for transitional flows with different levels of freestream turbulence in the ERCOFTAC Classic Database.

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