Effect of free-stream turbulence on flow over aerofoil section at high incidence

Unsteady flow past a NACA 0015 aerofoil is investigated for moderate Reynol ds numbers at high angles of attack by solving the full 2-D Navier-Stokes e quations with and without the presence of free-stream turbulence (FST). The investigation focusses on the by-pass mode of transition usually encounter ed in turbomachinery and wind engineering where the flow field around a blu ff-body can experience very high levels of FST. In this study, a 5% level o f FST is considered. While FST is all-pervasive, its effect has not been st udied at all theoretically. Here, this has been made possible by proposing a new model for FST based on a moving-average time-series and using it for long-time computation of the Navier-Stokes equations. The statistics of the modelled FST follows the statistics of a specific wind tunnel. The use of this model in conjunction with higher order upwinding, for the convection t erm to model the vorticity dynamics, gives the solution a very high degree of accuracy in the by-pass transitional flow regime. The present study is r elevant for understanding the implications of reduced order modelling propo sed for aeroelastic studies. The numerical results view the solution of the Navier-Stokes equations not only as the output of a dynamical system in th e presence of stochastic noise (FST), but which also produces the intermitt ency factor in and around the aerofoil dominated by differing pressure grad ient and unsteady effects. The last attribute is also a novel feature of th e present study and is relevant to bluff-body flow fields. The computed flo w field shows that the flow achieves a statistical stationarity even though the overall flow is chaotic and aperiodic. (C) 2001 Academic Press.