Effect of surface roughness and FST on crossflow instability

Författare: Kurian, T., Fransson, J. H. M.
Dokumenttyp: Konferens
Tillstånd: Publicerad
Tidskrift: 7th Eur. Fluid Mech. Conf. EFMC7, Manchester, 14-18 Sept. 2008.
År: 2008


In industrial applications knowledge about transition criteria is highly sought after, but still not well understood. Currently, the e^N -method is widely used even in cases where surface roughness is present and the free stream is affected by background noise. Today, this is known to trigger a different transition scenario with algebraic disturbance growth at sub-critical Reynolds numbers considering classical stability theory. More experiments are needed to enhance the fundamental knowledge of the receptivity mechanisms, which can lead to the establishment of receptivity coefficients. To this end an experimental investigation has been undertaken to study the growth of crossflow instabilities over a swept flat plate under different receptivity mechanisms, namely: surface roughness and FST. The experiments were carried out in the MTL wind tunnel at KTH Mechanics. The leading edge of the flat plate was a modified super-ellipse at a sweep angle of 25 . Taking into account the expected growth of the crossflow mode as well as the thickness of the boundary layer a bump, designed to give a strong acceleration with a Hartree parameter, beta = 0.19, was built. FST levels were varied by placing different turbulence generating grids upstream of the leading edge. By changing both the position as well as the actual grid (thus the mesh and bar widths), it is possible to vary the incoming free stream turbulence intensity as well as the characteristic length scales. Surface roughness was introduced by means of discrete roughness elements. Measurements were done with a single hot-wire that could be slanted in the flow to obtain both the streamwise and spanwise mean velocities as well as their variances and the covariance. Figure 1 shows that the contour lines of the base flow are parallel with the sweep angle up to approximately one metre from the leading edge and that the acceleration along the centreline matches with beta = 0.19. Disturbance energy evolution inside the boundary layer will be presented. This investigation has been performed within the pro ject TELFONA (TEsting for Laminar Flow On New Aircraft) under contract number AST4-CT-2005-516109.