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Effect of surface roughness and FST on crossflow instability
Fransson, J. H. M.,
||7th Eur. Fluid Mech. Conf. EFMC7, Manchester, 14-18 Sept. 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.