The Effect of Raised Freestream Turbulence on the Flow Around Leading Edge Radii 2008-01-0473

The turbulence environment in the real world is known to be significantly different to that found in a typical automotive wind tunnel. Various studies have shown that raising the level of freestream turbulence has an effect on the forces on generic bluff bodies and real vehicles. Previous work at Loughborough has shown a significant effect of raised freestream turbulence on edge radius optimisation using measurements of forces and moments, and in this paper the underlying changes in the flowfield are investigated using PIV.

Results are presented of the flowfield around the leading edge radius of the generic bluff body used in the previous work. The effect of changing the Reynolds number is investigated in the clean tunnel (0.2% turbulence), and it is found that, when the radius is small, there is a significant separation that persists up to a high speed, and then abruptly collapses. The Reynolds number examined ranged from 3*10⁴ to 1.1*10⁵ (based on edge radius - from 9*10⁵ to 2.3*10⁶ based on the square root of frontal area). Four levels of additional freestream turbulence intensity, up to a maximum of 5.13%, are used to investigate the effect on the flow around the radius. It is shown that there is some unsteadiness in the separated region around a small radius in 0.2% turbulence, and that the unsteadiness is increased by the addition of freestream turbulence. The separation is ultimately suppressed by the additional turbulence because the increased unsteadiness in the free shear layer causes the bubble to collapse. Time averaged flow fields are also used to show the changes in the flow that cause the time averaged forces to be affected by both Reynolds number and increased turbulence. It is shown that increasing the turbulence intensity changes the way the flow separates by different mechanisms than increasing the Reynolds number, by affecting the development of the free shear layer rather than causing the boundary layer to undergo transition upstream of the separation point.