DIRECT DRAG MEASUREMENTS IN A TURBULENT FLAT-PLATE BOUNDARY-LAYER WITH TURBULENCE MANIPULATORS

The effect of turbulence manipulators on the turbulent boundary layer above a flat plate has been investigated. These turbulence manipulator s are often referred to as Large Eddy Break Up (LEBU) devices. The bas ic idea is that thin blades or airfoils are inserted into the turbulen t flow in order to reduce the fluctuating vertical velocity component v' above the flat plate. In this way, the turbulent momentum transfer and with it the wall shear stress downstream of the manipulator should be decreased. In our experiments, for comparison, a merely drag-produ cing wire also was inserted into the boundary layer. In particular; th e trade-off between the drag of the turbulence manipulator and the dra g reduction due to the shear-stress reduction on the flat plate downst ream of the manipulator has been considered. The measurements were car ried out with very accurate force balances for both the manipulator dr ag and the shear stress on the flat plate. As it turns out, no net dra g reduction is found for a fairly large set of configurations. A singl e thin blade as a manipulator performed best, i.e., it was closest to break-even. However, a further improvement is unlikely, because the de vice drag of the thin blade elements used here has already been reduce d to only that due to laminar skin friction, and is thus the minimum p ossible drag. Airfoils performed slightly worse, because their device drag was higher. A purely drag-producing wire device performed disastr ously. The wire device, which consisted of a wire with another thin wi re wound around it to suppress coherent vortex shedding and vibration, was designed to have (and did have) the same drag as the airfoil mani pulator with which it was compared. The comparison showed that airfoil and blade manipulators recovered 75-90% of their device drag through a shear-stress reduction downstream, whereas the wire device recovered only about 25-30% of its device drag. Conventional LEBU manipulators with airfoils or thin blades produce between 0.25% and 1% net drag inc rease, whereas the wire device (with equal device drag) produces as mu ch as 4% net drag increase. These data are valid for the specific plat e length of our experiments, which was long enough in downstream exten t to realize the full effect of the LEBU manipulators. Turbulence mani pulators do indeed decrease the turbulent momentum exchange in the bou ndary layer by ''rectifying'' the turbulent fluctuations. This generat es a significant shear-stress reduction downstream, which is much more than just the effect of the wake of the manipulator. However, the dev ice drag of the manipulator cannot be reduced without simultaneously r educing the skin friction reduction. Thus, the manipulator's device dr ag exceeds, or at best cancels, the drag reduction achieved by the she ar-stress reduction downstream. A critical survey of previous investig ations shows that the suggestion that turbulence manipulators may prod uce net drag reduction is also not supported by the available previous drag force measurements. The issue had been stirred up by less conclu sive measurements based on local velocity data, i.e., data collected u sing the so-called momentum balance technique.