Volumetric Reconstruction Of Stagnated Wake Structures In Adverse Pressure Gradient

Abstract

To gather insights about turbulent structures of a wake in adverse pressure gradient, a wind tunnel model is investigated with PIV methods. A flat plate of L = 1.058 m at zero angle of attack is placed in the MUB wind tunnel at TU Braunschweig. The wake convects downstream into a carefully designed diffuser, where it undergoes a pressure increase of ∆c p = +0.6. Wakes, with the common example of a cylinder shedding van-Karman vortices, are generally prone to instability. The wake instability of the sharp-edged plate is attenuated by the adverse pressure gradient, manifesting in large coherent structures that carry up to 37 % of the turbulent kinetic energy. While the primary shedding motion is effectively a quasi-2D motion, a secondary mode of is also observed: Over the span S = 1.3 m of the wake, it organizes in three distinct cells. Their motion is similar to "owl-eyes", that are known to appear on separated flow over smooth 2D bodies. The presence of these modes reduces the average flow velocity in the wake centre up to stagnation in the mean sense. Because stagnation and shedding does occur far away from any solid surfaces, this test case offers excellent observability of the phenomenon with PIV methods. RANS results of this setup cannot predict the flow stagnation accurately. Because the shedding frequency is low compared to freestream velocity, scale and time resolving IDDES results need to run many convective time units until the observed instability develops. While the original goal of observing small scale turbulent wake structures is compromised by the large shedding modes, the observation gives an insight on wake-bursting phenomena that occur on multi-element high-lift devices. The mode shedding and consequent wake stagnation is sensitive to the Reynolds-number (Re = [1.6 ... 3.2] · 10 6 ) as well as to the initial boundary layer thickness of the flat plate.