Scale and origin of the dynamics of an air outlet

Abstract

The objective of this investigation is to measure the dynamical flow fluctuations that can occure in the wake of an an flapped Air Outlet (AO). Typical frequencies found in the wake of a bluff body are originated from the shear layer instability fsl, from periodic vortex shedding (e.g. a von Karman vortex street behind a cylinder) and from pumping of an shear layer enclosed fluid region (e.g. an recirculation zone). An flapped AO is used for internal flow systems of aircraft and basically consists of a channel with a deflection of 45° and an adjustable flap. Hot-wire measurements (HWA) are used to investigate the dynamic flow structures around an inclined three dimensional bluff body, with different aspect ratios, which represent the solid volume of an AO flap. Addiotionally mono PIV (2C2D-PIV) measurements were carried out in order to visualize the mean flow. The bluff body mainly consist of an inclined plate with sidewalls and an end section downstream of the bluff body. Results of the bluff body have than been compared with measurements made with the whole AO, also including the flowstream from the inside to the outer bulk flow. In total three different aspect ratios AR = 0.3, 0.6 and 0.8, which represent flap angles of 21, 33 and 45 degree, were investigated. Experiments were conducted in the Gottingen type wind tunnel Modell-Unterschallwindkanal (MUB) located at the Technische Universitat Braunschweig. Essential aspects, which include the flow structures in the wake of the bluff body and the AO, a comparison of the three different aspect ratios / flap angles and a comparison between the dynamic flow structures of the AO and the bluff body will be discussed. Power spectra of the hot-wire measurements show the existence of a shear layer frequency in the range between 1 to 4 kHz. Turbulent kinetic energy in the wake of the bluff body is about three times higher compared to the whole AO and increases with an increasing flap angle.