DLR cavity pressure oscillations, experimental

Kurzfassung

Windtunnel tests were carried out with the aim of establishing a measured unsteady surface pressure data set in and around a box-shaped shallow cavity, subject to tangential flow in the transonic Mach number range. Apart from the baseline case, for which systematic Mach number and Reynolds number variations were completed, the main purpose of the tests was to investigate the effect of certain upstream mounted passive flow control devices on the cavity oscillations for selected Mach numbers. This chapter contains the description of two baseline case data sets of unsteady surface pressures for freestream Mach number M*=0.8 and M*=1.33 respectively, made available to RTO. The main purpose of the experiment was to test techniques for the passive control of pressure occurring in and near cavities exposed to tangential transonic flows. Moreover, the phase relation among the different cavity modes were investigated since the design of devices (passive and especially active) for control, critically depends on the knowledge and an understanding of the underlying physical mechanisms responsible for the resonances driving the phenomenon. Despite its long term investigation and the corresponding vast literature on cavity oscillations, reliable prediction schemes exist only for the frequencies of the oscillation modes. An insight into the phase relations among the modes however is necessary e.g. in order to lay out the characteristics of a controller for a closed loop active control of the oscillations. Therefore the present tests were also performed to reveal the spatiotemporal phase relation among the modes in the cavity. The tests were done in the DLR wind tunnel TWG (Transonic Windtunnel Göttingen) in November 1997. The closed system tunnel has a test section area of 1m x 1m and is operated continuously. The cavity oscillation model is mounted on a cropped sting and consists basically of a flat plate, containing the cutout for the box-shaped cavity of length L = 0.202m, width W = 0.03m and depth D = 0.05m, which in turn is hosted in the fuselage carrying the model (for details of the geometry see section 2 and Figures 1-5). Unsteady surface pressures were measured using flush mounted Kulite pressure transducers as specified in Table 1 and Figures 1 and 5. The static pressures at three positions on the plate surface upstream of the cavity (details see section 7) were measured in order to determine the actual Mach number of the flow above the cavity. A geometrical angle of attack of alpha = 1° was set in order to assure non-separating flow at the sharp leading edge of the plate. The cavity's bottom surface was made of an aluminium plate, which could be translated along the x-direction (streamwise) with the help of a remote-controlled electric motor. Six equally (in x) spaced Kulite sensors were flush mounted into the moveable plate. It was possible to take measurements at arbitrary x-positions of the cavity's bottom surface by moving the plate (and thus the six sensors) to the desired setting. For each flow parameter this was done for 12 positions of the plate. From one position to the next, the plate was advanced upstream in steps of 3mm. For each of these settings the time histories of all Kulite sensors (including all non-moveable sensors) were recorded simultaneously along with the static flow data. Thus for each of the 12 positions the phase relation between all sensors can be evaluated.