Experimental investigation of flow past open and partially covered cylindrical cavities

Flows past wall-mounted cavities are found in a wide variety of applications, including side-branches, organ pipes, automobile sunroofs, inter-car gaps in trains and aircraft bays. Under certain conditions, flow excited cavities can generate large pressure fluctuations, undesirable noise and significant structural loads. To date, most of the studies have been focused on rectangular cavities while little attention has been given to cylindrical cavities despite their widespread use. Two different types of cylindrical cavities were experimentally investigated in low speed wind tunnels: an open mouth cavity and a deep cavity with a small rectangular opening. The measurements included hot wire anemometry, particle image velocimetry (PIV) and unsteady surface pressure measurements. Additionally, numerical analysis of the test section/cavity systems were carried out with the finite element program COMSOL Multiphysics and with a wave expansion method (WEM) code developed by the Trinity College Dublin. Important flow features are described by evaluating the pressure measurements conducted in several positions over the walls of an open mouth cavity, the PIV measurements performed over horizontal planes inside the cavity and the hot-wire measurements on the shear layer and on the wake of the cavity. Pressure Fourier spectra evidence the presence of the first three shear layer hydrodynamic modes at frequencies well predicted by classical formulation for rectangular cavities (Rossiter). When the cavity is open, the acoustic modes of the test section are found to be excited by the flow but when the cavity is partially covered, the shear layer hydrodynamic modes are more likely to lock on the natural frequencies of the cavity. The position of the opening has an influence on the lock-on acoustic modes. The acoustic energy generated by the shear layer is calculated by applying the vortex sound theory of Howe: the flow velocity and the vorticity are extracted from the PIV data and the acoustic particle velocity field from the WEM calculation. The acoustic sources are localised in space and quantified over an acoustic period providing insight into the sound production of flow-excited partially covered cylindrical cavities.