Low Reynolds Number Two- and Three-Dimensional Cavity Flows and the Effect of a Cavity on Airfoil Tonal Noise

Abstract

This thesis discusses the flow over rectangular and modified cavity geometries at low Reynolds number, as well as the role of a cavity in an airfoil surface in the production of airfoil tonal noise. Cavity flows can be found on many land, air and water vehicles where they can be a significant source of tonal or broadband noise. Modifying the cavity geometry is established as an effective passive control technique for cavity flow noise. The flow about, and noise produced by, two-dimensional rectangular and modified cavity cutouts in an airfoil surface (`airfoil with cavity') were studied using an anechoic wind tunnel. As L/0o increased, the coherence of shear layer vortices decreased, with an increase in the number of cavity oscillation modes found, each with lower intensity. Mean convection velocity ratio data were reported for an extended range compared to the literature, with cavity oscillation mode numbers also reported. The effect of sloping the front and rear cavity walls was investigated, with a significant change in shear layer roll-up modes found and cavity oscillation modes detected, compared to the rectangular cavity. The production of airfoil tonal noise was unexpectedly found from the `airfoil with cavity' profile. As the cavity position was varied on the airfoil surface, the airfoil tonal frequencies were found to vary. The noise was attributed to an aeroacoustic feedback loop, of a similar form to that responsible for cavity tones - the existence of this feedback loop as it pertains to airfoil tonal noise had been debated in the literature. A region of receptivity was identified near the cavity trailing edge where inflectional velocity profiles were measured. Boundary layer disturbances at the frequencies of the airfoil tones were detected downstream of this region. These disturbances were found to be amplified over a separation bubble upstream of the airfoil trailing edge. When external acoustic forcing was applied, the airfoil tonal noise frequencies were selectively reinforced - rather than the boundary layer's entire unstable frequency range responding to the external forcing in a smooth curve peaking at the most unstable Tollmien-Schlichting wave frequency. This suggested that a constructive feedback loop existed between the airfoil trailing edge and the region of receptivity near the cavity. The flow about three-dimensional cavity cutouts in a flat plate were investigated using a recirculating water tunnel. The shear layer structure about a shallow, narrow rectangular cavity flow was characterised. At low Reynolds number, `spanwise' shear layer vortices were found to have significant curvature. The lateral growth of the cavity shear layer beyond the sides of the cavity was found to be significant, with a periodic flow pattern identified adjacent to the sides of the cavity. The pattern is explained in terms of an interaction of the streamwise-orientated portions of preceding and following shear layer vortices. This finally causes the formation of a tornado-like feature on the at plate adjacent to the sides of the cavity. An asymmetric flow structure was found about a modified chevron-like cavity geometry, which was attributed to a shear layer twisting mode.