Vortex-Surface Interactions

Abstract

The high efficiency of open rotor propulsion has led to a comeback of propeller propulsion systems in recent years. The main issue preventing wide application of the technology is its high noise level. One of the main contributors to this noise is the pressure fluctuations generated during the interaction between a propeller tip vortex and a downstream surface.

The goal of this research project is to create a better understanding of the vortex- surface interaction, which would allow for more effective noise mitigation strategies to be developed. An experimental approach was adopted to quantify the effect of a number of governing parameters on the pressure fluctuations over the surface induced by the vortex. Furthermore, an attempt was made to develop a conceptual model describing the vortex-surface interaction in order to study the relative contributions of the sub-phenomena of the interaction. The accuracy of the conceptual model was assessed using the data obtained from the experimental campaign.

From the results it was clear that the low pressure vortex core was the primary mechanism that generated fluctuations over the surface. The vortex path was clearly visible and dominant over a large part of the chord. In the slipstream region the wake generated the largest pressure fluctuations. The effect of the wake was contained at the leading edge, up to 4% of the wing chord.

Large discrepancies between the unsteady pressure, both in magnitude and in spatial distribution, were observed. Three contributing factors to the discrepancies were identified, namely the overestimation of the induced angle of attack effect, the use of the Lamb-Oseen vortex model and the method of determining the vortex core radius. These discrepancies implied that the conceptual model could not be used for its intended purposes.

The results of this study show that the advance ratio and incidence angle are the most critical parameters governing the vortex-surface interaction. Although the former is difficult to incorporate into noise mitigation strategies, the latter can be used as a parameter in the initial design phase. Interestingly, the geometry of the airfoil seems to have limited effect on both the magnitude and distribution of the interaction. Although the conceptual model discussed in this report is not accurate, the development of such a simplified model would still contribute a lot to the body of knowledge. It could be used to evaluate the relative contributions of the different vortex-surface interaction sub-phenomena, or to perform a 'parameter sweep' to investigate the effect of the governing parameters on the unsteady pressure distribution and magnitude.