A Computational Study on Tip Vortex Noise

Abstract

Tip vortex noise is an important component of aircraft high-lift device noise. It is imperative to gain a better understanding of this problem before appropriate noise reduction technologies can be developed and implemented effectively. This paper reports results from our ongoing work on tip vortex simulations. The simulations are performed using a high-order accurate, multi-block, large eddy simulation (LES) code with overset grid capability. The main emphasis of the present simulations is to simulate the formation of a tip vortex around a wing with a blunt tip and its interaction with the wing surface. The blunt tip geometry actually gives rise to the formation of two vortices. The primary vortex forms over the upper surface of the wing, while the secondary vortex forms off of the side edge. These two vortices merge together to form the tip vortex. Comparisons of the simulation results with available experimental data are shown. Some of the observed differences between the LES and the experiment are believed to be due to the Reynolds number differences between them. The Reynolds number of the LES is lower than that in the experiment due to computational limitations. The higher Reynolds number of the experiment implies that the boundary layers in the experiment transition to turbulence faster. It was originally anticipated that the state of the boundary layers on the wing surface might have significant effects on the formation and properties of the primary and secondary vortex. Thus, to alleviate the Reynolds number difference and promote quicker transition to turbulence in the LES wing boundary layers, we have experimented with a new technique in which isotropic turbulence fluctuations are injected into the freestream flow approaching the wing. It is shown that without turbulence injection, natural transition to turbulence on the upper wing surface takes place relatively fast due to the fine wall resolution on the wing surface. Freestream turbulence injection moves the upper wing surface transition point even closer to the leading edge. On the other hand, without turbulence injection, the lower wing surface boundary layers remain laminar. With turbulence injection, there is a dramatic change in the state of the lower wing surface boundary layers. It was initially thought that the significantly different state of the lower wing surface boundary layers in the second simulation would have an important effect on the secondary vortex forming off of the tip side edge. However, the comparisons made between the two simulations show that the two sets of results are not much different. These observations lead us to conclude that for the blunt tip geometry, the state of the lower wing surface boundary layers do not have much effect on the secondary vortex forming off of the tip side edge.