The study of simulated battle damage to a wing using particle image velocimetry

The effects of simulated battle damage on the aerodynamic performance of a wing is well documented; it is known that battle damage reduces lift, increases drag and changes pitching moment. However, there is a fundamental lack of understanding when it comes to the three-dimensional flow features that create such effects. The current knowledge of the flow field is predominantly based upon interpretation of surface flow visualisation techniques coupled with force balance measurements. In this work, a more modern technique in the form of Particle Image Velocimetry (PIV) has been used to map the three-dimensional flow field away from the wing surface. A PIV system was designed for the Loughborough University Low Turbulence Wind Tunnel in order to gain a deeper understanding of the flow mechanisms that are produced by simulated battle damage. The system was tested for suitability and the data quality assessed. The system was found to produce high quality vector output with low levels of uncertainty, it could also be used in multiple planes and orientations to provide the flow field data required. The technique was first applied to a single battle damage hole with a diameter of 20% chord, located at the wing s mid-chord. The wing model was of realistic construction and had a cavity. This damage case had previously been shown to produce different flow features across the incidence range and was typically a survivable damage case. The hole was seen to produce a jet at incidence angles above 2°, however the characteristics of the jet were different to those predicted in previous battle damage work and in jets in cross-flow research. The velocity ratio was very low peaking at around 0.25 at 8° incidence, much lower than the surface flow features and jet in cross-flow literature would suggest. No characteristic counter-rotating vortex pair was found in the jet due to the presence of a wing cavity and low velocity ratio. It is suggested that the wing s adverse pressure gradient has a magnified effect on the wake and is responsible for the increase in wake size at higher incidence angles, something previously thought to be due to increasing velocity ratio. A larger hole of 40% chord located at mid-chord, along with a straight through damage hole with no cavity, have been tested to isolate and highlight flow features to further explain the flow mechanics. Then, PIV was applied to several multiple hole damage cases to study the interaction caused by such damage. Side-by-side holes with different proximities, where the two damage holes had the same chordwise location, were studied to isolate interaction effects. It was found that at low incidence angles the force increments were double that of a single damage hole at the same chordwise location. However, as incidence angle increased above 2° the increments were less than double. This was because the wakes of the two damage holes were smaller than the wake of a single damage hole. From a purely aerodynamic point of view, it was seen that having two holes close together interacting was favorable compared with having two holes far apart with no interaction. Two in-line damage holes were also tested. Once again the two holes together produced smaller increments than predicted by the summation method, i.e. summing the increments created by each single hole tested individually. The presence of two holes in-line limited the effectiveness of the forward hole and hence limited the performance losses, whereas individually the hole at the forward chord location will produce much larger effects than an equivalent hole further aft.