William Salmon

William Salmon

Aerospace Engineering BEng

An Experimental Investigation of the Unsteady Wall Pressure for the Owl-inspired Trailing Edge Serrations on a Flat Plate

The unusually quiet flight of owls has been recognised since the 1930s but the features that cause this have only been exactly identified over the last 40 years. The feathers' texture, shape, arrangement and wing shape are all recognised as important factors in achieving low noise emissions from the wings. The understanding of how they all combine to reduce noise is not yet fully achieved. The ability to harness the owl's low noise performance into practical engineering applications has not yet been realised. This is partly due to the lack of understanding of the sound generating mechanisms that the owl manipulates to reduce the noise level. Key properties of the flow field that effect radiated noise and are changed by the serrations are unsteady pressure and lateral length scale, how the serrations effect these properties is published in the literature. This project has focused on measuring the effect of the serrations on unsteady pressure but has also made sure that the developed equipment will also be useful for measuring lateral length scale. These key properties of unsteady pressure and lateral length scale, could not be measured in the required locations with the existing equipment at Brunel. Most of the required effort for this project was focused on developing the equipment for measuring the unsteady pressure with the required spatial requirements. I developed a system that allows for measurements to be taken from pressure holes of 0.5mm diameter and with their centres a distance of 1mm of the trailing edge. Despite various constraints and unexpected setbacks to the project, the main aim of measuring unsteady pressure have been achieved however, over the course of the project, the main improvements have been noted and so the equipment should allow even better calibration and results once the various recommendations of the report have been fully implemented. One of the PhD students at Brunel has already created more copies of my system and is implementing them to create an even bigger data set and investigate more parameters and try to measure lateral length scales. This project has developed Brunel's capability to measure unsteady pressure and brought the capability to develop measurement techniques for longitudinal length scales and stochastic flow visualisations very close. The physical means to perform stochastic flow visualisations using the equipment developed in this project and existing hot wire equipment have now been achieved but the software needed to process the raw data has not yet been set up. This capability to measure unsteady pressure has in itself allowed the report to improve the understanding of the Far Field Radiated noise equation to model the effect of owls' wing shape to reduce Far Field Radiated noise and show that the unsteady pressure is not reduced by trailing edge serrations and therefore the unsteady pressure is not responsible for the noise reductions in the Far Field Radiated noise equation.