The current work details an experimental study that attempts to introduce significant alterations to the level of unsteady loading experienced by the lifting surface of a delta wing when vortex breakdown (also known as vortex burst) is present. Specifically, the study looks to make use of an array of synthetic jet actuators along the length of the wing leading edge, to alter the characteristic spectral energy signature of the spiral vortex filament known to be present in many burst flows. The experimental model is a delta wing with a 60° angle of sweep with a leading edge profile of 20mm diameter. 137 pressure tappings are distributed over the lifting surface for the measurement of steady pressure coefficients, unsteady pressures and power spectral density distributions. The leading edge has 18 discrete synthetic jet actuators distributed along its length, each with a 1.2mm diameter orifice and a piezoceramic diaphragm. Three different waveforms are used to drive the actuator array: a single sine wave, two summed sine waves and a pulsed wave, each with frequencies based on measurements of the characteristic spectra of the burst flow. Sine wave actuation with a frequency an order of magnitude greater than the characteristic burst frequencies is found to be particularly effective at altering the burst spectra and is seen to be able to produce changes in the unsteady pressure levels of the order of 30% to 40%. The general effect of this actuation is seen to be an increase in unsteady loading in the immediate post burst region but a significant decrease in loading in the far-burst region. Surface flow visualisation results show that actuation introduces time-averaged delays in the leading edge separation line, local to each orifice, resulting in localised variations in vortex diameter. Also, PlV data shows that in the initial burst region, actuation moves the vortex core toward the surface. Based on these results, an interpretive hypothesis is formulated that offers an explanation for the changes seen. Firstly, it is suggested that movement of the core toward the surface is responsible for the loading increases seen in the initial burst region. Secondly, it is postulated that the local changes in vortex diameter, seen in the flow visualisation results, introduce kinks in the spiralling vortex filament of the burst flow. These kinks will augment the rate of dissipation of the energy of the spiral due to increased levels of Biot-Savart self-induction with the result being lower levels of unsteady loading on the wing surface. Finally, based on the large amount of potential actuation parameters and the interpretive nature of the hypothesis suggested, a series of suitable avenues for future work are suggested.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:488291 |
| Date | January 2004 |
| Creators | Watson, Mark |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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