Reduction of surface pressure fluctuations is desirable in various aerodynamic and hydrodynamic applications. Over the past few years, studies on canopy surface treatments have been conducted to investigate the fundamental mechanisms of surface pressure attenuation termed as pressure shielding. This work talks about the design, development and experimental testing of unidirectional canopy surface treatments which are evenly spaced arrays of streamwise rods placed parallel to the wall without an entrance condition. The canopy designs are based on surface treatments tested by Clark et al. (2014) inspired by the downy coating on owl wings. The main objective of the work is to establish fundamental physical and mathematical basis for treatments that shield aerodynamic surfaces from turbulent pressure fluctuations, while maintaining the wall-normal transport of momentum and low aerodynamic drag.
Experimental testing of these canopy treatments are performed in the Anechoic Wall-Jet facility at Virginia Tech. Different canopy configurations are designed to understand the effect of various geometric parameters on the surface pressure attenuation. The treatment is found to exhibit broadband reduction in the surface pressure spectrum. Attenuation develops in two frequency regions which scale differently depending on two different mechanisms.
Canopies seems to reduce the large-scale turbulent fluctuations up to nearly twice the height.
Semi-analytical model is developed to predict surface pressure spectra in a wall-jet and canopy flow. The rapid term model shows that the inflection in the streamwise mean velocity profile is the most dominant source of surface pressure fluctuations. Synchronized pressure and velocity measurements elucidate significant features of the sources that could be affecting surface pressure fluctuations. Overall, this study explores the qualitative and quantitative physics behind pressure shielding mechanism which find application particularly in trailing edge noise reduction. / Doctor of Philosophy / Unsteady pressure fluctuations originating from interaction of turbulent flow over surfaces often cause undesirable effects. Trailing edge noise in wind turbines and helicopter blades, cabin noise and interior wind noise are some of noise sources which originate from surface pressure fluctuations. Previous studies have demonstrated that surface treatments help in reducing the unsteady surface pressure fluctuations therefore shielding surfaces and this phenomenon is termed as 'Pressure Shielding'. These are surface treatments inspired from the downy coating on owl's wings.
This study is motivated by recent works conducted at Virginia Tech on experimental investigation of unidirectional canopy treatments. These are evenly spaced arrays of streamwise rods held horizontal at the downstream end. Most previous surface treatments contain some entrance condition such as steps, supports or gaps which effect the surface pressure measurements and disturb the incoming flow. In this study, the canopies are developed without any entrance condition therefore assist in capturing the fundamental mechanisms of the flow interaction with the canopy rods.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/110954 |
| Date | 27 June 2022 |
| Creators | Nurani Hari, Nandita |
| Contributors | Aerospace and Ocean Engineering, Devenport, William J., Lowe, K. Todd, Glegg, Stewart, Alexander, William Nathan, Coutier-Delgosha, Olivier |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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