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Title: Investigation into noise emitted by bluff bodies with large roughness
Author: Alomar, Antoni
ISNI:       0000 0004 2743 3324
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2013
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A set of wind tunnel experiments were performed to study the effect of large surface roughness on circular cylinder noise, with the goal of improving landing gear noise predictions. Roughness increases vortex shedding noise levels, and shifts the peak to a lower Strouhal number. The noise levels in the fall-off range also increase, but no significant change in the fall-off rate is observed. The decrease of the vortex shedding peak frequency has been associated with early detachment caused by the effect of roughness on the TBLs, which is in agreement with previous experimental studies with smaller roughness. The high frequency range of the spectrum revealed a broadband, Strouhal-based peak, which is caused by roughness noise generated on the upstream face of the cylinder. The peak Strouhal number is well predicted by Howe's model using the maximum outer velocity around the cylinder. Cylindrical roughness presents a weaker roughness noise peak, but higher noise levels for higher frequencies, and is thought to be caused by sharp edge separation. A bluff body roughness noise model has been developed based on the model of Howe and a Green's function tailored to the bluff body geometry, calculated using the Boundary Element Method. The application to rough circular cylinders using a at wall (ZPG) TBL model shows good agreement with experiments for downstream observers, but the model overpredicts the levels in over-head observers. The disagreement is thought to be due to inaccuracy of the at wall TBL model. The transition from smooth regime to rough regime was studied experimentally by partially covering the cylinder with distributed roughness in spanwise uniform configurations. Transition regarding vortex shedding happens mainly when roughness is added or removed around the separation region. The results agree with the fact that roughness changes the separation location by perturbing the TBL close to separation. Sparse and dense two-dimensional roughness on a circular cylinder, studied using CFD, have similar effects than distributed roughness regarding the vortex shedding peak level and frequency.
Supervisor: Angland, David Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TL Motor vehicles. Aeronautics. Astronautics