Nathan Alex | University of Dar es Salaam (original) (raw)
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15th AIAA/CEAS Aeroacoustics Conference (30th AIAA Aeroacoustics Conference), 2009
Far field noise was recorded for the flow of a turbulent wall jet boundary layer over various dis... more Far field noise was recorded for the flow of a turbulent wall jet boundary layer over various discrete roughness elements including rocks, cubes, and cylinders. The elements were contained within the boundary layer but were a substantial fraction of the boundary layer thickness. Far-field sound measurements were used to infer non-dimensional drag spectra for the elements. The sound radiated from individual elements, from pairs of elements at various streamwise and spanwise separations, and from fetches of elements built progressively from a single element was studied in detail. The coherence between wall pressure fluctuations in the vicinity of a cubic element and the far-field sound radiated by that element was measured. Overall, the results show that that the aerodynamic interaction between elements has remarkably little impact on the sound they generate. Only when the elements are packed as tightly as possible does the total sound produced differ substantially from the sum of that which would be produced by the elements in isolation.
Journal of Sound and Vibration, 2010
This paper introduces a new method for measuring the spectral character of wall pressure fluctuat... more This paper introduces a new method for measuring the spectral character of wall pressure fluctuations under turbulent boundary layers by measuring the sound that they radiate in the presence of hydrodynamically smooth sinusoidal ridges in the surface. The theoretical basis for the method and experimental tests demonstrating its viability are described. The sound spectrum radiated by the sinusoidal surface reveals a cut through the full three-dimensional wavenumber frequency spectrum of the wall pressure at the wavenumber of the surface. Since sinusoidal ridges can be made with very small wavelengths, this technique can be used to probe the structure of the wall pressure spectrum on scales far smaller than those that can be reached using conventional wall-mounted transducers. Furthermore, the method reveals the wavenumber frequency spectrum directly, without the need for multi-point measurements. The wall pressure wavenumber frequency spectra measured using this technique bear a close qualitative and quantitative similarity to Chase's (1980, 1987) model forms, with the exception that they show a high frequency decay that occurs at different rates than assumed in the models.
15th AIAA/CEAS Aeroacoustics Conference (30th AIAA Aeroacoustics Conference), 2009
Far field noise was recorded for the flow of a turbulent wall jet boundary layer over various dis... more Far field noise was recorded for the flow of a turbulent wall jet boundary layer over various discrete roughness elements including rocks, cubes, and cylinders. The elements were contained within the boundary layer but were a substantial fraction of the boundary layer thickness. Far-field sound measurements were used to infer non-dimensional drag spectra for the elements. The sound radiated from individual elements, from pairs of elements at various streamwise and spanwise separations, and from fetches of elements built progressively from a single element was studied in detail. The coherence between wall pressure fluctuations in the vicinity of a cubic element and the far-field sound radiated by that element was measured. Overall, the results show that that the aerodynamic interaction between elements has remarkably little impact on the sound they generate. Only when the elements are packed as tightly as possible does the total sound produced differ substantially from the sum of that which would be produced by the elements in isolation.
Journal of Sound and Vibration, 2010
This paper introduces a new method for measuring the spectral character of wall pressure fluctuat... more This paper introduces a new method for measuring the spectral character of wall pressure fluctuations under turbulent boundary layers by measuring the sound that they radiate in the presence of hydrodynamically smooth sinusoidal ridges in the surface. The theoretical basis for the method and experimental tests demonstrating its viability are described. The sound spectrum radiated by the sinusoidal surface reveals a cut through the full three-dimensional wavenumber frequency spectrum of the wall pressure at the wavenumber of the surface. Since sinusoidal ridges can be made with very small wavelengths, this technique can be used to probe the structure of the wall pressure spectrum on scales far smaller than those that can be reached using conventional wall-mounted transducers. Furthermore, the method reveals the wavenumber frequency spectrum directly, without the need for multi-point measurements. The wall pressure wavenumber frequency spectra measured using this technique bear a close qualitative and quantitative similarity to Chase's (1980, 1987) model forms, with the exception that they show a high frequency decay that occurs at different rates than assumed in the models.