Experimental Investigation of the Free-End Pressure Distribution for Surface-Mounted Finite-Height Square Prisms
Date
2019-12-18
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Degree Level
Masters
Abstract
An experimental investigation was performed to study the mean free-end surface pressure distribution for surface-mounted finite-height square prisms. The prism aspect ratio (AR) was varied from 1 to 11, with a small increment of 0.5. For each aspect ratio, the effect of incidence angle (α) on the mean pressure distribution was investigated from 0° to 45°, with a small increment of 1°. Integration of the free-end pressure distribution was performed to determine the normal force coefficient due to pressure (CN, p). Measurements were also performed for the mean drag and lift force coefficients (CD and CL) and Strouhal number (St) from α = 0° to 45°, and vortex formation length at mid-span and at α = 0°. The freestream velocity (U∞) used was 22.5 m/s (equivalent to a Reynolds number of Re = 6.5 × 10⁴) for all the measurements, except for the measurement of forces, where U∞ = 40.0 m/s (equivalent to a Reynolds number of Re = 1.1 × 10⁵) was used. The boundary layer thickness developed on the ground plane (relative to the width of the prism) (δ/D) varied from 0.8 to 2.6 for five different cases.
The results demonstrated that the most complex pressure distributions, based on the range of pressures encountered and the severity of the pressure gradients, tend to occur at the highest incidence angles, and were most pronounced for the lowest and highest aspect ratios tested, which suggests the existence of three distinct flow regimes based on the pressure distribution. The effect of the boundary layer on the pressure distribution varies at different incidence angles, with the most appreciable impact is observed at α = 0° and 45°. There are seven different critical incidence angles determined in the present study, based on the minimum CD, maximum CD, maximum magnitude of CL, positive CL, primary and secondary peaks of CN, p, and maximum St. Based on the results of CD and St, there are only two flow regimes identified (instead of three regimes as based on the pressure distribution). The first flow regime is where the boundary layer effect dominates the flow, and results in high sensitivity of CD, CL to AR, but no well-defined peak is identified in the power spectra. The second flow regime shows the forces and dominant vortex shedding frequency are insensitive to the aspect ratio. The vortex formation length at mid-span was found to be maximum when AR = 9 for δ/D = 0.8.
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Keywords
free-end pressure distribution, bluff-body aerodynamics, finite-height square prisms
Citation
Degree
Master of Science (M.Sc.)
Department
Mechanical Engineering
Program
Mechanical Engineering