## AN EXPERIMENTAL INVESTIGATION OF ASPECT RATIO AND INCIDENCE ANGLE EFFECTS ON THE MEAN WAKE OF A SURFACE-MOUNTED FINITE-HEIGHT SQUARE PRISM

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##### Date

2016-09-21##### Author

Unnikrishnan, Sanjith 1989-

##### ORCID

0000-0002-6349-5034##### Type

Thesis##### Degree Level

Masters##### Abstract

The flow around two-dimensional or infinite square prisms has been extensively studied. Fewer published studies have been carried out for the flow around surface-mounted finite-height square prisms, especially where the prism is oriented at a non-zero incidence angle. In the present study, the flow around surface-mounted finite-height square prisms of aspect ratios AR = 9, 7 and 5 was studied experimentally for a range of incidence angles, from α = 0° to 45°. The experiments were conducted in a low-speed wind tunnel at a Reynolds number of Re = 3.7×104 and with a boundary layer thickness, relative to the prism width, of δ/D = 1.5. Time-averaged velocity measurements in the wake were made using a seven-hole pressure probe. The wake measurements were carried out in two vertical planes, one parallel to the approach flow, and one normal to the approach flow. The data obtained were compared with previous results obtained for finite square prisms and finite cylinders.
For all aspect ratios, a symmetric counter-rotating pair of streamwise vortices representing the tip vortices is seen in the upper part of the wake at x/D = 10 for the orientation of α = 0°. Also, small streamwise vorticity concentrations are seen at the outer edge of the wake close to the ground plane representing the horseshoe vortices. The base vortex structures were nearly absent for the square prism. The maximum length of the recirculation zone for a finite square prism is much higher than that of a finite cylinder. With an increase in aspect ratio from AR = 5 to AR = 9, at α = 0°, the maximum length of the mean recirculation zone and the maximum downwash velocity increase. On the other hand, the width of the wake and the minimum value of the streamwise velocity component decrease with an increase in aspect ratio.
As α is varied from 0° to 45°, the mean wake shifts sideways in the direction of rotation of the prism, with the counterclockwise tip vortex moving higher above the ground plane and the clockwise tip vortex moving closer to the ground plane. The asymmetry in the wake is most pronounced at the critical incidence angle, which was between α = 10° and 15° in the experiments. The critical incidence angle corresponds to the smallest magnitude of the mean downwash velocity component, the largest value of the minimum mean streamwise velocity component, the maximum value of positive (counterclockwise) mean streamwise vorticity within the wake, and the minimum length of the mean recirculation zone. These properties of the wake were generally consistent for all three aspect ratios.

##### Degree

Master of Science (M.Sc.)##### Department

Mechanical Engineering##### Program

Mechanical Engineering##### Supervisor

Sumner, David##### Committee

Bugg, Jim; Bergstrom, Donald##### Copyright Date

July 2016##### Subject

bluff body

finite square prism

wake

vortex structures

wind tunnel

seven-hole probe