Date of Award

8-2018

Document Type

Thesis

Publisher

Santa Clara : Santa Clara University, 2018.

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

First Advisor

Drazen Fabris

Abstract

In this thesis, the Strouhal numbers from the downstream cylinder in a two cylinder configuration have been measured experimentally. Special apparatuses were designed to secure the cylinders in the wind tunnel as well as to hold a hot film anemometer probe in the center of the downstream cylinder’s wake. The cylinders were smooth, of equal diameter and extended the entire width of the wind tunnel. Because the cylinders used in this thesis were sufficiently long compared to their diameters and measurements were taken at center span, they were assumed to be infinite. The anemometer was calibrated according to King’s Law, enabling anemometer voltage output to be related to wind tunnel flow velocity. Power spectra were then obtained from anemometer data using FFT. The peaks from these power spectra were considered the dominant vortex-shedding frequencies in the cylinder wake and were used to calculate the Strouhal number, St. St measurements were taken for different α, the angle between cylinders relative to the flow axis and different P/D (dimensionless cylinder pitch) spacing. In this experiment, α was varied between 0 and 90± and P/D of 2, 3 and 4 were tested. In addition, measurements were taken for three Reynolds numbers in the high sub-critical regime: ReD = 1.70e+04, 2.89e+04, and 3.98e+04. For P/D spacings of 2 and 3, the Strouhal number increased at low α to a peak value near α ≈ 6 ̶ 8±. St then decreased before increasing towards the single cylinder value at higher alpha, though, there was considerable scatter in P/D = 3 data after the peak St value. For P/D = 4, St monotonically increased towards the single cylinder value for all α. No discernible trend between St and ReD was observed from 0 < α < 20± for P/D = 2, 3 or 4. Additionally, there was no clear correlation between the location of peak St for P/D = 2 and 3 and Reynolds number ReD. However, from20 < α < 90±, St were generally higher for all P/D with increasing ReD. Many cylinder configurations displayed frequency doubling, or two dominant vortex-shedding frequencies, with the higher frequency measuring twice as large as the other. This indicated that the velocity signal that was obtained in the wake of the downstream cylinder was not an exact sine wave. This was confirmed by performing time-synchronous signal averaging. The presence of two modal frequencies represented in-phase vortex-shedding. Three vortex-shedding frequencies were measured for select cylinder configurations. When three modes were measured, the third was twice as large as the first, and the second mode was in between, representing out-of-phase vortex-shedding. Two modes of vortex-shedding were observed at a large number of P/D and α cylinder configurations, and three modes of vortex-shedding were found at α = 30 and 40± for P/D = 3.

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