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30 points

A long cylinder with diameter D = 0.02 m is placed in a uniform flow of speed U. Assume that

fluid density p= 800 kg/m³, and fluid viscosity = 0.08 Pa-s. For a certain range of incoming flow

speeds, an interesting flow pattern will emerge: the cylinder will create a clockwise vortex in its

wake, then a counterclockwise vortex, then another clockwise, then another counterclockwise, and

on and on. This alternating pattern is called the Von Kármán Vortex Street, and there is a constant

frequency of vortex production, f (for a given spoed). A video: https://youtu.be/3mULL606f38

Part a: How many independent dimensionless groups govern this problem?

Part b: Assume that one of the dimensionless groups is the Reynolds number. Find another di-

mensionless group, this one with the shedding frequency / in the numerator. (Hint: it involves

speed and one other parameter.)

Part c: Experimental data is shown below, with the z-axis being Reynolds Number and the y-axis

the dimensionless number called St. (this is what you determined in (b)). Suppose you don't know

exactly what the incoming flow speed U is, other than it is less than 10 m/s. But you can measure

the oscillation in the wake (for example if the cylinder is piezoelectric) and know f= 25.6 Hz. Use

the experimental data in the figure to estimate the incoming flow velocity.

(Hint: May need to guess and check, and of course your answer will be a bit approximate.)

0.22

St

0.20

0.18

0.16

0.14

0.12

Fig: 1