A wind tunnel operates by drawing air through a contraction, passing this air through a

Chapter 9, Problem 9.68

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A wind tunnel operates by drawing air through a contraction, passing this air through a test section, and then exhausting the air using a large axial fan. Experimental data are recorded in the test section, which is typically a rectangular section of duct that is made of clear plastic (usually acrylic). In the test section, the velocity should have a very uniform distribution; thus, it is important that the boundary layer be very thin at the end of the test section. For the pictured wind tunnel, the test section is square with a dimension of W = 457 mm on each side and a length of L = 914 mm. Find the ratio of maximum boundary-layer thickness to test section width [S(x = L)I"W] for two cases: minimum operating velocity (1 m/s) and maximum operating velocity (70 m/s). Assume air properties at I atm and 20C.