# After completing your calculations, examine and compare your answers for tunnels (a) and (b). Which requires the smaller overall pressure difference?

We wish to operate a low-speed subsonic wind tunnel so that the flow in the test section has a velocity of200 . Consider two different types of wind tunnels (see figure below): (a) a nozzle and a constant-area test section, where the flow at the exit of the test section simply dumps out to the surrounding atmosphere (that is, there is no diffuser); and (b) a conventional arrangement of nozzle, test section, and diffuser, where the flow at the exit of the diffuser dumps out to the surrounding atmosphere. For both wind tunnels (a) and (b), calculate the pressure differences across the entire wind tunnel required to operate them so as to have the given flow conditions in the test section. For tunnel (a), the cross-sectional area of the entrance is 20 ft2, and the cross-sectional area of the test section is 4 ft2• For tunnel (b), a diffuser is added to (a) with a diffuser exit area of 18 ft2• After completing your calculations, examine and compare your answers for tunnels (a) and (b). Which requires the smaller overall pressure difference? What does this say about the value of a diffuser in a subsonic wind tunnel?

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