f20_mud_0304 - The subsonic leg occurs right at choking...

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Lecture F20 Mud: Laval Nozzle Flows 1. In the PRS, isn’t the answer 14.14, not 45? (1 student) I had a typo on the PRS missing an extra zero for p e and p r , which explains the 10 discrepancy. Sorry about that. The corrected PRS will be posted along with all the others for lectures F11-F20. 2. Is there a way to tell if a flow will be subsonic or supersonic? (1 student) As a minimum, the information required to determine the type of flow is: duct geometry A ( x ), or just A e and A throat reservoir p r , h r back pressure p e or p B . From this info one can determine what type of flow is present (choked/unchoked?, and if choked: shocked/overexpanded/matched/underexpanded?). p 3. How do we know if the flow behind the throat is subsonic or supersonic? (1 student) It depends on what’s the imposed p e (or p B ). You get partial or full supersonic flow if e is below the pressure needed to reach choking. 4. What are the dashed lines in the p ( x ) and M ( x ) plots? (1 student) Those are the pressure and Mach distributions for choked isentropic flow, either sub- sonic or supersonic.
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Unformatted text preview: The subsonic leg occurs right at choking onset. The supersonic leg occurs if p B < p (no shock inside duct). 5. Why do the waves reflect back into the jet rather than “escaping”? (1 student) The waves cannot propagate outside the jet because we have assumed the flow outside the jet is still ( M = 0), or at least subsonic ( M < 1). Stationary shocks or waves are possible only if M ± 1. However, the waves in the exhaust jet of a supersonic aircraft will indeed undergo only a partial reflection, with part of each wave “excaping” from the jet into the surrounding supersonic flow. We haven’t looked at this more complicated case. 6. No mud (6 students)...
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This note was uploaded on 01/28/2012 for the course AERO 16.01 taught by Professor Markdrela during the Fall '05 term at MIT.

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