University:
Massachusetts Institute of Technology
Course:
16.01 | Unified Engineering I, II, III, & IV
Academic year:

2019
Views:

432

Pages:

1
Author:

Rowan Mendoza

Lecture F18 Mud: Expansion Fans 1. Can you see expansion waves like you can see shocks? (1 student) Yes, but not as easily. Supersonic ﬂow viz techniques sense ﬁrst or second gradients of the air density. The gradients in a wave are much smaller in an expansion wave than in a shock. 2. Do you have any pictures of a supersonic razor blade? (1 student) No, but you can ﬁnd pictures of supersonic ﬂows around various other shapes which show shocks and waves. The one I showed in class is one good example. A great book for showing all sorts of ﬂow viz photos is “An Album of Fluid Motion”, by Milton Van Dyke, Parabolic Press, ISBN 0-915760-02-9 (paperback), ISBN 0-915760-03-7 (hardcover) 3. Does the change in zero moment location (center of pressure) aﬀect the design of supersonic aircraft? (1 student) Yes, a great deal. The shift in center of pressure must be trimmed out with substantial tail loads. This causes additional drag, among other problems. 4. Does the sign of � matter when calculating the airfoil pressures? (1 student) By the convention used in our notes (and Anderson), is positive “into the ﬂow” for an oblique shock, and positive “away from the ﬂow” for a wave. So for the airfoil, = +� on both sides. 5. Is an expansion fan a series of shock waves? (1 student) In eﬀect, yes, but they are “negative” shock waves, with the pressure decreasing rather than increasing as the ﬂow goes through them. Each one is also inﬁnitesimally weak. 6. Does the expansion fan contribute to wave drag, or just the shock? (1 student) Only the shock causes disspation, so in that sense the wave does not contribute to the drag. However, it’s not possible to have only expansion waves on a body in a supersonic freestream — some shocks must be present as well. So there’s always nonzero wave drag. 7. No mud (3 students)