Lecture F17 Mud: Oblique Waves 1. In the 1st PRS, why did the streamlines have to become horizontal and parallel to the wall again? (2 students) Since each streamline sees the same two shocks, they are all turned the same amount. So if they all started parallel, they must end up parallel. Also, they must all end up V · ˆ parallel to the wall, since we must have n =0 at a wall. 2. How do you tell what kind of shock will occur (oblique, bow, normal)? (1 student) Mainly by the ± , M 1 combination. The M 1 determines ± max . Then if ± < ± max , we’ll get an oblique shock. If ± > ± max , we’ll get a bow shock. A bow shock is actually a combination: it’s a normal shock at the frontmost point, and becomes an oblique shock out in the ﬂow. 3. What makes strong shock waves unstable? (1 student) Perhaps “unstable” was too strong a term. It can be set up inside a supersonic wind tunnel. However, a strong shock on a body in an external ﬂow can very easily jump ahead of the body and form a bow shock. 4. Is a bow shock desirable or undersirable on a vehicle? (1 student) It’s bad if you want low drag, like on a supersonic aircraft. It’s good if you want high drag, like on an orbit re-entry vehicle.
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