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Unformatted text preview: Lecture F1 Mud: Intro Concepts & Deﬁnitions
(40 respondents) 1. Explain PRS #1 (1 student)
For a ﬂuids situation to b e Hydrodynamic or Aerodynamic in nature, the ﬂuid must
b e moving relative to an object. In the PRS question, the water does not move past
the buoy. So that’s an example of Hydrostatics.
2. Why is p = lim �Fn /�A, and not just F /A ? (1 student)
This is just notation convention. The deﬁnition is similar to that of a derivative,
where �A b ecomes dA in the limit. Later in the course we will write dFn = p dA when
integrating all the pressure forces on a wing.
3. If the observer frame changes, wouldn’t p(x, y , z , t) be diﬀerent functions? (6
students)
Very good question. I think I was unclear on what I meant by “same” or “diﬀerent”,
b oth on the PRS question and in the discussion. Yes, the two observers will see
diﬀerent p(t) time traces at each observer’s origin (x, y , z ) = (0, 0, 0), for example. But
at the instant when the two origins cooincide (obesrvers are momentarily on top of
each other), the two p(t) values will be numerically equal. Likewise for the two �(t)
traces. But the V (t) and po (t) traces will not be numerically equal.
4. How can you tell when a scalar ﬁeld is the same for diﬀerent observers? (1
student)
There’s no guaranteed litmus test. You have to look at the physical situation and
decide. This is one job of a physicist.
5. If po is a “pressure quantity”, why does it depend on the observer, unlike
p? (1 student)
It’s partly b ecause po is a somewhat hypothetical quantity which isn’t directly measur
able at a p oint, unless you intentionally bring it to rest (drive V � 0). One observer’s
deﬁnition of “at rest” is diﬀerent from another’s. We’ll cover this more later.
6. In which frame will po b e most useful? (1 student)
Great question! In most cases, po is most useful in that frame in which the ﬂow is
steady. It’s useful to an observer moving with the airplane (steady ﬂow). It’s less
useful to a stationary observer who sees the plane ﬂy by (unsteady ﬂow).
7. No mud (29 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.
 Fall '05
 MarkDrela

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