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ESDU Challenge: Shock Wave

Posted August 08, 2014 12:00 AM by SavvyExacta
Pathfinder Tags: ESDU

In this series from IHS ESDU, challenge questions will be posted for the community. Some questions may require calculations and others are general knowledge questions.

Here's this week's question:

1. A shock wave travels across a lake and meets the entrance of a 2 km long water filled pipe draining from the lake. The first 1 km of the pipe (the lake end) is steel and the remainder is plastic. Which of the following are correct?

a) The wave speed of water in the lake is higher than the speed of sound.

b) A shock is reflected from the pipe entrance and travels back across the lake.

c) The shock wave increases its speed as it passes along the steel pipe.

d) The shock wave slows down as it passes along the steel pipe.

e) The shock wave increased speed again as it passes along the plastic pipe.

f) The shock wave slows down again as it passes along the plastic pipe.

g) The wave speed does not change in the pipe.

And the answer is:

a, b, d, and f are correct. The speed of propagation of pressure waves in contained and uncontained liquids and gases can be found in ESDU 83046.


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#1

Re: ESDU Challenge: Shock Wave

08/08/2014 2:31 AM

I'll buy a B and a G, Vanna.

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#2
In reply to #1

Re: ESDU Challenge: Shock Wave

08/08/2014 2:33 AM

I'm not Vanna. I only sell bowels, as in these challenges.

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#3
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Re: ESDU Challenge: Shock Wave

08/08/2014 4:00 AM

I am in! What about Vanna?

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Re: ESDU Challenge: Shock Wave

08/08/2014 4:42 AM

As far as I know, she is still on "Wheel of Fortune", where contestants guess letters that fill in some phrase or title. The contestants can "buy" vowels to help this fill-in scheme. Her official IRS occupational title is "letter turner", but I forget the 4-digit code for it. I doubt we are each other's type, but she is major cool, and would probably be a conversational blast.

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#5

Re: ESDU Challenge: Shock Wave

08/08/2014 7:52 AM

Question a) is a bit vague. The speed of the shock wave in water is the same as the speed of any sound in water, which is about 4 times faster than the 'speed of sound' in air.

There is no change in 'impedance' of the water in the pipe, so I don't see why there would be a reflection. There will be some diffraction from the edges of the pipe, as there is anytime a wave encounters a boundary; but diffraction is not physically the same as reflection. So I'd say b) is false.

Using my experience with fiber optics, I'd say that aside from my comments on a) and b), the only one I believe is true exactly as stated is g).

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#6
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Re: ESDU Challenge: Shock Wave

08/08/2014 8:09 AM

Yeah, that was worded a little vaguely, but I think that A also is true.

Tsunamis do the same thing. The actual physical wave you see on the shore is a byproduct of the tsunami.

I am going to argue against you on B. Sort of... While it is technically refraction (at least predominantly), you still get a wave front from that diffraction that travels not only backward, but in all directions from the mouth of the tube edge.

This is a case where the wording is not technically correct, perhaps the were asking if the small inlet would provide a reflection, which it will not. However, the edges will (to a small degree) and diffraction will as well to a larger degree.

I agree with you on g. The velocity of the shock wave will not change, but the amplitude will!

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#7
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Re: ESDU Challenge: Shock Wave

08/08/2014 3:30 PM

More about b): It's not really refraction, since there is no change in the medium. You get refraction of light (for ex.) passing from air to water, or water to glass, and so forth. The diffraction at the boundary will create ripples that travel in all directions, that's true. But I'm sticking with my comment that the phenomenon is not, per se, a reflection.

This is assuming that the pipe has a fairly large diameter. It the pipe opening was smaller than the effective wave pulse then yes, it would reflect, just as the metal mesh in the window of a microwave oven reflects the microwaves.

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#11
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Re: ESDU Challenge: Shock Wave

08/08/2014 6:31 PM

No, refraction can be caused when a layer that a wave is propagating against changes direction.

A great example is the corner of a speaker cabinet where the sound wave suddenly crosses the corner a refraction wave is created. Refraction can be quite noticeable and in the case of the speaker cabinet it will interfere with the performance of the sound.

Even a slit where a wave travels through will cause refraction. Try it in a bath tub, or remember the double slit experiment with light?

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#12
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Re: ESDU Challenge: Shock Wave

08/10/2014 2:39 AM

Remember, we're talking a "shock wave" here, which have decidedly different qualities than normal waves.

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#8

Re: ESDU Challenge: Shock Wave

08/08/2014 4:12 PM

I'll buy a) and g)

And say that a shock wave will be reflected directly back from the shore only if the shore is vertical. Otherwise it will be reflected at some angle, depending on the angle of incidence.

The resulting shock wave will indeed be reflected in all directions away from the wall (think doughnut) so would move vertically across the pipe opening as well.

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#9

Re: ESDU Challenge: Shock Wave

08/08/2014 4:40 PM
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#10

Re: ESDU Challenge: Shock Wave

08/08/2014 6:01 PM

OK, I'm not afraid to show my ignorance in public. So I'm gunna say a., d., and e., are true.

So much for pride.

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