Re: Pressure Wave Reflection

Sep 12, 2003 2 Replies
E

Should be in the FAQ


I would be glad if an expert could advise on the following.
>
> Suppose that we have an anular tube and in the middle of this tube a
> needle which section increase from zero to a certain diameter.
> A flow of air is introduced inside the tube at a supersonic speed.
> The shape of the needle is designed in order to avoid any crossing of
> the pressure wave in order to avoid the formation of a pressure shock. >
> Can someone tell me what happens with the reflection of each pressure
> wave on the wall of the tube ? Is there any formation of pressure
> shock due to the crossover of a reflected wave with a pressure wave ?
> Is there a danger of instability ? interferences ?
>
> Thanks
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E

Since no experts stepped forward, I will try to answer.

If there is supersonic flow in a tube, any disturbance, such as the needle you describe, will make a cone shaped shock wave. The shock wave will reflect off of the walls over and over again downstream.

This will also occur if the tube is just a boundary between air masses of different density. This causes the thrust diamonds sometimes visible in the supersonic exhaust of jet engines and rocket motors. As the shock waves reflect off of the cold air boundary, the waves collide in the center of the tube, heating the air to incandescence.

Supersonic wind tunnel usually have porous walls in the test section with a vacuum system to draw off air. This helps to reduce nuisance shock wave reflection off of the walls.

Lazy B Engineer

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J

And the related answer This should be in the FAQ.

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