Pressure calculation

[es335]

Hi.

I need some guidance.

In an air duct with airflow of say 20°C, 1bara and u=20m/s I want to
calculate the effect of a short air pulse of say dp=1000mmWG, dt=1sek
in the opposite direction of the airflow.

How to calculate the travelled distance of the effect and the
magnitude. Where do I start?

Any suggestions are welcome.


Cheers

 

[poetix99]

Rather strange set-up; I think the behavior depends on the details of the ramp rate of the pressure pulse. As an extreme example, if the dp "rise time" was very long, you would very smoothly stop the original flow, flow your system backward and then flow in the original direction again.

My intuition is that if the pulse onset is rapid (step-like), then there will be acoustic waves bouncing around in your duct, even though (p + dp)/p < critical pressure ratio.

For some additional info on this (without being certain that I'm correct), try a keyword search of this website on &quot;Method of Characteristics&quot; or &quot;MOC&quot;

I don't think that you will have much bulk flow upstream after the pulse has ended, but you would have some interesting tranverse velocity profiles (for a while) because of the reversed flow. This is similar to what happens in the main airways to the lungs, with regular flow reversals.