When a partial vacuum is suddenly relieved by unrestricted air at normal atmospheric pressure, is it possible for there to be a momentary high pressure pulse of many 'Bars'. This appears to be the observation of a chartered engineer,(sadly deceased) whose notes I am currently trying to order.His early career was in steam and diesel locomotives and his comments related to vacuum brakes.He seems to be refering to some sort of inertial or percussive effect.
Is this an accepted physical principle,does it have a name and is there any information about it.
Any help gratefully received !

This effect is well known, and it is more pronounced with water than air, because water is harder to compress. Basically, the fluid (liquid or gas, either one) builds up speed as it rushes into the vacuum, pushed by the fluid behind it. When it hits the back wall of the vacuum container, it comes to an abrupt halt. Since it stops much more suddenly than it starts, more force is required to stop it, which translates into a higher pressure.

In the case of water, this effect is known as "hydraulic ram." It is also known to every plumber as "water hammer" and causes loud knocking sounds (and perhaps burst pipes) when water is turned on too suddenly when pipes are empty or turned off suddenly when it is flowing rapidly. In order to prevent pipes from bursting, plumbers install a vertical branch pipe, filled with air and capped. This gives the water somewhere to go, and by compressing the air, it is brought to a more gentle halt.

What you are discussing is the air version of this well-known water effect.

Hope that helps!

--Stuart Anderson

Thanks for that.I have come across working examples of the ram or 'shock syphon', some still operating after nearly 100 years, -wonderful elegant design, but hadnt considered a similar effect in air. I am wondering now which branch of physics would cover it to get a better quantitative understanding of the effect (although it does sound pretty complex from a maths point of view!).This is in order to evaluate the viability of a practical design by the engineer mentioned, which I am struggling to understand.
Richard

That would be compressible fluid dynamics. It is one of the most mathematically complicated parts of classical physics. The basic theory comes from the Navier-Stokes equations, but in many cases, the math is so hard to solve that people resort to tabulating experimental data and fitting curves to them. Often, you must try to find a relevant set of data or empirical curve to work from, and sometimes there is nothing published that is of any relevance to the system you are studying. That's just a warning that it's a tough subject; you may be lucky enough to find that your case is covered by a strong theory or good, relevant empirical data.

--Stuart Anderson

Wow........ many thanks for that rather daunting information.A bit beyond my meagre talents,but I had suspected as much.The enquiry was prompted by a couple of rather intriguing practical designs by said engineer but without any supporting calcs. or descriptions, making them difficult to evaluate.I think it is going to take a fair bit of detective work and perhaps some 'proof of principle ' experimentation to sort out.
Thanks for your contribution.
Richard