Reply To: Locomotive Exhaust Visualisation

[Martin Johnson]

Well yes but………….
I am not sure if there are any other explanations, but I could not get any sound on the link, but it looks to me as though the Reynolds number is very low in this visualisation – I think we are looking at the Laminar / Turbulent transition as the flow regime changes shortly after the nozzle to the vortex producing turbulent. At low Reynolds numbers, viscous forces dominate and it is viscosity that generates vortices – vortices are impossible in a completely inviscid fluid.

Real blastpipes will be working at high Reynolds numbers, not the lazy flow you see in the video.

Also note that the accepted theory of blast pipe design relies on Bernoulli, Momentum exchange and Conservation of Energy – notice that list does not mention vortices.

The case of the “Jimmy” is also misleading, because the Jimmy is probably reducing efficiency of the system, but such devices also increase back pressure and hence the energy input to the system and hence the overall performance increases. That is not to be confused with well designed multiple nozzles, which will have no or very marginal efficiency drop.

So while the flow visualisation is interesting, I would be wary of reading too much into it.

Martin