Let's suppose I have a gravity-fed stream of a non-compressible fluid. A vertical pipeline from the bottom of a tank, with a valve inside (controlled to get constant flow), and then after a while (half meter?) i have the ending of the pipe.
Imagine now that i may have some troubles of coherence of the free-flow after the lower end of the pipe: instead of having the flow in the shape of a perfect, laminar trumpet (starting diam equal to the pipe internal diam, then shrinking parabolically), i have formation of droplets out of the main stream.
Question 1) Would I improve the coherence of the flow if i make the ending of the pipe with an angled cut (e.g. 45dgr), rather than a plain, 90dgr. pipe ending? Why? Would this result in a deflection of the free-fall stream direction?
Question 2) What if, from the valve to the open end of my pipe, the internal diameter of the line is tapered rather than cylindrical? (I mean with diameter decreasing towards the end, therefore the angled 45dgr cut being done in a conical pipe, which reduces its diam in the direction of the gravity-fed flow)
In my country we call a 45dgr cut as the "salami cut" so, google will not help me much, other than suggesting me how to fill my next sandwich... I know it's a easy technique to silent any compressible-fluid outlet line, or to improve open-air flow of water, but i lack some analytical explanation about.
Thanks so much for any hint and/or reference to literature
Regards
EF
Thanks but that's out of my budget, both money and timewise (= i should learn a code from zero :-) ) probably less expense to try it with a real-scale model. But i was looking for some literature or sort of.
Hi Enzo
Try rounding the internal sharp circumferential edge at the pipe end. Make it as smooth as you can. First use a file and then a very fine sandpaper, at least No. 200. The issuing fluid jet (a liquid would guarantee better the incompressibility you mentioned) induces flow on the basically static air surrounding the pipe end. This “new” fluid-fluid interaction also affects the jet stability.
Thanks for Your answer R.P.!
If i got your point, the way for a more coherent stream shall be to make the transition from "flow-in-pipe" to "flow-in-air" the more progressive possible? This (i imagine) is to promote a toroidal air-vortex to penetrate somehow in that filleted edge?
A toroidal air vortex could develop in the gap between rounded pipe wall and free jet. However, the gap will be continuously fed with air from the nearby regions. Air will flow into the gap close to the solid rounded wall and then will have its velocity reverted by the issuing jet itself. The inner most part of entering air will be incorporated to the vortex. The outer part would be thrown away along with the jet. You could add some color (tracer) to the nearby air, to visualize the vortex. Nice topic for research! Regarding the original problem (to obtain a coherent jet) I would go, if possible, to a laminar regime to avoid eddies inside jets. Micro eddies will diffuse from the issuing jet core (high velocity) to the jet surface (low velocity) making it rough and prone to splashing.
Thanks a lot! very useful considerations for me. Indeed should be an interesting topic for research... but about this habit of cutting the pipes angled, what do You think is the most suitable academic explanation for such a "practical" habit?
Hi Enzo
The lack of coherence for the issuing jet is also affected by the boundary layer abrupt detachment (flow separation) from the pipe inner surface at the exit. The wall responsible for the no slip/adherence condition vanishes of a sudden. By rounding the pipe internal edge, you are in fact giving it a sort of bell shape, that is, a slightly increasing cross section in the flow direction and consequently a slightly decreasing velocity (assuming constant flow rate). Notice, in this short domain the flow is 3-D and decelerating (negative convective acceleration). A smoother boundary layer detachment would explain a more coherent issuing jet. Very complex even for CFD.
Hi Enzo
As for your question regarding the habit of cutting pipes at 45 degrees, I’ll give it a further thought. Not simple. Very asymmetrical geometry involving profiles of fluid-fluid interaction and boundary layer detachment. What is claimed to be the advantage or purpose?