Waterfall laminar flow to mist
Waterfall laminar flow to mist
(OP)
I was wondering if anyonw knew a formula for the situation when you have free falling water, such as a waterfall to indicate the point where sheet flow becomes droplets?
I know it is a function of flow depth and velocity, but was hoping for some sort of formula to enusre it does not happen.
I am designing a number of free overflow weirs and the landscape architect has requested a "water curtain" effect.
some help would be greatly appreciated.
I know it is a function of flow depth and velocity, but was hoping for some sort of formula to enusre it does not happen.
I am designing a number of free overflow weirs and the landscape architect has requested a "water curtain" effect.
some help would be greatly appreciated.





RE: Waterfall laminar flow to mist
I was thinking it was something to do with Q=VA as the velocity of the water increases with gravity the area has to reduce. But, at what point does the area become small enough that the flow stops being laminar.
Any pointers on an approach to this or where some information might be available would be appreciated.
RE: Waterfall laminar flow to mist
RE: Waterfall laminar flow to mist
BobPE
RE: Waterfall laminar flow to mist
KRS Services
www.krs-services.com
RE: Waterfall laminar flow to mist
I would disagre, I have seen artistic fountains that have controlled streams of water that launch from pipes in laminar flow that do no mist at all over extremely large distances.
BobPE
RE: Waterfall laminar flow to mist
RE: Waterfall laminar flow to mist
RE: Waterfall laminar flow to mist
The depth at the overfall should by 0.715*Yc (Yc being the critical depth of the channel). The length from the overfall that the water will flow is approximately computed by the following equation:
L = 4.30*z*(Yc/z)^0.09
(with z being the difference in the ground elevation of the fall. This can be found in any weir or spillway chapter of a book.)
The length calculation should give you the location of the fall's landing.
RE: Waterfall laminar flow to mist
Everything in fluids that we look at as engineers is related to the Re number, including open channel flow. The Froude number is a measure how well the fluid is moving through a gravity system. The critical depth is a measure of where the fluid in gravity flow will go sub or supercritical. I would think you are right in your thinking that having the fluid exit the system in subcritical flow before it travels may be a way to reduce turbulance, but i am not sure. I think the question was how will the flow respond after it flows as a waterfall which is not channel flow. Re is for gravity and pressure systems so don't be confused, as d = diameter can be substituted for d = depth and can be applied to the free falling water sheet.
BobPE
RE: Waterfall laminar flow to mist
Like many of these forums, the topic being addressed strays from the original question. The original inquiry asked about free overflow weirs and a water curtain. This is an open channel situation. Therefore the Froude number is more fitting within the Energy Equation.
Also, I meant to describe the state of the water passing through critical depth upstream of the weir (about 3*Yc).
That is if it was originally in sub-critical, which is most likely since the question is about landscaping. At the bottom of the fall, of course it is going to have turbulence.
The equation presented would give the distance away from the weir that the water will travel. This will give an approximate location of the location of the water wall from the fall.
RE: Waterfall laminar flow to mist
There I found many interesting sites, such as -
http://www.atlanticfountains.com/architectural.htm
http://www.fountains-direct.co.uk/display.htm
I would contact the manufactures and get info about their items. You can pump a manufacturer's rep for a lot of good info.
I would consider speaking to the architect about ordering an "off the shelf" unit, especially if I never designed one before or paying for a special fountain consultant.
It sounds like the architect has great ideas and not enough budget. I would hate to be the goat if the requested water curtain effect did not occur or unforseen problems occured. Just understanding the underlying physical laws is not enough. Engineering is emperics and this is a rather specialized item.
Clifford H Laubstein
FL Certified PE #58662
RE: Waterfall laminar flow to mist
Start with a level pool upstream and shallow depth over the weir for best results. Use plywood or plexiglass for the weir. Build it a foot wide and to true vertical scale. Experiment with sharp edged versus broad crested weirs. Vary the height to determine the effect of fall distance.