[roi247]

Hello all,

I am simulating hydraulic jump in a circular open channel (partially full pipe).
geometry
geometry.jpg
Model: standard k-e, implicit VOF, open channel sub-model

pressure inlet and pressure outlet with water surface level specified. velocity is also given at the inlet.

initialize with 0 velocities and then patch the upstream channel and downstream channel separately. an upstream channel region is marked with upstream depth before jump, downstream channel region is marked with depth after the jump (Both velocity and water volume fraction are patched based on the calculations, respectively, upstream and downstream) As shown in the figure below

Capture.PNG

CASE 1 a pseudo-transient steady flow simulation has been implemented. automatic timestep with timescale factor 0.1
steady.jpg
(continuity k epsilon VOF cannot converge to 10e-6; currently after 20000 iterations it is 10e-4. )
CASE 2 a transient (unsteady flow) simulation, with timestep calculated based on courant number
transient 2020-34.jpg

(last time step after approximately 10 flow-through times based on the steep channel velocity, it should be longer time but it does not seem to have a tendancy to back up from the outlet)
As we can see from the figures above, both the transient and steady simulations show the jump reluctantly right before the pressure outlet, where we specified the water surface. However, it does not sense the change of channel bottom slope at all and keep flowing supercritical.

could you give some suggestions to let the jump happen at the turning of channels?

Thank you

Rui

[vinerm]

I do not have much idea about hydraulic jump, however, if it is supposed to happen with the conditions you are simulating, then you might be able to test the conditions for it to exist. So, I suppose you are checking the variation of Fr because you mentioned that the flow remains supercritical. What is the initial Fr number, i.e., of the upstream of slope change? And at what rate is it changing?

Few other important things are operating pressure reference location and operating density. This would change the pressure gradient significantly and affect the solution.

[roi247]

Hi Vinerm, thank you for your reply

every flow in an infinitely long channel tends to become the normal flow condition (velocity, flow depth, cross-section does not change with time and location). The related depth we call it normal flow depth

therefore, in this case, we assume the flow rate Q is the same through the channel (done by velocity inlet with specified water surface), after changing the slope S of the channel, the normal flow depth will change accordingly. that is the reason why we have higher normal flow depth yn with a smaller slope and lower normal flow depth with a larger slope. a subcritical flow downstream, a supercritical flow upstream.

for my simulation, I only know the two theoretical slopes and the respective normal flow depths. I am expecting something like this to happen .




However, as you can see from my post at the beginning. My jump seems to be forced to generate by the outlet water surface instead of the change of the slopes.
it is as if I put a weir to adjust my water surface there.

####
yes, I remembered that part, you helped me with the set up of operating density and pressure reference location in my open channel expansion case. THere ore I am still using the highest point in the domain and air density as the input.

[vinerm]

Thank you, Rui. The important part would be to understand what causes this jump. Why would Fr reduce while the flow is going downward in an open channel. As long as there is steep slope, velocity will increase, i.e., Fr will increase. Since the flow rate is same, higher velocity implies smaller depth. As soon as water hits the mild slope, its velocity has to drop. But what could be the reason for that? One, as it happens in water hammer, is the compressibility. If compressibility of water is important for hydraulic jump to occur due to slope change, then you have to model water as compressible liquid. Option is available in Fluent. Second reason is change from turbulent flow to laminar. This happens in case of water jet impinging on a flat plate. Until unless the reason is known, it would be difficult to make it happen. So, a little more study might be required/