## Fluids

#### Negative pressure problem

• javat33489
Subscriber

Hi all. I have a problem. Help me please. Good knowledge in CFD is required. I'm counting a jet downhole choke. This fitting creates a vacuum when liquid is supplied to it, due to the vacuum, it sucks out the liquid after the fitting. I set the flow rate to 5 liters per second, which is fed into this fitting (red), goes through the radial channels and enters the chamber where it creates a vacuum due to the round flow. There is atmospheric pressure at the outlet of this nozzle (blue) and where the nozzle also sucks out atmospheric pressure (pink). When calculating in the vacuum chamber, it shows negative pressure. Why? What else can be asked? What could be the problem?

• Rob
Ansys Employee

I think you have a swirling flow in the centre section? That's creating a vortex core, and that can create the suction effect. Some CFD images will help explain what's going on: tangential velocity and pressure would be good.

• javat33489
Subscriber

Yes, that's right, there are vortices that create a suction effect and in this place the pressure is -1.5 MPa, this does not happen

• Rob
Ansys Employee

How does the convergence look? What is the operating pressure, remember you're (probably) reporting gauge. Please post some images.

• javat33489
Subscriber

Sorry I can't attach images. The flow rate at the inlet is 5 liters per second, which creates a pressure of 6 MPa. The task of the device is to cause an influx of fluid due to the vacuum in the tangential chamber. Due to this, the liquid should be sucked out of the pink area (see picture). But in the rarefaction zone there is a negative pressure, this does not happen in nature. The mesh is very good, at least 5 elements across the narrowest channels. Ke turbulence model. Convergence is achieved in 300 iterations.

I tried to make a calculation under the condition of cavitation, then there is no convergence at all, the outlet pressure constantly jumps.

• Rob
Ansys Employee

With high swirl you'll need RSM: read the theory on the 2-equation turbulence models. How much mesh have you got in the swirl region?

The insert images button is in the menu below where you're typing the words.

• javat33489
Subscriber

>>With high swirl you'll need RSM: read the theory on the 2-equation turbulence models. How much mesh have you got in the swirl region?

I used K-epsilon. What model would you recommend?

>>The insert images button is in the menu below where you're typing the words.

Images on a working PC. I am not allowed to send them((

• Rob
Ansys Employee

RSM: read the documentation to understand the limitations of 2-equation models in high swirl. Read the various threads on cyclones: they'll give you further advice on what's needed.

OK, are you working with Student, Research or Commercial licences? The former are for learning, so confidential work is against the terms, the latter means you should have access to direct support.

• javat33489
Subscriber

I am writing it here. So that if someone has such a problem, they can solve it. Let's solve it together.

Noticed this feature. If I set the outlet pressure and the place where the vacuum will suck out the liquid. Then there is no negative pressure in the center, the pressure is simply less.

• Rob
Ansys Employee

If you're setting pressure differently, then you're solving a different model. How are you setting pressure in the swirling region?

• javat33489
Subscriber

In a swirling area, I do not set pressure. Only at the outlets of the medium and the places where the suction comes from

In a swirling area, the program calculates automatically

• javat33489
Subscriber

Maybe a non-stationary analysis is needed for the turbulent zone? How to choose the right calculation time and substeps? I think the calculation time is 1 second and the substep is 1e-6

• Rob
Ansys Employee

I think you need to carefully work through how the device works, and then review the model settings.

• javat33489
Subscriber

The jet pump that I expect works on the principle of an ejector. In the area where the liquid is lifted through the constriction, a decrease in pressure occurs, which creates a suction effect for the second liquid. Highlighted in red where the program shows negative pressure.