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-   -   Asymmetric water free jet study (http://www.cfd-online.com/Forums/main/3623-asymmetric-water-free-jet-study.html)

 Norberto Parreira June 20, 2001 04:03

Asymmetric water free jet study

Dear Sirs, I want to study an asymmetric water free jet discharging into the air (like the one found in Cross-Flow hydraulic turbine), using CFD codes. In my case, the free jet geometry must came out as a result, since it is unknown at the beginning. There are methods for potential flow analysis, but I never found anything for viscous flow. I don't know if there are any CFD software packages that can handle this kind of problem. I emphasize that we don't know the free jet surface shape. It can be shown that the pressure is constant on the jet surface (and, therefore, the velocity is also constant). Although we know the nozzle geometry, the flow calculation inside this region must take into account the shape of the free jet, which is not known at the start. I thank in advance for any help given.

 Kike June 20, 2001 11:09

Re: Asymmetric water free jet study

Dear Norberto

I doubt that the pressure on the interface remains contant when a jet enter into the surrounding atmosphere, even if the air is "at rest".

If the flow is viscous then you will have shear stresses and therefore, a spatial development of Kelvin-Helmholtz instabilities (maybe Rayleigh and Taylor intabilities), which are closely related to a pressure distribution on the inteface.

If you know the geometry of your nozzle just send the water into the computation domain by this arbitrary shape orifice. Track the interface and you will find the shape. That is what we do.

We (our group) works on liquid jets with and without air coflows, but I can't understand your last sentence. "the flow calculation inside this region must take into account the shape of the free jet, which is not known at the start". Please, be more clear if you can, and maybe we can help you a little.

Regards

Kike

 Norberto Parreira June 21, 2001 14:26

Re: Asymmetric water free jet study

 John C. Chien June 21, 2001 17:37

Re: Asymmetric water free jet study

(1). Most applications using Navier-Stokes code require the specification of the boundary conditions and the corresponding locations. (2). In your case, a model must be developed first to take the constant pressure as the boundary condition and the boundary location must be the part of the solution. (3). So, a method has to be developed first to do that. I don't know whether current commercial code can handle it or not. A code which can handle free surface might be a good place to start.

 Jim Park June 21, 2001 20:52

Re: Asymmetric water free jet study

The commercial code FLOW-3D was originally developed for this kind of application. The company, Flow Science, is a sponser of this web site.

It's antecedents, SOLA-VOF, NASA-VOF/2D and NASA-VOF/3D also have a free service capability.

I have no relationship to this company.

 Kike June 22, 2001 05:18

Re: Asymmetric water free jet study

Dear Norberto

OK, I thought you were interested in atomization, but I have not assumed at all you want to discretize all the turbine geometry to study the liquid jet dynamic. In your first message you had written "Although we know the nozzle geometry, the flow calculation inside this region must take into account the shape of the free jet, ...", and I suppose you want to study the same thing I studied in my PhD.: liquid sheets behind thick wall injectors.

Unfortunately, I will can't help you because I don't know how to handle a "constant pressure dynamical liquid-gas interface"; but I would like you help me to understand what is the instability-atomization mechanism you have in your turbine. It is just for professional curiosity.

As far as I know there are several atomization (I know, you had said you are not interested on it): high pressure jets, coflow assited, rotatory, ultrasound resonants systems. In all of them the pressure distribution on the interface is not constant.

Near the outlet, even at zero air velocity, you will have a pressure distribution along the axial coordinate. It allows the die-swel problem (low liquid velocity or high viscosities) in which the shape of the interface depends on liquid outlet velocity and contact angle dynamic. If the liquid outgoing velocity is high enough (I am sorry not to give an order of magnitude) the pressure have a singular behaivour behind the thick wall injector (your nozzle) and that is explained by a tripple-deck structure of the flow at the outlet.

I have studied the liquid sheet instability under the action of coflows with velocities between 5 and 100 m/s. with good agreement with experimental observations. I can confirm you that for low air velocities (and liquid outlet velocity of 1m/s) the pressure distribution on the interface (just behind the nozzle) is soft (low variations) but a sinusoidal function of the dowstream coordinate and, at a length about 10*sheet_thickness, the nonlinear effects turns this distribution into a very complex function.

"Dynamics of thin planar viscous liquid sheet in the presence of viscous gas coflows behind thick wall injectors"; C. Dopazo, E. Lopez-Pagés, N. Fueyo; Session of FN Interfacial and Thin-Film Instabilities III; 52nd. Annual Meeting of the Division of Flow Dynamics APS; New Orleans 1999.

"The instability of a thin liquid sheet between two parallel high-speed gas streams"; E. Lopez-Pagés, C. Dopazo and N Fueyo; V Latin American and Caribbean Congress on Fluid Mechanics; Caracas 2001.

To (maybe) ansuwer your first questions I had used PHOENICS, which is a well know powerfull and accurate commercial code (despite some people in this FORUM think). You can add to it your specific requirements via a wide-purpose subroutine. I can give you more details if you decide for this option.

Besides this, I think any CFD comercial code (FLUENT, CFX) can handle your problem with adaptative grids and high order schemes if you need them. The problem is that they ere expensive in most cases.

Dear colleague, I am sorry if I missunderstood your problem, I was trying to help.

Regards

Kike

 Marko van der Smitte June 24, 2001 07:56

Re: Asymmetric water free jet study

dear Norberto,

Regards, Marko van der Smitte

ps. do you have information about the origination of this asymmetric phenomenon? I suppose that it has to do with a difference in pressure gradients, but do not know the full story.

 A. Hassaneen June 25, 2001 08:23

Re: Asymmetric water free jet study

I have used (FIDAP) code to handle a Free Surface problem like this when I was simulating the fluid flow and heat transfer characteristics of a free liquid jet impinging on a hot plate.

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