Equations of State for Water in FLUENT
 

HI All,
I am currently constructing a "flow over sphere" scenario that required a Re of ~10. I have the box with sphere set up but I need an equation of state that best fits water. I am a novice to the entire program and I am finding it difficult to choose the correct EoS. Thank you for any help you can provide I am deeply appreciative.

Hello!

Reynolds number for your problem is very little, and so is the freestream velocity. One should use eos for compressible fluids if M>0.6 (for example, ideal gas eos). That means for your problem it is enough to use incompressible fluid eos.

Regards!

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Sorry I should have specified more; it is for compressible liquids. What other information should i provide? I just want to make sure you get the whole picture.

Why do you need compressible liquid eos? For this velocity there is no compression at all.

What do you want to find out with this calculation? Maybe the pressure on the wall or stresses? Boundary layer separation? Is it a tutorial for you?

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I am using it to measure the resulting flow of a sphere as a laminar flow rolls over it. A Re of ~10 seems to mimic the shapes generated by the objects we observe (in superfluid He). I can also alter the temperature as well in order to increase the velocity of the fluid. I hope this helps explain my predicament.

Why don't you use liquid helium material instead of water? It's built in fluent.

You can investigate how eos choice influences the flow pattern for such low velocity value. There must be no difference between these regimes.

Why do you think there are reasons for superliquid helium to show compressibility effects?

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My Lab's data indicates that superfluid helium functions as a compressible fluid. We are also trying to run similar experiments in water at room temperature so I am trying to run a simulation for that as well. Is the Re too low for water?

As Re = rho * U * L/ mu = 10, so for sphere radius R = L = 0.1 m we get U = 0.0001m/s. The regime of the flow will be 100% incompressible. Do you need it?

Is it possible for you to conduct an experiment with such low velocity? Remember, that for experiment one should model not only Re, but also M.

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Talking about data please tell also about the conditions of observation. For example, you can observe air flow with M=0.8 - the air is compressible. You can also observe the wind blowing over a house - the air is incompressible.

Or do you mean the superliquid helium is always compressible (with no dependence on the conditions)? I'm not specialized in such special fluids as yours

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Let me give you the background to the experiment. Immersed in of a pool of liquid helium, a metal target (copper) is hit with a laser causing metal particles to explode off the target and travel into the helium. The same experiment is also performed in water and I am trying to model the hydrodynamic effects these particles exhibit when traveling through both the water and superfluid helium (respectively).
These metal particles are micrometer-sized in both experiments with the ambient temperature of the helium experiment at ~2K and 298K for the water experiment. From the images taken from the experiments, we have determined that the Re should be around 10. We also know that both fluids are compressible as we use shadowgraph imaging to visualize the effects these particles have in the fluid.
In Fluent, I have constructed a box containing a sphere (all micrometer-sized) with liquid flowing from an inlet on one face of the cube projecting fluid onto the sphere. I hope this information helps and thank you for your insight.

Sounds interesting.

Water IS compressible, but to show its ability to compress in a flow around a body you need to accelerate the stream velocity to 0.6 Mach number or more. That is 900 m/s. Your velocity value is muuuch lower, so the incompressible model will work fine.

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If that is the case, which equation of state should I use?

In fluent choose constant density in properties of the material.

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Ok I'll try that. Sorry to ask again but which equation of state should I use? (e.g. Redlich-Kwong)

You should use the constant density equation (also known as the incompressible fluid equation). Redlich-Kwong is used for modeling real gases at very high pressure - you don't need it for you case.

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Oh, I see! I'll give that a shot and let you know what I find. Thanks Again!

You are welcome [emoji106]

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Hi,
Sorry to bother you again. I have finished running the simulation and it was successful but now the focus of the simulation has changed. How would you model super-cavitation of a sphere moving at 1500m/s in water at room temperature? Which equation of state would you choose and would you make the water compressible?

Hello! In the very beginning I would try to find out something about similar researches, because I never tried to model water flowing at such high velocity. I think ideal eos would be enough. But that's only my opinion. You should read articles about problems like this.

Regards!

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