[piyusharas]

I have done all calculation in fluent but i think i need for accuracy.
I searched a lot a found that my increasing quality of mesh i can get better results .can anyone tell me how to improve quality of mesh.
Is there any other ways to improve results.
I already have done calculations on second order pressure and momentum.
My model only involves mass equation no energy equations used.

[CeesH]

Dissolve your problem in some water, beat a horsehair-stuffed saddle a couple of times, and then dilute your dissolved problem by adding a drop of it into a bucket of water 30 times in succession.

In case that does not solve it, please provide us with some more information about the problem What do you want to solve? Which equations do you use for flow. In what way is the solution not accurate enough, what do you compare it with? Because with the current information, we cannot give a clear answer - only some rough guidelines, but you seem to be aware of those, and they only help in case your mesh is indeed not fine enough.

Best,
Cees

[piyusharas]

first line for hell of a sarcastic statement.
by the way some pictures of model to be clear enough what i am talking about.
https://drive.google.com/file/d/0B9c...FSbGhhYlU/view
https://drive.google.com/file/d/0B9c...JhaFlXTkk/view
https://drive.google.com/file/d/0B9c...g2T3Z0Q2M/view

here i have given inlet of 12lpm and in rest 6 pressure outlets i am getting some mass flow rates. I am also doing the same set up experimentally and the values are not matching so i think the mess quality has something to do with it as I have not done anything other than setting mesh sizing from coarse to fine.
My model only involves mass balance equation . I have given some inlet and i have to find mass flow rates in outlets.

Thank You for help....

[CeesH]

I just like to do some Homeopathybashing every now and then Let's call it an outlet of FLUENT frustrations.

Anyway, to be serious; your simulation is more complex than it may seem at first glance. I have no idea of the size of the domain, but guessing it's not too large, you have rather small pressure differences between the outlets which completely determine the outflow.

I think there are a few things you need to take into account;
- since the pressure difference between the outlets depends mostly on the outlets themselves, you will need a rather fine mesh around these outlets. Preferably one that allows you to resolve the velocity profile completely (without using wall functions if you use turbulent flow)
- So second, is your flow turbulent and which model do you use for turbulence?
- third, is your phase single or multiphase? How is this in reality, is the tank completely topped off with liquid (such that gravity does not play a role if the exits are in the bottom), or is there air in there as well? If so, the situation may become more complex.

[piyusharas]

My tank is not too large, its dimensions is 270*270 mm and the slit openings is of 1 mm width . My inlet is of 1 inch diameter.

I have define not a good mesh. I just have changed the sizing of mesh to fine and smoothing to high. What else should i do to make a better mesh?

I have considered a laminar model so i didn't use any concept of turbulance.

My flow is single phase. I am using water as liquid that to at room temperture.
And the thing about whether tank is full or not that will depend on what flow rate i keep. Will there be different cases if it is full or not. Will it not be automatically calculated by changing the flow rate.
Currently i m working with low flow rates so it wont be full but as I have filled fluid domain to full in the tank i.e. i have choosen upper layer of water and top layer of tank to be one of interface surface. Have i done wrong considering the upper surface at interface.

[CeesH]

With such dimensions, I'm not too sure about a lack of turbulence in your domain. In fact, with water, I'm pretty sure there will be turbulence.

Use inflation layers around your outlets to get a more accurate solution in that region. use local refinement too. Do you need 3D? Maybe you want to consider 2D - even though this will result in a different inlet geometry. Make sure you use a hexahedral mesh, which should be possible with your geometry.

And yes, of course it matters whether the tank is full or not. If the tank is not full, and you are considering the full tank with inlet geometry, your problem is not single phase anymore - and you need to specify a multiphase problem (volume of fluid) and include the gasphase. Otherwise, the tank is just assumed to be full, and the total mass flow rate out will always balance the inflow.

Furthermore, it matters what the pressure and density outside of the tank are. It matters wheter the exiting liquid falls into air, or is submerged in water. I have no experience with modeling such flows - perhaps you need to include part of the room below your tank to actually model this accurately. But I cannot be sure about that, perhaps someone else can.

[piyusharas]

Yes my model is 3D.
And I am confused the thing about multiphase.Are you sure I will have to use multiphase model. Won't my outlet will always be equal to inlet , I am talking about steady state condition. I have not used multiphase model. Can you please tell me something about multiphase model.
My whole tank is kept in atmosphere only so exiting water is water falls in air only.

[CeesH]

Well yes, in steady state the in and outflow are the same and the liquid level is constant. But you do not know that liquid level a-priori because it depends on the balance of the static head and pressure drop over the outlets. So it may be that there is air in the tank as well, and you need to account for that, unless you are sure the tank is full (or you do know the height a-priori, and model the top as a free surface. But then modelling the inlet is a new challenge)

Regarding outflow, as said I have little experience with such flows. I hope someone else can help you out there. Or maybe there are tutorials on tank drainage by gravity.

Best,
Cees

[piyusharas]

Ok I got the point and I think you are very much correct about the model to be multiphase. But now what should i do. I think you saw how I have made my model and how I have filled it to make it water domain. It is kind of small cylinder kept on a cuboidal block and I have named upper part of cylinder as inlet. But now as you are saying I have to do multiphase . So what I will have to change? can i make only the cuboidal part as multiphase.?

Thank you for giving your time.

[piyusharas]

I am still confused about the VOF model.
I read about VOF model and i don't think it will be used as in steady state volume of air is still and not moving .