But the lid doesn't cover the entire top surface. So a part of it (white in the picture) is exposed to air

That makes it a multiphase problem then

Does that change what the k and omega boundary conditions are on that part of the surface? That's my main question here :)

If it's exposed to air you need to explicitly model the air, which means your question doesn't make a lot of sense because you're suddenly going into an entirely new mode of simulation. What you're asking is like asking what incompressible solver you need to model supersonic flow (it's impossible to do this btw). I unfortunately have no experience with multiphase so I can't offer any input there

Thanks for the help! :)

Hi everyone!, I'm facing a little problem trying to simulate the effect of a bluff object on streamlines for a flow of Reynolds number between 1e5 to 1e6, the cavity that "grows" behind the obstacle doesn't seem to change in size with my Reynolds number, I think this might be a problem with my BC's or my fvSchemes files, anyone has faced this problem before?, also wanna add that I'm trying with the k-w SST model but I don't get better results. Sorry if I'm mistaken , I'm pretty much new to CFD in general, Thanks in advance.

What do you mean by cavity? Do you have a pic or some graphs to show what you're observing?

My problem is that for example for these 3 simulations, the cavity behind the object has the same dimensions but the Reynolds number at the inlet is different from one to another ( 1e5, 2,5e5, 1e6), I'm also confused about my BC's because I'm trying to use "free surface" boundary conditions at the top patch ( p = 0, Slip bc's, etc) but I'm not interested in the interaction water - air ( I'm trying to avoid Multiphase simulation), ¿What can be wrong with my set up? ¿Is my approach bad?

I'm not sure if this is a problem but I'm surprised you're using a free-surface. If you can assume that your container is sufficiently tall you can just pretend that there is only the fluid (which I assume is the water) with no air on top and you can just use something like a pressure outlet at the top

My best guess is that your mesh resolution at the cavity might not be fine enough to capture the physical effects that would cause the recirculation zone to lengthen

So, in order to apply "slip" and "atmospheric pressure" BC's at my top patch and "neglect" the air-water interaction I must extend my domain in the vertical direction?, also u said that the cavity length maybe is not a problem but I feel that these phenomena are very unphysical as I expect longer cavity's for much faster flows (bigger Reynolds number at the inlet patch), also does anyone know where can I get more information about BC's for k, epsilon, and omega for this type of situations?, Thanks for ur help!

It depends on the actual physical problem you are trying to solve. If you are trying to validate an experimental model that is exposed to air on the top surface you will likely need to model the air with something like your current approach, unless the experimental model is very deep in the water and the air interface is guaranteed to have no effect on your recirculation zone. If you are instead working on a pure cfd case then you will need to think very carefully about the implications of the BCs you choose (eg a pressure outlet represents an "infinitely" tall domain such that pressure doesn't change at the top surface) Keep in mind that air and water are both fluids. If your non-dimensional flow properties match (things like Reynolds number, Mach number, etc) then it doesn't actually matter what "material" your flow is. If the physical problem allows, you can simply pretend that there is no water-air interface, for example if this was on the sea bed you could safely ignore water surface-air interactions, or if you wanted to simply focus on the Reynolds number effects instead and neglect the air boundary

I'm trying to understand the effect of changing the obstacle geometry and the Reynolds number on the dimensions of the cavity and curvature of the streamlines, I think u made it very clear, I will try to get a better idea of the BC's that I need to use for this purpose, Thank you @User

glad I could help It sounds like you can get away with a pressure outlet at the top but yeah there's a lot of physical implications for each of your boundary conditions. It looks like your project is a variation of the classic backwards-facing step, so there might be some experimental results you can compare to

I will check pressure outlet BC as u said, thank you very much for ur help!

Hi everyone! I'm looking at a shallow lid-driven cavity where I managed to simplify the k-omega equations to these 2 ODEs with 2 additional constraints for p(x) and b(x)

I would like to try seeing what I get with matched asymptotic expansion, by scaling z with \nu (z = Z \nu)

Could someone please help me understand what assumptions I can make in the inner region / boundary layer? I should assume that \nu_T is of the same order as \nu, right? So \nu_T / (\nu + \nu_T)^2 is of order 1/\nu ?

And can I get rid of the last term in 37b in the inner region? Since dk/dz is decreasing and d omega/dz is increasing

Thank you 🙂

eddy viscosity is magnitudes larger than molecular viscosity in a turbulent boundary layer

Thanks @User 🙂 I thought that this was the case in the main flow region, but near the walls that eddy viscosity \nu_T = k / omega would decrease since dissipation is high near the walls

Am I wrong?

(And in the outer solution I set \nu = 0 )

exactly at the wall its 0 but as soon as you move away its not 0 anymore and quickly bigger than molecular viscosity as soon as you reach the end of the viscous sublayer

Well in my matched asymptotics I thought it would be good to look at this inner region where they're of the same order :)

Why would you want to do that exactly?

Well I'm just trying to make some analytic progress with my ODEs above

Seeing how I could deal with the boundary layer

I am hoping to get something like wall functions basically