Learning Forum

[Rozpruwacz10000]

Hi, nI'm modelling transient natural convection in closed cavity using ideal gas law. I've noticed that total mass in the cavity is changing with each time step. Could you suggest me what i'm doing wrong? nI have two wall temperature BC's forcing the flow. Operating density value is taken for the average temperature between two walls ((T_hot+T_cold)/2). I'm using floating operating pressure. nResiduals, and other monitors converge to steady state solution.n

[Rozpruwacz10000]

Well, residuals do not converge to steady state I mean that they look fine (at least three orders of magnitude drop in each time step). Other monitors reach steady state. n

[Rob]

How much is it changing by? How are you monitoring the mass?n

[Rozpruwacz10000]

It's changing by 15%. I'm using report type: mass. n

[Rob]

That sounds right assuming it's the volume report. Re-run but with a smaller time step and see how the convergence is. Do you have any other models switched on? Fix the operating pressure too. n

[Rozpruwacz10000]

Reducing time step and fixing operating pressure did not help. n

[Rob]

Do you have any other models switched on? n

[YasserSelima]

How do you calculate the mass inside the cavity?nWhat are the operating Temperatures? nCan you post pressure contours in the cavity?n

[Surya Deb]

Hello, nFloating operating pressure is generally suitable for cases where there is a gradual buildup of static pressure inside a closed domain. In your case, there might be some change in the static pressure during the solution but will there be a build up with time?nCan you turn off the floating pressure and check? Also check your operating density as that is important if you are using ideal gas.nRegards,nSDn

[Rozpruwacz10000]

Thanks for taking interest in my question. nAs to models used - I use laminar flow model and of course energy equation. Material properties such as heat capacity, viscosity, etc are taken as piecwise-linear form property data table. nMass is calculated as Volume-Report -> mass. nIt is my understanding that operating temperature is relevant only if I use Boussinesq model, wright? nI've turned off floating operating pressure and fixed it - this did not help. Operating density is calculated according to guidelines for closed convection (average of maximal and minimal density) and set as fixed. nI should also mention that case is 2D.nHere are my pressure contours. Wall of the cavity is 10mm. Wall to the right has 373.15K, wall to the left 300K. I'm doing this simulation to compare different density models. nnn

[Rozpruwacz10000]

I think I've solved the problem. I've set variable operating density (using function calculating density for average conditions in the domain). I also turned on floating operating pressure. I think that floating operating pressure is necessary to account for the pressure buildup in the domain. That pressure buildup corresponds to ensuring that overall mass in the domain is constant despite changes caused by temperature change. Am I correct? nI have a follow-up question. Can I define this floating operating pressure using UDF? This is needed when I use different density models. I can define my operating pressure using expressions but then I don't see a way to initialize absolute pressure in the domain. With floating pressure I can do that in initialize tab. n

[Rozpruwacz10000]

Also, say that I have two fluid domains, with different initial pressures in them. Can I somehow set separate operating pressure for them? n

[Rob]

With ideal gas as the temperature rises the pressure will also rise as it's in a fixed volume: you say you're using ideal gas? Average density should be unchanged as there's nowhere for the material to go. nFor two fluid domains you can't set different operating pressures: that function has been requested. n