ACUSOLVE

Altair Forum User

I am facing problem with boudary layer.

When I am not maintaining BLs result is converging easily but showing wrong result. Only55 to 60% accuracy I am getting.

When I am maintaining BL, result is not converging. The values are fluctuating so much,

Actually modal is EGR coolar and has two flow i.e. coolant & Exhaust gas.

In first trail, I solved with no BL, I got 60% accuracy. I got result trendy at very low time step 60, and I solved it for ten days till TS 1000, but result was same. There is no change.

 

In 2nd trail, I am solving with BL at wall seperating coolant & gas. But here, result I am not getting trendy atleast. 300 TS, already completed, still no result.

 

Solver is also very slow. Taking too much time. For each TS it is taking nearlly 20mins.

 

I have attached inp & log file please find it.

Please do needful as I wasted nearlly 1 month on it.

 

Regards

Rajendra Prasad

BorgWarner Emission System

Unable to find an attachment - read this blog

ydigit

CFD solvers in general are mesh dependent. Depending on the mesh (or BL), the solver ends up capturing or ignoring certain phenomenon. Since a lot is happening in BL region and you do not have enough nodes there, solver is unable to simulate those and ends up simplifying that and hence the convergence. 

In case you solve with BL, solver is capturing more physics and hence the issue with convergence. 

 

As per your input file, you are solving till 5000 iterations for steady state. Default for AcuSolve is 100 iterations for steady state.  So 5000 is obviously an overkill. If you are not getting converged solution in 200 iterations, then you are unlikely to get converged solution anyways. AcuSolve is FEM based solver and has much more happening in the background per iteration than FVM based solvers. Hence the default 100 and I would recommend typically not going beyond 200 iterations. 

 

There are bc_warnings being generated. That needs to be checked it those are for fluid nodes or for solid. Solids are ok, fluid nodes need BC assigned to them. 

 

Not sure why used absolute_temperature_offset         = 353.15        # K
    That would offset all values of by 353.15K. Remove that unless you are sure. 

 

By specifying, the inlet and outlet temperature, you are creating an overdefined system and any solver is bound to be instable for such a case.

SIMPLE_BOUNDARY_CONDITION( 'GasOutlet' ) {

    mass_flux                           = -0.0166666666667# -60.0 kg/hr
    temperature                         = 0.0           # K
 

There are lots of issue with your case. It would be best to probably talk to your local Altair support and arrange a webmeeting to clarify all these issues.