solver background

gokul prabu · August 12, 2016, 06:54

i am new to cfd domain, i have a doubt regarding the equations solved behind the fluent when i solve a problem.
when i solve a normal flow problem in a heat ex-changer without thermal condition, with two fluids the solvers shows solving one set of NS eqns in residue graph.
why there is not two set of eqns of for the two fluids, can anyone please help me with this?

LuckyTran · August 12, 2016, 21:12

In Fluent (and many software packages) there is also one set of residuals per each set of equations, i.e. one residual x-momentum, y-momentum, z-momentum.

The residual output is not the equations solved, because the system of equations being solved is highly vast. A 3D simulation of just the NS equations results in a matrix on the order of (4N)^3. It's a pedantic argument to ask why there are only one set of residuals and not one set of residuals for each domain because in reality there is a local residual for each cell, 4N residuals.

The way the residuals are computed are simply some mean over all domains for the sake of convenience.

gokul prabu · August 16, 2016, 08:04

hello luckytran thanks for your reply and information about the solver.
actually what you are saying is the residual graph shows the average residue from solving NS equation in each and every domain.
is this correct?

August 12, 2016, 06:54	solver background	#1
gokul prabu New Member gokul prabu Join Date: Aug 2016 Posts: 3 Rep Power: 9	i am new to cfd domain, i have a doubt regarding the equations solved behind the fluent when i solve a problem. when i solve a normal flow problem in a heat ex-changer without thermal condition, with two fluids the solvers shows solving one set of NS eqns in residue graph. why there is not two set of eqns of for the two fluids, can anyone please help me with this?

August 12, 2016, 21:12		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,674 Rep Power: 65	In Fluent (and many software packages) there is also one set of residuals per each set of equations, i.e. one residual x-momentum, y-momentum, z-momentum. The residual output is not the equations solved, because the system of equations being solved is highly vast. A 3D simulation of just the NS equations results in a matrix on the order of (4N)^3. It's a pedantic argument to ask why there are only one set of residuals and not one set of residuals for each domain because in reality there is a local residual for each cell, 4N residuals. The way the residuals are computed are simply some mean over all domains for the sake of convenience. Far and gokul prabu like this.

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August 16, 2016, 08:04		#3
gokul prabu New Member gokul prabu Join Date: Aug 2016 Posts: 3 Rep Power: 9	hello luckytran thanks for your reply and information about the solver. actually what you are saying is the residual graph shows the average residue from solving NS equation in each and every domain. is this correct?