[CFDBro]

Hello everyone. I have a question about RAS turbulence boundary conditions related to a suction inlet (i.e. uniform flow velocity out of the domain). While my k omega SST turbulence model converges, my k epsilon implementation blows up (though this might be because this isn't really a free shear layer problem). My y+ is uniform (for now) at about 20. After shadowing these forums extensively, it seemed that k omega SST with the omegaWallFunction, the nutUSpaldingWallFunction, the kqRWallFunction takes care of the wall boundary conditions.

My question is related to the suction inlet (which I hadn't thought of really). I have been using the usual CFD online tool (https://www.cfd-online.com/Tools/turbulence.php) to calculate my k, omega and epsilon assuming my turbulence length scale to be 0.07 the hydraulic diameter and specifying them through turbulentIntensityKineticEnergyInlet (for k), fixedValue (for omega), turbulentMixingLengthDissipationRateInlet (for epsilon).

However this link (https://www.openfoam.com/documentati...alarField.html) says "In the event of reverse flow, a zero-gradient condition is applied" for omega. Now technically in my suction inlet, a reverse flow IS happening. I'm at a loss here - what exactly do I specify for my inlet and outlet BCs for k, omega, epsilon. I have my boundary conditions added here.

U

internalField   uniform 10;

boundaryField
{
    walls
    {
        	type            kqRWallFunction;
		value           $internalField;
    }

    outlets
    {
		type 			zeroGradient;
    }

    inlet
    {
		type 			turbulentIntensityKineticEnergyInlet;
		intensity       	0.02;
		value 			uniform 0.00015;

    }
}

P

dimensions      [0 2 -2 0 0 0 0];

internalField   uniform 0;

boundaryField
{
    inlet
    {
        type            zeroGradient;
    }

    outlets
    {
        type            totalPressure;
	p0              uniform 0.0;
        gamma		0.0;
        value           $internalField;
    }

    walls
    {
        type            zeroGradient;
    }

}

nut

dimensions      [0 2 -1 0 0 0 0];

internalField   uniform 0.14;

boundaryField
{
    inlet
    {
        type            freestream;
        freestreamValue uniform 0.14;
    }

    outlets
    {
        type            freestream;
        freestreamValue uniform 0.14;
    }

    walls
    {
        type            nutUSpaldingWallFunction;
        value           uniform 0;
    }
}

k

dimensions      [0 2 -2 0 0 0 0];

internalField   uniform 10;

boundaryField
{
    walls
    {
        	type            kqRWallFunction;
		value           $internalField;
    }

    outlets
    {
		type 			zeroGradient;
    }

    inlet
    {
		type 			turbulentIntensityKineticEnergyInlet;
		intensity       	0.02;
		value 			uniform 0.00015;

    }
}

epsilon

dimensions      [0 2 -3 0 0 0 0];

internalField   uniform 100;

boundaryField
{
    walls
    {
        	type            epsilonWallFunction;
		value		$internalField;
    }

    outlets
    {
		type 			zeroGradient;
    }

    inlet 
    { 
		type turbulentMixingLengthDissipationRateInlet; 
		mixingLength 0.0378; 
		value uniform 200; // placeholder 
    } 
}

omega

dimensions      [0 0 -1 0 0 0 0];

internalField   uniform 1000;

boundaryField
{
    walls
    {
        type            omegaWallFunction;
        Cmu             0.09;
        kappa           0.41;
        E               9.8;
        value           $internalField;
    }
    inlet
	{
	    type 			fixedValue;
	    value    		        uniform 0.324;
	}
	outlets
	{
	    type 			zeroGradient;
	}
}

Thanks for reading! If you need anymore information, feel free to ask.