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• Turbulent Intensity in steady cases, is it valid?

Posted by on September 9, 2023 at 9:10 am

Good day dear Prof. Baris and collegues,

Turbulent Intensity I equals to u’ (fluctuanting velocity) divided by <U> (mean velocity), but also u’ is proportional to k (turbulente kinetic energy TKE).

We can simulate external flow in steady simulation in OpenFOAM and write into solution the I field viaÂ  functionObject.

My question is, how is it possible to compute I field that must be a time-dependent field in steady simulations in OpenFOAM? Is it valid the I values computed by OpenFOAM in steady simulations? I can guess u’ is solved from TKE, but <U> how is computed?

Thank you,

2 Members · 3 Replies
• 3 Replies
• MarÃ­a Manuela

Member
September 12, 2023 at 12:16 pm

Hello prof. and colleges,

The turbulent intensity as functionObject is defined in (https://www.openfoam.com/documentation/guides/latest/api/turbulenceFieldsTemplates_8C_source.html) as :

template<class Model>

Foam::tmp<Foam::volScalarField>

Foam::functionObjects::turbulenceFields::I

(

const Model& model

) const

{

// (P:p. 183)

// root-mean-square of velocity fluctuations – isotropic turbulence

const volScalarField uPrime(sqrt((2.0/3.0)*model.k()));

const dimensionedScalar U0(dimVelocity, SMALL);

return tmp<volScalarField>::New

(

“I.tmp”,

uPrime/max(max(uPrime, mag(model.U())), U0)

);

}

This is I = uPrime/max(U) at the end… but U max in steady case will be U max in space, also there is no averaging… is it valid to study the I field in steady cases?

Thank you,

• BarÄ±ÅŸ Bicer

Moderator
September 12, 2023 at 12:40 pm

Hi Maria,

steady-state is assumption which is independent of time. So, for a river or wind flow (whatever you are simulating) if you choose to simulate steady-state, then you are assuming that the mean flow stops developing. The fluctuations(turbulence) are kind of superimposed on the steady state flow. So, you are not intended to simulate developing turbulent flows anymore.

As you said U’ calculated from turbulence kinematic energy and U is computed from mean velocity (for steady flow that can be taken as inlet velocity since When setting boundary conditions for a CFD simulation it is often necessary to estimate the turbulence intensity on the inlets)

Best

• MarÃ­a Manuela

Member
September 12, 2023 at 12:48 pm

Thank you!