# DyOneEqEddy negative nuSgs

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 March 3, 2009, 19:43 dyOneEqEddy (and perhaps other #1 Member   Michael Roth Join Date: Mar 2009 Location: Guelph, Ontario, Canada Posts: 47 Rep Power: 10 dyOneEqEddy (and perhaps other dynamic models) depends upon a localized ck i.e. nuSgs = ck(D) * sqrt(k) * delta I've found that ck(D) can become negative leading to negative nuSgs. How about bounding nuSgs e.g. in dyOneEqEddy.C after the k solve with somebody like bound(nuSgs_, dimensionedScalar("zero", nuSgs_.dimensions(), 0.0)); Or upstream in the actual calculation of ck. Of course, perhaps we're simply trying to reverse the cascade. Michael

 March 3, 2009, 20:02 Hi Michael, Does the dyOneE #2 Senior Member   kumar Join Date: Mar 2009 Posts: 112 Rep Power: 10 Hi Michael, Does the dyOneEqEddy model calculate one single global constant, ck, or is it a localized costant. If it is local then in the presence of backsactter ck can be negative ( and nuSgs also negative). One the other hand if the constant is a global one, then the backscatter << forward scatter and the chance of the constant going negative is small. thanks kumar

 March 3, 2009, 20:34 Hi Kumar, dyOneEqEddy uses #3 Member   Michael Roth Join Date: Mar 2009 Location: Guelph, Ontario, Canada Posts: 47 Rep Power: 10 Hi Kumar, dyOneEqEddy uses a localized ck. Hmmm backscatter. Good point though I'm seeing far too much of it. I wonder if the ck needs some form of limiting or stabilization. Thanks, I'll investigate further. Thanks Michael

 March 4, 2009, 01:25 Hi Michael, I was looking #4 Senior Member   kumar Join Date: Mar 2009 Posts: 112 Rep Power: 10 Hi Michael, I was looking at the code for calculating ck in dynOneEqEddy.C (i have also pasted it below ///////////////////////////////// Begin Code/////////////////////////////////////////////////////////// dimensionedScalar dynOneEqEddy::ck(const volSymmTensorField& D) const { volScalarField KK = 0.5*(filter_(magSqr(U())) - magSqr(filter_(U()))); volSymmTensorField LL = dev(filter_(sqr(U())) - sqr(filter_(U()))); volSymmTensorField MM = delta()*(filter_(sqrt(k_)*D) - 2*sqrt(KK + filter_(k_))*filter_(D)); dimensionedScalar MMMM = average(magSqr(MM)); if (MMMM.value() > VSMALL) { return average(LL && MM)/MMMM; } else { return 0.0; } } //////////////////////////////// End Code /////////////////////////////////////////////////////////////// ck is average(LL && MM)/MMMM and MMMM = average(magSqr(MM)); // Is this not a single number for the whole domain ? average(LL && MM) // This also appears to be a single number for the whole domain ? Thus ck is just one number for the whole domain. or am i wrong here ?? Any ideas as to how locDynOneEqEddy is different from dynOneEqEddy. I thought the dynOneEqn gives a globalized ck and locDynOneEqEddy gives a local ck. any comments ? Thanks again. kumar

 March 4, 2009, 04:09 Hi Michael, Personally I am #5 Senior Member   Join Date: Mar 2009 Posts: 854 Rep Power: 15 Hi Michael, Personally I am not convinced by the argument that negative nuSgs represents back-scatter; it violates the entropy inequality. This is a big problem with localized dynamic models and the reason why I don't use them. Putting a clip in to stop nuSgs becoming negative is one "solution" but it's not very physical. H hua1015 likes this.

 March 4, 2009, 17:23 Hi Henry, What is the diffe #6 Senior Member   kumar Join Date: Mar 2009 Posts: 112 Rep Power: 10 Hi Henry, What is the difference between the locDynOneEqEddy and dynOneEqEddy ? Thanks a lot Best regards Kumar

 March 4, 2009, 18:03 locDynOneEqEddy uses locally e #7 Senior Member   Join Date: Mar 2009 Posts: 854 Rep Power: 15 locDynOneEqEddy uses locally evaluated coefficients. dynOneEqEddy uses globally averaged coefficients and is hence only formally applicable to homogeneous turbulence e.g. in a box with cyclic boundaries. H hua1015 likes this.

 March 4, 2009, 18:28 Hi Henry Thanks a lot for y #8 Senior Member   kumar Join Date: Mar 2009 Posts: 112 Rep Power: 10 Hi Henry Thanks a lot for your reply. Best Regards Kumar

September 26, 2016, 19:49
negative nuSGS
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louisiana
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Quote:
 Originally Posted by henry Hi Michael, Personally I am not convinced by the argument that negative nuSgs represents back-scatter; it violates the entropy inequality. This is a big problem with localized dynamic models and the reason why I don't use them. Putting a clip in to stop nuSgs becoming negative is one "solution" but it's not very physical. H
Hi Henry,

do you know why we have these negative nuSGS in dynamic models? Thanks.

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