Variable or temperature dependent viscosity
******************* This email originates from outside Imperial. Do not click on links and attachments unless you recognise the sender. If you trust the sender, add them to your safe senders list https://spam.ic.ac.uk/SpamConsole/Senders.aspx to disable email stamping for this address. ******************* Dear Nektar users, I'm using Nektar++ Incompressible Navier-Stokes Solver in fully 3D mode, with added temperature field, in a plane poiseuille flow heated at the bottom wall, solver running very well, and as if it wasn't already enough, I'm asking for more here ! 'hopefully', --> My question is about the possibility to handle variable viscosity or temperature dependent viscosity, So is there any approach to account for temperature dependent viscosity nu(T) ? I'm asking for this, since the temperature gradient at the heated wall is known to alter the viscosity (Ref: Physics of transitional shear flow, A. V. Boiko et al. Sec. 6.5 page 112) which I have to account for when carrying flow transition calculations. --> In case it is not directly achievable through Nektar's functionalities, is it conceivable to direct my effort toward employing an ad-hoc approach modifying the momentum transfer to account for lower liquid viscosity at higher temperature? That is, most likely adding a forcing term which mimics the effect of varying viscosity by altering the momentum transfer accordingly, since the stress tensor for a newtonian fluid tau=mu.du/dy. Any comment on the sanity of this approach ? Thank you all in advance, any help or general guidelines on this model variation greatly appreciated. Regards, Saad
Dear Saad, I am solving two phase flow with Nektar++. In my case, I should alter viscosity with the Phase function nu(phi). I implement variable viscosity by m_var in the HelmSolve function. m_fields[i]->HelmSolve(F[i], m_fields[i]->UpdateCoeffs(), NullFlagList, factors, m_var); that m_var is m_var[StdRegions::eVarCoeffD00] = VarCoeff; m_var[StdRegions::eVarCoeffD11] = VarCoeff; and VarCoeff = nu(phi) and in your case is nu(T). Noting that if temperature vary in the domain with time you should add factors[StdRegions::eFactorTime] = time; before the HelmSolve. This explanation also has been described in the LaplaceSolve (https://doc.nektar.info/doxygen/4.1.0/class_nektar_1_1_multi_regions_1_1_con... (https://doc.nektar.info/doxygen/4.1.0/class_nektar_1_1_multi_regions_1_1_con...)) , see the LaplaceSolve). By using FactorTime, if you do not use the last version of the nektar++, you will encounter memory leakage. This issue was solved in the last version of the Nektar++. Also if the coefficient is time-dependent, the Cholesky decomposition needs to be updated every step. The simulation would become very slow. Best Regards, Mohammad 14 Aabaan 1401 2:01 PM, "Debbahi Saad" <saad.debbahi@gmail.com (mailto:saad.debbahi@gmail.com?to=%22Debbahi%20Saad%22%20<saad.debbahi@gmail.com>)> wrote: This email from saad.debbahi@gmail.com (mailto:saad.debbahi@gmail.com) originates from outside Imperial. Do not click on links and attachments unless you recognise the sender. If you trust the sender, add them to your safe senders list (https://spam.ic.ac.uk/SpamConsole/Senders.aspx) to disable email stamping for this address. Dear Nektar users, I'm using Nektar++ Incompressible Navier-Stokes Solver in fully 3D mode, with added temperature field, in a plane poiseuille flow heated at the bottom wall, solver running very well, and as if it wasn't already enough, I'm asking for more here ! 'hopefully', --> My question is about the possibility to handle variable viscosity or temperature dependent viscosity, So is there any approach to account for temperature dependent viscosity nu(T) ? I'm asking for this, since the temperature gradient at the heated wall is known to alter the viscosity (Ref: Physics of transitional shear flow, A. V. Boiko et al. Sec. 6.5 page 112) which I have to account for when carrying flow transition calculations. --> In case it is not directly achievable through Nektar's functionalities, is it conceivable to direct my effort toward employing an ad-hoc approach modifying the momentum transfer to account for lower liquid viscosity at higher temperature? That is, most likely adding a forcing term which mimics the effect of varying viscosity by altering the momentum transfer accordingly, since the stress tensor for a newtonian fluid tau=mu.du/dy. Any comment on the sanity of this approach ? Thank you all in advance, any help or general guidelines on this model variation greatly appreciated. Regards, Saad
participants (2)
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Debbahi Saad
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m.r.rouhanian＠ut.ac.ir