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Dear Johan,
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<div>The outflow boundary condition, 'O ', is indeed the same as 'I ' for the thermal problem</div>
<div>since they both enforce grad T . nhat = 0.</div>
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<div>However, both also admit thermal flux by advection for cases where U . nhat is > 0, and that is</div>
<div>the intent of outflow --- to allow thermal flux to be carried out.</div>
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<div>For velocity, the situation is a bit more delicate. To leading order, however, 'O ' imposes</div>
<div>d/dn (U.nhat) = 0 and p=0. That is, Neumann conditions for the viscous fluxes and Dirichlet</div>
<div>for pressure.</div>
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<div>hth,</div>
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<div>Paul</div>
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<div id="divRpF973935" style="direction: ltr;"><font face="Tahoma" size="2" color="#000000"><b>From:</b> nek5000-users-bounces@lists.mcs.anl.gov [nek5000-users-bounces@lists.mcs.anl.gov] on behalf of nek5000-users@lists.mcs.anl.gov [nek5000-users@lists.mcs.anl.gov]<br>
<b>Sent:</b> Thursday, October 06, 2016 4:59 AM<br>
<b>To:</b> nek5000-users@lists.mcs.anl.gov<br>
<b>Subject:</b> [Nek5000-users] Open Boundary condition in NEK user Documentation<br>
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<p>Dear Neks,</p>
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<p>I am trying to understand what is actually meant by an "open boundary" in Nek.</p>
<p>Un the user documentation, eqn (4.5) sais that, for the temperature, it is</p>
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<p>grad(T) dot n = 0, where n is the normal to the boundary.</p>
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<p>Since the heat flux is q = - grad(T), I would rather interpret this as a zero flux boundary condition.</p>
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<p>And indeed, the insulated boundary described by equation (4.6) is identical to the open boundary condition.</p>
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<p>Is equation (4.5) a type-O or is the open boundary for the temperature actually the same as an insulated boundary?</p>
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<p>Also: Should not the open boundary condition for the "no-stress forumlation" also be given by the same expression as for the "stress-formulation" eqn (4.2)?<br>
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<p>Best Regards,</p>
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<p>Johan<br>
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