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<p style="margin-top:0;margin-bottom:0">Dear Johan,</p>
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<p style="margin-top:0;margin-bottom:0">I guess I'm confused about the "open boundary" part of your request.</p>
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<p style="margin-top:0;margin-bottom:0">Do you anticipate flow leaving through this part? Or no flow coming in?</p>
<p style="margin-top:0;margin-bottom:0">The reason I ask is that it's generally unstable to have any flow coming into</p>
<p style="margin-top:0;margin-bottom:0">the domain through an open boundary.</p>
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<p style="margin-top:0;margin-bottom:0">Aside from that, there are some technical issues to deal with. You need to</p>
<p style="margin-top:0;margin-bottom:0">change the boundary conditions, as Nicolas pointed out, but you also need to</p>
<p style="margin-top:0;margin-bottom:0">recompute the preconditioners that are set up at the beginning of the computation</p>
<p style="margin-top:0;margin-bottom:0">because they depend on the type of BC that is employed.</p>
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<p style="margin-top:0;margin-bottom:0">I think the easiest and best approach would be to evolve your initial condition to</p>
<p style="margin-top:0;margin-bottom:0">the point where you want to change the BC and then stop the calculation and </p>
<p style="margin-top:0;margin-bottom:0">use the second BC option from that point (i.e., by using a different rea file for</p>
<p style="margin-top:0;margin-bottom:0">the second part of the computation, or by applying Nicolas's suggestion in</p>
<p style="margin-top:0;margin-bottom:0">subroutine usrdat2, which is called before the preconditioners are established).</p>
<p style="margin-top:0;margin-bottom:0">Note that you can have your inflow BC be time dependent -- flow turned, then flow</p>
<p style="margin-top:0;margin-bottom:0">turned off -- which is all Dirichlet, and you don't need to start/stop the calculation.</p>
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<p style="margin-top:0;margin-bottom:0">hth</p>
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<p style="margin-top:0;margin-bottom:0">Paul</p>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Nek5000-users <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> Wednesday, April 11, 2018 7:08:12 AM<br>
<b>To:</b> nek5000-users@lists.mcs.anl.gov<br>
<b>Subject:</b> [Nek5000-users] Change boundary condition type with time?</font>
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<p>Hi Neks,</p>
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<p>I wonder if it is possible to have boundary conditions that chage type with time?</p>
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<p>I want to simulate a flow where I make a 'puff' of fluid through a hole in a wall.</p>
<p>After the puff is made it continues to evolve without beeing forced.</p>
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<p>Mathematically, this is what I want:</p>
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<li>Dirichlet boundary conditions (ux,uy,uz) = (0,0,1) for the velocity at a part B of the boundary up to a time T.</li><li><span>Open boundary condition for the velocity at B for time > T.</span></li></ol>
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<p>Is this possible to implement?</p>
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<p>Best,</p>
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<p>Johan<br>
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