<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div>I may have misunderstood your Q</div><div>Did you mean that c initially has a max = 1 and that mixing will only cause it to go down? </div><div>In this case check mass conservation</div><div>And make sure the initial density is accurately specified and that it gets updated correctly</div><div>Ammar</div><div><br>Sent from my iPhone</div><div><br>On Oct 13, 2013, at 9:01 AM, <a href="mailto:nek5000-users@lists.mcs.anl.gov">nek5000-users@lists.mcs.anl.gov</a> wrote:<br><br></div><blockquote type="cite"><div><meta http-equiv="content-type" content="text/html; charset=utf-8"><div><div style="direction: ltr;"><span style="-webkit-text-size-adjust: auto;">I am not sure that what you are solving for c is a normalized scalar like a mass fraction Y for example </span></div><div style="direction: ltr;"><span style="-webkit-text-size-adjust: auto;">If c is not normalized May be the formulation is equivalent to</span></div><div style="direction: ltr;"><span style="-webkit-text-size-adjust: auto;">C= rho* Y where y is a normalized scalar and rho is the total density..</span></div><div style="direction: ltr;"><span style="-webkit-text-size-adjust: auto;"><br></span></div><div style="direction: ltr;"><span style="-webkit-text-size-adjust: auto;">Finally check mass conservation</span></div><div style="direction: ltr;"><span style="-webkit-text-size-adjust: auto;">By integrating over the volume of the channel, the density equation</span></div><div style="direction: ltr;"><span style="-webkit-text-size-adjust: auto;">d/dt integral(rho) + integral (u. Grad rho) =error </span></div><div style="direction: ltr;"><span style="-webkit-text-size-adjust: auto;">And see how error changes over time</span></div><div style="direction: ltr;"><span style="-webkit-text-size-adjust: auto;"><br></span></div><div style="direction: ltr;"><span style="-webkit-text-size-adjust: auto;">Ammar</span></div><br><span style="-webkit-text-size-adjust: auto;">Sent from my iPhone</span></div><div style="-webkit-text-size-adjust: auto;"><br>On Oct 13, 2013, at 1:46 AM, <a href="mailto:nek5000-users@lists.mcs.anl.gov">nek5000-users@lists.mcs.anl.gov</a> wrote:<br><br></div><blockquote type="cite" style="-webkit-text-size-adjust: auto;"><div><div dir="ltr"><div class="gmail_extra"><br><div class="gmail_quote">On Sun, Oct 13, 2013 at 10:35 AM, <span dir="ltr"><<a href="mailto:nek5000-users@lists.mcs.anl.gov" target="_blank">nek5000-users@lists.mcs.anl.gov</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div>You can also try error function like profile with 0 value at the inlet and increases to a peak somewhere and stays constant up to the exit so gradient is0 at the exit<br>
Insulation normally refers to the diffusive flux (first form)</div><div class="im"><div>Ammar</div></div></blockquote></div><br>Thanks for the tips. I am using a smoothed initial condition for scalar now. However after some time the scalar does not stay within [0,1] which is important. My time step is already small with cfl ~= 0.1. What can I do to get better behaviour of the scalar ?</div>
<div class="gmail_extra"><br></div><div class="gmail_extra" style="">The diffusion coefficient for my scalar is quite small, D = 5*10^(-5).</div><div class="gmail_extra" style=""><br></div><div class="gmail_extra" style="">Regards</div>
<div class="gmail_extra" style="">praveen</div></div>
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