I introduced a velocity value of 3.54E-5  m/s in the momentum source term. After simulating for 10 seconds, the velocity magnitude ranges from 2.31E7 to 1.27E9 m/s, which should be the output of the Navier-Stokes equation. Before I introduced the velocity, the velocity is zero. My values for Dxx, Dxy, and Dxz are 1E18 and mu = viscosity = 1E-3. What is causing this velocity explosion? See the code below.

vel = .0000354;

source  = -(Dxx*mu*vel + Dxy*mu*C_V(c,t) + Dxz*mu*C_W(c,t));

dS[eqn] = -(Dxx*mu);

Can I get access to the code that solves the equation?