# DSolve Error 3D Navier Stokes (z,r,t)

I am essentially wanting to solve Navier Stokes in the [z,r,t] dimensions (2D+Unsteady Flow) for pressure driven flow. However, I keep getting an error I am unable to resolve in my Dsolve such as:

"To avoid possible ambiguity, the arguments of the dependent variable in u[z,r,t] should literally match the independent variables. "

R = .5;
rc = .47;
\[Mu] = 1;
\[Rho] = 1;

fz = {u[z, r, t]*D[u[z, r, t], z] - \[Mu]*D[u[z, r, t], z, z] +
D[P[z], z] == \[Mu]*(1/r*D[r*D[u[z, r, t], r], r]) -
v[z, r, t]*D[u[z, r, t], r] - \[Rho]*D[u[z, r, t], t]}

fr = {\[Rho]*(D[v[z, r, t], t] + v*D[v[z, r, t], r] +
u[z, r, t]*D[v[z, r, t], z]) == \[Mu]*(1/r*
D[r*D[v[z, r, t], r], r] - v[z, r, t]/r^2 + D[v[z, r, t], z])}
contEqu = {D[u[z, r, t], z] == -1/r*D[r*v[z, r, t], r]};

eq = {fz, fr}

bcs = {u[1, R, t] == 0, v[1, R, t] == 0, u[1, rc, t] == 0,
v[1, rc, t] == 0, u[0, rc, t] == 0, v[0, rc, t] == 0,
u[0, R, t] == 0, u[z, r, 0] == newIC[z]}

**DSolve[{eq}, {u[z, r, t], v[z, r, t]}, {z, 0, 5}, {r, rc, R}, {t, 0,
1}]**


Any help would be greatly appreciated.

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• 1) Your DSolve syntax incorrect; you seem to be using syntax that would be more appropriate for NDSolve. 2) your function v appears once without arguments in fr: it should probably appear as v[z, r, t] instead. Maybe you could start from there. – MarcoB Aug 21 '15 at 16:27

It is highly unlikely that your equations can be solved with DSolve, because they are nonlinear. Instead use, NDSolve To do so, P[z] and newIC[z] must be defined. For now, I set them to zero. Also, boundary conditions must be defined at surfaces, not corners, which I also fixed. Finally, there was one occurrence of v without arguments in fr, which I fixed. With these changes,

R = .5; rc = .47; μ = 1; ρ = 1;

fz = u[z, r, t]*D[u[z, r, t], z] - μ*D[u[z, r, t], z, z] ==
μ*(1/r*D[r*D[u[z, r, t], r], r]) - v[z, r, t]*D[u[z, r, t], r] - ρ*D[u[z, r, t], t];
fr = ρ*(D[v[z, r, t], t] + v[z, r, t]*D[v[z, r, t], r] + u[z, r, t]*D[v[z, r, t], z]) ==
μ*(1/r*D[r*D[v[z, r, t], r], r] - v[z, r, t]/r^2 + D[v[z, r, t], z]);
contEqu = D[u[z, r, t], z] == -1/r*D[r*v[z, r, t], r];
eq = {fz, fr}

bcs = {u[z, R, t] == 0, v[z, R, t] == 0, u[z, rc, t] == 0, v[z, rc, t] == 0,
u[0, r, t] == 0, v[0, r, t] == 0, u[5, r, t] == 0, v[5, r, t] == 0,
u[z, r, 0] == 1, v[z, r, 0] == 1}

{su, sv} = NDSolveValue[{eq, bcs}, {u, v}, {z, 0, 5}, {r, rc, R}, {t, 0, 1}]


produces

Plot3D[su[z, r, 1], {z, 0, 5}, {r, rc, R}, AxesLabel -> {z, r, u}] Define your two functions and provide alternative boundary conditions for more interesting results.

• Thank you for the direction bbgodfrey, that is what I needed to get off the ground. – A Stone Aug 21 '15 at 18:24
• @AStone Glad I could help. By the way, my sample answer generates a warning message that boundary conditions are incompatible. This happens at corners, where one boundary might have a value of one and the other of zero, for instance. Usually, not a problem but sometimes it is. Try to avoid this with your real boundary conditions. Thanks for accepting the answer. – bbgodfrey Aug 22 '15 at 2:00