problem with numerical solution

moving_domain/linear_growth.py

@@ -8,7 +8,7 @@ df.set_log_level(df.LogLevel.ERROR)
 df.parameters['form_compiler']['optimize'] = True
 
 # parameters
-tmax, dt, L, dx, b, m, D = 10, 0.01, 1, 0.01, 0.01, 2, 0.01
+tmax, dt, L, dx, b, m, D = 20, 0.01, 1, 0.01, 0.1, 2, 0.1
 Nt = int(tmax/dt)
 Nx = int(L/dx)
 
@@ -28,7 +28,7 @@ c_array = np.zeros((Nt+1, Nx+1))
 c_array[0] = c0.compute_vertex_values(mesh)
 
 # velocity 
-v = df.Expression('b/(L + b *t)*x[0]', b=b, L=L, t=0, degree=0)
+v = df.Expression('b*x[0]/(L + b *t)', b=b, L=L, t=0, degree=0)
 vh = df.project(v, function_space)
 
 # form