The axial velocity distribution of pipe flow is shown above across a cross-section of the pipe. This applet solves the incompressible Navier-Stokes equation in the presence of a constant axial pressure gradient. The red curve is the theoretical laminar steady-state velocity distribution; the black arrows show the current velocity distribution in the model.

The model is started by clicking the toggle marked "Stopped". Model speed is controlled with the "Faster" and "Slower" buttons. Different initial conditions can be applied with the "Clear" (constant initial velocity) and "Random" (random initial velocity) buttons.

The model starts out in laminar mode with constant viscosity everywhere. Clicking the "Laminar" toggle switches it to turbulent mode. In turbulent mode, the viscosity represents the turbulent eddy viscosity, which increases linearly with distance from the pipe wall.