Velocity Potential

Velocity potential is a scalar function that describes the movement of fluid particles in irrotational flows. In irrotational flow, the velocity field is defined as the gradient of this potential. Since the curl of a gradient is always zero, defining velocity through the potential ensures the flow has no vorticity, making it irrotational.

For incompressible flow, the continuity equation requires the divergence of the velocity vector to be zero. By substituting the velocity potential into this condition, the equation becomes Laplace's equation. This differential equation governs inviscid, incompressible, and irrotational flows in regions like pipe flow.

Cylindrical coordinates naturally align with pipe geometry, accounting for radial distance from the centerline, angular position, and axial distance along the pipe. Laplace's equation in cylindrical coordinates adjusts for these radial, angular, and axial variations, making it ideal for solving steady laminar flow in circular tubes.

Once the velocity potential is determined by solving Laplace's equation with boundary conditions, velocity components are derived by taking partial derivatives of the potential. In cylindrical coordinates, radial, angular, and axial velocity components are obtained through these spatial derivatives, revealing the complete flow pattern.

At pipe walls, boundary conditions enforce the no-slip condition, implying zero tangential velocity. In the central irrotational region, the potential function follows the pipe's symmetry. Solving Laplace's equation with these conditions defines the velocity potential across the entire flow field.

Velocity potential applies to three-dimensional flows and is derived from the irrotational condition. The stream function is specific to two-dimensional flows and results from mass conservation. Both describe flow fields but operate in different dimensional contexts and are based on different physical principles.

In incompressible flow, the continuity equation requires zero divergence of velocity. When velocity is expressed as the gradient of the potential, this condition mathematically transforms into Laplace's equation. This equation is fundamental because it ensures both the irrotational and incompressible conditions are simultaneously satisfied.