 Vortex stretching

Vortex stretching changes the strength of vorticity when a fluid velocity varies along the direction of the local vorticity. For an inviscid, constant-density incompressible flow, the vorticity equation reduces to $D\mathbf{\omega }/Dt=(\mathbf{\omega }\cdot \nabla )\mathbf{u}$. Thus a flow that stretches a vortex in its axial direction can amplify its vorticity without an applied rotational body force.

An ideal thin vortex tube gives a geometric explanation. Incompressibility preserves the volume of a material segment, so stretching its length reduces its cross-sectional area. Conserved circulation then requires a larger mean axial vorticity. Compression reverses that effect. The Burgers vortex adds a competing mechanism: radial momentum diffusion balances the axial stretching and selects a finite core size.

Vortex stretching is a phenomenon in fluid dynamics that occurs in turbulent flows. It refers to the process by which a vortex line, or a thin filament of vorticity, is stretched as the surrounding fluid moves. This stretching leads to an increase in the strength and intensity of the vortex, ultimately resulting in the formation of smaller vortices and a more complex flow structure.