Prandtl’s Mixing Length Hypothesis postulates that the eddy viscosity in a turbulent flow can be related to the characteristic length scale of turbulence and the velocity gradient in the direction perpendicular to the flow. It assumes that the turbulence consists of large eddies that transport momentum and energy. Prandtl’s Mixing Length Hypothesis is a simplified approach to modeling turbulent flows and is often used in conjunction with other turbulence models to predict flow behavior and performance in engineering applications. The mixing length represents the average distance over which these eddies exchange momentum with the surrounding fluid. Prandtl’s Mixing Length Hypothesis is typically expressed as:

Where:

• νt​ is the eddy viscosity, 
• $l$ is the mixing length,
• du/dy​ represents the velocity gradient in the direction perpendicular to the flow.