Vortex cutting by a blade

What is it about?

When a blade chops through a vortex structure, an interacting and complex sequence of physical events occurs. The vortex lines in the vortex are cut and reconnect to those within the blade boundary layer. The ambient axial flow within the vortex is also cut by the blade, leading to differences propagation of waves on the vortex both upstream and downstream of the blade and a transient lift force on the blade. This study examines a series of models for the blade lift force due to vortex cutting, and compares predictions of these models to results of numerical computations for the vortex cutting process.

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Why is it important?

Wind turbines, helicopters, hydraulic pumps, and ship propellers all operate in a turbulent environment, in which turbulent vortex structures are entrained into the rotor slip stream and chopped by the rotor blade. This is of particular importance for rotors operating in the wake of other rotors, such as the tail rotor of a helicopter or a wind turbine in a wind farm. The vortex-blade interaction leads to unsteady forces on the rotor, which can cause vibration and fatigue on machine components. This paper provides an explanation of and a simple expression for the transient vortex-cutting force on the blade, which can be then used in later studies of machine fatigue or noise generation due to vortex-blade interaction.