Écoulement irrotationnel

Irrotational flow occurs when fluid particles do not rotate around their axes, resulting in zero vorticity and a velocity field with zero curl. A uniform flow with constant velocity and no velocity gradients exemplifies irrotational motion. This condition requires specific relationships between velocity components, such that gradients in one direction balance those in another, preventing rotational effects throughout the flow field.

When a solid object enters a uniform irrotational flow, the flow remains irrotational far from the object but becomes rotational near its surface. The no-slip condition forces velocity to increase sharply from zero at the boundary to freestream value, creating steep velocity gradients and significant shear stress. In real viscous fluids, this generates a boundary layer with rotational effects, sometimes causing downstream separation.

Pipe flow initially exhibits an irrotational core with uniform velocity distribution at the entrance. As the fluid moves downstream, a boundary layer forms along the pipe walls due to viscosity, creating velocity gradients that develop into a fully developed parabolic profile. Understanding this transition is essential for analyzing steady laminar flow in circular tubes.

Bernoulli's equation applies to irrotational, incompressible, steady, and inviscid flow. In pipe flow, it is valid only in the entrance region and irrotational core where these assumptions hold. Once the boundary layer grows and rotational effects become significant near the walls, Bernoulli's equation breaks down because viscosity effects dominate and the irrotationality assumption no longer applies.

Irrotational flow requires zero curl of the velocity field, which imposes specific conditions on velocity gradients. For example, to maintain zero rotation about the z-axis, the gradient of v with respect to x must equal the gradient of u with respect to y. Similar conditions must be satisfied for other axes. Uniform flow satisfies these conditions because all velocity gradients are zero.

Viscosity causes fluid particles to experience shear stress, which induces rotation. Near fixed surfaces, the no-slip condition creates sharp velocity changes from zero to freestream value, generating steep gradients. These gradients produce rotational motion in the boundary layer, transforming the flow from irrotational to rotational. In inviscid flow, particles would glide past boundaries without this rotational effect.

Vorticity measures the local rotation of fluid particles, and irrotational flow is defined by zero vorticity everywhere in the field. This means fluid particles translate without spinning around their axes. Zero vorticity is equivalent to the curl of the velocity field being zero, a mathematical condition that constrains how velocity components vary spatially throughout the flow domain.