Lamb surface
In fluid dynamics, Lamb surfaces are smooth, connected orientable two-dimensional surfaces, which are simultaneously stream-surfaces and vortex surfaces, named after the physicist Horace Lamb. Lamb surfaces are orthogonal to the Lamb vector everywhere, where and are the vorticity and velocity field, respectively. The necessary and sufficient condition are
Flows with Lamb surfaces are neither irrotational nor Beltrami. But the generalized Beltrami flows has Lamb surfaces.