Secondary circulations in Holmboe waves

A sequence of direct simulations is used to study mechanisms for the growth of secondary circulations and turbulence in stratified shear flows. Five cases are examined, of which four deliver Holmboe waves as the primary instability and the fifth generates Kelvin-Helmholtz billows. Secondary circulations range in strength from weak, laminar motions to turbulence that destroys the parent wave. Processes that drive disturbance growth include shear production via the Orr mechanism, sheared convection in overturned regions, and sloping convection in stably stratified wave crests. Results are compared with previous predictions based on normal mode stability analysis.