**Peter J. Diamessis **
*Dept. of Applied Mechanics and Engineering Sciences University of California, San Diego La Jolla, CA 92093-0411, USA*

**Keiko K. Nomura**
*Dept. of Applied Mechanics and Engineering Sciences University of California, San Diego La Jolla, CA 92093-0411, USA*

### Abstract

The fully coupled, triadic interaction of vorticity ω, rate-of-strain S, and scalar (density) gradient *G* Ξ ∇_{ρ} in stratified sheared turbulence is investigated. Results of direct numerical simulations (DNS) of homogeneous sheared turbulence with uniform stable (supercritical) stratification are used in the analysis. Two cases are considered: HB-NISF, in which there is no initial fluctuating *G*, and HB-ISF, in which strong initial fluctuating *G* is present. The triadic interaction at the primary level involves the direct coupling of paired mechanisms. Interaction of ω and S is characterized by vortex stretching and locally-induced rotation of the S axes which are both influenced by mean ω and S. At early time, S axes rotation is enhanced in HB-ISF due to baroclinic torque generation of ω by initial *G*. In HB-NISF, S axes rotation is impeded. In time, the behavior of HB-ISF becomes similar to HB-NISF and shows limited vortex stretching. Interaction of ω and *G* involves an inherent negative feedback between baroclinic torque and reorientation of *G* by ω. The initial *G* in HB-ISF causes baroclinic torque to act as a source of ω early in time. Later, the inherent negative feedback in HB-NISF is established and baroclinic torque becomes a sink. Interaction of S and *G* is characterized by a positive feedback between differential acceleration and scalar gradient amplification by compressive straining. In HB-ISF, this interaction is strong due to the high amplitude of initial *G* and results in persistent *G*^{2}. In general, the behavior of ω, S, and *G* in HB-ISF tends towards that of HB-NISF as the flow settles into a similar state of decay.