Ramp-Cliffs and Buoyancy Flux Reversal: Connecting mean-field analysis with spatial structure in stratified turbulence

Bragg & de Bruyn Kops (JFM, 2024 Vol 991 A10) demonstrated that in stably stratified turbulence, the emergence of ramp-cliff structures in the density field causes the mean density gradient to aid (oppose) the production of fluctuating velocity (density) gradients. This explains the surprising Prandtl-number (Pr) dependence of the mixing coefficient seen in several previous studies (Riley et al. 2023 JoT 2023 Vol 24). Bhattacharjee et al. 2025 (in review) showed analytically how this same mechanism is also associated with the small-scale reversal of the buoyancy flux which has been observed in flows with Pr>1 (Legaspi et al. JFM 2020 Vol 903). However, the analysis in Bragg & de Bruyn Kops (2024) and Bhattacharjee et al. (2025) mainly focussed on the spatially averaged behaviour of the stratified flow. Given the strong spatial intermittency of stratified turbulence, an obvious and important question arises: how do the dynamically distinct flow regions, identified by Portwood et al. JFM (2016), vol. 807, R2; Couchman et al. JFM (2023), vol. 961, A20 contribute to the averaged mechanisms identified in the analysis of Bragg & de Bruyn Kops (2024)? Are there specific flow regions that contribute to the small-scale buoyancy flux, and are such regions also spatially intermittent? The objective of this work is to evaluate the relative importance of dynamically distinct regions of stably stratified turbulence on the production mechanisms that control mixing rates in the flow by analysing the conditioned flow statistics.