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We investigated the flow dynamics of tornado-like vortices, examining the influence of swirl ratio, S, defined as the ratio of tangential to radial momentum at the vortex base, on their structural characteristics. Using a combination of particle image velocimetry (PIV) in a custom-built simulator and large-eddy simulations (LES), we analyzed vortex flows at swirl ratios of S=4.66, 1.25, and 0.33. The results demonstrate that vortex flow characteristics strongly depend on S, with improved agreement between experimental and numerical data when employing flow-based swirl ratio definitions. Vortex wandering was quantified in experiments, and corrections were applied to refine tangential and radial velocity profiles. At S=0.33 and 1.25 in experiments and S=1.25 and 4.66 in simulations, the vortex transitioned from a single-celled to a double-celled structure, with further evolution into multi-celled vortices at the highest swirl ratio, substantially modifying circulation patterns. Proper orthogonal decomposition (POD) characterized the coherent structures governing vortex dynamics and their dependence on swirl ratio, revealing distinct physical features associated with each vortex regime. 

We explored the settling dynamics of vertically aligned particles in a quiescent, stratified two-layer fluid using particle tracking velocimetry. Glass spheres of$$d=4\,{\rm mm}$$diameter were released at frequencies of 4, 6 and 8 Hz near the free surface, traversing through an upper ethanol layer ($$H_1$$), whereHis height or layer thickess, varying from$$10d$$to$$40d$$and a lower oil layer. Results reveal pronounced lateral particle motion in the ethanol layer, attributed to a higher Galileo number ($$Ga = 976$$, ratio of buoyancy–gravity to viscous effects), compared with the less active behaviour in the oil layer ($$Ga = 16$$). The ensemble vertical velocity of particles exhibited a minimum just past the density interface, becoming more pronounced with increasing$$H_1$$, and suggesting that enhanced entrainment from ethanol to oil resulted in an additional buoyancy force. This produced distinct patterns of particle acceleration near the density interface, which were marked by significant deceleration, indicating substantial resistance to particle motion. An increased drag coefficient occurred for$$H_1/d = 40$$compared with a single particle settling in oil; drag reduced as the particle-release frequency ($$\,f_p$$) increased, likely due to enhanced particle interactions at closer proximity. Particle pair dispersions, lateral ($$R^2_L$$) and vertical ($$R^2_z$$), were modulated by$$H_1$$, initial separation$$r_0$$and$$f_p$$. The$$R^2_L$$dispersion displayed ballistic scaling initially, Taylor scaling for$$r_0 < H_1$$and Richardson scaling for$$r_0 > H_1$$. In contrast,$$R^2_z$$followed a$$R^2_z \sim t^{5.5}$$scaling under$$r_0 < H_1$$. Both$$R^2_L$$and$$R^2_z$$plateaued at a distance from the interface, depending on$$H_1$$and$$f_p$$. 

We experimentally explored the effect of single-sidewall cooling on Rayleigh–Bénard (RB) convection. Canonical RB was also studied to aid insight. The scenarios shared tank dimensions and bottom and top wall temperatures; the single sidewall cooling had the top wall temperature. Turbulence was explored at two canonical Rayleigh numbers, $$Ra=1.6\times 10^{10}$$ and $$Ra=2\times 10^9$$ under Prandtl number $Pr=5.4$ . Particle image velocimetry described vertical planes parallel and perpendicular to the sidewall cooling. The two $Ra$ scenarios reveal pronounced changes in the flow structure and large-scale circulation (LSC) due to the sidewall cooling. The density gradient induced by the sidewall cooling led to asymmetric descending and ascending flows and irregular LSC. Flow statistics departed from the canonical case, exhibiting lower buoyancy effects, represented by an effective Rayleigh number with effective height dependent on the distance from the lateral cooling. Velocity spectra show two scalings, $$\varPhi \propto f^{-5/3}$$ Kolmogorov (KO41) and $$\varPhi \propto f^{-11/5}$$ Bolgiano (BO59) in the larger $Ra$ ; the latter was not present in the smaller set-up. The BO59 scaling with sidewall cooling appears at higher frequencies than its canonical counterpart, suggesting weaker buoyancy effects. The LSC core motions allowed us to identify a characteristic time scale of the order of vortex turnover time associated with distinct vortex modes. The velocity spectra of the vortex core oscillation along its principal axis showed a scaling of $$\varPhi _c \propto f^{-5/3}$$ for the single sidewall cooling, which was dominant closer there. It did not occur in the canonical case, evidencing the modulation of LSC oscillation on the flow.