Direct numerical simulations are conducted in this study to investigate fully developed turbulent flows in spanwise rotating ducts with different aspect ratios. The global friction Reynolds number Re-tau is fixed at 180, while the global friction rotation number Ro(tau) varies from 0 to 10. The duct aspect ratios (AR) considered include 0.5, 1, and 2. The objective of this study is to explore in depth the kinematic and dynamic characteristics of secondary flows under different rotation numbers and aspect ratios, as well as their impact on the flow physics of wall-bounded turbulence. The results show that as the rotation rate increases, the secondary flow structures in both square (AR = 1) and AR = 2 ducts follow an evolution path from corner vortices, through additional vortices, to large-scale circulations dominated by the Coriolis force, although differences exist in the generation, development, and decay processes of each structure. In contrast, the AR = 0.5 duct directly evolves from corner vortices to Coriolis-dominated circulations. Analysis of the secondary flow intensity indicates that the expansion of the spanwise scale allows the secondary flow to fully develop, and the secondary flow intensity in the duct is significantly enhanced. The study further confirms that the secondary flow in rotating ducts is mainly driven by the Ekman layer, and the Ekman layers in ducts with different aspect ratios exhibit similarities. Turbulence statistics and transport analysis further reveal that the square duct with a smaller AR shows higher sensitivity of mean velocity and turbulence fluctuations to changes in rotation number, with more pronounced suppression effects by rotation, while the secondary flow significantly enhances the local turbulence intensity and transport capacity.
Numerical simulations of axial flow in a rectangular channel with two parallel fins were conducted using the Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations coupled with a Reynolds Stress Model (RSM). The primary objective of this study is to investigate the formation of quasi-periodic coherent structures and their significant role in fluid transport across the inter-fin region. Computational Fluid Dynamics (CFD) results were validated against experimental data available in the literature, specifically for Strouhal numbers and the spacings of coherent structures in the five studied test sections, with a ratio of fin height to inter-fin spacing ranging from 2.5 to 7.5. The Reynolds number, based on the hydraulic diameter of the channel and the bulk velocity, was held constant at 40 & times; 10(4). Velocity profiles and contours were analysed to capture and describe the presence of coherent structures within this family of finned rectangular channels.
We derive a two-equation model for the energy balance in statistically homogeneous turbulence. The present formulation is expressed in terms of the energy flux, unlike the classical approach, where the dissipation rate appears in the kinetic energy equation. This enables a unified description of both forced and decaying turbulence with a single set of model constants. The model also captures the time lag between the evolution of the kinetic energy and the dissipation rate. Reformulating the system as an equation for the dissipation rate further clarifies how non-equilibrium effects can be incorporated into existing turbulence models.
Direct numerical simulations are used to investigate the impact of compressibility and compositional variable-density (VD) effects on the growth of instability in buoyancy-driven shear layers (BSL), which exhibit features common to both Rayleigh-Taylor (RTI) and Kelvin-Helmholtz (KHI) instabilities. The two-dimensional simulations are performed in a periodic domain with alternating high- and low-density fluid columns under neutral background stratification. This study expands on previous work by Gat et al. [Incompressible variable-density turbulence in an external acceleration field. J Fluid Mech. 2017;827:506-535. doi: 10.1017/jfm.2017.490] conducted in the incompressible regime. The Atwood number, A, and the isothermal Mach number, Ma, are varied to isolate VD and compressibility effects. Two flow regimes are identified. At early times, shear dominates and the evolution resembles KHI with minimal sensitivity to A or Ma. Later, buoyancy becomes dominant and the flow exhibits RTI-like behaviour; increasing A accelerates this transition. Compressibility suppresses the growth of the mixing-layer, reduces bubble and spike penetration, and amplifies the VD-induced mixing asymmetry. Enhanced molecular mixing weakens the baroclinic torque, shifting vorticity production toward dissipative scales and eventually suppressing mixing-layer growth at sufficiently large Ma. The suppressive effect of compressibility strengthens with increasing A, indicating that VD and compressibility are coupled yet competing influences.
Adverse bed-slope transitions can strongly reorganise open-channel turbulence, but their impact on anisotropy, intermittency, and nonlinear dynamics is not yet well quantified. This study experimentally investigates turbulence over an adverse sloping bed in an open channel, aiming to elucidate how bed-induced geometric transitions shape turbulence characteristics. High-frequency three-dimensional velocity measurements were acquired at multiple streamwise stations and depths, and analysed using Reynolds-stress anisotropy diagnostics, intermittency indicators based on quadrant joint-PDFs, velocity spectra, and chaos-information theoretic measures. The results show that turbulence anisotropy evolves gradually along the bed transition. The flow shifts from predominantly bi-directional states upstream toward more mixed states over the slope, followed by partial recovery downstream. Anisotropy is strongest near the bed, exhibits peaks just below mid-depth, and becomes more vertically self-similar with increasing current magnitude. Positionally, no anomalous tendency is observed under stronger currents. In contrast, intermittency remains strongly localised, with slope-transition stations acting as hotspots where joint-PDF contours broaden toward ejection-sweep dominance and spectra steepen. This is consistent with sporadic bursting and sharper departures from local trends, whereas non-transitional stations retain tighter contours and flatter spectra indicative of steadier coherence. Chaos and information analyses further indicate increasing trajectory divergence and reduced memory with height as slope-driven upwellings are present. Collectively, these observations suggest that anisotropy trends and intermittency hotspots can exhibit distinct spatial signatures. Therefore, anisotropy should be interpreted alongside intermittency and memory metrics for position-aware turbulence assessment over adverse slopes.
