The present study reveals the turbulence dynamics and morphological adjustments of mobile dune-shaped bedforms in an alluvial stream. Results demonstrate acceleration of flow over the dune crest enhancing streamwise velocity, while the initial and the lee side sections of the dune experience flow circulation. The near-bed regions of the initial and lee sections experience peak Reynolds shear stress, marking zones of higher momentum exchange and active sediment entrainment. Turbulence is dominated by streamwise fluctuations, with spanwise and vertical components reinforcing lateral mixing and particle suspension. Octant analysis indicates that sweep events dominate in the near-bed regions of the initial, crest and lee sections of the dune, driving bedform migration and intensifying scour development on the lee side. Probability distribution functions highlight strong non-Gaussian behaviour and intermittency at crest and lee sections, linked to vortex shedding and flow separation. Higher-order structure functions further confirm the presence of intense turbulent bursts in the near-bed region, underscoring the role of coherent structures in driving sediment motion. Morphological analysis shows progressive scour development at the lee side and downstream crest erosion, resulting in continuous dune migration. These findings advance understanding of turbulence–morphology interactions and their control on sediment transport in alluvial channels.

Base-flow computations and stability analyses are performed for a hypersonic wind tunnel nozzle at a Mach number of 6. Isothermal and adiabatic wall boundary conditions are investigated, and moderate stagnation conditions are used to provide representative scenarios to study the transition in quiet hypersonic wind tunnel facilities. Under these conditions, the studied nozzle shows a small flow separation at the convergent inlet. Global stability analysis reveals that this recirculation bubble may trigger a classical three-dimensional stationary unstable global mode. Resolvent analysis reveals Görtler, first and second Mack modes affecting the divergent part of the nozzle, along with a Kelvin–Helmholtz instability induced by the bubble. The present study also highlights the key impact of perturbations located in the convergent inlet on the development of instabilities further downstream in the divergent outlet, helping to understand the need and efficacy of a suction lip upstream of the nozzle throat to mitigate instabilities in the divergent section. Detailed knowledge of all these mechanisms is essential for understanding flows in quiet hypersonic wind tunnel nozzles and, consequently, represents a key step towards the optimisation of such nozzles.

This study presents a high-fidelity direct numerical simulation (DNS) framework tailored for investigating turbulent flows through complex porous structures. It employs a compressible Navier–Stokes solver based on the spectral difference (SD) method, with immersed boundary conditions (IBCs) implemented via the Brinkman penalisation technique and integrated using a Strang splitting approach. A pressure gradient scaling (PGS) strategy is incorporated to improve computational efficiency. To provide realistic inflow conditions, synthetic turbulence is injected at the inlet using a random Fourier modes method. The methodology is validated in several stages. First, the IBC approach is tested against results from a body-fitted mesh, showing strong agreement in the mean velocity field. Next, the effectiveness of the PGS technique is demonstrated by comparing scaled and unscaled simulations, both of which yield consistent velocity fields and spectral content. Finally, the full DNS-SD framework is benchmarked against finite volume method results from the literature, successfully reproducing key turbulence characteristics, including two-point correlations. The validated solver is ultimately applied to simulate turbulent flow through a complex porous geometry. The results illustrate the robustness of the approach and highlight its potential for advancing the understanding of turbulence in porous materials.