arXiv subjects

Francesco Scarano

Publications and source records attributed to Francesco Scarano.

3 recordsLinked to original sources

arXiv2026

Effect of free-stream turbulence on a moderate adverse pressure gradient turbulent boundary layer developing over an airfoil

Turbulent boundary layers (TBLs) subjected to adverse pressure gradients (APGs) are common to industrial aerodynamic applications, yet the effect of freestream turbulence (FST) on TBLs developing under moderate APGs remains insufficiently understood. Wind-tunnel experiments were conducted to investigate the effects of FST on a developing TBL over a NACA 0015 airfoil. Varying the angle of attack (2 and 4$^\circ$) adjusted the pressure gradient, and hotwire anemometry measured boundary layer properties at different chordwise positions ($x/c$ = 0.400--0.625, with $\beta = \delta^*/\tau_0 dP/dx$ = 0.2-1.5). The FST level was increased using static grids, resulting in levels ranging from 0.15 to 6$\%$. The chord-based Reynolds number was kept constant at around 250,000 for all configurations. The results show that increasing FST systematically modifies the mean-flow development of the APG boundary layer. Higher FST levels reduce the shape factor and partially suppress the APG-induced wake in the mean velocity profile, while increasing the skin-friction coefficient towards values closer to canonical ZPG behaviour. The streamwise velocity variance is amplified in both the inner and outer regions, and spectral analysis shows that this increase is associated with energetic large-scale motions introduced by the freestream turbulence, with characteristic wavelengths of order $\lambda_x/\delta \approx 13$. These large scale structures penetrate into the boundary layer and contribute to the near-wall variance, with a stronger effect observed as the adverse pressure gradient increases. The results show that FST is a governing parameter in developing APG TBLs over airfoils and that its influence is amplified by the pressure gradient. It must therefore be considered when interpreting mean-flow evolution, turbulence statistics, and scale interactions in realistic aerodynamic environments.

physics.flu-dyn↗

arXiv2025

Noise dissipation mechanisms of an acoustic liner under grazing flow

High-fidelity lattice-Boltzmann very-large-eddy simulations are performed to describe the noise dissipation mechanisms in a single cavity acoustic liner subjected to grazing turbulent flow at a centreline Mach number of 0.3 and plane acoustic waves. The study examines the effects of sound pressure level (ranging from 130 to 160 dB) and source frequency, as well as of the direction of acoustic-wave propagation relative to the grazing flow. The acoustic energy dissipation mechanisms are the viscous losses within the shear layer forming along the internal walls of the orifice and the vortex-shedding. The latter is quantified through Howe's energy corollary. In the absence of grazing flow, acoustic energy is dissipated almost equally during both inflow and outflow phases, with vortex shedding dominating at high SPL and viscous losses at low SPL. The introduction of a grazing flow alters the flow topology; in particular, the shear layer past the orifice generates a quasi-steady vortex that confines the acoustic-induced flow to the downstream half of the orifice. This topological change alters the two noise dissipation mechanisms: viscous losses increase at low SPL because the grazing flow pushes the fluid toward the downstream lip of the orifice; vortex shedding becomes phase dependent, dissipating acoustic energy during the inflow phase and generating acoustic energy during the outflow phase. This explains why the net acoustic dissipation decreases in the presence of grazing flow, highlighting the crucial role of near-wall flow topology on liner performances.

physics.flu-dyn↗

arXiv2025

On the impact of the turbulent grazing flow development on the acoustic response of an acoustic liner

The interaction between acoustic waves and turbulent grazing flow over an acoustic liner is investigated using Lattice-Boltzmann Very-Large-Eddy simulations. A single-degree-of-freedom liner with 11 streamwise-aligned cavities is studied in a grazing flow impedance tube. The conditions replicate reference experiments from the Federal University of Santa Catarina. The influence of grazing flow (with a centerline Mach of 0.32), acoustic wave amplitude, frequency, and propagation direction relative to the mean flow is analysed. Impedance is computed using both the in-situ and the mode-matching methods. The in-situ method reveals strong spatial variations; however, averaged values throughout the sample show minimal differences between upstream and downstream propagating waves, in contrast to the mode-matching method. Flow analyses reveal that the orifices displace the flow away from the face sheet, with this effect amplified by acoustic waves and dependent on the wave propagation direction. Consequently, the boundary layer displacement thickness (${\delta}$*) increases along the streamwise direction compared to a smooth wall and exhibits localised humps downstream of each orifice. The growth of ${\delta}$* alters the flow dynamics within the orifices by weakening the shear layer at downstream positions. This influences the acoustic-induced mass flow rate through the orifices, suggesting that acoustic energy is dissipated differently along the liner. The role of near-wall flow features highlights the need to consider a spatially evolving turbulent flow when studying the acoustic-flow interaction and measuring impedance. The spatial development of the turbulent flow may also partly explain the upstream-downstream impedance differences, as current eduction methods do not account for it.

physics.flu-dyn↗