Effective control of frictional drag on a model surface is crucial for achieving total drag reduction at high Reynolds number conditions. This study presents wind tunnel tests at high Reynolds numbers, employing two sets of flat plate models with identical external dimensions. The tests cover incoming Mach numbers ranging from 0.3 to 0.85, corresponding to Reynolds numbers from 2.52 × 106 to 6.35 × 106. The velocity distributions and statistical properties within the turbulent boundary layer were measured using a combination of hot-wire probes and total pressure probes. Total model drag was assessed using a high-precision balance. The results demonstrate that the application of porous media significantly reduces frictional drag, with the cavity arrangement yielding superior drag reduction compared to the non-cavity arrangement. As the Reynolds number increases, the effectiveness of friction drag control diminishes. Moreover, the arrangement of porous media with appropriately chosen parameters can effectively reduce the total drag of the model. Flow field analysis reveals that porous media alter the vortex structure distribution within the boundary layer, increasing small-scale vortex structures and mitigating the impact of large-scale streamwise vortices on the wall, thereby reducing frictional drag. These findings provide important technical support and experimental evidence for the application of porous media in drag reduction at high Reynolds number conditions.

This study investigates the effect of Reynolds number on unsteady buffet characteristics of the OAT15A supercritical airfoil under transonic conditions (Ma = 0.73, AOA = 3.5°) using DDES based on the SST k-ω turbulence model coupled with the γ-Reθ transition model. Results show that, compared with fully turbulent conditions, the free-transition cases exhibit a more downstream shock position and higher lift. Under fully turbulent conditions, higher Reynolds numbers drive the shock downstream and enhance its stability. Under free-transition conditions, the shock moves downstream at low Reynolds numbers but shifts upstream at high Reynolds numbers due to changes in the transition location. During the unsteady buffet cycle at low Reynolds numbers, the lift increases as the shock moves downstream and the separation region shrinks. The lift reaches its maximum when the separation is minimal, corresponding to a quiet flow state with weak acoustic emission. As the lift decreases, a large separation region forms behind the shock, forcing the shock upstream and reducing the lift to its minimum. At high Reynolds numbers, the buffet cycle changes: the shock becomes more stable; trailing-edge vortex shedding intensifies; lift oscillation amplitude decreases; and buffet frequency increases.

Boundary layer suction is a critical technique in hybrid laminar flow control (HLFC) for delaying transition and reducing drag. While the effectiveness of suction is well-established, systematic studies on the parametric optimization of suction hole diameter, location, and coefficient for two-dimensional airfoils remain scarce. This study addresses this gap through numerical investigations using the validated γ-${\widetilde{Re}}_{\theta t}$ transition model. The research systematically analyzes the synergistic effects of suction coefficient (C_q), location (5%, 10%, and 15% chord), and suction hole diameter (0.2 mm, 0.6 mm, and 1.0 mm) on transition characteristics and aerodynamic performance. The results reveal that suction location predominantly governs the viscous drag coefficient (C_Dv), whereas suction hole diameter primarily influences the pressure drag coefficient (C_Dp). Consequently, suction location selection proves more critical for drag reduction than suction hole diameter. The maximum drag reduction (11.9% decrease in C_D) and optimal transition delay (11.8% chord shift) are achieved using a small suction hole (0.2 mm) located at an aft position (15% chord) with a high suction coefficient. Furthermore, an optimal matching range exists between suction location and coefficient, which widens with decreasing suction hole diameter. Based on these findings, this study proposes an energy-efficient design strategy: employing small apertures across the suction region while gradually increasing suction rates toward the trailing edge to achieve significant drag reduction with minimal energy penalty.

Shock buffet is one of the critical issues affecting the aerodynamic performance, flight quality, and flight safety of large aircraft. To overcome the limitations of traditional experimental measurement methods, such as insufficient capability in capturing flow features and high cost, an integrated experimental system tailored for extreme cryogenic and high-Reynolds-number conditions is developed based on the conventional tuft technique. This system comprises “preparation of low-flow-disturbance fluorescent mini-tufts, high-efficiency large-area tuft taping, automatic generation of digital streamline, and flow topology analysis”. Furthermore, a technique for assessing the transonic shock buffet onset using dynamic flow visualization with fluorescent mini-tufts is proposed. This paper takes a typical supercritical airfoil as the research object. First, through high-precision numerical simulations, it reveals that low-energy, unstable boundary-layer separation is the core driving force for the development and maintenance of shock buffet, and that flow separation characteristics serve as an important basis for determining the shock buffet onset. Subsequently, experimental validation is conducted in a 0.3 m high-Reynolds-number transonic wind tunnel. Using a dual-excitation-band composite light source, simultaneous measurements of pressure-sensitive paint (PSP) and fluorescent mini-tuft patterns are realized. The experimental results show that under extreme conditions, characterized by a wide total temperature range of 110 K to 280 K and strong scouring at Mach numbers from 0.6 to 0.9, the fluorescent mini-tufts (approximately 0.05 mm in diameter) exhibit excellent flow-following capability without any detachment. The digitized flow patterns of the fluorescent mini-tufts, obtained via computer image recognition algorithms, clearly reveal the location and area of boundary-layer separation. The trends show good agreement with the cryogenic PSP results, providing an important reference for determining the shock buffet onset.