Flow, Pressure, and Aerodynamic Loads on an Oscillating Airfoil Undergoing Dynamic Stall

Reynolds-Number-Dependent Microcavity Optimization for Drag Reduction on Airfoil

This study investigates the effectiveness of surface microcavities as a passive flow-control technique for reducing drag on a NACA 0012 airfoil over a range of Reynolds numbers and angles of attack. Two-dimensional RANS and URANS simulations were performed using the SU2 solver and validated against benchmark data for the smooth airfoil. A preliminary analysis at Reynolds numbers of [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] evaluated the sensitivity of cavity performance to flow conditions. A Taguchi design-of-experiments approach was then used to optimize cavity diameter, depth, and chordwise location at Reynolds numbers of [Formula: see text] and [Formula: see text]. Under specific optimized conditions, the two-dimensional simulations predict drag reductions of up to 39% and lift-to-drag ratio improvements exceeding 290%. However, the aerodynamic benefits were highly dependent on Reynolds number and angle of attack, with some configurations causing drag penalties at off-design conditions. Flowfield analysis showed that cavity-induced vortical structures can either suppress or intensify near-wall turbulence. This study provides the first systematic, multiparameter optimization of microcavities on an airfoil, addressing a gap in passive flow-control literature.

Mechanisms of local flexible membrane leeward surface on airfoil aerodynamic performance

Abstract Content of image described in text. An experimental investigation is conducted in a wind tunnel on a NACA0012 airfoil with a partially flexible polydimethylsiloxane membrane leeward surface extended from 16.7 % to 83.3 % of the chord length. Aerodynamic forces, membrane deformation and the surrounding flow field are measured simultaneously. The results show that membrane vibration effectively reduces the extent of the recirculation zone, thereby improving aerodynamic performance. Specifically, the stall angle is delayed by 3 Superscript ring 3∘ $3^\circ$ , and the maximum lift coefficient is increased by 12.4 % compared with that of the rigid airfoil. Novel insights into the flow–structure interaction are established from both spatial and temporal perspectives. Spatially, comparisons across angles of attack reveal that membrane vibrations driven by strong pressure fluctuations near the trailing edge can propagate upstream, accompanied by modifications in the distribution of turbulent kinetic energy and enhanced flow mixing. Temporally, a detailed analysis of the strongly periodic membrane motion and unsteady flow structures near stall further demonstrates that the membrane dynamics is tightly coupled with the evolution of the leading-edge vortex. This coupling is associated with the modulation of coherent flow structures through periodic absorption and release of mechanical energy. Overall, the flexible membrane can provide effective flow control by reorganising the spatio-temporal distribution of energy within the flow via flow–structure interaction, without external energy input. The findings provide insights into potential low-energy flow-control strategies.

Numerical Investigation on the Aerodynamic Performance of NACA Series Airfoils at Low Reynolds Numbers

In this paper, aerodynamic performance of three NACA airfoils, including NACA 2412, NACA 4412, and NACA 0012 are examined in two-dimensional CFD simulation to compare their aerodynamic performance. The current simulation is conducted at Re = 1.0 × 10 5 with turbulence model is able to predict adverse pressure gradient. The flow field is examined in a range of angle of attack between 0 and 16 degrees. Numerical results reveal that when Re is low, increasing camber not only increases both baseline lift and maximum lift coefficient, but also delay the stall. The symmetric airfoil NACA 0012 stalls at about 8°, while more cambered airfoil NACA 4412 delay stall at about 14°, obtain higher lift-to-drag ratio at peak and wider efficient operating range. The surface pressure, velocity and turbulent kinetic energy are further analyzed. It is found that when camber is increased, the adverse pressure gradient on upper surface is improved and transition point moves backward with angle of attack, which can suppress the large scale separation and vortex evolution.

Effects of a trapped vortex cell on thick wing profile

Decoupling the bifurcated effects of Reynolds and Mach numbers on the dynamic stall of a wind turbine airfoil

Modern ultra-large wind turbines can expose their outboard blade sections to Reynolds numbers above 1 × 107 and Mach numbers above 0.3. In conventional fixed-geometry tests, both parameters vary with inflow velocity. Their individual aerodynamic effects are, therefore, difficult to distinguish. This study presents a variable-chord sectional framework to examine the Reynolds- and Mach-number effects separately. Transitional unsteady Reynolds-averaged Navier–Stokes simulations are conducted for the FFA-W3-211 airfoil. The results reveal a regime-dependent competition between viscous scaling and compressibility. Increasing the isolated Reynolds number promotes earlier transition, strengthens boundary-layer momentum exchange, and accelerates flow reattachment. The lift-hysteresis intensity decreases by 67.2%. Increasing the isolated Mach number enhances lift in attached flow. During deep stall, however, local supersonic flow and shock–boundary-layer interaction promote earlier separation and delay pressure recovery. The downstroke aerodynamic efficiency decreases by 68.1% at the selected post-stall state. These findings indicate that extrapolating traditional uncorrected dynamic stall models to modern large-scale blades may substantially mispredict stall margins. The results suggest that incorporating distinct, decoupled time constants for viscous scaling and compressibility-induced structural persistence may improve predictions of unsteady sectional loads. Their quantitative implications for complete rotors remain to be established through three-dimensional rotating aeroelastic simulations.

Hydrodynamic force and vortex dynamics around an inclined cylinder in oscillatory flow

Large eddy simulation is performed to investigate oscillatory flow around a circular cylinder at a Reynolds number of Re = 3900 under Keulegan–Carpenter numbers of KC = 15, 30, and 60 and inclination angles of α = 0°, 30°, 45°, and 60°. The validity of the independence principle (IP) is analyzed, and the contributions to drag and lift are quantified using the force partitioning method (FPM). The results indicate that the validity of IP expands significantly with increasing KC. At KC = 15, the inline drag coefficient is comparable only for α ≤ 30°, whereas at KC = 60, it remains nearly invariant up to α = 60°. This is because the longer stroke length at high KC allows vortices to fully develop within each half-cycle, producing more coherent primary wake structures and helping preserve quasi-two-dimensional flow characteristics even at large inclination angles. FPM reveals that the forces in both the x and y directions are highly correlated with the vortex-induced forces. Decomposition of the vortex force shows that the spanwise–vortex contribution dominates the drag and lift force. Moreover, as the inclination angle increases, the reduced normal velocity and enhanced axial velocity weaken near-cylinder vortex formation and promote axial stretching of vortices, thereby reducing vortex coherence and lowering the peak magnitudes of the vortex-induced forces.