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

Effects of Reynolds number on boundary layer and wake development of embedded turbine vane stages

To address the aerodynamic challenges associated with low-Reynolds-number operation typical of high-altitude cruise conditions in aero-engines, this paper presents an experimental investigation—conducted on a large-scale, low-speed, multi-stage axial turbine facility—in which both the velocity triangles and the reduced wake-passing frequency were held constant. Synchronized hot-film measurements were conducted on the blade surface, and the dynamic flow field at the second-stage stator (S2) exit plane was acquired using a triple- and hot-wire probe. The study examines the effects of Reynolds number (Re = 7.4 × 104 and Re = 8.5 × 104) on the attached-flow transition and wake mixing characteristics of the S2 suction surface. The results indicate that at the lower Reynolds number, the thickened boundary layer and enhanced viscous effects lead to shear sheltering of external disturbances, resulting in a delayed transition onset and attenuated wall shear stress fluctuations. Consequently, the merging and evolution of turbulent spots are restricted, causing the calmed regions at the trailing edge to exhibit stronger intermittency. This process leads to wake broadening, an elevation of global turbulence levels, and a significant increase in velocity deficit magnitude (>1.5%), ultimately resulting in a significantly deeper and wider exit wake profile, indicative of increased aerodynamic loss.

Impact of freestream turbulence on the aerodynamics of a high-speed, low-Reynolds turbine rotor cascade usingWall-Resolved Large Eddy Simulations

High-speed low-pressure turbines (HS-LPTs) operate under transonic and low-Reynolds-number conditions, making their boundary layers highly sensitive to transition and separation. Accurate numerical prediction of these effects requires a realistic representation of inflow turbulence, which strongly influences separation, transition onset, and wake development. Conventional turbulence generation methods for internal compressible flows often fail to reproduce experimental turbulence characteristics, leading to significant discrepancies in performance predictions. This study proposes a numerical wind-tunnel replication strategy based on the Dynamic Actuator Line Method (DALM), designed to model the effects of passive turbulence grids commonly used in turbomachinery experiments. The approach generates realistic turbulent inflow conditions without explicitly meshing the grid geometry, significantly reducing computational cost. The method is applied to the SPLEEN C1 transonic cascade, experimentally tested at the von Karman Institute, at Reout,is = 70k and Mout,is from 0.70 to 0.95. Simulations are performed using the YALES2 explicit compressible solver and a wall-resolved LES framework. The DALM successfully reproduces experimental inflow turbulence characteristics: TIx = 2.5% and Λint = 13.5 mm. Accounting for realistic turbulence substantially improves predictions of boundary-layer behavior and wake losses compared to clean inflow conditions. In particular, suction-side separation is delayed, transition occurs earlier, and flow reattachment is promoted at low Mach numbers. Velocity and turbulence statistics in the blade passage and wake show good agreement with PIV measurements, highlighting the importance of realistic inflow turbulence under compressible HS-LPT operating conditions.

Analysis of Aerodynamic Characteristics of a Blunt-Trailing-Edge Wind Turbine Airfoil at High Angles of Attack

Effects of Reynolds number on flow structures and broadband forces of a pump-jet propulsor under non-uniform inflow

Under non-uniform wake conditions, the effects of Reynolds-number variation on the broadband-force response of a pump-jet propulsor remain unclear. In this study, an experimentally validated large-eddy simulation method is used to systematically examine Reynolds-number effects on the upstream wake structure, internal flow-field evolution, and spectral characteristics of rotor broadband forces. These results show that increasing Reynolds number leads to a marked contraction of the aft-body boundary layer and the low-speed wake upstream of the propulsor. At the higher Reynolds number, flow separation and wake diffusion in the stator region are suppressed, while rotor–stator shear interaction and local turbulence intensity near the rotor inlet are enhanced. Meanwhile, velocity fluctuations upstream of the rotor are attenuated, reducing the amplitudes of blade lift fluctuations. Flow separation over the rotor blade surface is weakened, and the regions of elevated skin-friction coefficient and pressure fluctuations from the aft half-chord to the trailing edge shrink substantially. These changes significantly reduce broadband fluctuations in the rotor thrust coefficient. The findings provide a physical basis for model-scale testing, numerical prediction, and design optimization of pump-jet propulsors.

Blockage-driven turbulent vortex dynamics and modal evolution in high-Reynolds-number cavity flows

This study presents two-dimensional particle image velocimetry measurements acquired at the mid-plane of the cavity, together with higher-order dynamic mode decomposition (HODMD) analysis, for lid-driven square-cavity flows containing cubical obstacles at high Reynolds numbers (Re = 1.161 × 105–3.483 × 105). Obstacle sizes of 3, 6, and 9 cm, corresponding to h/D = 0.10, 0.20, and 0.30, were systematically examined to clarify blockage-driven variations in vortex dynamics, velocity distribution, planar turbulent kinetic energy (TKE2D), and modal characteristics on the cavity middle plane. The results show that increasing Reynolds number drives the primary vortex toward the cavity center and promotes a transition from relatively fragmented, high-rotation structures to smoother large-scale recirculation. Meanwhile, increasing obstacle size strengthens geometric confinement, shifts the primary vortex toward the left wall, and induces vortex splitting at the largest blockage ratio. The low-velocity region expands markedly, reaching 43.29% for h/D = 0.30, while the high-velocity region is reduced to 0.8% at the highest Reynolds number. At Re = 2.322 × 105, increasing h/D from 0.10 to 0.30 increases the area proportion of the high normalized-TKE region from 0.83% to 4.74%, while decreasing that of the low normalized-TKE region from 50.72% to 36.52%, indicating a blockage-induced redistribution of the resolved in-plane fluctuation energy. HODMD further identifies a dominant zero-frequency mean-flow mode and higher-order modes with clear harmonic relations, corresponding to the multi-scale evolution of the cavity flow. The reconstructed flow fields show a root mean square error as low as 2.08%, substantially lower than that of standard DMD in strongly nonlinear conditions. These results provide experimental evidence for blockage-controlled turbulent vortex dynamics in high-Reynolds-number cavity flows and offer reference data for the validation of numerical simulations in confined flow systems.

Reynolds Number Effects on Transonic Aerodynamic Characteristics of the CHN-T1 Standard Model

Experimental data at flight-relevant Reynolds numbers remain scarce for publicly available transport aircraft standard models. This study analyses Reynolds number effects on the CHN-T1 large-aspect-ratio configuration using measurements from the cryogenic European Transonic Wind Tunnel, covering Mach 0.20–0.90 and unit Reynolds numbers from 3.6 × 106 to 37.8 × 106. A split transition strategy (fixed at low Re, free at high Re) is employed; the resulting trends are smooth and physically consistent with boundary-layer stability behaviour across the configuration change. At the cruise condition (M = 0.78), the non-induced drag coefficient CDV follows a power law CDV ∝ Re−n with n = 0.141, corresponding to about 70% of the canonical turbulent flat-plate exponent n = 1/5. At off-design Mach numbers, the exponent reduces to n = 0.126 (M = 0.60) and n = 0.125 (M = 0.85), reflecting growing pressure and wave drag contributions that scale more weakly with Reynolds number. The longitudinal static stability margin also exhibits a measurable Re dependence at the cruise condition, with the neutral point shifting aft by approximately 1% of the mean aerodynamic chord over the tested range. These findings provide quantitative benchmarks for Re scaling methodologies and CFD validation at flight-relevant conditions.