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

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.

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.

Wake Recovery of Vertical-Axis Wind Turbines: Effects of Rotor Solidity and Reynolds Number

The wake evolution of vertical-axis wind turbines (VAWTs) plays a critical role in turbine-array performance, yet the wake variations associated with rotor geometry, operating condition, and Reynolds-number-related factors remain insufficiently understood. In this study, the wake characteristics of H-type VAWTs were systematically investigated under varying rotor diameters (D = 2, 3, 4 m), chord lengths (c = 0.1–0.4 m), and incoming wind speeds (v∞ = 6–10 m/s) using a two-dimensional mid-span CFD approach based on Improved Delayed Detached Eddy Simulation (IDDES) built on the SST k-ω model. The simulations are intended to examine mid-span wake mechanisms rather than to directly predict full three-dimensional far-wake recovery, turbine-array interaction, or engineering layout performance involving tip vortices, spanwise momentum transport, and three-dimensional breakdown of coherent structures. The results show that, for the selected reference geometry and within the tested inflow-speed range, the lateral mean-velocity profiles at the same downstream location collapse reasonably well after normalization by v∞ and D, indicating weak sensitivity to incoming wind speed under these conditions rather than general Reynolds-number independence of VAWT wakes. For cases with different solidities, the observed wake differences should be interpreted as the combined effects of rotor solidity and the corresponding near-optimal operating condition. Overall, this study provides a mechanism-oriented numerical assessment of wake behavior in H-type VAWTs.

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.

Flow Separation and Wake Dynamics in High Reynolds Number Flows: Implications for Automotive Drag

Low pressure wake behind the body of a vehicle is the most important component of its aerodynamic drag at highway speeds, not the viscous friction along the body. In typical scales for vehicles, the Reynolds number is O ( 1 0 6 ) , which means that the boundary layer is turbulent and stays attached for the major part of the front and roof of the vehicle. The major problem is at the back where there is an adverse pressure gradient and the flow has a tendency to separate. So automotive drag is a more complex subject that is more related to a mechanism chain and not necessarily to speed. After separation a detached shear layer rolls up into coherent structures and a recirculating wake is formed. This wake size and structure will influence the mean base pressure and thus the drag. Based on the canonical Ahmed model, boundary-layer theory, separation criteria and wake scaling are used to demonstrate the order of variation in drag relative to relatively small changes in the rear geometry. From this point of view, the effective way of drag reduction is not to avoid separation, but to control the development of the separated wake and to enhance the pressure recovery.

Newly identified principle for aerodynamic heating in hypersonic flows