Reynolds Number and Roughness Effects on Thick Airfoils for Wind Turbines

Abstract

The effects of increased Reynolds number (Re) on clean and rough airfoils were investigated by wind tunnel experiments and by numerical simulation with an improved Navier-Stokes solver. By cooling the cryogenic wind tunnel in Cologne (KKK) of the German-Dutch Wind-Tunnel (DNW) down to 100 K, values of Re up to 10 × 106 were reached, based on a model chord of 500 mm. Whereas the clean surface configuration shows no drastic loss in maximum lift, the lift-to-drag ratio decreases from 95 to approximately 85, mainly due to an increase of drag. In contrast to that, rough surface configurations, including zig-zag tape for transition fixing and carborundum based distributed roughness around the nose, show increased performance for increased Re. In parallel, numerical simulations were carried out with a boundary layer stability code coupled to a Navier-Stokes solver. The trends in behaviour of the boundary-layer based properties were predicted well, in contrast to the separation pattern responsible for maximum lift.

Experimental detection of laminar‐turbulent transition on a rotating wind turbine blade in the free atmosphere

“…In any case, testing of newly designed profiles in a wind tunnel must cover the detection of the transition location.…”

Rotor Scaling Methodologies for Small Scale Testing of Floating Wind Turbine Systems

“…The performance match of the lift coefficient at various angles of attack pre-stall is good, the post stall performance was not considered as part of the design optimisation procedure. The drag coefficient for all angles of attack is greater at model scale than full; a result of the increase friction drag with reducing Reynolds number and unavoidable [27]. Given the relative contribution of the drag coefficient to the global rotor loads it is expected that an increase non-dimensionally between model and full scale will result in a general small increase in the model rotor thrust and a more pronounced decrease in the torque in comparison to the reference.…”

CFD in Wind Energy: The Virtual, Multiscale Wind Tunnel

“…Surface roughness can also play an important role in airfoil aerodynamics as it is known to increase viscous drag, decrease maximum lift and reduce abrupt stall. However, these general features are not necessarily always encountered as pointed out by Freudenreich et al [21], who studied the importance of transition and roughness, both experimentally and numerically, for the TUDelft airfoil series. In CFD simulations, roughness is usually introduced using the equivalent sand grain approach according to which the roughness height is considered small compared to the boundary layer thickness and the roughness effect on the flow is mimicked by increasing the turbulent eddy viscosity in the wall region to obtain higher skin friction [24].…”

“…The e N method, although regularly used in IBL methods, is seldom directly used in CFD methods due to inadequate resolution of the boundary layer characteristics [19]. The recent works of Windte et al [20] in the field of Unmmaned Aerial Vehicle (UAV) aerodynamics and of Freudenreich et al [21] on the TUDelft airfoil series, indicate however that the e N method can be coupled successfully with a CFD method to study transition for this range of Re. A recent approach consists in using a set of two transport equations modeling the intermittency of turbulence and the evolution of the transition Reynolds number based on the boundary layer momentum thickness Re θ,t as devised by Menter et al [22].…”