The prediction of the hydrodynamic performance of marine current turbines

Review of performance optimization techniques applied to wind turbines

This article reviews the most important aerodynamic research topics in the field of wind energy. Wind turbine aerodynamics concerns the modeling and prediction of aerodynamic forces, such as performance predictions of wind farms, and the design of specific parts of wind turbines, such as rotor-blade geometry. The basics of the blade-element momentum theory are presented along with guidelines for the construction of airfoil data. Various theories for aerodynamically optimum rotors are discussed, and recent results on classical models are presented. State-of-the-art advanced numerical simulation tools for wind turbine rotors and wakes are reviewed, including rotor predictions as well as models for simulating wind turbine wakes and flows in wind farms.

Aerodynamic Aspects of Wind Energy Conversion

We present the multiscale space---time techniques we have developed for fluid---structure interaction (FSI) computations. Some of these techniques are multiscale in the way the time integration is performed (i.e. temporally multiscale), some are multiscale in the way the spatial discretization is done (i.e. spatially multiscale), and some are in the context of the sequentially-coupled FSI (SCFSI) techniques developed by the Team for Advanced Flow Simulation and Modeling $${({\rm T} \bigstar {\rm AFSM})}$$ . In the multiscale SCFSI technique, the FSI computational effort is reduced at the stage we do not need it and the accuracy of the fluid mechanics (or structural mechanics) computation is increased at the stage we need accurate, detailed flow (or structure) computation. As ways of increasing the computational accuracy when or where needed, and beyond just increasing the mesh refinement or decreasing the time-step size, we propose switching to more accurate versions of the Deforming-Spatial-Domain/Stabilized Space---Time (DSD/SST) formulation, using more polynomial power for the basis functions of the spatial discretization or time integration, and using an advanced turbulence model. Specifically, for more polynomial power in time integration, we propose to use NURBS, and as an advanced turbulence model to be used with the DSD/SST formulation, we introduce a space---time version of the residual-based variational multiscale method. We present a number of test computations showing the performance of the multiscale space---time techniques we are proposing. We also present a stability and accuracy analysis for the higher-accuracy versions of the DSD/SST formulation.

Multiscale space---time fluid---structure interaction techniques

Near wake flow analysis of a vertical axis wind turbine by stereoscopic particle image velocimetry

The two-dimensional characteristics of airfoil NACA 0018 have been measured for Reynolds numbers between 0.15x106 and 1.0x106 to establish the lift, drag and moment curves that serve as input to performance calculations of vertical axis wind turbines. At the lower surface laminar separation occurs at low to medium angles of attack, which is of significant influence on the characteristics and the radiated noise. For the situation with a lower surface laminar separation bubble, span wise wake rake traverse measurements showed an irregular three-dimensional pattern. Noise reduction could be achieved with zigzag tape at the 70% to 80% lower surface chord station. Significant post-stall hysteresis loops occurred showing a high loss in lift.

/pdf/two-dimensional-low-reynolds-number-wind-tunnel-results-for-2e58v4m5df.pdf

Two-Dimensional Low-Reynolds Number Wind Tunnel Results for Airfoil NACA 0018:

This paper investigates the NACA 63 and 64 6-digit series of airfoils tested in the NACA LTPT in view to verify the RFOIL calculated airfoil characteristics for high Reynolds numbers. Some anomalies in the zero-lift angles of 15% and 18% thick airfoils from these series are identified, both in the airfoil clean case and in case of wrap-around roughness. It is found that RFOIL predicts the maximum lift coefficient at a Reynolds number of 3 million well, but consistently under predicts the Cl,max for Reynolds numbers of 6 and 9 million. It is, however, based on other comparisons at high Reynolds numbers unclear if this is due to an inability of the prediction code or to a deviation in the test results. The drag coefficient is under predicted with about 9% for a wide range of airfoils and Reynolds numbers. Due to wrap-around roughness the maximum lift coefficient decreases with 18% to 20%.

/pdf/an-overview-of-naca-6-digit-airfoil-series-characteristics-2i42lg9bm5.pdf

An overview of NACA 6 -digit airfoil series characteristics with reference to airfoils for large wind turbine blades.

This paper discusses the results of wind tunnel measurements performed on a modified DU 97-W300 airfoil at Reynolds numbers between 1x10 and 10x10 in the cryogenic wind tunnel of the DNW at Cologne, Germany. The airfoil was modified by reducing the trailing edge thickness from 1.74% to 0.49%. Although the measurements showed large scatter when flow separation occurred on the model it was possible to establish the variation with Reynolds number of the maximum lift coefficient, the maximum lift-drag ratio and the design-lift coefficient for a Mach number of 0.2. Furthermore the effect of wrap-around Carborundum roughness and zigzag tape of 0.4 mm thickness at upper and lower surface was studied. The experimental results were compared with RFOIL calculations. The measurements indicate that there was no significant variation in the maximum lift coefficient with Reynolds number for the clean airfoil. In contrast to the RFOIL calculations the experimental maximum lift-drag ratio decreased with Reynolds number from an average of 95 at R=3x10 to about 85 at R=10x10. The Carborundum 60 roughness had a larger negative effect on the airfoil performance than the zigzag tape, but in both cases the airfoil performance improved drastically with increasing Reynolds number.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/we.136

The effect of roughness at high Reynolds numbers on the performance of aerofoil DU 97‐W‐300Mod

Passive vane-type vortex generators (VGs) are commonly used on wind turbine blades to mitigate the effects of flow separation. However, significant uncertainty surrounds VG design guidelines. Understanding the influence of VG parameters on airfoil performance requires a systematic approach targeting wind energy-specific airfoils. Thus, the 30%-thick DU97-W-300 airfoil was equipped with numerous VG designs, and its performance was evaluated in the Delft University Low Turbulence Wind Tunnel at a chord-based Reynolds number of 2×106. Oil-flow visualizations confirmed the suppression of separation as a result of the vortex-induced mixing. Further investigation of the oil streaks demonstrated a method to determine the vortex strength. The airfoil performance sensitivity to 41 different VG designs was explored by analysing model and wake pressures. The chordwise positioning, array configuration, and vane height were of prime importance. The sensitivity to vane length, inclination angle, vane shape, and array packing density proved secondary. The VGs were also able to delay stall with simulated airfoil surface roughness. The use of the VG mounting strip was detrimental to the airfoil's performance, highlighting the aerodynamic cost of the commonly used mounting technique. Time-averaged pressure distributions and the lift standard deviation revealed that the presence of VGs increases load fluctuations in the stalling regime, compared with the uncontrolled case.

/pdf/experimental-parameter-study-for-passive-vortex-generators-czqny2ww9j.pdf

W.A. Timmer

Papers

Two-Dimensional Low-Reynolds Number Wind Tunnel Results for Airfoil NACA 0018:

An overview of NACA 6 -digit airfoil series characteristics with reference to airfoils for large wind turbine blades.

The effect of roughness at high Reynolds numbers on the performance of aerofoil DU 97‐W‐300Mod

Experimental parameter study for passive vortex generators on a 30% thick airfoil

Thick airfoils for HAWTs