Optimisation of the aerodynamic properties of wind turbine rotor blades

Increasing the area efficiency of wind farms

Research consortium

Numerical and experimental investigations of knitted wire mesh structures for aerodynamic optimizations of wind turbine blades

Prof. Dr. Franz-Josef Peitzmann, Jan Hauke Harmening

In the research project Aeroblade funded by the ZIM program a novel type of knitted wire mesh structures is developed using CFD simulations to optimize the aerodynamics of wind turbine blades. The objective is to increase the power output of wind parks by 2 % to 5 %.

Simulations demonstrate that the knitted wire mesh generates streamwise vortices that significantly reduce the flow separation at a backward-facing step. The knitted wire mesh not only delays the flow separation but also reduces the separation bubble by 27.1 % (Figure 1 and 2).

Figure 1: Induces vortices on the knitted wire mesh. The red arrows mark streamwise vortices that recuce flow separation.
Figure 2: Flow separation on a backward-facing ramp. Up: clean canal, down: including the knitted wire mesh to the left of the ramp

The detailed results of this study were published in a scientific journal. The article is freely available at https://www.mdpi.com/2311-5521/7/12/370.

First investigations of the NREL 5MW turbine also show a beneficial effect. At an angle of attack of 18° the separated flow on the DU99W350 airfoil is delayed. This reduces the drag and increases the lift of the blade (Figure 3).

Figure 3: Simulated flow speration on the DU99W350 airfoil.

Development of CFD simulation models
Optimisation of the aerodynamic surface of rotor blades
Design of innovative surface structures from fine wire meshes

Prof. Dr.-Ing. Volker Kassera, Klaus-Peter Helbig, René Thümmler

CFD Consultants GmbH, based in Rottenburg am Neckar, is a long-standing service provider in the field of computational fluid dynamics (CFD). As part of the Aeroblade research project, the aim is to be able to analyse a customer’s wind turbines (WT) in the future and optimise them by means of a CFD simulation and evaluation of the existing situation in the wind farm. Based on the simulations carried out, surface structures made of knitted wire mesh are applied to the rotor blades and contribute to an increase in the performance of the wind farm.

One challenge of the research project is to find solution approaches that are able to describe local, small-scale turbulence effects sufficiently accurate and to map these effects in a large-scale effective volume. In any case, this challenge places the highest demands on the use of the necessary computer hardware and on data handling due to the enormous amounts of data to be expected. For this purpose, both our own computing clusters and the computing capacities of the High-Performance Computing Center Stuttgart (HLRS) with the supercomputer HAWK are used.

Due to the occurrence of these enormous differences in the size of the physical phenomena, the numerical flow simulation (CFD) is divided into micro- and macrosimulations.

The micro-simulation, carried out by our research partner Westfälische Hochschule Bocholt, simulates all flow processes near the rotor blade of the wind turbine in order to map the fluid-dynamic boundary layer with the transitions from laminar to turbulent and other turbulences. The aim is to manipulate the boundary layer of the rotor blade for as long as possible over the length and width of the profile in such a way, that the wake contains as little macroscopic turbulence as possible and the aerodynamic efficiency of the blade increases by delaying or reducing the stall.

The unification of the simulation results from the microsimulation with the macrosimulation requires a manipulation of the simulation tools (algorithm, boundary layer calculation, insertion of an empirical or calculated model), because due to the scale differences (small-scale wires in the mesh with about 1mm diameter/ large-scale WT wake with up to 2km length) no resolution of the wire mesh in the macro model is possible.

Within the project, the macroscopic calculation model for one type of wind turbine is developed as an example, since no high-resolution calculation models exist for the wake of wind turbines.

In order to assess how fine the meshing of the calculation model must be for the macrosimulation, a measurement system is being developed that analyses turbulence variables in the wake of a wind turbine.

Figure 4: Measuring system for turbulence variables and flow velocity fluctuations as a function of time

With the help of this measuring system, changes in turbulence in the wake due to changes in the surface of the rotor blades through applications of wire mesh can be made visible.

Figure 5: Relative vectors and vortex shedding at a wind turbine (Postprocessing done with FieldView)

Development of material systems for microperforated, turbulence-inhibiting coatings for rotor blades

Robert Ruf

eloona GmbH is a manufacturer of technical knitted wire meshes for demanding applications.     

The Aeroblade research project aims to develop material systems based on knitted wire meshes in order to equip rotor blades with a turbulence-inhibiting coating or surface.

Figure 6: Knitted wire mesh (cut-out)

In the initial calculations, stainless steel wire meshes with defined, mutually coordinated mesh lengths and mesh spacings with an average wire diameter were used.

We are able to process almost all materials, especially stainless steels and special materials in the range of 0.05 – 0.80 mm depending on the system conditions. In addition, there are material combinations as well as multi-thread variants and designs.