Wind Energy

192 B. Hillmer et al.
For a better basic comprehension of the flow regime at discrete roughness
and for improved modelling further numerical flow simulations of the airfoil
with trip wire and zigzag tape have been carried out. In the simulations the
obstacle is aerodynamically represented by modifying the airfoil contour and
by modifying the turbulence energy equation.
35.2 Measurements
The numeric results are compared with measurements for the airfoil
DU97-300mod with a relative thickness of 30% and a modified trailing edge
thickness of 0.49%. The measurements have been carried out in cooperation
with the wind turbine manufacturer DeWind at DNW’s cryogenetic
wind tunnel in Cologne in 2003. Reynolds numbers between 1 and 10 M and
Mach numbers (Ma) between 0.1 and 0.2 were obtained by decreasing the
fluid temperature down to 100K. Surface roughness was investigated using
Carborundum of different grain sizes in the leading edge region as well as
zigzag tape and cylindrical trip wire at different locations. In this study
experimental results with 11 mm wide zigzag tape of 0.4 mm and 0.6 mm
height and trip wire of 1.0 mm height are analysed. The devices are located
on the suction side at 30% of the chord.
35.3 Modelling
The used CFD-code FLOWer (release 116.4) has been developed by the
German Aerospace Center (DLR) as a part of the research project
MEGAFLOW. The code requires a block structured grid and solves the
Reynolds averaged Navier–Stokes (RANS) equation. Krumbein [5] implemented
a prediction module for the laminar-turbulent transition. The module
is coupled to the boundary layer calculation allowing free transition simulations.
In the present study an e N -database method is used as transition
criterion. The parameter N is set to N = 3. According to Mack’s correlation
this corresponds to a turbulence intensity of 0.85%. The Wilcox k–ω-model is
applied.
Two dimensional C-grids are created with the hyperbolic grid generator
KGrid3D [6] developed at the University of Applied Sciences Westküste. The
number of cells for the used grid is between 46,000 and 48,000. The airfoil contour
line is segmented into 600 cells. In order to achieve a sufficient resolution
of the boundary layer calculation the height of the cells at the profile surface
is decreased down to 1 µm. In the normal direction to the surface 60–75 cells
are created. The wake flow area covers 30 times the chord length using 30
cells in this direction.
The first CFD-model uses a grid created with an obstacle as part of the
airfoil contour. Figure 35.1 illustrates the grid around the modelled obstacle.