XFOIL Tips – No.2 – Airfoil Reshaping (Part 2)

XFOIL Tips – No.2 – Airfoil Reshaping (Part 2)

After the initial explanatory steps on the XFOIL functionality integrated into QBlade we attempt a deeper description of the more specific functions that will allow you to refine the airfoil geometry properties in order to closely match wind turbine blade reality. Furthermore this added functionality will allow you to experiment within the flexible QBlade GUI with various airfoil shapes and thus to understand the effects of each airfoil modification on wind turbine performance.

L.E. Radius (Select: Foil->Set L.E. Radius)

Starting from the “Set L.E. Radius” option, it is possible to modify the radius of the leading edge of an airfoil by means of a scaling factor. For factors larger than 1.00 the L.E. radius is enlarged while factors lower than 1.00 reduce the leading edge radius. By defining the blending distance it is possible to limit the extend of the airfoil coordinates downstream of the L.E. where the airfoil smoothing code is allowed to automatically modify the airfoil shape while integrating the modified L.E. Radius.

T.E. Gap (Select: Foil->Set T.E. Gap)

The modification of the trailing edge gap, which was mentioned in the previous QBlade blog post, works in a similar way. The T.E. Gap is specified with chord-normalized length (%c) and the blending distance dictates the absolute percentage of airfoil chord, downstream of which the smoothing code is free to modify the airfoil shape in order to accommodate the  modified T.E.

Thickness and Camber modification (Select: Foil->Scale Camber and Thickness)

The airfoil shape operations however extend beyond the limited local airfoil modifications. Global airfoil shape modification functionality is also offered by means of airfoil Camber and Thickness selection. Both the absolute camber and thickness (as percentages of airfoil chord) as well as their locations along the airfoil can be defined by the user. The generated modified airfoil shapes are very helpful when one investigates the effects of these parameters on wind turbine performance.

NOTE: For the users of QBlade who are interested in Vertical Axis Wind Turbine (VAWT) design, the camber modification function is of special use. Flow curvature on VAWTs requires a camber and pitch-angle modification on the turbine airfoils* and such modifications are simply done by means of the aforementioned function.

*Migliore, P.G., Wolfe, W.P.and Fanucci, J.B., 1980, “Flow Curvature Effects on Darrieus Turbine Blade Aerodynamics,” Journal of Energy, 4(2), pp. 49-55.

L.E. and T.E. Flap (Select: Foil->Set Flap)

Even though wind turbines are not equipped with ailerons and flaps we have decided to maintain the L.E. and T.E. Flap generator of XFOIL inside QBlade. That proved to be a good idea for various researchers, users of QBlade who wanted to investigate the use of leading edge and trailing edge modifications. Currently the users with such needs are very limited but future developments of QBlade will definitely allow more experimentation with such…exotic functions.

Refine Locally (Select: Foil->Refine Locally)

The local Panel Refinement function is a sub-set of the more general Global Refinement function of QBlade. This is usually used for small refinements after local shape modifications such as flaps. When deflecting a flap it is necessary to refine the “flap hinge” area in order to avoid abrupt panel angle changes, which can lead to convergence problems. For this reason the local refinement is selected. The refinement region is given as a number from 0 to 1, with 0 representing the Leading Edge and 1 being the Trailing Edge point. The angle criterion defines the maximum permissible angle between panels and it usually is set to a relatively low value (<8.0°). The refinement code adds panels until the criterion is reached on both suction and pressure side.