XFOIL Tips – No.3 – Airfoil Generation, Export & Interpolation

XFOIL Tips – No.3 – Airfoil Generation, Export & Interpolation

As a conclusion of the airfoil shape operation information we add some more information on useful airfoil generation, export and interpolation functions. The next steps of our QBlade Blog posts will concentrate of airfoil Direct and Inverse Simulations, but please bare with us till then.

Spline airfoil (Menu: Splines)

A manual airfoil shape generator is available in XFOIL and the feature is maintained in QBlade. This function allows a complete airfoil shape to be described by a single spline with 14 control points. The user is able to manually adjust the control points in order to achieve the desired airfoil shape. If the spline behavior is not the desired one, it is possible to add more control points, or explicitly vary their coordinates as well as their polynomial degree (Select: Splines->Splines Params).

Splines are mostly helpful for automatic optimization routines, which will be introduced in QBlade at a later stage. However some specific airfoil shaping requirements benefit a lot from the existence of the spline functionality.

Circular Airfoil (Select: Foil->Generate a Circular Foil)

A wind turbine specific requirement is that of handling circular foils. These are necessary for the blade root region and therefore they have to be either imported through an airfoil file or be generated with the circular foil generator. The only variable that the user can change at this point is the drag coefficient of the generated circular airfoil. This happens because XFOIL is not able to compute the flow if such a bluff body with massive separation region. Therefore by default the typical experimental drag coefficient (Cd) value of 1.2 for a cylinder is given to that airfoil. The user however can modify this value. There is no lift from a non-rotating cylinder and therefore no lift coefficient is defined.

Export airfoils (Select: Foil->Export…)

The airfoil Export function is equally important with the Import function of the airfoil module of QBlade. All the generated, refined, optimized and modified airfoil shapes can be exported in the form of a coordinate .dat file that carries the airfoil name in its 1st line.

Tip: It is good practice to generate your own airfoil library with properly named and refined airfoils. It is sometimes easy to import/generate an airfoil and forget to refine or modify it…and then lose track of changes. A properly organized airfoil library helps keeping track of hundreds of different airfoil shapes!

Interpolate Foils (Select: Foil-> Interpolate Foils)

The interpolate foil function is an extremely useful one, especially in the field of Horizontal Axis Wind Turbine (HAWT) blade design. This tool gives the user the ability to linearly interpolate between two different airfoils and generate a mixed-airfoil shape. The interpolation routine is controlled by a slider which modifies the weight factor of each of the two airfoils in the final mix! Especially when designing the blade root region or when refining airfoil-transitions along the blade, the Interpolate Foil function is invaluable!!

The QBlade Team!