Implementation of a new transition prediction method in xfoil: Predicting transition in suction boundary layers

To reduce drag on wings, aerodynamicists have long been trying to keep the boundary layer on the wings laminar and to postpone transition to a turbulent boundary layer. To achieve this laminar _ow control by means of boundary layer suction can be used. At the Delft University of Technology xfoil has been adapted to design airfoils using boundary layer suction. This version is called xfoilsuc. The use of this design program was restricted because the transition point prediction methods in xfoilsuc could not predict the damping of disturbances in the boundary layer when suction was used. The objective of this study was therefore to implement a reliable transition prediction method into xfoilsuc that could predict transition for boundary layers with and without suction. To achieve this a new database eN method was implemented into xfoilsuc. This new method, dubbed the Improved eN-method, was developed by Van Ingen with some assistance from the present author. It uses the solutions of the Orr-Sommerfeld equation as calculated by Arnal, that describe the stability diagrams for _fteen values of Re_crit . These _fteen values of Re_crit represent boundary layers with shapefactors between 2.216 and 35.944. As characteristic parameter, the boundary layer shapefactor is used to correlate arbitrary boundary layers to a Re_crit value. By scaling and shifting some parameters, the data is stored in splines suitable for interpolation and some extrapolations. It was found that when using the Improved eN-method in xfoil due to the interaction between the boundary layer iterations and the transition prediction, most solutions would not converge. A method using forced transition was developed to remedy this problem. Using this method, convergence of the boundary layer solution is achieved without a_ecting the accuracy. The Improved eN-method was found to be able to accurately predict transition in boundary layers with and without suction. After the implementation of the Improved eN method a study into the quality of xfoils laminar boundary layer calculations revealed that the shapefactor can sometimes be a bit o_, compared to a _nite di_erence method using the pressure distribution as calculated in xfoil. This has an impact on the predicted transition point. Another problem is that for high suction velocities the shapefactor can go below the value of 2, which is considered erroneous. Further research into this is recommended. A case study concerning the DU99 airfoil, originally designed for the standard class ASW-28 sailplane, was done using the Improved eN-method. For this airfoil a suction distribution is designed that results in a 50 to 75 % reduction in drag in the low drag bucket, excluding the suction drag. Also the Clmax value is increased from 1.4 to 1.7. This airfoil is therefore well suited for further testing in a windtunnel.Aerospace Engineerin