Investigation into Boundary Layer Transition on the MEXICO Blade

Abstract

Boundary layer transition studies have been carried out using the unsteady surface pressure data from the EU project MEXICO and the later New-MEXICO experiment at the DNW wind tunnel, which have been subject of investigation in IEA Wind Task 29 Mexnext. The experiments were conducted on a specially designed 4.5 m diameter wind turbine with and without zig-zag roughness strips applied to the outboard part of the blades. Standstill tests with the MEXICO blades were also conducted at the TU Delft LTT wind tunnel including oil flow and stethoscope tests.Transition is determined by detecting and observing the growth of Tollmien-Schlichting (TS) packets through a Power Spectral Density plot in the frequency domain. The expected frequency range of these waves is determined using a database developed by Van Ingen. The transition results are compared to those determined by RFOIL, a program for the analysis of aerofoils which uses the eN method for transition detection. Generally a good agreement was observed between measurements and predictions. In addition to that the effectiveness of the roughness strips was studied. The result is a unique database in controlled conditions to be used for validation and improvement of transition modelling of wind turbine blades.