European Transonic Wind Tunnel

Since this investigation was funded by the European Commission as part of the ESWIRP project, the available budget only allowed for testing over a limited range of conditions. The test plan for the 5-day test campaign in the ETW was determined based on a compromise between test requirements from the European project group chaired by J.L. Goddard from ONERA-France which focused on acquiring data for CFD validations of unsteady wake flows and a repeat of the conditions at which the used CRM model had been tested in the NTF. A few polars were added at a very low Reynolds number to provide comparative aerodynamic data for the Japanese research organisation JAXA who have tested the CRM in a downscaled version in their transonic tunnel.

For achieving the scientific goal of the project, newly integrated measurement capabilities were operated during the campaign: unsteady PIV for wake flow analysis and unsteady and steady model deformation measurements combined with the recording of unsteady balance signals taking the benefit of an upgraded fast high capacity data acquisition system. In the frame of the present paper only aerodynamic data like force, moments and wing pressure distributions combined with the wing deformation are presented. Although, data were acquired at 12 different Mach numbers ranging from 0.25 to 0.87 the majority focussed on M=0.7 and the model design Mach number of 0.85. So, with respect to the intended comparison of results, the reference test conditions of the NTF at these two Mach numbers were carefully set and controlled. To cover the relevant Reynolds numbers of 5, 19.8 and 30 million the tunnel temperature was varied between 302 K and 117 K combined with corresponding pressures between 200 and 300 kPa. The operating envelopes of NTF and ETW do not allow achieving the minimum and maximum Reynolds number at the identical q/E value. Hence, it was decided to duplicate the 19.8 million Reynolds number at a lower and higher q/E value allowing an additional comparison of the model deformation assessment as a function of the different aeroelastic effects. By performing lift polars with the model in upright and inverted position the upwash could be assessed as 0.010 to 0.015 deg over the full operating range. The measured data were additionally corrected for wall interference based on the ETW experimental assessment established in the past. Extreme care is always given to the measurement of the model angle of attack. Before starting the test campaign the electrical offset and misalignment of the relevant inclinometer inside the model is checked even under load applied to it. Special care was also given to the application of the transition band classically used when testing at a chord Reynolds number of 5 million. Performing this work in close cooperation with the NTF experts minimised the risk for later mismatches in the results originated by this sensitive item.

Before facilities like NTF and ETW went into operation engineers were convinced of the rigidity of wind tunnel models not suffering remarkable deformations generated by aerodynamic loads especially present in pressurised facilities. Using the unique capabilities of these tunnels for establishing identical Reynolds numbers at different pressure or better q/E levels the opposite could be proven. Nowadays, the correct assessment of the wing shape under load is mandatory for all comparisons to CFD results. Starting with the developments of appropriate measurement systems with a Moirée tool today, ETW may operate 4 systems in parallel, e.g. for monitoring the shape of the main wing and all high lift components of a half-model. The Stereo Pattern Tracking systems (SPT) are capable of monitoring the shift in space of markers pasted on the lower wing surface. In the test campaign reported here one system was looking on the main wing with 58 frames/sec while a second one monitored unsteady HTP movements with 386 frames/sec.

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