PIV and BOS measurements at an airfoil in transonic buffeting with flow control

AVERT is an EC funded research project within FP6. The major objective of the AVERT project is the development and industrialisation of active flow control technologies for application to a realistic configuration and thereby reduce drag significantly. The achievement of the objective will give the aircraft manufacturers within AVERT confidence that emerging flow control technologies can be industrialised to the point of practical and beneficial application to an aircraft operating in a commercial environment. The project objective will be achieved through the evaluation of selected types of sensor, actuator and control systems, the assessment of these devices against two baseline aircraft configurations and the validation of the most promising technologies through large scale wind tunnel tests.

Fig. 1: Instantaneous velocity vector fields without blowing at the fluidic trailing edge device and with shock-induced buffeting and strong separation (click here to enlarge) . Fig. 2: Instantaneous velocity field realized at the same CL as in Fig 1 with blowing at 1.5 bar with a lower angle of attack which inhibits buffeting (click here to enlarge) . Fig. 3: Density gradient field visualization by BOS technique (click here to enlarge)

The PIV and BOS measurement techniques from the department of Experimental Methods have been applied to a transonic 2D-wing at Ma = 0.736 in the VZLU suck down wind tunnel A4 in Prague in order to study the fluid dynamical effects of a fluidic trailing edge control device in one task of AVERT. A 0.5 mm spanwise blowing slot at x/c = 0.94 driven with pressures between 1.5 and 3 bars blows nitrogen perpendicular to the pressure side surface of a trailing edge segment which has been developed by LEA (University Poitiers) and attached to the ONERA OAT15A wing with 200 mm chord length in order to postpone the buffeting limits of a transonic wing. This control will allow cruise at a higher Mach number and/or higher CL with lower angles of attack. The fluidic trailing edge device control enlarges the CL, so that for the same CL lower angles of attack can be applied reaching regions of the flight envelope without buffeting and reduced separation due to shock-wave boundary layer interaction.

The figures 1 and 2 show the instantaneous velocity vector fields at the conditions written on the plots. Figure 1 shows the velocity field without blowing at the fluidic trailing edge device and with buffeting and strong separation, while figure 2 shows a velocity field realized at the same CL with blowing at 1.5 bar with a lower angle of attack, which inhibits buffeting and reduces drag. In figure 3 density gradient field visualization by BOS shows the induced field caused by the blowing device and the flow separation at the trailing edge during transonic buffeting.