Experimental Investigations

As blade flitter in axial turbomachines is caused by an interaction of the blade motions and the motion-induced unsteady aerodynamic forces, the main parameters for flitter beside the cascade’s geometry and structural properties are the fl»w conditions and the damping properties of the structure. Hence, two different approaches for experimental flitter investigations are possible: in the so-called "aerodynamic approach” a certain level of structural damp­ing is needed for the blades to prevent self-excited vibrations if the cascade

is aerodynamically unstable. In order to determine the aerodynamic damp­ing the blades are forced into controlled harmonic vibrations in each traveling wave mode successively and the motion induced unsteady pressure distribu­tions is measured. The analysis of this data — in particular the out-of-phase unsteady harmonic pressure — leads to an estimation of the aerodynamic sta­bility of each traveling wave mode. In a so-called "aeroclastic approach" the aerodynamic instability can overcome for the structural damping and the cas­cade starts to Hitter. With the use of some safety devices (like hydaulic flitter brakes), the blade vibrations and the unsteady pressure distributions can be measured at the onset of flitter.

In order to drive the cascade into flitter each blade was tuned to a lower cigcnfrcqucncy of 183 Hz by increasing the mass moment of inertia. This cas­cade was mounted in the annular wind tunnel. Steady fbw conditions were achieved by adjusting the inlet total pressure, the inlet infbw angle, and the back pressure. A hydraulic brake prevented blade vibrations during the ad­justment of the steady ftm These fbw conditions were surveyed by probe measurements of upstream and downstream fbw field and by measuring the steady pressure distribution on the blades.

At the up – and down-stream fbw field an aerodynamic probe was used to scan one pitch of the cascade by taking small steps in the radial and circum­ferential directions. The measured probe pressures w’ere used to compute mass fbw averaged in – and out-fbw values such as p tl, pt2, pi, p->, etc. (Table 1). The steady pressure distribution the blades were measured with pressure taps on the pressure and suction surface at nine equidistant chordwise positions and the three radial positions z/h = 0.2, 0.5, and 0.8 on several blades. They were transformed to steady pressure coefficients

P ~ ‘P i Pti ~ Pi

using the mass fbw averaged values of the infbw total and steady pressure p ц and pi, respectively (Fig. 5).

Afterwards, the hydaulic brake w’as released to set the blade assembly to controlled pitching oscillations for the performance of the unsteady measure­ments ("aerodynamic approach") or to allow self-excited vibrations for the performance of the flitter experiments ("aeroelastic approach"). The unsteady pressure distribution was measured by 25 piezoelectric transducers, 15 of w’hich
are mounted on the suction side and 10 on the pressure side of the blades. The transducers were distributed in blocks on only four blades. In each block, the transducers are located close together in order to resolve the unsteady pressure distribution near possible shock positions (Fig. 6).

The blade vibrations were measured by an eddy-current displacement sensor for each blade.