Annular Test Facility Annular Wind Tunnel

The experimental investigations presented here were performed in the wind tunnel for annular cascades at the EPFL. The annular cascade tunnel was de­veloped for a research project between BBC and the EPFL to investigated the steady and unsteady ft>w in annular cascades without having to rotate them; Bolcs (1983). A spiral ft>w is generated in order to simulate real inflow an­gles such as those that would occur in a rotating cascade. Figure 1 shows the cross-section of the wind tunnel. The advantage of an annular cascade such as circumferential ft>w periodicity is combined with the advantage of a fixed cas­cade in respect of data aquisition and data transfer. Steady ft>w conditions are measured by aerodynamic probes in the upstream and downstream sections and by pressure taps on the blades’ surfaces. The aerodynamic probes were calibrated to obtain the total pressure ptl and pt2, the steady pressures p1 and p2, the ft>w angles в і and вг, and the Mach numbers Ma 1 and Ma2 from the measured pressure data.

1.1 Cascade

The compressor cascade used here is composed of 20 blades with a NACA 3506 profile of c = 80 mm in nominal chord length, a stagger angle of вд = 40° , and a pitch at midspan of s = 56.5 mm (Fig. 2). The profiles were shortened to a cred = 77.5 mm chord length with a round trailing edge (see

also Fig. 5 and 6). The blades are mounted via elastic spring suspensions which allow a torsional motion around midchord.

A NACA3506 profile was chosen for the investigations of tuned bending vibrations by Kbrbacher (1996). The fact that pitching motions of this pro­file should be more critical in aeroelastic stability – mainly for transonic fbw conditions – was the reason why investigations of the aerodynamic damp­ing using excited pitching vibrations were carried out by Hennings and Beiz (1999, 2000). For the fijtter measurements the blades are tuned to lower eigen – frequencies in order to reach an aerodynamically as well as aeroelastically un­stable case. For the latter one the aerodynamic self-excitation has to surpass the structural damping of the cascade, namely of the elastic spring suspension.

For experimental investigations concerning the aerodynamic damping, the blades are driven by electromagnetic exciters such that their motions represent a traveling wave modes with one of the possible interblade phase angles

(1)

given by the number of N = 20 blades. The unsteady pitching motions

ati(t) = a ■ cos(Ш — і og)

of the blades are controlled in both their amplitudes a and interblade phase angles (Tk■ Tn order to reach appropriate amplitudes, the inertia of each blade and the vibrating part of the inner wall of the wind tunnel had to be reduced and the blades had to be excited near resonance (Figure 3 shows the assembly of the cascade). Lowering the eigenfrequencies by a minor mass moment of inertia, the torsional stiffness had to be reduced as well. So, the demand of both lower torsional stiffness (in presence of a high transversal stiffness to avoid a heaving motion) and lower structural damping led to a new one-piece spring suspension.

The spring suspension was made out a cylindrical part by cutting sections out using electrical discharge machining. The remaining section consists of

eight rectangular beams orientated such that the torsional stiffness is low and the transverse stiffness is high. The use of electrical discharge machining made it possible to manufacture the spring suspension out of one part, which results in a very low structural damping (see Figure 4).