Experiments on annular cascades

Experiments on annular cascades

Aeroelastic investigations with vibrating cascades are performed in the Non-Rotating Annular Test Facility (RGP).


Figure 1 : scheme of the RGP streamtube and test section position

The main advantages of the test rig are (see Figure 1) :

  • No lateral walls in circumferential direction – no reflection of pressure waves
  • Periodic flow through blade passages
  • Non rotating cascade – extended instrumentation possible
  • Streamtube shape enables supersonic flows at the inlet of the test section

Most vibrating cascade experiments in the annular channel are performed in controlled vibration, gust or forced response and flutter, with or without structural mistuning.

Annular cascade

The annular vibrating cascades consist of 20 blades, which can be set in vibration with a selectable frequency (within a range of 150-330 Hz), amplitude and interblade phase angle. Each blade system comprises its individual vibration system, consisting of a spring (blade suspension element), a mass (designed to enable torsional or bending movements), a blade base and the blade itself (Figure 2). The spring element geometry determines the natural frequency and vibration modeshape of the cascade. Labyrinth seals are included in the blade base design to guarantee a free motion of the vibrating blades (prevent collisions), as well as to limit the flow leakage present between the 20 individual blade systems.

The blades are forced into vibration by means of 20 electromagnetic exciters. The blade oscillations are measured by inductive displacement transducers (Model type TQ 102) from Vibro-Meter (Switzerland). Each blade has its own feedback control device, working in closed loop. It allows to establish and maintain constant amplitude and constant interblade phase angle between blades (controlled vibration measurements). Since this system can only exert a limited force on the blades, the excitation frequencies have to be close to the natural frequencies of the mechanical system.

The blade vibration system can only excite the blades and not damp the blade movements. To block the blades during the steady-state experiments, a hydraulic brake is mounted, acting on the mass elements. It also prevents possible damages on the blade row in cases of flutter investigations, by limiting the vibration amplitude during unsteady measurement. The test facility casing wall is instrumented with 16 pressure taps, distributed axially in the vicinity of the blade tip. Combined with a step indexing feature, this enables the flow mapping (circumferential  and axial directions) in the blade tip area.


Figure 2 : A) CAD blade vibration system with spring, mass, blade base and blade element. B) Blade vibration system, equipped with static pressure taps for steady-state surface pressure measurements (tubings are the connected to a digital sensor array, to the data acquisition device). C). CAD model of the entire compressor row, consisting of 20 blade vibration systems and labyrinth seals D). Complete compressor cascade test model dedicated to the FUTURE project measurements (source : EU-FUTURE Project)

Cascade instrumentation

The cascade is instrumented with static pressure taps dedicated to steady-state and unsteady pressure measurements on the blade surface. The measurement positions are usually distributed along the blade chord. For transonic flow conditions, the shock position is detected by placing several pressure taps in the shock area. The tappings are placed on both blade pressure and suction side of neighboring blades, in order to assess the data in one interblade passage.

For unsteady pressure measurements, two types of pressure transducers are used. Cylindrical pressure transducers of type KULITE XCQ-062 are embedded directly in the blade profile, and connected to the blade surface pressure taps. Subminiatur pressure transducers of type KULITE LQ 1-062-25A are surface mounted. Compared to the other type of transducers, they can be placed closer together (< 3.5mm) and closer to the blade trailing edge. The disadvantage  of surface mounted transducers is essentially their influence on the blade surface profile, as well as their exposure to all possible damaging effects (particles in the flow, mistreatment, etc…).

The electrical signal of the transducer is amplified, filtered and digitized. The GTT  unsteady data acquisition system enables a sampling rate of 180000 samples per second and records up to 64 channels. The sampling rate per channel depends on the number of channels connected. Raw data is stored and evaluated on a data acquisition PC.

Steady-state flow field measurements

In order to compare and analyse the blade surface unsteady pressures, one has to characterize the steady-state flow field characteristics.

Steady-state flow conditions are measured by aerodynamic 5-hole probes, upstream and downstream of the test section (Figure 3). After a calibration process, the pressures measured by the probes enable the determination of total pressures pt1 and pt2, steady pressures p1 and p2, as well as the interpolation of Mach numbers Ma1, Ma2 and flow angles β1 and β2. Two-step motor driven probe-holders, combined with a step indexing feature allow the flow mapping over the radial and circumferential directions. Typically, 15 radial positions are measured over 18°, in steps of 1°, which corresponds to one inter-blade passage. 18 axial measuring locations are also included in the casing to provide flow field data in the blade tip area (18 axial position measured over one blade passage).

flow fields measurements_2.gif

Figure 3 :  Mach numbers, measured upstream and downstream of the test section, with casing steady-state measurements. Inlet and outlet flow field characteristics are measured with 5-hole aerodynamic probes (steady-state measurements). This yields 2-D information about flow-angles, Mach-numbers, total pressures and the mass-flow distribution in upstream and downstream of the test section  (source : results from H.Körbächer, 1996,” Experimental investigation of the unsteady flow in an oscillating annular compressor cascade”)

Blade steady-state surface pressure measurements

The blade static pressure distribution is measured on the blade surfaces at selected blade spans. This enables the calculation of the isentropic Mach number along the blade chord length (Figure 4). 3D flow effects can be measured by assessing the static pressures at different blade spans. Typical measurement spans are 20% (measurements in the hub vicinity ), 50% (mid-channel measurements) and 90% spans (measurements in the tip gap area).

Isentropic Mach number

 Figure 4: Isentropic Mach number based on the blade steady-state surface static pressure measurements, located at 50% of the blade span (source : results from H.Körbächer, 1996,” Experimental investigation of the unsteady flow in an oscillating annular compressor cascade”)


 Measurement of time-dependent pressures

Unsteady static pressures are measured with the cascades vibrating in single blade vibration mode (only one blade vibrates at a time) or in travelling wave mode (all blade vibrate with a constant amplitude and interblade phase angle). The time-dependent pressures are measured with unsteady pressure transducers embedded in the blades. Pressure tappings present on the test facility casing wall can be connected to unsteady pressure transducers as well. An ensemble averging technique is used to calculate the time-dependent pressures at a specific transducer location. The associated integration over the blade surfaces yields the unsteady forces which can be correlated to the vibration stability (self-excitation) or if the cascade vibration is damped (damping coefficient). Unsteady aerodynamic influence coefficients or each vibrating blade can be calculated from the measured data of different cascade vibration modes using a decomposition technique.

Figure 5 and Figure 6, Animations 1 and 2 provide a visualization of the measurement results :


Figure 5 : Comparison of unsteady pressure levels and isentropic Mach number isolines measured at the test rig casing wall (Source : results from H.Körbächer, 1996,” Experimental investigation of the unsteady flow in an oscillating annular compressor cascade”)


Figure 6 : A cube of colour coded unsteady pressure coefficients on a blade surface with the chordwise position, the interblade phase angles and the angle of attack as the three axis parameters for a constant inlet Mach number (source : results from H.Körbächer, 1996,” Experimental investigation of the unsteady flow in an oscillating annular compressor cascade”).

Animation 1 : Measured time-dependent pressures at the outer channel wall while the cascade is vibrating in travelling wave mode (IBPA = 180°). (Source : results from H.Körbächer, 1996,” Experimental investigation of the unsteady flow in an oscillating annular compressor cascade”).

Animation 2: Measured time-dependent pressures at the outer channel wall. The middle blade is vibrating in single blade vibration mode (source : results from H.Körbächer, 1996, “Experimental investigation of the unsteady flow in an oscillating annular compressor cascade”).

Rotor/stator interaction

In the frame of a european research project (Brite-Euram …) a rotor with elliptical struts was put in front of a turbine cascade in the annular channel in order to examine the influence of wakes on vibrating cascades. The wakes create periodic time-dependent disturbances. The unsteady forces caused by the wakes excite the blades to vibrate and the blade vibrations cause additional unsteady aerodynamic forces. One goal of the project was to study the superposition and interaction of the unsteady aerodynamic forces caused by the wakes and those created by blade vibration.

Animation 3: Calculated Mach number distribution in a turbine cascade with rotating elliptical struts in front (results from F.Rottmeier, 2000, “Experimental investigation of a vibrating axial turbine cascade in presence of upstream generated aerodynamic gusts”).