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Test Stand
To realize fl>w about the vane cascade with prescribed velocities and angles of attack, the cascade was placed in a wind tunnel (Figures 1, 2 ) in which Mach numbers up to 0.7 could be attained. The four central vanes in the cascade were secured to individual vibration units and could undergo prescribed vibrations with two degrees of freedom. Since there was some mechanical coupling even with vibration-proofing elements between the units, it was necessary to keep the vibrations in all of the units steady (whether there was a fbw or not) in order to prevent this coupling from affecting the measurement results. Proceeding on the basis of this requirement, we employed an eight – channel feedback system to automatically control the vibrations of the vanes (vibration units). The system also controls the voltages and, thus, the forces on the vibrator coils so as to reduce the difference between the signal of the master oscillator and the vane vibration signal obtained in each channel (the equipment used was described in more detail in Tsymaluk 1996).
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Figure 1. Test stand |
The considered cascade consists of 9 airfoils. The central three were fastened cantilever on the individual vibro-unit (see Figure 3), they could accomplish assigned displacements, aerodynamic loads were measured on them. Such airfoils are called active. An elastic suspension was designed for vanes that has two elastic elements of different widths. The auxiliary (narrow) elas-
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Figure 2. Linear Test Facility, 1 – nuzzle wall; 2 – Pito tube; 3 – blade; 4 – airfoil; 5,8 – outlet rotary screens; 6 – rotary disk; 7 – openings for static pressure release |
tic element does not impede the twisting of the main (wide) element about its own longitudinal axis, and during fexural vibrations the two elements form an elastic parallelogram. This setup ensures constant vibration parameters along the vane. The unit just described also makes it possible to change the natural frequencies of the suspension by using replaceable main elements differing only in thickness.
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Figure 3. Structure of the airfoil fhxible suspension 1 – airfoil; 2 – voice coil of the vibrator; 3 – elastic elements; 4 – strain gages |
There were used three active airfoils to asset aerodynamic loads on initial airfoil (n=0), which were induced by vibration of airfoils -2, using designed experimental workbench. This could be accounted for periodicity of inflience of airfoil n = -1onn=1 one to be the same as for n = -2 on n = 0, inflience of n = 1 airfoil on n = -1 – the same as for n = 2 on n = 0 airfoil.
In accordance with developed method, the required vibrations were induced for every active airfoil. The first harmonic of the unsteady aerodynamic force and moment was measured on them. It should be noted, that determined unsteady aerodynamic forces and moments, and thus aerodynamic influence coefficients (AIC) were related to the center of airfoil chord, about which its angular displacement occurred.
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The geometrical characteristics of the compressor cascade and oscillation regimes are presented by Tsimbalyuk et al. 2002. The blade length L=0.069 m, the chord length b= 0.05 m, the circular camber 10o, the thickness-to chord ration 0.07, the stager angle 60 deg., pitch-to – chord ration 0.78, the inlet Mach number 0.12 -0.35, the vibration amplitude ho =0.0007 m, ao =0.0084 rad. The cross-section of the blade is presented in Figure 4.
Figure 4. Airfoil coordinates, solid line-suction side, dashed line – pressire side
In order to calculate the AIC the vibrations of airfoils were induced by turns. The vibration amplitude of the blade was almost invariable along its length and equal to и = 85.85 Hz, which is equal to the natural frequency of the system.
