Ulrich Retze

Measurement of 2D dynamic stress distributions with a 3D-Scanning Laser Doppler Vibrometer

Recently a method has been developed to measure surface dynamic strain and stress fields by using a contactless 3D scanning laser Doppler vibrometer. Assuming a free visible surface of the test specimen this method, a combination of advanced measurement technique and a post-processing algorithm, enables the assessment of high resolution dynamic surface loadings including quantity and direction of strain/stress distributions. The first part of the article describes the basic principles of the method. The second part informs about the validation work of the measurement results. Different steps were performed in order to measure real structures. A numerical model was the base for correlation with measured deflection-shapes, strain- and stress-fields. During this analysis, the different influencing parameters for measuring the strain information could be analyzed systematically. The third part presents a comparison of vibrometer measurements to strain gauge values. The validation of the method shows a high level of agreement between measured values compared to numerical strain and stress results. The correctness of the absolute strain values could be validated by the comparison with strain gauges. Further research will be done to close the gap between simulation and measurement.

Contribution to Free and Forced Vibration Analysis of an Intentionally Mistuned Blisk

Most of the experimental mistuning studies are performed using a blisk with random mistuning only. Intentional mistuning is often investigated analytically with respect to aeroelasticity, as it is well known that intentional mistuning reduces the flutter risk due to less interaction between the blades.In this paper, an intentionally mistuned test blisk is investigated both analytically and experimentally with respect to free and forced vibrations. First, free vibrations are studied and aliasing effects for the intentionally mistuned blisk are analyzed in comparison with a tuned blisk. A comparison between the experimentally determined dominant nodal diameters and the computed ones shows good agreement.Then, the blisk is experimentally excited by a travelling wave for various engine orders. Similar investigations are performed with a FEM model of the blisk and a reduced-order code. The amplification factor for some modes and several blisks is compared. The influence of the disc onto the blade mode shapes is studied for the tuned and mistuned case without and with aerodynamic coupling effects.Cyclic spacing of vanes is a concept to reduce the vibration level of downstream rotor blades by distributing the excitation onto more engine orders while reducing the overall excitation level. In this paper it is shown for blisks with and without intentional mistuning that care should be taken in applying this concept in the vicinity of veering regions, because the amplification factor in a veering region may become much higher than compared to other nodal diameters.Copyright

Experimental and Analytical Mistuning Analysis of a Blisk at Lab Conditions and Under Rig Conditions Using Tip Timing

Determination of the amplification factor due to mistuning is an important task for the safe design of turbomachinery, specially for blisk-design with low mechanical damping. The complexity of the environment effects increases from measurements in the laboratory to measurements in test rigs and engines. Also the uncertainties of the boundary conditions increase. In this paper measurements of the amplification factors due to mistuning at various conditions are presented, starting from simple lab measurements up to rig measurements. Calculations with a Reduced-Order Code based on measured frequency distributions and FEM are performed and compared with the measurements. For the determination of the amplification factor a mistuning rig with travelling wave excitation was built. For a small demo-blisk the amplification factor was determined. Then a real blisk was tested. Afterward, the same blisk was built into a two-stage axial compressor rig. Here, tip-timing measurements were performed under rig conditions (centrifugal force, flow conditions). As tip-timing measures the vibration amplitude of each single airfoil, an amplification factor can be determined for each resonance.Copyright

Multi-Body Damping of a Vane Cluster

Spring-damper systems are standard for reducing blade vibration amplitude at vane clusters. Spring-dampers can only be used with an altered geometry of the inner shrouds. In most cases a separation of the inner shrouds is inevitable. In this paper an alternative damping system without changes of the outer inner shroud geometry is developed and analyzed. Two analytical models — a simplified Rigid Body Model and a 3D Finite Element Model show, based on similar results, a good comparison. The analytical results were validated by shaker tests. A high level of agreement between simulation and test was achieved.Copyright