**5. Conclusions**

A thorough review of contact models available for turbomachinery applications and the related calibration methods highlights the need for a method to solve the under-determinacy of the contact parameter estimation problem and, subsequently, to assess the adequacy of contact models. This chapter presents the evolution of the Piezo Damper Rig, a test facility for the experimental investigation of underplatform dampers. It was shown how its unique capability to provide kinematic and force related quantities while reproducing the real damper-platform kinematics allows for a trustworthy and univocal determination of contact parameters.

The measurement protocol and data processing technique ensure adequate uncertainty levels (i.e. <15%). The results can thus be used to perform safe and meaningful investigations on trends and variability of contact parameters.

The following conclusions can be drawn:


Heuristic models and sensible assumptions such as the uniformity of conforming contacts are nowadays considered a practical and adequate choice in turbomachinery applications. This is generally true, however special attention is required whenever a microscale phenomenon (e.g. nonuniform flat-on-flat contact, large rolling motion), not taken into account by the model, becomes prominent.

It was shown that the state-of-the-art heuristic contact model adopted in this chapter represents faultlessly run-in uniform flat-on-flat surfaces (i.e. Damper A). The same contact model CAN still be adapted to achieve simulated results matching the experimental evidence on dampers with irregular flat-on-flat contacts, but recalibrations are needed. For instance, a non-uniform distribution of ktL among contact points, adjustments of the dknL/dx and μL values. Unfortunately, at design stage, when it is not possible to know "a-priori" the condition of a given flat-on-flat contact surface, nor how long it will take for that surface to evolve towards a uniform distribution of contacts, this necessity for "adjustments" of contact parameters values translates into higher uncertainty levels. In other words, the state-of-the-art contact model used in this chapter is only partially adequate to represent all the complex phenomena observed. This adds its contribution to uncertainty.

On the other hand, other recalibrations (such as that needed for increasing normal loads at the flat-on-flat contact) or for very large rolling motions still signal that the heuristic model is not 100% adequate. Still, these dependences can be easily mapped and therefore do not add to the uncertainty.

One main outcome of this careful investigation, apart from the best fit values of the contact parameters (and the methodology used to obtain them), is an increased awareness of the limits and capabilities of heuristic contact models. The logical next step, the author is now working on, is the assessment of the influence that the uncertainty on contact parameters has at the blade response level.
