Friction damper optimization: Simulation of rainbow tests

Friction dampers have been used to reduce turbine blade vibration levels fo r a considerable period of time. However optimal design of these dampers ha s been quite difficult due both to a lack of adequate theoretical predictio ns and to difficulties in conducting reliable experiments. One of the diffi culties of damper weight optimization via the experimental route has been t he inevitable effects of mistuning. Also, conducting separate experiments f or different damper weights involves excessive cost. Therefore, current pra ctice in the turbomachinery industry has been to Conduct so-called "rainbow tests" where friction dampers with different weights are placed between bl ades with a predefined configuration. However, it has been observed that so me rainbow test results have been difficult to interpret and have been inco nclusive for determining the optimum damper weight for a given bladed-disk assembly. A new method of analysis-a combination of the harmonic balance me thod and structural modification approaches-is presented in this paper for the analysis of structures with friction interfaces and the method is appli ed to search for qualitative answers about the so-called "rainbow tests" in turbomachinery applications. A simple lumped-parameter model of a bladed-d isk model was used and different damper weights were modeled using friction elements with different characteristics. Resonance response levels were ob tained for bladed disks with various numbers of blades under various engine -order excitations. It was found that rainbow tests, where friction dampers with different weights are used on the same bladed-disk assembly, can be u sed to find the optimum damper weight if the mode of vibration concerned ha s weak blade-to-blade coupling (the case where the disk is almost rigid and blades vibrate almost independently from each other). Otherwise, it is ver y difficult to draw any reliable conclusion from such expensive experiments .