Remote identification of impact forces on loosely supported tubes: Analysis of multi-supported systems

Impact forces are useful information in field monitoring of many industrial components, such as heat exchangers, condensers, etc. In two previous pape rs we presented techniques-based on vibratory measurements remote fr om the actual impact locations-for the experimental identification of isolated im pacts (Araujo et al., 1996) and complex rattling forces (Antunes et at, 199 7). In both papers a single gap support was assumed. Those results concern systems which are simpler than the actual multi-supported tube bundles foun d in heat exchangers. Impact force identification is a difficult problem fo r such systems, because 1) when sensed by the remote motion transducers, th e traveling waves generated at several impact supports are mixed, and there is no obvious way to isolate the contribution of each support; 2) multisup ported tubes may be quite long, with significant dissipative effects (by in teracting flows or by frictional phenomena at the clearance supports), lead ing to some loss of the information carried by the traveling waves; 3) in m ulti-supported systems, some of the supports are often in permanent contact , leading to nonimpulsive forces which are difficult to identify. In this p aper we move closer towards force identification under realistic conditions . Only the first problem of wave isolation is addressed, assuming that damp ing effects are small and also that all clearance supports are impacting. A n iterative multiple-identification method is introduced, which operates in an alternate fashion between the time and frequency domains. This techniqu e proved to be effective in isolating the impact forces generated at each g ap support. Experiments were performed on a long beam with three clearance supports, excited by random forces. Beam motions were planar, with complex rattling at the supports. Experimental results are quite satisfactory as th e identified impact forces compare favorably,vith the direct measurements.