Handbook of Turbomachinery Second Edition Revised - Ventech!

working fluid interactions, couplings, seals, splines, internal damping, etc.
Some technical issues of concern to development engineers include the
frequency order of the response relative to the rotation speed, noise generation
(decibel level and steady versus beating), possible rotor–structure rubs and/or
rotor–rotor rubs, deflection of the rotating assembly at seal locations, and
bearing loads and their influence on bearing life and failure.
The dynamic characteristics of rotor systems that are of primary
interest to the machine analyst and designer include
Natural frequencies of whirl (whirl speeds) and associated mode
shapes
Critical speeds and associated mode shapes
Range of dynamically stable operation
Steady spin-speed unbalance response (deflections and bearing loads)
Steady response due to maneuvers (turns, rolls, etc.)
Transient response
Startup and shutdown operation (variable spin-speed)
Foreign substance ingestion (e.g., birds)
Blade-loss excitation (step change in unbalance)
Hard landings and maneuvers
Response from self-excitation mechanisms
Knowledge of these characteristics is helpful in assisting a design engineer in
selecting design parameters to avoid unstable regions of operation and/or
large-amplitude response within the operating range of the system.
Rotordynamic simulation may be used to assist the designer in the
preliminary design of a new system, in the redesign of an existing system,
or in determining the cause of undesirable response characteristics of an
existing system.
The motion of a rotor system is usually described in terms of physical
coordinates as observed from afixed reference (Fig. 2). Most analyses are
posed in terms of these physical coordinates as long as the number of
coordinates does not present significant computational problems. Many
coordinate reduction procedures and/or modal analysis procedures have
been successfully employed in the rotordynamic simulation of ‘‘very large’’order
systems. The interested reader is referred to more extensive treatment
of these topics in references such as Ehrich [6].
Rotordynamic systems are normally modeled as a finite set of
interconnecting flexible-shaft elements, bearings and dampers, working
fluid mechanisms, rigid and/or flexible disks, rotational springs to connect
rotor sections, and the support structure. The system components are
typically defined by using a lumped-mass or consistent-mass (finite-element)
idealization, or a combination of these two approaches. The system
Copyright © 2003 Marcel Dekker, Inc.