Self-excited instabilities

The first example is a small-scale laboratory combustor using an aeroengine gas turbine burner (power 30 kW) while the second one corresponds to a laboratory-scale staged burner in which self-excited instabilities can be easily triggered by changing the outlet acoustic boundary conditions. In staged combustors, fuel and air are premixed but they are introduced into the chamber at different locations and different equivalence ratios so that partially premixed flames are found inside the burner. All combustors are operated at atmospheric pressure. 

Forced or Resonant Vibration Self-Excited or Instability Vibration... 

Figure 5-22a. Hysteretic whirl. (Ehrich, F.F., Identification and Avoidance of Instabilities and Self-Excited Vibrations in Rotating Machinery, Adopted from ASME Paper 72-DE-21, General Electric Co., Aircraft Engine Group, Group Engineering Division, May 11, 1972.)...

One of the most serious forms of instability encountered in journal bearing operation is known as half-frequency whirl. It is caused by self-excited vibration and characterized by the shaft center orbiting around the bearing center at a frequency of approximately half of the shaft rotational speed as shown in Figure 13-15. 

Koch, W. (1985). Local instability characteristics and frequency determination of self excited wake flow. J. Sound and vib., 99, 53-83. 

Self-excited combustion instabilities are associated with the propagation and reflection of heat-release-induced acoustic waves and their interactions. Hence, flame sound represents the main source of these acoustic waves. Therefore, sound pressure level (SPL) data for turbulent nonpremixed jet flames have been obtained for two Turbulent Nonpremixed Flame (TNF) workshop flames, DLR-A and DLR-B [1]. The exit Reynolds numbers (Re) for the two flames based on injected gas properties at room temperature were 15.200 (DLR-A) and 22,800 (DLR-B). Air was used for studying the sound emission from equivalent nonreacting jets. The flow in each case had very low exit Mach numbers (M = 0.04-... 

An instability effect of a closed passive resonance system in a self-exciting oscillating amplitude because of a small perturbation of frequency, co, in the potential state given by... 

The self-excited loads are the portion of aerodynamic loads which are responsible for the instability of the bridge deck. A number of different aerodynamic phenomena, such as flutter, vortex-shedding, torsional divergence, etc., may play a role in the bridge instability. However, only the instability problem caused by flutter, which is considered to be most critical for long-span bridges, is considered herein. 

The sliding nature of the contact in lead screw drives puts great importance on the role of friction on their performance. In addition to efficiency concerns, driving torque requirements, or wear, friction can be the cause of dynamic instabilities, resulting in self-excited vibrations which deteriorates the performance of the system and may cause unacceptable levels of audible noise. 

Different models have been proposed to reproduce this type of velocity-dependent friction (see, e.g., [16, 17]). The important feature of these models is the existence of a region of negative slope in the friction-velocity curve, which may lead to self-excited vibrations. This type of instability is discussed in Sect. 4.1 and Chap. 6. 

Clean amplitude and phase curves are desirable in d3mamic-mode AFM, especially with the FM detection method. In FM-AFM, a cantilever is always oscillated at its resonance frequency using the self-oscillation circuit. This self-oscillation loop works as a feedback circuit to keep at —90°. This feedback control operates on the linear slope of the phase versus frequency curve around the resonance frequency, as shown in Fig. 18.1b. Thus, large distortions in the phase curve result in instability of the cantilever excitation and deteriorate the accuracy of the force measurements by FM-AFM. 

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