Investigations of the effect of eccentricity on the unsteady fluid forces on the centrebody of an annular diffuser

Abstract

In recent years several analytical and experimental studies have been carried out into the excitation of coaxial cylindrical bodies by annular flows, and an understanding of the mechanisms which lead to self-excited vibrations has been achieved. It has been observed experimentally, however, that sometimes geometries which are stable when concentric become unstable if eccentricity is introduced. Often in these cases the flow is severely nonaxisymmetric and unsteady and sometimes reverses over part of the annulus, a situation which cannot easily be modelled theoretically. These effects of eccentricity have been investigated experimentally on a vibration rig where the fluid forces on a centrebody fixed or forced to vibrate over a range of frequencies and amplitudes within an annular diffuser have been measured directly. It has been discovered that the flow in the annulus can exist in two distinct states characterized by the existence or absence of a Strouhal type of aerodynamic instability. With the instability present and the centrebody held fixed, large forces can be measured at a frequency proportional to flow velocity, but if the centrebody is shaken with sufficient amplitude the frequency of the instability can lock-on to the vibration in the classic fashion. A novel feature of this particular flow, however, is that the instability can be completely suppressed by carefully adjusting the imposed vibration amplitude and frequency, leaving the flow in its nonoscillatory state. It is found that the periodic instability can be reinitiated by increasing the forced vibration amplitude above a frequency-dependent threshold. Furthermore, by examining the phase relationships between the self-induced forces on the centrebody and its displacement, it is shown that the second state gives rise to negative fluid dynamic damping, the first to positive damping.