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Although quasi-periodic oscillations (QPOs) have been discovered in differentX-ray sources, their origin is still a matter of debate. Analytical studies ofhydrodynamic accretion disks have shown three types of trapped global modeswith properties that appear to agree with the observations. However, thesestudies take only linear effects into account and do not address the issues ofmode excitation and decay. Moreover, observations suggest that resonancesbetween modes play a crucial role. A systematic, numerical study of thisproblem is therefore needed. In this paper, we use a pseudo-spectral algorithmto perform a parameter study of the inner regions of hydrodynamic disks. Byassuming alpha-viscosity, we show that steady state solutions rarely exist. Theinner edges of the disks oscillate and excite axisymmetric waves. In addition,the flows inside the inner edges are sometimes unstable to non-axisymmetricperturbations. One-armed, or even two-armed, spirals are developed, whichprovides a plausible explanation for the high-frequency QPOs observed fromaccreting black holes. When the Reynolds numbers are above certain criticalvalues, the inner disks go through some transient turbulent statescharacterized by strong trailing spirals; while large-scale leading spiralsdeveloped in the outer disks. We compared our numerical results with standardthin disk oscillation models. Although the non-axisymmetric features have theiranalytical counterparts, more careful study is needed to explain theaxisymmetric oscillations.

Oscillations of the Inner Regions of Viscous Accretion Disks