High-speed compressor data immediately prior to rotating stall incepti
on are analyzed and compared to stability theory. New techniques for t
he detection of small-amplitude rotating waves in the presence of nois
e are detailed, and experimental and signal processing pitfalls discus
sed. In all nine compressors examined, rotating stall precedes surge.
Prior to rotating stall inception, all the machines support small-ampl
itude (< 1 percent of fully developed stall) waves traveling about the
circumference. Traveling wave strength and structure are shown to be
a strong function of corrected speed. At low speeds, a similar to 0.5
times shaft speed wave is present for hundreds of rotor revolutions pr
ior to stall initiation. At 100 percent speed, a shaft speed rotating
wave dominates, growing as stall initiation is approached (fully devel
oped rotating stall occurs at about 1/2 of shaft speed). A new, two-di
mensional, compressible hydrodynamic stability analysis is applied to
the geometry of two of the compressors and gives results in agreement
with data. The calculations show that, at low corrected speeds, these
compressors behave predominantly as incompressible machines. The wave
that first goes unstable is the 1/2 shaft frequency mode predicted by
the incompressible Moore-Greitzer analysis and previously observed in
low-speed compressors. Compressibility becomes important at high corre
cted speeds and adds axial structure to the rotating waves. At 100 per
cent corrected speed, one of these hitherto unrecognized compressible
modes goes unstable first. The rotating frequency of this mode is cons
tant and predicted to be approximately coincident with shaft speed at
design. Thus, it is susceptible to excitation by geometric nonuniformi
ties in the compressor. This new understanding of compressor dynamics
is used to introduce the concept of traveling wave energy as a real ti
me measure of compressor stability. Such a wave energy-based scheme is
shown consistently to give an indication of low stability for signifi
cant periods (100-200 rotor revolutions) before stall initiation, even
at 100 percent corrected speed.