  A new universal theory for flow instability and turbulent transition is
proposed in this study. Flow instability and turbulence transition have been
challenging subjects for fluid dynamics for a century. The critical condition
of turbulent transition from theory and experiments differs largely from each
other for Poiseuille flows. In this paper, a new mechanism of flow instability
and turbulence transition is presented for parallel shear flows and the energy
gradient theory of hydrodynamic instability is proposed. It is stated that the
total energy gradient in the transverse direction and that in the streamwise
direction of the main flow dominate the disturbance amplification or decay. A
new dimensionless parameter K for characterizing flow instability is proposed
for wall bounded shear flows, which is expressed as the ratio of the energy
gradients in the two directions. It is thought that flow instability should
first occur at the position of Kmax which may be the most dangerous position.
This speculation is confirmed by Nishioka et al's experimental data. Comparison
with experimental data for plane Poiseuille flow and pipe Poiseuille flow
indicates that the proposed idea is really valid. It is found that the
turbulence transition takes place at a critical value of Kmax of about 385 for
both plane Poiseuille flow and pipe Poiseuille flow, below which no turbulence
will occur regardless the disturbance. More studies show that the theory is
also valid for plane Couette flows and Taylor-Couette flows between concentric
rotating cylinders.
