The Mediterranean city of Alexandria has seen heavy rains amid an unstable wave
that hit the country, and expected to remain in place for three days, according to meteorological experts.
For the primary breakup of spray, the breakup process is mainly controlled by the Kelvin-Helmholtz (KH) unstable wave at the gas-liquid interface.
When the equivalent radius of droplet ([R.sub.e]) is greater than the critical radius of the KH unstable wave ([R.sub.KH]), small droplets are assumed to be peeled off from parent droplets, .
where [B.sub.0] = 0.61 is the constant of critical radius of the KH unstable wave, and [B.sub.1] = 20 is the a constant for the breakup time of KH unstable wave.
 studied the viscous potential flow analysis of Rayleigh-Taylor instability and observed that the wavelength of the most unstable wave
increases strongly with viscosity.
An important factor in conducting the experiments is selection of the amplitude of the pressure disturbance such that the linearly unstable wave
reaches its saturation amplitude sufficiently prior to the die exit so it can undergo a supercritical bifurcation near the exit of the die that would result in an interfacial wave with a sharp front and a gradual tail (i.e., mechanical interlocking).
(2) With the disturbance evolving from the upstream to the downstream, most of disturbance waves in the boundary layer decrease, and only special frequency ranges ([f.sub.1] - [f.sub.5]) of unstable wave exist in the downstream boundary layer, indicating mode competition exists during the disturbance wave evolution along streamwise in the boundary layer.
In recent years, with the development of computational fluid dynamics (CFD) and computer performance, a series of numerical studies are performed to understand how the freestream disturbance wave interacts with shock wave and boundary layer, how the unstable waves are induced, how the induced disturbance waves interact with shock wave and boundary layer, and how they evolve [7-11].
This also means that some modes in the boundary layer are suppressed during response and period phase; mode competition between these modes and the main disturbance modes exists in the temporal evolution of boundary layer disturbance wave, which can change the characteristics of nonlinear evolution of the unstable waves in the boundary layer.
Fedorov and Khokhlov  investigated the prehistory of instability in a hypersonic boundary layer and presented a detailed analysis about how the forcing environmental disturbances enter into boundary layer and produce unstable wave that further develops and induces typical unstable wave in boundary layer.
The interactions between boundary layer and unstable wave become more complicated due to the reflection between bow shockwave and wall.
This conclusion shows that when [[gamma].sub.1][m.sub.1] = [[gamma].sub.2][m.sub.2], two modulated unstable waves
, the interaction of the two waves is still unstable.