What is meant by "surge" in fluid mechanics? When reading about the action of opening a volume at vacuum submerged in a fluid at pressure, the fluid is said to "surge", i.e. to fill the void space (the vacuum).  Per Google, surge is defined as a sudden powerful forward or upward movement, especially by a crowd or by a natural force such as the waves or tide.  Another source says 

Transient flow is usually referred to as surge or water hammer, and
  the terms are commonly interchanged. But strictly speaking surge is
  where the mass oscillation of the fluid is the dominating force and
  the compressibility of fluid is not significant, for example two
  connecting reservoirs oscillating up and down

So it seems in the study of fluid mechanics, "surge" has a more defined meaning and is related to a force.  Can someone tell me another example of fluid surge and what equations that explain/demonstrate the forces producing/(causing by?) surge?
In the above description, it is said that the compressibility of fluid is not significant, and then describe fluid (presumably "incompressible" liquid) flowing back and forth between containers.  But I am still having trouble understanding the difference caused by compressibility.  Can someone give an example of a compressible fluid in situation that would cause surge flow  for an incompressible fluid and show how the compressibility of fluid limits the surge?
 A: Surge is a highly complex, nonlinear behavior that can occur in pumps and blowers. It occurs in centrifugal pumps, for example, at specific states of flow pressure; usually higher pressure at low flow where the vanes spend more of their energy compressing the fluid in the downstream compliance rather than moving the gas through the pump. So there is a kind of tug of war between the gas trying to expand back into the pump and the pump trying to push gas downstream.
Surge may manifest itself and be observed as solitary transients, but can also occur as a sustained limit cycle or chaotic oscillation, and at the low flow actual cause transient flow reversals in the pump. In liquids such as water for example, pressures may be reduced below the vapor pressure causing water vapor or dissolved gases to precipitate, and this multi-phase state can lead to surge.
Surge is very difficult to predict since it requires 3 dimensional rotational (CFD)modeling of the fluid flow, and this can even be a poor predictor. Designing a pump that has a low tendency to surge involves allot of trial and error in shaping the blower vanes, pump plenum, and inlet/outlet flow areas, and design relies mostly on experimental and empirical results.
Surge can suddenly drop or raise pressure on the rotating impeller, and so unless the pump has closed loop speed and torque control, the pressure fluctuations may cause sudden racing or delay in the rotor rotational speed - a 'surging' of the rotor from which the name comes I believe.
A: In the context of fluid dynamics, surge is a transient flow event that occurs in internal flow systems such as flow in piping systems or flow in compressors. It refers to a flow transient event in which fluid stream is forced to go in the reverse direction due to adverse pressure gradient, either caused by a sudden drop in inlet pressure or sudden rise in outlet pressure.
For example, in a centrifugal compressor system, surge (reverse in flow direction) can happen if the compressor suddenly loses power (rotor stops rotating). In this case, the compressor is not able to supply the pressure that has been built up at the outlet of the compressor, so the flow in side the gas passages will start to go in reverse direction due to the adverse (reverse in the direction of the flow stream)) pressure gradient. Surge can also happen if there is a sudden blockage in the exit of the compressor. In this case, if the compressor continue to run at the same speed (assuming there is enough power to keep it running) the pressure starts to build up at the exit, until it is high enough to reverse the flow direction in the gas passages and causes surge.
Whether the flow is compressible or not is not a criterion to determine if surge will happen, although it might affect the prediction of the onset of the surge event or how deep the surge event is.
