In very basic terms, flow is movement with continuous deformation while regular old movement is without continuous deformation, but there could be discrete deformation I suppose.
Water will flow when spilled, it tumbles and rolls and breaks up and rejoins. But when you push a ball across the table, the ball stays together, moving rigidly. It could be a rubber ball, which will deform locally and may even stay deformed if it's inelastic, but as a whole, the object moves as one unit.
How an object moves and it's relation to shear stresses is what defines states of matter which I describe very simply in the linked answer. Flowing really implies movement with no fixed shape (apart from shape imposed by the boundaries, such as containers or channels) which distinguishes it from the movement of solids.
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.
Best Answer
From the Wikipedia article for Reynolds number:
In addition to measuring the ratio of inertial to viscous forces in a flow, the incompressible Navier-Stokes equations can be written in non-dimensional form such that the only parameter is the Reynolds number (ignoring body forces). This is very nice because it is the basis for the validity of wind tunnel testing.
Suppose we would like to measure the aerodynamics of the flow around a Boeing 747. Two (at least) options exist:
But how do we know that the flow we measure in the wind tunnel is what really happens in flight? We match the Reynolds numbers and the exact same equations model both situations--therefore the aerodynamics must be the same. (Ignoring compressibility effects.)