Yeay, finally a discussion I can add something to! Speaking of flow, we're revisiting an old, old, topic. Head over to APC and search for it and you'll see. Color me lazy too. I don't want to go through explaining hydrodynamics in detail again.
Here's a quick and dirty of flow/current/hydrodynamics using the simple formula for discharge; Q=v/a (Discharge = Velocity divided by Area):
1. High flow doesn't necessarily mean high velocity. This, and #2 are what Barr was talking about when he mentioned "mass flow". Depending on how you work the hardware, you can have good "mass flow", aka laminar flow. This is what Lily Pipes were designed to do. They spread out the area of the point of discharge to reduce velocity enough to achieve laminar flow.
2. What is laminar flow? https://en.wikipedia.org/wiki/Laminar_flow
3. Why is it important/useful in an aquarium? Your plants will tend to be bent over in a current less and therefore look nicer. Fish tend to find it easier to move around in as they typically don't have to correct for turbulence/eddies. It tends to distribute water more evenly and can create inertial patterns that help transport entrained solids into a filter intake or allow them to settle out of the water column, depending on placement (again, look at ADA's filter in/out design and typical placement).
5. Is laminar flow achievable in an aquarium? Yes, and no. If your point of discharge is wide enough then it's possible to get laminar flow in the water column until it hits a barrier like hardscape and plants, or some shearing force/friction in the water column. At that point you start creating eddies, however small, that cause turbulence. On to turbulent flow!
What is turbulent flow? https://en.wikipedia.org/wiki/Turbulence
Scroll down to the section "Onset of Turbulence" for a great image and relatively simple description.
1. Does turbulent flow mean high velocity? Not necessarily, but high velocity in an enclosed system, a point of discharge that's got a small area, flow makers that create cross-currents, and aquariums with a lot of stuff in the water column tend to create turbulent flow.
2. Is turbulence bad? No, but it does tend to create micro-zones of lower relative flow where solids settle. This is why debris tends to congregate around the base of hardscape and plants.
3. Eddies also tend to create zones of circulation that feed themselves. Think of a powerhead creating a flow loop where the water being moved circulates back to the powerhead rather than being distributed throughout the tank, therefore circulating nutrients more evenly to the plants.
4. Depending on the point of discharge, turbulent flow can also entrain settled solids back into the water column, but not necessarily transporting them to a filter intake. This can cause the solids (not sediment; sediment is solids that have settled out of the water column. Likewise, aquarium substrate, even soil-based stuff like Aquasoil isn't sediment, it's substrate. Sorry, pet peeve.) to settle onto plant leaves, thereby releasing their nutrients into the water column. DOC, anyone?
TL;DR- Laminar flow is desirable, but rarely achieved throughout an entire aquarium. Turbulent flow is most common, but isn't necessarily a bad thing. Filter outlets with wide points of discharge (Lily Pipes and the like or propeller pumps like Hydor Corallias or Eco-Tech Vortechs) help achieve laminar flow by reducing the velocity at the point of discharge even though total discharge isn't reduced. Filter outlets or powerheads with low-area/small points of discharge tend to create turbulent flow and potentially reducing overall water mixing to achieve even nutrient distribution. However, as long as this is accounted for, it's not always a bad thing.
Clear as mud?