“Ambient” refers to whatever environment a device is exposed to; it could be understood differently as the temperature of the airstream just before it begins passing over the device. Note that thermal resistance figures for heatsinks are based on some particular test condition, and that actual results in a particular application can vary significantly depending on the fluid flow properties within an enclosure, which are not the easiest thing in the world to predict. Leaving oneself some wiggle room is usually a good idea.
The question as I understand it might be rephrased as “how does one select a fan to produce a linear flow speed across a heat sink of 5m/s?”
It’s not an easy question to answer precisely. Assuming one’s heatsink is contained within a tight -fitting duct, the cross-sectional area of that duct multiplied by the desired flow speed yields some minimum volumetric flow rate (m2*m/s=m3/s). That’d represent an absolute minimum CFM rating required.
But because fan CFM ratings are given at zero static pressure, and forcing air through an enclosure at some useful rate will present a non-zero static pressure to the fan, a fan with a higher CFM rating than the estimate above will be necessary. (See the post on fan & system curves)
Estimating static pressure drops across a straight, round, unobstructed duct using hand calculations is doable to a reasonable degree of accuracy; but for any system of significantly greater geometric complexity one’s going to be relying on some rather expensive fluid mechanics modeling software, and doing physical tests afterward to validate the results.
It’s pretty common for folks to skip the fancy software part, and move on to testing a system designed using generous assumptions, estimated guesstimation, and decent self-protection mechanisms.