Hello Again Simon.
Having inadvertently done what I should have done in the first place, I have slept on your questions and I hope that this will be a more compact, possibly more useful reply. For a start I can avoid spending so much time on discussing points you already are happy with.
As I see it your difficulty is:
>Bird is high, in say 20kts wind, dives down and increases it's own airspeed as it goes, glides a long way at the ocean level in calm air using it's own momentum. Then as it loses momentum (airspeed) it climbs again to gain height. THAT is where I am unstuck, HOW is it going from a low energy state (low airspeed) AND CLIMBING AT THE SAME TIME using just the wind shear to 'add energy'? I can't understand how a bird can dynamically soar (endlessly) given a flat surface and JUST a wind gradient. <
We can kill that easily (as I see it anyway, correct me if necessary) if we bear a few already familiar principles in mind at crucial points.
1: In the absence of waves and similar luxuries, the bird’s energy assets are height, and momentum relative to the air. “Groundspeed” is irrelevant, and in this case so is vertical shear (notionally). It does however expend its energy profit on lateral travel, which from our point of view is the main incentive.
2: As the bird lost its airspeed and height in gliding, it did not lose its available wind shear as long as it was in a generous wind gradient; as long as it kept enough airspeed or height, or is willing to do an occasional flap, it can change its altitude, and as soon as it does that it picks up airspeed again. The velocity might not have a convenient direction, but the bird can use its energy profit to correct for that.
3: So, as the bird reaches the end of its glide, it can use its initially low(-ish, not zero!) airspeed to climb, all at the price of a few small adjustments. As soon as it climbs it picks up some airspeed; not necessarily in a desired direction, but that doesn’t matter; it can use it to gain height, which it then can cash in on energy for progress in any direction.
4: The apparent violation of thermodynamics, “something for nothing”, is an illusion: the energy for most of its climb comes from moving through a shear gradient that contributes airspeed. It is now harvesting more energy than it is expending. In still air that would not apply. The bird can go on rising and falling as long as it likes, gaining energy on each pass across the gradient, and topping up on the occasional scrap of fish or carrion (whale dreck, drowned, pipe-smoking sailors and the like). You could consider surfing as an analogy – how can the surfer slide down a wave and run up a wave, and not run out of energy? Because by the nature of the mechanism he is harvesting energy from the wave all the time.
5: One could imagine a problem if the bird is in a high-speed consistent prevailing wind in the direction opposite to which it wanted to go. If it could not gain a higher airspeed than the wind speed near the surface, then it could not travel upwind at all. It would have to use its gains to move sideways as far as possible until it got out of that wind zone and could resume business as usual. In practice however, one would expect a strong swell in such a wind, and the bird could move upwind in the low windspeed in the lee of the swells, hopping over each swell as it came. Sound like hard work though!
I hope that is more useful.
Cheers,
Jon
