Tuesday, February 16, 2016

But Would the Nested Flow Dividers Really Work?

People coming from different backgrounds will have trouble with different parts of the gravity balloon concept for space colonies. However, the one criticism I am most excited to receive is that the flow dividers might not work. This came up on a reddit thread sharing this blog. I am overjoyed to hear this criticism - because it means that the critic has understood all the big details. They are up to speed, and that means they are ready for the meat of the conversation.
Now, having something spin in an atmosphere presents issues of its own, and the author proposes a complicated scheme involving nested shells to avoid turbulence. This feels like the sketchiest part of it to me - there's a lot of handwaving involved.
For too long, I have neglected to argue the core mechanical details on this blog. I can't give it the full treatment it deserves in limited time, but I'll break out the big guns (even if that only means labeling them).

Why the Balloon is Not Disputed

Other scientists and engineers have already covered the basic physical mechanism of a gravity balloon with no rotating structures inside. In every case, educated people who looked at the problem said "yeah, of course that would work". The core prediction comes from Newtonian gravity.

Take a moment to appreciate this fact: A gravity balloon construction has never existed. Even if someone tried to replicated it within a present-day space station, the other (mostly molecular) forces would dwarf self-gravitation. It is a purely hypothetical construction. Yet we are all agreed (all of the informed, for whatever it matters) with 100% certainty that it would work.

Conservative Approach to Flow Dividers

What is the "sketchy" part of the flow dividers? Like any engineering, the concept originates directly from the equations, given specific assumptions.
  • Equations - Parallel plate turbulent flow (or laminar, if needed)
  • Assumptions - The geometry and movement of the flow dividers

You probably need a fluids expert to comment on this. One problem might be that those equations are not exact... but this is unconvincing. Turbulent flow models don't run the risk of dramatically underestimating the drag. The transition point from laminar to turbulent is also highly uncertain. That would chip away at the laminar flow designs I have entertained before.

Also, there's more to flow than the global sheer forces. We have eddy currents. Those can form resonant patterns of certain kinds, you could posit that those might be destructive. But that claim is just plain wrong - because the exact problem has been studied before. It's called Taylor-Couette flow. For the most part, this leaves the flow circling in cylinders between the sheets. No, I don't have the exact flow description for the (very turbulent, very big) geometry described here, but there's nothing spectacular about the flow regime.

Geometry is the most challenging part of this all. The flow solution is all well-and-good, but it assumes that the sheets are in certain places. This requires them to be held there. That could be difficult, maybe even impossible. That might demand large steel scaffolding holding the flow dividers in place, along with mechanical joints and wheels to maintain separation between the nested sheets. This could become quite expensive. I'm not even willing to concede that this scenario makes it totally nonviable in all foreseeable circumstances.

Just take a moment to accept, however, that demanding assumptions of large structural supports (to resist air currents) is the most conservative academically honest position you could take. The flow regimes have already been in literature. All I'm asking is to apply them to a fictional geometry.

Very Liberal Approaches

The sell gets difficult when we start attempting to strip down those supports for maintaining the geometry. As I've argued, you can try using flimsy sheets. Perhaps you apply some positive pressure to them so that they hold a pseudo-rigid cylinder shape. But maybe not. We can just handwave these complications away.

In fact, there are two components to maintaining the geometry.
  1. Keeping the flow dividers from colliding or jostling
  2. Keeping the flow dividers shape in tact
Violating #2 might also imply a violation of #1, but I'm not worried as much about #2. Balancing the pressure in each layer will likely maintain shape as a side effect.

I've received one interesting response that seems to argue that the sheets may not even be necessary because the transition to laminar flow isn't clearly defined and may not necessarily exist if certain precautions are made (what exactly, I don't know). That sort of position is too liberal for me.

You might even build flow dividers with massive holes in them. Flow dividers which are more of a suggestion for the flow than a solid rule might be entirely sufficient. As for myself, I pull back a little bit from that vision. There is a lot of energy in the system, and the movement makes it difficult to identify a clear lines to the isobars in a mostly open system. If this were simply cylindrical geometry, I would be more inclined to the idea, but the end tapers wreck havoc on the flow complexity. I'm mainly speaking from intuition here, and I think that partially-open flow dividers are in the engineering battleground.


Mechanical Stability (the meat of the discussion)

There is something called the "wedge effect", but it might not be called exactly this depending on the source. Lots of large machinery levitate a rotor on a fluid. Some of that machinery rotates at tremendously high speeds. Essentially, the combination of the rotation in conjunction with the

You can find plenty of literature on the subject. Go look at chapters 2 and 3 of this book for some basic theory. This is what makes me largely an optimist, because it argues for a mostly passive answer to component #1. Additionally, it's an answer that is quite hopeful to aid in the concerns of component #2. Oscillations aren't really going to lead up to a Bernoulli effect like a naive reading might seem to suggest. The forces in involved have most to do with friction and a roughly static pressure profile.... if its fully laminar. As we get into turbulent territory, there is some wiggle room for pessimists, but it's more unknown than anything else.

Well, I just wanted to get that out there. Consider the surface to be scratched.

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