[qchris]

Measuring low-force, high-temp flows like can be pretty difficult, especially in academic labs where researchers are typically pretty limited in terms of instrumentation. I don't have 1-to-1 experience in this field but have briefly worked in plasma physics and ion thruster propulsion.

Doing a quick scan of the group's AIP paper, I'm generally okay with the methodology and the results they've drawn from it. It seems like their results are mostly establishing a rough thrust value with a force ceiling on each measurement, not a high-precision continuous dataset. Assuming the quartz tube and sphere don't seriously deviate from one another, the equations and physical assumptions they're using check out fine. It's not something you'd want to doing a full characterization on a flight model or anything, but for lab-scale experiment, I think it works.

[mannykannot]

One question I have is this: would the point at which the hollow ball rattles possibly be dependent on the Reynolds number, which in turn is dependent on the temperature? And if so, would that not have to be compensated for when comparing plasma-on and -off measurements?

[qchris]

I don't think so--it doesn't look like the rattling of the sphere is caused by flow around the sphere itself, but rather by the rate of the displacement/return to equilibrium of the sphere over the opening of the quartz tube. I'm assuming (and I could be wrong) that the vibration patterns between plasma-on/off varies pretty significantly, so that rattling, as opposed to occasionally shifts only happens when the plasma flow is actually happening and that the force is significant enough to be consistently displacing the sphere rather than the long-period pressure cycling you'd otherwise see. I don't think that turbulence in the classical sense would play much into it.

Not sure if this is helpful, but based on the video I'd guess that rattling only occurs in the plasma regime (as opposed to the normal, standard temp airflow), at which point I'm don't think the physical basis of the Reynold's number holds. Plasmas, especially under an external magnetic field, are only quasi-stable and so their bulk behaviours don't necessarily track with those of classical fluids.

[NGC404]

Thank you for you insight.