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- Title
High‐Speed Drag Measurements of Aluminum Particles in Free Molecular Flow.
- Authors
DeLuca, Michael; Sternovsky, Zoltan
- Abstract
High‐speed drag in a free molecular flow is still poorly understood despite playing an important role in a variety of physical situations, including meteor ablation in the upper atmosphere, the orbits of satellites, and the dynamics of cosmic dust grains. To measure drag at high speeds, small aluminum spheres 0.1–2.1 μm in radius were shot at 1–10 km/s into air, N2, Ar, and CO2 using an electrostatic dust accelerator. The measured drag coefficient in air is Γ = 1.29 ± 0.13, with similar values for the other gases. The measurement constrains the heating coefficient to Λ = 0.58 ± 0.37 in air assuming the gas molecules reflect diffusely from the particles' surfaces. The drag appears to be independent of the molecular structures of the four gases tested but has a slight dependence on molecular mass. The drag is also higher than frequently assumed, which has implications for the modeling of high‐speed objects in free molecular flows. Plain Language Summary: When an object is moving in a free molecular flow, the average distance that a gas molecule travels before colliding with another molecule is much larger than the object's size. The object's slowdown then depends on individual molecules interacting with its surface. High‐speed gas‐surface interactions are relevant to meteor entry into planetary atmospheres, satellite drag, and the dynamics of interstellar dust but are still not fully understood. To measure high‐speed drag, we use a dust accelerator to shoot spherical aluminum particles into a chamber holding rarefied air, nitrogen, argon, or carbon dioxide. The particles are timed as they fly through the chamber, allowing us to calculate the drag coefficient, which measures the amount of momentum transfer between the particles and the gas. We measured a drag coefficient of about 1.3 in all four gases, which is higher than usually assumed for meteors and spacecraft. Assuming that gas molecules bounce off of a particle's surface diffusely, we estimate that in air about 58% of the kinetic energy of the incoming gas molecules goes into heating each particle. The heating and drag appear to be independent of the molecular structure of the gas, although a slight dependence on molecular mass is observed. Key Points: The free molecular drag coefficient of micron‐sized aluminum particles was measured to be about 1.3 in air, N2, Ar, and CO2The measured drag corresponds to an energy accommodation or heating coefficient of about 0.6 in air assuming a diffuse reflection modelThe results are applicable to modeling basic drag physics at high speeds relevant to meteors, satellites, and cosmic dust
- Subjects
DRAG measurements; ALUMINUM; PARTICLES; FREE molecular flow; UPPER atmosphere
- Publication
Journal of Geophysical Research. Space Physics, 2019, Vol 124, Issue 5, p3743
- ISSN
2169-9380
- Publication type
Article
- DOI
10.1029/2019JA026583