Onset of cavity deformation upon subsonic motion of a projectile in a fluid complex plasma

We study the deformation of a cavity around a large projectile moving with subsonic velocity in the cloud of small dust particles. To solve this problem, we employ the Navier-Stokes equation for a compressible fluid with due regard for friction between dust particles and atoms of neutral gas. The solution shows that due to friction, the pressure of a dust cloud at the surface of a cavity around the projectile can become negative, which entails the emergence of a considerable asymmetry of the cavity, i.e., the cavity deformation. Corresponding threshold velocity is calculated, which is found to decrease with increasing cavity size. Measurement of such velocity makes it possible to estimate the static pressure inside the dust cloud.

Figure 1

Deformation of a cavity behind a moving projectile (large bullet in the center) moving with the velocity $u$ through the dust crystal (small bullets around) in a carrier gas. The projectile is surrounded by a deformed cavity with initial radius $R$.

Figure 2

Positions of dust particles and the projectile (negative images) at successive instants. The time interval between individual frames is 0.08 s. Frames (a) and (b) illustrate a spherical cavity; (c) and (d) illustrate a deformed cavity. The projectile is accelerated from (a) to (d).

Figure 3

Deformed cavity around a large subsonic projectile moving with the velocity $u=1.8\mathrm{cm}/\mathrm{s}$. Reproduction of an enlarged fragment of Fig. 3a in Ref. [11].