We analyze the impact of gas rarefaction on the two-dimensional aerodynamic ground effect over a flat plate. Focusing on highly rarefied flow conditions, we formulate the free-molecular problem based on the collisionless Boltzmann equation and the Maxwell boundary conditions. A semianalytical solution is derived, where specular and diffuse surface reflections are studied separately. The calculated ballistic field is compared with direct simulation Monte Carlo computations at finite Knudsen numbers to test its validity and breakdown with decreasing rarefaction. The specific effect of ground reflections is illustrated through comparison with the nonconfined (in the absence of ground) flow field. The results indicate that the ground invariably increases the aerodynamic loading on the plate and shifts the maximum lift value to lower angles of attack compared with the nonconfined configuration. While the ground may yield a negative contribution to the lift in the continuum (ideal-flow) limit, its relative difference compared with the nonconfined setup is found significantly larger and consistently positive at high Knudsen numbers.

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• Noncontinuum effects
• Rarefied flows