Collisionless jet expanding into vacuum from a circular aperture

Document Type


Publication Date



Department of Mechanical Engineering-Engineering Mechanics


Collisionless flows expanding into a vacuum from a circular slit are investigated using the gaskinetic theory. Both accurate and approximate expressions to the plume fields along a centerline and a constant radius are obtained, including the density, velocity components, and translational temperature. Special non-dimensional parameters such as the exit semi-opening angle, gas speed ratio at the exit, and positions are included in these expressions. Particle simulations with the direct simulation Monte Carlo method are performed to validate the above results. These expressions are complex and may need computers for evaluations; however, the cost is minor when compared to particle simulations. With the assumptions of a high or low exit speed ratio and small exit angle, simple explicit expressions for properties along the centerline and a constant radius are obtained. The investigations also find out that the cosine law relations can provide fast and crude estimations, but it is not recommended to use the cosine law relation to compute collisionless plume flowfields with a large exit speed.

Publisher's Statement

© 2022 Author(s). Published under an exclusive license by AIP Publishing. Publisher’s version of record: https://doi.org/10.1063/5.0094596

Publication Title

Physics of Fluids