A time-dependent study of dust devil vortex structure and evolution

Abstract In this study, the radius of a central low-pressure core is incorporated into the stream function, as described by [Onishchenko et al 2014. Physica Scripta , 89 , 075606]. The presence of such a central low-pressure core is crucial for the formation and persistence of dust devils. The flow analysed here is unsteady, axisymmetric, inviscid, and incompressible. The radial and axial velocity components are derived from the stream function, while the azimuthal component is obtained using the method of separation of variables. The analysis presents the corresponding radial distributions of all velocity components. Notably, the inclusion of the central low-pressure zone results in a distinct feature: the axial velocity reaches its maximum value at a certain distance from the edge of this zone. This observation aligns with the characteristics of dust devil vortices, which typically have a pronounced low-pressure core around their central axis. The azimuthal velocity exhibits enhanced exponential behaviour governed by the combined temporal and spatial exponential terms in the analytical solution. The pressure field is derived from the radial momentum equation, which shows a strong outward pressure gradient originating from the central low-pressure core. This pressure distribution increases monotonically with radial distance and is essential for driving the inward radial flow and maintaining the vortex structure. Unlike conventional models, the azimuthal velocity in this analysis does not asymptotically approach zero; rather, it decreases to zero over a finite radial extent. This behaviour provides a more realistic representation of dust devils, which are generally columnar structures with heights significantly greater than their radius. Consequently, the common assumption of asymptotic decay of azimuthal velocity is not applicable in this context.