Starting vortex

About: Starting vortex is a research topic. Over the lifetime, 4785 publications have been published within this topic receiving 100419 citations.

Linear Motion of a Shallow-Water Barotropic Vortex as an Initial-Value Problem

Abstract: This paper revisits calculation of motion for a shallow-water barotropic vortex with fixed mean axisymmetric structure. The algorithm marches the linear primitive equations for the wavenumber 1 asymmetry forward intime using a vortex motion extrapolated from previous calculations. Periodically, it examines the calculated asymmetry for the apparent asymmetry due to mispositioning of the vortex center, repositions the vortex to remove the apparent asymmetry, and passes the corrected vortex motion on to the next cycle. This approach differs from the author's earlier variational determination of the steady-state motion after initial transients had died away. The steady-state approach demonstrated that the vortex had normal modes at zero frequency and, when an annulus of weak anticyclonic flow encircled the cyclonic inner vortex, at the most anticyclonic rotation frequency of the mean flow. Forcing of the former model led to too rapid steady-state poleward motion on a beta plane. At least for the line...

Wave propagation in vortices and vortex bursting

Abstract: The propagation of pressure waves in a Lamb–Oseen vortex has been investigated both by three-dimensional direct numerical simulations as well as a set of large-eddy simulations. The pressure wave is initiated by locally increasing the core radius of a Lamb–Oseen vortex at its edge. This wave travels along the vortex axis towards the region with a thinner core radius. Behind the wave the axial velocity increases so that sufficient swirl may trigger the helical instability. An abrupt change of flow structure in the vortex core is observed in the case of intersecting pressure waves: this phenomenon is known as vortex bursting. A vortex system composed of two symmetric counter-rotating vortices, which is similar to that of an aircraft has also been investigated by means of large-eddy simulations. In the far-field region the system develops linear instabilities such as the Crow instability, which is characterized by a large-scale symmetric sinusoidal deformation resulting in the reconnection of the two vortices. It also demonstrates the occurrence of helical instability and vortex bursting.

Nonlinear Lift on a Triangular Airfoil in Low-Reynolds-Number Compressible Flow

Abstract: Numerical and experimental analyses of the aerodynamic performance of a triangular airfoil in low-Reynolds-number compressible flow are performed. This airfoil is one of the candidates for propeller blades on a possible future Martian air vehicle design. Based on past experimental studies conducted in the Mars Wind Tunnel at Tohoku University, this airfoil is known to exhibit nonlinear lift behavior. In the present study, direct numerical simulations of low-Reynolds-number compressible flow over a spanwise periodic triangular airfoil are conducted to identify the source of nonlinear lift. The numerical results reveal that the source of the nonlinear aerodynamic behavior is the lift enhancement provided by the large leading-edge vortex generated. For compressible low-Reynolds-number flow, the wake structure becomes elongated, causing the nonlinear lift enhancement to appear at higher angles of attack compared to the case of incompressible flow.

Origin and dynamics of vortex rings in drop splashing

Abstract: A vortex is a flow phenomenon that is very commonly observed in nature. More than a century, a vortex ring that forms during drop splashing has caught the attention of many scientists due to its importance in understanding fluid mixing and mass transport processes. However, the origin of the vortices and their dynamics remain unclear, mostly due to the lack of appropriate visualization methods. Here, with ultrafast X-ray phase-contrast imaging, we show that the formation of vortex rings originates from the energy transfer by capillary waves generated at the moment of the drop impact. Interestingly, we find a row of vortex rings along the drop wall, as demonstrated by a phase diagram established here, with different power-law dependencies of the angular velocities on the Reynolds number. These results provide important insight that allows understanding and modelling any type of vortex rings in nature, beyond just vortex rings during drop splashing.

Delay of vortex ring pinchoff by an imposed bulk counterflow

Abstract: The Kelvin–Benjamin variational principle has been previously used to predict that vortex ring pinchoff can be delayed if the energy of the leading vortex ring is decreased during formation and/or the energy delivered by the vortex generator is increased. We present experimental results in which the former and latter energy effects were simultaneously accomplished by imposing a bulk axisymmetric counterflow during vortex ring formation. Measurements indicate that the formation number is retarded sufficiently to allow increased ingestion of fluid circulation by the leading vortex ring. This serves as a first demonstration of artificial manipulation of vortex ring formation for potential propulsive benefits in starting jets.