2024 Volume 19 Issue 2 Pages JFST0014
As a new perspective on the influences of the electrically charged air and electric field on the air flow, the natural air flow on a negatively charged forward-inclined step is analyzed. The electric field formed by the charged step pushes away the negative charge and attracts the positive charge in natural air, so that only the positive charge density is analyzed. A two-dimensional model of flow and electric field is set and solved on COMSOL Multiphysics FEM solver coupling three physics, that is, flow, electrostatics, and charge-transport. The pressure power spectra at the point around the vortex shedding region are calculated in the case of neutral air / uncharged step and charged air / charged step combinations. Whereas the former has large power below 1Hz, the latter has small power below 1Hz but with a larger peak at 30Hz, that is the vortex shedding frequency. In order to analyze the cause of the difference, the electric force effects are evaluated. The result shows that the upper portion of the vortex has forces in the same direction to the mainstream, and it degrades to the lower portion, which accelerates the vorticity of the vortex. Also, the vortex region has a strong downward force. Adding that, the curl of the electric force on the charged air is calculated to show that the upstream half of the vortex has the same direction curl as the vortex rotation, and the downstream half has an opposite curl because of the gradient charge density. These forces strengthen the vortices and raise their negative pressures and are estimated to suppress the fluctuation of the pressures so that the pressure’s spread spectrum reduces. In addition, the results of an experiment using a desktop wind tunnel are given as extra information.
