Gas dynamics and vorticity generation in laser-induced breakdown of air

Research has shown that the ignition characteristics of laser-induced plasmas in fuel-air mixtures are influenced by the gas dynamics effects induced during the gas breakdown stage. Here, we present the numerical modeling of the fluid mechanics induced by breakdown (plasma formation) from a nanosecond near-infrared (NIR) laser pulse in air. The simulations focus on the post-discharge kernel dynamics with the goal of developing a better understanding of how vorticity is generated during the kernel cooling phase. Initial conditions (ICs) of kernel shape, temperature, and pressure (corresponding to the end of the laser pulse) are found from experimental Rayleigh scattering data. It is shown that this method for determining ICs is preferred versus the use of the Taylor-Sedov blast wave theory as it provides a more accurate description of the starting field. Past experimental observations have revealed that the gas dynamics of nanosecond laser sparks typically lead to the formation of an asymmetric torus with a frontal lobe propagating towards the laser source. We show that the development of the asymmetric torus is governed by strong vorticity generated through baroclinic torque arising from the blast wave that forms at the kernel boundary. Initially, the blast takes the shape of the teardrop kernel but then evolves into a spherical front during the first ∼10 µs because the blast wave strength varies along its circumference. This spatial variation leads to a misalignment between the pressure and density gradients and generation of vorticity by baroclinic torque. Ultimately, the observed flow-field is dictated by how the energy was initially deposited around the beam waist during breakdown. As such, one can tailor the aerodynamics induced during the cooling and recombination phase by controlling the energy deposition profile.