Compressible vortex ring propagating faster than the speed of sound simulated for the first time

Compressible vortex rings (CVRs) are spinning flows of fluid with rotational cores and irrotational outer edges formed under a high-pressure difference when a sudden sonic blast occurs.

Gaining a fundamental understanding of how CVRs form and how they affect supersonic aircraft, rocket launching or natural phenomena, like volcanic eruptions, remain a challenge due to complex variations in local velocity, density and vorticity in the flow field.

Poudel et al. adapted a numerical tool to simultaneously measure all the necessary physical and thermodynamic properties for a more accurate study of CVR formation and propagation. Their model obtained a spinning CVR traveling at supersonic speed, which is quantified by a Mach number greater than 1.

To achieve a 1.08 Mach number, the researchers used diatomic hydrogen as the driver gas in a short driver section shock tube, with the initial pressure equivalent to 50 times the atmospheric pressure. They compared results to rings with lower Mach numbers, where rings are formed with smaller secondary counter-rotating rings in front of the primary one.

But when the Mach number gradually increases to exceed 1, the ring’s spinning flow interacts with the shock waves generated during the sonic boom, giving rise to multiple secondary counter-rotating rings at different radial locations behind the main ring.

They discovered the secondary rings are generated from the shock tube’s outer wall. The layer instability creates another series of opposite circulating rings, which later interferes with the primary ring, complicating the flow field considerably.

The researchers plan to investigate the role of barrel shocks in the supersonic regime and understand the potential implications of chemical reactions with different gases in the driver section of the shock tube.

Source: “Characteristics of shock tube generated compressible vortex rings at very high shock Mach numbers,” by Sajag Poudel, Lakshmana Chandrala, Debopam Das, and Ashoke De, Physics of Fluids (2021). The article can be accessed at https://doi.org/10.1063/5.0063164 .