Direct numerical simulations are conducted to investigate in detail the effect of Mach number on vortex pairing in a mixing layer. The pairing process is found to be delayed at higher Mach numbers and the paths followed by the vortices change. To investigate the effect of the initial shape of the vortices a simple vortex dynamical model of pairing is constructed which accurately models pairing at low Mach numbers. Results from the model suggest that a variation in the initial shape of the vortices is not sufficient to explain the changes in the pairing process due to Mach number. Further simulations are conducted for isolated vortex pairs. There is little departure from the expected rotation rate as Mach number is increased, but strong core effects. Overall, changes in the pairing process reflect changes in the evolution of the primary instability, with vortex trajectories becoming more elongated as the Mach number is increase
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