  The interaction of trailing and leading vortex structures in low Reynolds
number forced (varicose) jet with coflow is studied using Direct Numerical
Simulation (DNS), where we report the dynamics of these vortices for a range of
varying parameters. In a circular forced jet with Strouhal number 0.2,
continuous formation of these vortices is observed which undergo pairing,
tearing and disintegration. The forced jet is analyzed for varying frequency of
forced perturbations, coflow temperature, turbulence intensity, momentum
thickness, coflow intensity and Reynolds number at a constant forcing
amplitude. The observation reveals that any reduction in momentum thickness
increases the circulation as well as energy capacity of the leading and the
trailing vortices, thereby enhancing the vortex pairing time. However, the hot
coflow increases the strength of the leading vortex to such an extent that the
generated trailing vortex is weak and dissipates even before it undergoes
vortex pairing, while an incomplete disintegration of the leading vortex occurs
in this case. Moreover, the strength of leading and trailing vortices is found
to decrease with an increase in coflow power, thereby leading to a weaker
vortex pairing. The decrease in Reynolds number also reduces the intensity of
trailing vortices; whereas, at a lower Reynolds Number (Re) the trailing
vortices dissipate soon after they form. The findings are achieved based on the
analysis done by Proper Orthogonal Decomposition (POD), Dynamic Mode
Decomposition (DMD), fast Fourier transform (FFT), Q-criterion and vortex
tracking method to understand the dynamic interaction and behavior of the
vortices.
