Comparative assessment and analysis of Rortex vortex in swirling jets

Recently, a new definition, called rortex, has been proposed to quantify the purely rotational motion of fluids. In this work, based on the DNS data we employed the rortex to assess and visualize the rotational motion and structure of vortex in swirling jets in comparison with other kinds of vortex criteria of Q, (2), vorticity and Omega. The rortex vector, Omega, Q, and (2) criteria are found to be better than vorticity for vortex core identification. The vector triangle formed by the Rortex R, nonrelational shear S, and vorticity (V) is analyzed to give mechanical explanations, especially on the effect of non-rotational shear on rotation of fluids. In addition, the probability density distributions (PDF) of Rortex R, nonrotational shear S, and vorticity (V) have been computed. The peak value of PDF of vorticity could be used to explain the pure rigid rotation effect and combination effects of rigid rotation and non-rotational shear.