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Calculation of velocity structure functions for vortex models of isotropic turbulence

Velocity structure functions (u'p–up)m are calculated for vortex models of isotropic turbulence. An integral operator is introduced which defines an isotropic two-point field from a volume-orientation average for a specific solution of the Navier–Stokes equations. Applying this to positive integer powers of the longitudinal velocity difference then gives explicit formulas for (u'p–up)m as a function of order m and of the scalar separation r. Special forms of the operator are then obtained for rectilinear stretched vortex models of the Townsend–Lundgren type. Numerical results are given for the Burgers vortex and also for a realization of the Lundgren-strained spiral vortex, and comparison with experimental measurement is made. In an Appendix, we calculate values of the velocity-derivative moments for the Townsend–Burgers model.