“…The evolution of D is of interest : its divergence, for example, plays a role in the production of jet-noise in compressible flows, wherever the mean of div D = 0. For recent discussions of the utility of the this vector as a diagnostic in fluid dynamics or as an important source of jet noise, see Rousseaux, Seifer, Steinberg & Wiebel (2007), Hamman, Klewick & Kirby (2008) and Cabana, Fortuné & Jordan (2008), respectively. In the incompressible case, existence of solutions is assured provided the Beale, Kato, Majda (1984) condition is fulfilled.…”