Multiscale issues in DNS of multiphase flows

Direct numerical simulations (DNS) have now become a well established tool to examine complex multiphase ows. Such ows typically exhibit a large range of scales and it is generally necessary to use different descriptions of the ow depending on the scale that we are examining. Here we discuss multiphase ows from a multiscale perspective. Those include both how DNS are providing insight and understanding for modeling of scales much larger than the “dominant scale” (defined where surface tension, viscous forces or inertia are important), as well as how DNS are often limited by the need to resolve processes taking place on much smaller scales. Both problems can be cast into a language introduced for general classes of multiscale problems and reveal that while the classification may be new, the issues are not.