For Navy relevant geometries, separation of wall bounded flows is a highly complex phenomenon. Because of the relatively high Reynolds numbers involved, separation is always associated with considerable unsteadiness. This unsteadiness is caused by large coherent structures that are a consequence of hydrodynamic instability mechanisms of the mean flow. In addition, due to the shape of underwater vehicles (submarines, torpedoes, low aspect ratio lifting or control surfaces) the separation is three-dimensional (3D). The combination of three-dimensionality and unsteadiness results in a highly complex time-dependent topology of the separated region. In a combined numerical/experimental effort, we are studying laminar separation bubbles in external flows. For these simulations, we employ highly-resolved direct numerical simulations (DNS) to obtain a deeper understanding of the various physical mechanisms governing separation, transition, and reattachment of 3D bubbles. Ultimately, such understanding may pave the way for the development of effective and efficient strategies for preventing separation for practical applications. We are also evaluating hybrid turbulence models for high Reynolds number flows. In particular, we describe DNS, Reynolds-Averaged Navier-Stokes (RANS), and hybrid simulations of a turbulent square duct flow. Based on these simulations we decided on two hybrid strategies for simulating the asymmetric diffuser experiments that were conducted at Stanford University by J. Eaton et al. The first mean flow results look very encouraging. If successful, this research will result in hybrid models that are suitable for a wide variety of flow topologies and Reynolds numbers.