Direct numerical simulations (DNS) are employed to investigate laminar boundary layer separation and its control by pulsed vortex generator jets (VGJs), i.e. by injecting fluid into the flow through a spanwise array of small holes. Particular focus is directed towards identifying the relevant physical mechanisms associated with VGJ control of low-Reynolds-number separation, as encountered in low-pressure turbine applications. Pulsed VGJs are shown to be much more effective than steady VGJs when the same momentum coefficient is used for the actuation. From our investigations we have found that the increased control effectiveness of pulsed VGJs can be explained by the fact that linear hydrodynamic instability mechanisms are exploited. When pulsing with frequencies to which the separated shear layer is naturally unstable, instability modes are shown to develop into large-scale, spanwise coherent structures. These structures provide the necessary entrainment of high-momentum fluid to successfully reattach the flow.

A high-order-accurate numerical method for solving the incompressible Navier-Stokes equations in general orthogonal coordinates is presented. The method is applied to a test case of the NASA Langley Computational Fluid Dynamics Validation Workshop 2004, a turbulent flow over a wall-mounted hump geometry. Results of direct numerical simulations (DNS) for the unforced flow as well as for a case with steady suction are presented and compared to the available experimental data. The DNS predictions are shown to agree well with the experiments, except in the vicinity of the experimental reattachment locations. The simulations predict slightly longer recirculation regions for both the unforced and the controlled case. The results presented in this work suggest that, with the rapidly increasing computational resources of modern supercomputers such as the Cray X1, DNS is becoming a viable alternative to the use of turbulence models for investigating complex turbulent flows at moderately high Reynolds numbers.

The role of hydrodynamic instability mechanisms in the presence of laminar boundary layer separation is investigated by means of Direct Numerical Simulations. In a series of simulations involving generic laminar separation bubbles we show that the "natural" onset of unsteadiness (i.e. the development of visible vortex shedding) is not necessarily caused by an absolute/global instability. Our results indicate that the entrainment of high-momentum fluid required to "close" the separation bubble is primarily provided by 2-D or "2-D coherent" structures, which are a consequence of the (inviscid) hydrodynamic instability of the separated shear layer. In a series of highly resolved simulations for a flat-plate boundary layer subjected to low-pressure turbine blade conditions, we demonstrate that this natural instability mechanism (with respect to two-dimensional disturbances) can be exploited for effective control of separation using pulsed vortex generator jets.

The Flow Simulation Methodology (FSM) is evaluated for the case of a separated ow subject to control by oscillatory forcing. The geometry chosen is the hump geometry from the NASA Langley CFD Validation Workshop 2004. In addition to the FSM, Direct Numerical Simulation (DNS) results are also presented. Simulation data for the uncontrolled ow, control by steady suction, and control by unsteady forcing all agree very well with the experimental data for both DNS and FSM. In addition, the ability of the FSM to produce accurate results over a wide variety of model lter widths is demonstrated. Finally, the relative strengths of the two approaches (FSM and DNS) are compared.