High Reynolds number simulation and drag reduction techniques

The first part of this thesis is an attempt to perform high Reynolds number DNS and large Eddy Simulation (LES). In order to conduct DNS at high Reynolds number, we have implemented and benchmarked several different parallel models. Domain decomposition has been done in either stream-wise only or in both stream-wise and span-wise directions, based on MPI or OpenMP to parallelize the code. In order to overcome in part the limitations of Reynolds number on DNS, LES has been implemented whereby the large scale dynamics of flow are accurately simulated, but the small scale features are parameterized by an approximate model. LES requires fewer grid points and less computer time for a simulation. We have conducted relatively high Reynolds number DNS, and LES at Re* = 600,1000 using the Spectral Vanishing Viscosity (SVV) method. SVV has been combined with standard and dynamic Smagorinsky models. In order to improve the numerical stability, SVV has been implemented implicitly. The LES results have been compared with DNS, and show that SVV has potential to be a good approach for LES.; In the second half of thesis, we have investigated several turbulent drag reduction techniques using DNS. Turbulence drag reduction by adding micro-bubbles into a turbulent boundary layer has been well established in experiments. However, it has been difficult until now to capture such effects in numerical simulations due to a lack of an accurate interaction model between turbulence and micro-bubbles. In this thesis, a series of DNS of small bubbles seeded in turbulent channel flow at average volume fractions of up to 13% have been carried out. The results show that about 10% drag reduction is reached. This is consistent with low speed and low void fraction experiments, but significantly less than 70% to 80% reductions in skin friction reported in some experiments. A nondeformable, spherical bubble shape has been assumed, and the Force Coupling Method (FCM) has been used to simulate bubbles in turbulent flow. It is found that the bubble size should be small enough to produce a sustained level of drag reduction over time. Drag reduction effects have been investigated in detail at different Reynolds numbers.; Finally, motivated by the apparently lower levels of drag reduction found in the numerical simulation as compared to many experiments, we consider the possibility of other physical process not captured so far. One such process is a partial slip flow condition at the wall. A slip boundary condition can arise from effects of hydrophobic surfaces or the formation of a thin gas film on the wall. The simulation results show that a large level of drag reduction can be achieved by applying slip boundary condition. The effect of combining slip boundary conditions with micro-bubbles has also been investigated, and a detailed analysis has been carried out. (Abstract shortened by UMI.)...