This thesis discusses work on three fundamental turbulent flow problems.

Passive scalar mixing in a spatially evolving turbulent jet is studied using direct numerical simulations. Simulations are performed for a turbulent jet at Reynolds number of 2400 and Schmidt number of 1. Good agreement with experimental data is obtained for the velocity and passive scalar statistics. The importance of entrainment into the jet in the vicinity of jet exit is examined in detail. A simple way of account for the entrainment near the jet exit is proposed by extending the jet exit plane further upstream. The nature of convection and diffusion terms in the passive scalar transport equation were studied. Convection dominates toward the jet centerline while the contribution of diffusion is found to be comparable to convection closer to the jet shear layer. A simple kinematic model for the intensity of scalar fluctuations is proposed. The model relates the amplitude of scalar fluctuations at a fixed point in the jet, to the oscillation of scalar fronts whose thickness decreases with Reynolds and Schmidt numbers, and increases with radial distance.

Bubble transport in a turbulent flow field is modeling using a Lagrangian formulation. An efficient method to track the bubbles in an unstructured code is developed. The bubbles are seamlessly integrated with the fluid-phase solver with one-way coupling. The importance of using an adaptive time integration scheme while solving for bubble dynamics is shown.

Aerodynamics of cactus-shaped cylinders are studied using direct numerical simulations at Reynolds numbers of 20, 100 and 300. The results are contrasted to those from smooth cylinders at the same Reynolds numbers. The cavities in the cactus-shaped cylinders are seen to reduce the forces acting on them. At higher Reynolds numbers, both mean and the fluctuating forces are found to decrease considerably. The Strouhal number is also found to decrease by about 10%. These reductions in force magnitudes and observed wake instabilities are attributed to the presence of large-scale, quiescent, recirculating flow within the cactus cavities.