COMPUTATIONAL & THEORETICAL FLUID DYNAMICS
Predictions of urban wind environments
Turbulent flow and dispersion characteristics over a real urban street canyon, `Teheran Street' (of Seoul), were simulated by CFD (Computational Fluid Dynamics) using LES (Large-Eddy Simulation) modeling. Contour sections of plumes of pollutants from a point source in the street canyon show spatial distributions of time averaged pollution concentrations. Isosurfaces near buildings represent eddy-viscosity.
Behavior of particles in turbulence over a wavy boundary
Understanding the mechanism controlling the particle motion in flow over a complex boundary is of great importance for many engineering fields. It is well known that fluid flowing over a wavy boundary creates turbulence that is remarkably different from the flow over a flat wall. Previous studies have presented the role of vortical structures over a wavy boundary, which is associated with particle clustering near the boundary. In this study, behavior of particles in turbulence over a wavy wall is investigated by direct numerical simulation of a turbulent wavy channel flow with laden particles. Our investigation shows that particles interact selectively with vortical structures and accumulate in a specific region along the wavy wall. Active deposition and resuspension occur in the upslope region where streamwise vortices are concentrated. Depending on the particle Stokes number and waviness of the wall, a strip or streak pattern of particle clustering near the wall is observed due to the combined effect of the wavy wall and vortical structures. However, due to the particle inertia, particle velocity and its fluctuation do not change as much as fluid velocity along the wavy boundary.
Blast wave propagations after nuclear explosion
Pressure waves develop immediately after nuclear explosions and start to move outward from the fireball. The most of initial damages are caused by the blast waves. We performed the blast wave propagations by solving two-dimensional and axisymmetric Euler equations. A clean air burst of fireball above the ground zero is considered. Depending on surface types (convex, concave blocks and flat surface) of the ground, formations of Mach stem was significantly changed. The highest peak overpressure was found for the case of convex block, while distributions of peak overpressure for all the cases are similar at a region sufficiently far from the ground zero.
Human-induced contaminant transports
A large eddy simulation is used to investigate contaminant transport due to complex human and door motions and vent-system activity in room compartments. Human and door motions are simulated with an immersed boundary procedure. We demonstrated the details of contaminant transport due to human- and door-motion induced wake development during a short-duration event involving the movement of a person (or persons). Isosurfaces represent fine-scale vortical structures colored by contaminant concentration levels.
Turbulence modulation in particle-laden isotropic turbulent flow
Direct numerical simulations of stationary and decaying isotropic turbulence were carried out for a Taylor micro-scale Reynolds number of Rλ ∼ 70. The effect of O(106) solid particles with a different Stokes number (St) was implemented as a point-force approximation in the Navier-Stokes equation. Turbulence modulation in decaying turbulence is qualitatively and quantitatively inconsistent compared to that in stationary turbulence, particularly when the particles have St ≤ 1, mainly due to the artificial forcing necessary for stationary status. For the particles with St > 1, however, decaying and stationary turbulence are qualitatively similar. A simple analysis confirms that stationary turbulence is not appropriate for the study of turbulence modulation by particles with St ≤ 1.
Immersed boundary method for incompressible flows
We develop a new formulation of immersed boundary (IB) method based on direct forcing for incompressible viscous flows. The new algorithm for the present IB method is derived using a block LU decomposition and Taylor series expansion, and the direct forcing for imposing no-slip condition on the IB surface is calculated in an iterative procedure. We perform simulations of two-dimensional flows around a circular cylinder and three-dimensional flows over a sphere for low and moderate Reynolds numbers. Figure shows pressure distributions and streamlines for flow over circular cylinder at Re=1. The present method yield a better imposition of no-slip condition on IB surface for low Reynolds number with a fairly larger time step than other IB methods based on direct forcing.