Hyungoo Lee

Drag and lift forces on a spherical particle moving on a wall in a shear flow at finite Re

Recent research (Zeng, PhD thesis, 2007; Zeng et al ., Phys. Fluids , vol. 21, 2009, art. no. 033302) has shown that both the shear- and wall-induced lift contributions on a particle sharply increase as the gap between the wall and the particle is decreased. Explicit expressions that are valid over a range of finite Re were obtained for the drag and lift forces in the limiting cases of a stationary particle in wall-bounded linear flow and of a particle translating parallel to a wall in a quiescent ambient. Here we consider the more general case of a translating and rotating particle in a wall-bounded linear shear flow where shear, translational and rotational effects superpose. We have considered a modest Reynolds number range of 1–100. Direct numerical simulations using immersed boundary method were performed to systematically figure out the characteristics of hydrodynamic forces on a finite-sized particle moving while almost in contact with a wall. We present composite correlation for the hydrodynamic forces which are in agreement with all the available low-Reynolds-number theories.

A direct numerical simulation study of higher order statistics in a turbulent round jet

Up until recently direct numerical simulation (DNS) studies involving round turbulent jets have focused on first and second order statistics and vortical behavior near the source of the jet. The third order statistics necessary to compute the turbulent kinetic energy and Reynolds stress transport equations have been examined using LES studies. However, further examination with DNS is important as, on the subgrid scale, LES uses models for Reynolds stress. In this study a DNS of a turbulent free jet with a Reynolds number equal to ReJ = 2000 is computed using a second order accurate, time splitting finite volume scheme. First, second, and third order statistics are compared with previous experimental and numerical studies. All terms of the turbulent kinetic energy balance are calculated directly. The results are compared to experimental studies such as those of Hussein et al. [“Velocity measurements in a high-Reynolds-number, momentum-conserving, axisymmetric, turbulent jet,” J. Fluid Mech. 258, 31–75 (199...

Work-based criterion for particle motion and implication for turbulent bed-load transport

A simple work-based criterion for the onset of downstream migration of a particle sitting on a rough bed in a turbulent flow is developed in the present work. The criterion is motivated by the fact that the geometric pocket formed by other bed particles within which the mobile particle is sitting can be viewed as a potential well and the gravitational and frictional mechanisms impose an energy barrier for the particle to fully escape the pocket and initiate irreversible downstream migration. The energy barrier is clearly a statistical quantity, as it depends on the shape, size, and other details of the mobile particle and the geometry of the pocket. The energy barrier imposes a threshold value for the hydrodynamic work that must be done on the particle in order to initiate downstream migration. The simple work-based criterion developed here is related to the critical force and critical impulse criteria that have been advanced in the past. The fluctuating nature of the effective hydrodynamic force that wor...

A study of similarity solutions for laminar swirling axisymmetric flows with both buoyancy and initial momentum flux

Using an asymptotic approach, the established similarity solution for a plume is extended to the case of a plume with a small amount of angular momentum flux. Numerical simulations of laminar flows are performed in order to verify the similarity solution and to further examine the behavior of plumes with and without swirl and to compare with those of jets and swirling jets. Vortex breakdown of jets and plumes is explored, and an analytical estimate for the critical swirl in plumes is obtained, which compares well with the simulation results. A comparison is made between vortex breakdown in jets, where vortex breakdown destroys the jet-like behavior, and plumes, where vortex breakdown has a weak effect and the flow continues to have plume-like behavior. Simulation results of buoyant jets and injected plumes are presented, and the behavior of vortex breakdown of these hybrid flows is discussed in reference to the pure cases.

An examination of the high-order statistics of developing jets, lazy and forced plumes at various axial distances from their source

In this study, direct numerical simulations of a turbulent free jet (Re = 2000), a lazy plume (), and a forced plume (Re = 1684, Ri = 0.025) are compared. The evolution of the various fluxes and the so-called source parameter, Γ, are examined as a function of distance from the source. The first-, second-, and third-order statistics of the flows are calculated and discussed. The radial profiles of such statistics, as well as that of the turbulent kinetic energy balance and other second-order transport equations are examined at two axial distances, one axial distance before the flows have adjusted to their similarity solution, and the other beyond the similarity adjustment length scale. Vortical structures are visualised and discussed along with entrainment. The source term Γ was not found to monotonically decrease with axial distance from the source as predicted by past researchers. While the mean flow and turbulent velocity statistics of the simulated lazy and forced plumes took on similar behaviour far from the sources turbulent statistics which involve buoyancy did not.

A Numerical Study of Swirling Buoyant Laminar Jets at Low Reynolds Numbers

The effect of swirl on laminar buoyant jets with low Reynolds numbers is explored. Three dimensional direct numerical simulations are performed to solve the time-dependent, incompressible Navier-Stokes equations. We use a body fitted grid system and employ the finite volume method to discretize the governing equations. A second-order central difference scheme is employed for all spatial derivative terms. The numerical simulation is advanced in time by a fractional step method with the second-order Adams-Bashforth scheme for explicit-convection terms and the Crank-Nicholson scheme for implicit-diffusion terms. The amount of swirl and buoyancy is varied from zero to very large values and the effect on the velocity field, jet width, entrainment and vortex are examined. Comparisons with analytical and experimental models are discussed.Copyright

A Numerical Study of Natural Convection in a Square Enclosure with a Circular Cylinder at Different Vertical Locations

Numerical calculations are carried out for the natural convection induced by temperature difference between a cold outer square cylinder and a hot inner circular cylinder. A two-dimensional solution for unsteady natural convection is obtained, using the immersed boundary method (IBM) to model an inner circular cylinder based on finite volume method, for different Rayleigh numbers varying over the range of . The study goes further to investigate the effect of an inner cylinder location on the heat transfer and fluid flow. The location of inner circular cylinder is changed vertically along the center-line of square enclosure. The number, size and formation of cell strongly depend on Rayleigh number and the position of inner circular cylinder. The changes in heat transfer quantities have been presented.

Numerical Simulation of Thermal Lattice Boltzmann Model with a Modified In-Ternal Energy Non-Equilibrium First-Order Extrapolation Boundary Condition

In this paper, we adapt a modified internal energy non-equilibrium first-order extrapolation thermal boundary condition to the thermal lattice Boltzmann model (TLBM). This model is the double populations approach to simulate hydrodynamic and thermal fields. The bounce-back boundary condition which is a traditional boundary condition of lattice Boltzmann method has only a first order in numerical accuracy at the boundary and numerical instability. A non-equilibrium first-order extrapolation boundary condition has been verified to be of better numerical stability than the bounce-back boundary condition and this boundary condition is proved to be of second-order accuracy for the flat boundaries. The two-dimensional natural convection flow in a square cavity with Pr