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 very fast, electroless, microwave method is described to synthesize electrically conductive CdS nanowires on DNA just within 60 s. The electrical characterization indicates that the CdS wires are continuous, have very low contact resistance, and exhibit Ohmic behavior. Highly selective deposition on DNA is obtained by specific complexation between the Cd(II) ion and DNA, followed by decomposition of thioacetamide to S(2-) to form CdS. The nanowires are found to have a diameter of 140-170 nm and a length of approximately 8-12 microm. The one-step process developed here does not perturb the overall conformation of the DNA chain. The nanowires we fabricated can be used as building blocks for functional nanodevices, tiny computers, sensors, and optoelectronics.

No abstract

2016-11-03T14:11:40

[1] The main objectives of the present study are to obtain improved models of hydrodynamic forces and torque on a particle sitting on a bed and to use these models for the investigation of incipient motion and resuspension of particles. The improved models for force and torque are obtained from numerical simulations of a particle sitting on a bed with a turbulent flow of logarithmic mean velocity profile approaching the particle. Since the mean turbulent velocity profile can depart from the logarithmic profile in case of macroscale rough beds or flow down steep slopes, we have also considered forces and torque on a particle due to both linear and uniform mean flow profiles. The computed drag and lift coefficients and the predicted critical shear stress for incipient particle motion and resuspension are compared against available experimental results. The improved force and torque models are also used to evaluate the effect of turbulent velocity fluctuations on the critical shear stress for incipient motion and resuspension. The present results are of direct relevance to cases where the particle is mostly exposed to the ambient flow. In cases where the particle protrusion is small and is submerged mostly within a pocket of other particles, the above formulation can be used with a redefined area of exposure and flow velocity seen by the particle. However, the drag, lift, and torque coefficients and the resisting forces will be influenced by the partial exposure and the details of pocket geometry, which require further investigation.

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 (1994)], Panchapakesan and Lumley [“Turbulence measurements in axisymmetric jets of air and helium. Part 1. Air jet,” J. Fluid Mech. 246, 197–233 (1993)], and others. The assumptions made by the various experimental studies in order to solve the dissipation and pressure diffusion terms are discussed and examined using the data from the current study. The Reynolds stress transport equations are also calculated and discussed. Vortical structures are visualized by the λci method and discussed along with entrainment of ambient fluid into the jet. The one dimensional energy spectra in the azimuthal direction are calculated directly and are also discussed.

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 works towards dislodging the particle out of the pocket and migrating it downstream is explored with data from a direct numerical simulation of turbulent channel flow and the probabilities of instances when the force, impulse, and work-based criteria for particle motion are satisfied and computed. The work-based criterion for particle migration is used to obtain an expression for the bed load transport that can be applied on an instantaneous basis at any local region of the bed. The average nondimensional bed load transport rate, or Einstein number, computed based on the present work-based model is shown to compare well with existing experimental data and empirical models. In particular, at low mobility regime, the present model is able to naturally recover the well-accepted rapid increase in bed load transport as 16th power of average bed shear stress and at high mobility regime the present model captures the slower increase in bed load transport with increasing average bed shear stress.

No abstract