Mohsen Cheraghi

A detailed laboratory study was conducted to examine the effects of particle size on hysteretic sediment transport under time-varying rainfall. A rainfall pattern composed of seven sequential stepwise varying rainfall intensities (30, 37.5, 45, 60, 45, 37.5 and 30 mm h‑1), each of 20-mins duration, was applied to a 5-m × 2-m soil erosion flume. The soil in the flume was initially dried, ploughed to a depth of 20 cm and had a mechanically smoothed surface. Flow rates and sediment concentration data for seven particle size classes (< 2, 2-20, 20-50, 50-100, 100-315, 315-1000 and > 1000 µm) were measured in the flume effluent. Clockwise hysteresis loops in the sediment concentration versus discharge curves were measured for the total eroded soil and the finer particle sizes (< 2, 2-20 and 20-50 µm). However, for particle sizes greater than 50 µm, hysteresis effects decreased and suspended concentrations tended to vary linearly with discharge. The Hairsine and Rose (HR) soil erosion model agreed well with the experimental data for the total eroded soil and for the finer particle size classes (up to 50 µm). For the larger particle size classes, the model provided reasonable qualitative agreement with the measurements although the fit was poor for the largest size class (> 1000 µm). Overall, it is found that hysteresis varies amongst particle sizes and that the predictions of the HR model are consistent with hysteretic behavior of different sediment size classes.

Heat-transfer enhancement in a uniformly heated slot mini-channel due to vortices shed from an adiabatic circular cylinder is numerically investigated. The effects of gap spacing between the cylinder and bottom wall on wall heat transfer and pressure drop are systemically studied. Numerical simulations are performed at Re=100Re=100, 0.1⩽Pr⩽100.1⩽Pr⩽10 and a blockage ratio of D/H=1/3D/H=1/3. Results within the thermally developing flow region show heat transfer augmentation compared to the plane channel. It was found that when the obstacle is placed in the middle of the duct, maximum heat transfer enhancement from channel walls is achieved. Displacement of circular cylinder towards the bottom wall leads to the suppression of the vortex shedding, the establishment of a steady flow and a reduction of both wall heat transfer and pressure drop. Performance analysis indicates that the proposed heat transfer enhancement mechanism is beneficial for low-Prandtl-number fluids.

Heat convection from the uniformly heated walls of a straight channel in presence of a rotationally oscillating cylinder (ROC) is simulated at Re = 100. Heat transfer enhancement due to vortex shedding from the ROC is investigated. Systematic studies are performed to explore the rotation angle and frequency influences on heat transfer by varying the latter in range of the lock-in regime and the former from 0 to 2π/3. All simulation results are based on the numerical solutions of two-dimensional, unsteady, incompressible Navier–Stokes and energy equations using an h/p type finite element algorithm. Considering time periodicity of the resulting flow and temperature fields, time averaged wall Nusselt number is reported to quantify the heat transfer enhancement for Pr = 0.1, 1.0, 5.0 and 10.0 fluids. Performance analyses of the ROC device based on its total power consumption and heat transfer enhancement are also presented.

Oscillating structures and actuators can induce flow kinematics that enhances mixing. This approach is specifically effective for mixing enhancement in meso-scale channels, where the flow kinematics can be actively controlled using micro-electro-mechanical-systems (MEMS). In this paper, numerical results for mixing of two incompressible ideal gas (Schmidt number of 1.0) streams through a 2D minichannel via a rotationally oscillating circular cylinder are presented and discussed. Simulations are performed for blockage ratio of D/H=l/3 and Reynolds number of 100 and oscillation amplitudes of π/3, 2π/3,π/2 and π for subharmonic (F < 1), harmonic (F = 1) and superharmonic (F > 1) regimes. Numerical results indicate that mixing performance is improved by about 70% compare to the plane channel at oscillation amplitude of π and excitation frequency of 25% higher than the natural frequency of vortex shedding of a stationary cylinder. It is shown that the mixing efficiency is increased by increasing of amplitude in all the cases except at very low excitation frequencies. This study also shows that when the excitation frequency is equal to the vortex shedding frequency the maximum power is required for mixing of two gases.