Trade Liberalisation Policies, Intra-regional Trade and Opportunities for Sustainable Agricultural Development

Many of the Near East (NE) countries are currently opening their agricultural markets at three distinct but interacting levels: unilateral liberalisation, regional integration schemes and multilateral trade liberalisation. These changes hold important implications for intra- and extra-regional trade, use of agricultural resources and sustainability of agricultural development in the NE countries. Unilaterally, and since the late 1980s, most countries of the region have liberalised their agriculture sectors by eliminating or reducing input subsidies, removing or reducing guaranteed producer prices, reducing the number of subsidised commodities and liberalising the exchange rate and the trade regime. Most of the implicit and explicit subsidies for agricultural inputs and outputs were withdrawn. However, some of the NE countries were able to continue supporting agriculture mainly for food security reasons. Experiences showed that domestic reform is necessary but not sufficient condition for economic growth.

Length-scale cascade and spread rate of atomizing planar liquid jets

The primary breakup of a planar liquid jet is explored via direct numerical simulation (DNS) of the incompressible Navier-Stokes equation with level-set and volume-of-fluid interface capturing methods. PDFs of the local radius of curvature and the local cross-flow displacement of the liquid-gas interface are evaluated over wide ranges of the Reynolds number ($Re$), Weber number ($We$), density ratio and viscosity ratio. The temporal cascade of liquid-structure length scales and the spread rate of the liquid jet during primary atomization are analyzed. The formation rate of different surface structures, e.g. lobes, ligaments and droplets, are compared for different flow conditions and are explained in terms of the vortex dynamics in each atomization domain that we identified recently. With increasing $We$, the average radius of curvature of the surface decreases, the number of small droplets increases, and the cascade and the surface area growth occur at faster rates. The spray angle is mainly affected by $Re$ and density ratio, and is larger at higher $We$, at higher density ratios, and also at lower $Re$. The change in the spray spread rate versus $Re$ is attributed to the angle of ligaments stretching from the jet core, which increases as $Re$ decreases. Gas viscosity has negligible effect on both the droplet-size distribution and the spray angle. Increasing the wavelength-to-sheet-thickness ratio, however, increases the spray angle and the structure cascade rate, while decreasing the droplet size. The smallest length scale is determined more by surface tension and liquid inertia than by the liquid viscosity, while gas inertia and liquid surface tension are the key parameters in determining the spray angle.Comment: Submitted for publication to International Journal of Multiphase Flow. 37 pages; 33 figure

Understanding liquid-jet atomization cascades via vortex dynamics

Temporal instabilities of a planar liquid jet are studied using direct numerical simulation (DNS) of the incompressible Navier-Stokes equations with level-set (LS) and volume-of-fluid (VoF) surface tracking methods. $\lambda_2$ contours are used to relate the vortex dynamics to the surface dynamics at different stages of the jet breakup, namely, lobe formation, lobe perforation, ligament formation, stretching, and tearing. Three distinct breakup mechanisms are identified in the primary breakup, which are well categorized on the parameter space of gas Weber number ($We_g$) versus liquid Reynolds number ($Re_l$). These mechanisms are analyzed here from a vortex dynamics perspective. Vortex dynamics explains the hairpin formation, and the interaction between the hairpins and the Kelvin-Helmholtz (KH) roller explains the perforation of the lobes, which is attributed to the streamwise overlapping of two oppositely-oriented hairpin vortices on top and bottom of the lobe. The formation of corrugations on the lobe front edge at high $Re_l$ is also related to the location and structure of the hairpins with respect to the KH vortex. The lobe perforation and corrugation formation are inhibited at low $Re_l$ and low $We_g$ due to the high surface tension and viscous forces, which damp the small scale corrugations and resist hole formation. Streamwise vorticity generation - resulting in three-dimensional instabilities - is mainly caused by vortex stretching and baroclinic torque at high and low density ratios, respectively. Generation of streamwise vortices and their interaction with spanwise vortices produce the liquid structures seen at various flow conditions. Understanding the liquid sheet breakup and the related vortex dynamics are crucial for controlling the droplet size distribution in primary atomization.Comment: Submitted for publication in Journal of Fluid Mechanics. 56 pages; 52 figure

Liquid jet pumped by rising gas bubbles

A two-phase mathematical model is proposed for calculating the induced turbulent vertical liquid flow. Bubbles provide a large buoyancy force and the associated drag on the liquid moves the liquid upward. The liquid pumped upward consists of the bubble wakes and the liquid brought into the jet region by turbulent entrainment. The expansion of the gas bubbles as they rise through the liquid is taken into account. The continuity and momentum equations are solved numerically for an axisymmetric air jet submerged in water. Water pumping rates are obtained as a function of air flow rate and depth of submergence. Comparisons are made with limited experimental information in the literature