One of the outstanding challenges in the field of porous materials is the design and synthesis of chemical structures with exceptionally high surface areas Such materials are of critical importance to many applications involving catalysis, separation and gas storage The claim for the highest surface area of a disordered structure is for carbon, at 2,030 m2 g(-1) (ref 2) Until recently, the largest surface area of an ordered structure was that of zeolite Y, recorded at 904 m2 g(-1) (ref 3) But with the introduction of metal-organic framework materials, this has been exceeded, with values up to 3,000 m2 g(-1) (refs 4-7) Despite this, no method of determining the upper limit in surface area for a material has yet been found Here we present a general strategy that has allowed us to realize a structure having by far the highest surface area reported to date We report the design, synthesis and properties of crystalline Zn4O(1,3,5-benzenetribenzoate)2, a new metal-organic framework with a surface area estimated at 4,500 m2 g(-1) This framework, which we name MOF-177, combines this exceptional level of surface area with an ordered structure that has extra-large pores capable of binding polycyclic organic guest molecules--attributes not previously combined in one material

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A route to high surface area, porosity and inclusion of large molecules in crystals

In this review, we present control methods for flow over a bluff body such as a circular cylinder, a 2D bluff body with a blunt trailing edge, and a sphere. We introduce recent major achievements in bluff-body flow controls such as 3D forcing, active feedback control, control based on local and global instability, and control with a synthetic jet. We then classify the controls as boundary-layer controls and direct-wake modifications and discuss important features associated with these controls. Finally, we discuss some other issues such as Reynolds-number dependence, the lowest possible drag by control, and control efficiency.

Control of Flow Over a Bluff Body

In this Letter we present a detailed mechanism of drag reduction by dimples on a sphere such as golf-ball dimples by measuring the streamwise velocity above the dimpled surface. Dimples cause local flow separation and trigger the shear layer instability along the separating shear layer, resulting in the generation of large turbulence intensity. With this increased turbulence, the flow reattaches to the sphere surface with a high momentum near the wall and overcomes a strong adverse pressure gradient formed in the rear sphere surface. As a result, dimples delay the main separation and reduce drag significantly. The present study suggests that generation of a separation bubble, i.e., a closed-loop streamline consisting of separation and reattachment, on a body surface is an important flow-control strategy for drag reduction on a bluff body such as the sphere and cylinder.

Mechanism of drag reduction by dimples on a sphere

Stepped spillways have been used for about 3,500 years. The last few decades have seen the development of new construction materials, design techniques and applications : e.g., embankment overtopping protection systems. Although it is commonly acknowledged that free-surface aeration is significant in stepped chutes, experimental data are scarce, often limited to very steep slopes. The paper presents an experimental study conducted in a large-size stepped chute. Observations demonstrate the existence of a transition flow pattern for intermediate flow rates between nappe and skimming flows. Detailed air-water flow measurements were conducted in both transition and skimming flows, immediately downstream of the inception point of free-surface aeration where uniform equilibrium flow conditions were not achieved. In skimming flows, a complete characterisation is developed for the distributions of void fraction, bubble count rate and velocity, and flow resistance is estimated including drag reduction effects. Transition flows exhibit significantly different air-water flow properties. They are highly aerated, requiring the design of comparatively high chute sidewalls. The design of embankment overflow stepped spillway is discussed in the light of the new results and design recommendations are provided. Major design issues include the step stability at the plunge point for high tailwater levels, the interactions between free-surface and seepage flows which could lead to further drag reduction, and secondary currents at the connection between steps and abutments.

https://espace.library.uq.edu.au/view/UQ:11096/ce158.pdf

Experimental Investigations of Air Entrainment in Transition and Skimming Flows down a Stepped Chute: Application to Embankment Overflow Stepped Spillways

Characterization of the microscopic fluctuations in systems that are far from equilibrium is crucial for understanding the macroscopic response. One approach is to use an 'effective temperature'--such a quantity has been invoked for chaotic fluids, spin glasses, glasses and colloids, as well as non-thermal systems such as flowing granular materials and foams. We therefore ask to what extent the concept of effective temperature is valid. Here we investigate this question experimentally in a simple system consisting of a sphere placed on a fine screen in an upward flow of gas; the sphere rolls because of the turbulence it generates in the gas stream. In contrast to many-particle systems, in which it is difficult to measure and predict fluctuations, our system has no particle-particle interactions and its dynamics can be captured fully by video imaging. Surprisingly, we find that the sphere behaves exactly like a harmonically bound brownian particle. The random driving force and frequency-dependent drag satisfy the fluctuation-dissipation relation, a cornerstone of statistical mechanics. The statistical mechanics of near-equilibrium systems is therefore unexpectedly useful for studying at least some classes of systems that are driven far from equilibrium.

Statistical mechanics of a gas-fluidized particle

Experiments were conducted in water and wind tunnels on spheres in the Reynolds number range 6 × 103 to 6.5 × 105 to study the effect of natural ventilation on the boundary layer separation and near-wake vortex shedding characteristics. In the subcritical range of Re (  4 × 105), ventilation caused a downstream shift in the mean locations of boundary layer separation and reattachment; these lines showed significant axisymmetry in the presence of venting. No distinct vortex shedding frequency was found. Instead, a dramatic reduction occurred in the wake unsteadiness at all frequencies. The reduction of wake unsteadiness is consistent with the reduction in total drag already reported. Based on the present results and those reported earlier, the effects of natural ventilation on the flow past a sphere can be categorized in two broad regimes, viz., weak and strong interaction regimes. In the weak interaction regime (subcritical Re), the broad features of the basic sphere are largely unaltered despite the large addition of mass in the near wake. Strong interaction is promoted by the closer proximity of the inner and outer shear layers at supercritical Re. This results in a modified and steady near-wake flow, characterized by reduced unsteadiness and small drag.

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Effect of natural ventilation on the boundary layer separation and near-wake vortex shedding characteristics of a sphere

The flowfield around a sphere with and without ventilation was investigated in a wind tunnel over a range of Reynolds numbers in an incompressible flow. At supercritical Re, the pressure drag of a sphere can be nearly nullified by venting only 2% of the frontal area of the sphere to the base through a smooth internal duct. The drag reduction is achieved by increased pressures in the separated flow region close to the base. At high Re, the vent flow breaks through the near wake and brings about symmetry in the global flowfield. When the internal shear is increased by using a rough internal duct, the base pressure is unchanged, but the external flow is accelerated to velocities beyond that achieved by the potential flow around the basic sphere. The findings can be explained by a flow model in which the near wake is aerodynamically streamlined by a pair of counterrotating vortex rings at the base. A roughness element can be made to partially destroy the vortex system at the base and result in a steady asymmetric wake. A 1.2 mm diameter wire placed at 70° was found to overtrip the boundary layer and completely destroy the vortex system. Simultaneously, the turbulent separation is advanced and the drag increased.

/pdf/effect-of-ventilation-on-the-flowfield-around-a-sphere-3z2qyk3xpm.pdf

G. K. Suryanarayana

Papers

Effect of natural ventilation on the boundary layer separation and near-wake vortex shedding characteristics of a sphere

Effect of ventilation on the flowfield around a sphere