Pressure Drop for Low Reynolds-Number Flows Through Regular and Random Screens

Creeping flow through both regular and irregular screens was simulated by the lattice-Boltzmann method, and the dependence on screen porosity and Reynolds number of the pressure drop across the screen was analyzed. Regular structures were planar arrays of straight fibers or woven one-layer structures. The irregular planar structures were composed of randomly located and oriented fibers of finite length. A simple function of screen porosity based on partly numerical scaling arguments was found to describe accurately the simulated pressure drop across all regular screens. Due to their bigger surface area, the flow resistance of woven screens was found to be about 15% larger than that of regular planar screens with the same porosity. The pressure drop across irregular planar screens was found to be described by the same screen-porosity function with a slightly different ‘scaling’ exponent which thus appears to be dependent on the structure of the screen. The flow resistance of irregular structures was found to be clearly smaller than that of regular structures because of channelling of the flow through very few largest pores.     

Extension of the problem of designing a wing airfoil with active flow controls to the case of systems involving a screen

The solution of the inverse boundary-value problem for a wing airfoil with slot air suction from the outer flow on its upper surface and control jet injection from its rear is generalized to include the case of the flow around such an airfoil in the presence of a screen. The complications due to the double connectivity of the flow region are overcome by means of introducing a fictitious flow beneath the screen. The solution is constructed using a numerical-analytical iteration procedure. Formulas for recalculating the aerodynamic characteristics and the velocity distribution over the airfoil surface to different flight regimes and suction-injection device operation regimes are derived. The effect of the injected jet energy, the airfoil-screen spacing, and the slope of the rectilinear airfoil undersurface on the aerodynamic and geometric characteristics of the airfoil, namely, the shape and the lift coefficient, is studied. The reliability of the results is confirmed by the results of calculations using the Fluent program.     

Plane jets impinging on porous walls

The flow of a two-dimensional plane turbulent jet impinging on a porous screen has been studied experimentally. It is shown how the overall flow structure depends on the porosity of the surface. For low screen porosity (β < 0.41, say), transverse wall jets can be formed on both sides of the screen and in extreme cases the axial momentum flux some way downstream of the screen falls to zero, so that the screen has the same drag as would a solid wall. For high screen porosity (β > 0.57, say) the axial volume flux is largely preserved through the screen, but the dominant eddy structures present in the upstream jet are largely destroyed, so that entrainment rates downstream of the screen can be very low. The relatively small, intermediate range of porosities (0.41 < β < 0.57, where β is the screen open area ratio) is associated with dramatic changes in flow pattern and recirculating regions can exist on the upstream side of the screen. These flows, although all geometrically very simple, provide a serious challenge for computational modelling. Received: 25 May 2000 / Accepted: 22 February 2001     

Double porous screen element for gas–liquid phase separation

We consider an assembly of two parallel porous screens suspended in a tube at a distance L. The screens are connected by wicking aids. If one screen is brought into contact with a wetting liquid, the other screen will be wetted as well enclosing gas in between. Due to surface tension in the screen pores, the gas can only be removed from the chamber when the pressure difference across one screen exceeds the bubble point. With such a double porous screen element it is therefore possible to block liquid flow using trapped gas as plug. We present a model approach, experiments and numerical calculations on the performance of such a screen element. The model is based on capillary transport in vertical and radial capillaries and allows to predict how fast the element will trap the gas to become operational. For the experiments, we have built such an element using Dutch Twilled weaves made of stainless steel. Placed in a vertical tube and initially dry, it is wetted from below or above and submitted to an increasing pressure difference until breakthrough occurs where the element fails. Corresponding numerical calculations elucidate what happens within the element when it fails. Our results confirm the concept of the double porous screen element and encourage its application as liquid management device.     

Axisymmetric jets impinging on porous walls

The flow of axisymmetric turbulent jets impinging on porous walls has been studied experimentally. It is shown how the overall flow structure depends on the porosity of the surface. For high porosities (open area ratios, β, in excess of around 40% say) the porous wall, or screen, leads to a sudden increase in jet width and decrease in mean and fluctuating velocities, a direct consequence of the momentum flux extracted because of the screen drag. Lower porosities can lead to the appearance of radial wall jets on the upstream side of the screen but, in contrast to the corresponding case of planar jet impingement (Cant et al. in Exp Fluids 32:16–26, 2002), such wall jets never occur on the downstream side. The axial downstream velocities thus remain positive for all porosities. Jet growth rates for are initially increased by the screen, but once β≤0.4 momentum extraction by the screen is virtually complete, so that velocities become very small. Again, unlike in the corresponding planar case (for β≈0.4), recirculating regions upstream of the screen never occur. A simple argument is suggested to explain the fundamental differences in flow behaviour between planar and axisymmetric jet impingement onto porous screens and it is concluded that in the latter case the effects of the screen are generally more benign and unsurprising. Nonetheless, these axisymmetric flows, like the corresponding planar ones, provide a serious challenge for computational modelling.     

Darcy–Brinkman Flow Over a Screen Embedded in an Anisotropic Porous Medium

A screen composed of in-plane thin strips is embedded in a porous medium. The screen is either normal or parallel to the applied pressure gradient which forces a flow through the anisotropic porous medium. The principal axes of anisotropy are assumed to be aligned with that of the screen. The governing equation is fourth order and cannot be factored as in the isotropic case. The solutions are found by eigenfunction superposition (with complex eigenvalues) and point match. Anisotropy has first-order effects on the flow and the drag on the screen. Extrapolation yields fundamental results for the drag of a single slat in an anisotropic porous medium.

     

Heat and mass transport in nonisothermal turbulent flow when there is a screen formed by a gas of a different nature

We obtain a limiting relative law for heat and mass transport when there is a gas screen in a turbulent boundary layer, which makes it possible to take into account the effect of nonisothermal flow on the turbulent heat and mass transport beyond the region where the foreign gas is injected. The theoretical results are compared with experimental data on the intensity of burn-up of a graphite surface in an air flow when helium is injected through a tangential slit. The experimental data were obtained from the diffusion region of the burn-up.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 152–156, September–October, 1971.     

Bluff-body drag reduction by passive ventilation

The drag of a sphere at highRe can be reduced to more than half its value by passive ventilation from the stagnation region to the base. Simultaneously, the flow field around the base is stabilized and made symmetric, leading to reduction of unsteady aerodynamic forces. At highRe, the vent flow breaks through the dead water region associated with the near wake and aerodynamically streamlines the base. The streamlining is done by virtue of a base-vortex-ring beyond the point of turbulent boundary layer separation. A mean flow model for the flow around the vented sphere is proposed.Smoke flow visualized on a laser light screen placed at two diameters behind the base of the sphere shows the effectiveness of the method in suppressing the flow oscillations.The drag reduction achieved is very sensitive to the quality of the external surface and relatively insensitive to disturbances in the internal flow. Surface roughness or boundary layer tripping wire on the external flow can completely offset the benefit obtained.     

A device to achieve low Reynolds numbers in an open surface water channel

When investigating flow structures, and especially flow transitions, research projects often seek to increase insight using complementary numerical and physical experiments. Obtaining exact Reynolds number correspondence can frequently be difficult in experiments, particularly when relatively low values are required. Often, available test facilities were designed and optimised for a specific velocity range, meaning they have restrictions on the minimum flow velocity. This study describes a device to reduce the flow velocity locally in an open surface water channel. The underlying idea is to divert a controlled fraction of the incoming flow from the working section by increasing the pressure there, resulting in reduced velocity. This idea is realised using a ‘sub-channel’ that can be inserted into the main test chamber, with a variable porosity perforated screen at its downstream end. This study assesses and optimises the flow quality inside this structure, such as usable test section length, uniformity of the velocity profiles and turbulence intensity. The results demonstrate that the device creates high quality low Reynolds number flows, which is exemplified with the canonical circular cylinder in cross-flow.     

Heat‐and mass‐transfer characteristics in a spatial subsonic flow around a blunt body

A study was performed of methods for controlling thermal regimes in a spatial supersonic flow around a blunt body with the simultaneous use of gas injection from the surface of the porous bluntness and heat flow in the shell material. The effect of the nonisothermicity of the shell wall on the heat and masstransfer characteristics in the boundary layer was taken into account by solution of the problem in a conjugate formulation. It is shown that heat conducting materials can be used to advantage to reduce the maximum temperatures in the screen zone.     

Hydrodynamic Solution to the Problem of Water Permeability of a Screen with Damaged Continuity

The problem of water permeability of a thin impervious screen made of a polymeric geomembrane with flaws (damages) is considered. The screen consists of a covering layer and a ground base underlaid by a drainage bed. The solution is implemented using methods of theory of flow through a porous medium by means of the conformal mapping and velocity hodograph methods. The characteristic feature of this solution is the study of free pressurized–pressureless flow in a porousmediumthrough a continuous slit in the plane formulation. The basic computational dependences are presented and the calculations of the water permeability are carried out by means of the formulas obtained in comparison with the well-known dependences for a particular case.     

Numerical prediction of turbulent flow in a conical diffuser usingk-ε model

Fully developed incompressible turbulent flow in a conical diffuser having a total divergence angle of 8° and an area ratio of 4∶1 has been simulated by ak-ε turbulence model with high Reynolds number and adverse pressure gradient. The research has been done for pipe entry Reynolds numbers of 1.16×10⁵ and 2.93×10⁵. The mean flow velocity and turbulence energy are predicted successfully and the advantage of Boundary Fit Coordinates approach is discussed. Furthermore, thek-ε turbulence model is applied to a flow in a conical diffuser having a total divergence angle of 30° with a perforated screen. A simplified mathematical model, where only the pressure drop is considered, has been used for describing the effect of the perforated screen. The optimum combination of the resistance coefficient and the location of the perforated screen is predicted for high diffuser efficiency or the uniform velocity distribution.     

Unsteady flow through screens

Summary Reflected and transmitted pressure waves take place when an incident pressure wave reaches a screen placed in a duct and crossed by a previous steady flow. The thickness of the screen is assumed to be negligible and essure losses through it in the quasi steady flow, which takes place after the passage of the incident pressure wave, are assumed be steady flow ones. The strength of reflected and transmitted pressure waves is related to the strength of the incident wave, to the porosity of the screen and to the previous flow through it. Subsonic and just supersonic quasi steady flow inside the screen are considered. Five commercial screens have been tested with incident shock waves and the experimental results have been compared with the theoretical ones: the agreement is quite satisfactory.

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Sommario L'interazione di un'onda di pressione con una rete intubata attraversata da un flusso preesistente produce un'onda riflessa ed una trasmessa. Si presenta un modello teorico basato essenzialmente sulle potesi che lo spessore della rete sia trascurabile e che le perdite carico attraverso di essa, nel flusso quasi stazionario indotto dalla suddetta interazione, coincidano con quelle proprie di un oto permanente. Si analizza il caso di flusso indotto nella rete ia subsonico che sonico. Si eseguono prove sperimentali su cinque reti di diverse porosità differenti condizioni di flusso, e si verifica l'attendibilità del odello teorico proposto.

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The present work has been supported by the “Consiglio Nazionale delle Ricerche” (C.N.R.) under Contract no. 115.4075.69.01220.

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The suffixes “u” and “d” pertain to the conditions upstream and downstream of the screen with reference to the flow through it. The suffix “j” refers to the region “j” of the wave diagrams in Fig. 1.     

Collaborative optimization of screening efficiency and screen surface load — Part I: Modeling and evaluation

The screen surface load (SSL) caused by granular materials is an important factor affecting the structural performance of vibrating screen. Based on virtual experiment, a multi-objective collaborative optimization method is proposed to control the SSL under high screening efficiency (SE) in this work. Firstly, a DEM model was established to study the influence of process parameters on SE and SSL. Secondly, the NSGA-II (Non-dominated Sorting Genetic Algorithm) was employed to optimize the screening parameters with both SE and SSL as targets. The optimization method proves to be effective implementing on a linear vibrating screening. With SE equals to 98.5%, the SSL optimizable range is 39.2%. While compromising the SE to 88.7%, the SSL optimizable range improves to 48.6%. The result shows that the collaborative optimization could effectively control the SSL while maintaining a high SE, which is of great significance to improve the service life of screen surface and screen body.     

Experimental study on wake flow structures of screen cylinders using PIV

Experiments were conducted in a water flume using Particle Image Velocimetry (PIV) to study the evolution of the vortical structures in the wakes of four types of screen cylinders at a Reynolds number of about 3200. The results were compared with that of a bare cylinder. The screen cylinders were made of stainless steel screen meshes of various porosities (37%, 48%, 61% and 67%) rolled into cylindrical shapes. Smoke wire flow visualisations in a wind tunnel were also conducted in support of the PIV tests. Depending on the porosity of the screen mesh, two vortex formation mechanisms for the screen cylinder wakes were identified. One was associated with wake instability and the other was associated with shear-layer (Kelvin-Helmholtz) convective instability which involved merging through pairing and tripling of small-scale vortices within the shear layers. The former was responsible for the formation of large-scale vortices in the bare cylinder and the screen cylinder wakes with 37% and 48% porosities, while the latter was responsible for the screen cylinder wakes with 61% and 67% porosities. The results also showed that with increasing porosity, the vortex formation region was extended farther downstream and the Reynolds shear stress, the Turbulent Kinetic Energy (TKE) and vortex intensity were decreased constantly.     

Enhancement of pool boiling heat transfer in a horizontal water layer through surface roughness and screen coverage

An experimental investigation was carried out to study the augmentation of heat transfer in saturated pool boiling of a liquid water layer on a heated horizontal stainless steel plate by roughing the surface and/or covering it with a single layer of stainless steel screen. The results were presented in terms of the boiling curves. Effects of various parameters – the surface roughness, liquid level and size of the stainless steel mesh on the boiling heat transfer were examined in detail. The measured data clearly indicated that a lowering of the liquid level from 60 to 5?mm in water depth causes heat transfer reduction. Roughing the surface was found to sig- nificantly enhance the heat transfer. Use a layer of metal screen to cover the heated surface was shown to substantially augment the heat transfer especially for a shallow water layer if the mesh size is comparable with the bubble departure diameter. Covering the rough surface with the metal mesh, however, reduced the heat transfer.     

Some features of the flow around rapidly rotating bodies made of cellular-porous materials

Two types of gas flows arising near a rapidly rotating cellular-porous disk are studied numerically and experimentally. Steady-state limits for the flow around a disk rotating in free space and the type and scenario of the loss of stability are determined. Transitional flows are characterized by formation of a vortex sheet at the boundary of the exhausting jet. Numerical simulations of the flow around a cellular-porous disk rotating near a flat screen show that it is possible to form a closed swirl flow responsible for redistribution of swirl in the gap between the disk and the flat screen. The computed results offer an explanation for the experimentally observed excess of tangential velocity of the flow in the gap over the velocity of disk rotation. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 1, pp. 86–96, January–February, 2007.     

水平井砾石充填数值模拟研究进展

综述了水平井数值模拟研究的最新进展,内容包括：水平井砾石充填 机理及技术难题、充填过程中平衡砂床高度、临界流速的计算模型,描述了筛管/井筒环空砂 浆流动、冲管/筛管环空纯携砂液流动以及它们之间流体的交换和向地层的滤失过程. 重点介 绍了$\alpha$波充填过程中流动数学模型的研究现状及影响水平井砾石充填的各个 因素和存在的问题.     

Experimental investigation of the characteristics of unsteady-state breakaway zones arising in a supersonic flow at a needle with a screen

An experimental investigation of unsteady-state flow in a breakaway zone arising at a needle installed ahead of a conical screen and ahead of a screen located in a cylindrical resonance cavity was made.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 98–104, July–August, 1977.     

The theoretical study of the efficiency of diffusion deposition of nanoaerosols in the extended range of the Peclet numbers

The efficiencies of the diffusion deposition of nanoaerosols for a single fiber for the models of aerosol filter and wire mesh screen are studied numerically in the extended range of the Peclet number Pe. The rectangular periodic cell model for fluid flow and convective-diffusive transport of small aerosol particles is used. Most of the previous theoretical and experimental studies of single fiber diffusion deposition efficiency were for the case of Pe > 1. The array with uniform square or chess grid of fibers and of a row of circular cylindrical fibers are considered as the filter and wire mesh screen models. The flow and particles transport equations are solved numerically using the Boundary Element Method.The obtained numerical data are used to derive the approximate formulas for the deposition efficiency in the entire range of the Peclet number for the various porosities of the filter medium or distances between fibers in a wire mesh screen. The derived dependencies take into account nonlinearity of the deposition efficiency at the low Peclet numbers. The obtained analytical dependencies compare well with the numerical and experimental data.