Drag reduction in riblet-lined pipes

The possibilities of reducing the drag in pipes with a circular cross section by lining them with riblets have been investigated experimentally for developed turbulent air flow. The maximum drag reduction of 6–7% in the riblet-lined as compared with the smooth pipe was obtained for a dimensionless riblet pitch, expressed in law-of-the-wall parameters,s ⁺=14–18.Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 57–61, January–February, 1995.     

Potential flow in helical pipes

Summary In this paper we studied the potential flow in a helical pipe using an orthogonal system of coordinates fixed to the helical axis. The analysis was conducted both analitically by applying perturbative methods and numerically. Results show that the torsion of the helical axis has little effect on axial velocity in a torus provided with the same curvature of the helix. At the same time it induces a large secondary flow that is expected to have a relevant effect on the boundary layer on the walls of the helical pipe.

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Sommario In questo lavoro si è studiato il flusso potenziale in un tubo ad asse elicoidale per mezzo di un sistema di coordinate ortogonali fisse all'asse del tubo. Il flusso è stato studiato sia analiticamente con metodi perturbativi che numericamente. I risultati ottenuti mostrano che la torsione dell'asse elicoidale ha piccoli effetti sulla velocitá assiale, approssimativamente data da quella che si avrebbe in un toro di uguale curvatura, mentre produce forti flussi secondari nel piano normale all'asse. Questi flussi secondari possono avere un'effetto rilevante per quel che riguarda lo strato limite sulle pareti del tubo.

     

Periodic flows of pseudoplastic fluids in pipes

Experimental results on the flow enhancement during flow of clay slurries (n = 0.15) through oscillating pipes are compared with theoretical predictions. The agreement is fairly good, especially in the oscillating boundary layer flow regime. Flow enhancement of order 10⁸ was found with the slurries used.     

Developing turbulent flow in smooth pipes

A numerical and experimental investigation of steady incompressible developing turbulent flow in smooth pipes is presented. Finite difference techniques are used to solve simultaneously the vorticity transport and stream function equations utilising a modified form of the Van Driest effective viscosity model. The numerical solutions are verified experimentally using air as a working fluid at pipe Reynolds 1 × 10⁵, 2 × 10⁵ and 3 × 10⁵.     

So-called constants of turbulence in rough pipes

A direct analysis of Nikuradse's experiments on flow in rough pipes has shown that on the quadratic resistance interval the value of the second constant of turbulence depends on the relative roughness of the pipe. It is established that the universal logarithmic relation for the velocity profile in a pipe in the quadratic resistance regime is an approximation of the more accurate relation obtained by the authors, which takes into account the effect of the relative roughness of the pipe. Where the roughness is significant, the proposed expression for the skin-friction coefficient is more accurate than the well-known Prandtl-Nikuradse formula.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 182–185, March–April, 1990.     

Flow of plastico-viscous media in noncircular pipes

Journal of Applied Mechanics and Technical Physics -     

Condenser instability in pressure-controlled heat pipes

The vapour-gas interface of pressure-controlled heat pipes loses heat to the condenser. This heat is supplied by condensation of vapour, forming a mist which is supported by flow of vapour from below. Convecting gas transports vapour and mist to cool parts of the condenser where they liquefy and flow back past the interface. The balance of these flows is easily disturbed and regular fluctuations of the interface commonly occur, especially at low operating pressures. This accentuates transport of vapour via the gas, and solid condensate can form and isolate the heat pipe from the gas ballast, especially when the interface is well inside the condenser.

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Kondensatorinstabilität in druckgesteuerten Wärmerohren

Zusammenfassung Die Dampf-Gas-Grenzfläche von druckgesteuerten Wärmerohren verliert Wärme an den Kondensator. Diese Wärme wird durch Kondensation vom Dampf geliefert unter Bildung eines Tröpfchenschleiers, der vom Dampfstrom von unten her getragen wird. Konvektionsgas überträgt Dampf und Tröpfchen zu den kalten Teilen des Kondensators, wo sie sich verflüssigen und an der Grenzfläche vorbei zurückfließen. Das Gleichgewicht dieser Strömungen kann leicht gestört werden und regelmäßige Schwankungen der Grenzfläche kommen, besonders bei niedrigen Betriebsdrücken, oft vor. Dies verstärkt die Dampfübertragung durch das Gas. Festkondensat kann sich bilden und das Wärmerohr vom Gasballast isolieren, besonders wenn die Grenzfläche weit innerhalb des Kondensators liegt.

     

Surface phenomena in elasticity

The characteristic properties of free surfaces of elastic bodies, such as propagation speeds of characteristic waves and conditions of instability of the surface shape (wrinkling), are considered. The results are generalized whenever a free surface is streamlined by an inviscid flow. The theory is illustrated by example.     

The effects of casting speed on steel continuous casting process

A three dimensional simulation of molten steel flow, heat transfer and solidification in mold and “secondary cooling zone” of Continuous Casting machine was performed with consideration of standard k−ε model. For this purpose, computational fluid dynamics software, FLUENT was utilized. From the simulation standpoint, the main distinction between this work and preceding ones is that, the phase change process (solidification) and flow (turbulent in mold section and laminar in secondary cooling zone) have been coupled and solved jointly instead of dividing it into “transient heat conduction” and “steady fluid flow” that can lead to more realistic simulation. Determining the appropriate boundary conditions in secondary cooling zone is very complicated because of various forms of heat transfer involved, including natural and forced convection and simultaneous radiation heat transfer. The main objective of this work is to have better understanding of heat transfer and solidification in the continuous casting process. Also, effects of casting speed on heat flux and shell thickness and role of radiation in total heat transfer is discussed.     

Modeling and analysis of water-hammer in coaxial pipes

The fluid-structure interaction is studied for a system composed of two coaxial pipes in an annular geometry, for both homogeneous isotropic metal pipes and fiber-reinforced (anisotropic) pipes. Multiple waves, traveling at different speeds and amplitudes, result when a projectile impacts on the water fillinhe annular space between the pipes. In the case of carbon fiber-reinforced plastic thin pipes we compute the wavespeeds, the fluid pressure and mechanical strains as functions of the fiber winding angle. This generalizes the single-pipe analysis of J. H. You, and K. Inaba, Fluid-structure interaction in water-filled pipes of anisotropic composite materials, J. Fl. Str. 36 (2013). Comparison with a set of experimental measurements seems to validate our models and predictions.     

Flow of drag-reducing surfactant solutions in rough pipes

The paper reports the results of experimental study of the flow of hexadecyltrimethylammonium chloride (CTAC) solutions with addition of sodium salicylate (NaSal) in the rough pipes. Measurements were performed in the range of the surfactant concentration from 200 to 400 ppm at a constant molar ratio CTAC/NaSal of 1:2. Five pipes of the relative roughness k/D varying from 1.2 × 10^?2 to 5.6 × 10^?2, obtained by the covering of inner surface of the pipes with glued silicon carbide particles of different size, were studied. The roughness was observed to increase the drag of flow of CTAC/NaSal solutions already at Reynolds numbers higher than 800. With increasing relative roughness k/D, the critical value of Reynolds number, at which the drag reduction disappears, was found to decrease. However, no influence of the roughness on the critical shear stress was noted. The ratio of the critical Reynolds number for rough pipes to that of hydraulically smooth pipes was independent of the surfactant concentration. The degree of drag reduction by the flow of surfactants was greater in rough pipes than in smooth pipes.     

Calculations of stratified wavy two-phase flow in pipes

Calculations of fully developed, stratified wavy gas–liquid pipe flow is presented. The wavy interface is represented by an equivalent interfacial roughness obtained from experimental data, which is made non-dimensional following the Charnock formulation. The two-dimensional, steady-state axial momentum equation is solved together with a two-layer turbulence model, which is modified to account for the roughness introduced at the interface. The governing equations are discretized using a finite difference method on a composite, overlapping grid with local grid refinement near the interface and the wall. The immersed interface method is used to make the numerical scheme well-defined across the interface, and a level set function is used to represent the interface. Numerical calculations are found to compare satisfactorily with experimental data.     

Hydromagnetic convective heat transfer in vertical pipes

In the present analysis, we consider the effect of radial magnetic field on the steady flow produced by the combined free and forced convection in an annulus between two coaxial vertical cylinders. A numerical solution of the problem is obtained by using Runge-Kutta-Merson method. For Rayleigh number Ra<0, that is, when the temperature of the pipes decreases as their height increases, the velocity increases with |Ra|. However, it reduces as the Hartmann number M increases. On the other hand, when Ra>0, there occurs back flow controlled by the effect of the magnetic field. Further, the influence of Rayleigh number and Hartmann number on the temperature is also discussed.Nomenclature c _p specific heat at constant pressure - g acceleration due to gravity - H _r applied magnetic field - H _z induced magnetic field - p pressure - T temperature of the fluid - T ₁, T ₂ temperatures of the inner and outer cylinders at z=0 - U _z velocity - coefficient of volume expansion - density - _w reference density - coefficient of viscosity - _e magnetic permeability - _e electrical conductivity - thermal conductivity - _m magnetic diffusivity     

Torsional deformations in incompressible fibre-reinforced cylindrical pipes

The first part of the paper deals with an extension of the classical Rivlin's solution of the torsion problem of a neo-Hookean pipe. The second part concerns a study of the passive torsional deformation processes in a fibre-reinforced cylindrical dummy of the beating heart. Especially, the dependence of the torsional and volumetric stiffness of the cylindrical pipe on different geometric and material parameters is discussed through a set of numerical simulations.