Dam-break flows at a jump in the width of a rectangular channel

The problem of dam-break flow at a jump in the width of a rectangular channel is studied in the first shallow-water approximation. Two cases where the upstream channel width is greater or smaller than the downstream channel width are considered. It is shown that in the first case, the problem is uniquely solvable under the assumption that the total energy of the flow is conserved at the jump in the channel width, and in the second case, a solution of the problem for some initial data exists only provided that the total energy of the flow is lost at this jump.     

Pressure losses in transitions between square and rectangular ducts of the same cross-sectional area

Experimental results are presented for the pressure loss in transitions between square and rectangular ducts where the two ends have the same cross-sectional area. The aspect ratios at the rectangular end ranged from 0.3 to 0.625, and the transition length from 1 to 2 times the hydraulic diameter. Reynolds numbers ranged from 50 000 to 125 000. The pressure drop may be divided into components arising from friction and velocity profile distortion. The friction component, which may be evaluated by normal pipe flow methods, accounts for the observed variation with Reynolds number. The velocity profile component increases as the aspect ratio of the rectangular end falls, and is significantly higher for rectangular to square than for square to rectangular transitions. There is an optimum length to hydraulic diameter ratio, for which the pressure loss is a minimum; it has not been found exactly, but is less than 2 and probably below 1.     

A numerical experiment on the interaction between a film and a turbulent jet

This work is focused on the study of the impingement of a turbulent plane jet on a moving film. A computational fluid dynamics code has been used to simulate the interaction between the turbulent plane jet and the moving film. Since the problem of coupling between turbulence and free surface flow is poorly understood and experiments in this problem are difficult to carry out, this new numerical tool has been designed to give insight into global and local parameters of the free surface flow. To cite this article: D. Lacanette et al., C. R. Mecanique 333 (2005).     

Effects of inlet conditions and surface roughness on the performance of transitions between square and rectangular ducts of the same cross-sectional area

A study has been made of the effects of inlet conditions and surface roughness on the performance of transitions between square and rectangular ducts of the same cross-sectional area. The conditions at entry were varied by using different approach lengths of straight duct and by means of a square screen of woven wire cloth. The surface roughening was accomplished by coating the surface of the transition with graded waterproof silicon carbide paper, whose surface roughness was measured with a Talysurf 4 instrument. All tests were run at Reynolds number 10⁵.

The results indicate that the static pressure loss coefficient significantly increases as the inlet boundary layer thickness increases. This variation is a function of aspect ratio at the rectangular end; the loss coefficient rises as the aspect ratio falls. The pressure drop slightly increases when the wall surface is roughened and is higher at low aspect ratios.     

Waves resulting from partial dam break with the formation of a breach in the form of a cut to the channel bottom

This paper gives experimental data on the propagation speed and height of a dam-break wave arising in the tailwater region during a partial dam break event. These data were used to confirm the Khristianovich calculation method. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 50, No. 3, pp. 97–103, May–June, 2009.     

Interfacial separation between elastic solids with randomly rough surfaces: Comparison between theory and numerical techniques

We study the distribution of interfacial separations at the contact region between two elastic solids with randomly rough surfaces. An analytical expression is derived for the distribution of interfacial separations using Persson's theory of contact mechanics, and is compared to numerical solutions obtained using (a) a half-space method based on the Boussinesq equation, (b) Green's function molecular dynamics technique and (c) smart-block classical molecular dynamics. Overall, we find good agreement between all the different approaches.     

Motion of a circular cylinder in a rectangular channel

When bodies move in fluids, the parameters of their motion depend strongly on the interaction of the bodies with the surrounding fluid [1, 2]. The present paper is devoted to determination of the hydrodynamic forces that act on a cylinder moving in an infinite rectangular channel in an ideal incompressible fluid that is at rest.     

One-dimensional interfacial area transport of vertical upward bubbly flow in narrow rectangular channel

The design and safety analysis for miniature heat exchangers, the cooling system of high performance microelectronics, research nuclear reactors, fusion reactors and the cooling system of the spallation neutron source targets requires the knowledge of the gas–liquid two-phase flow in a narrow rectangular channel. In this study, flow measurements of vertical upward air–water flows in a narrow rectangular channel with the gap of 0.993 mm and the width of 40.0 mm were performed at seven axial locations by using the imaging processing technique. The local frictional pressure loss gradients were also measured by a differential pressure cell. In the experiment, the superficial liquid velocity and the void fraction ranged from 0.214 m/s to 2.08 m/s and from 3.92% to 42.6%, respectively. The developing two-phase flow was characterized by the significant axial changes of the local flow parameters due to the bubble coalescence and breakup in the tested flow conditions. The existing two-phase frictional multiplier correlations such as Chisholm, 1967, Mishima et al., 1993 and Lee and Lee (2001) were verified to give a good prediction for the measured two-phase frictional multiplier. The predictions of the drift-flux model with the rectangular channel distribution parameter correlation of Ishii (1977) and several existing drift velocity correlations of Ishii, 1977, Hibiki and Ishii, 2003 and Jones and Zuber (1979) agreed well with the measured void fractions and gas velocities. The interfacial area concentration (IAC) model of Hibiki and Ishii (2002) was modified by taking the channel width as the system length scale and the modified IAC model could predict the IAC and Sauter mean diameter acceptably.     

The critical role of channel cross-sectional area in microchannel flow boiling heat transfer

Experiments are conducted with a perfluorinated dielectric fluid, Fluorinert FC-77, to identify the critical geometric parameters that affect flow boiling heat transfer and flow patterns in microchannels. In recent work by the authors (Harirchian and Garimella, 2009), seven different silicon test pieces containing parallel microchannels of widths ranging from 100 to 5850 μm, all with a depth of 400 μm were tested and it was shown that for a fixed channel depth, the heat transfer coefficient was independent of channel width for microchannels of widths 400 μm and larger, with the flow regimes in these microchannels being similar; nucleate boiling was also found to be dominant over a wide range of heat fluxes. In the present study, experiments are performed with five additional microchannel test pieces with channel depths of 100 and 250 μm and widths ranging from 100 to 1000 μm. Flow visualizations are performed using a high-speed digital video camera to determine the flow regimes, with simultaneous local measurements of the heat transfer coefficient and pressure drop. The aim of the present study is to investigate as independent parameters the channel width and depth as well as the aspect ratio and cross-sectional area on boiling heat transfer in microchannels, based on an expanded database of experimental results. The flow visualizations and heat transfer results show that the channel cross-sectional area is the important governing parameter determining boiling mechanisms and heat transfer in microchannels. For channels with cross-sectional area exceeding a specific value, nucleate boiling is the dominant mechanism and the boiling heat transfer coefficient is independent of channel dimensions; below this threshold value of cross-sectional area, vapor confinement is observed in all channels at all heat fluxes, and the heat transfer rate increases as the microchannel cross-sectional area decreases before premature dryout occurs due to channel confinement.     

One-group interfacial area transport equation and its sink and source terms in narrow rectangular channel

The characteristics of two-phase flow in a narrow rectangular channel are expected to be different from those in other channel geometries, because of the significant restriction of the bubble shape which, consequently, may affect the heat removal by boiling under various operating conditions. The objective of this study is to develop an interfacial area transport equation with the sink and source terms being properly modeled for the gas–liquid two-phase flow in a narrow rectangular channel. By taking into account the crushed characteristics of the bubbles a new one-group interfacial area transport equation was derived for the two-phase flow in a narrow rectangular channel. The random collisions between bubbles and the impacts of turbulent eddies with bubbles were modeled for the bubble coalescence and breakup respectively in the two-phase flow in a narrow rectangular channel. The newly-developed one-group interfacial area transport equation with the derived sink and source terms was evaluated by using the area-averaged flow parameters of vertical upwardly-moving adiabatic air–water two-phase flows measured in a narrow rectangular channel with the gap of 0.993 mm and the width of 40.0 mm. The flow conditions of the data set covered spherical bubbly, crushed pancake bubbly, crushed cap-bubbly and crushed slug flow regimes and their superficial liquid velocity and the void fraction ranged from 0.214 m/s to 2.08 m/s and from 3.92% to 42.6%, respectively. Good agreement with the average relative deviation of 9.98% was obtained between the predicted and measured interfacial area concentrations in this study.     

Characteristics of fiber suspension flow in a rectangular channel

Velocity profile of fiber suspension flow in a rectangular channel is measured by pulsed ultrasonic Doppler velocimetry (PUDV), and the effect of fiber concentration and Reynolds number on the shape of the velocity profile is investigated. Five types of flow behavior are observed when fiber concentration increases or flow rate decreases progressively. The turbulent velocity profiles of fiber suspension can be described by a correlation with fiber concentration, nl³, and Reynolds number, Re as the main parameters. The presence of fiber in the suspension will reduce the turbulence intensity and thus reduce the turbulent momentum transfer. On the other hand, fibers in the suspension have the tendency to form fiber networks, which will increase the momentum transfer. The relative contribution of these two types of momentum flux will determine the final shape of the velocity profile.     

Mechanical and acoustical study of a structural bond: comparison theory/numerical simulations/experiment

To carry out numerical simulations able to describe the behavior of a structural bond during a mechanical test, it is common to use a theory of damage. This approach consists in modeling the bonded zone by a surface distribution of springs with or without mass. The elasto-plastic with damage model and the numerical simulations are carried out with a finite element code. The use of this model requires two types of data: the critical energies in modes I and II measured during mechanical tests and the stiffnesses of the springs. These ones cannot be identified by mechanical measurements and, in this paper we propose an ultrasonic method to measure them. The ultrasonic approach and its experimental validation are first presented. Then, the mechanical model is detailed. The whole identification strategy is applied on aluminum/epoxy/aluminum samples.     

Steady flow of a liquid metal in a rectangular MHD channel

The known experimental studies of steady flows of a liquid metal in magnetohydrodynamic (MHD) channels of rectangular section [1–4] were performed only for a few values of the Reynolds number, which does not permit a clear delineation of the fundamental governing laws of the flow in the zone of transition from laminar to turbulent flow. In addition, the study of turbulent MHD flows has been limited to two-dimensional channels.Below we present some results of experimental studies of the effect of a transverse magnetic field on the resistance coefficient for mercury flow in an MHD channel with side ratio 1 to 2.5. The choice of a channel with this side ratio was dictated by the need for studies of the intermediate case between flows in two-dimensional and square channels, which differ significantly from one another because of the different effect of the walls parallel to the magnetic field. In our studies, for each value of the Hartmann number the investigations were made for 30–50 values of the Reynolds number.Notation B₀ flux density of the applied magnetic field - M Hartmann number - R Reynolds number - _tm resistance factor of turbulent MHD flow - _* critical value of the resistance factor - geometric parameter of channel - the component of resistance factor in ordinary hydrodynamics due to pulsations - normed function - electric conductivity of metal - viscosity of metal - R₀ shydraulic radius - N smagnetic field parameter     

Experimental and numerical investigations on flashing-induced instabilities in a single channel

During the start-up phase, natural circulation BWRs (NC-BWRs) need to be operated at low pressure conditions. Such conditions favor flashing-induced instabilities due to the large hydrostatic pressure drop induced by the tall chimney. Moreover, in novel NC-BWR designs the steam separation is performed in the steam separators which create large pressure drops at the chimney outlet, which effect on stability has not been investigated yet.In this work, flashing-induced oscillations occurring in a tall, bottom heated channel are numerically investigated by using a simple linear model with three regions and an accurate implementation for estimating the water properties. The model is used to investigate flashing-induced instabilities in a channel for different values of the core inlet friction value. The results are compared with experiments obtained by using the CIRCUS facility at the same conditions, showing a good agreement. In addition, the experiments on flashing-induced instabilities are presented in a novel manner allowing visualizing new details of the phenomenon numerical stability investigations on the effect of the friction distribution are also done. It is found that by increasing the total restriction in the channel the system is destabilized. In addition, the chimney outlet restriction has a stronger destabilizing effect than the core inlet restriction. A stable two-phase region is observed prior to the instabilities in the experiments and the numerical simulations which may help to pressurize the vessel of NC-BWRs and thus reducing the effects of flashing instabilities during start-up.     

Localised dynamics of laminar pulsatile flow in a rectangular channel

The exploitation of flow pulsation in low-Reynolds number micro/minichannel flows is a potentially useful technique for enhancing cooling of high power photonics and electronics devices. Although the mechanical and thermal problems are inextricably linked, decoupling of the local instantaneous parameters provides insight into underlying mechanisms. The current study performs complementary experimental and analytical analyses to verify novel representations of the pulsating channel flow solutions, which conveniently decompose hydrodynamic parameters into amplitude and phase values relative to a prescribed flow rate, for sinusoidally-pulsating flows of Womersley numbers 1.4 ≤ Wo ≤ 7.0 and a fixed ratio of oscillating flow rate amplitude to steady flow rate equal to 0.9. To the best of the authors’ knowledge, the velocity measurements – taken using particle image velocimetry – constitute the first experimental verification of theory over two dimensions of a rectangular channel. Furthermore, the wall shear stress measurements add to the very limited number of studies that exist for any vessel geometry. The amplification of the modulation component of wall shear stress relative to a steady flow (with flow rate equal to the amplitude of the oscillating flow rate) is an important thermal indicator that may be coupled with future heat transfer measurements. The positive half-cycle time- and space-averaged value is found to increase with frequency owing to growing phase delays and higher amplitudes in the near-wall region of the velocity profiles. Furthermore, the local time-dependent amplification varies depending on the regime of unsteadiness: (i) For quasi-steady flows, the local values are similar during acceleration and deceleration though amplification is greater near the corners over the interval 0 – 0.5π. (ii) At intermediate frequencies, local behaviour begins to differ during accelerating and decelerating periods and the interval of greater wall shear stress near the corners lengthens. (iii) Plug-like flows experience universally high amplifications, with wall shear stress greater near the corners for the majority of the positive half-cycle. The overall fluid mechanical performance of pulsating flow, measured by the ratio of bulk mean wall shear stress and pressure gradient amplifications, is found to reduce from an initial value of 0.97 at Wo = 1.4 to 0.28 at Wo = 7.0, demonstrating the increasing work input required to overcome inertia.     

A problem for a micropolar rectangular region in the fifth approximation

An isotropic micropolar two-dimensional region is considered. Several equations of the fifth approximation for displacements and rotations are derived in terms of moments with respect to the Legendre polynomials. Based on these equations, the solutions obtained in the framework of the micropolar theory are compared with the solutions obtained in the framework of the classical theory of elasticity.