Numerical Calculation of Turbulence Structure in Depth-Varying Unsteady Open-Channel Flows

Unsteady depth-varying open-channel flows are really observed in flood rivers. Owing to highly accurate laser Doppler anemometers (LDA), some valuable experimental databases of depth-varying unsteady open-channel flows are now available. However, these LDA measurements are more difficult to conduct in open-channel flows at higher unsteadiness, in comparison with unsteady wall-bounded flows such as oscillatory boundary layers and duct flows. Therefore, in this study, a low-Reynolds-number k–ε model involved with a function of unsteadiness effect was developed and some numerical calculations were conducted using the volume of fluid method as a free-surface condition. The present calculated values were in good agreement with the existing LDA data in the whole flow depth from the wall to the time-dependent free surface. These values were also compared with those of unsteady wall-bounded flows. The present calculations were able to predict the distributions of turbulence generation and its dissipation, and consequently the unsteadiness effect on turbulence structure was discussed on the basis of the outer-variable unsteadiness parameter α, which is correlated with the inner-variable unsteadiness parameter ω+ in unsteady wall-bounded flows.     

Numerical Sensitivity Study of Unsteady Friction in Simple Systems with External Flows

This paper investigates the importance of unsteady friction effects when performing water hammer analyses for pipe systems with external fluxes due to demands, leaks, and other system elements. The transient energy equation for a system containing an orifice-type external flow is derived from the two-dimensional, axial momentum equation. A quasi-two-dimensional flow model is used to evaluate the relative energy contribution of total friction, unsteady friction, and the external flow, in a 1,500?m pipeline, with orifice flows ranging from steady-state flows of 2–70% of the mean pipe flow, and a Reynolds number of 600,000. It is found that for initial lateral flows larger than around 30% of the mean flow, unsteady friction effects can probably be neglected, whereas for external flows smaller than this, unsteady friction should generally be considered. Overall, the relative role of unsteady friction is found to diminish as the external flux increases, implying that unsteady friction is not critical for systems with large external flows. These results imply that unsteady friction may have a significant impact on the validity of transient leak detection techniques that have been derived assuming quasi-steady friction. To demonstrate this point, an existing transient leak detection method, originally derived under quasi-steady conditions, is tested with unsteady friction included.     

Dam Break Wave of Thixotropic Fluid

Thixotropy is the characteristic of a fluid to form a gelled structure over time when it is not subjected to shearing, and to liquefy when agitated. Thixotropic fluids are commonly used in the construction industry (e.g., liquid concrete and drilling fluids), and related applications include some forms of mud flows and debris flows. This paper describes a basic study of dam break wave with thixotropic fluid. Theoretical considerations were developed based upon a kinematic wave approximation of the Saint-Venant equations down a prismatic sloping channel. A very simple thixotropic model, which predicts the basic rheological trends of such fluids, was used. It describes the instantaneous state of fluid structure by a single parameter. The analytical solution of the basic flow motion and rheology equations predicts three basic flow regimes depending upon the fluid properties and flow conditions, including the initial “degree of jamming” of the fluid (related to its time of restructuration at rest). These findings were successfully compared with systematic bentonite suspension experiments. The present work is the first theoretical analysis combining the basic principles of unsteady flow motion with a thixotropic fluid model and systematic laboratory experiments.     

Coherent Structures in Flat-Bed Abutment Flow: Computational Fluid Dynamics Simulations and Experiments

Numerical computations and laboratory experiments are carried out to investigate the three-dimensional structure of large-scale (coherent) vortices induced by bridge abutments on a flat bed. A finite-volume numerical method is developed for solving the unsteady, three-dimensional Reynolds-averaged Navier–Stokes equations, closed with the k–ω turbulence model, in generalized curvilinear coordinates and applied to study the flow in the vicinity of a typical abutment geometry with a fixed, flat bed. The computed flowfields reveal the presence of multiple, large-scale, unsteady vortices both in the upstream, “quiescent,” region of recirculating fluid and the shear-layer emanating from the edge of the foundation. These computational findings motivated the development of a novel experimental technique for visualizing the footprints of large-scale coherent structures at the free surface. The technique relies on digital photography and employs averaging of instantaneous images over finite-size windows to extract coherent eddies from the chaotic turbulent flow. Application of this technique to several abutment configurations yielded results that support the numerical findings.     

Unsteady Effluent Dispersion in a Round Jet Interacting with an Oscillating Cross-Flow

Dispersion of a vertical round jet issuing into an unsteady cross-flow is investigated by laboratory and numerical experiments. An experimental technique has previously been devised to simulate a sinusoidally oscillating cross-flow situation with a nonzero mean flow velocity. The parameters of the cross-flow can be selected with ease. With this experimental technique, 12 cross-flow situations with systematic varying flow parameters are produced. The dispersion pattern of a jet in each cross-flow situation is studied by phase-locked dye visualizations and the dilution level of jet effluent is estimated using image processing. It is found that in a cross-flow of a large unsteadiness parameter, the jet dispersion pattern is significantly different from that of the same jet in a steady cross-flow. The jet effluent is organized into successive large-scale effluent clouds which are connected on the inner side by a bent-over effluent fetch. Specially designed experiments using time-controlled dye ejection are performed to investigate the formation mechanism of the effluent structures. Computational fluid dynamics (CFD) studies are carried out to supplement the experimental concentration data. In addition, the CFD results help to support the formation mechanism of the effluent flow structures and to explore their dynamics. In the time-averaged sense, unsteadiness in the oscillating cross-flow leads to a two- to threefold increase in jet width. The reduction in time-averaged concentration level of jet effluent is not as dramatic because there still remains high concentration of effluent inside the effluent clouds.     

Experimental Simulation of a Vertical Round Jet Issuing into an Unsteady Cross-Flow

This paper investigates the discharge of a vertical round jet into an unsteady cross-flow that consists of a mean flow and a sinusoidally oscillating component. An experimental technique is devised to simulate the unsteady cross-flow situation in the laboratory. A vertical jet-pipe-nozzle assembly is physically oscillated backward and forward in the steady flow stream of a laboratory flume, and the flow patterns are viewed by an observer moving with the jet. Dispersion patterns of the dye-marked jet fluid are studied with a phase-locked analysis of digitized flow images. Oscillations in cross-flow velocity are found to organize the jet fluid into regular large-scale fluid patches, which, after time averaging, lead to a widened jet width and enhanced dilution. Experiments are also carried out on a vertical jet discharging into a current with surface waves, a situation that approximates to a genuine oscillating cross-flow with a nonzero mean velocity. The two sets of experimental observations are found to match with each other. The validity of the experimental simulation technique is further supported by a computational fluid dynamics study of the flow problem, in which it is possible to produce an idealized oscillating cross-flow situation. The numerical results agree well with the experimental observations.     

Investigation of a Lorentz force biomagnetometer

This work evaluates an approach to the noninvasive measurement of small ionic current flows by a technique of Lorentz force magnetometry. An instrument was constructed that is basically a very sensitive force-balance that can measure Lorentz forces experienced by ionic currents flowing in small objects when exposed to strong oscillating magnetic fields. For objects that can fit on a microscope slide, the system is sensitive to ion current dipole moments as low as 180 pA-m. Images were made of ionic currents flowing in thin profiles by a process of scanning a localized magnetic field over the object, measuring generated Lorentz forces, and using a computer to reconstruct images. It can be shown that this method of Lorentz magnetometry has an immunity to ambient magnetic noise and has system characteristics that might suggest its possible use in biomagnetometry of small thin specimens.     

Prediction of Flow in Centrifugal Blower Using Quasi-Steady Rotor–Stator Models

A computational fluid dynamics (CFD)-based computational tool, named STREAM, is used to analyze fluid flow in a centrifugal blower. The unsteady interaction of the flow in the rotating impeller (rotor) and the stationary volute (stator) is modeled via quasi-steady rotor–stator models. Two such models are developed: one based on a local exchange of information and the other based on a circumferential averaging procedure at the rotor–stator interface. Due care is exercised to ensure that inadequate grid resolution and numerical dissipation do not smear out the small pressure rise typical of the blower considered here. Computed results based on the proposed models with multiblock structured grids are presented; global quantities such as static pressure rise, horsepower, and static efficiency, available from test data for different mass flow rates, are used to evaluate the trends predicted by the CFD results. Overall, the predictions by the proposed models are satisfactory.     

Numerical and Experimental Study on Two-Dimensional Flood Flows with and without Structures

The behavior of two-dimensional (2D) flood flows and the hydrodynamic force acting on structures are investigated numerically and experimentally. Numerical simulations are performed using a model based on the finite-volume method with an unstructured grid system and the flux-difference splitting technique. Experiments on flood propagation in a flood plain, with and without structures were conducted so as to obtain a comprehensive verification of the model. Front positions, depths, and surface velocities of flood flows as well as hydrodynamic forces on structures were observed. Comparisons of numerical results against these experimental data show that the model can predict 2D flood flows and the force on structures with reasonable accuracy.     

Well-Balanced Scheme between Flux and Source Terms for Computation of Shallow-Water Equations over Irregular Bathymetry

Although many numerical techniques such as approximate Riemann solvers can be used to analyze subcritical and supercritical flows modeled by hyperbolic-type shallow-water equations, there are some difficulties in practical applications due to the numerical unbalance between source and flux terms. In this study, a revised surface gradient method is proposed that balances source and flux terms. The new numerical model employs the MUSCL–Hancock scheme and the HLLC approximate Riemann solver. Several verifications are conducted, including analyses of transcritical steady-state flows, unsteady dam break flows on a wet and dry bed, and flows over an irregular bathymetry. The model consistently returns accurate and reasonable results comparable to those obtained through analytical methods and laboratory experiments. The revised surface gradient method may be a simple but robust numerical scheme appropriate for solving hyperbolic-type shallow-water equations over an irregular bathymetry.     

Approximation of Turbulent Wall Shear Stresses in Highly Transient Pipe Flows

Theoretical predictions of wall shear stresses in unsteady turbulent flows in pipes are developed for all flow conditions from fully smooth to fully rough and for Reynolds numbers from 103 to 108. A weighting function approach is used, based on a two-region viscosity distribution in the pipe cross section that is consistent with the Colebrook–White expression for steady-state wall friction. The basic model is developed in an analytical form and the resulting weighting function is then approximated as a sum of exponentials using a modified form of an approximation due to Trikha. A straightforward method is presented for the determination of appropriate values of coefficients for any particular Reynolds number and pipe roughness ratio. The end result is a method that can be used relatively easily by analysts seeking to model unsteady flows in pipes and ducts.     

Differential effect of steady versus oscillating flow on bone cells

Loading induced fluid flow has recently been proposed as an important biophysical signal in bone mechanotransduction. Fluid flow resulting from activities which load the skeleton such as standing, locomotion, or postural muscle activity are predicted to be dynamic in nature and include a relatively small static component. However, in vitro fluid flow experiments with bone cells to date have been conducted using steady or pulsing flow profiles only. In this study we exposed osteoblast-like hFOB 1.19 cells (immortalized human fetal osteoblasts) to precisely controlled dynamic fluid flow profiles of saline supplemented with 2% fetal bovine serum while monitoring intracellular calcium concentration with the fluorescent dye fura-2. Applied flows included steady flow resulting in a wall shear stress of 2 N m(-2), oscillating flow (+/-2 Nm(-2)), and pulsing flow (0 to 2 N m(-2)). The dynamic flows were applied with sinusoidal profiles of 0.5, 1.0, and 2.0 Hz. We found that oscillating flow was a much less potent stimulator of bone cells than either steady or pulsing flow. Furthermore, a decrease in responsiveness with increasing frequency was observed for the dynamic flows. In both cases a reduction in responsiveness coincides with a reduction in the net fluid transport of the flow profile. Thus. these findings support the hypothesis that the response of bone cells to fluid flow is dependent on chemotransport effects.     

Cold Model-Based Investigations to Study the Effects of Operational and Nonoperational Parameters on the Ruhrstahl–Heraeus Degassing Process

The circulation rate of steel is known to play a vital role in the superlative performance of the Ruhrstahl–Heraeus (RH) degasser. Numerous experiments were conducted on a physical model for the RH degassing process, which was established at IEHK, RWTH-Aachen University. The model was developed with a scale ratio of 1:3 to study the RH process. This study is conducted to show the effects of operational and nonoperational parameters on the circulation rate of liquid water in the model. The effects of lift gas flow rate, submerged depth of snorkels, water level in vessel, etc. on the circulation rate are studied. The mixing characteristics are studied with the help of current conductivity experiments for different lift gas flow rates and water levels in the vacuum vessel. Finally, the relationship between dimensionless numbers is derived with the help of the experimental data obtained from the cold model.     

Combustion and motion of coke and gases at blast-furnace tuyeres

The issues relating to coke circulation at the air tuyeres in high-productivity blast furnaces may be elucidated by systematic analysis of literature and production data and model experiments. Research on the forces associated with cavity formation and destruction in combustion ahead of the tuyeres indicates that three states are possible in the combustion zone: stability, instability, and collapse. In the stable state, when the uplift force of the gases from the cavity significantly exceeds the vertical pressure of the batch column, there will be no coke circulation. Coke circulation occurs in the unsteady state and at collapse. Rationalization of the terminology is proposed. In mechanical terms, it is expedient to divide the combustion zone into loose and dense sections, rather than into a circulation zone and a dense section.     

Wall Shear Stress in the Early Stage of Unsteady Turbulent Pipe Flow

Conventionally, wall shear stress in an unsteady turbulent pipe flow is decomposed into a quasi-steady component and an “unsteady wall shear stress” component. Whereas the former is evaluated by using “standard” steady flow correlations, extensive research has been carried out to develop methods to predict the latter leading to various unsteady friction models. A different approach of decomposition is used in the present paper whereby the wall shear in an unsteady flow is split into the initial steady value and perturbations from it. It is shown that in the early stages of an unsteady turbulent pipe flow, these perturbations are well described by a laminar-flow formulation. This allows simple expressions to be derived for unsteady friction predictions, which are in good agreement with experimental and computational results.     

The Impact of a Vertically Travelling Magnetic Field on the Flow in a Cylindrical Liquid Metal Bubble Plume

This article describes laboratory experiments for the investigations of flow structures and related transport processes in liquid metal bubbly flows under the influence of a traveling magnetic field (TMF). The melt flow is driven by central gas injection into a cylindrical container filled with the low-melting-point alloy GaInSn. The velocity fields of both the liquid and the gaseous phase were measured nonintrusively using the ultrasound Doppler method. Depending on the traveling direction of the magnetic field, the TMF mainly imposes either a concurrent flow or counterflow with respect to the original bubble-driven circulation. In general, the application of a downward TMF significantly increases the liquid velocity all over the fluid volume. An upward TMF gives rise to the more complex structures of the velocity field resulting in alternately arranged upstream and downstream regions. Both the upward and downward TMF promote the occurrence of nonsteady motions with distinct velocity fluctuations leading to an intensification of related transport processes in the melt and providing the perspective of enhanced mixing efficiencies.     

Modeling of noncatalytic fluid-solid reactions: The quasi-steady state assumption

An isothermal, topochemical model for noncatalytic fluid-solid reactions, in spherical pellets, is formulated, solved numerically, and investigated experimentally. The equations are transformed into dimensionless form in order to introduce characteristic dimensionless groups. The dimensionless groups represent the relative effects of the diffusional, fluid film transport, and chemical reaction rates on the system, as well as the ratio of the molar density of the reactant fluid to that of the reactant solid. The model is solved with and without the quasi-steady state assumption in order to investigate the effects of the values of the dimensionless groups on the error introduced by this assumption. The model, without the quasi-steady state assumption, is shown to be an accurate representation of a resin exchange system, and reduces to the quasi-steady state model for appropriate values of the dimensionless groups.