Mechanism Of Flow Instability Research Articles

Experimental Study on the Helical Flow in a Concentric Annulus With Rotating Inner Cylinder

This experimental study concerns the characteristics of vortex flow in a concentric annulus with a diameter ratio of 0.52, whose outer cylinder is stationary and inner one is rotating. Pressure losses and skin friction coefficients have been measured for fully developed laminar flows of water and of 0.4% aqueous solution of sodium carboxymethyl cellulose, respectively, when the inner cylinder rotates at the speed of 0-600rpm. The results of the present study show the effect of the bulk flow Reynolds number Re and Rossby number Ro on the skin friction coefficients. They also point to the existence of a flow instability mechanism. The effect of rotation on the skin friction coefficient depends significantly on the flow regime. In all flow regimes, the skin friction coefficient is increased by the inner cylinder rotation. The change in skin friction coefficient, which corresponds to a variation of the rotational speed, is large for the laminar flow regime, whereas it becomes smaller as Re increases for transitional flow regime and, then, it gradually approaches to zero for turbulent flow regime. Consequently, the critical bulk flow Reynolds number Rec decreases as the rotational speed increases. The rotation of the inner cylinder promotes the onset of transition due to the excitation of Taylor vortices.

Non-Darcy flow stability of mixed convection in a vertical channel filled with a porous medium

The linear stability analysis of mixed convection in a heated vertical channel filled with a porous medium was investigated. The Galerkin method was employed in this calculation. The results indicated that the fully developed laminar flow could become unstable under a mild heating condition for higher modified Darcy numbers of Da ∗=1 and 10 −2, but the critical Rayleigh number increases substantially for Da ∗=10 −4 . The disturbance kinetic energy budget analysis showed that, for Da ∗=1 and 10 −2, there are generally three instability types, the thermal-buoyant, mixed, and thermal-shear instabilities for low, intermediate, and high Reynolds numbers, respectively, while for Da ∗=10 −4 , the thermal-buoyant instability is the only driving mechanisms of flow instability regardless of the value of Reynolds number.

Role of Marangoni convection in Si-Czochralski melts, part I: 3D predictions without crystal

Three-dimensional numerical simulations of flow field and heat transfer in a Czochralski crucible without crystal were carried out under pulling conditions. The study concentrated on the influence of the Marangoni convection in a silicon melt adopting a block-structured finite-volume code. The existing numerical code was extended to implement the surface tension boundary condition in order to investigate the influence of the Marangoni convection on the flow instability mechanism. The transient flow patterns reveal a significant role of Marangoni convection on the fluid mechanical instability at the free surface of the melt. A strong radial flow due to the presence of a surface tension effect is seen to increase the intensity of the buoyant plumes and enhance the net inward radial motion of the plumes. At later stages, the Coriolis force due to large crucible rotation suppresses the radial buoyant convection while substantial radial flow is observed in the presence of the Marangoni effect.

Experimental study on the vortex flow in a concentric annulus with a rotating inner cylinder

This experimental study concerns the characteristics of vortex flow in a concentric annulus with a diameter ratio of 0.52, whose outer cylinder is stationary and inner one is rotating. Pressure losses and skin friction coefficients have been measured for fully developed flows of water and of 0.4% aqueous solution of sodium carboxymethyl cellulose (CMC), respectively, when the inner cylinder rotates at the speed of 0-600 rpm. Also, the visualization of vortex flows has been performed to observe the unstable waves. The results of present study reveal the relation of the bulk flow Reynolds number Re and Rossby number Ro with respect to the skin friction coefficients. In somehow, they show the existence of flow instability mechanism. The effect of rotation on the skin friction coefficient is significantly dependent on the flow regime. The change of skin friction coefficient corresponding to the variation of rotating speed is large for the laminar flow regime, whereas it becomes smaller as Re increases for the transitional flow regime and, then, it gradually approach to zero for the turbulent flow regime. Consequently, the critical (bulk flow) Reynolds number Rec decreases as the rotational speed increases. Thus, the rotation of the inner cylinder promotes the onset of transition due to the excitation of Taylor vortices.

Spectral/hp element technology for global flow instability and control

AbstractThe objective of our paper is to demonstrate the use of spectral/hp element technology in unravelling global flow instability mechanisms. Understanding these mechanisms is central to devising flow control approaches based on theoretically-founded physical principles. Global instability theory is concerned with prediction and control of linear and nonlinear disturbances developing in flows that are inhomogeneous in more than one spatial direction. As such, this theory encompasses the classic analysis of Tollmien which is valid for simple geometries of academic interest (e.g. a flat-plate) and therefore broadens the scope of this well-established but simplified methodology to include realistic problems encountered in aeronautical engineering. Compared with a direct numerical simulation approach, global instability theory can be used to explore efficiently far wider parameters ranges and deliver physical information to be used as handle for effective flow control.

Spectral/hp element technology for global flow instability and control

AbstractThe objective of our paper is to demonstrate the use of spectral/hp element technology in unravelling global flow instability mechanisms. Understanding these mechanisms is central to devising flow control approaches based on theoretically-founded physical principles. Global instability theory is concerned with prediction and control of linear and nonlinear disturbances developing in flows that are inhomogeneous in more than one spatial direction. As such, this theory encompasses the classic analysis of Tollmien which is valid for simple geometries of academic interest (e.g. a flat-plate) and therefore broadens the scope of this well-established but simplified methodology to include realistic problems encountered in aeronautical engineering. Compared with a direct numerical simulation approach, global instability theory can be used to explore efficiently far wider parameters ranges and deliver physical information to be used as handle for effective flow control.

Experimental Study on the Helical Flow Field in a Concentric Annulus with Rotating Inner Cylinders

This experimental study concerns the characteristics of a transitional flow in a concentric annulus with a diameter ratio of 0.52, whose outer cylinder is stationary and inner one rotating. The pressure drops and skin-friction coefficients have been measured for the fully developed flow of water and that of glycerine-water solution (44%) at a inner cylinder rotational speed of rpm, respectively. The transitional flow has been examined by the measurement of pressure drops and the visualization of flow field, to reveal the relation of the Reynolds and Rossby numbers with the skin-friction coefficients and to understand the flow instability mechanism. The present results show that the skin-friction coefficients have the significant relation with the Rossby numbers, only for laminar regime. The occurrence of transition has been checked by the gradient changes of pressure drops and skin-friction coefficients with respect to the Reynolds numbers. The increasing rate of skin-friction coefficient due to the rotation is uniform for laminar flow regime, whereas it is suddenly reduced for transitional flow regime and, then, it is gradually declined for turbulent flow regime. Consequently, the critical (axial-flow) Reynolds number decreases as the rotational speed increases. Thus, the rotation of inner cylinder promotes the early occurrence of transition due to the excitation of taylor vortices

Inter–relationships between ionic transport and composition in amorphous anodic oxides

The inter–relationships of alloy composition, film composition and ionic transport for formation of amorphous anodic oxide films are addressed quantitatively through systematic study of sputter–deposited Al–Ta alloys containing up to 39 at.% Ta. The work reveals the dependence of electric field, ionic transport number, incorporation of species into the anodic film at the alloy–film interface and mobility and distribution of species within the anodic film on alloy composition. Anodic oxidation, at high current efficiency, of alloys containing 2.8, 15, 32 and 39 at.% tantalum results in formation of two–layered anodic films by migration of cations outwards and by migration of anions inwards: an outer layer, 20% or less of the total film thickness, composed of relatively pure alumina and an inner layer containing units of Al2O3 and Ta2O5 distributed relatively homogeneously. Two–layered films develop due to the slower migration rate of Ta5+ ions relative to A13+ ions in the inner layer of the growing anodic films, which changes progressively from about 0.6 for dilute alloys to about 0.9 for Al–39 at. per cent Ta. The average nm V−1 ratios, total transport numbers of cations and average Pilling–Bedworth ratios for the films change almost linearly with alloy composition between the values for anodic alumina and anodic tantala. A tantalum–enriched layer, about 1 nm thick, is formed in the Al–2.8 at.% Ta and Al–15 at.% Ta alloys just beneath the anodic film, indicating prior oxidation of aluminium in the initial stages of anodizing. In contrast, aluminium and tantalum in the alloys containing more than 30 at.% tantalum are immediately incorporated into anodic films in their alloy proportions, without development of a tantalum–enriched layer, at the available resolution. Boron species, incorporated from the electrolyte into the outer parts of the films, are immobile in films on alloys up to 15 at.% Ta but migrate outwards in other films, possibly due to the increased Lorentz field. Though the inter–relationships between film parameters and alloy composition are established for Al–Ta alloys specifically, the findings are considered to be equally relevant to amorphous anodic oxides formed on alloys and semiconductors generally.

Surface Defect Formation in the Injection Molding of Acrylonitrile-Styrene-Acrylate Polymers

Abstract Surface defects in the injection molding of acrylonitrile-styrene-acrylate (ASA) weatherable polymers, such as “chevrons”, “non-uniform haze” and “step defect “, are examined. A criterion is proposed based on a flow instability mechanism which suggests that recoverable shear strain, shear stress, and coefficient of friction are the key factors controlling the formation of surface defects. The criterion is supported by various experiments.

On the mechanism of shear flow instabilities

In homogeneous and density-stratified inviscid shear flows, the mechanism for instability that is most commonly invoked and discussed is Kelvin–Helmholtz instability, as it occurs for a simple velocity discontinuity. There is a second mechanism, the wave-interaction mechanism, which is much more general, and is the subject of this paper. This mechanism depends on two free waves that propagate in opposite directions in a stratified shear flow, and which may become stationary relative to each other because of the shear. If this occurs, and their relative phase is suitably chosen, the velocity field of each wave increases the displacement of the other, and so the disturbance grows.We show that this mechanism is responsible for instability in a general class of symmetric but otherwise arbitrary velocity and density profiles, provided that the Richardson number Ri < ¼ in a central region of arbitrarily small thickness. A critical layer exists in this central region for the growing disturbance, but its role in the instability process is incidental. When Ri > ¼ everywhere, the flow is stable because the free waves described above are absorbed by the critical layer, and hence are heavily damped. The necessary criteria of Rayleigh and Fjortoft for instability in homogeneous fluid are seen to provide a suitable geometry for two interacting waves. Some specific examples are given, including a succinct explanation of Holmboe waves.

Some Issues Concerning Fluid-Elastic Instability of a Group of Circular Cylinders in Cross-Flow

Abstract Since the early 1970s, extensive studies of fluid-elastic instability of circular cylinders in cross-flow have been reported. A significant understanding of the phenomena involved now exists. However, some confusion, misunderstanding, and misinterpretation still remain. The objective of this paper is to discuss, based on the current state of the art, a series of the most asked questions. Emphasis is placed on the determination of the critical flow velocity, nondimensional parameters, stability criteria, and instability mechanisms.

Hydrodynamic stability threshold of liquid metal current collectors

A mechanism derived from hydrodynamic theory to explain the ejection instability of liquid metal current collectors is presented. The ejection mechanism is shown to be caused by the onset of the Kelvin–Helmholtz instability resulting from the gradient of azimuthal (primary) flow at the interface between the liquid metal and cover gas. This new mechanism differs from the previous theory developed by Eriksson [in Electrical Engineering Series, no. 48, edited by M. Luukkala (The Finnish Academy of Technical Sciences, Helsinki, Finland, 1982), who analyzed the onset of the Kelvin–Helmhotz instability resulting from the gradient of meridional (secondary) flow at the interface. Considering the solution to the linearized Navier–Stokes equations at the liquid metal and gas interface, the azimuthally driven (primary flow) instability mechanism for the onset of ejection is much more prevalent than the meridional (secondary) flow driven mechansim. Furthermore, Eriksson’s theory requires an empirical multiplicative fractional factor that is not physically justified to predict experimentally measured ejection points, whereas the present theory is more self-consistent. Calculations of minimum ejection values from both theories were compared with corresponding experimental ejection data. The present theory appears to give significantly better engineering estimates, both quantitively and qualitatively, for minimum ejection threshold than Eriksson’s theory. The basic mathemtical model presented can serve as the basis for developing a more complex mathematical model for liquid metal ejection.

Glow discharge in a gas flow

The status of research on glow discharges in a gas flow, used in pumping fast-flow lasers, is reviewed. Systematic study of this discharge began about 10 years ago. This discharge form, according to a number of properties (current flow mechanism, nature of the development of instabilities, etc.), differs considerably from the well-studied glow discharge in tubes. One of the distinguishing features of such a discharge is the negligibly small role of ionization in a large part of the positive column, so that narrow regions near the electrodes contribute most of the positive ions and electrons. Negative ions, which compensate the charge of the positive column, are generated in its volume. The discharge turns out to be weakly inhomogeneous in the direction from the cathode to the anode and the ion currents can form an appreciable part of the total current. These facts have not yet been sufficiently widely discussed in reviews and monographs. In this review, results of experimental investigations of the mechanism of current flow, energy balance, plasmochemical processes, and discharge instabilities are presented, the mathematical models used in their analysis are analyzed, and the possibilities for increasing the stability and efficiency of the discharge in fast-flow lasers are examined.

A unified concept of melt flow instability during extrusion

AbstractThe mechanism of melt flow instability at drawdown rates below and up to that necessary to compensate for die swell and at drawdown rates exceeding the former is explained with the aid of diagrams and appropriate comment. It is suggested that there is no fundamental difference between the causes of commonly encountered extrudate defects and “draw resonance” which appears in fiber spinning at excessive drawdown rates. It is argued that both are manifestations of viscoelastic phenomena which affect extrudate appearance either not at all due to effective suppression or which give rise to defects of increasing severity when control of the process—including die design and proper consideration of the forces at the die/melt interface—is inadequate.

Downflow of solids in standpipes

General two-phase theory is applied to the flow of gas-solid mixtures in standpipes. It is shown that at a fixed flow rate both fluidized bed flow and packed bed flow can coexist in the same standpipe. Matsen's analysis of standpipe flow is extended and a general method for determining flow pattern and gas entrainment rate in standpipe flow is described. Data from a commercial standpipe in a fluid bed catalytic cracking unit are analysed. The analysis shows that fluidized bed flow occurs in the standpipe in support of Matsen's work on standpipe flow in another fluid bed catalytic cracking system. Flow instability, in standpipe is discussed in the light of this work and a mechanism of flow instability based on transition in flow pattern is postulated.

Extrusion of Polymer Melts and Melt Flow Instabilities. II. Site of Initiation and Mechanism of Melt Flow Instability

Abstract In the second part of this paper we will make a critical survey of experimental studies on the site of initiation, the structure of the unstable flow regime, criteria for the inception of the instability and mechanisms for initiation of the unstable flow regime. Some new experimental data will be cited in the discussion of criteria.