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In this present study, burning velocities of [H.sub.2]--CO mixtures (primary constituents of syngas fuels) are measured. Impact of different burner systems, [H.sub.2] concentration, and different measurement techniques on the burning velocities of [H.sub.2]--CO mixtures are discussed. The burning velocities of [H.sub.2]--CO mixtures increase as the He contents in the mixture increase. Even with different burner systems using different methods, the burning velocities are aligned in the lean condition, but at rich conditions, they deviate from each other. The burning velocities of actual syngas compositions (based on different sources of coal) are also measured and analyzed. It was found that the effect of carbon dioxide had a more dominating role compared with nitrogen on the burning velocities. [DOI: 10.1115/1.4002762] Keywords: syngas, burning velocity

The transition to turbulence in a constant-acceleration pipe flow from an initial laminar state was investigated in a custom-made apparatus permitting visual access to the water flow in the pipe. The apparatus allowed both laser Doppler velocimetry measurements and flow visualization using a tracer. The experiment was carried out by accelerating the flow from a steady laminar state to a steady turbulent state. The relation between the critical Reynolds number for transition to turbulence and the acceleration was found to be similar to that in a constant-acceleration pipe flow started from rest. In addition, with increased acceleration, the turbulent transition was found to be delayed to higher Reynolds numbers using flow visualization with simultaneous laser Doppler velocimetry measurements. [DOI: 10.1115/1.4002358] Keywords: turbulence, transition, accelerating flow, initial steady flow, critical Reynolds number

Laser-induced oxidative labeling of proteins provides insights into biomolecular structures and interactions. In these experiments, the hydroxyl radical (*OH) formed by photolysis of [H.sub.2][O.sub.2] generates covalent modifications that are detectable by mass spectrometry. Under conditions where individual protein molecules are irradiated only once, the short *OH lifetime (~1 [micro]s) ensures that covalent modifications are formed before any oxidation-induced conformational changes take place. This feature implies that the method should be free of structural artifacts. It has been proposed that single-exposure conditions can be achieved by passing the solution through a capillary where successive laser pulses generate a string of irradiated flow segments that are well separated from one another. The current work explores the convection phenomena within the labeling capillary in more detail. The experiments are conducted at Reynolds numbers < 10.1021/ac101326f

When fluid flows through a channel, pipe, or duct, there are two basic forms of motion: smooth laminar motion and complex turbulent motion. The discontinuous transition between these states is a fundamental problem that has been studied for more than 100 yr. What has received far less attention is the large-scale nature of the turbulent flows near transition once they are established. We have carried out extensive numerical computations in pipes of variable lengths up to 125 diameters to investigate the nature of transitional turbulence in pipe flow. We show the existence of three fundamentally different turbulent states separated by two distinct Reynolds numbers. Below [Re.sub.1] [??] 2,300, turbulence takes the form of familiar equilibrium (or longtime transient) puffs that are spatially localized and keep their size independent of pipe length. At [Re.sub.1] the flow makes a striking transition to a spatio-temporally intermittent flow that fills the pipe. Irregular alternation of turbulent and laminar regions is inherent and does not result from random disturbances. The fraction of turbulence increases with Re until [Re.sub.2] [??] 2,600 where there is a continuous transition to a state of uniform turbulence along the pipe. We relate these observations to directed percolation and argue that [Re.sub.1] marks the onset of infinite-lifetime turbulence. intermittency | puff | turbulence | directed percolation | lifetime doi/ 10.1073/pnas.0909560107

Agglomerated multigrid techniques used in unstructured-grid methods are studied critically for a model problem representative of laminar diffusion in the incompressible limit. The studied target-grid discretizations and discretizations used on agglomerated grids are typical of current node-centered formulations. Agglomerated multigrid convergence rates are presented using a range of two- and three-dimensional randomly perturbed unstructured grids for simple geometries with isotropic and stretched grids. Two agglomeration techniques are used within an overall topology-preserving agglomeration framework. The results show that a multigrid with an inconsistent coarse-grid scheme using only the edge derivatives (also referred to in the literature as a thin-layer formulation) provides considerable speedup over single-grid methods, but its convergence can deteriorate on highly skewed grids. A multigrid with a Galerkin coarse-grid discretization using piecewise-constant prolongation and a heuristic correction factor is slower and also can be grid dependent. In contrast, nearly grid-independent convergence rates are demonstrated for a multigrid with consistent coarse-grid discretizations. Convergence rates of multigrid cycles are verified with quantitative analysis methods in which parts of the two-grid cycle are replaced by their idealized counterparts. DOI: 10.2514/1.J050055

In this technical brief we report the results of a systematic numerical investigation of developing laminar pipe flow of yield stress fluids, obeying models of the Bingham-type. We are able to show that using a suitable choice of the Reynolds number allows, for high Reynolds number values at least, the development length to collapse to the Newtonian correlation. On the other hand, the development length remains a weak, nonmonotonic, function of the Bingham number at small values of the Reynolds number (Re [less than or equal to] 40). [DOI: 10.1115/1.4001079]

In this paper, the nonsimilarity boundary-layer flows of second-order fluid over a flat sheet with arbitrary stretching velocity are studied. The boundary-layer equations describing the steady laminar flow of an incompressible viscoelastic fluid past a semi-infinite stretching flat sheet are transformed into a partial differential equation with variable coefficients. An analytic technique for highly nonlinear problems, namely, the homotopy analysis method, is applied to give convergent analytical approximations, which agree well with the numerical results given by the Keller box method. Furthermore, the effects of physical parameters on some important physical quantities, such as the local skin-friction coefficient and the boundary-layer thickness, are investigated in detail. Mathematically; this analytic approach is rather general in principle and can be applied to solve different types of nonlinear partial differential equations with variable coefficients in physics. [DO]: 10.1115/1.3173764] Keywords: nonsimilarity boundary-layer flow, second-order fluid, series solution, homotopy analysis method

Large eddy simulation of flow around a NACA-0012 airfoil at a Reynolds number of 50,000 has been used to study the behavior of a laminar separation bubble near stall The effects of the subgrid-scale model and explicit filtering were studied for a test case in which direct numerical simulation results were available. It was found that a method incorporating a mixed-time-scale model in addition to explicit filtering gave improved results compared with a method with filtering alone. Statistical results as well as snapshots of the flow below stall exhibit good agreement with the direct numerical simulations. For a configuration near stall, the effect of the spanwise domain width was investigated by increasing the spanwise length from 20 to 50% chord. Two-point velocity correlations showed a significant improvement for the wider computational domain, in which the simulation was able to capture a low-frequency flow oscillation, in which intermittent bursting of the bubble was observed. The bubble bursting observed here is more irregular than in experiments at higher Reynolds number. The amplitude and frequency are compared with experimental results and with an unsteady viscons-inviscid interaction method which is shown to be capable of capturing unsteady behavior during stall. DOI: 10.2514/1.44298

The laminar near-wake flow behind a circular cylinder at hypersonic speeds has been examined experimentally and analytically. Surface pressure and heat flux measurements were obtained in a free-piston shock tunnel at a nominal Mach number of 10. The freestream unit Reynolds numbers were 3.02 x [10.sup.5]/m and 11.7 x [10.sup.5]/m at total specific enthalpies of 13.35 and 3.94 MJ/kg, respectively. The experimental data of surface pressure and heat flux showed good agreement with theory based on perfect gas. Unlike surface pressure, the surface heat flux depended strongly on the wall-to-total-temperature ratio, and it increased with increase in the ratio. The surface pressure, however, depended on the Reynolds number, although this dependency was found to be weak. The flow separation angles behind the cylinder showed an inverse of the square root of Reynolds number dependence similar to the low-speed laminar flow separation behind a circular cylinder. DOI: 10.2514/1.44167

In this experimental study, the pressure dependence of the extinction strain rate of flames of jet fuel and other hydrocarbons was measured and quantified. Laminar nonpremixed opposed-jet flames of fuel/nitrogen mixtures versus oxygen-enriched air were progressively strained to extinction by lowering pressure over the pressure range 0.1 to 0.68 atm. The fuels investigated were methane; ethylene; Jet-A; n-decane; 1, 2, 4-trimethyl benzene; and blends of n-decane and 1, 2, 4-trimethyl benzene. All liquid fuels were vaporized and diluted to a mass fraction of 21.5% in nitrogen gas; the gaseous fuels were not diluted. The oxidizer was a 50/50% molar mixture of oxygen and nitrogen. The global strain rate at extinction was seen to monotonieaHy increase with pressure in the subatmospheric pressure range explored. Overall reaction orders calculated from density-weighted extinction strain rates were between 1.3 and 2.0. Jet-A flames were extinguished at conditions similar to those for the decane/trimethyl benzene blends, indicating that these blends can serve as a surrogate for Jet-A. DOI: 10.2514/1.42742

Transport phenomena in three-dimensional branching channel are important because of their relevance in polymer processing. In this article, an experimental study on viscoelastic flow in a three-dimensional cylindrical branching channel is carried out to investigate variations of flow pattern. Flow visualization in representative symmetric planes is made both for the viscoelastic fluid and Newtonian flow. From the results of the present investigation, the flow field in the three-dimensional cylindrical branching channel is clarified within the range of laminar flow. It is confirmed that corner vortex, shedding vortex, and secondary vortex flow are all obviously changed with the fluid concentration and the Reynolds number, which are much more three-dimensional and complex than the Newtonian fluid, and the flow pattern of the viscoelastic fliud flow largely depends on the Reynolds number and fluid concentration. Even for the viscoelastic flow at the low Reynolds number, shedding vortex and secondary vortex and complex three-dimensional flow occur in the cylinder. The flow field is not symmetric space for the viscoelastic flow and however is fairly symmetric for the Newtonian fluid. The above reasons explain why the flow deflection happens even at the low Reynolds number flow. POLYM. ENG. SCI., 50:32-42, 2010. [C]2009 Society of Plastics Engineers, INTRODUCTION Internal laminar flows in an abrupt change in the area of cross-section are widely studied and mechanisms and basic hydrodynamic characteristics are fairly well understood for simple channel geometries. [...]

The behavior of unsteady velocity profiles in laminar and turbulent water hammer flows is numerically investigated. In this way, the governing equations for the quasitwo-dimensional equations of transient flow in pipe are solved by using the modified implicit characteristics method. A k-[omega] turbulence model which is accurate for two-dimensional boundary layers under adverse and favorable pressure gradients is applied. The numerical results for both steady and unsteady turbulent pipe flows are in good agreement with the experimental data. The results indicate that both decelerating and accelerating flows are produced in a wave cycle of water hammer. During deceleration of the flow, a region of reverse flows and also strong gradients is formed near to the pipe wall. In case of the turbulent water hammer, this region is very close to the pipe wall compared with the laminar water hammer. Moreover, point of inflection and also point of zero velocity are formed in the unsteady velocity profile due to the water hammer problem. The results show that the point of zero velocity does not move very far from its initial location, while the point of inflection moves rapidly from the wall [DOI: 10.1115/1.4000557] Keywords: water hammer, unsteady velocity profile, k-[omega] turbulence model, inflection point

This paper presents a study of the laminar flow in a channel with longitudinal moving bars arrayed along the channel width. The governing equations describing the fluid, which flows along the direction of the bar's length, are expressed with double Poisson equations and are solved by eigenfunction-expansion and point-match method. The result shows that when the solid bars move forward, the fluid flow will move in the same direction, and the f Re decreases as the positive velocity of bars increases. However, when the bars move backward, a reverse flow will occur in the channel, and the f Re is higher at larger negative velocity of bars. For a channel flow with moving bars, the f Re value is not a constant, such as a classical one without moving bars, in which the f Re value is a constant. Furthermore, when the area of the cross section of the bar is fixed, both the mean velocities and the f Re values of the fluid can be obtained under different velocities and aspect ratios of the bars. [DOI: 10.1115/1.4000261] Keywords: channel flow, moving bars, point-match method

A heated ([approximately equal to]90 [degrees]C) laminar flow interface has been designed to assist in the development of an argon electrospray sample introduction system for low-flow rate applications using inductively coupled plasma (ICP) spectrometry. Previously, the stability and robustness of the ICP were compromised by the entrainment of air, [N.sub.2], or gas mixtures (e.g., Ar-[N.sub.2]) from the electrospray source. Also, more concentrated organic solvents (e.g., 50% (v/v) methanol-water), typically introduced by electrospray, could generate carbon deposits that obstruct the entrance lens to an ICP optical emission spectrometer (ICP-OES) or the sampler/skimmer cone interface in an ICP mass spectrometer (ICP-MS), decreasing analyte sensitivity. With the new interface design, a stable spray of 5% (v/v) methanol-water in a pure argon environment is achieved, eliminating the aforementioned problems. The turbulence and the consequent droplet loss caused by high gas velocity around the electrospray capillary are mitigated by the use of a laminar-flow gas with the aid of a flow diffuser. The argon electrospray interface is successfully installed on an ICP-OES and an ICP-MS for the first time.

Numerical investigations show that the spontaneous break of symmetry in annular incompressible jets occurs in the laminar flow regime and is controlled by both the Reynolds number and the blockage ratio. In the blockage ratio range between 0.50 and 0.89 the transition critical Reynolds number decreases with increasing blockage ratio, according to a defined formula. Transition to asymmetry happens in the steady regime, before the transition to the unsteady flow. Asymmetry is characterized by a preferential flow direction from one side of the jet boundary layer to the diametrically opposite side. The plane of preferential direction passes through the geometry centerline and represents the single plane of flow symmetry. Experiments reported in literature have confirmed the existence of flow asymmetry in an annular jet flow. [DOI: 10.1115/1.3176960] Keywords: annular jets, break of symmetry, laminar flow, CFD

We numerically investigate the application of steady blowing to three-dimensional stalled flows around low-aspect-ratio rectangular flat-plate wings at a Reynolds number of 300. The objective of this study is to explore techniques to enhance lift by directly modifying the dynamics of the wake vortices. Out of various combinations of forcing location and direction considered, we identify two configurations that provide significant lift enhancement. In these cases, actuation appears to strengthen the tip vortices for increased downward induced velocity upon the leading-edge vortices. This in turn moves the low-pressure core directly above the top surface of the wing to greatly enhance lift.

The development of any particular analytical or preparative applications using electrochemical techniques in microfluidic devices requires integration of microelectrodes. This involves detailed predictions for optimizing the design of devices and selecting the best hydrodynamic conditions. For this purpose, we undertook a series of works aimed at a precise investigation of mass transport near electrodes with focus on analytical measurements. Part I of this series (Anal. Chem. 2007, 79, 8502-8510) evaluated the common case of a single microband electrode embedded within a microchannel under laminar flow. The present work (Part 2) investigated the case of a pair of microband electrodes operating either in generator-generator or generator-collector modes. The influence of the confining effect and flow velocity on the amperometric responses was examined on the basis of numerical simulations under steady-state regime. Several situations were identified, each of them corresponding to specific interactions taking place between the electrodes. Related conditions were extracted to establish a zone diagram describing all the situations. These predictions were systematically validated by experimental measurements. The results show that amperometric detections within microchannels can be performed at dual electrodes with higher analytical performances than at single ones.

This paper describes in-flight measurements in low- and high-speed experiments to investigate laminar-turbulent transition on airfoils. The low-speed experiments were carried out using a laminar wing glove for a sailplane. High-speed experiments on an modified A320 hybrid laminar fin were conducted in close collaboration with Airbus. Different surface sensor measuring techniques (piezofoil, surface hot wires, wall microphones) were used in the low-speed experiments. The piezofoil technique was applied in the high-speed in-flight experiments to prove the operation robustness, the functional capability, durability, and reliability under atmospheric conditions in cruise flight on modern airplanes. All measuring techniques were successfully implemented for the particular task and the laminar-turbulent transition was detected with a high degree of certainty. For the high-speed investigations a complete piezofoil sensor array was glued directly to the fin without causing any interference between the boundary layer flow and the sensor foil. This technique therefore offered the good potential to detect transition in the boundary layer under flight conditions especially in high-speed tests.

Sheet cavitation has been traditionally analyzed with ideal fluid theory that employs the cavitation number as the single parameter. However, characteristics of cavitation can significantly depend on location of cavity detachment. According to known experimental data, this location is influenced by the freestream speed and the body/hydrofoil size. As shown in this paper, it takes place because of the combined effect of the Reynolds number and Weber number. Here, sheet cavitation is considered as a special kind of viscous separation caused by the cavity itself The viscous-inviscid interaction concept is employed to analyze the entire flow. Validation of the suggested approach is provided for hydrofoils and bodies of revolution. The effects of flow speed, the body size, and its surface wettability are illustrated by comparison of computed cavity length/shape to the known experimental data. The difference between cavity detachment in laminar and turbulent boundary layers is discussed. [DOI: 10.1115/1.2754312]

Abstract: The laminar boundary-layer flow of a micropolar fluid on a fixed or continuously moving flat plate with uniform surface heat flux is investigated. The plate is assumed to move [...]

This study examines low Reynolds number flow over vegetated sloping ground. Instead of using the traditional empirical formula related to resistance or frictional roughness, a new approach is presented by considering the flow inside the vegetation layer as porous media flow governed by Blot's poroelastic theory as well as inside the soil layer. There is a discrepancy in the velocity distributions of flow over vegetative mantle area and bare area, respectively. Due to the change of the vegetation density, the effects of vegetation porosity and vegetation blockage on surface water flow are discussed. DOI: 10.1061/(ASCE)0733-9429(2007)133:3(335) CE Database subject headings: Laminar flow; Vegetation; Reynolds number; Slopes; Surface waters.

A numerical study has been carried out for the laminar flow of Newtonian and non-Newtonian power-law fluids through a suddenly expanded axisymmetric geometry. Mathematical correlations are proposed for the prediction of the length of the recirculating eddy in terms of Reynolds number, expansion ratio and rheological parameters. A wide range of expansion ratios (1.25 [less than or equal to] ER [less than or greater than] 8.0) has been covered for the Newtonian fluid and both the shear-thinning and shear-thickening flow characteristic fluids have been considered for the non-Newtonian fluids. [DOI: 10.1115/1.2409361]

Heat transfer coefficients in a surfactant drag-reduction additive in the entrance region of a uniformly heated horizontal cylindrical pipe were measured. The measured Nusselt numbers were found to be in good agreement with theoretical results for laminar flow, due to the absence of circumferential buoyancy effects.

The origin of the exact, closed-form, self-similar exponential solution for the laminar decay of an isolated two-dimensional vortex in a viscous incompressible fluid is studied to find a proper name for the function. Two major contributions to the solution are the similarity parameter derived by Boltzmann and the derivation of the form of the solution by Carslaw for the diffusion of heat from a line source, that is, a spark or lightning stroke. Lamb adapted the heat transfer solution derived by Carslaw to the time-dependent diffusion of vorticity from a potential line vortex, and, after publication of his book, has provided a short, complete, and readily available derivation. Oseen's derivation followed about four years later and is not as complete. Various titles for the solution are believed to be appropriate, including a generic name such as exponential vortex solution, or the name of Lamb. The name of Lamb is probably the most appropriate because he was the first in existing literature to put the entire derivation together and because the presentation is compact, complete, and readily available.

A liquid metal forced-convection fully developed laminar flow inside a square duct, whose surfaces are electronically insulated and subjected to a constant temperature in a transverse magnetic field, is solved numerically using the spectral method. The powerful spectral method resolves the sharp velocity gradient near the duct walls very well leading to accurate calculation of friction factor.

Test results are presented for laminar-flow seals that are representative of buffered-flow oil seals in centrifugal compressors. The seals are short (L/D[congruent to]0.21), with a diameter of 117 mm and a clearance-to-radius ratio 0.0007. A smooth seal, a seal with one central groove, and a seal with three grooves were tested. Groove geometries employed are representative of industrial practice for compressors with a groove-depth to clearance ratio on the order of 6. Tests were conducted at 4000, 7000, and 10,000 rpm shaft speed with delta pressures across the seals of 21, 45, and 69 bars. For all cases, the flow was laminar. The seals were tested from a centered position out to an eccentricity ratio of 0.7. Static data included leakage and equilibrium loci for a range of loads. Direct and cross-coupled stiffness and damping coefficients and direct mass coefficients were determined from dynamic tests. For the smooth seal, comparisons between measurements and predictions were reasonable for the direct and cross-coupled stiffness and damping coefficients; however, measured added mass coefficients were roughly ten times larger than predicted. Predictions for the grooved seals from a 'deep-groove' model that assumed zero pressure oscillations in the grooves greatly over predicted the influence of the grooves. In a centered position, smooth and grooved seals had comparable leakage performance. At higher eccentricity ratios, the grooved seals leaked modestly more. For eccentricity ratios less than approximately 0.3, the grooved seals and the smooth seal had qualitatively similar static and dynamic characteristics. In terms of cross-coupled stiffness coefficients, the grooved seals were less stable than the smooth seal at eccentricity ratios greater than approximately 0.5 but had significantly lower cross-coupled coefficients at reduced eccentricity ratios between zero and 0.3. A grooved centered seal is more stable than a smooth centered seal. The smooth seal had higher damping than the grooved seals and had moderately better centering capabilities. [DOI: 10.1115/1.2164468]

A heat transfer correlation of flow boiling at low liquid Reynolds number for small diameter channels is developed. The developed heat transfer correlation works satisfactorily with mean deviation and rms errors of 19.1% and 24.3%, respectively, under different channel geometries like circular and rectangular and flow orientations like vertical and horizontal for some test fluids such as water, R11, R12 and R113.

Results of an experimental investigation of the heat transfer and pressure drop characteristics of laminar flow of viscous oil through horizontal rectangular and square plain ducts and ducts inserted with full-length twisted tapes, short-length twisted tapes, and regularly spaced twisted-tape elements are presented. Performance evaluation reveals that short-length twisted tapes are worse and regularly spaced twisted-tape elements are better than the full-length twisted tapes.

We report on flow phenomena such as wall shear stress and periodic oscillations that occur in the right-angle branch during laminar steady flow in the upstream trunk. The side-branch bifurcates from the trunk at a 90-deg angle, and both the upstream and the downstream corners of the entrance of the side-branch are square-edged. As the flow approaches the entrance of the side-branch through the trunk, the wall shear stress steeply increases along the near wall and this stress at the upstream corner is comparable to that generated around the flow divider. Periodic velocity oscillations occur downstream in the separation region in the side-branch. The Strouhal number based on the flow conditions in the side-branch is independent of the flow division ratio for the sidebranch radii ranging from 4 to 9 mm. Furthermore, it appears that the Strouhal number is weakly dependent on the Reynolds number. The periodic oscillation in the side-branch exhibits a characteristic flow behavior, which is a universally recognized phenomenon in the right-angle branch. [DOI: 10.1115/1.1852480]

Pressure drops and friction factors associated with the forced flow of de-ionized water over staggered and in-line circular/diamond shaped micro pin-fin bundles 100/[micro]m long with hydraulic diameter of 50 and 100 [micro]m have been experimentally investigated over Reynolds number ranging from 5 to 128. Pin fins were arranged according to two different horizontal and vertical pitch ratios (1.5 and 5). The applicability of conventional scale correlations to evaluate micro flow tests results has been assessed. It is shown that the available large-scale correlations do not adequately predict the pressure drop obtained at the micro scale. A modified correlation, based on the experimental results obtained using micro scale devices, has been proposed. The refined correlation accounts for fin density and the endwall effects encountered in micro scale configurations.

In the present paper, some peculiar characteristics of transient laminar flow are discussed. After presenting a review of the existing literature, attention is focused on transient energy dissipation phenomena. Specifically, results of both laboratory and numerical experiments are reported, the latter by considering one-dimensional (1D) along with two-dimensional (2D) models. The need of modifying a criterion for simulating unsteady friction proposed some years ago by one of the writers, and extensively used for water-hammer calculations, is pointed out. Differences between accelerating and decelerating flows as well as between transients in metallic and plastic pipes are also highlighted. [DOI: 10.1115/1.1839926] Keywords: Transient, Laminar, Energy, Dissipation, Viscoelasticity

'Supply Air' windows work as a heat reclaim device by intercepting a substantial proportion of incident solar energy as well as entraining heat escaping from rooms. This series of experiments [...]

An improved low-Reynolds-number k-[epsilon] model is adopted to predict turbulent flows with a nonstationary boundary. The performance of the adopted model is first contrasted with direct numerical simulation data of the turbulent plane Couette-Poiseuille flow. Detailed flow structure is captured accurately by the model, in particular the reduction of the shear stress and, hence, the turbulent kinetic energy due to the presence of the moving wail. The validity of the present model in computing complex flows within rotating disk cavities is further examined. Flows with two different rotational Reynolds numbers are investigated, and the predicted mean and turbulence results are also contrasted with measurements. The influence of rotation on the flowfield and turbulence modeling is investigated by sensitizing the turbulence model coefficient to rotational Richardson number, and the effect is found to be marginal. Because the internal flow structure of the rotating disk cavities is induced by the diffusive transport of the tangential momentum from the rotor into the interior, the predicted thickness of the Ekman layer is found to be critical to the correct predictions. The adopted model reproduces correctly the gradual thickening of the Ekman layers from small to large radii, which is due to the transition from laminar to turbulent regimes, especially on the rotor side within the disk cavities. The elevated level of turbulence on the stator side compared to that on the rotor side is also predicted correctly by the present model.

This paper explores the effects of mild sweep on active separation control at high Reynolds numbers and incompressible Mach numbers. The model, which was tested in a cryogenic pressurized wind tunnel, simulates the upper surface of a 20% thick Glauert-Goldschmied type airfoil at zero angle of attack. The boundary-layer flow is turbulent because the tunnel side-wall boundary layer flows over the model, eliminating laminar-turbulent transition from the problem. Without control the flow separates at the highly convex area, and a large turbulent separation bubble is formed at the lee side of the model. Periodic excitation and steady mass transfer were applied to eliminate the separation bubble gradually. During the test, the Reynolds numbers ranged from 7 x [10.sup.6] to 26 x [10.sup.6], and the Mach numbers were 0.2 and 0.25. The test sweep angles were 0 and 30 deg. It was found that the excitation must be introduced slightly upstream of the separation region regardless of the sweep angle at low Mach numbers, as in the two-dimensional flow. The conventional swept flow scaling is valid for controlled, fully and even partially attached flow, but different scaling is required for the separated three-dimensional flow. The effectiveness of the active control is not reduced by mild sweep, and the effective frequencies do not change.

Interlaminar stress distribution in smart composite shells using a coupled thermal-piezoelectric-mechanical model is investigated. To maintain local accuracy of stress distributions, the trial displacement field is assumed layerwise higher order and Co continuous through the entire laminate thickness, accommodating zigzag in-plane warping and interlaminar shear stress continuity. The temperature and electrical fields are modeled using higher. order descriptions that can satisfy surface flux boundary conditions at structural surfaces and equipotential conditions at electrode surfaces. These assumptions ensure computational efficiency. A variational principle, addressing the interaction between thermal, piezoelectric, and mechanical fields, is used to derive the governing equations of equilibrium. The proposed theory is used to investigate the cylindrical bending problem of simply supported composite host structures with attached piezoelectric actuators, subject to a combination of mechanical, piezoelectric, and thermal loading. The interlaminar stress distributions under comprehensive loading are presented for different geometries and stacking sequences. The effects of two-way piezoelectric and thermal coupling on the stress distributions are investigated. The significance of the thermal mismatch effect on interlaminar stress distribution is also discussed. The results from present theory are validated with available exact elasticity solutions.

The shapes (flame-sheet and luminous-flame boundaries) of steady weakly buoyant round hydrocarbon-fueled laminar-jet diffusion flames in still and coflowing air were studied both experimentally and theoretically. Flamesheet shapes were measured from photographs using a CH optical filter to distinguish flame-sheet boundaries in the presence of blue C[O.sub.2] and OH emissions and yellow continuum radiation from soot. Present experimental conditions included acetylene-, methane-, propane-, and ethylene-fueled flames having initial reactant temperatures of 300 K, ambient pressures of 4-50 kPa, jet-exit Reynolds numbers of 3-54, initial air/fuel velocity ratios of 0-9, and luminous flame lengths of 5-55 mm; earlier measurements for propylene- and 1,3-butadiene-fueled flames for similar conditions were considered as well. Nonbuoyant flames in still air were observed at microgravity conditions; essentially nonbuoyant flames in coflowing air were observed at small pressures to control effects of buoyancy. Predictions of luminous flame boundaries from soot luminosity were limited to laminar smoke-point conditions, whereas predictions of flame-sheet boundaries ranged from soot-free to smoke-point conditions. Flame-shape predictions were based on simplified analyses using the boundary-layer approximations along with empirical parameters to distinguish flame-sheet and luminous-flame (at the laminar smoke point) boundaries. The comparison between measurements and predictions was remarkably good and showed that both flame-sheet and luminous-flame lengths are primarily controlled by fuel flow rates with lengths in coflowing air approaching 2/3 of the lengths in still air as coflowing air velocities are increased. Finally, luminous flame lengths at laminar smoke-point conditions were roughly twice as long as flame-sheet lengths at comparable conditions because of the presence of luminous soot particles in the fuel-lean region of the flames.

Transition characteristics of a supersonic natural laminar flow wing with a sweep angle of more than 60 deg were measured using hot-film sensors and an infrared camera at Mach 2. The pressure falls very rapidly in the vicinity of the leading edge of the wing, such that crossflow instabilities are reduced. The flow slightly accelerates downstream such that streamwise instabilities are stabilized. Tests were conducted in two wind tunnels to estimate the transition location in flight: a quiet tunnel with a low Reynolds number and a moderately quiet tunnel with variable Reynolds number. As a result, the natural laminar flow effect of the wing was confirmed. The unit Reynolds number had no effect on the transition Reynolds number of the wing. The transition Reynolds number was 8 x 10[.sup.5] at the design angle of attack at Mach 2, but there is room for further investigation.

A small trailing-edge flap, often referred to as a cruise flap or camber-changing flap, can be used to extend the low-drag range of a natural-laminar-flow airfoil. Automation of such a cruise flap is likely to result in improved aircraft performance over a large speed range without an increase in the pilot work load. An important step in achieving the automation is to arrive at a simple approach for determination of the optimum flap angle for a given airfoil lift coefficient. This optimum flap angle can then be used in a closed-loop control system to set the flap automatically. Two pressure-based schemes are presented for determining the optimum flap angle for any given airfoil lift coefficient. The schemes use the pressure difference between two pressure sensors on the airfoil surface close to the leading edge. In each of the schemes, for a given lift coefficient, this nondimensionalized pressure difference is brought to a predetermined target value by deflecting the flap. It is shown that the drag bucket is then shifted to bracket the given lift coefficient. This nondimensional pressure difference, therefore, can be used to determine and set the optimum flap angle for a specified lift coefficient. The two schemes differ in the method used for the nondimensionalization. The effectiveness of the two schemes is verified using computational and wind-tunnel results for two NASA laminar flow airfoils. Finally, an aircraft performance simulation approach is used to analyze the potential aircraft performance benefits while addressing trim drag considerations.

In this work, the onset of turbulence inside a rectangular chamber is investigated, with and without side-wall injection, in the presence of an oscillatory pressure gradient. Two techniques are used to define the transition from laminar to turbulent regimes: statistical analysis and flow visualization. Calibrated hot film anemometry and a computer data acquisition system are used to record and analyze acoustical flow data. Four classifications of flow regimes are reported: (a) laminar, (b) distorted laminar, (c) weakly turbulent, and (d) conditionally turbulent. Despite numerous attempts to promote turbulence, a fully turbulent flow does not develop at any of the driving frequencies tested. Statistical measurements reveal that a periodic drop in standard deviation of axial velocity fluctuations always occurs, indicating relaminarization within each cycle. Transition between flow regimes is assessed from the standard deviation of velocity data correlated as a function of the acoustic Reynolds number [Re.sub.A]. Under predominantly laminar conditions, the standard deviation is found to vary approximately with the square of the acoustic Reynolds number. Under turbulent conditions, the standard deviation becomes almost directly proportional to the acoustic Reynolds number. Inception of turbulence in the oscillatory flow with side-wall injection is found to be reproducible at the same critical value of [Re.sub.A] [congruent to] 200. [DOI: 10.1115/1.1490375]

A method is presented for estimating the laminar/turbulent transition location in three-dimensional boundary layers for computational fluid dynamics (CFD) applications requiring numerous transition estimates with no user intervention. Given the Reynolds number and the [C.sub.P] distribution, the location of transition is estimated based on the attachment-line state, the potential for relaminarization, the occurrence of laminar separation, and the growth of instabilities. Transition caused by instability is estimated based on N factors calculated for Tollmien-Schlichting waves and for stationary crossflow instabilities. A neural network is used (in place of solving the Orr-Sommerfeld stability equation) for determining the instability growth rates. The current version of the method assumes incompressible flow. The boundary-layer flow and instabilities are calculated based on an infinite-sweep (strip boundary layer) approximation; the instability calculations also employ the parallel-flow approximation. Comparisons with traditional stability codes show good N-factor agreement over a practical range of [C.sub.P] distributions. The method is several hundred times faster than traditional stability calculations, and it is robust enough to function as a simple 'subroutine' in CFD codes. The method is biased toward application efficiency and simplicity as balanced against improvements in the detailed physical modeling.

Even with all of the advances in airfoil and aircraft design, there remains little guidance on how to tailor an airfoil to suit a particular aircraft. A two-pronged approach is presented to tailor an airfoil for an aircraft: 1) an approach in which aircraft performance simulations are used to study the effects of airfoil changes and to guide the airfoil design and 2) an analytical approach to determine expressions that provide guidance in sizing and locating the airfoil low-drag range. The analytical study shows that there is an ideal value for the lift coefficient for the lower corner of the airfoil low-drag range when the airfoil is tailored for aircraft level-flight maximum speed. Likewise, there is an ideal value for the lift coefficient for the upper corner of the low-drag range when the airfoil is tailored for maximizing the aircraft range. These ideal locations are functions of the amount of laminar flow on the upper and lower surfaces of the airfoil and also depend on the geometry, drag, and power characteristics of the aircraft. Comparison of the results from the two approaches for a hypothetical general aviation aircraft are presented to validate the expressions derived in the analytical approach.

A three-dimensional transient finite element flow analysis code that includes inertia and free surface modeling is used to predict uniform (axisymmetric) and nonuniform (nonaxisymmetric) filling patterns in a thick-walled tool with a diaphragm gate. The simulation for a metal injection molding compound, which is strongly influenced by thermal effects, predicted several observed flow patterns: initial annular free surface flow (Jetting), bypass and folding flow to form surface defects, and the transition from uniform (axisymmetric) flow to nonuniform (nonaxisymmetric flow) with increasing fill time and lower temperatures. Simulations of filling through a thicker diaphragm gate showed jetting and the associated problems were eliminated. Simulations also demonstrate the effects of filling time, mold temperature, inertia, yield stress, and wall slip on filling patterns, particularly the processing conditions that separate regions of uniform and nonuniform flow., INTRODUCTION Injection molding of powdered metal and ceramic compounds (PIM) opens new opportunities in the manufacturing of complex parts for medical, automotive, aerospace, leisure, and telecommunication industries. The technique consists [...]

The esterification of the maleic anhydride residue (MA) in a hydrogenated styrene-butadiene-styrene block copolymer (SEBS-g-MA) with a polycaprolactone diol (POL) was carried out in a melt. Experiments in static melts found that the reaction had an induction period followed by a rapid consumption of the reactants. The gel content accumulation was found synchronous to the consumption of the acidic groups during the reaction process, indicating that the crosslinking between the copolymer molecules prevailed over the esterification of the pendant acid group generated by the ring opening of the MA residue. The reaction rate was strongly affected by both the intensity and the pattern of flow. The reaction rate increased as the flow intensity increased. A back-and-forth shear was more effective in promoting the reaction rate than a one-way shear. This reaction rate enhancement is due to the increase in the interfacial area and concentration gradient caused by the breakup of the dispersed droplets in the melt., INTRODUCTION Blending has been used for decades to obtain desired properties superior to the individual polymers used in the blend. Various blending processes have been applied to achieve property enhancements. [...]

The purpose of this study is to gain better understanding of flow patterns and mixing conditions in a particular single-screw extruder: the Buss Cokneader. To this end, the residence time distribution (RTD) of the polymer was investigated experimentally for different combinations of the operating variables (i.e. feed rate. screw rotation speed). The measurement of RTD used a standard stimulus-response technique. Two kinds of tracer were used: free anthracene and anthracene grafted on the polymer. It was shown that only the second could characterize the actual flow of the polymer in the extruder. It does not perturb the flow and has the same rheological behavior as the studied fluid. Thanks to the RTD data, a model of the extruder based on the combination of ideal reactors, such as continuous stirred tank reactors or plug flow reactors, was finally set up. The establishment of relationships between model parameters and extrusion conditions allowed the prediction of RTD with good agreement., INTRODUCTION This study deals with the elaboration of composite materials having absorbent properties at ultra-high-frequency waves. These materials are typically polymers filled with carbon black particles, and their end-use properties [...]

The aim of this study was to investigate first departures from laminar conditions in both impulsively started and steady pipe entrance flows. Wall shear stress measurements were conducted of transition in impulsively started pipe flows with large disturbances. These results were reconciled in a framework of displacement thickness Reynolds number and a velocity profile shape parameter, with existing measurements of pipe entrance flow instability, pipe-Poiseuille and boundary layer flow responses to large disturbances, and linear stability predictions. Limiting critical Reynolds number variations for each type of flow. were thus inferred, corresponding to the small and gross disturbance limits respectively. Consequently, insights have been provided regarding the effect of disturbance levels on the stability of both steady and unsteady pipe flows. [DOI: 10.1115/1.1445137]

We have measured the efficiency over the range 125-225 [Angstrom] of a bare ion-etched plane laminar holographic grating made of fused silica and with 1000 grooves/mm. The measured efficiency of each order oscillates with wavelength because of constructive and destructive interference between radiation diffracted from the lands and the grooves. We measured the grating groove profile with an atomic force microscope, and the resulting groove depth of 434 [+ or -] 6 [Angstrom] agrees well with the values determined independently from the oscillatory behavior of the efficiency measurements. Grating efficiency in the +1 order peaked at values of 0.027%, 0.011%, and 0.005% at wavelengths of 191, 157, and 132 [Angstrom], respectively; and the derived groove efficiencies are 27%, 25%, and 27%. The irregular shape at the land--groove edges dominates the large grating roughness of 23-45-[Angstrom] rms, but even regions far from the edges have a roughness of 10-18-[Angstrom] rms. The average groove profile was used to model the grating efficiency, and the resulting wavelengths predicted for different order maxima and minima agree well with measured wavelengths, although the calculated efficiencies are greater than the measured results by 10-20%. OCIS codes: 050.1950, 120.6200, 120.6660, 260.7200, 350.1260.

The recent progress in compliant-wall designs for aeronautical applications is reviewed, focusing on flow-induced surface instabilities, multiple-panel compliant walls and laminar flow control.

Ozone simulations are performed in an attempt to simulate laminar events with the frequency observed in balloon ozone sondes. The winds are taken from the Goddard Earth Observing System Data Assimilation System (GEOS DAS), the importance of horizontal and vertical resolution to production of lamina are investigated. A simulation with a high horizontal reolution (grid spacing 1 degree latitude by 1.25 degrees longitude) and high vertical resolution (approximately 300 m grid spacing) isentropic model produces lamination frequencies close to the balloon sonde climatology near the polar vortex edge but exhibits too much lamination in the subtropics. This indicates that the GEOS DAS winds contain the information to produce laminar events, although such small-scale features are not manifest in the more commonly used 2 degrees latitude by 2.5 degrees longitude transport model, which uses the hybrid sigma-pressure vertical coordinate. The zonal average ozone tendencies due to horizontal mixing in the lamina-producing models are similar to the tendencies in coarser resolution models that show no lamination, suggesting that it is not necessary to resolve laminar events to maintain a realistic ozone budget. The comparison of the modeled lamination frequency with the balloon sonde climatology indicates that the model horizontal mixing at the vortex edge is accurate but in the subtropics the mixing is excessive.

The problem of optimizing suction distribution on multi-panel hybrid laminar flow aircraft to minimize total energy usage is discussed. Experimental results are included.