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A transition model for internal flows which enables the prediction of the change of flow regime from turbulent through intermittent to laminar has been implemented by numerical simulation. This model had previously been demonstrat...
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A transition model for internal flows which enables the prediction of the change of flow regime from turbulent through intermittent to laminar has been implemented by numerical simulation. This model had previously been demonstrated to be effective for the prediction of the breakdown of an initially laminar internal flow and its subsequent transitions to either intermittent or turbulent states. The model was employed here to study a flow which is decelerated by passing through a conical diffuser. The flow enters the diffuser with a fully developed turbulent velocity profile and exhibits transitions either in the diffuser or in a pipe situated downstream of the diffuser exit. The presence or absence of flow separation affected the onset of laminarization. Proof that laminarization actually occurred is provided by the values of the fully developed friction factors in the pipe downstream of the exit of the diffuser. These friction factors spanned the range from pure laminar flow through intermittent flow to fully turbulent flow. Comparisons were made with established benchmarks for each of these three flow regimes.
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An analysis of the quasi-steady streaming of the liquid in a vertically vibrated horizontal soap him is reported. The air around the soap film is seen to play a variety of roles: it transmits normal and tangential oscillatory stre...
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An analysis of the quasi-steady streaming of the liquid in a vertically vibrated horizontal soap him is reported. The air around the soap film is seen to play a variety of roles: it transmits normal and tangential oscillatory stresses to the film, damps out Marangoni waves, and forces non-oscillatory deflection of the film and tangential motion of the liquid. Non-oscillatory volume forcing originating inside the liquid is also analysed. This forcing dominates the quasi-steady streaming when the excitation frequency is close to the eigenfrequency of a Marangoni mode of the soap him, while both volume forcing in the liquid and surface forcing of the gas on the liquid are important when no Marangoni mode resonates. Different manners by which the combined forcings can induce quasi-steady streaming motion are discussed and some numerical simulations of the quasi-steady liquid flow are presented. [References: 26]
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Heat transfer and pressure drop results in laminar, transitional and turbulent flow are presented for the 0.7 to 126 Prandtl number range. The working fluids for the experiments were air, water, ethylene glycol and ethylene glycol...
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Heat transfer and pressure drop results in laminar, transitional and turbulent flow are presented for the 0.7 to 126 Prandtl number range. The working fluids for the experiments were air, water, ethylene glycol and ethylene glycol/water mixtures. For air or water, the Reynolds number was varied between 600 and 50,000, while tests with pure glycol and glycol/water mixtures covered the 200-3,000 and 200-10.000 Reynolds number range, respectively. (C) 1997 Elsevier Science Ltd. [References: 13]
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Surface-piercing vegetation often captures particles that flow on the water surface, where surface tension forces contribute to capture. Yet the physics of capillary capture in flow has not been addressed. Here we model the captur...
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Surface-piercing vegetation often captures particles that flow on the water surface, where surface tension forces contribute to capture. Yet the physics of capillary capture in flow has not been addressed. Here we model the capture of floating particles by surface-piercing collectors at moderately low Reynolds numbers (Re＜10). We find a trade-off between the capillary force, which increases with the collector diameter, and the relative size of the meniscus, which decreases with the collector diameter, resulting in an optimal collector diameter of ～1-10 mm that corresponds to the regime in which many aquatic plant species operate. For this diameter range the angular distribution of capture events is nearly uniform and capture can be orders of magnitude more efficient than direct interception, showing that capillary forces can be major contributors to the capture of seeds and particulate matter by organisms.
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We have fabricated and characterized a novel superhydrophobic system, a meshlike porous superhydrophobic membrane with solid area fraction Φ_s , which can maintain intimate contact with outside air and water reservoirs simultaneo...
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We have fabricated and characterized a novel superhydrophobic system, a meshlike porous superhydrophobic membrane with solid area fraction Φ_s , which can maintain intimate contact with outside air and water reservoirs simultaneously. Oscillatory hydrodynamic measurements on porous superhydrophobic membranes as a function of Φ_s reveal surprising effects. The hydrodynamic mass oscillating in phase with the membranes stays constant for 0.9 ≤Φ_s ≤ 1, but drops precipitously for Φ_s < 0.9. The viscous friction shows a similar drop after a slow initial decrease proportional to Φ_s . We attribute these effects to the percolation of a stable Knudsen layer of air at the interface.
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When two jets of fluid collide, they can "bounce" off each other, due to a thin film of air which keeps them separated. We describe the phenomenon of stable noncoalescence between two jets of the same fluid, colliding obliquely wi...
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When two jets of fluid collide, they can "bounce" off each other, due to a thin film of air which keeps them separated. We describe the phenomenon of stable noncoalescence between two jets of the same fluid, colliding obliquely with each other. Using a simple experimental setup, we carry out a parametric study of the bouncing jets by varying the jet diameter, velocity, angle of inclination, and fluid viscosity, which suggests that the contact time of bouncing jets scales as the square root of the normal Weber number We. A dimensionless parameter K = (We Re~(1/2)/ sinα)~(1/2), where Re is the normal Reynolds number and a the angle of inclination of the jets, quantitatively captures the transition of colliding jets from bouncing to coalescence. This parameter draws parallels between jet coalescence and droplet splashing and indicates that the transition is governed by a surface instability. Stable and continuous noncoalescence between fluid jets makes it a good platform for experimental studies of the interaction between fluid interfaces and the properties of the interfacial air films.
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Centrifugal forces break the degenerate closed-streamline configuration that occurs in simple shear flow past a neutrally buoyant torque-free particle in the inertialess limit. The broken symmetry allows heat or mass to be convect...
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Centrifugal forces break the degenerate closed-streamline configuration that occurs in simple shear flow past a neutrally buoyant torque-free particle in the inertialess limit. The broken symmetry allows heat or mass to be convected away in an efficient manner in sharp contrast to the inertialess diffusion-limited scenario. The dimensionless transfer rate, characterized by the Nusselt number, is found to be Nu = 0.33(RePe)~(1/3) + O(1) for small but finite Re when RePe >> 1. Here, the particle Reynolds number (Re) is a dimensionless measure of the inertial forces, while the Peclet number (Pe) measures the relative importance of the convective and the diffusive transfer mechanisms. The symmetry-breaking bifurcation is expected to occur in generic shearing flows, and represents a possible means for heat or mass transfer enhancement from the dispersed phase in multiphase systems.
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The Differential Mobility Analyzer (DMA) is currently the most effective instrument for sizing sub-micrometer aerosol particles. An important requirement to ensure good performance in terms of sizing accuracy and resolution is tha...
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The Differential Mobility Analyzer (DMA) is currently the most effective instrument for sizing sub-micrometer aerosol particles. An important requirement to ensure good performance in terms of sizing accuracy and resolution is that the flow field remains laminar and undisturbed along its classification zone. To achieve that, a flow straightener (or flow laminarizer) is employed at the sheath flow inlet, located at the top of the classification column. In this study, we assess the performance of a custom-made DMA using different sheath flow straighteners made out of plastic fabric materials or built by 3D printing. Our tests show that 3D-printed flow straighteners can achieve comparable, and in some cases better, results to those used in commercial DMAs (e.g., fine nylon meshes; Dacron (R)). Considering the great flexibility and ease in manufacturing offered by 3D printing, our findings show that this technology provides a promising alternative for building enhanced flow straightening systems.
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Recently, it has been observed that simple geometry characterized by a low level of symmetry present interesting peculiarities in the process of transition from laminar Poiseuille flow to turbulent flow. Examples of this type of g...
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Recently, it has been observed that simple geometry characterized by a low level of symmetry present interesting peculiarities in the process of transition from laminar Poiseuille flow to turbulent flow. Examples of this type of geometry are eccentric channels and, more generally, parallel channels containing a narrow gap. In the present work, a global linear stability analysis for the flow in this class of geometry has been performed. The problem is discretized through spectral collocation and the eigenvalue problem has been solved with the Arnoldi-method based algorithms and the QZ algorithm. Since no numerical studies of this type have yet been performed to address the issue of transition in this geometry, the codes have been validated toward results obtained in simplified geometries (e.g., concentric annular channel and square channel). The eigenvalue spectra of the Poiseuille flow in eccentric channels and a U-shaped channel have then been computed and analyzed for a wide range of geometric parameters. After comparison with spectra typical of channel flow and pipe flow it is shown that an additional linear mechanism of instability is present, related to the spanwise variation of the laminar velocity profile.
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Enhanced heat transfer in tubes under laminar flow conditions can be found in coils or corrugated tubes or in the presence of high wall relative roughness, curves, pipe fittings or mechanical vibration. Modeling these cases can be...
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Enhanced heat transfer in tubes under laminar flow conditions can be found in coils or corrugated tubes or in the presence of high wall relative roughness, curves, pipe fittings or mechanical vibration. Modeling these cases can be complex because of the induced secondary flow. A modification of the Graetz problem for non-Newtonian power-law flow is proposed to take into account the augmented heat transfer by the introduction of an effective radial thermal diffusivity. The induced mixing was modeled as an increased radial heat transfer in a straight tube. Three experiments using a coiled tube and a tubular heat exchanger with high relative wall roughness are presented in order to show how this parameter can be obtained. Results were successfully correlated with Reynolds number. This approach can be useful for modeling laminar flow reactors (LFR) and tubular heat exchangers available in the chemical and food industries.
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