Transitional Flow Phenomena Research Articles

Transitional Flow and Related Transport Phenomena in Curved Microchannels

In microchannels with typical dimensions from 10 μm to a few hundreds of micrometers, the flow is dominated by viscous forces, often leading to laminar flow conditions. At the entrance or in bends and curves, where the flow changes its velocity or direction, inertial forces generate transverse flow velocities. Due to continuity, vortex pairs, such as Dean flow in circular bends, are generated, which are still laminar, steady, and showing no statistically distributed fluctuations typical for turbulent flow. This deviation from straight laminar conditions is called transitional flow, often occurring in channels larger than 500 μm at higher flow rates. Transitional flow phenomena include the first occurrence of flow bifurcation, pulsating vortices, period doubling of vortex pairs, and regularly fluctuating wake flow or vortex shedding. Chaotic flow phenomena are the first evidence of turbulence. Transitional flow augments the transport characteristics in microchannels for enhanced heat and mass transfer and for performing chemical reactions in microreactors. The profound understanding of how to generate and control vortices in microchannels guides the design of advanced microstructured devices for various applications.

MHz-rate nitric oxide planar laser-induced fluorescence imaging in a Mach 10 hypersonic wind tunnel

Nitric oxide planar laser-induced fluorescence (NO PLIF) imaging at repetition rates as high as 1 MHz is demonstrated in the NASA Langley 31 in. Mach 10 hypersonic wind tunnel. Approximately 200 time-correlated image sequences of between 10 and 20 individual frames were obtained over eight days of wind tunnel testing spanning two entries in March and September of 2009. The image sequences presented were obtained from the boundary layer of a 20° flat plate model, in which transition was induced using a variety of different shaped protuberances, including a cylinder and a triangle. The high-speed image sequences captured a variety of laminar and transitional flow phenomena, ranging from mostly laminar flow, typically at a lower Reynolds number and/or in the near wall region of the model, to highly transitional flow in which the temporal evolution and progression of characteristic streak instabilities and/or corkscrew-shaped vortices could be clearly identified.

Modelling coupled hydraulics and sediment transport of a high-magnitude flood and associated landscape change

AbstractProcesses and mechanisms of erosion, transport and deposition within high-magnitude outburst floods such as jökulhlaups and lahars are poorly understood and remain largely unquantified. This study therefore applies a two-dimensional or depth-averaged hydrodynamic model, with fully integrated sediment transport, to reconstruct a Holocene jökulhlaup to have occurred from Kverkfjöll volcano, Iceland. Results indicate simultaneous inundation of multiple channels, flow around islands, hydraulic jumps and multi-directional flow including backwater areas and hydraulic ponding. These flow characteristics are typical of outburst floods that are volcanically triggered, flow through steep volcanic terrain and contain high concentrations of volcaniclastic sediment. Kverkfjöll jökulhlaups had low frontal flow velocities but as stage increased, velocities reached 5–15ms–1. Peak stage was prolonged in zones of hydraulic ponding, but generally attenuated in magnitude and duration downstream. Suspended load transport persisted over the entire hydrograph but bed load transport was spatially discontinuous and comprised distinct pulses. A hierarchy of landforms is proposed, ranging from highest energy zones (erosional gorges, scoured bedrock, cataracts and spillways) to lowest (valley-fills, bars and slackwater deposits). Bedrock erosion was generally where flow exceeded ∽3m flow depth, ∽7ms–1 flow velocity, ∽1×102Nm–2 shear stress and 3 ×102Wm–2 stream power. Deposition occurred below ∽8m flow depth, 11 ms–1 flow velocity, 5 ×102Nm–2 shear stress and 3 ×103Wm–2 stream power. Hydraulic ranges associated with erosion and deposition have considerable overlap due to transitional flow phenomena, transitions in sediment concentration and the influence of upstream effects, such as hydraulic ponding behind topographic constrictions. These results are the first of coupled hydraulic and sediment transport phenomena in high-magnitude outburst floods with fluid rheology and high sediment content, such as jökulhlaups and dilute lahars. Modelled changes in sediment mass closely resembled field-mapped zones of erosion and deposition. This paper therefore introduces a capability to simulate rapid landscape change due to high-magnitude outburst flood.

Hydrodynamics and geomorphic work of jökulhlaups (glacial outburst floods) from Kverkfjöll volcano, Iceland

AbstractJökulhlaups (glacial outburst floods) occur frequently within most glaciated regions of the world and cause rapid landscape change, infrastructure damage, and human disturbance. The largest jökulhlaups known to have occurred during the Holocene within Iceland drained from the northern margin of Vatnajökull and along the Jökulsá á Fjöllum. Some of these jökulhlaups originated from Kverkfjöll volcano and were routed through anastomosing, high gradient and hydraulically rough channels. Landforms and sediments preserved within these channels permit palaeoflow reconstructions.Kverkfjöll jökulhlaups were reconstructed using palaeocompetence (point measurements), slope–area (one‐dimensional), and depth‐averaged two‐dimensional (2D) hydrodynamic modelling techniques. The increasing complexity of 2D modelling required a range of assumptions, but produced information on both spatial and temporal variations in jökulhlaup characteristics. The jökulhlaups were volcanically triggered, had a linear‐rise hydrograph and a peak discharge of 50 000–100 000 m3 s−1, which attenuated by 50–75% within 25 km. Frontal flow velocities were ∼2 m s−1; but, as stage increased, velocities reached 5–15m s−1. Peak instantaneous shear stress and stream power reached 1 × 104 N m−2 and 1 × 105 W m−2 respectively.Hydraulic parameters can be related to landform groups. A hierarchy of landforms is proposed, ranging from the highest energy zones (erosional gorges, scoured bedrock, cataracts, and spillways) to the lowest energy zones (of valley fills, bars, and slackwater deposits). Fluvial erosion of bedrock occurred in Kverkfjallarani above ∼3 m flow depth, ∼7m s−1 flow velocity, ∼1 × 102 N m−2 shear stress, and 3 × 102 W m−2 stream power. Fluvial deposition occurred in Kverkfjallarani below ∼8 m flow depth, 11 m s−1 flow velocity, 5 × 102 N m−2 shear stress, and 3 × 103 W m−2 stream power. Hence, erosional and depositional ‘envelopes’ have considerable overlap, probably due to transitional flow phenomena and the influence of upstream effects, such as hydraulic ponding and topographic constrictions, for example.Holocene Kverkfjöll jökulhlaups achieved geomorphic work comparable to that of other late Pleistocene ‘megafloods’. This work was a result of steep channel gradients, topographic channel constrictions, and high hydraulic roughness, rather than to extreme peak discharges. The Kverkfjöll jökulhlaups have implications for landscape evolution in north‐central Iceland, for water‐sediment inputs into the North Atlantic, and for recognizing jökulhlaups in the rock record. 2D hydrodynamic modelling is likely to be important for hazard mitigation in similar landscapes and upon other glaciated volcanoes, because it only requires an input hydrograph and a digital elevation model to run a model, rather than suites of geomorphological evidence and field‐surveyed valley cross‐sections, for example. Copyright © 2006 John Wiley & Sons, Ltd.

A numerical study of cavitation foot‐prints in liquid‐lubricated asymmetrical herringbone grooved journal bearings

A numerical model is presented in this paper to better describe the cavitated fluid flow phenomena in liquid‐lubricated Asymmetrical Herringbone Grooved Journal Bearings (HGJBs). An effective “follow the groove” grid transformation method is used in the present study to capture all the groove boundaries. A singularity at the groove edges is avoided with this approach. Symmetrical groove patterns as well as asymmetrical groove patterns can be accurately computed with the proposed method. The difficult problem of abrupt changes of oil film thickness in the liquid‐lubricated HGJB physical domain is modeled here through a series expansion approach. Results are comparable with available experimental and known numerical data from other investigators. Cavitation footprints, pressure distributions and their corresponding load characteristics are presented in this study. Effects of the critical transitional flow phenomena on the performance of the asymmetrical HGJBs are also determined through the present study.

Visualization of vortex shedding at the proximal side-to-end artery-graft anastomosis

The motion of guanine particles was recorded by video to visualize transitional flow phenomena in models of a proximal side-to-end anastomosis. Close examination of successive video fields revealed that above a critical Reynolds number, particles were periodically shed into the graft from a vortex situated near the anastomosis heel, and this disturbed the flow patterns in the graft causing vortex shedding to occur near to the toe of the anastomosis. The images clearly demonstrated that periodic flow structures propagated distally along the graft for over 15 tube diameters from the proximal anastomosis. The frequency of the vortex shedding was found to increase with Reynolds number. Under pulsatile conditions, the primary vortex at the heel of the anastomosis became unstable during the deceleration phase of the flow cycle and particles were shed downstream into the graft. Although it was possible briefly to observe the characteristic banded structure in the bypass graft, the flow patterns were highly three-dimensional and were quickly broken up by the accelerating flow. Dynamic flow visualization using guanine particles was found to be a complementary technique to particle tracer flow visualization and was highly effective in identifying transitional flow phenomena and the mass transport mechanisms associated with them.

Laminar and turbulent entry flow of polymer solutions

AbstractEntry lengths and contraction losses have been determined for 0.01, 0.2 and 0.6% aqueous solutions of Separan AP‐30. The Reynolds numbers ranged between 102 and 105 in 1/2‐in. and 2‐in. tubes. At Reynolds numbers in excess of 2100, the entry lengths and contraction losses for the polymer solutions are significantly larger than those for Newtonian fluids. These effects are shown to be related to the transitional flow phenomena which occur with drag reducing polymer solutions.At low Reynolds number the contraction loss data show expected trends but are in quantitative disagreement with predictions based on recent studies in capillary tubes. These indicate that the analyses based on small tubes are less general than anticipated.

Laminar‐turbulent transitional flow phenomena in isosceles triangular cross‐section ducts

AbstractA theoretical analysis of the laminar‐turbulent transition phenomenon is presented for the case of steady, isothermal, fully developed flow of Newtonian fluids in straight ducts of constant isosceles triangular cross section. As a result of this analysis, the simultaneous existence of macroscopically large stable regions of laminar and turbulent flow are predicted to occur during the transition phenomenon. The existence of this special type of flow field, which has not been previously predicted theoretically, is shown to be a consequence of the presence of an asymmetric shear stress distribution. The results of an experimental study are also presented. These data verify the predictions made by the theoretical analysis.

Birefringent flow visualization of transitional flow phenomena in an isosceles triangular duct

AbstractA flow visualization study was made with an optically birefringent solution of milling yellow dye in water flowing through a transparent duct of isoceles triangular cross section. The present data confirm a number of theoretical predictions concerning transitional phenomena in triangular ducts. One of the most interesting of these phenomena is the existence of a region of simultaneous laminar and turbulent flow in the duct. The present results, which agree with the theory, indicate an order of laminar and turbulent flow which is inverse to previous observations made with smoke filament tracings.