Increased Momentum Transfer Research Articles

Future sea ice weakening amplifies wind-driven trends in surface stress and Arctic Ocean spin-up

Arctic sea ice mediates atmosphere-ocean momentum transfer, which drives upper ocean circulation. How Arctic Ocean surface stress and velocity respond to sea ice decline and changing winds under global warming is unclear. Here we show that state-of-the-art climate models consistently predict an increase in future (2015–2100) ocean surface stress in response to increased surface wind speed, declining sea ice area, and a weaker ice pack. While wind speeds increase most during fall (+2.2% per decade), surface stress rises most in winter (+5.1% per decade) being amplified by reduced internal ice stress. This is because, as sea ice concentration decreases in a warming climate, less energy is dissipated by the weaker ice pack, resulting in more momentum transfer to the ocean. The increased momentum transfer accelerates Arctic Ocean surface velocity (+31–47% by 2100), leading to elevated ocean kinetic energy and enhanced vertical mixing. The enhanced surface stress also increases the Beaufort Gyre Ekman convergence and freshwater content, impacting Arctic marine ecosystems and the downstream ocean circulation. The impacts of projected changes are profound, but different and simplified model formulations of atmosphere-ice-ocean momentum transfer introduce considerable uncertainty, highlighting the need for improved coupling in climate models.

The flow topology transition of liquid\u2013liquid Taylor flows in square microchannels

This work investigates the change of the flow topology of Taylor flow and qualitatively relates it to the excess velocity. Ensemble-averaged 3D2C-upmu PIV measurements simultaneously resolve the flow field inside and outside the droplets of a liquid–liquid Taylor flow that moves through a rectangular horizontal microchannel. While maintaining a constant Capillary number Ca = 0.005, the Reynolds number (0.52 le {text{Re}} le 2.14), the viscosity ratio (0.24 le lambda le 2.67) and surfactant concentrations of sodium dodecyl sulfate (0–3 CMC) are varied. We experimentally identified the product of the Reynolds number Re and the viscosity ratio lambda to indicate the momentum transport from the continuous phase (slugs) into the droplets (plugs). The position and size of the droplet’s main vortex core as well as the flow topology in the cross section of this vortex core changed with increased momentum transfer. Further, we found that the relative velocity of the Taylor droplet correlates negatively with the evoked topology change. A correlation is proposed to describe the effect quantitatively.Graphical abstract

Toward predictive understanding of the rapidly evolving Arctic Ocean—An overview

Some of the most rapid climate changes on the planet are experienced in the Arctic. In particular, the Arctic has been warming at a quicker pace than any other place on Earth, what is recognized as Arctic Amplification (AA). This warming has been most visibly manifested through a declining perennial sea ice cover, increasing the potential for its transition from the permanent toward a seasonal coverage. Those changes also affect air-sea heat fluxes and amplify ice-albedo feedback, which strongly influences ocean’s absorption of solar radiation. In addition, they also alter the Arctic Ocean acoustical regime, as the thinning sea ice moves faster and deforms easier, while its reduced coverage allows increased momentum transfer from the atmosphere to the upper ocean. This talk will provide an updated overview of the recent changes and trends in the Arctic Ocean of relevance to acoustical oceanography. We will focus on the evolution of the upper ocean stratification and water masses, mesoscale processes, and their linkages to the changing regime of the sea ice cover from multi-year to first-year sea ice. Also, the latest advancements and outstanding challenges in modeling and prediction of Arctic climate change at sub-seasonal to interannual time scales will be discussed.

Effects of Charge State, Charge Distribution, and Structure on the Ion Mobility of Protein Ions in Helium Gas: Results from Trajectory Method Calculations.

Collision cross section (Ω) values of gas-phase ions of proteins and protein complexes are used to probe the structures of the corresponding species in solution. Ions of many proteins exhibit increasing Ω-values with increasing charge state but most Ω-values calculated for protein ions have used simple collision models that do not explicitly account for charge. Here we use a combination of ion mobility mass spectrometry experiments with helium gas and trajectory method calculations to characterize the extents to which increases in experimental Ω-values with increasing charge state may be attributed to increased momentum transfer concomitant with enhanced long-range interactions between the protein ion and helium atoms. Ubiquitin and C-to-N terminally linked diubiquitin ions generated from different solution conditions exhibit more than a 2-fold increase in Ω with increasing charge state. For native and energy-relaxed models of the proteins and most methods for distributing charge, Ω-values calculated using the trajectory method increase by less than 1% over the range of charge states observed from typical solution conditions used for native mass spectrometry. However, the calculated Ω-values increase by 10% to 15% over the full range of charge states observed from all solution conditions. Therefore, contributions from enhanced ion-induced dipole interactions with increasing charge state are significant but without additional structural changes can account for only a fraction of the increase in Ω observed experimentally. On the basis of these results, we suggest guidelines for calculating Ω-values in the context of applications in biophysics and structural biology.

An Investigation of Fuel Mixing and Reaction in a CH4/Syngas/Air Premixed Impinging Flame with Varied H2/CO Proportion

For industrial applications, we propose a concept of clean and efficient combustion through burning syngas on an impinging burner. We performed experimental measurements of particle image velocimetry, OH radical (OH*) chemiluminescence, flame temperature, and CO emission to examine the fuel mixing and reaction of premixed impinging flames of CH4/syngas/air with H2/CO in varied proportions. The velocity distribution of the combustion flow field showed that a deceleration area in the main flow formed through the mutual impingement of two jet flows, which enhanced the mixing of fuel and air because of an increased momentum transfer. The deceleration area expanded with an increased CO proportion, which indicated that the mixing of fuel and air also increased with the increased CO proportion. Our examination of the OH* chemiluminescence demonstrated that its intensity increased with increased CO proportion, which showed that the reaction between fuel and air accordingly increased. CO provided in the syngas hence participated readily in the reaction of the CH4/syngas/air premixed impinging flames when the syngas contained CO in a large proportion. Although the volume flow rate of the provided CO quadrupled, the CO emission increased by only 12% to 15%. The results of this work are useful to improve the feasibility of fuel-injection systems using syngas as an alternative fuel.

Effects of surface roughness on shear viscosity.

This paper investigates the effect of surface roughness on fluid viscosity using molecular dynamics simulations. The three-dimensional model consists of liquid argon flowing between two solid walls whose surface roughness was modeled using fractal theory. In tandem with previously published experimental work, our results show that, while the viscosity in smooth channels remains constant across the channel width, in the presence of surface roughness it increases close to the walls. The increase of the boundary viscosity is further accentuated by an increase in the depth of surface roughness. We attribute this behavior to the increased momentum transfer at the boundary, a result of the irregular distribution of fluid particles near rough surfaces. Furthermore, although the viscosity in smooth channels has previously been shown to be independent of the strength of the solid-liquid interaction, here we show that in the presence of surface roughness, the boundary viscosity increases with the solid's wettability. The paper concludes with an analytical description of the viscosity as a function of the distance from the channel walls, the walls' surface roughness, and the solid's wetting properties. The relation can potentially be used to adjust the fluid dynamics equations for a more accurate description of microfluidic systems.

Momentum enhancement due to hypervelocity impacts into pumice

Momentum enhancement refers to the effect of more momentum being transferred to a target than resides in the impactor. This increased momentum transfer is due to crater ejecta leaving the impacted body in the direction opposite to the impactor travel direction. An application for this effect is using hypervelocity impactors to deflect potentially hazardous asteroids and comet nuclei. A concern with historical work with consolidated material is the effect of porosity in the impacted body on momentum enhancement. Hypervelocity impact tests were performed of aluminum spheres striking pumice boulders. The pumice boulders ranged in mass from 70 to 486 kg and had an estimated porosity of 70 – 75%. Two different projectiles diameters were used (2.54 cm and 4.45 cm) to study the effect of size scale. Impacts occurred at roughly 2.1 km/s. Depth of penetration was measured, an estimated crater radius was measured (though the crater geometries were irregular), and momentum enhancement was measured. Significant scatter was observed in the results even though the tests were carefully controlled, with measured momentum enhancements ranging from 1 to 1.69 and the average being β = 1.3.

Wind‐driven mixing at intermediate depths in an ice‐free Arctic Ocean

AbstractRecent seasonal Arctic Ocean sea ice retreat is a major indicator of polar climate change. The Arctic Ocean is generally quiescent with the interior basins characterized by low levels of turbulent mixing at intermediate depths. In contrast, under conditions of reduced sea ice cover, there is evidence of energetic internal waves that have been attributed to increased momentum transfer from the atmosphere to the ocean. New measurements made in the Canada Basin during the unusually ice‐free and stormy summer of 2012 show previously observed enhancement of internal wave energy associated with ice‐free conditions. However, there is no enhancement of mixing at intermediate depths away from significant topography. This implies that contrary to expectations of increased wind‐induced mixing under declining Arctic sea ice cover, the stratification in the central Canada Basin continues to suppress turbulent mixing at intermediate depths and to effectively isolate the large Atlantic and Pacific heat reservoirs from the sea surface.

Low Temperature Cu Etching Using CH4-Based Plasmas

Methane- (CH4) based subtractive plasma etching of Cu was investigated at low temperature. The Cu etch rate using a CH4 plasma (17 nm/min) was higher than that of an H2 plasma (13 nm/min) under the same plasma conditions, despite the fact that hydrocarbon deposition was evident with CH4. The higher Cu etch rate is ascribed to increased momentum transfer due to the heavier mass of CH+ and C+ relative to H+ and to the more thermodynamically stable etch products of CH3Cu or CH3CuH− relative to CuHx. Methyl-containing Cu etch product species were not detected by optical emission spectroscopy, most likely because Cu-CH3 was readily dissociated by energetic electron impact collisions after desorption from the Cu surface. Patterns generated in Cu displayed a sidewall angle of ∼80°, similar to that of H2 plasma patterning, but unlike the result in an H2 plasma, a photoresist mask was not significantly degraded during the etch process.

On the influence of windward edge structure and stand density on the flow characteristics at forest edges

Experimental investigations in an atmospheric boundary layer wind tunnel were carried out in order to study both the influence of the windward edge structure and the stand density on the flow characteristics near the canopy top of forest edges. For the edge structure investigation, two different types of forest edges were used. The taper angle was varied three times for both edge types, and all tapered edges were combined with both dense and sparse forest stands. In addition, a configuration was investigated where the airflow in the trunk space of the dense forest was blocked completely by an impermeable wall. In order to study the influence of stand density, forest stands were varied three times by removing whole rows of the originally dense model forest. The measurements show that a forest edge induces strong disturbances in the flow field at the near-edge region leading to an increased momentum transfer and to an increased wind loading on trees. The region of significant disturbances extends to a distance of about eight tree heights. Both edge density and taper angle determine the amount of volume influx through the edge, which seems to be primarily responsible for the flow field properties near the canopy top. It was found that for sparse forest stands, sloping tapered edges reduce the wind velocities nearest to the canopy. It could also be shown that the positive skewness of the near-canopy longitudinal velocity distribution is diminished with decreasing taper angle indicating that higher wind velocity fluctuations are reduced.

Formation Criterion for Synthetic Jets

Af ormation criterion for synthetic jets is proposed and validated. A synthetic jet actuator is a zero-net mass-flux device that imparts momentum to its surroundings. Je tf ormation is defined as the appearance of a time-averaged outward velocity along the jet axis and corresponds to the generation and subsequent convection or escape of a vortex ring. It is shown that over a wide range of operating conditions synthetic jet formation is governed by the jet Strouhal number Sr (or Reynolds number Re and Stokes number S). Both numerical simulations and experiments are performed to supplement available two-dimensional and axisymmetric synthetic jet formation data in the literature. The data support the jet formation criterion 1/Sr = Re/S 2 > K, where the constant K is approximately 1 and 0.16 for two-dimensional and axisymmetric synthetic jets, respectively. In addition, the dependence of the constant K on the normalized radius of curvature of a rounded orifice or slot is addressed. The criterion is expected to serve as a useful design guide for synthetic jet formation in flow control, heat transfer, and acoustic liner applications, in which a stronger jet is synonymous with increased momentum transfer, vorticity generation, and acoustic nonlinearities.

Near-wall physics of a shear-driven three-dimensional turbulent boundary layer with varying crossflow

The near-wall physics and turbulence structure of a planar shear-driven three-dimensional turbulent boundary layer (3DTBL) with varying strengths of crossflow are examined in a specialized facility. Spanwise shear modifies the near-wall turbulence structure. Flow visualization reveals a reduction of mean streak length by as much as 50%, while streak spanwise spacing remains constant. Power spectra of velocity confirm this shift towards higher temporal frequencies, corresponding to decreased streamwise length scales. Particle image velocimetry (PIV) measurements indicate that the spanwise shear increases the number and strength of flow structures that interact in the inner region of the boundary layer. This leads to increased momentum transfer between low- and high-speed fluids, resulting in a thickening of the inner region of the boundary layer. In addition, the organized two-dimensional boundary layer spanwise vorticity layer along the wall surface is disrupted and forms strong vortical structures that are lifted off the wall surface and diffuse into the boundary layer with streamwise distance. The transverse vorticity experiences a similar alteration. Streamwise velocity profiles exhibit an increasing velocity deficit with increased crossflow, consistent with the thickening of the inner region of the boundary layer. Increases in the normal Reynolds stresses are associated with interaction of the secondary flow structures and modifications to vortical structures with increasing three-dimensionality and higher relative wall speed. Significant increases are observed for the most highly sheared cases with translating wall velocities 2.0 and 2.75 times the free-stream velocity. This increase in the normal stresses leads to an increase in the turbulent kinetic energy over the belt surface, which diffuses into the inner region of the boundary layer with streamwise distance. The Reynolds shear stresses also exhibit a significant increase in magnitude over the belt surface owing to enhanced turbulence production over the translating wall section. In general, three-dimensional effects are confined to the area close to the wall and the crossflow increases the interaction of secondary flow structures leading to modifications of the vorticity in the inner region of the boundary layer. These flow distortions disrupt the near-wall streak structure and result in increased levels of turbulent kinetic energy and Reynolds stresses, particularly over the translating wall section.

Flow measurements on a turbulent fiber suspension by laser Doppler anemometry

AbstractThe flow and transition of fiber suspensions to turbulence in a rotary shear tester were studied. Both torque and LDA measurements were performed on a refractive‐index‐matched glass‐fiber suspension in the 3–20 wt. % concentration range. The flow of such suspensions underwent two transitions: the first one coincided with complete agitation, a sharp increase in torque, and fluctuating velocities close to zero. The mean velocities were similar to those of single‐phase flow above this transition. The fluctuation velocities approached those of single‐phase flow with increasing rotational speed, until they were nearly equal. This second transition was interpreted as fully developed turbulent flow. CFD, assuming single‐phase flow, is a helpful tool for describing the flow behavior in this region. The network strength was useful for the study of flow transitions. LDA measurements of the boundary layer at the bottom wall showed that the velocity profile is linear close to the wall even with fibers present. The presence of the fibers also flattened the profiles, indicating an increased momentum transfer.

The mutual interference of spruce canopy structural elements

The drag force on fourteen three-prong spruce shoots (Picea rubens) has been measured in a wind tunnel in velocities ranging from 0.4 to 4.5 m s −1. The influence of wind velocity and shoot needle density on momentum transfer and the mutual interference to momentum transfer are evaluated. The influence of the shoot needle density (ranging from 0.6 to 1.7) on momentum transfer is velocity dependent. The shoot needle density does not influence the momentum transfer to the shoot when the wind velocity is less than about 1.5 m s −1. However, a slight trend of increased momentum transfer to the shoot with decreased shoot needle density was found for wind velocities between 1.5 and 4.5 m s −1. This trend is not statistically significant. The shelter factor, a measure of the mutual interference to momentum transfer to the shoot structure, is shown to be dependent on the model used to describe the flow around and through the shoot. This suggests that any apparent velocity dependence of the shelter factor may actually be due to a change in flow conditions. It is concluded that: (1) the needles are the vegetative component involved in mutual interference within the shoot for wind speeds between 0.4 and 1.5 ms −1, and (2) the shoots themselves are the vegetative component involved in mutual interference within a shoot array for wind speeds between 1.5 and 4.5 ms −1. The shelter factor at the low wind speeds is estimated to be 2.1 The shelter factor at the higher wind speeds should be based on interference between shoots and not between needles within a single shoot.

Breaking of wind waves and the sea surface wind stress

In the conventional treatment of the coefficient of sea surface wind stress by plotting it against 10-m wind speed, there are inevitable discrepancies among results of various investigators. The reason is considered to lie primarily in the fact that the state of the sea surface or of waves is disregarded, which may have great influence on the sea surface wind stress. Former concepts concerning the conditions which control the sea surface wind stress are discussed, and it is shown that a more universal expression may be obtained by plotting the coefficient against a kind of roughness Reynolds number:Re 2 *=u*H/ν, whereu* is the friction velocity of air, ν the kinematic viscosity of air, andH the characteristic wave height.H is used here to treat some data in wind-wave tunnels, as a tentative variable, one step towards a more rigorous approach to the problem. This variableRe 2 *, orRe 4 *=u *w /L/v w =2πgu *w /v w n 1, where the subscript ω represents values for water,L andn 1 the characteristic wave length and frequency, respectively, is also the condition describing the air entrainment or the breaking of wind waves. In this case, these Reynolds numbers are interpreted as the quantity describing the intensity of turbulence of the water surface itself. It is shown, using data from our wind-wave tunnel experiments, that the breaking commences asRe 2 * reaches 1×103, or asRe 4 * reaches 3×103. Simultaneously, the stress-coefficient begins to increase sharply at this value ofRe 2 *. This phenomenon is understood as an increased momentum transfer from the air to the water through “boundary penetration of turbulence” caused by the breaking of wind waves. Further, it is suggested that there is a possibility that this excess momentum transfer does not increase wave momentum, but reinforces drift current.