Upwelling Flow Research Articles

Relocation of the 8 January 2022 Menyuan (Ms 6.9), China, earthquake sequence: A conjugated type of rupturing event with a seismic gap in the Qilian-Haiyuan fault in northeast Tibet

On 8 January 2022, an earthquake with Ms 6.9 struck Menyuan County, Qinghai Province, China, which provides an opportunity to investigate the seismotectonics of the Qilian-Haiyuan fault in northeast Tibet. We collect seismic observation data from China Seismic Network Center from 8 January to 6 February 2022 around the 2022 Menyuan mainshock. The double-difference relocation algorithm is applied to relocate 904 earthquakes, and 820 high-precision hypocenter locations are obtained. The mainshock epicenter was located at 10.2 km depth in the Tuolaishan fault. The aftershock sequence extended towards nearly E-W along the Tuolaishan fault and NW-SE along the Lenglongling fault, parts of the Qilian-Haiyuan fault, but there were fewer aftershocks at the beginning of the rupture near the intersection between the Lenglongling and Tuolaishan faults, suggesting a conjugated type of rupturing with a seismic gap. Integrating with focal mechanism solutions of mainshock and one large aftershock, we suggest that the rupture is a left-lateral strike-slip event with a high dip. The stress change around the faults intersection. The 1986 Ms 6.4 earthquake may trigger the 2016 Ms 6.4 earthquake, and then the extrusion stress could transfer to around the 2022 Ms 6.9 earthquake source area due to the Indo-Eurasian collision. Previous tomographic models revealed low-velocity anomalies under the Menyuan earthquakes, reflecting that fluids could decrease friction coefficient and normal stress on the fault planes and further promote the generation of these earthquakes. These fluids could be contained in the lower crustal flow and mantle upwelling flow under northeast Tibet.

The Four‐Stage Evolution of Martian Mantle Inferred From Numerical Simulation of the Magmatism‐Mantle Upwelling Feedback

AbstractTo understand how mantle dynamics exerts control over the history of magmatic activities and surface environment on Mars, I used a numerical model of magmatism in the convecting mantle that is nominally anhydrous and internally heated. Magmatism occurs as an upward permeable flow of basaltic magma generated by decompression melting through matrix. The mantle evolves in four stages in preferred models. In Stage I, high initial temperature in the uppermost mantle causes an extensive magmatism intensified by two types of positive feedback that operate between magmatism and mantle upwelling flow, the MMUb and MMUc feedbacks: the buoyancy and volume change of matrix, respectively, caused by migrating magma that a mantle upwelling flow generates intensify the flow itself to generate more magma. The stratification suppresses mantle convection and magmatism for the next tens to hundreds of millions of years, allowing heat to build up in the mantle by internal heating (Stage II). Eventually, magma is generated at depth, and the MMUb feedback operates to cause an episodic plume magmatism that releases water from the interior of Mars (Stage III). The plume magmatism also stirs the mantle to make it more homogeneous and extracts heat producing elements from the deep mantle to let the magmatism itself wane and cease. In the final stage IV, mantle convection becomes more akin to a thermal convection. The episodic magmatism and water outgassing in Stage III account for the volcanism and clement surface environment observed for early Mars.

Winter diet of Japanese macaques from Chubu Sangaku National Park, Japan incorporates freshwater biota

The Japanese macaque (Macaca fuscata) is native to the main islands of Japan, except Hokkaido, and is the most northerly living non-human primate. In the Chubu Sangaku National Park of the Japanese Alps, macaques live in one of the coldest areas of the world, with snow cover limiting the availability of preferred food sources. Winter is typically a bottleneck for food availability potentially resulting in marked energy deficits, and mortality may result from famine. However, streams with groundwater upwelling flow during the winter with a constant water temperature of about 5 °C are easily accessible for Japanese macaques to search for riverine biota. We used metabarcoding (Cytochrome c oxidase I) of fecal samples from Japanese macaques to determine their wintertime diet. Here we provide the first robust evidence that Japanese macaques feed on freshwater biota, including brown trout, riverine insects and molluscs, in Chubu Sangaku National Park. These additional food sources likely aid their winter survival.

High Fluid‐Pressure Patches Beneath the Décollement: A Potential Source of Slow Earthquakes in the Nankai Trough off Cape Muroto

AbstractPore pressure plays a key role in the generation of earthquakes in subduction zones. However, quantitative constraints for its determination are quite limited. Here, we estimate the subsurface pore pressure by analyzing the transient upwelling flow of drilling mud from borehole C0023A of the International Ocean Discovery Program (IODP) Expedition 370, in the Nankai Trough off Cape Muroto. This upward flow provided the first direct evidence of an overpressured aquifer in the underthrust sediments off Cape Muroto. To estimate the pre‐drilling pore pressure in the overpressured aquifer around a depth of 950–1,050 m below sea floor, we examined the measured porosities of core samples retrieved from nearby IODP wells; we then proceeded to explain the observed time evolution of the flow rate of the upwelling flow by modeling various sized aquifers through solving a radial diffusion equation. It was observed that for a permeability of 10−13 m2, the aquifer possessed an initial excess pore pressure of ∼5–10 MPa above the hydrostatic pressure, with a lateral dimension of several hundred meters and thickness of several tens of meters. The overpressure estimates from the porosity‐depth profile at Site C0023 differ from those at other drill sites in the region, suggesting the possible existence of multiple overpressured aquifers with a patchy distribution in the underthrust sediments of the Nankai Trough. As pore pressure is relevant in maintaining fault stability, the overpressured aquifers may be the source of slow earthquakes that have been observed around the drilling site.

New insights from 2- and 3-D numerical modelling on fluid flow mechanisms and geological factors responsible for the formation of the world-class Cigar Lake uranium deposit, eastern Athabasca Basin, Canada

The Cigar Lake uranium deposit in the Athabasca Basin is the world’s second largest high-grade unconformity-related uranium deposit. Its distinct geological architecture includes heterogeneous basement lithologies, a local basement high and sub-vertical faults. The sub-vertical faults are classified further into two sub-types: faults that are restricted to the basement, termed ‘basement faults’, and faults that are distinctly related to post-Athabasca fault reactivation, in that they extend upward into the sandstone, termed ‘extended basement faults’. This study aims to evaluate the effects of the aforementioned geological factors on the fluid flow patterns under two different driving forces, buoyancy due to variation of fluid density or ‘thermal convection’ and deformation or a combination of them, and to determine the most probable fluid flow scenarios for the formation of the deposit. The numerical results show that fluid flow is strongly affected by the two types of faults. While the basement faults represent fluid paths with complex fluid flow patterns, depending on the driving forces, providing favourable physical conditions for different chemical processes, such as fluid-fluid and fluid-rock interactions, the extended basement faults enhance the permeability of an E-W corridor within the sandstone and significantly strengthen the overall upwelling flow above the basement faults, promoting sandstone-hosted mineralization. The numerical results also suggest that the main deposit likely formed via fluid convection during tectonically quiet periods, although faulting played a critical role in increasing permeability, in turn, enhancing thermal convection.

Exsolving two-phase flow in oil wells

ABSTRACTOil wells contain two-phase liquid and gas mixtures driven upwards due to a pressure gradient. In this paper, we study a two-fluid model for vertical upwelling flow and explicitly account for the exsolution of the dissolved gas as the pressure decreases along the well. We find that the application of Henry's law for the dissolved gas concentration predicts a rapid transition to a foam, which runs counter to intuition. In order to study ways in which this rapid transition could be avoided, we examine rate limiting non-equilibrium dynamics by incorporating nucleation and bubble growth dynamics in the two-phase model.

Experimental investigation on natural convection and thermal stratification of IRWST using PIV measurement

Passive Residual Heat Removal Heat Exchanger (PRHR HX) immerged in the In-containment Refueling Water Storage Tank (IRWST) plays an important role in removing the core decay heat under non-Loss of Coolant Accident (LOCA) accident. In the initial stage of the Passive Residual Heat Removal System (PRHRS) activating, the natural convection is the main way to exchange heat between the secondary side of the PRHR HX with IRWST. However, it is almost impossible to acquire the thermohydraulic performance in IRWST since the influence of the natural convection and thermal stratification in IRWST happens simultaneously. In this paper, Hierarchical Two-Tiered Scaling (H2TS) method is employed to get scaling criteria based on control equations. Anda visualized water tank with five C-shape electrical heating tubes was scaled-down and applied to investigate the thermohydraulic performance for IRWST of CAP1400 Nuclear Power Plants (NPPs). Particle Image Velocimetry (PIV) measurement was used to investigate the evolution of the flow field of total twenty-five surfaces in three axial directions under three steady- and two variable-heating conditions. Meanwhile, three thermocouple bundles were adopted to acquire temperature data in three regions. The experimental results show that the temperature deviation in the same height of the IRWST is flattened by plenty of vortexes. As a result, local circulation and the temperature difference is less than 1 °C. While the thermal stratification along the vertical direction is obvious, the distribution of flow field and temperature demonstrate that a “dead zone” exists in bottom of the IRWST. Besides, a “thermal interface” region forms in the lower middle region as hot and cold fluid mixing. An obvious upwelling flow is observed near the PRHR HX heating rod bundle from flow fields of three axial directions. At the same time, the maximum height of the upwelling flow decreases with time increasing. On the one hand, the natural convection is induced by the thermal stratification. What is more, the thermal stratification will impair the scale and strength of natural convection, which will intensify the thermal stratification conversely.

Effect of the Wall on Semi-open Upwelling Flow Caused by Air Bubbler

Effect of the Wall on Semi-open Upwelling Flow Caused by Air Bubbler

The UK22/4b blowout 20 years on: Investigations of continuing methane emissions from sub-seabed to the atmosphere in a North Sea context

The 22/4b blowout occurred in the UK sector of the North Sea in November 1990, but during a survey in September 2011 strong gas emissions still were occurring. This manuscript summarizes the findings of the 2011 survey and subsequent studies, considering them in the context of previous investigations, and the regional geologic and oceanographic environments.The seabed crater formed during the initial event is still there, as is a previously-undiscovered secondary crater. Seabed bubble flux estimates indicate methane emission rates of 90 L s−1 at the time; however, this methane's fate(s) remains unclear. The very strong thermocline that persists for more than half the year acts as an effective barrier to upward migration, despite the presence of strong upwelling flows around the bubble plume. Clearly a large proportion of the methane is advected away from the site to be either oxidized microbially in the water column, or released to the atmosphere as a result of normal sea:air gas exchange processes. Nevertheless, a significant atmospheric methane anomaly persists in the vicinity of the blowout site. This has been constrained to likely less than 0.72 Mscfd (5 kTon yr−1) and possibly less than 0.36 Mscfd (2.5 kton yr−1).During the late-fall to early spring months (when there is no thermocline), direct methane emissions to the atmosphere are expected to increase significantly. Also, long-term monitoring has shown that periodic eruptive events occur, which likely expel great quantities of methane. These demonstrate the dynamic nature of the system and suggest that migration pathways in and between the deep sub-seabed and seabed remain active.The 22/4b Study resulted in the development and adaptation of novel techniques that are applicable to other studies of seabed seepage and the development of a number of critical hypotheses with application to megaplume seepage by natural migration pathways or from the result of anthropogenic intervention.

Megaplume bubble process visualization by 3D multibeam sonar mapping

MultiBeam echosounder data were collected during a surface-ship survey of the 22/4b well site in the North Sea in September 2011 using a Teledyne-Reson 7125. Modern multibeam echosounders are instrumental in providing detection and accurate localization of weak to strong bubble plumes. Two survey profiles effectively insonified the bubble plumes rising from the main crater at the well site, providing snapshot data of bubble plume processes. Additionally, three profiles insonified bubble plumes rising from, in, and to the south of a secondary crater, 1.2 km southeast of the main crater. Data processing included a simple algorithm that muted mislocated echoes from incomplete sidelobe suppression. The data processing produced a Cartesian volume of echo intensity from the water column and seabed.Plume geometry was analyzed to investigate a number of important large-scale plume processes, including plume bubble detrainment due to currents and stratification, downwelling flows, sea surface interaction, plume heterogeneity, and other fluid transport processes. The data showed strong upwelling flows, with bubble vertical motions generally much faster than currents. One important finding was that megaplumes create intrusions above the general thermocline, in part because their extensive upwelling flow lifts the thermocline locally. As a result, the intrusion layer deposits dissolved gases in the upper wave-mixed layer of the water column where it is not isolated from the atmosphere, unlike dissolved gases in the lower water column.The analysis shows that high fidelity multibeam echosounder data can provide a wealth of remote sensing information on bubble plume characteristics and processes, with important applications, including blowout monitoring and response, better understanding of megaplumes such as used in lake destratification, and improved characterization of natural seep emission processes.

Numerical Study on Effect of Wall on Upwelling Flow

Numerical method is done on effect of wall on upwelling flow induced by air bubbler in a tank. It shows that: this reverse flow causes the increasing upwelling flow with height; when the bubbler distance to wall decreases, it is gradually only influenced at one side and then it affects the upwelling flow; the change of velocity is small in the region below bubbler. So the numerical method is helpful to describe the flow field as figures clearly and design bubbler devices in air-lift engineering applications.

Stream and bed temperature variability in a coastal headwater catchment: influences of surface-subsurface interactions and partial-retention forest harvesting

Stream temperature was recorded between 2002 and 2005 at four sites in a coastal headwater catchment in British Columbia, Canada. Shallow groundwater temperatures, along with bed temperature profiles at depths of 1 to 30 cm, were recorded at 10-min intervals in two hydrologically distinct reaches beginning in 2003 or 2004, depending on the site. The lower reach had smaller discharge contributions via lateral inflow from the hillslopes and fewer areas with upwelling (UW) and/or neutral flow across the stream bed compared to the middle reach. Bed temperatures were greater than those of shallow groundwater during summer, with higher temperatures in areas of downwelling (DW) flow compared to areas of neutral and UW flow. A paired-catchment analysis revealed that partial-retention forest harvesting in autumn 2004 resulted in higher daily maximum stream and bed temperatures but smaller changes in daily minima. Changes in daily maximum stream temperature, averaged over July and August of the post-harvest year, ranged from 1.6 to 3 °C at different locations within the cut block. Post-harvest changes in bed temperature in the lower reach were smaller than the changes in stream temperature, greater at sites with DW flow, and decreased with depth at both UW and DW sites, dropping to about 1 °C at a depth of 30 cm. In the middle reach, changes in daily maximum bed temperature, averaged over July and August, were generally about 1 °C and did not vary significantly with depth. The pre-harvest regression models for shallow groundwater were not suitable for applying the paired-catchment analysis to estimate the effects of harvesting. However, shallow groundwater was warmer at the lower reach following harvesting, despite generally cooler weather compared to the pre-harvest year. Copyright © 2012 John Wiley & Sons, Ltd.

Application of the Robinson biodynamical theory to turbulence

In the late 1990s Allan Robinson developed a theory of NPZ interactions in a laminar upwelling flow field. His approach was to use the advection reaction (AR) equation in a Lagrangian coordinate system. Recently, his theory was extended to turbulent flow by applying a probability density function to the solution of the AR equation. A review of this work is presented as well as new work examining the role of the turbulent induced biodynamical interaction (TIBI) effect, which is typically neglected in advection diffusion reaction (ADR) formulations for NPZ problems. The TIBI effect is associated with turbulence inducing fluctuations in nonlinear biological constituent interactions and is separate from the effect of turbulent mixing in dispersing the constituents. A simple example of the application of the theory – that of nutrient and phytoplankton fields being upwelled into a uniform optically active turbulent mixed layer – is presented. For this example, not including the TIBI term in an ADR formulation results in an overestimate of the primary production, increasing with decreasing turbulent Peclet number.

Large-eddy simulation of a curved open-channel flow over topography

Large-eddy simulation (LES) is performed of a curved open-channel flow over topography based on the laboratory experiment by Blanckaert [“Topographic steering, flow circulation, velocity redistribution and bed topography in sharp meander bends,” Water Resour. Res., doi:10.1029/2009WR008303 (in press)]. In the experiment, the large-scale bed topography had developed to a more or less stationary shape which was prescribed in the LES model as boundary conditions neglecting the small-scale dune forms by means of a straightforward immersed boundary scheme in combination with a simple wall-modeling approach. The small-scale dunes are accounted for in the numerical model by means of parametrization. Sensitivity of the flow to this roughness parametrization is examined by simulating the flow for three different roughness heights. It was found that, notwithstanding the coarse method of representing the dune forms, the qualitative agreement of the experimental results and the LES results is rather good. Comparison of the LES results with the Reynolds averaged numerical simulation results of Zeng et al. [“Flow and bathymetry in sharp open-channel bends: Experiments and predictions,” Water Resour. Res. 44, W09401, doi:10.1029/2007WR006303 (2008)] reveals surprisingly good agreement. This good agreement is explained by the minor importance of turbulence stress gradients in the contribution to the transverse and streamwise momentum balance. Moreover, it is found that in the bend the structure of the Reynolds stress tensor shows a tendency toward isotropy which enhances the performance of isotropic eddy viscosity closure models of turbulence. This observation is remarkable since highly anisotropic turbulence might well be expected considering the complex nature of the geometry. Furthermore, the LES results reveal a pronounced recirculation zone near the convex inner bank of the flume due to the shallowness of the flow and strong curvature of the flume. At the interface between the recirculation zone and the main flow, a curved mixing layer is identified as well as strong upwelling flow motion that is accompanied with large production of turbulent kinetic energy.

STRANGE SHAPE AFFECTS SAND DOLLAR LARVAE SWIMMING

Most youngsters look a lot like their parents. They have the same number of arms and legs in roughly the same arrangement – but not sand dollar larvae. While their parents are disc shaped bottom dwellers, the shuttlecock shaped larvae bob around freely in the ocean. ‘They are plankton that drift with the currents,’ explains Tansy Clay from the University of Washington. Over the course of their development the larvae grow additional arms. Starting off with four, they develop another two and finally a total of eight before eventually settling on the seabed. And sand dollars are not the only larvae to take this strange shape. The larvae of several unrelated species are also shaped like shuttlecocks. ‘So what is the purpose of this crazy shape and why would different organisms independently evolve it?’ puzzles Clay. Intrigued by the larvae's bizarre appearance, Clay and her supervisor, Daniel Grünbaum, decided to find out how the different larval stages swim. Knowing that their coastal water habitats are turbulent, the duo decided to focus on how the larvae fare in the flows in turbulent eddies (p. 1281).According to Clay, previous attempts to understand how plankton move in turbulent eddies had represented them as simple spheres and ellipsoids. The calculations suggested that rounded plankton tilt in vertically circulating water so that they begin travelling horizontally toward downwelling water and are sucked down. Would the shuttlechock-shaped larvae suffer the same fate or would their strange morphology direct them into up-welling water?The duo took a two-pronged attack: running computer simulations of the movements of all three larval life stages; and filming real larvae bobbing about in a tank. Basing the computer simulations on model larvae built from cylinders, Clay calculated how all three-life stages moved and found that they were directed into up-welling flows in mild turbulence found in a calm estuary. But as the turbulence increased, Clay's calculations suggested that the four- and eight-armed larvae tilted and swam horizontally until they became trapped in downward plumes of water, while the intermediate six-armed larvae were tilted and drawn toward up-welling flows.Clay admits that she was surprised that the differences in the larvae's shapes had such a significant effect on their calculated movements. ‘I expected some differences but not that extreme,’ she says.Turning to live larvae bobbing about in tanks, Clay generated vertical water flows in the middle of the tank by warming one side of the chamber and cooling the opposite side while she filmed the larvae's movements in the central portion. Tracking the fluid movements with algal cells, Clay could see that the four- and eight-armed larvae behaved as her simulations had predicted, travelling horizontally until they were engulfed in a plume of down-welling water. But when she looked at the six-armed larvae, they were doing something completely different: they didn't move horizontally like the simulations. ‘It seems that they were resisting horizontal movement towards down-welling water,’ says Clay.So the larvae's strange shapes do change the ways in which they swim and could allow them to move selectively to different locations within the water column at different stages of development.

Engineered and Natural Marine Seep, Bubble-Driven Buoyancy Flows

Abstract Bubble-plume upwelling flows were studied in the marine environment through dye releases into engineered plumes and a natural hydrocarbon seep plume. For engineered plumes, these experiments measured the water column–averaged upwelling flow Vup(zo) from release depth zo to the sea surface, for a wide range of flows Q, and zo. From Vup(zo), the local upwelling flow Vup(z), where z is depth, was calculated and found to vary with Q as Vup(z) ∼ Q0.23 for plumes strong enough to penetrate a shallow, thermally stratified layer, which was in good agreement with published relationships between Vup(z) and Q. These data were used to interpret data collected at a natural marine seep. For the seep, the upwelling flow decelerated toward the sea surface in contrast to the engineered plumes, which accelerated toward the sea surface. Data showed the seep bubble-plume upwelling flow lifted significantly colder and more saline water. The increased density difference between this upwelling fluid and the surrounding fluid most likely caused the deceleration. Midwater-column bubble measurements showed downcurrent detrainment of smaller bubbles from the bubble plume.

Variations in Ocean Surface Temperature due to Near-Surface Flow: Straining the Cool Skin Layer

Abstract The aqueous thermal boundary layer near to the ocean surface, or skin layer, has thickness O(1 mm) and plays an important role in controlling the exchange of heat between the atmosphere and the ocean. Theoretical arguments and experimental measurements are used to investigate the dynamics of the skin layer under the influence of an upwelling flow, which is imposed in addition to free convection below a cooled water surface. Previous theories of straining flow in the skin layer are considered and a simple extension of a surface straining model is posed to describe the combination of turbulence and an upwelling flow. An additional theory is also proposed, conceptually based on the buoyancy-driven instability of a laminar straining flow cooled from above. In all three theories considered two distinct regimes are observed for different values of the Péclet number, which characterizes the ratio of advection to diffusion within the skin layer. For large Péclet numbers, the upwelling flow dominates and increases the free surface temperature, or skin temperature, to follow the scaling expected for a laminar straining flow. For small Péclet numbers, it is shown that any flow that is steady or varies over long time scales produces only a small change in skin temperature by direct straining of the skin layer. Experimental measurements demonstrate that a strong upwelling flow increases the skin temperature and suggest that the mean change in skin temperature with Péclet number is consistent with the theoretical trends for large Péclet number flow. However, all of the models considered consistently underpredict the measured skin temperature, both with and without an upwelling flow, possibly a result of surfactant effects not included in the models.

High‐order essentially local extremum diminishing schemes for environmental flows

AbstractThe accuracy of numerical simulation is greatly affected by the applied convective schemes for high Reynolds number environmental flows. The convection for any scalar equation should theoretically be a purely transport process. Simple monotonic convective schemes produce smooth and stable solutions. However, the results are not reliable and possibly introduce excessive artificial diffusion (AD). We develop a new high‐order monotonic formulation of essentially local extremum diminishing (ELED) schemes, using a simple algorithm to minimize the AD introduced by the convective schemes. The high‐order scheme is based on the Quadratic Upstream Interpolation for Convective Kinematics formula in order to compare the results with Simple High‐Accuracy Resolution Program. The resulting algorithm is applied to a two‐dimensional standing interfacial wave problem and extended to three‐dimensional turbulent coastal upwelling flows using large eddy simulation (LES). These examples allow sharp density interfaces in the domain, which may cause severe dispersion errors when high‐order schemes are used, whereas most monotonic schemes produce significant AD. When a finer grid is used, the AD based on the Jameson, Schmidt and Turkel formula (Int. J. Comput. Fluid Dyn. 1995; 4:171–218) drops dramatically. The new high‐order ELED schemes exhibit comparable accuracy to other high‐order monotonic schemes and introduce less diffusion in the unsteady environmental flow simulations. The current results of LES show that the energy transfer inherent in the monotonic convection schemes may contribute non‐negligibly to the turbulence. It is found that a dynamic‐mixed model is able to compensate the effect of the convection schemes and provides correct flow behaviors in reality. Copyright © 2008 John Wiley & Sons, Ltd.

On the theory of advective effects on biological dynamics in the sea. III. The role of turbulence in biological–physical interactions

A nonlinear model for biological and physical dynamical interactions in a laminar upwelling flow field in parts I and II of this study is extended to turbulent flow. In the previous studies, a prescription for obtaining quadrature solutions to the fundamental biodynamical equations was developed. In this study, we use a probability density function approach on these solutions to obtain statistics of the biodynamical state variables and their self-interaction for the case of turbulent advection. To illustrate the theory, a simple nutrient ( N ), phytoplankton ( P ) problem is considered, that of upwelling into a surface turbulent layer. Biological interaction is modelled as bilinear, representing the uptake of N by P in a uniform light euphotic zone. A random walk model is used to obtain the appropriate probability density function for the advective turbulent field. The mean quantities, , , as well as the biological interaction term are calculated. The term has two contributions, , and the turbulence-induced interaction term, . It is shown that the often neglected turbulence-induced coupling term is of the order and opposite in sign. This results in, over a wide range of Peclet numbers, the mean interaction term being significantly smaller than either of its constituent terms, and .

A numerical study on the evolution and structure of a stress-driven free-surface turbulent shear flow

Turbulent shear flow beneath a flat free surface driven by a surface stress is simulated numerically to gain a better understanding of the hydrodynamic processes governing the scalar transfer across the air-water interface. The simulation is posed to mimic the subsequent development of a wind-driven shear layer as in a previous experiment except that the initiation of the surface waves is inhibited, thus focusing on the boundary effect of the stress-imposed surface on the underlying turbulent flow and vice versa. Despite the idealizations inherent in conducting the simulation, the computed flow exhibits the major surface features, qualitatively similar to those that appear in the laboratory and field experiments. Two distinct surface signatures, namely elongated high-speed streaks and localized low-speed spots, are observed in the simulated flow. Including temperature as a passive tracer and describing an upward heat flux at the surface, we obtain high-speed streaks that are colder and low-speed spots that are warmer than the surrounding regions. The high-speed streaks, arranged with somewhat equal cross-spacing of centimetres scale, are formed by an array of streamwise jets within the viscous sublayer immediately next to the surface. Beneath the streaks, counter-rotating streamwise vortex pairs are observed among other prevailing elongated vortices. However, they are significantly shorter in length and more irregular than their corresponding high-speed streaks at the surface. Accompanying the more organized high-speed streaks, localized regions of low streamwise velocity emerge randomly on the surface. These low-speed spots are attributed to strong upwelling flows which disrupt the viscous sublayer and also bring up the submerged fluids of low streamwise velocity. The occasional interruptions of the streamwise high-speed jets by the upwelling flows account for bifurcation or dislocation of the surface streaks. Statistics of the turbulence are presented and their implications for the formation of the flow structures are discussed.