Showing papers on "Secondary circulation published in 2018"

Solar and volcanic forcing of North Atlantic climate inferred from a process-based reconstruction

Abstract: . The effect of external forcings on atmospheric circulation is debated. Due to the short observational period, the analysis of the role of external forcings is hampered, making it difficult to assess the sensitivity of atmospheric circulation to external forcings, as well as persistence of the effects. In observations, the average response to tropical volcanic eruptions is a positive North Atlantic Oscillation (NAO) during the following winter. However, past major tropical eruptions exceeding the magnitude of eruptions during the instrumental era could have had more lasting effects. Decadal NAO variability has been suggested to follow the 11-year solar cycle, and linkages have been made between grand solar minima and negative NAO. However, the solar link to NAO found by modeling studies is not unequivocally supported by reconstructions, and is not consistently present in observations for the 20th century. Here we present a reconstruction of atmospheric winter circulation for the North Atlantic region covering the period 1241–1970 CE. Based on seasonally resolved Greenland ice core records and a 1200-year-long simulation with an isotope-enabled climate model, we reconstruct sea level pressure and temperature by matching the spatiotemporal variability in the modeled isotopic composition to that of the ice cores. This method allows us to capture the primary (NAO) and secondary mode (Eastern Atlantic Pattern) of atmospheric circulation in the North Atlantic region, while, contrary to previous reconstructions, preserving the amplitude of observed year-to-year atmospheric variability. Our results show five winters of positive NAO on average following major tropical volcanic eruptions, which is more persistent than previously suggested. In response to decadal minima of solar activity we find a high-pressure anomaly over northern Europe, while a reinforced opposite response in pressure emerges with a 5-year time lag. On centennial timescales we observe a similar response of circulation as for the 5-year time-lagged response, with a high-pressure anomaly across North America and south of Greenland. This response to solar forcing is correlated to the second mode of atmospheric circulation, the Eastern Atlantic Pattern. The response could be due to an increase in blocking frequency, possibly linked to a weakening of the subpolar gyre. The long-term anomalies of temperature during solar minima shows cooling across Greenland, Iceland and western Europe, resembling the cooling pattern during the Little Ice Age (1450–1850 CE). While our results show significant correlation between solar forcing and the secondary circulation pattern on decadal ( r=0.29 , p ) and centennial timescales ( r=0.6 , p ), we find no consistent relationship between solar forcing and NAO. We conclude that solar and volcanic forcing impacts different modes of our reconstructed atmospheric circulation, which can aid in separating the regional effects of forcings and understanding the underlying mechanisms.

Secondary Circulation Asymmetry in a Meandering, Partially Stratified Estuary

Abstract: Numerical model experiments are used to study the effects of multiple channel bends on estuarine dynamics and, in particular, on secondary flows. These effects are demonstrated by comparing experiments with two different idealized trumpet-shaped estuaries, one straight and another one with a 8 km meandering section in the middle of the estuary. Meanders complicate the flow field by introducing secondary processes. For instance, meanders increase turbulence and associated mixing locally within the water column, as well as outside the meandering portion. Furthermore, meanders transform up to 30% of the along-channel momentum into secondary circulation. Production of turbulence and secondary currents is different at flood and ebb tidal phases. At flood, meanders lead to unstable stratification and increased turbulence. At ebb, the flow develops a helical pattern and adjusts to the channel curvature with minimal decrease in density stability. The secondary circulation asymmetry is caused by an interplay between the across-channel baroclinic pressure gradient force and the centrifugal force. During ebb both forces enhance each other, whereas they oppose during flood. As a consequence of this interaction between baroclinic forcing and curving morphology, ebb flows and horizontal buoyancy fluxes increase relative to flood. The enhanced ebb dominance shifts a density front toward the mouth of the estuary, thus reducing salt intrusion.

Ageostrophic Secondary Circulation at a Submesoscale Front and the Formation of Gravity Currents

Abstract: Large-eddy simulations are performed to investigate the development of the ageostrophic secondary circulation (ASC) and associated transport in a submesoscale front. Based on the observatio...

Pattern of vertical velocity in the Lofoten vortex (the Norwegian Sea)

Abstract: Mean radial distributions of various dynamic characteristics of the permanently existing anticyclonic Lofoten vortex (LV) in the Norwegian Sea are obtained from an eddy-permitting regional hydrodynamic MIT general circulation model. It is shown that the model adequately reproduces the observed 3D thermohaline and dynamic structure of the vortex. The obtained radial distribution of the mean vertical velocity is found to form a complex structure: with the upward fluxes along the axis in and above the anticyclonically rotating LV core, compensated by the downward fluxes in the vortex skirt. These vertical motions maintain the vortex potential energy anomaly against dissipation. This secondary circulation is generated by the centrifugal force and, to a lesser extent, by the horizontal dispersion of the vortex energy, both intensified towards the sea surface. Below the vortex core, the maximum downward vertical velocity converges towards the vortex axis with depth. At these depth levels, the secondary circulation is forced by Ekman divergence in the bottom mixed layer. The theory of columnar vortices with helical structure, applied to the LV, relate the radial profiles of the vertical velocity with those of the horizontal circulation. The theoretically predicted the radial patterns of the mean vertical velocity in the LV were close to those, obtained from the primitive equation ocean model, when approximating the radial patterns of the azimuthal velocity with the Rayleigh profile.

The Emergence of Shallow Easterly Jets within QBO Westerlies

Abstract: A configuration of an idealized general circulation model has been obtained in which a deep, stratospheric, equatorial, westerly jet is established that is spontaneously and quasi-periodically disrupted by shallow easterly jets. Similar to the disruption of the quasi-biennial oscillation (QBO) observed in early 2016, meridional fluxes of wave activity are found to play a central role. The possible relevance of two feedback mechanisms to these disruptions is considered. The first involves the secondary circulation produced in the shear zones on the upper and lower flanks of the easterly jet. This is found to play a role in maintaining the aspect ratio of the emerging easterly jet. The second involves the organization of the eddy fluxes by the mean flow: the presence of a weak easterly anomaly within a tall, tropical, westerly jet is demonstrated to produce enhanced and highly focused wave activity fluxes that reinforce and strengthen the easterly anomalies. The eddies appear to be organized by the ...

Sensitivity of Tropical Cyclone Intensity and Structure to Planetary Boundary Layer Parameterization

Abstract: The advanced weather research and forecasting model is used to investigate the influence of planetary boundary layer (PBL) processes on intensity and structure of the storm Phailin (2013). Five simulations are conducted with five PBL schemes; Yonsei University (YSU), Mellor−Yamada−Nakanishi−Niino order2.5 (MYNN2), Assymetric Convective Model2 (ACM2), Medium Range Forecast (MRF), and Bougeault and Lacarrere (BouLac). The simulation duration includes the pre − intensification and rapid intensification phase of Phailin before landfall. Results indicate that during the pre − intensification phase, storm’s track and intensity are not much sensitivity to PBL but structural changes are noted. A significant sensitivity of track and intensity to PBL parameterizations are found during rapid intensification phase. BouLac and MRF produced two extremes with 39 hPa intense and 16 km compact storm for BouLac than MRF. Further analysis reveals an outward movement of air parcel just above the boundary layer which causes spin-down for YSU and MYNN2. BouLac is associated with stronger eddy diffusivity and moisture fluxes within the boundary layer and stronger cyclonic vorticity just above the boundary layer than other experiments. Stronger cyclonic vorticity above the boundary layer in BouLac favors intense updraft, facilitating more moisture transport from the boundary layer to upper layers aiding stronger secondary circulation and robustly intensifying the storm. A relatively deeper and drier inflow layer associated with weaker cyclonic vorticity just above the boundary layer reduces the moisture transport and weaken the secondary circulation for MRF than others.

A numerical modelling investigation of the role of diabatic heating and cooling in the development of a mid-level vortex prior to tropical cyclogenesis – Part 1: The response to stratiform components of diabatic forcing

Abstract: . Mid-tropospheric mesoscale convective vortices have been often observed to precede tropical cyclogenesis. Moreover, recent cloud-resolving numerical modelling studies that are initialized with a weak cyclonic mid-tropospheric vortex sometimes show a considerable intensification of the mid-level circulation prior to the development of the strong cyclonic surface winds that characterize tropical cyclogenesis. The objective of this two-part study is to determine the processes that lead to the development of a prominent mid-level vortex during a simulation of the transformation of a tropical disturbance into a tropical depression, in particular the role of diabatic heating and cooling. For simplicity simulations are initialized from a quiescent environment. In this first part, results of the numerical simulation are described and the response to stratiform components of the diabatic forcing is investigated. In the second part, the contribution of diabatic heating in convective cells to the development of the mid-level vortex is examined. Results show that after a period of intense convective activity, merging of anvils from numerous cells creates an expansive stratiform ice region in the upper troposphere, and at its base a mid-level inflow starts to develop. Subsequently conservation of angular momentum leads to strengthening of the mid-level circulation. A 12 h period of mid-level vortex intensification is examined during which the mid-level tangential winds become stronger than those at the surface. The main method employed to determine the role of diabatic forcing in causing the mid-level inflow is to diagnose it from the full physics simulation and then impose it in a simulation with hydrometeors removed and the microphysics scheme turned off. Removal of hydrometeors is achieved primarily through artificially increasing their fall speeds 3 h prior to the 12 h period. This results in a state that is in approximate gradient wind balance, with only a weak secondary circulation. Then, estimates of various components of the diabatic forcing are imposed as source terms in the thermodynamic equation in order to examine the circulations that they independently induce. Sublimation cooling at the base of the stratiform ice region is shown to be the main factor responsible for causing the strong mid-level vortex to develop, with smaller contributions from stratiform heating aloft and low-level melting and evaporation. This contrasts with the findings of previous studies of mid-latitude vortices that indicate sublimation plays a relatively minor role. An unanticipated result is that the central cool region that develops near the melting level is to a large degree due to compensating adiabatic ascent in response to descent driven by diabatic cooling adjacent to the central region, rather than in situ diabatic cooling. The mid-level inflow estimated from stratiform processes is notably weaker than for the full physics simulation, suggesting a moderate contribution from diabatic forcing in convective cells.

Trajectory of a jet in crossflow in a channel bend

Abstract: Mixing in rivers is an important issue with many applications in water quality and water resource management. Mixing of effluents with ambient river water is especially important, particularly in river bends, where secondary circulation complicates the mixing process. By comparing measured trajectories from dye tests to velocimetry data measured with an acoustic Doppler velocimeter, this paper models the trajectory of a jet in an open channel bend using a modified formula for a jet trajectory in a straight crossflow. The original formula is shown to be insufficient for modeling the trajectory in the bend. Modifications are proposed using the position of the centre of the main secondary circulation cell to account for the bend effects. In the absence of secondary circulation, the modified formula reduces to the original formula. Once the secondary circulation has developed, the proposed formula is shown to have better residuals, lower root mean squared error, and higher $$R^2$$ than the original formula.

Hydrodynamic Drivers of Juvenile-Salmon Out-Migration in the Sacramento River: Secondary Circulation

Abstract: The entrances to the two lowest-survival migration routes for juvenile Chinook salmon in the tidal Sacramento River are located in the outside of a river bend where secondary circulation oc...

Advanced 3D Mapping of Hydrodynamic Parameters for the Analysis of Complex Flow Motions in a Submerged Bedrock Canyon of the Tocantins River, Brazil

Abstract: Validation and scaling of sophisticated physical and numerical fluvial hydraulic models to real field conditions are limited by temporal and spatial constraints of field measurement technologies. These limitations increase when analyzing hydraulic properties of complex river forms such as submerged bedrock canyons. The analysis of flow under these conditions has demonstrated non-linear behavior, strong secondary circulation and a high level of turbulence. The objective of this paper is to obtain and analyze turbulence and secondary current information for flow analysis, by evaluating the spatial distribution of bed shear stress and eddy viscosity with acoustic Doppler current profiler (ADCP) measurements. The real field-scale case study was the Lourenco Rock Canyon in Tocantins River, Brazil. A total of 10,531 velocity profiles were measured with an ADCP adapted to a moving boat. The data were post-processed by decomposing the velocity data measurements and calculating the standard deviations. Three different methods are compared for bed shear stress: total kinetic energy (TKE), law of the wall and the depth–slope product. The eddy viscosity was estimated with the Boussinesq approach. The initial data processing confirmed turbulence and flow velocity characteristics described in similar literature findings. The results for the bed shear stress for the canyon area were 8 times greater than the outside and for eddy viscosity, 10 times higher. For the surroundings area, the eddy viscosity showed reasonable results with values around the regular 1.0 m2/s used for rivers. It is concluded that turbulent values can be calculated to better represent physical processes with the intention to improve hydrodynamic numerical models calibrations.

Distribution of primary and secondary currents in sine-generated bends

Abstract: The secondary circulation in a meandering channel redistributes the velocity over the bend. However, the shift of primary flow by secondary currents is not quantitatively understood, due to the difficulty in isolating the role of curvature-driven secondary flow from that of topography-driven secondary flow in bed-deformed meanders. The influences of curvaturedriven and topography-driven secondary currents on the redistribution of primary flow in sine-generated meandering channels were examined by CCHE2D. The model is calibrated using data measured in two sets of laboratory experiments including flat-bed flow and mobile-bed flow. Analysis indicated that topography-induced current mainly contributes to the redistribution of primary flow from inner to outer bank in the curved channels, rather than the secondary flow driven by curvature. Keywords: primary flow, secondary flow, meander, numerical analysis

Cold start apparatus for hydrogen fuel cell system

Abstract: The invention provides a cold start apparatus for a hydrogen fuel cell system. The hydrogen fuel cell system includes a fuel cell pile, a secondary circulation circuit and a water supply path; the secondary circulation circuit includes a liquid coolant circulation pipeline and a secondary circulation heating device arranged on the liquid coolant circulation pipeline and used for heating a liquid coolant in the liquid coolant circulation pipeline; and the water supply path includes an expansion water tank for supplementing the liquid coolant to the liquid coolant circulation pipeline and a water supply path heating device arranged on the pipeline between the expansion water tank and the liquid coolant circulation pipeline and used for heating the supplied liquid coolant. In the cold start apparatus, the water supply path heating device heats the supplemented liquid coolant supplemented to the liquid coolant circulation pipeline, and the secondary circulation heating device heats the liquid coolant in the liquid coolant circulation pipeline. Two heating devices work together in the invention, so the heating process of the secondary circulation circuit the fuel cell pile is accelerated, and the cold start time is shortened.

Flooded ice making system and control method thereof

Abstract: The invention relates to the technical field of ice manufacturing, and discloses a flooded ice making system and a control method thereof The flooded ice making system comprises a refrigerant circulation loop system, a secondary refrigerant circulation loop system and a heat exchanger which is shared by the refrigerant circulation loop system and secondary refrigerant circulation loop system; theheat exchanger is used for conducting heat exchange between a refrigerant in the refrigerant circulation loop system and a secondary refrigerant in the secondary refrigerant circulation loop system;and the secondary refrigerant circulation loop system comprises an ice making tank and a circulating pump, wherein a channel for allowing the refrigerant to flow is formed inside the groove wall of the ice making groove, the circulating pump is used for providing power for circulating flowing of the secondary refrigerant in the secondary circulation loop system, and the secondary refrigerant flowsthrough the channel inside the groove wall of the ice making groove during circulation flowing of the secondary refrigerant in the secondary refrigerant circulation loop system According to the flooded ice making system and the control method thereof, the advantages of brine ice making and direct cooling type ice making are integrated, so that prepared block ice is more uniform in density and not prone to damage in transportation and use processes

Secondary circulation in river junctions even at very low flow momentum ratios : The legacy effects of point bar formation

Abstract: River confluences remain a challenging subject because of their 3D geometry which leads to a complex, three-dimensional mean and turbulent velocity processes. Since secondary circulation plays an important role in flow hydrodynamics and the development of bank erosion, bed scour and bar formation, several numerical models, laboratory experiments and field studies have paid special attention to this subject. However, for small tributaries, associated with low flow momentum ratio, there has been a lack of understanding as to how momentum ratio and sediment supply regime can control the formation of secondary circulation. Of interest here is the possibility that sediment delivery and tributary bar formation in the main channel leaves a morphological legacy that is capable of inducing significant secondary circulation at the junction even if the tributary has a low flow momentum. Laboratory experiments have tended to study the equilibrium morphology formed by the interaction between an imposed flow driven sediment supply regime, rather than the effects of that morphology when tributary flows are lower and there is no sediment supply. This paper tests the hypothesis that even where the flow momentum of a tributary is very low, it is still possible to identify significant secondary circulation because of the legacy of morphological changes associated with higher momentum tributary flow. Secondary circulation at river confluences is thought to be dominated by streamwise-oriented vortical cells produced by the convergence of the surface flow through the center of the main channel, with downwelling motion in the zone of maximum flow convergence and upwelling motion at the channel margins. Considerable agreement exists among numerical, field and laboratory experiments that secondary circulation is an important element in developing bank erosion, bed scour, bar formation and boundary shear. Such studies have focused on situations where the momentum ratio is close to one and there have been fewer investigations of confluences where the momentum ratio is much less than one, such as Alpine tributaries, which produce confluences characterized by low momentum ratio and a high sediment delivery ratio during extreme events. To close this gap, intensive field data collection was used to investigate three upper Rhone river confluences in Switzerland, using an aDcp. Results show that high rates of tributary sediment delivery into the main channel during extreme events, lead to tributary mouth bar formation and associated bed discordance, which creates a two-layer flow. It results in the penetration of the tributary flow into the upper part of the main channel water column. Due to this two layer flow, the mixing layer is shifted toward the opposite bank and is associated with a large recirculation zone as well as a pronounced scour hole at this bank and an inner bank attached bar. Through time, the main channel may erode the inner bank attached bar, reducing bed discordance, and causing the mixing layer to form within the center part of the downstream channel. Analyses of these results confirms that even small tributaries can significantly influence confluence secondary circulation due to the legacy effects of sediment delivery.

Microfluidic paper chip for chemiluminescence time resolution and detection as well as preparation method and application thereof

Abstract: The invention discloses a microfluidic paper chip for chemiluminescence time resolution and detection as well as a preparation method and application thereof. The paper chip takes filter paper as a substrate; the substrate is provided with a sample feeding hole, a circulation channel and a detection hole; the circulation channel comprises a main circulation channel and a plurality of secondary circulation channels; one end of the main circulation channel is connected with the sample feeding hole and the other end of the main circulation channel is connected with the detection hole; deflectionflow splitting is carried out in the middle of the main circulation channel at a certain angle to form a second-grade circulation channel; one end of the second-grade circulation channel is connectedwith the middle of the main circulation channel and the other end of the second-grade circulation channel is connected with the detection hole, and so on; chitosan, specific oxidase and a chemiluminescence catalyst are sequentially filled into the detection hole. According to the microfluidic paper chip disclosed by the invention, a microfluidic flow path is designed so that time resolution and detection of chemiluminescence is realized; the microfluidic paper chip does not need to be modified, has a small area and can be used for simultaneously detecting glucose, uric acid, cholesterol and the like.

Calculation Method of Secondary Circulation Current for MMC-UPFC Based on Simulated Annealing Optimization

Abstract: The modular multilevel converter (MMC) based unified power flow controller (UPFC) is the newest flexible AC transmission system technology at present, which can realize the adjustment of active power and reactive power with voltage regulation, series compensation and shift equalization. With secondary circulation in the MMC bridge arm during UPFC running, a method for calculating secondary circulation current of modular multilevel converters is proposed, which establishes a calculation method of secondary circulation current by simplifying averaging control to be double proportional regulator, based on capacitance volt-ampere characteristic and equivalent switching function. The topological structure of MMC-UPFC is analyzed, and the specific process of the above model is displayed based on understanding the reason of producing secondary circulation current. In order to improve calculation accuracy, correction model of secondary circulation current is established, and the optimal parameters are obtained by using simulated annealing based on least squares. An important conclusion that secondary circulation current is associated with K V1 and K I1 , and it increase with the increase of K V1 and decrease with the increase of K I1, is obtained by analyzing the above model, which provides a theoretical basis for suppressing secondary circulation current and selecting power device for UPFC. Finally PSCAD simulation results show that the method can accurately calculate secondary circulation current, which verifies its validity and feasibility.

Heat exchange system for cardiac surgery requiring cardioplegic arrest

Abstract: A heat exchange system for a cardiac surgery requiring cardioplegic arrest, comprising: an ice water tank (15), a primary circulation water tank (11), and a secondary circulation water tank (16) Side walls of the primary circulation water tank (11) and the secondary circulation water tank (16) are each provided with an overflow orifice Each overflow orifice is connected to the ice water tank (15) through a circulation pipe (14) A first roller pump (12) is mounted on a first hose (13) A second roller pump (18) is mounted on a second hose (17) Water in the ice water tank (15) is pumped into the primary circulation water tank (11) and the secondary circulation water tank (16) through the first hose (13) and the second hose (17), respectively The primary circulation water tank (11) is mounted on a first loop (5) and a second loop (6) The secondary circulation water tank (16) is mounted on a third loop (1) According to the heat exchange system, a conventional refrigeration device, ie, a compressor, of a heat exchange water tank is replaced with the ice water tank (15), so that the cold source is high in safety and low in costs All parts, in contact with water, of the ice water tank (15), the primary circulation water tank (11), the secondary circulation water tank (16), and conduits of the first to third loops (5, 6, 1) become disposable medical articles, so that breeding of bacteria is fundamentally avoided

Towards the development of analytical tornado-like models

Abstract: In this paper, we analyze and develop modifications to two well-known analytical tornado-like models. First, the vortex sink with axial flow (VSAF) model is compared with results obtained from the vortex generator. It is shown that in a cylindrical coordinate system a secondary circulation at the tornado corner region can be responsible for structural discrepancies and vorticity distribution. A tangential velocity field is implemented in the VSAF model to predict an experimental distribution. A novel model is proposed to capture the secondary circulation. A modified model of the VSAF is introduced with piecewise-constant coefficients that enables to predict a shape of a tornado vortex. Further scaling analysis confirms dimensional similarity of tornadoes. The VSAF model is also modified using a Boussinesq-like approximation to take into account the effect of a buoyancy force within a tornado. Finally, the two-fluid model by Yih is used to show the influence of the atmospheric stratification on the radius of tornado.In this paper, we analyze and develop modifications to two well-known analytical tornado-like models. First, the vortex sink with axial flow (VSAF) model is compared with results obtained from the vortex generator. It is shown that in a cylindrical coordinate system a secondary circulation at the tornado corner region can be responsible for structural discrepancies and vorticity distribution. A tangential velocity field is implemented in the VSAF model to predict an experimental distribution. A novel model is proposed to capture the secondary circulation. A modified model of the VSAF is introduced with piecewise-constant coefficients that enables to predict a shape of a tornado vortex. Further scaling analysis confirms dimensional similarity of tornadoes. The VSAF model is also modified using a Boussinesq-like approximation to take into account the effect of a buoyancy force within a tornado. Finally, the two-fluid model by Yih is used to show the influence of the atmospheric stratification on the radius ...

Application of a methodological advance to calculate 3D flow fields in river channel junctions

Abstract: Acoustic Doppler current profiler (aDcp) vessel-mounted flow measurements are now commonly used to quantify discharge and velocity in shallow water fluvial environments. Here, we consider the benefits of improving secondary circulation estimates in river confluences through the manner in which moving vessel aDcp data are handled. Secondary circulation in Alpine river confluences involves a rotational movement of flow, orthogonal to the main flow. It results in a spatial and temporal variation of fluid motion and a relatively high level of morphological change. It is well established that measuring such flows requires repeated surveys at the same cross-section. However, less attention has been given to how to process these data. Most techniques used to process vessel-mounted aDcp data use the assumption of homogeneity between the measured radial components of velocity. This assumption can be problematic where acoustic beams diverge with distance from the aDcp probe. Divergence between the beams increases the volume in which the flow must be assumed homogeneous. In the presence of secondary circulation cells, and where there are strong rates of shear in the flow, the homogeneity assumption may not apply, especially deeper in the water column. To reduce the volume assumed homogeneous, a method proposed by Vermeulen et al. (2014) has been applied for the first application to Sontek Riversurveyor data, collected in medium sized (∼60 m wide) gravel-bed river confluences. The method combines radial velocities in a predefined mesh, based on their position. In this paper, we present the results of this method and compare them with more conventional data processing approaches. The proposed method suggests an improvement in secondary flow cell representation, comparing to more conventional methods whilst also confirming that repeated transects are required to achieve meaningful secondary flow and turbulence estimation. Use of this method resolves two counter-rotating cells in the confluence zone more clearly, with downward velocity in the channel centre. This pattern helps to explain the development of confluence scour holes.