Baroclinic Effects Research Articles

The contribution of hurricane remote ocean forcing to storm surge along the Southeastern U.S. coast

The dynamics controlling the spatial and temporal expressions of storm surges over the coastal wetlands and communities of the South Atlantic Bight (SAB) is complex and not well understood. Leveraging a newly developed high-density hyper-local network of water level sensors in the North Georgia coast, we implement and test an unstructured numerical coastal ocean model (up to 10-m horizontal resolution) that can resolve and diagnose the storm-induced sea-level rise during the two Hurricanes Matthew (2016) and Dorian (2019) that have shore-parallel tracks. Using a set of model sensitivity analyses we decompose the drivers of the storm surge into a component that is associated with direct surface forcing by the hurricanes over the targeted area (e.g., local atmospheric wind and pressure condition in the nested model domain) and remote ocean forcing that is connected to hurricane-induced sea level anomalies and baroclinic effect through the open boundary of the model. For both hurricanes, we find that local surface atmospheric forcing leads to a uniform alongshore response in water level along the entire North Georgia coast with amplitudes that are proportional to how close to shore are the hurricane tracks (e.g., stronger in Matthew and weaker in Dorian). However, the alongshore structure and location of maximum storm surge are determined entirely by the arrival timing of ocean remote forcing. In the case of Matthew, the remote forcing arrives within 2 h of the direct passage of the hurricane over North Georgia and drives peak surges in the northern region of the domain (e.g., the City of Savannah and Tybee Island). In contrast, during Dorian, there is a 14-h difference between the remote and local forcing, and maximum storm surges are found in the southern region around Sapelo Island. We estimate that if local and remote forcing were to be simultaneous, the peak storm surge and the water level would be amplified by up to 30% for Matthew and 50% for Dorian. While this sensitivity analysis only includes two hurricanes and is focused on a case study around North Georgia, it is clear that predicting and understanding the regional expressions and timing of the hurricanes' coastal-wide remote ocean forcing in the SAB is important for estimating worst-case scenarios for coastal communities as they face emergency and management decisions. • Newly developed high-resolution observation and model reveal the hidden role of extreme water level drivers. • The timing and structure of the remote ocean forcing exerts a dominant control on the regional distribution of storm surge. • The alignment in the peak timing of the local and remote forcing can increase the peak surge level by ∼50%.

Baroclinic Effect on Inner-Port Circulation in a Macro-Tidal Estuary: A Case Study of Incheon North Port, Korea

This study investigated the flow patterns and affecting factors in the North Port of Incheon, South Korea, to understand the inner-port circulation characteristics by applying an unstructured grid finite volume community ocean model (FVCOM) together with a field survey. The FVCOM considered the tide, river discharge, surface winds, and atmospheric pressure with the highest resolution of 50 m around the port. The model results were validated with current velocity and salinity observations. In the main channel, the difference in salinity between the sea surface and bottom increased during the neap tide due to weakened tidal currents, thus strengthening the stratification. However, at the inner port, strong stratification was observed during spring tide as the near-surface freshwater from upstream of the estuary moved relatively farther south of the port than that during the neap tide. Freshwater flowed into the port during the flood current and was trapped in the semi-closed geometry of the inner port. The horizontal salinity gradient between the trapped fresher water in the port and saltier water in the main channel increased during the flood current. As a result, density-driven circulations associated with near-bed currents towards the port were maintained for more than 3 h after high tide. This result implies that the baroclinic effect, mainly due to the salinity gradient in the North Port, could significantly affect residual circulation at the inner harbor in a macro-tidal environment and the mass transport mechanism, such as sediment transport. In general, such enhanced baroclinic effects due to salinity and tides are not limited to the North Port and can have greater effects on inner-port circulation in other macro-tidal harbors that suffer from severe sedimentation problems.

A High‐Resolution 3‐D Circulation Model in a Complex Archipelago on the Coastal Scotian Shelf

AbstractA high‐resolution coastal Finite‐Volume Community Ocean Model (FVCOM) has been configured to simulate the water circulation in the Eastern Shore Islands (ESI) archipelago on the coastal Scotian Shelf. Circulation in this area is characterized by complex interactions between the irregular coastline, a dense archipelago, tides, wind, and subtidal currents. Model outputs show close agreement with the tides, subtidal current, and hydrographic observations. Two circulation regimes within the study area were identified “inshore” and “offshore” of the 60‐m isobath. The balance between pressure gradient and Coriolis effect controls the dominant southwestward current (Nova Scotia Current) in the offshore. Rotary spectra and numerical experiments showed that the tidal current was an important component to the circulation in the inshore. Within the study area there are several clockwise and counter‐clockwise gyres related to the Coriolis effect, surface wind stress, pressure gradient, and the interaction between the irregular topography and current. The combined effects of topography, wind, tide, and Nova Scotia Current regulate the inshore circulation. The topographic features complex the cross‐shore transport. The increased bathymetric slope and strengthened relative vorticity contribute to the inshore cross‐shore current. Here, we found that the joint effect of baroclinicity and bottom relief drives the cross‐isobath transport. The ability of our model to resolve complex coastal circulation patterns provides an important basis on which to better understand how physical oceanography influences the unique ecological processes of this coastal archipelago and ultimately will help to evaluate dispersal and connectivity of coastal species in this area.

The vertical shear instability in poorly ionized, magnetized protoplanetary discs

ABSTRACT Protoplanetary discs should exhibit a weak vertical variation in their rotation profiles. Typically, this ‘vertical shear’ issues from a baroclinic effect driven by the central star’s radiation field, but it might also arise during the launching of a magnetocentrifugal wind. As a consequence, protoplanetary discs are subject to a hydrodynamical instability, the ‘vertical shear instability’ (VSI), whose breakdown into turbulence could transport a moderate amount of angular momentum and facilitate, or interfere with, the process of planet formation. Magnetic fields may suppress the VSI, however, either directly via magnetic tension or indirectly through magnetorotational turbulence. On the other hand, protoplanetary discs exhibit notoriously low ionization fractions, and non-ideal effects, if sufficiently dominant, may come to the VSI’s rescue. In this paper, we develop a local linear theory that explores how non-ideal magnetohydrodynamics influences the VSI, while exciting additional diffusive shear instabilities. We derive a set of analytical criteria that establish when the VSI prevails, and then show how it can be applied to a representative global model of a protoplanetary disc. Our calculations suggest that within ∼10 au the VSI should have little trouble emerging in the main body of the disc, but beyond that, and in the upper regions of the disc, its onset depends sensitively on the size of the preponderant dust grains.

Improvement of Accuracy of Temperature Measurement Method around Discharge Field Based on Shock Wave Velocity

The shock wave mitigation technique by using the electromagnetic energy is one of the promising methods. The baroclinic effect has the important role in this method. This effect has the possibility to apply to the generation the vortex phenomena in the compressible flow field. Therefore, the comprehension of the effect is inevitable for exploring the further vortex control- ing application in the supersonic flow field. Until now, the temperature distribution measurement has been conducted in order to the comprehension of the baroclinic effect by the interaction between the shock wave and the temperature modulated field due to the electric discharge energy, based on the shock wave velocity. However, the conventional method depends on the Mach number at the down stream region of the discharged field. This Mach number has a large impact on the temperature determination.Based on this background, in this study, we tried to improve the temperature determination method. As a result, the determination accuracy improved.

Comprehension of shock wave modulation phenomena due to electromagnetic energy by using three-dimensional numerical simulation

Recently, the development of the supersonic transportation system is desired. For the development, the reduction of the wave drag and sonic boom is crucial problem. For the solution of the problem, the shock wave modulation technique by using the electro-magnetic power is one of the promising methods. This technique makes use of the baroclinic effect. For the application of this method to the other method of vortex generation method in the supersonic flow field, such as the scram jet engine inlet, the comprehension of the baroclinic effect is inevitable. In this study, for the comprehension of the baroclinic effect due to the electro-magnetic power, the three-dimensional numerical analysis code to simulate the interaction between the shock wave and the temperature modulated field was developed. The results were verified by comparing with the experimental visualization data.

Capturing the baroclinic effect in non-Boussinesq gravity currents

Direct numerical simulations of two-dimensional gravity currents with small and medium density variations are performed using different non-Boussinesq buoyancy approximations. Taking the full low-Mach-number approximation as the reference, the accuracy of several buoyancy terms are examined. It is found that all considered buoyancy terms performed well in the cases with small density variation. In the cases with medium density variation, the classical gravitational Boussinesq’s buoyancy term showed the lack of accuracy, and a simple correction did not make any improvement. In contrast, the recently introduced second-order buoyancy term showed a significantly higher accuracy. The present results and our previous derivations indicate that simple algebraic buoyancy approximations extended from the Boussinesq’s gravitational buoyancy are unlikely to achieve an accuracy beyond first order. Instead, it seems necessary to solve at least one extra Poisson equation for buoyancy terms to capture the higher-order baroclinic effect. An approximate analysis is also provided to show the leading term of the non-Boussinesq effect corresponding to gravity.

Modelling study on estuarine circulation and its effect on the turbidity maximum zone in the Yalu River Estuary, China

The along-channel, lateral estuarine circulation and the related baroclinic effects on the turbidity maximum zone (TMZ) of the Yalu River Estuary (YRE) were explored in our study based on the Finite Volume Coastal Ocean Model (FVCOM). The results show that along-channel estuarine flow in YRE had a two-layer-structure with seaward flow in the surface layer and landward flow in the bottom layer. The along-channel bottom landward flow can transport sediment to the upper estuary and influence the location of the TMZ, which was mainly induced by asymmetric tidal mixing. Then, for the variation in cross-channel circulation (lateral circulation), the TMZ moved westwards after reclamation, due to the decreased eastward residual flow resulting from weakened eastward barotropic and baroclinic pressure gradient forcing. This study qualitatively demonstrated the baroclinic effects and human-induced impacts on estuarine subtidal flow and sedimentation. Although site-specific, these findings have wider implications for sediment transport in other medium-scale estuaries.

On eddy transport in the ocean. Part II: The advection tensor

This study considers the isopycnal eddy transport of mass and passive tracers in eddy-resolving double-gyre quasigeostrophic oceanic circulation. Here we focus on advective transport, whereas a companion paper focuses on eddy-induced diffusive tracer transport. To work towards parameterising eddy tracer transport we quantify the eddy tracer flux using a transport tensor with eddies defined using a spatial filter, which leads to results distinct from those obtained via a temporal Reynolds eddy decomposition. The advection tensor is the antisymmetric part of the transport tensor, and is so named since the associated tracer transport can be expressed as advection of the large-scale tracer field by a rotational eddy-induced velocity (EIV) u∗c with streamfunction A. The EIV u∗c is fastest (∼1 m s−1) where eddy activity is strongest, e.g., in the upper layer, near the eastward jet and western boundary current. Our results suggest that a stochastic closure for the eddy transport would be most suitable since A exhibits a probabilistic distribution when conditioned on, for example, the large-scale relative vorticity. Consistent with closures in ocean circulation models, we quantify eddy mass (isopycnal layer thickness) fluxes as eddy-induced advection by the thickness EIV u∗h. The divergent part of u∗h – the only part relevant for mass transport in the quasigeostrophic limit – tends to be oriented down the thickness gradient suggesting it quantifies some baroclinic eddy effects similar to those parameterised by the Gent & McWilliams (GM90) EIV. Although u∗h has some qualitative similarities to u∗c, our results suggest that eddy-induced tracer advection is driven by more than just the thickness-determined EIV and, in turn, more than just the GM90 EIV.

Period bubbling bifurcation and transition to chaotic state of convective flow on a top-open cylinder

Period bubbling bifurcation and transition to chaotic state of a convective flow on the top-open cylinder beneath heating are investigated using direct numerical simulation owing to its fundamental significance and extensive application. Numerical results show that a primary convective flow on the cylinder is axisymmetric and dominated by the effect of baroclinicity for small Rayleigh numbers. There exists a complex route of the transition to chaotic state involving a succession of bifurcations when the Rayleigh number is increased; that is, the pitchfork bifurcation occurs between Ra = 2.7 × 103 and Ra = 2.8 × 103 for which an axisymmetric convective flow is broken, and the first Hopf bifurcation occurs between Ra = 3.3 × 105 and Ra = 3.4 × 105. An interesting phenomenon is that the convective flow may change from periodic back to steady state with the increase in the Rayleigh number for 7.0 × 105 ≤ Ra ≤ 1.5 × 106, termed as a period bubbling bifurcation. The whole route of the transition to chaotic state can be described by a steady, a periodic, a steady again, a period doubling, a quasiperiodic, and a chaotic state as the Rayleigh number is increased. Further, the convective flow in the transition is discussed by employing the topologic index, the spectral analysis, the attractor, and the fractal dimension. Additionally, heat transfer is also quantified.

Observation and Analysis of Water and Salt Transports in the North Branch of the Changjiang Estuary

Gu, J.; Zhu, J., and Lyu, H., 2021. Observation and analysis of water and salt transports in the North Branch of the Changjiang Estuary. Journal of Coastal Research, 37(3), 518–527. Coconut Creek (Florida), ISSN 0749-0208.The North Branch (NB) is the first-order bifurcation of the Changjiang Estuary and is an extremely shallow water channel. The greatest characteristic of saltwater intrusion is the saltwater spillover from the NB into the South Branch (SB). A field observation was conducted in the NB from 21 to 27 February 2017 to understand the water current and salinity variation. The residual unit width water and salt fluxes, as well as their decomposed terms, were calculated to discuss the mechanisms of water and salt transport. The tidal and vertical variations in ebb and flood duration, current velocity, and salinity during the neap-middle-spring tide were presented and dynamically analyzed in detail. The direction of the Eulerian water and salt transports and tidal pumping salt transport varied by site, vertical layer, and tidal type, whereas the Stokes drift water and salt transports were landward. During neap tide, the total residual water and salt fluxes in the upper layer of the whole NB were seaward transported because of the runoff effect; the total water and salt fluxes in the lower layer were landward in the lower-middle and lower reaches of the NB, reflecting the existence of a two-layer estuarine circulation. During the moderate tide, the total residual water and salt fluxes were seaward in the whole NB except in the lower layer in the low reaches of the NB, which were landward due to the strong baroclinic effect. During the spring tide, all the total residual water and salt fluxes were landward because some flood water flowed into the SB, resulting in an imbalance between the flood and ebb water volumes in the NB. The dynamic processes of water and salt transport from the NB into the SB during spring tide were revealed by the observation and flux decomposition method.

The Alfvénic nature of chromospheric swirls

Context.Observations show that small-scale vortical plasma motions are ubiquitous in the quiet solar atmosphere. They have received increasing attention in recent years because they are a viable candidate mechanism for the heating of the outer solar atmospheric layers. However, the true nature and the origin of these swirls, and their effective role in the energy transport, are still unclear.Aims.We investigate the evolution and origin of chromospheric swirls by analyzing numerical simulations of the quiet solar atmosphere. In particular, we are interested in finding their relation with magnetic field perturbations and in the processes driving their evolution.Methods.The radiative magnetohydrodynamic code CO5BOLD is used to perform realistic numerical simulations of a small portion of the solar atmosphere, ranging from the top layers of the convection zone to the middle chromosphere. For the analysis, the swirling strength criterion and its evolution equation are applied in order to identify vortical motions and to study their dynamics. As a new criterion, we introduce the magnetic swirling strength, which allows us to recognize torsional perturbations in the magnetic field.Results.We find a strong correlation between swirling strength and magnetic swirling strength, in particular in intense magnetic flux concentrations, which suggests a tight relation between vortical motions and torsional magnetic field perturbations. Furthermore, we find that swirls propagate upward with the local Alfvén speed as unidirectional swirls driven by magnetic tension forces alone. In the photosphere and low chromosphere, the rotation of the plasma co-occurs with a twist in the upwardly directed magnetic field that is in the opposite direction of the plasma flow. All together, these are clear characteristics of torsional Alfvén waves. Yet, the Alfvén wave is not oscillatory but takes the form of a unidirectional pulse. The novelty of the present work is that these Alfvén pulses naturally emerge from realistic numerical simulations of the solar atmosphere. We also find indications of an imbalance between the hydrodynamic and magnetohydrodynamic baroclinic effects being at the origin of the swirls. At the base of the chromosphere, we find a mean net upwardly directed Poynting flux of 12.8 ± 6.5 kW m−2, which is mainly due to swirling motions. This energy flux is mostly associated with large and complex swirling structures, which we interpret as the superposition of various small-scale vortices.Conclusions.We conclude that the ubiquitous swirling events observed in numerical simulations are tightly correlated with perturbations of the magnetic field. At photospheric and chromospheric levels, they form Alfvén pulses that propagate upward and may contribute to chromospheric heating.

Improved representation of river runoff in Estimating the Circulation and Climate of the Ocean Version 4 (ECCOv4) simulations: implementation, evaluation, and impacts to coastal plume regions

Abstract. In this study, we improve the representation of global river runoff in the Estimating the Circulation and Climate of the Ocean Version 4 (ECCOv4) framework, allowing for a more realistic treatment of coastal plume dynamics. We use a suite of experiments to explore the sensitivity of coastal plume regions to runoff forcing, model grid resolution, and grid type. The results show that simulated sea surface salinity (SSS) is reduced as the model grid resolution increases. Compared to Soil Moisture Active Passive (SMAP) observations, simulated SSS is closest to SMAP when using daily, point-source runoff (DPR) and the intermediate-resolution LLC270 grid. The Willmott skill score, which quantifies agreement between models and SMAP, yields up to 0.92 for large rivers such as the Amazon. There was no major difference in SSS for tropical and temperate coastal rivers when the model grid type was changed from the ECCO v4 latitude–longitude–polar-cap grid to the ECCO2 cube–sphere grid. We also found that using DPR forcing and increasing model resolution from the coarse-resolution LLC90 grid to the intermediate-resolution LLC270 grid elevated the river plume area, volume, stabilized the stratification and shoal the mixed layer depth (MLD). Additionally, we find that the impacts of increasing model resolution from the intermediate-resolution LLC270 grid to the high-resolution LLC540 grid are regionally dependent. The Mississippi River Plume is more sensitive than other regions, possibly because the wider and shallower Texas–Louisiana shelf drives a stronger baroclinic effect, as well as relatively weak sub-grid vertical mixing and adjustment in this region. Since rivers deliver large amounts of freshwater and anthropogenic materials to coastal regions, improving the representation of river runoff in global, high-resolution models will advance studies of coastal hypoxia, carbon cycling, and regional weather and climate and will ultimately help to predict land–ocean–atmospheric feedbacks seamlessly in the next generation of Earth system models.

The Richtmyer–Meshkov instability of concave circular arc density interfaces in hydrodynamics and magnetohydrodynamics

Concave circular arc density interfaces (CDIs) are relevant to a deformed diaphragm separating different pressure gases in a shock tunnel or an expansion tube, where it is known that the Richtmyer–Meshkov instability (RMI) limits facility performance. Considering CDIs characterized by different curvatures (κ), numerical investigations of the RMI in both hydrodynamics (hydro) and magnetohydrodynamics (MHD) are performed. In the hydro cases, the largest curvature case appears to be the most unstable one, with the largest amounts of vorticities deposited on the CDIs. In the MHD cases, the interplay between the RMI and the magnetic field is investigated. On the one hand, the RMI can be compressed by magnetic fields. The stronger the magnetic field is, the smoother the density interface will be. The magnetic pressure alleviates pressure deviations along two sides of the CDIs, reducing baroclinic effects. Meanwhile, the magnetic tension force induces Alfvén waves, which transport vorticities away from density interfaces. On the other hand, magnetic fields can be amplified by the RMI, indicating that more amplification occurs when the initial magnetic field is weak, and magnetic lines are severely distorted in such cases. Besides, the evolutions of the kinetic energy and the magnetic energy are discussed. The results indicate that there is no energy transfer between them, and the magnetic energy mainly concentrates on the MHD wave fronts. The change of the enstrophy against time demonstrates that the vorticity energy decreases when the strength of initial magnetic fields increases.

New Insight Into the Onshore Intrusion of the Kuroshio Into the East China Sea

AbstractThe Kuroshio Current, a vigorous western boundary current in the Pacific, is responsible for bringing high temperature, high salinity, and phosphate‐rich open ocean water into the East China Sea (ECS) when it flows past Taiwan and generates a branch current, known as the Kuroshio Branch Current (KBC), which intrudes onto the continental shelf of the ECS. Based on observations and numerical modeling, a 1.5‐layer reduced‐gravity model is evaluated and modified to simulate the flow pattern of the Kuroshio intrusion (KI) northeast of Taiwan. We revised the analytical solution derived by Hsueh (1992), https://doi.org/10.1029/92JC01401 to conserve the horizontal momentum balance in the along‐shelf and cross‐shelf directions. The new analytical solution not only explains the KI angle but also addresses the KBC recirculation mechanism. To sustain the steady‐state momentum balance in the area adjacent to the shelf break northeast of Taiwan, there must be a recirculation that plays the same role in vorticity dynamics as the combination of the joint effect of baroclinicity and relief, bottom friction, and nonlinear advection. Unlike previous studies, our results agree well with the observations and could account for seasonal variations in the KI direction. Moreover, the new model predicts a reasonable velocity maximum (0.42 m s−1) over the shelf break. Satellite data and numerical results confirm that the new solution reflects a reasonable estimation for KI direction. This theory may potentially be applied to regions with trenches or valleys with completely different flow patterns by using similar analytical methods.

Effects of corner rounding on aerothermodynamic properties in rarefied hypersonic flows over an open cavity

A direct simulation Monte Carlo (DSMC) method is utilized to numerically investigate rarefied hypersonic flows over a group of two-dimensional rectangular cavities with rounded separation/reattachment corner. The effects of the rounded separation/reattachment corner on flow characteristics are discussed. The sensitivities of aerodynamic surface quantities are hence assessed comprehensively. Flows over the cavity configurations specified in the study hold the “open-cavity” characteristics and do not show any unsteadiness. The rounded separation corner does not significantly alter the flow patterns, while the rounded reattachment corner has a remarkable effect. Rounding the separation and reattachment corners can weaken the baroclinic effect in the separation and reattachment regions, respectively. For the rounded separation corner, it is easier for external gas molecules to enter the cavity and hence promote the momentum and energy exchanges with gas molecules inside the cavity. The fluid density inside the cavity, peak density at the reattachment point, pressure on and heat fluxes to the cavity floor are augmented, compared to those in the rectangular cavity case. However, the rounded reattachment corner may enhance the escape of gas molecules from the cavity and weaken the exchanges of momentum and energy. This leads to a significant decrease in the gas density inside the cavity, peak gas density at the reattachment point, pressure on and heat fluxes to the cavity floor. It is concluded that the approach of rounding the reattachment corner can achieve the most dramatic drop in the peak pressure and heat flux at the reattachment point. The research is guidable to the aerodynamic design of local configuration of hypersonic vehicles.

Oscillations of 2D ESTER models

Context.Recent numerical and theoretical considerations have shown that low-degree acoustic modes in rapidly rotating stars follow an asymptotic formula. In parallel, recent studies have revealed the presence of regular pulsation frequency patterns in rapidly rotatingδScuti stars that seem to match theoretical expectations.Aims.In this context, a key question is whether strong gradients or discontinuities can adversely affect the asymptotic frequency pattern to the point of hindering its identification. Other important questions are how rotational splittings are affected by the 2D rotation profiles expected from baroclinic effects and whether it is possible to probe the rotation profile using these splittings.Methods.In order to address these questions, we numerically calculate stellar pulsation modes in continuous and discontinuous rapidly rotating models produced by the 2D Evolution STEllaire en Rotation (ESTER) code. This code self-consistently calculates the rotation profile based on baroclinic effects and uses a spectral multi-domain approach, thus making it possible to introduce discontinuities at the domain interfaces without loss of numerical accuracy. The pulsation calculations are carried out using an adiabatic version of the Two-dimensional Oscillation Program (TOP) code. The variational principle is then used to confirm the high numerical accuracy of the pulsation frequencies and to derive an integral formula for the generalised rotational splittings. Acoustic glitch theory, combined with ray dynamics, is applied to the discontinuous models in order to interpret their pulsation spectra.Results.Our results show that the generalised rotational splittings are very well approximated by the integral formula, except for modes involved in avoided crossings. This potentially allows the application of inverse theory for probing the rotation profile. We also show that glitch theory applied along the island mode orbit can correctly predict the periodicity of the glitch frequency pattern produced by the discontinuity or Γ1dip related to the He II ionisation zone in some of the models. Furthermore, the asymptotic frequency pattern remains sufficiently well preserved to potentially allow its detection in observed stars.

The effects of turbulence and pressure gradients on vorticity transport in premixed bluff-body flames

The current study experimentally evaluates the effects of turbulence and pressure gradients on vorticity transport in premixed bluff-body flames. The flow field pressure gradient is modified using a variable-geometry test section. In tandem, a custom-designed turbulence generator is used to modulate the turbulence intensity in the test section. The objective of the combined pressure gradient and turbulence tailoring is to alter the balance between the individual terms of the vorticity transport equation and ultimately identify the subsequent impacts on the overall behavior of the flame and flow field. Simultaneous high-speed particle image velocimetry and CH* chemiluminescence are employed to measure the turbulent reacting flow field. The vorticity dynamics are analyzed using a mean, conditionally filtered, fluid element tracking method to calculate individual terms of the vorticity transport equation as fluid elements evolve across the flame front. The analysis shows that increasing the pressure gradient increases the reacting vorticity production for all vorticity terms. In contrast, the overall vorticity budget, or relative contribution of each vorticity term, is much more sensitive to turbulence effects. At low turbulence conditions, vorticity is dominated by baroclinic torque, regardless of the pressure gradient. As the turbulence intensity is increased, the exothermic effects of baroclinicity and dilatation are diminished, and the reacting flow field approaches that of a non-reacting turbulent flow. The relative balance of vorticity transport terms is shown to influence the overall flow field, as well as the structure and evolution of the flame front.

Investigations on interactions between vortex flow and the induced string cavitation characteristics in real-size diesel tapered-hole nozzles

An experimental and computational study was carried out to investigate the characteristics of vortex-induced string-type cavitation in the real-size diesel tapered-hole injector nozzle. A transparent-nozzle visualization experiment bench with high-speed imaging technology was applied to capture the string cavitation development in nozzles. Numerical simulations were conducted with the Reynolds stress turbulence model combined with the Schnerr–Sauer cavitation model. The numerical results are in good agreement with the experimental data. Intermittent string cavitation is mainly concentrated in the SAC chamber and near the hole exit due to the complex distribution of vortexes. The recirculated SAC flow encounters upstream injection flow resulting in a large vortex field which induced the formation of string cavitation. A large amount of cavitation vapor entrained by swirling flow is positioned in the near-field spray. Due to the higher dynamic pressure of cavitating flow in this region, the cavitation vapor bubbles are barely changed to liquid phase under the ambient pressure (101325 Pa). Three velocity components: axial, radial and tangential velocity are presented to characterize the flow field of string cavitation. Analysis based on the vorticity transport equation shows that the vortex stretching term is the dominant factor for the formation and development of string cavitation. The effect of dilatation term is secondary, followed by the baroclinic torque term effect on vorticity distribution.

Vortex formation in starting buoyant jets at moderate Richardson numbers

In this paper, the formation process of the leading vortex ring in positively and negatively buoyant starting jets with moderate Richardson number in the range of 0.06 < |Ri| ≤ 0.321 has been investigated numerically and theoretically. Using the similarity variables |Ri| and |Ri|2/5 for the positively and negatively buoyant starting jets, respectively, fitting equations can be obtained to predict the buoyant jet penetration rate. Based on these fitting equations, a revised circulation model is proposed by incorporating the effects of both over-pressure and buoyancy. The revised model is well consistent with the numerical results for all positively buoyant starting jets. However, for the negatively buoyant starting jets, this model can predict well only during the initial period. The over-pressure has little influence on the vortex ring characteristics of the starting jets with positive buoyancy, whereas it can significantly affect the vortex ring formation of the negatively buoyant starting jets at moderate Richardson numbers. As the positive Richardson number increases, the instabilities of the trailing shear layer occur earlier. At moderately negative Richardson numbers, a “double plume” structure (an inner sinking circular forced plume and an outer rising annular plume) can be observed. The outer negative vorticity layers develop gradually due to the baroclinic effect. Consequently, the size and strength of the leading vortex progressively decrease. As the negative Richardson number decreases, the negative vorticity layers occur earlier and grow faster.