Prograde Flow Research Articles

Abyssal Slope Currents

Abstract Realistic computational simulations in different oceanic basins reveal prevalent prograde mean flows (in the direction of topographic Rossby wave propagation along isobaths; a.k.a. topostrophy) on topographic slopes in the deep ocean, consistent with the barotropic theory of eddy-driven mean flows. Attention is focused on the Western Mediterranean Sea with strong currents and steep topography. These prograde mean currents induce an opposing bottom drag stress and thus a turbulent boundary-layer mean flow in the downhill direction, evidenced by a near-bottom negative mean vertical velocity. The slope-normal profile of diapycnal buoyancy mixing results in down-slope mean advection near the bottom (a tendency to locally increase the mean buoyancy) and up-slope buoyancy mixing (a tendency to decrease buoyancy) with associated buoyancy fluxes across the mean isopycnal surfaces (diapycnal downwelling). In the upper part of the boundary layer and nearby interior, the diapycnal turbulent buoyancy flux divergence reverses sign (diapycnal upwelling), with upward Eulerian mean buoyancy advection across isopycnal surfaces. These near-slope tendencies abate with further distance from the boundary. An along-isobath mean momentum balance shows an advective acceleration and a bottom-drag retardation of the prograde flow. The eddy buoyancy advection is significant near the slope, and the associated eddy potential energy conversion is negative, consistent with mean vertical shear flow generation for the eddies. This cross-isobath flow structure differs from previous proposals, and a new one-dimensional model is constructed for a topostrophic, stratified, slope bottom boundary layer. The broader issue of the return pathways of the global thermohaline circulation remains open, but the abyssal slope region is likely to play a dominant role.

Radiation RMHD Accretion Flows around Spinning AGNs: A Comparative Study of MAD and SANE State

In our study, we examine a 2D radiation, relativistic, magnetohydrodynamics accretion flow around a spinning supermassive black hole. We begin by setting an initial equilibrium torus around the black hole, with an embedded initial magnetic field inside the torus. The strength of the initial magnetic field is determined by the plasma beta parameter, which is the ratio of the gas pressure to the magnetic pressure. In this paper, we perform a comparative study of the magnetically arrested disc (MAD) and standard and normal evolution (SANE) states. We observe that the MAD state is possible for comparatively high initial magnetic field strength flow. Additionally, we also adopt a self-consistent two-temperature model to evaluate the luminosity and energy spectrum for our model. We observe that the total luminosity is mostly dominated by bremsstrahlung luminosity compared to the synchrotron luminosity due to the presence of a highly dense torus. We also identify similar quasi-periodic oscillations for both MAD and SANE states based on power-density spectrum analysis. Furthermore, our comparative study of the energy spectrum does not reveal any characteristic differences between MAD and SANE states. Last, we note that the MAD state is possible for both prograde and retrograde accretion flow.

Prograde and Retrograde Gas Flow around Disk-embedded Companions: Dependence on Eccentricity, Mass, and Disk Properties

We apply 3D hydrodynamical simulations to study the rotational aspect of gas flow patterns around eccentric companions embedded in an accretion disk. We sample a wide range of companion mass ratios q and disk aspect ratios h 0, and confirm a generic transition from prograde (steady tidal interaction dominated) to retrograde (background Keplerian shear dominated) circumcompanion flow when orbital eccentricity exceeds a critical value e t . We find e t ∼ h 0 for subthermal companions while for superthermal companions, and propose an empirical formula to unify the two scenarios. Our results also suggest that e t is insensitive to modest levels of turbulence, modeled in the form of a kinematic viscosity term. In the context of stellar-mass black holes (sBHs) embedded in active galactic nucleus (AGN) accretion disks, the bifurcation of their circumstellar disk rotation suggests the formation of a population of nearly antialigned sBHs, whose relevance to low spin gravitational wave events can be probed in more detail with future population models of sBH evolution in AGN disks, making use of our quantitative scaling for e t ; in the context of circumplanetary disks (CPDs), our results suggest the possibility of forming retrograde satellites in situ in retrograde CPDs around eccentric planets.

Jupiter's Banding and Jets May Be Caused by Tides

The stratification parameters for Jupiter’s outer 3000 km shell are calculated using a density profile recently derived from observations of the NASA Juno spacecraft currently in Jovian orbit. Using these parameters, the equations of classical tidal theory for a stratified, nonhydrostatic, compressible fluid are numerically solved for sectoral tidal forcing by Io. The results support a long-standing though little discussed proposal that the banding/jets (and possibly the unexplained endogenic heat) are caused by the tides. First, general arguments from eigenmode analyses expect resonantly forced tidal modes and the scattering of the tidal response to higher spatial degrees by Jupiter’s fast rotation, with time-averaged tidal effects appearing in bands between critical latitudes (±50° for forcing by Io). Second, resonant tides and banding are specifically demonstrated in the tidal model configured with the Juno-derived stratification. While banding in the time-averaged tidal features is a robust expectation (from the well-prescribed forcing and rotation parameters) and is independent of the internal parameters, the details of the banding (e.g., number, width) are highly dependent. Hence, comparison of the tidal model with observations provides a test of the tidal hypothesis of the bands as well as assumptions of interior parameters and processes. Here, dissipation parameterized as a simple pressure relaxation term in the vertical balance equation shows a time-mean banded structure between the critical latitudes that can drive geostrophic jets matching the major observed features, including strong prograde flow at the equator. By contrast, alternate stratification/dissipation assumptions produce banded structures that do not agree with observations.

Evolution of solar surface inflows around emerging active regions

Context.Solar active regions are associated with Evershed outflows in sunspot penumbrae, moat outflows surrounding sunspots, and extended inflows surrounding active regions. Extended inflows have been identified around established active regions with various methods. The evolution of these inflows and their dependence on active region properties as well as their effect on the global magnetic field are not yet understood.Aims.We aim to understand the evolution of the average inflows around emerging active regions and to derive an empirical model for these inflows. We expect that this can be used to better understand how the inflows act on the diffusion of the magnetic field in active regions.Methods.We analyzed horizontal flows at the surface of the Sun using local correlation tracking of solar granules observed in continuum images of the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We measured average flows of a sample of 182 isolated active regions up to seven days before and after their emergence onto the solar surface with a cadence of 12 h. About half of the active regions in the sample developed sunspots with moat flows in addition to the surrounding inflows. We investigated the average inflow properties with respect to active region characteristics of total flux and latitude. We fit a model to these observed inflows for a quantitative analysis.Results.We find that converging flows of about 20–30 m s−1are first visible one day prior to emergence, in agreement with recent results. These converging flows are present regardless of the active region properties of latitude or flux. We confirm a recently found prograde flow of about 40 m s−1at the leading polarity during emergence. We find that the time after emergence when the latitudinal inflows increase in amplitude depends on the flux of the active region, ranging from one to four days after emergence and increasing with flux. The largest extent of the inflows is up to about 7 ± 1° away from the center of the active region within the first six days after emergence. The inflow velocities have amplitudes of about 50 m s−1.

Successful repair of an anomalous right coronary artery arising from the pulmonary artery in a 10 month-old

Echocardiogram (Figure 1A-B) in an asymptomatic 6 month-old raised suspicion for anomalous Right Coronary Artery (RCA) from the Pulmonary Artery (ARCAPA) with retrograde flow from the RCA to the main pulmonary artery. The patient’s left ventricular systolic function was normal. Subsequent cardiac catheterization confirmed the diagnosis as angiography showed that the RCA arose from the anterior aspect of the main pulmonary artery and filled retrograde via a collateral network from the Left Coronary Artery (LCA) system (Figure 1C-D). At 10 months of age, the patient underwent surgical translocation of the RCA to the aorta. On direct visualization, the RCA arose from the anterior wall of the main pulmonary artery and the robust collateral supply of the RCA from the LCA was visualized on the heart’s epicardium (Figure 2). The patient did well and was discharged home several days later. Three months after the operation, the patient continues to do well with normal left ventricular systolic flow and prograde flow into the reimplanted RCA.

Granular flow on a rotating and gravitating elliptical body

Abstract

Fetal Echocardiographic Parameters and Surgical Outcomes in Congenital Left-Sided Cardiac Lesions.

This study aimed to evaluate fetal echocardiographic parameters associated with neonatal intervention and single-ventricle palliation (SVP) in fetuses with suspected left-sided cardiac lesions. Initial fetal echocardiograms (1/2002-1/2017) were interpreted by the contemporary fetal cardiologist as coarctation of the aorta (COA), left heart hypoplasia (LHH), hypoplastic left heart syndrome (HLHS), mitral valve hypoplasia (MVH) ± stenosis, and aortic valve hypoplasia ± stenosis (AS). The cohort comprised 68 fetuses with suspected left-sided cardiac lesions (COA n = 15, LHH n = 9, HLHS n = 39, MVH n = 1, and AS n = 4). Smaller left ventricular (LV) length Z score, aortic valve Z score, ascending aorta Z score, and aorta/pulmonary artery ratio; left-to-right shunting at the foramen ovale; and retrograde flow in the aortic arch were associated with the need for neonatal intervention (p = 0.005-0.04). Smaller mitral valve (MV) Z score, LV length Z score, aortic valve Z score, ascending aorta Z score, aorta/pulmonary artery ratio, and LV ejection fraction, as well as higher tricuspid valve-to-MV (TV/MV) ratio, right ventricular-to-LV (RV/LV) length ratio, left-to-right shunting at the foramen ovale, abnormal pulmonary vein Doppler, absence of prograde aortic flow, and retrograde flow in the aortic arch were associated with SVP (p < 0.001-0.008). The strongest independent variable associated with SVP was RV/LV length ratio (stepwise logistical regression, p = 0.03); an RV/LV length ratio > 1.28 was associated with SVP with a sensitivity of 76% and specificity of 96% (AUC 0.90, p < 0.001). A fetal RV/LV length ratio of > 1.28 may be a useful threshold for identifying fetuses requiring SVP.

Uhl's anomaly: Clinical spectrum and pathophysiology

A 28 year-old woman from Afghanistan was referred for prenatal evaluation. She and her husband were consanguineous and had a previous female baby with a rare congenital cardiac defect that consisted of Uhl's anomaly, Ebstein's malformation and pulmonary atresia. The child died after the first palliative step towards a Fontan circulation. A fetal echocardiogram performed at 18 + 2 weeks' gestation showed distinctive right-sided cardiac abnormalities similar to those found in their previous daughter. A dilated, extremely thin walled right ventricle (RV) with poor systolic function was found with a markedly dysplastic tricuspid valve (Fig. 1A, Video 1). There appeared only to be a rudimentary anterior tricuspid valve leaflet. The tricuspid orifice was larger than the mitral valve annulus, 4.9 mm (Z-score +1.5) and 3.6 mm (Z-score -1.5) respectively [1] and free regurgitation through the rudimentary tricuspid valve was present. The oval fossa was wide open with right to left shunting over it (Fig. 1A–B, Video 2). The pulmonary valve annulus diameter was small (2.3 mm, Z-score -0.9) and the pulmonary arteries were underdeveloped (Fig. 1E). The left side of the heart appeared structurally normal (Fig. 1A–C) with normal aortic annulus dimension (3.0 mm, Z-score +2.9). No prograde flow could be detected over

Magnetic field induced flow pattern reversal in a ferrofluidic Taylor-Couette system.

We investigate the dynamics of ferrofluidic wavy vortex flows in the counter-rotating Taylor-Couette system, with a focus on wavy flows with a mixture of the dominant azimuthal modes. Without external magnetic field flows are stable and pro-grade with respect to the rotation of the inner cylinder. More complex behaviors can arise when an axial or a transverse magnetic field is applied. Depending on the direction and strength of the field, multi-stable wavy states and bifurcations can occur. We uncover the phenomenon of flow pattern reversal as the strength of the magnetic field is increased through a critical value. In between the regimes of pro-grade and retrograde flow rotations, standing waves with zero angular velocities can emerge. A striking finding is that, under a transverse magnetic field, a second reversal in the flow pattern direction can occur, where the flow pattern evolves into pro-grade rotation again from a retrograde state. Flow reversal is relevant to intriguing phenomena in nature such as geomagnetic reversal. Our results suggest that, in ferrofluids, flow pattern reversal can be induced by varying a magnetic field in a controlled manner, which can be realized in laboratory experiments with potential applications in the development of modern fluid devices.

Parallel Endografts in the Treatment of Distal Aortic and Common Iliac Aneurysms

Endovascular treatment of distal abdominal aortic aneurysms (D-AAA) and bilateral common iliac artery aneurysms (BCIAA) may present technical challenges for standard EVAR. Parallel iliac leg endografts (ILEs) of standard aortic devices and covered stents have been successfully employed to treat patients with D-AAA and BCIAA. The perioperative and long-term results of this straightforward endovascular technique are presented. Beginning in 2009, patients deemed unfit for open surgery underwent parallel endografts D-AAA and BCIAA exclusion. Avoiding the use of a main body, ILEs are simultaneously delivered from both femoral arteries, landing parallel into the aortic neck (parallel grafts: PG). Distal landing zones including external iliac arteries (EIAs) are reached using appropriate ILEs. A third parallel covered stent graft (Viabahn, Gore) is delivered from a left brachial approach to maintain prograde blood flow to one internal iliac artery (IIA) when needed. Eighteen patients were successfully treated using parallel endografts, nine for BCIAA and nine for D-AAA. All D-AAA presented an irregular saccular shape, including three penetrating aortic ulcers and two pseudoaneurysms of previous aortic grafts. Prograde flow to one IIA was successfully maintained using a Viabahn graft in five patients with BCIAA. Mean aneurysm size was 50 mm in D-AAA and 43 mm in BCIAA. One patient required a perioperative ILE extension to treat a type Ib endoleak. One patient suffered a minor stroke 24 hours after the procedure. Two type II endoleaks were observed postoperatively. Five patients died of non-aneurysm related causes during follow-up. No new endoleaks, graft displacements or occlusions were observed during follow-up (median: 26 months, range 12-42 months). Successful exclusion of D-AAA and BCIAA was achieved in high-risk patients using parallel endografts, allowing antegrade blood flow to one IIA when needed. Commercially available endografts were used in a simple and effective approach, with excellent follow-up results.

Coronary Anatomy in the Newborn: What Do We Need to Know and When?

Coronary Anatomy in the Newborn: What Do We Need to Know and When?

On spin dependence of relativistic acoustic geometry

This work makes the first ever attempt to understand the influence of the black hole background spacetime in determining the fundamental properties of the embedded relativistic acoustic geometry. To accomplish such task, we investigate the role of the spin angular momentum of the astrophysical black hole (the Kerr parameter a—a representative feature of the background black hole metric) in estimating the value of the acoustic surface gravity (the representative feature of the corresponding analogue spacetime). Since almost all astrophysical black holes are supposed to posses some degree of intrinsic rotation, the influence of the Kerr parameter on classical analogue models is very important to understand. We study the general relativistic, axially symmetric, non-self-gravitating inflow of the hydrodynamic fluid onto a rotating astrophysical black hole from the dynamical systems point of view. In this work the location of the acoustic horizon inside such fluid flow is identified and the associated acoustic surface gravity is estimated. We study the dependence of such surface gravity as a function of the Kerr parameter as well as with other dynamical and thermodynamic variables governing the fluid flow under strong gravity, and demonstrate that for retrograde flow, the surface gravity (and hence the associated analogue Hawking temperature) correlates with the black hole spin in general, whereas for the prograde flow, the surface gravity as well as the analogue temperature correlates with the black hole spin for slow to moderately rotating holes, but anti-correlates with the spin for fast to extremely rotating holes. We found that for certain values of the initial boundary conditions, more than one acoustic horizons, namely two black hole types and one white hole type, may form, and the surface gravity may become formally infinite at the acoustic white hole. We discuss the possible connection between the corresponding analogue Hawking temperature and astrophysically relevant observables associated with the spectral signature of the black hole candidates. Our result indicates that the modified dispersion relation evaluated at the close proximity of the acoustic horizon (and hence the nonuniversal feature of Hawking-like effects) is a sensitive function of the spin angular momentum of the astrophysical black hole. We propose that the black hole spin dependence of such dispersion relation may be used to distinguish a corotating flow from a counter rotating flow for axisymmetric accretion onto a Kerr black hole.

On the efficiency of the Blandford–Znajek mechanism for low angular momentum relativistic accretion

ABSTRACT The Blandford–Znajek (BZ) mechanism has usually been studied in the literature for accretion with considerably high angular momentum leading to the formation of either a cold Keplerian disc or a hot and geometrically thick sub-Keplerian flow as described within the framework of advection-dominated accretion flow/radiatively inefficient accretion flow. However, in nearby elliptical galaxies, as well as for our own Galactic Centre, accretion with very low angular momentum is prevalent. Such quasi-spherical strongly sub-Keplerian accretion has complex dynamical features and can accommodate stationary shocks. In this Letter, we present our calculation for the maximum efficiency obtainable through the BZ mechanism for complete general relativistic weakly rotating axisymmetric flow in the Kerr metric. Both shocked and shock-free flows have been studied in detail for rotating and counter-rotating accretion. Such a study has never been done in the literature before. We find that the energy extraction efficiency is low, about 0.1 per cent, and increases by a factor of 15 if the ram pressure is included. Such an efficiency is still much higher than the radiative efficiency of such optically thin flows. For the BZ mechanism, shocked flow produces a higher efficiency than the shock-free solutions and retrograde flow provides a slightly larger value of the efficiency than that for the prograde flow.

On the relationship between zonal jets and dynamo action in giant planets

[1] Jupiter and Saturn exhibit similar large-scale dynamical features. Each planet has a prograde equatorial jet and a deeply seated dipolar magnetic field. Compared to Jupiter, Saturn's jet is broader and faster, while its magnetic field is weaker and more axially symmetric. The Sun also has prograde equatorial flow and a large-scale axial magnetic field. While the depth of the Sun's differential rotation is well constrained by helioseismology, the depth to which the zonal winds penetrate in the giant planets is not known and has been a subject of debate. Although magnetic braking has been invoked as the mechanism to slow the winds at depth, such a mechanism has not previously been demonstrated. Here we present the first self-consistent numerical planetary dynamo models in which slow convection in the interior dynamo source region coexists with strong zonal flow near the outer surface. The models include radially variable electrical conductivity and show that prograde zonal flow penetrates to a depth where Lorentz forces balance the Reynolds stress, which drives the equatorial jet. Our results imply that major differences between the surface zonal flows of Jupiter and Saturn arise from the different depths and conditions of a transition layer analogous to the solar tachocline. This transition layer, the planetary tachocline, separates the high velocity, semiconducting molecular envelope from the slow moving liquid metal interior dynamo.

Endovascular Repair of Traumatic Aortic Injury Using a Custom Fenestrated Endograft to Preserve the Left Subclavian Artery

To describe the use of custom fenestrated endografts to preserve left subclavian artery (SCA) flow when requiring coverage during endovascular repair of blunt aortic injury (BAI). A 39-year-old male involved in a motor vehicle accident sustained injuries including intracranial hemorrhage, BAI, and extremity fractures. Immediate neurosurgical intervention was required. Once neurologically stabilized, endovascular repair was performed with a commercially available device modified with a custom fenestration to preserve flow into the left SCA. Serial follow-up CT angiography (CTA) demonstrates satisfactory repair with prograde left SCA flow and no evidence of endoleak. Left SCA coverage is often required for successful endovascular repair of BAI. A subgroup of patients who undergo left SCA coverage will require revascularization. The use of custom fenestrated endografts for preserving left SCA during thoracic endovascular aortic repair (TEVAR) for BAI is an innovative and feasible option in patients who require revascularization.

Prograde and retrograde flow past cylindrical obstacles on a β-plane

The problem of viscous prograde (eastward) and retrograde (westward) flow past a cylindrical obstacle on a β-plane is considered. The barotropic vorticity equation is solved using a numerical method that combines finite difference and spectral methods. A modified version of the β-plane approximation is proposed to avoid computational difficulties associated with the traditional β-plane approximation. Numerical results are presented and discussed for flow past a circular cylinder at selected Reynolds numbers (Re )a nd non-dimensional β-parameters ( ˆ β) as well as for flow past an elliptic cylinder of a fixed aspect ratio (r = 0.2) at inclination angles of ±15 ◦ , 90 ◦ and selected Reand ˆ β. In prograde flows, it is found that the β-effect acts to suppress boundary-layer separation and to excite a standing Rossby lee wavetrain, as observed in previ- ous works. In retrograde flows, the boundary-layer separation region is elongated and westward propagating Rossby waves are excited.

Experimental and numerical study of mean zonal flows generated by librations of a rotating spherical cavity

We study both experimentally and numerically the steady zonal flow generated by longitudinal librations of a spherical rotating container. This study follows the recent weakly nonlinear analysis of Busse (J. Fluid Mech., vol. 650, 2010, pp. 505–512), developed in the limit of small libration frequency–rotation rate ratio and large libration frequency–spin-up time product. Using particle image velocimetry measurements as well as results from axisymmetric numerical simulations, we confirm quantitatively the main features of Busse's analytical solution: the zonal flow takes the form of a retrograde solid-body rotation in the fluid interior, which does not depend on the libration frequency nor on the Ekman number, and which varies as the square of the amplitude of excitation. We also report the presence of an unpredicted prograde flow at the equator near the outer wall.

Evaluation of the myocardial performance index and tissue doppler imaging by comparison to near-simultaneous catheter measurements in pediatric cardiac transplant patients

Parameters derived from Doppler echocardiography, such as the myocardial performance index (MPI) and the ratio of peak velocity of early diastolic prograde flow (E) across the atrioventricular valve (AVV) divided by the peak velocity of early diastolic tissue motion at the AVV annulus (E') (E:E' ratio), are routinely used to assess ventricular function. However, they have not been compared with measurements obtained by cardiac catheterization (CC) in children. We prospectively studied post-cardiac transplant children undergoing CC with near-simultaneous echocardiographic imaging for MPI and E:E'. CC data included right atrial pressure (RAp), pulmonary artery wedge pressure (PAWp) and cardiac index (CI). One investigator, who was blinded to the CC data, measured right ventricular (RV) and left ventricular (LV) MPI and E:E' offline. Correlations between echocardiographic and catheter measurements were calculated. Receiver-operating-characteristics (ROC) curves were created to evaluate the utility of echocardiographic measurements that exhibited statistically significant correlations with CC measurements. Twenty-four patients (age range 0.8 to 21 years) underwent 142 CCs during the study period. Of the 12 correlations studied, 3 met statistical significance (p < 0.05) (RV E:E' vs RAp, RV E:E' vs PAWp and LV MPI vs CI). The ROC areas for these correlations were 0.38, 0.42 and 0.68, respectively. There were no cut-off points where both the sensitivity and specificity for any measurement were >0.80. Our data suggest that MPI and E:E' are poor predictors of simultaneously obtained, catheter-derived hemodynamic parameters in post-transplant children. These limitations should be carefully considered before ascribing diagnostic value to these measurements.

Compressible convection in the deep atmospheres of giant planets

Fast rotating giant planets such as Jupiter and Saturn possess alternate prograde and retrograde zonal winds which are stable over long periods of time. We consider a compressible model of convection in a spherical shell with rapid rotation, using the anelastic approximation, to explore the parameter range for which such zonal flows can be produced. We consider models with a large variation in density across the layer. Our models are based only on the molecular H/He region above the metallic hydrogen transition at about 2 Mbar, and we do not include the hydromagnetic effects which may be important if the electrical conductivity is significant. We find that the convective velocities are significantly higher in the low density regions of the shell, but the zonal flow is almost independent of the z-coordinate parallel to the rotation axis. We analyse how this behaviour is consistent with the Proudman–Taylor theorem. We find that deep prograde zonal flow near the equator is a very robust feature of our models. Prograde and retrograde jets alternating in latitude can occur inside the tangent cylinder in compressible as well as Boussinesq models, particularly at lower Prandtl numbers. However, the zonal jets inside the tangent cylinder are suppressed if a no-slip condition is imposed at the inner boundary. This suggests that deep high latitude jets may be suppressed if there is significant magnetic dissipation. Our compressible calculations include the viscous dissipation in the entropy equation, and we find this is comparable to, and in some cases exceeds, the total heat flux emerging from the surface. For numerical reasons, these simulations cannot reach the extremely low Ekman number found in giant planets, and they necessarily also have a much larger heat flux than planets. We therefore discuss how our results might scale down to give solutions with lower dissipation and lower heat flux.