Axisymmetric Inflow Research Articles

An Observational Study of the Symmetric Boundary Layer Structure and Tropical Cyclone Intensity

This study analyses Global Positioning System dropsondes to document the axisymmetric tropical cyclone (TC) boundary-layer structure, based on storm intensity. A total of 2608 dropsondes from 42 named TCs in the Atlantic basin from 1998 to 2017 are used in the composite analyses. The results show that the axisymmetric inflow layer depth, the height of maximum tangential wind speed, and the thermodynamic mixed layer depth are all shallower in more intense TCs. The results also show that more intense TCs tend to have a deep layer of the near-saturated air inside the radius of maximum wind speed (RMW). The magnitude of the radial gradient of equivalent potential temperature (θe) near the RMW correlates positively with storm intensity. Above the inflow layer, composite structures of TCs with different intensities all possess a ring of anomalously cool temperatures surrounding the warm-core, with the magnitude of the warm-core anomaly proportional to TC intensity. The boundary layer composites presented here provide a climatology of how axisymmetric TC boundary layer structure changes with intensity.

Impact of nonlinear surface inflows into activity belts on the solar dynamo

We examine the impact of surface inflows into activity belts on the operation of solar cycle models based on the Babcock–Leighton mechanism of poloidal field regeneration. Towards this end we introduce in the solar cycle model of Lemerle & Charbonneau (2017.ApJ834: 133) a magnetic flux-dependent variation of the surface meridional flow based on the axisymmetric inflow parameterization developped by Jiang et al. (2010.ApJ717: 597). The inflow dependence on emerging magnetic flux thus introduces abona fidenonlinear backreaction mechanism in the dynamo loop. For solar-like inflow speeds, our simulation results indicate a decrease of 10–20% in the strength of the global dipole building up at the end of an activity cycle, in agreement with earlier simulations based on linear surface flux transport models. Our simulations also indicate a significant stabilizing effect on cycle characteristics, in that individual cycle amplitudes in simulations including inflows show less scatter about their mean than in the absence of inflows. Our simulations also demonstrate an enhancement of cross-hemispheric coupling, leading to a significant decrease in hemispheric cycle amplitude asymmetries and temporal lag in hemispheric cycle onset. Analysis of temporally extended simulations also indicate that the presence of inflows increases the probability of cycle shutdown following an unfavorable sequence of emergence events. This results ultimately from the lower threshold nonlinearity built into our solar cycle model, and presumably operating in the sun as well.

Structure and Maintenance Mechanism of Long-Lived Concentric Eyewalls Associated with Simulated Typhoon Bolaven (2012)

Abstract Typhoons with long-lived concentric eyewalls (CEs) are more intense than those with short-lived CEs. It is important for more accurate prediction of typhoon intensity to understand the maintenance mechanism of the long-lived CEs. To study the mechanism of the long-term maintenance of CEs, a numerical experiment of Typhoon Bolaven (2012) is performed using a nonhydrostatic model with full physics. Two aspects of the maintenance of simulated CEs are investigated: the maintenance of the inner eyewall and the contraction of the outer eyewall. To examine the maintenance of the inner eyewall, the equivalent potential temperature budget and air parcel trajectories of the simulated inner eyewall are calculated. The results show that the entropy supply to the inner eyewall is sufficient to maintain the inner eyewall after secondary eyewall formation (SEF). During the early period after SEF, entropy is supplied by an axisymmetric inflow, and later it is supplied by nonaxisymmetric flows of the outer eyewall. To examine the contraction of the outer eyewall, the potential vorticity (PV) budget of the outer eyewall is diagnosed. The result reveals that the negative contribution to the contraction of the outer PV peak (i.e., the outer eyewall) in the early period is the negative PV generation due to axisymmetric advection and diabatic heating just inside of the outer PV peak. In the later period, the negative PV generation due to nonaxisymmetric structure is important for the prevention of contraction. The present study reveals that the structure of the outer eyewall plays important roles in the maintenance of long-lived CEs.

Analog rotating black holes in a magnetohydrodynamic inflow

We present a model of the analog geometry in a magnetohydrodynamic (MHD) flow. For the MHD flow with magnetic pressure-dominated and gas pressure-dominated conditions, we obtain the magnetoacoustic metric for the fast MHD mode. For the slow MHD mode, on the other hand, the wave is governed by the advective-type equation without an isotropic dispersion term. Thus, the "distance" perpendicular to the wave propagation is not defined and the magnetoacoustic metric cannot be introduced. To investigate the properties of the magnetoacoustic geometry for the fast mode, we prepare a two-dimensional axisymmetric inflow and examine the behavior of magnetoacoustic rays which is a counterpart of the MHD waves in the eikonal limit. We find that the magnetoacoustic geometry is classified into three types depending on two parameters characterizing the background flow:~analog spacetimes of rotating black holes, ultra spinning stars with ergoregions, and spinning stars without ergoregions. We address the effects of the magnetic pressure on the effective geometries.

Aerodynamic Simulation of the MEXICO Rotor

CFD (Computational Fluid Dynamics) simulations are a very promising method for predicting the aerodynamic behavior of wind turbines in an inexpensive and accurate way. One of the major drawbacks of this method is the lack of validated models. As a consequence, the reliability of numerical results is often difficult to assess. The MEXICO project aimed at solving this problem by providing the project partners with high quality measurements of a 4.5 meters rotor diameter wind turbine operating under controlled conditions. The large measurement data-set allows the validation of all kind of aerodynamic models. This work summarizes our efforts for validating a CFD model based on the open source software OpenFoam. Both steady- state and time-accurate simulations have been performed with the Spalart-Allmaras turbulence model for several operating conditions. In this paper we will concentrate on axisymmetric inflow for 3 different wind speeds. The numerical results are compared with pressure distributions from several blade sections and PIV-flow data from the near wake region. In general, a reasonable agreement between measurements the and our simulations exists. Some discrepancies, which require further research, are also discussed.

Insight into Rotational Effects on a Wind Turbine Blade Using Navier–Stokes Computations

Rotational effects are known to influence severely the aerodynamic performance of the inboard region of rotor blades. The underlying physical mechanisms are however far from being well understood. The present work addresses this problem using Reynolds averaged Navier–Stokes computations and experimental results of the MEXICO (Model Experiments in Controlled Conditions) rotor. Four axisymmetric inflow cases with wind speeds ranging from pre-stall to post-stall conditions are computed and compared with pressure and particle image velocimetry (PIV) experimental data, obtaining, in general, consistent results. At low angles of attack, the aerodynamic behavior of all of the studied blade sections resembles the one from the corresponding 2D airfoils. However, at high angles of attack, rotational effects lead to stall delay and/or lift enhancement at inboard positions. Such effects are shown to occur only in the presence of significant radial flows. Interestingly, the way in which rotational effects influence the aerodynamics of the MEXICO blades differs qualitatively in certain aspects from the descriptions found in the literature about this topic. The presented results provide new insights that are useful for the development of advanced and physically-sound correction models.

Hurricane Sea Surface Inflow Angle and an Observation-Based Parametric Model

Abstract This study presents an analysis of near-surface (10 m) inflow angles using wind vector data from over 1600 quality-controlled global positioning system dropwindsondes deployed by aircraft on 187 flights into 18 hurricanes. The mean inflow angle in hurricanes is found to be −22.6° ± 2.2° (95% confidence). Composite analysis results indicate little dependence of storm-relative axisymmetric inflow angle on local surface wind speed, and a weak but statistically significant dependence on the radial distance from the storm center. A small, but statistically significant dependence of the axisymmetric inflow angle on storm intensity is also found, especially well outside the eyewall. By compositing observations according to radial and azimuthal location relative to storm motion direction, significant inflow angle asymmetries are found to depend on storm motion speed, although a large amount of unexplained variability remains. Generally, the largest storm-relative inflow angles (<−50°) are found in the fastest-moving storms (>8 m s−1) at large radii (>8 times the radius of maximum wind) in the right-front storm quadrant, while the smallest inflow angles (>−10°) are found in the fastest-moving storms in the left-rear quadrant. Based on these observations, a parametric model of low-wavenumber inflow angle variability as a function of radius, azimuth, storm intensity, and motion speed is developed. This model can be applied for purposes of ocean surface remote sensing studies when wind direction is either unknown or ambiguous, for forcing storm surge, surface wave, and ocean circulation models that require a parametric surface wind vector field, and evaluating surface wind field structure in numerical models of tropical cyclones.

Axisymmetrical gas inflow in the central region of NGC 7331

New Integral Field Spectroscopy of the central region of NGC 7331 reveals strong Hα emission in the well-known CO and HI ring of NGC 7331. The [NII]/Hα ratio indicates that a large scale stellar formation process is taken place at the ring in agreement with previous hypothesis about the exhaustion of gas in the inner to the ring region. The dynamics of stars and gas are not coupled. There is a ring of peculiar velocities in the ionized gas velocity map. These peculiar velocities can be well interpreted by the presence of an axisymmetric inflow of 40 km s-1 at the inner boundary of the large-scale gaseous ring. We infer an inwards total flux of 1.6 yr-1. This value is typical of the accretion rates in hypothetical large nuclear black holes. Despite the large differences in the scales of the nucleus and the gas ring of NGC 7331, we suggest that this inwards flux is feeding the nucleus.

Noncircular Gas Kinematics and Star Formation in the Ringed Galaxy NGC 4736

We analyze the gas kinematics and star formation properties of the nearby RSab galaxy NGC 4736 using interferometric and single-dish CO(1-0) data and previously published Hα and H I data. The CO morphology is dominated by a central molecular bar and tightly wound spiral arms associated with a bright ring of star formation. Strong H I emission is also found in the ring, but H I is absent from the central regions. Comparison of the H I and Hα distributions suggests that H I in the ring is primarily dissociated H2. Modeling of the CO kinematics reveals gas motion in elliptical orbits around the central bar, and we argue that the ring represents both the outer Lindblad resonance of the bar and the inner Lindblad resonance of a larger oval distortion. The H I kinematics show evidence for axisymmetric inflow toward the ring and are inconsistent with streaming in aligned elliptical orbits, but the highly supersonic (~40 km s-1) inflow velocities required, corresponding to mass inflow rates of ~2 M☉ yr-1, suggest that more sophisticated models (e.g., gas orbiting in precessed elliptical orbits) should be considered. The radial CO and Hα profiles are poorly correlated in the vicinity of the nuclear bar but show a better correlation (in rough agreement with the Schmidt law) at the ring. Even along the ring, however, the azimuthal correspondence between CO and Hα is poor, suggesting that massive stars form more efficiently at some (perhaps resonant) locations than at others. These results indicate that the star formation rate per unit gas mass exhibits strong spatial variations and is not solely a function of the available gas supply. The localization of star formation to the ring is broadly consistent with gravitational instability theory, although the instability parameter Q ~ 3 on average in the ring, only falling below 1 in localized regions. Large-scale dynamical effects, by concentrating gas at resonances and influencing the star formation rate, appear to play a key role in this galaxy's evolution.

Axisymmetric withdrawal and inflow in a density-stratified container

The axisymmetric withdrawal of fluid from a linearly stratified container is studied over the full parameter range. When only buoyancy and inertia are important the flow in the withdrawal layer is influenced by a virtual control point and is not analogous to that observed in the two-dimensional withdrawal problem. Two further flow regimes are shown to exist in which viscous forces are important : one in which convection of species is important, and a second in which diffusion of species is important. Theoretical arguments and laboratory experiments are used to show that S = (Q2N/v3)~ is the appropriate flow parameter to differentiate between these possibilities. It is also argued that these results may be generalized to describe the features of several related flows : axisymmetric drawdown (or drawup) in withdrawal from a layered density structure, axisymmetric inflow into a linearly stratified environment and the axisymmetric spreading of density currents.