Momentum Mixing Research Articles

Effects of Double-Side Labyrinth Seals on Aerodynamic Performance in a Transonic Shrouded Turbine Stage

Labyrinth seals on both rotor casing and blade tip as an effective method to control the leakage flowrate of the shroud and improve aerodynamic performances in a transonic turbine stage are investigated in this study. Compared to the case without the labyrinth seal structure, the cases with three different types of sealing teeth have been shown to reduce significantly the tip leakage flow by computational simulations. The double-side sealing teeth case reduces the leakage flowrate from 3.4% to 1.3% and increases the efficiency by 1.4%, which is the maximum efficiency improvement of all cases. The sealing structures increase the loss inside the shroud while reducing the momentum mixing between shroud leakage flow and mainstream. Therefore, the circumferential distribution of leakage velocity is changed, as well as the distribution of high-loss zones at turbine outlet. Furthermore, the leakage-vortex loss, which is associated with the blockage effect of sealing structure to the tip leakage flow, gains more improvement than the passage-vortex at the rotor outlet section in double-side seal case. In addition, it has also been found that with a larger gap at tip, the double-side seal has better effects of reducing the leakage flow and improving the aerodynamic performance in the transonic turbine stage.

Towards a physics-based understanding of fruit frost protection using wind machines

Wind machines are used in the agricultural sector to prevent or mitigate the adverse effects of night frost in spring. In this study we aim to quantify the impact of wind machine operation on the local temperature field in an orchard. To this end, a field experiment is conducted and experimental analysis is combined with numerical simulation studies in order to assess the functional relations between wind machine performance and the dominating physical processes occurring during radiative frost events.Experimental observations showed that the temperature response strongly depends on the radial distance to the fan and the height above the surface. In agreement with previous studies, the wind machine was able to achieve rotation-averaged temperature increases of up to 50% of the inversion strength ( ≈ 3 K) in an area of 3–5 ha at 1 m height. Furthermore, it was observed that even weak ambient winds (<1 m/s) already may cause strong upwind-downwind asymmetries in the protected area, the downwind area being larger.The numerical model, inspired by the field experiment, showed similar spatial temperature responses as compared to observations. Interestingly, it was found that slower rotation times of the wind machine (3 to 6 min) lead to a significant increase of affected area, while the temperature enhancement itself stayed relatively constant. Variation of the horizontal tilt angle showed that, in our model, temperature enhancement was maximized between 8∘ and 16∘. This nearly horizontal flow already facilitates efficient vertical mixing of momentum and heat, presumably due to generation of shear instabilities at the lower edge of the jet. Finally, like in the observations also the numerical result showed strong upwind-downwind asymmetry in the affected area due to background wind.

The Arctic Front and its variability in the Norwegian Sea

Abstract. The Arctic Front (AF) in the Norwegian Sea is an important biologically productive region which is well-known for its large feeding schools of pelagic fish. A suite of satellite data, a regional coupled ocean–sea ice data assimilation system (the TOPAZ reanalysis) and atmospheric reanalysis data are used to investigate the variability in the lateral and vertical structure of the AF. A method, known as “singularity analysis”, is applied on the satellite and reanalysis data for 2-D spatial analysis of the front, whereas for the vertical structure, a horizontal gradient method is used. We present new evidence of active air–sea interaction along the AF due to enhanced momentum mixing near the frontal region. The frontal structure of the AF is found to be most distinct near the Faroe Current in the south-west Norwegian Sea and along the Mohn Ridge. Coincidentally, these are the two locations along the AF where the air–sea interactions are most intense. This study investigates in particular the frontal structure and its variability along the Mohn Ridge. The seasonal variability in the strength of the AF is found to be limited to the surface. The study also provides new insights into the influence of the three dominant modes of the Norwegian Sea atmospheric circulation on the AF along the Mohn Ridge. The analyses show a weakened AF during the negative phase of the North Atlantic Oscillation (NAO−), even though the geographical location of the front does not vary. The weakening of AF during NAO− is attributed to the variability in the strength of the Norwegian Atlantic Front Current over the Mohn Ridge associated with the changes in the wind field.

Baroclinic Instability with a Simple Model for Vertical Mixing

AbstractHere, we examine baroclinic instability in the presence of vertical mixing in an idealized setting. Specifically, we use a simple model for vertical mixing of momentum and buoyancy and expand the buoyancy and vorticity in a series for small Rossby numbers. A flow in subinertial mixed layer (SML) balance (see the study by Young in 1994) exhibits a normal mode linear instability, which is studied here using linear stability analysis and numerical simulations. The most unstable modes grow by converting potential energy associated with the basic state into kinetic energy of the growing perturbations. However, unlike the inviscid Eady problem, the dominant energy balance is between the buoyancy flux and the energy dissipated by vertical mixing. Vertical mixing reduces the growth rate and changes the orientation of the most unstable modes with respect to the front. By comparing with numerical simulations, we find that the predicted scale of the most unstable mode matches the simulations for small Rossby numbers while the growth rate and orientation agree for a broader range of parameters. A stability analysis of a basic state in SML balance using the inviscid QG equations shows that the angle of the unstable modes is controlled by the orientation of the SML flow, while stratification associated with an advection/diffusion balance controls the size of growing perturbations for small Ekman numbers and/or large Rossby numbers. These results imply that baroclinic instability can be inhibited by small-scale turbulence when the Ekman number is sufficiently large and might explain the lack of submesoscale eddies in observations and numerical models of the ocean surface mixed layer during summer.

Enhanced Diapycnal Mixing Between Water Masses in the Western Equatorial Pacific

AbstractDiapycnal mixing of momentum and heat is crucially important to ocean dynamics, as it affects the state of the ocean and its interactions with the atmosphere. For realistic climate and circulation predictions, the magnitude and distribution of diapycnal mixing must be accurately represented in ocean models. Based on full‐depth microstructure data obtained in the western equatorial Pacific, we demonstrate that there is an enhanced diapycnal diffusivity layer nested below the thermocline of the western equatorial Pacific. Diapycnal diffusivities in this layer range from 5×10−6 to 5× 10−4 m2 s−1, one to two orders of magnitude higher than those predicted by the wave‐wave interaction theory (5 ×10−7−5×10−6 m2 s−1). These enhanced diapycnal mixings are strongly related to the South Pacific Tropical Water intrusion. Stratification is largely weakened by the South Pacific Tropical Water intrusion; thus, the shear can easily trigger shear instabilities and induce strong diapycnal mixing. This enhanced diapycnal diffusivity layer occupies the water column between ~250 and 750 m and spans from the equator to 10°N. Such widespread enhanced diapycnal mixing related to water mass intrusions may play an important role in the dynamics of the equatorial ocean, and it should be taken into account in ocean models.

Numerical study of desalination by vacuum membrane distillation – Transient three-dimensional analysis

The performance of vacuum membrane distillation (VMD) modules can be optimized through careful selection of design parameters. The present study examines how the addition of cylindrical filaments in the feed channel increases momentum mixing and the overall performance of VMD modules under different operating inlet conditions. Three-dimensional transient Computational Fluid Dynamics (CFD) simulations are conducted using Wall-Adapting Local Eddy-Viscosity (WALE) subgrid-scale Large Eddy Simulation (LES) turbulence model. Local concentration, temperature, and flux are coupled at the membrane surface to predict the rate of water vapor diffused through the membrane by Knudsen and viscous diffusion mechanisms. The predicted and measured vapor flux agrees reasonably well; validating the employed model. The small-scale eddies induced by the presence of spacer filaments promote mixing in the module, thus the temperature and concentration polarization is alleviated and the water vapor flux is immensely improved. The insertions of filaments in the feed channel increase the water permeate rate by more than 50% at higher feed flow rates and inlet temperatures. The pressure drop by the spacer reduces the allowable module length by one order of magnitude, but the module length increases two folds at feed temperature 80°C. Even though the power consumption of the module containing the filaments is increased, the addition of filaments is strongly recommended since the required power for the process could be supplied from readily available low-grade heat source.

Gas accretion and galactic fountain flows in the Auriga cosmological simulations: angular momentum and metal redistribution

ABSTRACT Using a set of 15 high-resolution magnetohydrodynamic cosmological simulations of Milky Way formation, we investigate the origin of the baryonic material found in stars at redshift zero. We find that roughly half of this material originates from subhalo/satellite systems and half is smoothly accreted from the intergalactic medium. About $90 {{\ \rm per\ cent}}$ of all material has been ejected and re-accreted in galactic winds at least once. The vast majority of smoothly accreted gas enters into a galactic fountain that extends to a median galactocentric distance of ∼20 kpc with a median recycling time-scale of ∼500 Myr. We demonstrate that, in most cases, galactic fountains acquire angular momentum via mixing of low angular momentum, wind-recycled gas with high angular momentum gas in the circumgalactic medium (CGM). Prograde mergers boost this activity by helping to align the disc and CGM rotation axes, whereas retrograde mergers cause the fountain to lose angular momentum. Fountain flows that promote angular momentum growth are conducive to smooth evolution on tracks quasi-parallel to the disc sequence of the stellar mass-specific angular momentum plane, whereas retrograde minor mergers, major mergers, and bar-driven secular evolution move galaxies towards the bulge sequence. Finally, we demonstrate that fountain flows act to flatten and narrow the radial metallicity gradient and metallicity dispersion of disc stars, respectively. Thus, the evolution of galactic fountains depends strongly on the cosmological merger history and is crucial for the chemodynamical evolution of Milky-Way-sized disc galaxies.

Lower-Tropospheric Influences on the Timing and Intensity of Afternoon Severe Convection over Modest Terrain in a Convection-Allowing Ensemble

AbstractA 50-member convection-allowing ensemble is used to examine effects of daytime PBL evolution and ambient flow interacting with modest terrain features on convection initiation (CI) in the lee of the Rocky Mountains. The examined case (4 June 2015) has isolated supercell storms that initiate during mid- to late afternoon along the northern portion of the Palmer Lake Divide, which is a ~0.5-km-deep zonally oriented terrain feature in east-central Colorado that extends eastward from the Rocky Mountains. To diagnose factors most crucial to storm development, two 10-member subensembles are constructed from the full 50-member ensemble. One subensemble (STRONG) has storm locations with mature storm intensities, and average timing of CI similar to that observed. The other subensemble (WEAK) has fewer storms, with generally weaker intensity, and delayed CI. Environmental composites constructed from these subensembles reveal a stronger surface horizontal convergence zone and moisture gradient in STRONG, resulting from 2–3.5 m s−1 stronger southerly winds on the south flank of the convergence zone. The stronger southerlies result from accelerated PBL growth and momentum mixing in the presence of strong low-to-midtropospheric vertical shear, which is facilitated by reduced above-PBL static stability in the composite STRONG initial condition. Stronger time-averaged low-to-midtropospheric upward motion coincides with the surface convergence zone in STRONG, and individual CI locations occur at the northeastern edge of the composite vertical motion maximum. Trajectory analysis with STRONG members confirms that the CI locations are consistent with large vertical displacements, and corresponding relative humidity increases leading to decreases in convective inhibition, as the southerly airstream ascends across the convergence zone.

Numerical Simulations of Flow and Heat Transfer in a Wall-Bounded Pin Matrix

A detailed computational study was performed for the case of a wall-bounded pin-fin-array in a staggered arrangement, representative of industrial configurations designed to enhance heat transfer. In order to evaluate the level of turbulence modelling necessary to accurately reproduce the flow physics at three (3) different Reynolds numbers (3,000, 10,000 and 30,000), four models were selected: two eddy-viscosity URANS models (k-ω-SST and ϕ-model), an Elliptic Blending Reynolds-stress model (EB-RSM) and Large Eddy Simulation (LES). Global comparisons for the pressure loss coefficients and average Nusselt numbers were performed with available experimental data which are relevant for industrial applications. Further detailed comparisons of the velocity fields, turbulence quantities and local Nusselt numbers revealed that the correct prediction of the characteristics of the flow is closely related to the ability of the turbulence model to reproduce the large-scale unsteadiness in the wake of the pins, which is at the origin of the intense mixing of momentum and heat. Eddy-viscosity-based turbulence models have difficulties to develop such an unsteadiness, in particular around the first few rows of pins, which leads to a severe underestimation of the Nusselt number. In contrast, LES and EB-RSM are able to predict the unsteady motion of the flow and heat transfer in a satisfactory manner.

Orbital Angular Momentum (OAM) Mode Mixing in a Bent Step Index Fiber in Perturbation Theory

Within the framework of perturbation theory, we explore in detail the mixing of orbital angular momentum(OAM) modes due to a fiber bend in a step-index multimode fiber. Using scalar wave equation, we develop a complete set of analytic expressions for mode-mixing, including those for the $2\pi$ walk-off length, which is the distance traveled within the bent fiber before an OAM mode transforms into its negative topological charge counterpart, and back into itself. The derived results provide insight into the nature of the bend effects, clearly revealing the mathematical dependence on the bend radius and the topological charge. We numerically simulate the theoretical results with applications to a few-mode fiber and a multimode fiber, and calculate bend-induced modal crosstalk with implications for mode-multiplexed systems. The presented perturbation technique is general enough to be applicable to other perturbations like ellipticity and easily extendable to other fibers with step-index-like profile as in the ring fiber.

The Role of the Divergent Circulation for Large-Scale Eddy Momentum Transport in the Tropics. Part I: Observations

Abstract This work investigates the role played by the divergent circulation for meridional eddy momentum transport in the tropical atmosphere. It is shown that the eddy momentum flux in the deep tropics arises primarily from correlations between the divergent eddy meridional velocity and the rotational eddy zonal velocity. Consistent with previous studies, this transport is dominated by the stationary wave component, associated with correlations between the zonal structure of the Hadley cell (zonal anomalies in the meridional overturning) and the climatological-mean Rossby gyres. This eddy momentum flux decomposition implies a different mechanism of eddy momentum convergence from the extratropics, associated with upper-level mass convergence (divergence) over sectors with anomalous westerlies (easterlies). By itself, this meridional transport would only increase (decrease) isentropic thickness over regions with anomalous westerly (easterly) zonal flow. The actual momentum mixing is due to vertical (cross isentropic) advection, pointing to the key role of diabatic processes for eddy–mean flow interaction in the tropics.

Resolvent-analysis-based design of airfoil separation control

We use resolvent analysis to design active control techniques for separated flows over a NACA 0012 airfoil. Spanwise-periodic flows over the airfoil at a chord-based Reynolds number of$23\,000$and a free-stream Mach number of$0.3$are considered at two post-stall angles of attack of$6^{\circ }$and$9^{\circ }$. Near the leading edge, localized unsteady thermal actuation is introduced in an open-loop manner with two tunable parameters of actuation frequency and spanwise wavelength. To provide physics-based guidance for the effective choice of these control input parameters, we conduct global resolvent analysis on the baseline turbulent mean flows to identify the actuation frequency and wavenumber that provide large perturbation energy amplification. The present analysis also considers the use of a temporal filter to limit the time horizon for assessing the energy amplification to extend resolvent analysis to unstable base flows. We incorporate the amplification and response mode from resolvent analysis to provide a metric that quantifies momentum mixing associated with the modal structure. This metric is compared to the results from a large number of three-dimensional large-eddy simulations of open-loop controlled flows. With the agreement between the resolvent-based metric and the enhancement of aerodynamic performance found through large-eddy simulations, we demonstrate that resolvent analysis can predict the effective range of actuation frequency as well as the global response to the actuation input. We believe that the present resolvent-based approach provides a promising path towards mean flow modification by capitalizing on the dominant modal mixing.

Stratified shear instability in a field of pre-existing turbulence

Turbulent mixing of heat and momentum in the stably-stratified ocean interior occurs in discrete events driven by vertical variations of the horizontal velocity. Typically, these events have been modelled assuming an initially laminar stratified shear flow which develops wavelike instabilities, becomes fully turbulent, and then relaminarizes into a stable state. However, in the real ocean there is always some level of turbulence left over from previous events. Using direct numerical simulations, we show that the evolution of a stably-stratified shear layer may be significantly modified by pre-existing turbulence. The classical billow structure associated with Kelvin–Helmholtz instability is suppressed and eventually eliminated as the strength of the initial turbulence is increased. A corresponding energetics analysis shows that potential energy changes and dissipation of kinetic energy depend non-monotonically on initial turbulence strength, with the largest effects when initial turbulence is present but insufficient to prevent billow formation. The mixing efficiency decreases with increasing initial turbulence amplitude as the development of the Kelvin–Helmholtz billow, with its large pre-turbulent mixing efficiency, is arrested.

SST front anchored mesoscale feature of surface wind in the southern Indian Ocean

Using 28-year satellite-borne Special Sensor Microwave Imager observations, features of high-wind frequency (HWF) over the southern Indian Ocean are investigated. Climatology maps show that high winds occur frequently during austral winter, located in the open ocean south of Polar Front in subpolar region, warm flank of the Subantarctic Front between 55°E–78°E, and south of Cape Agulhas, where westerly wind prevails. The strong instability of marine atmospheric boundary layer accompanied by increased sensible and latent heat fluxes on the warmer flank acts to enhance the vertical momentum mixing, thus accelerate the surface winds. Effects of sea surface temperature (SST) front can even reach the entire troposphere by deep convection. HWF also shows distinct interannual variability, which is associated with the Southern Annual Mode (SAM). During positive phase of the SAM, HWF has positive anomalies over the open ocean south of Polar Front, while has negative anomalies north of the SST front. A phase shift of HWF happened around 2001, which is likely related to the reduction of storm tracks and poleward shift of westerly winds in the Southern Hemisphere.

Onset of non-linear internal gravity waves in intermediate-mass stars

Internal gravity waves (IGW) propagate in the radiation zones of all stars. During propagation, their amplitudes are affected by two main features: radiative diffusion and density stratification. We have studied the implications of these two features on waves travelling within the radiative zones of non-rotating stars with stellar parameters obtained from the one-dimensional stellar evolution code, MESA. As a simple measure of induced wave dynamics, we define a criterion to see if waves can become non-linear and if so, under what conditions. This was done to understand the role IGW may play in angular momentum transport and mixing within stellar interiors. We find that the IGW generation spectrum, convective velocities, and the strength of density stratification all play major roles in whether waves become non-linear. With increasing stellar mass, there is an increasing trend in non-linear wave energies. The trends with different metallicities and ages depend on the generation spectrum.

Forecasting wind gusts in winter storms using a calibrated convection‐permitting ensemble

Windstorms associated with low‐pressure systems from the North Atlantic are the most important natural hazards for central Europe. Although their predictability has generally improved over the last decades, forecasting wind gusts is still challenging, due to the multiple scales involved. One of the first ensemble prediction systems at convection‐permitting resolution, COSMO‐DE‐EPS, offers a novel 2.8‐km dataset over Germany for the 2011–2016 period. The high resolution allows representation of mesoscale features that are barely captured by global models, while the long period allows both investigation of rare storms and application of statistical post‐processing. Ensemble model output statistics based on a truncated logistic distribution substantially improve forecasts of wind gusts in the whole dataset. However, some winter storms exhibit uncharacteristic forecast errors that cannot be reduced by post‐processing. During the passage of the most severe storm, gusts related to a cold jet are predicted relatively well at the time of maximum intensity, whereas those related to a warm jet are poorly predicted at an early phase. Wind gusts are overestimated during two cases of frontal convection, which suggests that even higher resolution is needed to resolve fully the downward mixing of momentum and the stabilization resulting from convective dynamics. In contrast, extreme gusts are underestimated during a rare case involving a possible sting jet, but this arises from the representation of the synoptic rather than the mesoscale. The synoptic scale also controls the ensemble spread, which is inherited mostly from the initial and boundary conditions. This is unsurprising, but leads to high forecast uncertainty in the case of a small, fast‐moving cyclone crossing the model domain. These results illustrate how statistical post‐processing can help identify the limits of predictability across scales in convection‐permitting ensemble forecasts. They may guide the development of regime‐dependent statistical methods to improve forecasts of wind gusts in winter storms further.

Twisted hollow fiber membrane modules for reverse osmosis-driven desalination

We show how a simple design adaptation creates desirable flow structures that dramatically enhance performance in hollow fiber membrane (HFM) water desalination modules using computational fluid dynamics (CFD) simulations. Conventional HFM desalination modules encase thousands of co-axially aligned HFMs in a mutually parallel flow of brackish water. HFMs are subject to a phenomenon called concentration polarization (CP), which leads to fouling and will eventually prevent clean water production. We found that the twisted HFM module mitigates CP effects and increases transmembrane permeate flux by 5–9% for three flow rates considered. Twisted HFM bundles induce swirling flow structures inside desalination modules that increase momentum mixing throughout. Frictional energy losses and increased pumping power associated with this subtle design alteration are small relative to projected gains in clean water production. We predict system performance increases about 70% for the twisted modules herein considered, and there are in principle no additional required components associated with this geometry adaptation. With our findings, we identify how the twisted module design induces desirable flow structures that increase membrane separation performance by mitigating CP effects and increasing HFM efficacy.

Theoretical study of asymmetric super-rotors: Alignment and orientation

We report a theoretical study of the optical centrifuge acceleration of an asymmetric top molecule interacting with an electric static field by solving the time-dependent Schr\"odinger equation in the rigid rotor approximation. A detailed analysis of the mixing of the angular momentum in both the molecular and the laboratory fixed frames allows us to deepen the understanding of the main features of the acceleration process, for instance, the effective angular frequency of the molecule at the end of the pulse. For the case of the ${\mathrm{SO}}_{2}$ molecular super-rotor, we show numerically that it rotates around one internal axis and that its dynamics is confined to the plane defined by the polarization axis of the laser, in agreement with experimental findings. Furthermore, we consider the orientation patterns induced by the dc field, showing the characteristics of their structure as a function of the strength of the static field and the initial configuration of the fields.

Sensitivity of Coupled Tropical Pacific Model Biases to Convective Parameterization in CESM1

AbstractSix month coupled hindcasts show the central equatorial Pacific cold tongue bias development in a GCM to be sensitive to the atmospheric convective parameterization employed. Simulations using the standard configuration of the Community Earth System Model version 1 (CESM1) develop a cold bias in equatorial Pacific sea surface temperatures (SSTs) within the first two months of integration due to anomalous ocean advection driven by overly strong easterly surface wind stress along the equator. Disabling the deep convection parameterization enhances the zonal pressure gradient leading to stronger zonal wind stress and a stronger equatorial SST bias, highlighting the role of pressure gradients in determining the strength of the cold bias. Superparameterized hindcasts show reduced SST bias in the cold tongue region due to a reduction in surface easterlies despite simulating an excessively strong low‐level jet at 1‐1.5 km elevation. This reflects inadequate vertical mixing of zonal momentum from the absence of convective momentum transport in the superparameterized model. Standard CESM1simulations modified to omit shallow convective momentum transport reproduce the superparameterized low‐level wind bias and associated equatorial SST pattern. Further superparameterized simulations using a three‐dimensional cloud resolving model capable of producing realistic momentum transport simulate a cold tongue similar to the default CESM1. These findings imply convective momentum fluxes may be an underappreciated mechanism for controlling the strength of the equatorial cold tongue. Despite the sensitivity of equatorial SST to these changes in convective parameterization, the east Pacific double‐Intertropical Convergence Zone rainfall bias persists in all simulations presented in this study.

Frontogenesis and frontal arrest of a dense filament in the oceanic surface boundary layer

The evolution of upper ocean currents involves a set of complex, poorly understood interactions between submesoscale turbulence (e.g. density fronts and filaments and coherent vortices) and smaller-scale boundary-layer turbulence. Here we simulate the lifecycle of a cold (dense) filament undergoing frontogenesis in the presence of turbulence generated by surface stress and/or buoyancy loss. This phenomenon is examined in large-eddy simulations with resolved turbulent motions in large horizontal domains using${\sim}10^{10}$grid points. Steady winds are oriented in directions perpendicular or parallel to the filament axis. Due to turbulent vertical momentum mixing, cold filaments generate a potent two-celled secondary circulation in the cross-filament plane that is frontogenetic, sharpens the cross-filament buoyancy and horizontal velocity gradients and blocks Ekman buoyancy flux across the cold filament core towards the warm filament edge. Within less than a day, the frontogenesis is arrested at a small width,${\approx}100~\text{m}$, primarily by an enhancement of the turbulence through a small submesoscale, horizontal shear instability of the sharpened filament, followed by a subsequent slow decay of the filament by further turbulent mixing. The boundary-layer turbulence is inhomogeneous and non-stationary in relation to the evolving submesoscale currents and density stratification. The occurrence of frontogenesis and arrest are qualitatively similar with varying stress direction or with convective cooling, but the detailed evolution and flow structure differ among the cases. Thus submesoscale filament frontogenesis caused by boundary-layer turbulence, frontal arrest by frontal instability and frontal decay by forward energy cascade, and turbulent mixing are generic processes in the upper ocean.