Turbulent Energy Research Articles

Turbulent Energy and Carbon Fluxes in an Andean Montane Forest—Energy Balance and Heat Storage

High mountain rainforests are vital in the global energy and carbon cycle. Understanding the exchange of energy and carbon plays an important role in reflecting responses to climate change. In this study, an eddy covariance (EC) measurement system installed in the high Andean Mountains of southern Ecuador was used. As EC measurements are affected by heterogeneous topography and the vegetation height, the main objective was to estimate the effect of the sloped terrain and the forest on the turbulent energy and carbon fluxes considering the energy balance closure (EBC) and the heat storage. The results showed that the performance of the EBC was generally good and estimated it to be 79.5%. This could be improved when the heat storage effect was considered. Based on the variability of the residuals in the diel, modifications in the imbalances were highlighted. Particularly, during daytime, the residuals were largest (56.9 W/m2 on average), with a clear overestimation. At nighttime, mean imbalances were rather weak (6.5 W/m2) and mostly positive while strongest underestimations developed in the transition period to morning hours (down to −100 W/m2). With respect to the Monin–Obukhov stability parameter ((z − d)/L) and the friction velocity (u*), it was revealed that the largest overestimations evolved in weak unstable and very stable conditions associated with large u* values. In contrast, underestimation was related to very unstable conditions. The estimated carbon fluxes were independently modelled with a non-linear regression using a light-response relationship and reached a good performance value (R2 = 0.51). All fluxes were additionally examined in the annual course to estimate whether both the energy and carbon fluxes resembled the microclimatological conditions of the study site. This unique study demonstrated that EC measurements provide valuable insights into land-surface–atmosphere interactions and contribute to our understanding of energy and carbon exchanges. Moreover, the flux data provide an important basis to validate coupled atmosphere ecosystem models.

Coherent turbulent flow structure under plunging breaker on movable grain bed simulated by 3D DEM-MPS method

ABSTRACT Breaking waves transport momentum, gas, and sediment vertically. Recent particle image velocimetry (PIV) measurements have elucidated the coherent vortex and turbulence structures under breaking waves. However, clarifying three-dimensional fluid motion under breaking waves with PIV remains challenging. Although particle methods have performed wave-breaking simulations, they have never been validated through velocity distributions for breaking waves above movable sediment beds. This study simulated the sediment transport process under plunging waves using a 3D Lagrangian fluid-particle mixture model. The investigation focuses on the first wave plunging for a fundamental investigation. Our simulation shows the instantaneous velocity distributions at different wave phases, which agree with the PIV measurement. A physically-based turbulence extraction method is introduced: a numerical filter based on the coherence between the water elevation and horizontal velocity. Visualized three-dimensional vortex and turbulence structures are consistent with previous studies. The transport equation of the kinetic turbulence energy investigates the turbulence budget near the bed surface and emphasizes the negative contribution of the fluid-particle interaction. Our findings show that a contribution of the pressure gradient to offshore sediment transport. The pressure gradient is difficult to correlate to the water surface gradient and is derived from the large vortex induced by the plunging wave.

Two-point statistics in non-homogeneous rotating turbulence

Time-resolved two-dimensional two-component particle image velocimetry measurements with high spatial resolution are carried out in a water tank agitated by four blades rotating at constant speed. Different blade geometries and rotation speeds are tested for the purpose of modifying turbulent flow conditions. In all cases where no baffles are used to break the rotation, the Zeman length is an order of magnitude smaller than the Taylor length. Compared with the cases with baffles which break the rotation, in the unbaffled cases turbulence production and/or mean advection are significant and the turbulence nonlinearity is dramatically reduced for the horizontal (i.e. normal to the axis of rotation) two-point turbulence fluctuating velocities. This nonlinearity reduction is manifest not only in the interscale turbulent energy transfer but also in the interspace turbulent energy transfer, which nearly vanishes. However, the nonlinearity is not reduced for the vertical two-point turbulence fluctuating velocities: the corresponding interscale turbulent transfer rate is in fact intensified, and its dependence on the two-point separation distance, as well as that of the corresponding interspace turbulent transfer rate, which does not vanish, is significantly modified. Even though non-homogeneities are very different for different blades and rotation speeds in the unbaffled cases, the horizontal fluctuating velocity's second-order structure function collapses with scalings which resemble predictions for homogeneous turbulence subject to strong rotation. The vertical fluctuating velocity's second-order structure function does not collapse for different blade geometries by neither these nor the Kolmogorov predictions.

Orifice Versus Converging-Nozzle Grid Turbulence: A Wavelet Perspective

Grids such as perforated plates are of fundamental importance in flow turbulence study and are commonly utilised to promote mixing. An orificed perforated plate (OPP) and its reversed counterpart, the converging-nozzle perforated plate (CNPP), were applied to produce quasi-isotropic turbulence inside a wind tunnel. The three orthogonal velocity components were measured using a triple hotwire at 10D downstream of the perforated plate for Reynolds numbers, ReD, 18,700 and 28,400, where D is the diameter of the perforated holes. The statistics of the grid-generated turbulence was analysed using the time-averaged local velocity profile and turbulence intensity, which revealed a more homogeneous distribution of the flow field with a higher level of turbulence for the OPP. Fourier and wavelet analyses were employed to investigate the energy of the eddies as a function of frequency and multiscale characteristics of the fluctuating velocity, respectively. At ReD = 18,700, the turbulent energy remains prominently with large-scale vortical structures which are non-intermittently present in the flow for both perforated plates. The thickness of the converging channels of the CNPP appears to provide the venue for spawning intermittent fluctuations. At higher ReD 28,400, the effect of this intermittent behaviour becomes evident for the CNPP, leading to a multiscale distribution of turbulent energy.

Direct observation of ion cyclotron damping of turbulence in Earth’s magnetosheath plasma

Plasma turbulence plays a key role in space and astrophysical plasma systems, enabling the energy of magnetic fields and plasma flows to be transported to particle kinetic scales at which the turbulence dissipates and heats the plasma. Identifying the physical mechanisms responsible for the dissipation of the turbulent energy is a critical step in developing the predictive capability for the turbulent heating needed by global models. In this work, spacecraft measurements of the electromagnetic fields and ion velocity distributions by the Magnetospheric Multiscale (MMS) mission are used to generate velocity-space signatures that identify ion cyclotron damping in Earth’s turbulent magnetosheath, in agreement with analytical modeling. Furthermore, the rate of ion energization is directly quantified and combined with a previous analysis of the electron energization to identify the dominant channels of turbulent dissipation and determine the partitioning of energy among species in this interval.

THE IMPACT OF SOLAR RADIATION ON THE TEMPERATURE REGIME OF A ROOM IN WINTER

The article is devoted to the computational studies of the air-thermal state of office premises taking into account the effect of solar radiation coming through window openings. The study was conducted for a room with two windows using two heating devices installed under them. The air-temperature regime of premises in winter, characterized by the highest thermal energy consumption for heat supply, is considered. The study is based on the solution of a three-dimensional nonlinear heat transfer problem described by a system of equations of turbulent momentum and energy transfer. The k-e turbulence model is used to close this system. The results of numarical modeling of the physical situation under study are presented. The research data on the features of the air-thermal state of the premises under solar radiation conditions are given. The results of a comparative analysis of the air-temperature conditions of office premises corresponding to the solution of the specified heat exchange problems in the presence and absence of solar radiation are presented and the effects of solar radiation on the structure of the air flow and the thermal state of the premises are established. It is shown that in the presence of solar radiation, the air flow picture and the character of the temperature fields in the premises change significantly. In particular, in these conditions, the increase in the average temperature of the premises for the studied period is 2.5 °С. The possibility of a certain reduction of the load on the heating system in the presence of solar radiation is noted. Bibl. 17, Fig. 5.

High-turbulence fine particle flotation cell optimization and verification

Microfine mineral particles have a small size, light weight, and low inertia, making it difficult for them to deviate from streamlines and collide with bubbles. Conventional flotation operations consume a large amount of reagents and exhibit poor flotation indicators. This study employs computational fluid dynamics (CFD) simulation and hydrodynamic testing to investigate the flow field within a high-turbulence microfine particle flotation machine equipped with a multilayer impeller–stator configuration, and validates the practical application performance of the microfine particle flotation machine through single-batch flotation experiments. Result shows that the impeller region of the traditional mechanical stirring flotation machine has a turbulent energy dissipation rate of 20 m²/s³, whereas that for the microfine particle flotation machine averages over 120 m²/s³. In the flotation verification, the cumulative recovery rate of the fine particle flotation machine is increased by 28% compared with that of the traditional KYF flotation machine. The flotation rate is also 1.3 times that of the KYF, demonstrating stronger selectivity for fine particle concentrates. It has certain guiding significance for the resource utilization of fine particle minerals.

Distinct Impacts of Topographic versus Planetary PV Gradients on Baroclinic Turbulence

Abstract The impacts of topographic versus planetary potential vorticity (PV) gradients on fully developed geostrophic turbulence are often treated as dynamically equivalent in theoretical understanding and parameterizations of ocean mesoscale turbulence. Using a suite of homogeneous, two-layer quasigeostrophic (QG) turbulence simulations, we identify similarities and distinctions of topographic versus planetary PV gradients in modulating turbulent eddy energy and fluxes. We show that, while an elevated background PV gradient can suppress geostrophic turbulence, a positive (negative) bottom slope with respect to the orientation of isopycnal slope barotropizes (debarotropizes) the turbulent energy at large scales, which contrasts with a dynamically inert planetary PV gradient in the modewise energy transfer. Then, in the presence of weak bottom drag, a positive slope energizes large-scale along-slope jets and limits small-scale barotropic eddies, both of which yield stronger eddy suppression effects than from a planetary PV gradient; by contrast, a negative slope hinders along-slope jet formation by enhancing the dual-energy cascade cycling, which alleviates the topographic suppression on eddies. In the presence of strong bottom drag, a positive slope elevates barotropic eddy energy, which further enhances the eddy-driven fluxes; by contrast, a negative slope confines turbulent energy to the baroclinic mode, which is strongly damped, causing further weakened turbulent energy and eddy fluxes. A flow regime captured by linear QG theories also emerges as the turbulent energy cascade is jointly suppressed by negative slopes and strong bottom drag. This study provides insights into parameterizing mesoscale eddy effects across sloping bathymetry in predictive ocean models.

Polymer-doped two-dimensional turbulent flow to study the transition from Newtonian turbulence to elastic instability

Detecting the flow regimes of Newtonian turbulence (NT), elasto-inertial filament (EIF), elasto-inertial turbulence (EIT), and maximum drag reduction (MDR) of polymer solution and their transition has been a hot topic in the last decade. We attempted to detect NT, EIF, EIT, and MDR by visualizing vortex shedding downstream of an array of cylinders that was inserted perpendicular to polymer-doped two-dimensional (2D) flow. Since polymers are stretched at the cylinders, the consequent vortex shedding is affected by viscoelasticity. The flow regimes are characterized based on Weissenberg (Wi) and Reynolds numbers (Re) with the relaxation time of the polymeric solution estimated from capillary-thinning experiments. The flow regimes are observed for different molecular weights of polyethylene oxide and polyacrylamide in solution and are categorized as either vortex type 1, type 2, and type 3 on a Re–Wi map based on flow visualization using particle image velocimetry. In addition, turbulent statistics of these flow regimes are calculated to more fully quantify these flow regimes. We found that vortex types from 1 to 3 have a similarity to NT, EIF, EIT, and MDR. In addition, characteristic turbulent energy transfer without an increase in turbulent energy production was found in the flow regimes of vortex types 2 and 3 of each polymer solution. Our results suggest intriguing parallels between pipe, jet, and 2D turbulent flow for drag-reducing polymeric solutions.

A Magnetized Strongly Turbulent Corona as the Source of Neutrinos from NGC 1068

The cores of active galactic nuclei are potential accelerators of 10–100 TeV cosmic rays, in turn producing high-energy neutrinos. This picture was confirmed by the compelling evidence of a TeV neutrino signal from the nearby active galaxy NGC 1068, leaving open the question of what is the site and mechanism of cosmic-ray acceleration. One candidate is the magnetized turbulence surrounding the central supermassive black hole. Recent particle-in-cell simulations of magnetized turbulence indicate that stochastic cosmic-ray acceleration is nonresonant, in contrast to the assumptions of previous studies. We show that this has important consequences on a self-consistent theory of neutrino production in the corona, leading to a more rapid cosmic-ray acceleration than previously considered. The turbulent magnetic-field fluctuations needed to explain the neutrino signal are consistent with a magnetically powered corona. We find that strong turbulence, with turbulent magnetic energy density higher than 1% of the rest-mass energy density, naturally explains the normalization of the IceCube neutrino flux, in addition to the neutrino spectral shape. Only a fraction of the protons in the corona, which can be directly inferred from the neutrino signal, are accelerated to high energies. Thus, in this framework, the neutrino signal from NGC 1068 provides a testbed for particle acceleration in magnetized turbulence.

Mapping and characterizing magnetic fields in the Rho Ophiuchus-A molecular cloud with SOFIA/HAWC+

Context. Together with gravity, turbulence, and stellar feedback, magnetic fields (B-fields) are thought to play a critical role in the evolution of molecular clouds and star formation processes. The polarization of thermal dust emission is a popular tracer of B-fields in star-forming regions. Aims. We aim to map the morphology and measure the strength of B-fields of the nearby molecular cloud, rho Ophiuchus-A (ρ Oph-A), to understand the role of B-fields in regulating star formation and in shaping the cloud. Methods. We analyzed the far-infrared (FIR) polarization of thermal dust emission observed by SOFIA/HAWC+ at 89 and 154 μm toward the densest part of ρ Oph-A, which is irradiated by the nearby B3/4 star, Oph-S1. These FIR polarimetric maps cover an area of ~4.5′ × 4.5′ (corresponding to 0″.18 × 0″.18 pc2) with an angular resolution of 7.8″ and 13.6″ respectively. Results. The ρ Oph-A cloud exhibits well-ordered B-fields with magnetic orientations that are mainly perpendicular to the ridge of the cloud toward the densest region. We obtained a map of B-field strengths in the range of 0.2–2.5 mG, using the Davis-Chandrasekhar-Fermi (DCF) method. The B-fields are strongest at the densest part of the cloud, which is associated with the starless core SM1, and then decrease toward the outskirts of the cloud. By calculating the map of the mass-to-flux ratio, Alfvén Mach number, and plasma β parameter in ρ Oph-A, we find that the cloud is predominantly magnetically sub-critical, sub-Alfvénic, which implies that the cloud is supported by strong B-fields that dominate over gravity, turbulence, and thermal gas energy. The measured B-field strengths at the two densest subsregions using other methods that account for the compressible mode are relatively lower than that measured with the DCF method. However, these results do not significantly change our conclusions on the roles of B-fields relative to gravity and turbulence on star formation. Our virial analysis suggests that the cloud is gravitationally unbound, which is consistent with the previous detection of numerous starless cores in the cloud. By comparing the magnetic pressure with the radiation pressure from the Oph-S1 star, we find that B-fields are sufficiently strong to support the cloud against radiative feedback and to regulate the shape of the cloud.

Cosmic-Ray Feedback on Bistable Interstellar Medium Turbulence

While cosmic rays (E ≳ 1 GeV) are well coupled to a galaxy’s interstellar medium (ISM) at scales of L > 100 pc, adjusting stratification and driving outflows, their impact on small scales is less clear. Based on calculations of the cosmic-ray diffusion coefficient from observations of the grammage in the Milky Way, cosmic rays have little time to dynamically impact the ISM on those small scales. Using numerical simulations, we explore how more complex cosmic-ray transport could allow cosmic rays to couple to the ISM on small scales. We create a two-zone model of cosmic-ray transport, with the cosmic-ray diffusion coefficient set at the estimated Milky Way value in cold gas but smaller in warm gas. We compare this model to simulations with a constant diffusion coefficient. Quicker diffusion through cold gas allows more cold gas to form compared to a simulation with a constant, small diffusion coefficient. However, slower diffusion in warm gas allows cosmic rays to take energy from the turbulent cascade anisotropically. This cosmic-ray energization comes at the expense of turbulent energy which would otherwise be lost during radiative cooling. Finally, we show our two-zone model is capable of matching observational estimates of the grammage for some transport paths through the simulation.

A CFD‐PBM‐ANN framework to simulate the liquid–liquid two‐phase flow in a pulsed column

AbstractCFD‐PBM numerical simulation is a powerful tool in the research of droplet swarm behavior. In this work, an artificial neural network (ANN) based droplet breakage frequency function is established based on the directly measured data from our previous studies. Then, the weights and biases of ANN are embedded into the CFD‐PBM code in the form of matrices and vectors. For the first time, a CFD‐PBM‐ANN simulation framework is established. Simulation results are in good agreement with the experimental data under different operation conditions. The cumulative droplet size distribution decreases with the increase of interfacial tension and pulse intensity. It is also found by the simulation that the droplet breakage frequency is relatively high at the edge of disc and doughnut plate, which is accordant with the distribution of turbulent energy dissipation and velocity gradient.

Dynamical generation of macroscale magnetic fields and fast flows in a four-component astrophysical plasma

We explore the possibility of the generation or amplification of macroscale magnetic fields and flows in a four-component astrophysical dusty plasma composed of mobile massless electrons and positrons, inertial positive ions and negatively charged static dust particles. The investigation demonstrates that when microscopic turbulent ambient plasma energy is predominantly kinetic, a straight dynamo (DY) mechanism is feasible. Conversely, a unified reverse-dynamo/dynamo (RDY/DY) mechanism is possible when the microscopic turbulent ambient plasma energy is primarily magnetic. Additionally, the evolution of Alfvén Mach numbers at the macro- and microscale are significantly affected by plasma species densities and invariant helicities. The potential implications of the present study for astrophysical settings are also highlighted.

Functionalized super-hydrophobic nanocomposite surface integrating with anti-icing and drag reduction properties

Anti-icing and drag reduction are two significant issues for aircraft, but functional surfaces integrating both properties are rarely reported. Here, we propose a functional super-hydrophobic nanocomposite surface with sharkskin-like groove structures, which achieves the combination of anti-icing and drag reduction properties. The nanocomposite surface was prepared by stripping polydimethylsiloxane doped with carbon nanotubes from a laser-prepared sharkskin-like structure mold and then spraying a layer of polytetrafluoroethylene hydrophobic coating. The microstructures and nanoparticles with low surface energy provide the surface with hydrophobicity and icephobicity, enabling droplet transitions between Cassie and Wenzel states during icing-melting process. Carbon nanotubes enhance the Joule heating performance when voltage is applied. Ice at interface melts into water film, significantly reducing adhesion and enabling efficient anti-icing. Under the influence of the bionic sharkskin riblet structure, the vortices are lifted and pinned above the riblet tips, which reduces the near-wall velocity gradient and total wall shear stress, ultimately leading to lower drag. In addition, the flexible surface undergoes adaptive deformation to more effectively conform to the fluid flow during motion. The riblet protrusions increase the contact area between the flexible surface and the fluid, thereby enabling the absorption of more turbulent energy and fluctuations, further reduces drag. This study presents a new method for manufacturing functional surfaces for practical anti-icing and drag reduction applications.

Blue in green: forestation turns blue water green, mitigating heat at the expense of water availability

Abstract In order to meet a stringent carbon budget, shared socioeconomic pathways (SSPs) aligned with the Paris Agreement typically require substantial land-use changes (LUC), such as large-scale forestation and bioenergy crop plantations. What if such a low-emission, intense-LUC scenario actually materialized? This paper quantifies the biophysical effects of LUC under SSP1-2.6 using an ensemble of regional climate simulations over Europe. We find that LUC projected over the 21st century, primarily broadleaf-tree forestation at the expense of grasslands, reduce summertime heat extremes significantly over large swaths of continental Europe. In fact, cooling from LUC trumps warming by greenhouse gas (GHG) emissions, resulting in milder heat extremes by 2100 for about half of the European population. Forestation brings heat relief by shifting the partition of turbulent energy fluxes away from sensible and towards latent heat fluxes. Impacts on the water cycle are then assessed. Forestation enhances precipitation recycling over continental Europe, but not enough to match the boost of evapotranspiration (green water flux). Run-off (blue water flux) is reduced as a consequence. Some regions experience severe drying in response. In other words, forestation turns blue water green, bringing heat relief but compromising water availability in some already-dry regions.

The interaction between vegetation patchiness and tidal flows in a shortleaf seagrass meadow

AbstractSeagrasses are critical coastal habitats that provide numerous ecosystem services. The inherent patchiness within meadows exerts a significant influence on the flow‐vegetation interaction, which, in turn, affects the array of services provided by these environments. We present field observations of vegetation distribution and hydrodynamic measurements within and surrounding an approximately 2 m diameter gap (bare sediment) in a fragmented seagrass meadow. We show that variability in mean flows and turbulence is correlated with meadow structure at small (O (100 m)) and large (O (102 m)) spatial scales. Our observations reveal that bare gaps within seagrass meadows lead to faster flows and lower bed elevations. Despite slower flow speeds above the dense seagrass adjacent to the gap, the rates of dissipation of turbulent energy above the vegetation are typically around an order of magnitude larger than above the bare bed. Associated with this enhanced dissipation of turbulent energy, we observed a dominance of down‐deceleration events promoting fluid exchange and mixing and driving mass flux into the canopy. Analysis of directional variograms demonstrates that the major continuity axis within NDVI (normalized difference vegetation index) imagery (used as a proxy for vegetation biomass) coincides with the major axis of flow on both gap and meadow scales. Conversely, along the axis of minor flow variance, vegetation remains dense and exhibits greater uniformity. These findings indicate a feedback mechanism between seagrass meadow patchiness and spatial structure through flow modification, which may be beneficial for plant development.

On dissipation timescales of the basic second-order moments: the effect on the energy and flux budget (EFB) turbulence closure for stably stratified turbulence

Abstract. The dissipation rates of the basic second-order moments are the key parameters playing a vital role in turbulence modelling and controlling turbulence energetics and spectra and turbulent fluxes of momentum and heat. In this paper, we use the results of direct numerical simulations (DNSs) to evaluate dissipation rates of the basic second-order moments and revise the energy and flux budget (EFB) turbulence closure theory for stably stratified turbulence. We delve into the theoretical implications of this approach and substantiate our closure hypotheses through DNS data. We also show why the concept of down-gradient turbulent transport becomes incomplete when applied to the vertical turbulent flux of potential temperature under stable stratification. We reveal essential feedback between the turbulent kinetic energy (TKE), the vertical turbulent flux of buoyancy, and the turbulent potential energy (TPE), which is responsible for maintaining shear-produced stably stratified turbulence for any Richardson number.

Requirements for partial turbulence simulations using nondimensional turbulence energy contributions

Quantifying turbulence effects is crucial for understanding building aerodynamics and for ensuring accurate wind tunnel test methods. This is especially important in wind tunnel methods that require post-experiment adjustments because approximate wind fields are used, such as the Partial Turbulence Simulation (PTS) approach. Understanding and analyzing these effects enables load adjustments since the PTS method only requires matching the high frequency portions of the upstream spectra of the longitudinal velocity component in model and full-scale. However, the limits for which the PTS method is applicable are unclear in terms of the allowable range of wind field characteristics that can be used in the wind tunnel simulation. To address this, the paper utilizes two nondimensional parameters, one representing the small-scale turbulence energy, ES, and the other the large-scale turbulence energy, EL, to elaborate the aerodynamic effects of turbulence intensity and integral length scales in the upstream wind. The results show that the maximum allowable mismatch ratio of integral length scales and of Jensen numbers between model and full-scale simulations depend on the target small-scale turbulence energy and the maximum allowable deviation of small-scale energy. By quantifying the effects of ES and EL on area-averaged pressure coefficients, the allowable limits are identified for wind tunnel test parameters that lead to negligible differences in the resultant pressure coefficient statistics in regions of separated-reattaching flow on the roof of a low-rise building.

Regulating star formation in a magnetized disc galaxy

ABSTRACT We use high-resolution magnetohydrodynamic simulations of isolated disc galaxies to investigate the co-evolution of magnetic fields with a self-regulated, star-forming interstellar medium (ISM). The simulations are conducted using the ramses adaptive mesh refinement code on the standard agora initial condition, with gas cooling, star formation, and feedback. We run galaxies with a variety of initial magnetic field strengths. The fields evolve and achieve approximate saturation within 500 Myr, but at different levels. The galaxies reach a quasi-steady state, with slowly declining star formation due to both gas consumption and increase in the field strength at intermediate ISM densities. We connect this behaviour to differences in the gas properties and overall structure of the galaxies. Stronger magnetic fields limit supernova bubble sizes. Different cases support the ISM using varying combinations of magnetic pressure, turbulence, and thermal energy. Initially, $\gtrsim\!\! 1\ \mu \mathrm{ G}$ magnetic fields evolve modestly and dominate support at all radii. Conversely, initially weaker fields grow through feedback and turbulence but never dominate the support. This is reflected in the stability of the gas disc. This interplay determines the overall distribution of star formation in each case. We conclude that an initially weak field can grow to produce a realistic model of a local disc galaxy, but starting with typically assumed field strengths ($\gtrsim\!\! 1\ \mu \mathrm{ G}$) will not.