Turbulent Layer Research Articles

Controlling the differential resistance of axial blown arcs

Transfer switches for future HVDC systems can be based on passive oscillation principle to create a current zero crossing. The principle relies on a negative differential arc resistance of the mechanical switch. Present investigation aims to understand and quantify under which conditions the is negative. For this, results from electrical arc measurements are compared to results from a 1D arc model. Based on this, the authors conclude that ablation is not the reason for non-negative . In fact it is rather the location of the smallest arc radius which determines the negative characteristic. As soon as the turbulent shear layer is thick enough compared to the total arc diameter that it influences the temperature at the arc center, the turns negative. The current at which this happens scales with p 2/3, can be influenced by the nozzle shape and is rather insensitive to the chosen type of blowing gas.

A spark ignition scenario in a temporally evolving mixing layer

The paper presents the numerical studies on spark ignition in a turbulent mixing layer formed between a fuel stream (H2/N2) and air flowing in the opposite directions. Compared to an ignition mechanism observed in stationary mixing layers or in premixed homogeneous mixtures studied previously by many authors, the present results show a significantly different flame formation process. At an early stage of the ignition process, the flame kernel, which is initially spherical, is strongly torn by shear stresses and vortical structures formed in a region of the mixing layer. Depending on the spark location with respect to a vortical structure three different ignition scenarios were identified: (i) the flame kernel is formed, but at the successive time instants, it is destroyed by vortices and eventually vanishes; (ii) the flame develops around the initial spark position; (iii) the flame develops being simultaneously transported along the mixing layer. In the former case the flame grows significantly faster and its volume is found to be even two times larger.

Integral field spectroscopy of Green Peas – I. Disentangling disc-like turbulence and strong outflow kinematics in SDSS J083843.63+385350.5

ABSTRACT Integral field spectroscopy is well known for providing detailed insight of extended sources thanks to the possibility of handling space resolved spectroscopic information. Simple and straightforward analysis such as single line fitting yields interesting results, although it might miss a more complete picture in many cases. Violent star-forming regions, such as starburst galaxies, display very complex emission line profiles due to multiple kinematic components superposed in the line of sight. We perform a spatially resolved kinematical study of a single Green Pea (GP) galaxy, SDSS J083843.63+385350.5, using a new method for analysing integral field unit observations of emission line spectra. The method considers the presence of multiple components in the emission line profiles and makes use of a statistical indicator to determine the meaningful number of components to fit the observed profiles. We are able to identify three distinct kinematic features throughout the field and discuss their link with a rotating component, a strong outflow, and a turbulent mixing layer. We also derive an updated star formation rate for SDSS J0838 and discuss the link between the observed signatures of a large-scale outflow and of the Lyman continuum leakage detected in GP galaxies.

Analyzing the atmospheric boundary layer using high-order moments obtained from multiwavelength lidar data: impact of wavelength choice

Abstract. The lowest region of the troposphere is a turbulent layer known as the atmospheric boundary layer (ABL) and characterized by high daily variability due to the influence of surface forcings. This is the reason why detecting systems with high spatial and temporal resolution, such as lidar, have been widely applied for researching this region. In this paper, we present a comparative analysis on the use of lidar-backscattered signals at three wavelengths (355, 532 and 1064 nm) to study the ABL by investigating the high-order moments, which give us information about the ABL height (derived by the variance method), aerosol layer movement (skewness) and mixing conditions (kurtosis) at several heights. Previous studies have shown that the 1064 nm wavelength, due to the predominance of particle signature in the total backscattered atmospheric signal and practically null presence of molecular signal (which can represent noise in high-order moments), provides an appropriate description of the turbulence field, and thus in this study it was considered a reference. We analyze two case studies that show us that the backscattered signal at 355 nm, even after applying some corrections, has a limited applicability for turbulence studies using the proposed methodology due to the strong contribution of the molecular signature to the total backscatter signal. This increases the noise associated with the high-order profiles and, consequently, generates misinformation. On the other hand, the information on the turbulence field derived from the backscattered signal at 532 nm is similar to that obtained at 1064 nm due to the appropriate attenuation of the noise, generated by molecular component of backscattered signal by the application of the corrections proposed.

Characteristics of Atmospheric Turbulence Retrieved From High Vertical‐Resolution Radiosonde Data in the United States

AbstractIn this study, we estimate atmospheric turbulence in the free atmosphere in terms of the Thorpe scale (LT) and eddy dissipation rate (ε) using U.S. high vertical‐resolution radiosonde data over 4 years (September 2012 to August 2016) at 68 operational stations. In addition, same calculations are conducted for 12 years (October 2005 to September 2017) at four stations among the 68 stations. These high vertical‐resolution radiosonde data have a vertical resolution of approximately 5 m and extend to an altitude of approximately 33 km, and thus, turbulence can be retrieved in the entire troposphere and lower stratosphere. There are thicker and stronger turbulent layers in the troposphere than in the stratosphere, with mean ε values of 1.84 × 10−4 and 1.37 × 10−4 m2/s3 in the troposphere and stratosphere, respectively. The vertical structure of ε exhibits strong seasonal variations, especially in the upper troposphere and lower stratosphere, with the largest ε values in summer and the smallest in winter. In the horizontal distribution of ε, large ε is seen mainly above the mountainous region in the troposphere, but this pattern is not seen in the stratosphere. Although ε is estimated by the square of LT multiplied by the cube of the Brunt‐Väisälä frequency (N), the regions of large ε are matched with large LT regions where N is relatively small. For the time series of ε near the tropopause for 12 years at four stations, an annual variation is prominent at all stations without significant interannual variations. There is, however, a slightly increasing trend of ε at two stations.

Internal wave mixing in warming lake Grevelingen

Seasonal hypoxia or even anoxia can occur in some local deep basins of coastal waters. Such low summertime oxygen contents especially affect benthic life. The seasonal coastal hypoxia is commonly related to biological increased respiration and to physical limited vertical turbulent exchange that is associated with increased vertical stable density stratification. However, the same stratification can support internal waves that may break and locally generate turbulence. Here, we investigate the physics of internal wave motions in saltwater Lake Grevelingen (SW-Netherlands) during warming in mid-spring. Grevelingen is refreshed by weak tidal motions through an open sluice in its dam to the North Sea. The outer North Sea has a surface tidal range of about 3 m, but the lake surface tidal range is negligible (<0.1 m). To quantify vertical turbulent exchange, high-resolution temperature sensors were moored in conjunction with a current meter in 36 m water depth for three days. The site is known for anoxic conditions near the bottom in summer. While a 3-day, 30-m mean eddy diffusivity of <[Kz]> = 4 ± 2 × 10−5 m2 s−1 is found, the overall mean turbulence dissipation rate (∝turbulent flux) is <[ε]> = 1.1 ± 0.6 × 10−7 m2 s−3. Turbulent mixing occurs episodically, via near-surface cooling during night and increased winds, via sparse shear-driven breaking of internal waves at the main pycnocline, and via sheared near-bottom currents. Shear-driven turbulence is not commonly found in fresh-water lakes. Just below the main pycnocline around mid-depth a layer of weak turbulence is observed, as in fresh-water lakes. The observed turbulent exchange is sufficient to warm the near-bottom waters over the course of summer, but insufficient to prevent the biology from over-consuming oxygen contents.

Kelvin–Helmholtz instability in self-gravitating streams

ABSTRACT Self-gravitating gaseous filaments exist on many astrophysical scales, from sub-pc filaments in the interstellar medium to Mpc scale streams feeding galaxies from the cosmic web. These filaments are often subject to Kelvin–Helmholtz Instability (KHI) due to shearing against a confining background medium. We study the non-linear evolution of KHI in pressure-confined self-gravitating gas streams initially in hydrostatic equilibrium, using analytic models and hydrodynamic simulations, not including radiative cooling. We derive a critical line mass, or mass per unit length, as a function of the stream Mach number and density contrast with respect to the background, μcr(Mb, δc) ≤ 1, where μ = 1 is normalized to the maximal line mass for which initial hydrostatic equilibrium is possible. For μ &lt; μcr, KHI dominates the stream evolution. A turbulent shear layer expands into the background and leads to stream deceleration at a similar rate to the non-gravitating case. However, with gravity, penetration of the shear layer into the stream is halted at roughly half the initial stream radius by stabilizing buoyancy forces, significantly delaying total stream disruption. Streams with μcr &lt; μ ≤ 1 fragment and form round, long-lived clumps by gravitational instability (GI), with typical separations roughly eight times the stream radius, similar to the case without KHI. When KHI is still somewhat effective, these clumps are below the spherical Jeans mass and are partially confined by external pressure, but they approach the Jeans mass as μ → 1 and GI dominates. We discuss potential applications of our results to streams feeding galaxies at high redshift, filaments in the ISM, and streams resulting from tidal disruption of stars near the centres of massive galaxies.

Wave propagation effect on differential image motion monitor measurements

ABSTRACT The differential image motion monitor (DIMM) is widely used to measure the atmospheric optical turbulence (OT) integral over the line of sight. However, it is known that because of the wave propagation effect, the DIMM’s sensitivity falls with the distance to the turbulent layer and the variance of differential image motion is only approximately proportional to the OT integral. In this paper, we study this problem using the formalism of so-called weighting functions (WFs). We develop a method for calculating these WFs, including the case of the wide spectral band of detected radiation. The effect of underestimating the OT integral for different DIMM devices involved in previous site-testing campaigns is estimated using real measurements of OT vertical profiles. For instruments with small apertures (4–6 cm), the underestimation is ≈10 per cent, whereas for larger devices with apertures of 8–10 cm, the underestimation is less than 5 per cent, on average. This leads to a negative bias in seeing estimation of 7 and 3 per cent, respectively.

Structure of vorticity field in compressible turbulent mixing layers

The structure of the vorticity field in compressible mixing layers is studied using direct numerical simulations of temporally evolving mixing layers. The probability distribution of the inclination angles of the vorticity vectors with respect to various axes are analyzed. Spanwise projections of vorticity vectors have a tendency to align at angles −45° and 135° with respect to the streamwise direction. On streamwise and transverse planes, the projected vorticity vectors tend to align at very small angles with respect to the mean vorticity vector. These distributions are uninfluenced by changes in compressibility. Orientations of vortex filaments, which are dominant contributors to the vorticity covariances, are also identified. These orientations are independent of the transverse location within the turbulent core. The orientations of the projections of these filaments on to different planes are found out. Compressibility has no effect on the orientation of vorticity vectors contributing most to covariances.

Formation and evolution of flame kernels in autoignition of a turbulent hydrogen/air mixing layer at 50 atm

Autoignition of a turbulent stratified mixing layer between nitrogen-diluted hydrogen and hot air under an elevated pressure of 50 atm is studied using direct numerical simulations (DNS) in this work. Homogeneous isotropic turbulence is superimposed on the flow field. Reduced chemical mechanisms and a detailed multicomponent diffusion model are employed. In addition to turbulent mixing ignition (TMI), homogeneous mixing ignition (HMI) and laminar mixing ignition (LMI) are also investigated for comparison. Autoignition chemistry over a wide range of pressures is studied in HMI and LMI, which shows different behaviors at elevated pressures versus low pressures. The importance of H2O2 and HO2 in TMI is highlighted as radical sinks during the ignition process and can also be used as an indicator for locating the ignition spots. Moreover, OH radicals can be used as a marker variable for the transition of autoignition to flame propagation under high pressures. According to the present study, turbulence has some influence on the radical explosion stage especially for the conservation of H2O2 under the elevated pressure of 50 atm. Autoignition kernels forming away from the most reactive mixture fraction iso-surface are identified for the first time, which is a hybrid of autoignition and diffusive-ignition.

Local and Remote Controls on Arctic Mixed‐Layer Evolution

AbstractIn this study Lagrangian large‐eddy simulation of cloudy mixed layers in evolving warm air masses in the Arctic is constrained by in situ observations from the recent PASCAL field campaign. A key novelty is that time dependence is maintained in the large‐scale forcings. An iterative procedure featuring large‐eddy simulation on microgrids is explored to calibrate the case setup, inspired by and making use of the typically long memory of Arctic air masses for upstream conditions. The simulated mixed‐phase clouds are part of a turbulent mixed layer that is weakly coupled to the surface and is occasionally capped by a shallow humidity layer. All eight simulated mixed layers exhibit a strong time evolution across a range of time scales, including diurnal but also synoptic fingerprints. A few cases experience rapid cloud collapse, coinciding with a rapid decrease in mixed‐layer depth. To gain insight, composite budget analyses are performed. In the mixed‐layer interior the heat and moisture budgets are dominated by turbulent transport, radiative cooling, and precipitation. However, near the thermal inversion the large‐scale vertical advection also contributes significantly, showing a distinct difference between subsidence and upsidence conditions. A bulk mass budget analysis reveals that entrainment deepening behaves almost time‐constantly, as long as clouds are present. In contrast, large‐scale subsidence fluctuates much more strongly and can both counteract and boost boundary‐layer deepening resulting from entrainment. Strong and sudden subsidence events following prolonged deepening periods are found to cause the cloud collapses, associated with a substantial reduction in the surface downward longwave radiative flux.

Global Distribution of Far-ultraviolet Emissions from Highly Ionized Gas in the Milky Way

Abstract We present all-sky maps of two major far-ultraviolet cooling lines, C iv and O vi, of highly ionized gas to investigate the nature of the transition-temperature gas. From the extinction-corrected line intensities of C iv and O vi, we calculated the gas temperature and the emission measure of the transition-temperature gas assuming isothermal plasma in the collisional ionization equilibrium. The gas temperature was found to be more or less uniform throughout the Galaxy with a value of (1.89 ± 0.06) × 105 K. The emission measure of the transition-temperature gas is described well by a disk-like model in which the scale height of the electron density is kpc. The total mass of the transition-temperature gas is estimated to be approximately . We also calculated the volume-filling fraction of the transition-temperature gas, which was estimated to be f = 0.26 ± 0.09, and it varies from f ∼ 0.37 in the inner Galaxy to f ∼ 0.18 in the outer Galaxy. The spatial distribution of C iv and O vi cannot be explained by a simple supernova remnant model or a three-phase model. The combined effects of supernova remnants and turbulent mixing layers can explain the intensity ratio of C iv and O vi. Thermal conduction front models and high-velocity cloud models are also consistent with our observation.

Substorm Activity and Orientation of the Front of a Shock Wave of an Interplanetary Magnetic Cloud

The paper draws attention to the complex structure of the fast magnetic clouds of the solar wind, which, in addition to the actual cloud body, contain a turbulent transition layer (cloud sheath) with a large and irregular magnetic field following the shock wave. The orientation of the plane of the magnetic-cloud shock wave with respect to the interplanetary magnetic field modified by the shock wave propagating in the solar wind for 33 cases of the registration of fast magnetic clouds has been calculated. The dependence of the substorm activity in the auroral zone on the level of turbulent processes occurring in the sheath of magnetic clouds has been studied. It was taken into account that the turbulent phenomena in the sheath are largely determined by the orientation of the shock-wave plane with respect to the interplanetary magnetic field. It is shown that the level of magnetic activity in the auroral zone, which is characterized by the integral AL index, increases with a decrease in the angle between the direction normal to the shock-wave front and the vector of the interplanetary magnetic field. Thus, the most geoeffective are magnetic clouds with a quasi-parallel shock wave, and the least geoeffective are those with a quasi-perpendicular shock wave. It was concluded that the intensity of turbulent processes in the cloud sheath increases with a decrease in the magnetic field penetrating the sheath, which plays a stabilizing role for turbulent magnetohydrodynamic perturbations.

A Giant Intragroup Nebula Hosting a Damped Absorber at z = 0.313

Abstract This Letter reports the discovery of spatially extended line-emitting nebula, reaching to ≈100 physical kpc (pkpc) from a damped absorber (DLA) at z DLA = 0.313 along the sightline toward quasi-stellar object (QSO) PKS 1127−145 (z QSO = 1.188). This DLA was known to be associated with a galaxy group of dynamical mass M group ∼ 3 × 1012 M ⊙, but its physical origin remained ambiguous. New wide-field integral field observations revealed a giant nebula detected in [O ii], Hβ, [O iii], Hα, and [N ii] emission, with the line-emitting gas following closely the motions of group galaxies. One of the denser streams passes directly in front of the QSO with kinematics that are consistent with the absorption profiles recorded in the QSO echelle spectra. The emission morphology, kinematics, and line ratios of the nebula suggest that shocks and turbulent mixing layers, produced as a result of stripped gaseous streams moving at supersonic speed across the ambient hot medium, contribute significantly to the ionization of the gas. While the DLA may not be associated with any specific detected member of the group, both the kinematic and dust properties are consistent with the DLA originating in streams of gas stripped from sub-L * group members at ≲25 pkpc from the QSO sightline. This study demonstrates that gas stripping in low-mass galaxy groups is effective in releasing metal-enriched gas from star-forming regions, producing absorption systems in QSO spectra, and that combining absorption and emission-line observations provides an exciting new opportunity for studying gas and galaxy co-evolution.

A comprehensive study of flamelet tabulation methods for pulverized coal combustion in a turbulent mixing layer — Part I: A priori and budget analyses

In this work, a comprehensive study of flamelet tabulation methods for pulverized coal combustion in a turbulent mixing layer is conducted. At first, a priori analyses are conducted to evaluate the suitability of the premixed and non-premixed flamelet models for the studied pulverized coal flame with multiple combustion modes. Then, to clarify why a certain flamelet model does work or not work in certain regions, a more in-depth investigation of the premixed and non-premixed flamelet models is conducted through a budget analysis. The results show that the first and second derivatives in physical space can be well reproduced by the tabulated manifolds in trajectory variable space for both the premixed and non-premixed flamelet models, between which the non-premixed flamelet model performs slightly better. For the time derivative, large discrepancies can be observed, although the predicted variation trend overall follows the reference results. Through the analysis of the individual budget terms in the trajectory variable space, the individual trajectory variable’s contributions to the convection and diffusion of thermo-chemical variables are quantified. Through the analysis of the individual budget terms for the sensible enthalpy and the CO mass fraction governing equations, the influences of the space transformation on the individual transport process (e.g., convection, diffusion, etc.) are clarified. Overall, the findings obtained from the budget analyses are consistent with those obtained from the a priori analyses.

Frozen waves in turbulent mixing layers

The horizontal vibrations of an interface between miscible fluids trigger a parametric instability, leading to frozen wave patterns. Theory and numerical simulations reveal that the mixing layer size varies as the square of the forcing amplitude in the turbulent regime.

Stochastic flow approach to model the mean velocity profile of wall-bounded flows.

There is no satisfactory model to explain the mean velocity profile of the whole turbulent layer in canonical wall-bounded flows. In this paper, a mean velocity profile expression is proposed for wall-bounded turbulent flows based on a recently proposed stochastic representation of fluid flows dynamics. This original approach, called modeling under location uncertainty, introduces in a rigorous way a subgrid term generalizing the eddy-viscosity assumption and an eddy-induced advection term resulting from turbulence inhomogeneity. This latter term gives rise to a theoretically well-grounded model for the transitional zone between the viscous sublayer and the turbulent sublayer. An expression of the small-scale velocity component is also provided in the viscous zone. Numerical assessments of the results are provided for turbulent boundary layer flows, pipe flows and channel flows at various Reynolds numbers.

Local equilibrium in the inertial layer of wall bounded turbulence

ABSTRACTRecently Brouwers [Dissipation equals production in the log layer of wall-induced turbulence. Phys Fluids. 2007;19:101702] carried out an asymptotic analysis using the RANS based turbulence energy transport equation and showed that the energy dissipation equals its production in the inertial layer of wall-induced turbulence. Assuming log-law profile to the mean velocity, pressure, viscous and energy diffusion terms were estimated and shown to be negligibly small compared to the production and dissipation terms thereby proving local equilibrium. However, based on scale relations Tennekes and Lumley [A first course in turbulence. Cambridge (MA): MIT Press; 1994] have already established that the pressure and energy diffusion terms appearing in the energy transport equation are of the same order of magnitude especially in the inertial layer thus leading to a contradiction. Hence we have attempted here to re-estimate the turbulence energy budgets in a different way by invoking the Kolomogrov’s similarity hypotheses and (4/5)th law. Magnitudes of pressure and energy diffusion terms are determined explicitly and found to match well with the DNS data. The striking point of the present analysis is that no prior assumption is enforced on the mean velocity profile. Further, two main advantages of the present study are, (i) reasonable estimates for both the diffusion terms are obtained explicitly that were unavailable before and (ii) these estimates help us to tweak the production/dissipation terms to reflect the influence of turbulent diffusion mechanisms without the necessity to model them as in the case of elliptic relaxation and Reynold stress RANS models.

3D Modeling of the Structure and Dynamics of a Main-Sequence F-type Star

AbstractCurrent state-of-the-art computational modeling makes it possible to build realistic models of stellar convection zones and atmospheres that take into account chemical composition, radiative effects, ionization, and turbulence. The standard 1D mixing-length-based evolutionary models are not able to capture many physical processes of the stellar interior dynamics. Mixing-length models provide an initial approximation of stellar structure that can be used to initialize 3D radiative hydrodynamics simulations which include realistic modeling of turbulence, radiation, and other phenomena.In this paper, we present 3D radiative hydrodynamic simulations of an F-type main-sequence star with 1.47 solar mass. The computational domain includes the upper layers of the radiation zone, the entire convection zone, and the photosphere. The effects of stellar rotation is modeled in the f-plane approximation. These simulations provide new insight into the properties of the convective overshoot region, the dynamics of the near-surface, highly turbulent layer, and the structure and dynamics of granulation. They reveal solar-type differential rotation and latitudinal dependence of the tachocline location.

Numerical study of combustion effects on the development of supersonic turbulent mixing layer flows with WENO schemes

This paper mainly investigates the combustion effects on the development of supersonic turbulent mixing layer flows. Firstly, the positivity-preserving limiter proposed by Hu et al. is extended to solve the unphysical negative species mass fraction problem of traditional WENO schemes for the first time. Then, the large eddy simulation (LES) of the supersonic turbulent mixing layer flow with convective Mach number being 0.61 under the inert and combustion conditions are conducted separately to investigate the combustion effects. The LES is conducted with the extending fifth-order positivity-preserving WENO scheme and great efforts are spent to reproduce the real flow situation by adopting the three dimensional space-developing method with detailed chemical reaction mechanism. The following conclusions are drawn. It is found that the combustion will obviously change the flow structures. The kinetic energy spectra are nearly not affected by the combustion effects but the temperature fluctuations are strengthened among all the resolved scales. The decreasing mechanism of mixing layer growth rate due to combustion heat release effects is found. The combustion heat release effects decrease the production of Reynolds stress. Then the Reynolds stress is decreased, which is mainly a mean density effect. Finally, the growth rate is decreased according to the positive correlation between the growth rate and the Reynolds stress. Moreover, the old momentum thickness definition is no longer appropriate and a new one is introduced specifically for the space-developing LES. The relation between the growth rate and Reynolds stress originally deducted under the time-developing frame is also found to be valid under the space-developing frame.