Turbulent Dissipation Research Articles

Submesoscale Kinetic Energy Induced by Vertical Buoyancy Fluxes During the Tropical Cyclone Haitang

AbstractSubmesoscale process is an important part in the kinetic energy cascade from large‐scale circulation to turbulent dissipation, and a key component of the global heat budget. Its dynamic response to weather event is an important process in forecasting marine bio‐chemical matter transport. So how will submesoscale instabilities response to tropical cyclones (TCs) is worth studying. Based on underwater glider observations and 1‐km high resolution numerical modeling, we investigated two TCs (Roke and Haitang)‐induced submesoscale baroclinic instabilities and their dynamic mechanisms in the Northern South China Sea. The TC Haitang induced significant surface cooling, mixed layer deepening, front sharpening, and enhanced the mixed layer baroclinic and symmetric instabilities. The submesoscale kinetic energy also enhanced sharply after TC Haitang, which was higher correlated with increased mesoscale strain rates. The submesoscale energetics analysis revealed that the enhanced frontal submesoscale kinetic energy after TC Haitang was mainly from potential energy release via baroclinic energy conversion. Four groups of sensitivity numerical experiments revealed that the turbulent heat buoyancy flux and the Ekman buoyancy flux contributed equally to the positive baroclinic energy conversion during the TC Haitang. This study helps us to understand the multiscale oceanic energy transfers and submesoscale air‐sea interaction processes.

Wake effect on bubble–particle collision: An experimental study using 4D particle tracking velocimetry

The analysis of particle–bubble collisions in turbulent flow is a fundamental problem of high technological relevance, e.g., for the separation of valuable mineral particles by froth flotation. This relevance contrasts with an apparent lack of experimental data and understanding of this collision process. To this end, a periodic bubble chain was used to study the collision of millimeter-sized bubbles with polystyrene particles. The collision process between these entities was measured using 4D particle tracking velocimetry (PTV). By analyzing the collision data as a function of the polar angle along the bubble surface, we show that the collision took place not only at the leading edge but also at the trailing edge of the bubble. To understand the underlying mechanisms of the trailing edge collision, the flow field around a rising bubble chain was measured with Tomographic Particle Image Velocimetry (TPIV). The vortex formed in the bubble wake led to a velocity in the direction of the bubble surface that enabled trailing edge collisions. This effect is amplified by an increase in the turbulent kinetic energy and dissipation rate in the bubble wake. Overall, the investigation reveals different collision mechanisms and advances our understanding of the role of the wake in the bubble–particle collision.

Design and Material Optimization of Oil Plant Piping Structure for Mitigating Erosion Wear

Erosion in piping structures poses a significant challenge for oil industries as the conveyance of solid particles leads to operational malfunctions and structural failures affecting the overall oil plant operation. Conventional oil recovery methods have historically dominated, while in response to the challenges imposed by declining conventional oil production, the global shift towards non-conventional methods necessitates a reevaluation of erosion mitigation strategies due to increased piping infrastructure. Therefore, in this study research has been conducted to reduce erosion and optimize the piping structure. Variables impacting the erosion in piping were investigated from the literature, and simulation cases were made based on the impacted variables. Computational Fluid Dynamics (CFDs) analysis was performed using the Discrete Phase Model (DPM) to determine the erosion wear rate in each simulation case; based on the CFD results, variables with low Turbulent Dissipation Rates (TDRs) and Erosion Rates (ERs) were determined, and the optimized piping structure was designed. As a result, the optimized piping structure showed an 80% reduction in the turbulent dissipation rate and a 99.2% decrease in the erosion wear rate. These findings highlight a substantial improvement in erosion control, ensuring the safety and longevity of piping structures in oil plant operations.

CO in the Draco nebula: the atomic–molecular transition

ABSTRACT This paper presents maps of the J=2-1 transition of CO towards the Draco nebula intermediate-velocity cloud (IVC). The maps cover 8500 square arcmin with a velocity resolution of 0.33 km s$^{-1}$ and angular resolution of 38 ${\rm arcsec}$, or 0.11 pc at the cloud distance of 600 pc. The mapped area includes all the emission detected by the Herschel satellite with 250 $\mu$m intensity >5 MJy sr−1. Previously published observations of the far-infrared (far-IR) emission and the 21 cm line of HI are used to derive the column density distribution of H$_2$ and the abundance ratio CO/H$_2$, as well as the distribution of the molecular fraction of hydrogen, which approaches 90 per cent over much of the brighter parts of the nebula. The CO emission is highly clumpy and closely resembles the structures seen in far-IR images. The kinematics of the CO show supersonic motions between clumps but near-thermal to trans-sonic motions within clumps, consistent with model predictions that the scale length for dissipation of supersonic turbulence should be $\sim 0.1$ pc, mediated by kinematic viscosity and/or ambipolar diffusion. Different parts of the nebula show evidence for a spread of molecular formation time-scales of a few 10$^5$ yr, comparable with the dynamical time-scale of the infalling gas. The IVC will likely merge with the Galactic interstellar medium in $\sim 10^7$ yr, and the densest clumps may form an unbound cluster of low-mass stars.

Dissipation Rates of Mesospheric Stratified Turbulence From Multistatic Meteor‐Radar Observations

AbstractStratified turbulence (ST) has been proposed as a model for the dynamics of the mesosphere‐lower thermosphere (MLT) region. This theory postulates that for horizontal mesoscales (∼1–400 km), the kinetic energy of horizontal winds dissipates from large to small scales with an approximately mean constant rate. In this investigation, dissipation rates are quantified using meteor‐radar observations conducted in Northern Norway. The observed seasonal variability of dissipation rates exhibits maxima during the summer and winter, and minima near the equinoxes, between 80 and 95 km altitude. The results are compared with model predictions and earlier medium frequency radar, rocket, lidar, and satellite observations of MLT turbulence. The findings suggest that multi‐static meteor radar measurements of ST can provide a novel way to continuously monitor turbulent dissipation rates in the MLT region.

Numerical investigation of the vorticity transportation and energy dissipation in a variable speed pump-turbine

Variable speed technology is utilized in pump turbines to enhance their operational efficiency and stability. The unsteady internal flows of a variable speed pump turbine operating at different rotation speeds are numerically investigated with SST k-ω turbulence model. Firstly, the ideal rotating speed is selected for each output. Amongst three researched output levels, the lowest entropy production consistently occurs at minimum rotation speed,1350 rpm. Under similar outputs, higher rotation speeds result in higher entropy production. Difference between various rotation speed is up to 40 %. Secondly, Liutex method is used to structure the vortex while vorticity transport equation is applied to analyze its evolution. Results show that variable speed operation can effectively reduce pressure fluctuation amplitude with approximately 20 %–40 % amplitude drop. Subsequently, the relationship between vortex rope and entropy production in the draft tube is deeply explored as it reveals that entropy production in the draft tube correlates with vortex rope evolution. Moreover, direct dissipation's entropy production and turbulence dissipation's entropy production are two primary components leading to irreversible energy loss. It is clearly revealed that the entropy production reduces with rotation speed. The relationship between entropy production and vortex rope in the draft tube is confirmed.

Large-Eddy Simulations of Flow over a Backward-Facing Ramp with a Wall-Mounted Cube

Passive flow control devices, such as vortex generators (VGs), can effectively modulate the turbulent boundary layer flow near regions of adverse pressure gradients, but the interactions between the salient flow structures produced by VGs and those of the separated flow are not fully understood. In this study, a spatially evolving turbulent boundary layer interacting with a wall-mounted cube ahead of a backward-facing ramp is investigated using wall-resolved large-eddy simulations for a Reynolds number of 19,600, based on the inlet boundary layer thickness and freestream velocity. Different cube configurations are examined to isolate the effects of cube height and streamwise position. The large counter-rotating flow produced by the larger cubes generated more turbulent kinetic energy, thereby resulting in smaller separated regions over the ramp. Although significantly lower levels of turbulent kinetic energy dissipation than production are observed in the outer flow regions of the horseshoe vortex and in the induced counter-rotating flow, the spatial distribution of dissipation is similar to that of the turbulent kinetic energy. When the upstream position of an isolated single cube, relative to the leading ramp edge, is more than three cube heights, the streamwise decay of the counter-rotating flow results in lower levels of turbulent kinetic energy.

Usage of high-fidelity large eddy simulation to improve the turbulence modeling of Reynolds averaged navier stokes simulation in film cooling applications via a neural network

In this study, high-Fidelity Large eddy simulation (LES) data was used to create a surrogate turbulence model to enhance the estimation of turbulent heat flux in a Reynolds averaged Navier Stokes (RANS) solver for film cooling applications. The LES simulation (A flat plate film cooling application with a Reynolds number of 17,382, a blowing ratio of 1 and a density ratio of 2) was validated by comparison with experimental data. A correlation analysis was performed to identify the most influential parameters, and temperature gradient, velocity gradient, turbulent dissipation rate, shear strain rate, and turbulent viscosity ratio were selected as the most effective parameters. A second model was developed to create a Galilean-invariant model that remains unaffected by changes in the coordinate system. Neural network was applied to the LES training data to propose a modified surrogate dynamic turbulent Prandtl number to be applied to the k − ε realizable RANS simulation. The first surrogate model was implemented to two different geometries, one geometry was a flat plate film cooling similar to the training data but with different flow conditions, and another one was a more complex geometry with pressure gradient. The second model was only implemented on the second geometry and produced similar results. The surrogate models predicted a more accurate thermal field near the cooling wall (up-to 58 % error reduction) with a computational time complexity similar to the base RANS solver.

Direct numerical simulation of turbulent flow over irregular rough surfaces

Direct numerical simulations of turbulent channel flow at a shear Reynolds number of Re*=360 in smooth and rough channels have been performed. Made of irregular undulations, surface roughness was such that the ratio of the channel half-height to the root mean square roughness height is equal to 48, and the root mean square and the maximum crest and trough heights are equal to 7.5 and 23 wall units, respectively. The simulation results confirm that turbulence in the outer layer is not directly affected by the rough surface. The roughness effects on the turbulent stresses, the mean momentum balance, and the budget of turbulence kinetic energy are confined to the layer between 0 and 25 wall units; beyond which the profiles collapse with those for smooth channels. In the roughness sublayer, the peak value of the streamwise normal stress is reduced, while the spanwise and wall-normal components are increased. The largest increase is for the Reynolds shear stress, resulting in a significant increase in the turbulence production near the wall, even though the velocity gradient is decreased. The kinetic energy budget shows that turbulence production dominates the mean viscous diffusion of turbulence kinetic energy, and both mechanisms are balanced by turbulent dissipation. The friction factor using the Colebrook–White correlation calculated by specifying the sand–grain roughness equal to the root mean square of the roughness height predicts the friction velocity and the bulk velocity accurately. The streaky structures that exist near smooth walls were observed to be broken by the roughness elements, leading to a denser population of coherent structures near the wall, which increases the velocity fluctuations. The coherent structures developed in the roughness layer do not seem to penetrate into the outer layer.

Influence of impeller energy dissipation characteristics of multi-blade axial flow pump impeller by the cascade density

Abstract This paper investigates the influence of cascade density on energy dissipation in the impeller of a multi-blade axial-flow pump. Three-dimensional transient numerical simulations were conducted using the SST k-ω turbulence model for impeller schemes with different cascade density, corresponding to different blade chord length and pitch. Entropy production theory was applied to locate the regions with high energy loss in the impeller. The relationships between local entropy generation, energy loss, and unsteady flow were analyzed for different cascade density. The results indicate that impeller entropy output of the multi-blade axial-flow pump is highly consistent with the head loss during testing. Turbulence dissipation accounts for more than 50% of the total energy loss, followed by wall friction, while direct dissipation is the smallest contributor. The relative loss of the impeller can be minimized by decreasing the number of blades or increasing the blade chord length. By comparing energy dissipation in blade rotation with different chord length and pitch, it is found that turbulent dissipation ability can be controlled more effectively by changing chord length, while the ability of direct dissipation can be controlled more effectively by changing the pitch.

Time-resolved tomographic particle image velocimetry of flow structures in non-circular orifice impinging jets

In this study, time-resolved tomographic particle image velocimetry (Tomo-PIV) was implemented on two different non-circular orifice impinging jets, i.e., elliptical and square orifices, and the circular one was employed as a reference for comparison, with the same equivalent diameter De=20 mm, impinging distance-to-diameter H/D = 3.0, and the Reynolds number (Re) at 1.6×103. A particular concern was placed on examining the coherent structure dynamics and turbulence dissipation of these impinging jets. The dominant Strouhal number (St) of all three jets has the component of 0.53, representing the large-scale Kelvin–Helmholtz (K–H) vortex ring, particularly for the square orifice, the dominant St is 0.70 at the central axis and 0.18 at the diagonal axis near the impinging surface. In free jet region, the streamwise velocity profile of the square orifice jet always maintains a rhombic development with a 45° difference relative to the outlet shape. In the impingement region, the circular orifice jet has the strongest K–H structure, with two opposite wall jets generated inside and outside, while in elliptical jet impinging, the upturned short axis of the vortex ring after axis-switching invariably contacts the impinging surface first, and then the wall jet vortex ring re-stretches to a circular shape due to the higher velocity of the wall jets generated from the upturned short axis, and the square orifice impinging jet contains no obvious wall K–H vortex rings but undergoes an irregular merging with the vortex ring downstream and stagnation. The time-averaged flow field statistics show that the circular orifice impinging jets have stronger wall jets, while the square orifice is the weakest, due to the strongest turbulent dissipation generated by the more fragmented flow upstream.

Numerical Investigation on internal flow characteristics of a mixed-flow pump with Tandem malposed blades

Abstract Mixed-flow pump plays an important role in national major projects and national economic construction. In this paper, a mixed-flow pump is taken as the research object, and the internal flow characteristics and the distribution of turbulent kinetic energy in the pump under different malposed angles of blade are studied by using numerical simulation and blade modification method. The results show that the pump efficiency and head are higher when the tandem blade angles (α) are 15° and 45°, and the pump head is the largest at the α =15°, increasing by 4.37%. Meanwhile, the high-pressure area inside the impeller was large, which had a higher pressurization performance. Under the rated flow condition, the velocity curve of α =15° changes most gently, which is close to the prototype velocity curve, and the single hump zone is not obvious. The tandem malposed blades can restrain the velocity fluctuation and improve the flow field in the impeller. At α =45°, the turbulent kinetic energy dissipation and the velocity fluctuation were the smallest; the flow field was the most stable. The research results have reference significance for the optimal design and selection of mixed flow pump.

Probing the physics of star formation (ProPStar)

Context. Electron fraction and cosmic-ray ionization rates in star-forming regions are important quantities in astrochemical modeling and are critical to the degree of coupling between neutrals, ions, and electrons, which regulates the dynamics of the magnetic field. However, these are difficult quantities to estimate. Aims. We aim to derive the electron fraction and cosmic-ray ionization rate maps of an active star-forming region. Methods. We combined observations of the nearby NGC 1333 star-forming region carried out with the NOEMA interferometer and IRAM 30 m single dish to generate high spatial dynamic range maps of different molecular transitions. We used the DCO+ and H13CO+ ratio (in addition to complementary data) to estimate the electron fraction and produce cosmic-ray ionization rate maps. Results. We derived the first large-area electron fraction and cosmic-ray ionization rate resolved maps in a star-forming region, with typical values of 10−65 and 10−16.5 s−1, respectively. The maps present clear evidence of enhanced values around embedded young stellar objects (YSOs). This provides strong evidence for locally accelerated cosmic rays. We also found a strong enhancement toward the northwest region in the map that might be related either to an interaction with a bubble or to locally generated cosmic rays by YSOs. We used the typical electron fraction and derived a magnetohydrodynamic (MHD) turbulence dissipation scale of 0.054 pc, which could be tested with future observations. Conclusions. We found a higher cosmic-ray ionization rate compared to the canonical value for N(H2) = 1021−1023 cm−2 of 10−17 s−1 in the region, and it is likely generated by the accreting YSOs. The high value of the electron fraction suggests that new disks will form from gas in the ideal-MHD limit. This indicates that local enhancements of ζ(H2), due to YSOs, should be taken into account in the analysis of clustered star formation.

Turbulence Over Camarinal Sill and Its Impact on Water Mixing—Strait of Gibraltar

AbstractStraits and narrows are the place of intensified turbulent mixing. Deep understanding of the turbulent dynamics at these locations is of crucial importance as it conditions the properties of the water masses flowing in the open ocean. A new extensive field experiment, PROTEVS GIB20, with high frequency measurements has been conducted in the Strait of Gibraltar. It allows us to infer dissipation rates of the turbulent kinetic energy, ϵ, from two consistent methods. The range of ϵ is depicted for the different processes which developed in the vicinity of Camarinal Sill, the main topographic feature of the Strait of Gibraltar. It evidences that the bottom boundary layer, hydraulic jumps and large overturns are the main loci of intensified turbulence reaching 10−3W.kg−1. The variability of the turbulence is mainly controlled by semi‐diurnal, diurnal and fortnightly tidal oscillations. Spatially, the western flank of Camarinal Sill is evidenced as the hotspot for turbulent mixing. We confirm a weak variation of the spatially averaged vertically integrated turbulent dissipation rate. This result needs to be qualified in view of the differentiated impact of the various processes on adjacent water masses. The dynamics of the spring tide directly mix Atlantic and Mediterranean waters, resulting in a large spreading of the T‐S diagram.

Surface and Sub‐Surface Kinetic Energy Wavenumber‐Frequency Spectra in Global Ocean Models and Observations

AbstractThis paper examines spectra of horizontal kinetic energy (HKE) in the surface and sub‐surface ocean, with an emphasis on internal gravity wave (IGW) motions, in global high‐resolution ocean simulations. Horizontal wavenumber‐frequency spectra of surface HKE are computed over seven oceanic regions from two global simulations of the HYbrid Coordinate Ocean Model (HYCOM) and three global simulations of the Massachusetts Institute of Technology general circulation model (MITgcm). In regions with high IGW activity, high surface HKE variance in the horizontal wavenumber‐frequency spectra is aligned along IGW linear dispersion curves. For both HYCOM and MITgcm, and in almost all regions, finer horizontal resolution yields more energetic supertidal IGW continuum spectra. The ratio of high‐horizontal‐wavenumber variance in semi‐diurnal and supertidal motions relative to lower‐frequency motions, a quantity of great interest for swath altimetry, depends on the model employed and the horizontal resolution within the model, implying that quantitative predictions of the partition between low‐ and high‐frequency motions taken from particular simulations should be treated with care. The frequency‐vertical wavenumber spectra, frequency spectra, and vertical wavenumber spectra from the models are compared to spectra computed from McLane profilers at nine locations. In general, MITgcm spectra match the McLane profiler spectra more closely at high frequencies (|ω| > 4.5 cpd). In both models, vertical wavenumber spectra roll off more steeply than observations at high vertical wavenumbers (m > 10−2 cpm). The vertical wavenumber spectra in such models is an important target for improvement, due to turbulence production and dissipation that takes place at high vertical wavenumbers.

Construction of anti-fouling ceramic tubular membranes with corrugated inner surfaces using DLP 3D printing

The challenge of membrane fouling persistently hinders the advancement of membrane technology. The construction of patterned membrane surfaces is anticipated to mitigate the effects of membrane fouling significantly. In this study, ceramic microfiltration membranes with corrugated patterns were fabricated by 3D printing combined with the dip-coating process. To assess the impact of surface patterning on ceramic membrane performance, we initially compared the pore size and pure water flux between straight and corrugated membrane tubes. Both configurations exhibited similar parameters: a support pore size of approximately 1 μm, a membrane layer pore size of about 110 nm, and a pure water permeance of around 950 L m−2 h−1·bar−1. The corrugated ceramic membranes demonstrated superior anti-fouling properties compared to their straight counterparts during the filtration of nanoparticle suspensions, oil-in-water emulsions, and bacterial suspensions. Specifically, under a nanoparticle suspension concentration of 2000 ppm, a transmembrane pressure of 0.2 MPa, and a flow rate of 1.0 m s−1, the corrugated membrane achieved a stable flux of approximately 1.7 times greater than that of the straight membrane. Furthermore, we explored the effects of surface patterning on fluid dynamics through numerical simulations. The enhanced turbulence dissipation rate in the patterned membrane tubes suggests increased surface turbulence and an improved capacity to remove contaminants. This research offers novel insights into the development of ceramic membranes with enhanced anti-fouling capabilities for water treatment.

Numerical investigation on characteristics of vortex dissipation in multi-horizontal submerged jets stilling basin.

Multi-horizontal submerged jets stilling basins have been utilized in large-scale water conservancy and hydropower projects due to its stable flow pattern, high energy dissipation rate and less atomization. This study employs vorticity criterion, Q criterion, λ2 criterion and Ω criterion to investigate the characteristics of vortex formation and turbulent dissipation in multi-horizontal submerged jets stilling basins with various configurations, including crest overflowing orifice alone (COO), combination of crest overflowing orifice and mid-discharge orifice (COO-MO) and mid-discharge orifice alone (MO). The results indicate that the Q criterion and λ2 criterion are effective in identifying vortex structure within multi-horizontal submerged jets stilling basin. Specifically, the stronger intensity of vortex structure and vortex dissipation are mainly distributed in the vicinity of the vertical drop, which gradually weakens for the increasing distance to the vertical drop. Furthermore, the intensity and number of vortexes with COO-MO are the largest. This conclusion can provide guidance for energy dissipation and bottom protection of stilling pool.

ATOMIX benchmark datasets for dissipation rate measurements using shear probes

Turbulent mixing in the ocean, lakes and reservoirs facilitates the transport of momentum, heat, nutrients, and other passive tracers. Turbulent fluxes are proportional to the rate of turbulent kinetic energy dissipation per unit mass, ε. A common method for ε measurements is using microstructure profilers with shear probes. Such measurements are now widespread, and a non-expert practitioner will benefit from best practice guidelines and benchmark datasets. As a part of the Scientific Committee on Oceanographic Research (SCOR) working group on “Analysing ocean turbulence observations to quantify mixing” (ATOMIX), we compiled a collection of five benchmark data of ε from measurements of turbulence shear using shear probes. The datasets are processed using the ATOMIX recommendations for best practices documented separately. Here, we describe and validate the datasets. The benchmark collection is from different types of instruments and covers a wide range of environmental conditions. These datasets serve to guide the users to test their ε estimation methods and quality-assurance metrics, and to standardize their data for archiving.

On the Heating of the Slow Solar Wind by Imbalanced Alfvén-wave Turbulence from 0.06 to 1 au: Parker Solar Probe and Solar Orbiter Observations

In this work we analyze plasma and magnetic field data provided by the Parker Solar Probe and Solar Orbiter missions to investigate the radial evolution of the heating of Alfvénic slow wind by imbalanced Alfvén-wave (AW) turbulent fluctuations from 0.06 to 1 au. in our analysis we focus on slow solar-wind intervals with highly imbalanced and incompressible turbulence (i.e., magnetic compressibility C B = δ B/B ≤ 0.25, plasma compressibility C n = δ n/n ≤ 0.25, and normalized cross helicity σ c ≥ 0.65). First, we estimate the AW turbulent dissipation rate from the wave energy equation and find that the radial profile trend is similar to the proton heating rate. Second, we find that the scaling of the empirical AW turbulent dissipation rate Q W obtained from the wave energy equation matches the scaling from the phenomenological AW turbulent dissipation rate Q CH09 (with Q CH09 ≃ 1.55Q W ) derived by Chandran & Hollweg based on the model of reflection-driven turbulence. Our results suggest that, as in the fast solar wind, AW turbulence plays a major role in the ion heating that occurs in incompressible slow-wind streams.

Rapid Restratification Processes Control Mixed Layer Turbulence and Phytoplankton Growth in a Deep Convection Region

AbstractThe Gulf of Lion, Northwestern Mediterranean Sea, is one of few oceanic regions where deep convection occurs. We investigate the restratification following a convection event using measurements from an ocean glider equipped with turbulence microstructure sensors. This unique combination of instruments provides a high‐resolution description of the mixed layer with regard to turbulence, stratification and chlorophyll. We observe a rapid restratification process that proceeds over a timescale of days to one week. We find that restratification exerts a leading order control on surface mixed layer turbulence variability, as abrupt changes in turbulence dissipation rates are associated with the formation of near‐surface stratification. The near‐surface formation of stratification occurs through both the diurnal variability in surface buoyancy fluxes and through lateral advective processes. We conclude that daily near‐surface processes that influence stratification control mixed layer turbulence levels, and thus the phytoplankton response in the critical transition period to spring bloom.