Turbulent Energy Research Articles

“Turbulence effect on total mechanical energy budget and energy loss of turbulent flows with different hydraulic regimes in open-channel transitions”

ABSTRACT Transitions are needed in hydraulic structures which are flumed. In expanding transitions, flow separates from the boundary if the angle of divergence exceeds about 5 degrees. The present paper gives an insight into the turbulence effect and energy loss in expanding transition with small angle of divergence. In wide angle expansion, flow separates from the boundary resulting in highly non-uniform velocity distribution at the exit of expansion resulting in erosion of down stream channel. Author performed experiments to control separation of flow in wide angle expansion. The paper discusses about the devices used by the author to control separation in wide angle expanding transition.

Buoyancy effects on very-large-scale motions and amplitude modulation in convective atmospheric surface layers

A large amount of high-Reynolds-number experimental data was obtained from long-term observations of the atmospheric surface layer in the Qingtu Lake Observation Array under different thermal stability conditions. To reveal the thermal effects, the variations of the scales of turbulent coherent structures, energy distribution and amplitude modulation as a function of thermal stability are investigated. Results show that in the convective atmospheric surface layer, the superposition of buoyancy and shear drives leads to a large enhancement of turbulent kinetic energy, an increase in the scales of coherent structures, and in particular, a more significant enhancement of the very-large-scale motions kinetic energy. This results in an increased energy proportion of very-large-scale motions, and thus enhancing the amplitude modulation effect on small scales. Moreover, the variations in the coherent structure scale, energy proportion of the very-large-scale motions, amplitude modulation coefficient and the corresponding modulating and carrier signal scales with the thermal stability parameter can all be characterized by the same parametric equation form. This study contributes to a better understanding of turbulence dynamics driven by both buoyancy and shear.

Influence of runner blade number on hydraulic performance and flow control in draft tube of Francis turbine

As a widely used core component of hydropower generation, the stable and safe operation of the Francis turbine plays a very important role in engineering applications. Therefore, the effects of different rotor blade numbers on the hydraulic performance and flow control of the draft tube of the Francis turbine are of great research value. This paper focuses on the rotor blade number 13, 14, 15, 16, and 17 in the prohibited and stabilized operating zones, each of which is taken as a working condition point. (Condition αref - HM and Condition 2 αref - HM). Computational Fluid Dynamics (CFD) numerical simulations were performed and analyzed. In terms of hydraulic performance, the increase in the number of blades increases the efficiency and power of the turbine under Condition αref - HM by 2.26% and 0.0021 MW respectively. The efficiency and power of the turbine under Condition αref - HM is also elevated, with the efficiency reaching a maximum of 91.77% at blade number 15, and the power of the same turbine reaching a maximum of 0.182 MW at runner blade number 13. From the 3D flow analysis of the turbine, the increase in the number of blades does not significantly change the flow state of the turbine’s draft tube. By defining the turbulence energy E, the fast Fourier transform of the turbulence energy signals on the two monitoring surfaces of the draft tube is used to obtain its main frequency, and the amplitude and phase at the typical main frequency are visualized, through which we can analyze the vortex band motion state on the two monitoring surfaces of the draft tube, and speculate the change of pressure pulsation of the draft tube. Such an analysis method has a guiding significance for us to improve the performance and stability of the turbine, which can be achieved by increasing or decreasing the number of blades.

Laboratory Investigation of Energy Dissipation and Turbulent Mixing During Internal Solitary Wave Breaking on the Rough Shelf Slope

AbstractWhen internal solitary waves (ISW) propagate to the continental shelf, they typically runup and break on the forereef. This process might lead to periodic temperature drops at the reef slope, which has potential to protect coral reefs from bleaching threats. A series of laboratory experiments were conducted to investigate turbulence characteristics and energy dissipation during the ISW breaking events on the shelf slope with different bottom roughness to mimic the forereef environment. Particle image velocimetry (PIV) and planar laser‐induced fluorescence (PLIF) were coupled to obtain simultaneous high‐resolution velocity and density fields, based on which we then calculate the energy budget and turbulent dissipation rate. We found that the ISW breaking can be divided into two turbulent processes, the production of shear instability and coherent structures. Ratio of energy loss is weakened with the increase of bottom roughness and the collapse height, while the rough structure substantially enhanced the turbulent dissipation rate. The mixing efficiency is between 0.1 and 0.3, close to its maximal rate at 0.25. Our results reveal that the bottom roughness has impact on the mixing efficiency and turbulent process during the ISW breaking events. We found that rough structures at the shelf slope inhibit the mixing efficiency, especially in cases with small amplitude incline ISW. This finding may help us to better understand the reef damage and coral reef decline and support the management of coral protection strategy.

Generation of nearly homogeneous isotropic turbulence using a novel oscillating grid system

The creation and control of homogeneous isotropic turbulence in laboratory settings is of practical significance. In this paper, we propose a novel design for a turbulence generation apparatus driven by two pairs of oscillating grids, with the object of striking a balance between the stochastics of momentum injection and the flexibility of control of turbulent kinematic energy (TKE). To validate the performance of this system, tomographic particle image velocimetry measurements are conducted. Based on Reynolds decomposition, we characterize the secondary flow pattern and examine the impact of the mean velocity on the TKE transport. Analysis of one- and two-point statistics indicates that the generated turbulence exhibits reasonable approximations of homogeneity and isotropy. The TKE and the dissipation rate are linearly proportional to the grid oscillation frequency, with the Taylor-scale Reynolds number Reλ ranging from 47 to 154, suggesting a convenient control of turbulence with low-to-moderate intensity.

Molecular Bubble and Outflow in S Mon Revealed by Multiband Data Sets

We identify a molecular bubble, and study the star formation and its feedback in the S Mon region, using multiple molecular lines, young stellar objects (YSOs), and infrared data. We revisit the distance to S Mon, ∼722 ± 9 pc, using Gaia Data Release 3 parallaxes of the associated Class II YSOs. The bubble may be mainly driven by a massive binary system (namely 15 Mon), the primary of which is an O7V-type star. An outflow is detected in the shell of the bubble, suggesting ongoing star formation activities in the vicinity of the bubble. The total wind energy of the massive binary star is 3 orders of magnitude higher than the sum of the observed turbulent energy in the molecular gas and the kinetic energy of the bubble, indicating that stellar winds help to maintain the turbulence in the S Mon region and drive the bubble. We conclude that the stellar winds of massive stars have an impact on their surrounding environment.

Heat release and flame scale effects on turbulence dynamics in confined premixed flows

As industry transitions to a net-zero carbon future, turbulent premixed combustion will remain an integral process for power generating gas turbines, aviation engines, and high-speed propulsion due to their ability to minimize pollutant emissions. However, accurately predicting the behavior of a turbulent reacting flow field remains a challenge. To better understand the dynamics of premixed reacting flows, this study experimentally investigates the effects of combustion heat release and flame scales on the evolution of turbulence in a high-speed, confined bluff-body combustor. The combustor is operated across a range of equivalence ratios from 0.7 to 1 to isolate the role of chemical heat release, flame speed, and flame thickness on the evolution of turbulence as the flow progresses from reactants to products. High-speed particle image velocimetry and CH* chemiluminescence imaging systems are simultaneously employed to quantify turbulent flame and flow dynamics. The results notably demonstrate that the flame augments turbulence fluctuations as the flow evolves from reactants to products for all cases, which opposes most simulations of premixed turbulent reactions. Notably, turbulence fluctuations increase monotonically with the heat of combustion and corresponding turbulent flame speed. Spatial profiles of turbulence statistics are conditioned on the mean flame front, and nondimensionalizing the turbulence profiles using laminar flame properties is shown to collapse all conditions onto a single curve. The resulting nondimensional profile confirms that turbulence dynamics scales with the heat of combustion and was used to develop a novel correlation to predict the increase in turbulent fluctuations across the premixed flame. A Reynolds averaged Navier Stokes decomposition is also explored to further characterize the effects of combustion heat release on the dominant mechanisms of turbulent energy transport. The cumulative results can guide modeling capabilities to better predict flame and flow dynamics and accelerate design strategies for premixed turbines with carbon-free fuels.

New insights into experimental stratified flows obtained through physics-informed neural networks

We develop a physics-informed neural network (PINN) to significantly augment state-of-the-art experimental data of stratified flows. A fully connected deep neural network is trained using time-resolved experimental data in a salt-stratified inclined duct experiment, consisting of three-component velocity fields and density fields measured simultaneously in three dimensions at Reynolds number $= O(10^3)$ and at Prandtl or Schmidt number $=700$ . The PINN enforces incompressibility, the governing equations for momentum and buoyancy, and the boundary conditions at the duct walls. These physics-constrained, augmented data are output at an increased spatio-temporal resolution and demonstrate five key results: (i) the elimination of measurement noise; (ii) the correction of distortion caused by the scanning measurement technique; (iii) the identification of weak but dynamically important three-dimensional vortices of Holmboe waves; (iv) the revision of turbulent energy budgets and mixing efficiency; and (v) the prediction of the latent pressure field and its role in the observed asymmetric Holmboe wave dynamics. These results mark a significant step forward in furthering the reach of experiments, especially in the context of stratified turbulence, where accurately computing three-dimensional gradients and resolving small scales remain enduring challenges.

Formation of Unipolar Outflow and Protostellar Rocket Effect in Magnetized Turbulent Molecular Cloud Cores

Observed protostellar outflows exhibit a variety of asymmetrical features, including remarkable unipolar outflows and bending outflows. Revealing the formation and early evolution of such asymmetrical protostellar outflows, especially the unipolar outflows, is essential for a better understanding of the star and planet formation because they can dramatically change the mass accretion and angular momentum transport to the protostars and protoplanetary disks. Here we perform three-dimensional nonideal magnetohydrodynamics simulations to investigate the formation and early evolution of the asymmetrical protostellar outflows in magnetized turbulent isolated molecular cloud cores. We find, for the first time to our knowledge, that the unipolar outflow forms even in the single low-mass protostellar system. The results show that the unipolar outflow is driven in the weakly magnetized cloud cores with the dimensionless mass-to-flux ratios of μ = 8 and 16. Furthermore, we find the protostellar rocket effect of the unipolar outflow, which is similar to the launch and propulsion of a rocket. The unipolar outflow ejects the protostellar system from the central dense region to the outer region of the parent cloud core, and the ram pressure caused by its ejection suppresses the driving of additional new outflows. In contrast, the bending bipolar outflow is driven in the moderately magnetized cloud core with μ = 4. The ratio of the magnetic to turbulent energies of a parent cloud core may play a key role in the formation of asymmetrical protostellar outflows.

Flocculation characteristics of suspended Mississippi River mud under variable turbulence, water and salt sources, and salinity: a laboratory study

Muddy sediment constitutes a major fraction of the suspended sediment mass carried by the Mississippi River. Thus, adequate knowledge of the transport dynamics of suspended mud in this region is critical in devising efficient management plans for coastal Louisiana. We conducted laboratory tank experiments on the sediment suspended in the lower reaches of the Mississippi River to provide insight into the flocculation behavior of the mud. In particular, we measure how the floc size distribution responds to changing environmental factors of turbulent energy, sediment concentration, and changes in base water composition and salinity during summer and winter. We also compare observations from the tank experiments to in situ observations. Turbulence shear rate, a measure of river hydrodynamic energy, was found to be the most influential factor in determining mud floc size. All flocs produced at a given shear rate could be kept in suspension down to shear rates of approximately 20 s−1. At this shear rate, flocs on the order of 150–200 μm and larger can settle out. Equilibrium floc size was not found to depend on sediment concentration; flocs larger than 100 μm formed in sediment concentrations as low as 20 mgL−1. An increase in salinity generated by adding salts to river water suspensions did not increase the flocculation rate or equilibrium size. However, the addition of water collected from the Gulf of Mexico to river-water suspensions did enhance the flocculation rate and the equilibrium sizes. We speculate that the effects of Gulf of Mexico water originate from its biomatter content rather than its ion composition. Floc sizes in the mixing tanks were comparable to those from the field for similar estimated turbulent energy. Flocs were found to break within minutes under increased turbulence but can take hours to grow under conditions of reduced shear in freshwater settings. Growth was faster with the addition of Gulf of Mexico water. Overall, the experiments provide information on how suspended mud in the lower reaches of the Mississippi might respond to changes in turbulence and salinity moving from the fluvial to marine setting through natural distributary channels or man-made diversions.

Understanding the effects of revegetated shrubs on fluxes of energy, water, and gross primary productivity in a desert steppe ecosystem using the STEMMUS–SCOPE model

Abstract. Revegetation is one of the most effective ways to combat desertification and soil erosion in semiarid and arid regions. However, the impact of the perturbation of revegetation on ecohydrological processes, particularly its effects on the interplay between hydrological processes and vegetation growth under water stress, requires further investigation. This study evaluated the effects of revegetation on the energy, water, and carbon fluxes in a desert steppe in Yanchi County, Ningxia Province, northwest China, by simulating two vegetated scenarios (shrub–grassland ecosystem and grassland ecosystem) using the STEMMUS–SCOPE (Simultaneous Transfer of Energy, Mass and Momentum in Unsaturated Soil–Soil Canopy Observation of Photosynthesis and Energy fluxes) model. The model was validated by field observations from May to September of 2016–2019. The evaluation of revegetation effects relied on comparing simulated fluxes between two vegetated scenarios in 2016 and 2019. In both scenarios, turbulent energy was dominated by latent heat flux, which was stronger in the shrub–grassland ecosystem (+7 %). A higher leaf area index and root water uptake of C3 shrubs (Caragana intermedia) resulted in increased carbon fixation (+83 %) and transpiration (+72 %) of the shrub–grassland ecosystem compared to the C3 grassland ecosystem. Accompanied by a marked increase in root water uptake (+123 %), revegetation intensified water consumption beyond the levels of received precipitation. These results highlight the critical importance of considering both energy and water budgets in water-limited ecosystems during ecological restoration to avert soil water depletion.

Comparison of pressure-loss evaluation fidelity in turbulent energy dissipation models of poppet check valves using computational fluid dynamics (CFD) software

Check valves are critical components of fluid systems and have various applications, including house appliances. This article presents a methodology for mapping geometry-specific constriction pressure loss as a function of flow and turbulence in a check valve. This study aimed to gain insight on which Ansys Fluent available turbulent energy dissipation model should be used for further design optimization. This methodology consists of a statistical comparison of computational fluid dynamics (CFD) simulation results obtained using the turbulent energy dissipation models. The key components of the simulation process are discussed. The study’s main results are a comparison of empirical results among flow models’ estimated pressure loss, shown as a function of flow rate in specific geometry and identification of the most suitable model for the considered application. This study concludes that the K-Epsilon (Standard) model best represents the empirically measured behavior of naturally occurring flow energy losses in the considered geometry.

Estimation of turbulent energy mixing factor in PWR sub‐channel by DNS

AbstractTurbulent mixing within sub‐channels plays a crucial role in understanding the thermal hydraulics of reactor channels. It serves as an empirical parameter in sub‐channel analysis and has long been a challenge in the nuclear industry. Conducting experiments in this context is challenging due to the stringent requirement of maintaining pressure balance among sub‐channels to prevent convection effects. Fortunately, direct numerical simulation (DNS) is emerging as an invaluable tool for addressing this persistent issue. DNS enables the direct computation of turbulent mixing by analyzing fluctuating lateral velocities, offering a more profound understanding of the underlying phenomena. In this study, DNS was conducted at six Reynolds numbers ranging from 17,640 to 1.5 × 105 in pressurized water reactor (PWR) geometry to investigate the lateral mixing driven by turbulence. By studying intricate mechanisms governing the turbulent mixing, the valuable insights into reactor thermal performance and safety are provided. Furthermore, a correlation for turbulent mixing of energy based on the DNS data has been derived, enhancing our ability to model and predict this critical aspect of reactor behaviour. Additionally, this paper explores temperature fluctuations occurring at the fuel rod surface due to turbulence. A probabilistic distribution for temperature fluctuation under specific reactor conditions is presented.

LES analysis of local isotropic turbulence depending on the spatial scale of the external forcing field

The study investigates the effect of the parameterisation of an external force scheme based on trigonometric functions on turbulence. In particular, the dependence of the generated turbulence on the Reynolds number and the influence of its spatial parameters are investigated. The turbulence used is generated by a large-eddy simulation based on a high-precision finite volume method, and the turbulence energy and isotropy of the turbulence are also investigated. The parameter of the external force field used in this study characterises the wavelength of the trigonometric function, and the influence of this parameter on the spatial scale is studied in detail. As a result, steady turbulent fields are generated and isotropic turbulence is observed at different spatial scales of the external force field with M=2, 3 and 4. The property of increasing turbulence energy with increasing Reynolds number is confirmed.

Balanced Turbulence and the Helicity Barrier in Black Hole Accretion

Horizon-scale observations from the Event Horizon Telescope (EHT) have enabled precision study of supermassive black hole accretion. Contemporary accretion modeling often treats the inflowing plasma as a single, thermal fluid, but microphysical kinetic effects can lead to significant deviations from this idealized picture. We investigate how the helicity barrier influences EHT-accessible electromagnetic observables by employing a simple model for electron heating based on kinetic physics and the cascade of energy and helicity in unbalanced turbulence. Although the helicity barrier plays only a minor role in regions with high plasma β, like in standard and normal evolution (SANE) disks, it may substantially impact regions with more ordered magnetic fields, such as the jet and its surrounding wind in SANE flows as well as throughout the entire domain in magnetically arrested disk (MAD) flows. In SANE flows, emission shifts from the funnel wall toward the lower-magnetization disk region; in MAD flows the emission morphology remains largely unchanged. Including the helicity barrier leads to characteristically lower electron temperatures, and neglecting it can lead to underestimated accretion rates and inferred jet powers. The corresponding higher plasma densities result in increased depolarization and Faraday depths thereby decreasing the amplitude of the β 2 coefficient while leaving its angle unchanged. Both the increased jet power and lower β2 may help alleviate outstanding tensions between modeling and EHT observations. We also find that the estimated ring diameter may be underestimated when the helicity barrier is neglected. Our results underscore the significance of the helicity barrier in shaping black hole observables and inferred accretion system parameters.

Turbulence Across the Antarctic Circumpolar Current in the Indian Southern Ocean: Micro‐Temperature Measurements and Finescale Parameterizations

AbstractTurbulence structures across the Antarctic circumpolar current (ACC) in the Indian Ocean at around 50°E were revealed using microstructure measurements. Depth‐averaged turbulent energy dissipation rates (ε) in 320 m segments estimated using a conductivity‐temperature‐depth (CTD)‐attached micro‐temperature fast‐response thermistor (FP07) were well reproduced using existing finescale parameterizations excluding near‐surface and near‐bottom 160 m. The spatial variations of ε are explained by density stratification N2, bottom topographic roughness, and depth‐averaged current speed U. Diapycnal diffusivity Kρ(=0.2εN−2) is found proportional to the squared internal wave energy () and increases toward the bottom, especially over rough bottom topography. The decay scale height is >1000m and the turbulence remains large even in the upper pycnocline. Existing shear‐based finescale parameterizations were generally consistent with the FP07 measurements but tended to overestimate in the areas with large shear‐strain ratio Rω and low ε in the Continental Zone (CZ) south of the ACC fronts (58–65°S). This overestimation is improved by using the strain‐based finescale parameterization with a constant Rω = 3.

Non-linear response of kinetic energy in an inviscid Taylor-Green flow obtained by OpenFOAM-LES with respect to time increments

This study investigates the conservation error of kinetic energy with respect to time increment using OpenFOAM LES. The energy conservation error is verified using an inviscid Taylor-Green vortex and the effect on energy conservation is checked by varying the time increment by a factor of 30. Differences in the response of turbulent energy conservation between large and small time increments are also verified. Higher order accuracy for time increments is expected when the kinetic energy conservation accuracy is high, however the actual behaviour in OpenFOAM needs to be investigated: as the implicit time integration method is used in OpenFOAM, a non-linear response is expected. As a result, it is evident that kinetic energy is not fully conserved in the inviscid flow of OpenFOAM. The kinetic energy distribution is similar for different time increments, with a different relationship between the conservation error observed in the range of large and small time increments.

Amplification and Saturation of Turbulent Magnetic Fields in Collapsing Primordial Gas Clouds

Recent numerical studies suggest that magnetic fields play an important role in primordial star formation in the early Universe. However, the detailed evolution of the magnetic field in the collapse phase still has uncertainties because of the complicated physics associated with turbulence in a collapsing magnetized system. Here, we perform a suite of numerical MHD simulations that follow the collapse of magnetized, turbulent primordial gas clouds to investigate the evolution of the magnetic field associated with the turbulence, assuming a polytropic equation of state with exponent γ eff and with various numerical resolutions. In addition, we generalize the analytic theory of magnetic field growth/saturation so that it can deal with various exponents γ eff and turbulence energy spectra. We find that the numerical results are well reproduced by the theory for various γ eff through the collapse phase during the formation of the first stars. The magnetic field is eventually amplified by a factor of 1012–1015 due to kinematic and nonlinear turbulent dynamo effects and reaches 3%–100% of the equipartition level, depending on γ eff. We also find that the transition between the kinematic and nonlinear stages can be analytically estimated. These results indicate that the strong magnetic field accompanied by supersonic turbulence is a general property and suggest that it can play a crucial role in the formation of the first stars.

Biologically generated turbulent energy flux in shear flow depends on tensor geometry.

It has been proposed that biologically generated turbulence plays an important role in material transport and ocean mixing. Both experimental and numerical studies have reported evidence of the nonnegligible mixing by moderate Reynolds number swimmers, such as zooplankton, in quiescent water, especially at aggregation scales. However, the interaction between biologically generated agitation and the background flow, as a key factor in biologically generated turbulence that could reshape our previous knowledge of biologically generated turbulence, has long been ignored. Here, we show that the geometry between the biologically generated agitation and the background hydrodynamic shear can determine both the intensity and direction of biologically generated turbulent energy flux. Measuring the migration of a centimeter-scale swimmer-as represented by the brine shrimp Artemia salina-in a shear flow and verifying through an analog experiment with an artificial jet revealed that different geometries between the biologically generated agitation and the background shear can result in spectral energy transferring toward larger or smaller scales, which consequently intensifies or attenuates the large-scale hydrodynamic shear. Our results suggest that the long ignored geometry between the biologically generated agitation and the background flow field is an important factor that should be taken into consideration in future studies of biologically generated turbulence.

Dissipation closure of fluid turbulence from energy spectrum

Direct method of the turbulence closure consists of obtaining expression for small-scale velocity and inserting this expression in the Reynolds tensor and in the transport equations for turbulent energy, dissipation, eddy and so on, implementing some averaging. Previously such approaches were used for Burgers-Lundgren vortex. Here we used ‘naïve’ perturbation method for the transport equation for small-scale velocity incompressible fluid - the equation suggested by one of the authors earlier. It is assumed that the energy spectrum of turbulence is given.