High Turbulence Levels Research Articles

A Study of The Turbulent Flows behind A Rectangular Sharp Crested Weir in Open Channel Using Laser Doppler Velocimetry (Dept.C)

This paper describes the experimental investigation using laser Doppler velocimetry) (LDV) in the downstream of .be rectangular sharp crested weir in a horizontal rectangular channel of constant width. The study was carried out for flows on smooth and rough weir. For precise and accurate measurements of mean and fluctuating a ting flow characteristics such as streamwise and vertical turbulence intensity components and streamwise and vertical mean velocity components. The depthwise measurements were carried out for different heights of the weir along to e centerline at different cross sections downstream of weir in the wake region . The results show that, the measured values of turbulence intensities are found to have high level of turbulence in the Dear wake region but are low in the far wake region. The turbulence intensities depict the occurrence of a constant turbulence close to the wall in the far wake region. The roughness was found to increase the turbulence. Also, it can be seen that with increasing weir height, the turbulence intensities are Increased.

Теоретико-прикладні засади антисипативного контролювання на підприємствах

The current stage of functioning of business structures is characterized by a high level of uncertainty and turbulence. In such circumstances, the earliest identification of threats or opportunities in the operating environment of enterprises is provided by anticipative management, which is designed to identify and evaluate the first weak signals of their occurrence.

An experimental assessment of the enhancement of fuel droplet vaporization in a very high turbulence intensity environment

A zero-mean flow fan-stirred chamber is used to gather data on suspended fuel droplet evaporation at turbulence intensities, q1/2, approaching 4.30 m/s. This research is driven by the oft-cited but unconfirmed belief that the droplet evaporation rate, K, eventually plateaus with increasing turbulence kinetic energy. Further motivation comes from numerous real-world examples, including combustion systems, where droplets are exposed to turbulence intensities far beyond the experimental capabilities reported in the literature to date. Decane, ethanol, and heptane fuels are selected and grouped into two categories based on the similarity of their vapor pressures, Pv, and mass diffusivity coefficients, DAB, in an effort to discern which thermophysical properties are important to vaporization enhancement, or lack thereof, in high turbulence. Individual droplets with constant initial diameters, d0, of 500 µm are placed in the central region of a highly homogeneous and isotropic turbulent flow field using a novel multi-fiber intersection technique. Neither alkane fuel exhibits any sign of reduced effect of turbulence, as K/K0 remains a strongly linear function of turbulence intensity throughout the test conditions. Conversely, the normalized evaporation rate of ethanol begins to plateau at the higher levels of intensity. Although equalizing the vapor pressures of ethanol and heptane was sufficient to match K/K0 values at low intensity, their divergence as q1/2 increases implies that the thermodynamically-predicted vapor quantity at the droplet surface does not enforce an upper limit on turbulence effectiveness at moderately elevated temperatures. This is confirmed by decane, which has the lowest volatility and yet experiences the greatest improvement in evaporation rate at the highest levels of turbulence.

Analytical Determination of Thresholds of LDPC Codes in Free Space Optical Channel

Free-space optical (FSO) communication is crucial for the next-generation 5G+ wireless networks. The FSO links suffer from atmospheric-turbulence-induced bit errors. For the increasing link's performance, low-density parity-check (LDPC) codes, complemented by the belief-propagation (BP) algorithm, are an excellent option. The bit error rate (BER) of the LDPC code is characterized by a parameter called the threshold. The threshold is the signal-to-noise ratio (SNR), after which the BER falls arbitrarily and becomes close to zero. We derive the threshold for the LDPC codes under the BP algorithm for an uncorrelated flat FSO channel. The determination of the FSO channel threshold is a tedious task as the density of the log-likelihood ratio from the FSO channel cannot be assumed as Gaussian and is available only in a numerical form. It, thus, requires testing different values of SNR as a possible threshold systematically. Therefore, we propose the divide and conquer algorithm. The threshold depends on the degree distributions, channel state information (CSI), and the turbulence level. When CSI is known, we obtain the threshold at an SNR of 8.10 dB in high turbulence for a regular (3, 6) LDPC code. This threshold steps up to 12.48 dB when the CSI is unknown at the receiver. We evaluate the threshold values for various degree distributions (regular and irregular LDPC codes) under high, moderate, and low turbulence levels for both channel models (CSI known and unknown at the receiver). We also confirm the derived threshold values with MATLAB simulations.

Effect of impeller geometry on bubble breakage and the contributions of different breakage mechanisms in a stirred tank

The effect of impeller geometry on bubble breakage in a stirred tank was investigated for a range of impeller Reynolds number (Re) using a high speed imaging method. The bubble dynamic behavior and breakage mechanism were investigated for four different impeller geometries namely, two-flat blades impeller, four-flat blades impeller, four-twisted blades impeller, and two-pinned blades impeller. The performance of each geometry was investigated by determining the breakage probability and number of fragments (daughter bubbles) produced. The contributions of dominant breakage mechanisms for each geometry were specified and discussed by identifying the breakage locations relative to the impeller. Three main breakage mechanisms were observed, namely: bubble collision with the blade, bubble breakage by blade shear, and breakage by turbulent fluctuation away from the blade. The number of fragments by each breakage mechanism was specified for the entire range of Re. The four-flat blades impeller exhibited the highest breakage probability and produced the highest number of fragments. The Pinned blades gave a high performance compared to the smooth blades, especially at higher Re. This is considered to be due to the high turbulence level provided by this type of impeller. The twisted blades impeller showed low bubble breakage probability compared with the other geometries. Breakages by collision with the blades and by shearing effect resulted in a higher number of fragments compared to the breakages caused by turbulent fluctuations. The number of fragments produced by ‘collision with blade’ exhibited a higher dependence on Re than by ‘blade shear’ or by ‘turbulent fluctuations’.

Mesoscale Eddy Dissipation by a “Zoo” of Submesoscale Processes at a Western Boundary

AbstractMesoscale eddies are ubiquitous dynamical features that tend to propagate westward and disappear along ocean western boundaries. Using a multiscale observational study, we assess the extent to which eddies dissipate via a direct cascade of energy at a western boundary. We analyze data from a ship‐based microstructure and velocity survey, and an 18‐month mooring deployment, to document the dissipation of energy in anticyclonic and cyclonic eddies impinging on the topographic slope east of the Bahamas, in the North Atlantic Ocean. These observations reveal high levels of turbulence where the steep and rough topographic slope modified the intensified northward flow associated with, in particular, anticyclonic eddies. Elevated dissipation was observed both near‐bottom and at mid depths (200–800 m). Near‐bottom turbulence occurred in the lee of a protruding escarpment, where elevated Froude numbers suggest hydraulic control. Energy was also radiated in the form of upward‐propagating internal waves. Elevated dissipation at mid depths occurred in regions of strong vertical shear, where the topographic slope modified the vertical structure of the northward eddy flow. Here, low Richardson numbers and a local change in the isopycnal gradient of potential vorticity (PV) suggest that the elevated dissipation was associated with horizontal shear instability. Elevated mid‐depth dissipation was also induced by topographic steering of the flow. This led to large anticyclonic vorticity and negative PV adjacent to the topographic slope, suggesting that centrifugal instability underpinned the local enhancement in dissipation. Our results provide a mechanistic benchmark for the realistic representation of eddy dissipation in ocean models.

A cellular automata model for expanding turbulent flames.

Cellular automata models based on population dynamics, introduced by Von Neumann in the 1950s, has been successfully used to describe pattern development and front propagation in many applications, such as crystal growth, forest fires, fractal growth in biological media, etc. We, herein, explore the possibility of using a cellular automaton, based on the population dynamics of flamelets, as a low-order model to describe the dynamics of an expanding flame propagating in a turbulent environment. A turbulent flame is constituted by numerous flamelets, each of which interacts with their neighborhood composed of other flamelets, as well as unburned and burnt fluid particles. This local interaction leads to global flame dynamics. The effect of turbulence is simulated by introducing stochasticity in the local interaction and hence in the temporal evolution of the flamefront. Our results show that the model preserves various multifractal characteristics of the expanding turbulent flame and captures several characteristics of expanding turbulent flames observed in experiments. For example, at low turbulence levels, an increase in global burning rate leads to an increase in the turbulence level, while beyond a critical turbulence level, the expanding flame becomes increasingly fragmented, and consequently, the total burning rate decreases with increasing turbulence. Furthermore, at an extremely high turbulence level, the ignition kernel quenches at its nascent state and consequently loses its ability to propagate as an expanding flame.

An Extended Version of an Algebraic Intermittency Model for Prediction of Separation-Induced Transition at Elevated Free-Stream Turbulence Level

An algebraic intermittency model for boundary layer flow transition from laminar to turbulent state, is extended using an experimental data base on boundary layer flows with various transition types and results by large eddy simulation of transition in a separated boundary layer. The originating algebraic transition model functions well for bypass transition in an attached boundary layer under a moderately high or elevated free-stream turbulence level, and for transition by Kelvin–Helmholtz instability in a separated boundary layer under a low free-stream turbulence level. It also functions well for transition in a separated layer, caused by a very strong adverse pressure gradient under a moderately high or elevated free-stream turbulence level. It is not accurate for transition in a separated layer under a moderately strong adverse pressure gradient, in the presence of a moderately high or elevated free-stream turbulence level. The extension repairs this deficiency. Therefore, a sensor function for detection of the front part of a separated boundary layer activates two terms that express the effect of free-stream turbulence on the breakdown of a separated layer, without changing the functioning of the model in other flow regions. The sensor and the breakdown terms use only local variables.

Numerical Simulation of Melting Kinetics of Metal Particles during Tapping with Argon-Bottom Stirring

Molten steel is alloyed during tapping from the melting furnace to the argon-bottom stirred ladle. The metallic additions thrown to the ladle during the ladle filling time are at room temperature. The melting rates or kinetics of sinking-metals, like nickel, are simulated through a multiphase Euler–Lagrangian mathematical model during this operation. The melting rate of a metallic particle depends on its trajectory within regions of the melt with high or low turbulence levels, delaying or speeding up their melting process. At low steel levels in the ladle, the melting rates are higher on the opposite side of the plume zone induced by the bottom gas stirring. This effect is due to its deviation after the impact of the impinging jet on the ladle bottom. The higher melting kinetics are located on both sides at high steel levels due to the more extensive recirculation flows formed in taller baths. Making the additions above the eye of the argon plume spout increases the melting rate of nickel particles. The increase of the superheat makes the heat flux more significant from the melt to the particle, increasing its melting rate. At higher superheats, the melting kinetics become less dependent on the fluid dynamics of the melt.

Keeping dispute resolution internal: Exploring the role of the industrial relations climate, organizational embeddedness and organizational turbulence

Drawing on job embeddedness theory, this study investigates how and when the industrial relations climate influences employees’ preference for particular labour dispute resolution procedures. Analyses of multisource and multilevel data from China show that organizational embeddedness mediates the relationship between the industrial relations climate and preference for internal dispute resolution (IDR) and this indirect effect is stronger in organizations with a high level of turbulence. These findings provide novel theoretical explanation of the linkage between the industrial relations climate and employees’ preference for IDR and highlight the interactions between the industrial relations climate and organizational turbulence. Practical implications are also offered to facilitate employees’ preferences for IDR by improving the quality of the industrial relations climate.

Evaluation of wave-turbulence decomposition methods applied to experimental wave and grid-generated turbulence data

Tidal energy turbines are significantly impacted by surface gravity waves and high turbulence levels, which lead to increased blade loads and consequent device fatigue. Accurately describing turbulence quantities is a key factor to improve numerical models and ensure the minimum necessary device longevity. The application of an appropriate wave-turbulence decomposition technique substantially improves estimates and must be debated by the tidal energy community. This work analyses two wave-turbulence decomposition techniques: i) linear wave theory and ii) Synchrosqueezing Wavelet Transform (SWT) based on physical modelling. The experiments were conducted in a towing tank and the physical model used Froude scaling to simulate wave heights and frequencies based on field measurements from a tidal energy candidate site in Australia. Turbulence was generated using a grid with 47 mm openings replicating turbulence intensities of 17%, 23% and 25%. Both decomposition techniques present limitations in wave frequencies above 0.7 Hz. Our results suggest that the SWT technique is more versatile and more efficient under various conditions. Restrictions to the linear wave theory method are discussed with regards to ADCP geometry and possible non-linearity. The variability obtained from the wave-turbulence decomposition techniques emphasizes the importance of establishing guidelines for turbulence characterization in tidal energy sites.

Phenomenology of Turbulence Production in Merging Process of Supersonic Streamwise Vortices

Merging of corotating vortices is of primary interest in the field of supersonic mixing due to the ability of two or more structures to coalesce and form a larger one which enhances the stirring motion and straining in the flow, thus promoting turbulence production and mixing augmentation. During experimental efforts aimed at quantifying mean and fluctuating flowfields produced by the merging of two supersonic streamwise corotating vortices through stereoscopic particle image velocimetry, it was noticed that both the strain rates and turbulent kinetic energy would peak within the elongated merger’s vorticity patch. The article discusses how this is in fact a counterintuitive phenomenon and shows that a possible explanation can be given by studying the flow within a frame of reference which rotates with the merger itself. It will be shown how the merger is surrounded by a different flowfield than that measured in the laboratory frame of reference, characterized by the presence of recirculation zones referred to as ghost vortices. These vortices introduce large rates of strain within the merger, thus providing a possible justification for the high levels of turbulence attained within the rotating structure. The experimental findings are strongly corroborated by reduced-order, inviscid simulations, showing the formation of similar structures and clarifying the presence of peaks of strain rates within the merger.

Structure and stability characteristics of turbulent planar flames with inhomogeneous jet in a concentric flow slot burner

Turbulent flames with compositionally inhomogeneous mixtures are common in many combustion systems. Turbulent jet flames with a circular nozzle burner were used earlier to study the impact of inhomogeneous mixtures, and these studies showed that the nozzle radius affects the flame stability. Accordingly, planar turbulent flames with inhomogeneous turbulent jet are created in a concentric flow slot burner (CFSB) to avoid this effect in the present study. The stability characteristics, the mixing field structure, and the flame front structure were measured, and the correlations between stability and the mixing field structure were investigated. The mixture fraction field was measured in non-reacting jets at the nozzle exit using highly resolved Rayleigh scattering technique, and the flame front was measured in some selected turbulent flames using high-speed Planar Laser-Induced Fluorescence (PLIF) of OH technique. The data show strong correlations between flame stability and the range of mixture fraction fluctuations. The flames are highly stabilized within a mixing field environment with the range of fluctuation in mixture fraction close to the range of the flammability limits. The mixing field structure is also illustrated and discussed using a mixing regime diagram and showed that the scatter of the data of the different cases is consistent with the classified mixing regimes. Lean flames are stabilized in the current slot burner. The flame front structure topology varies consistently from thin, small curvature at the low level of turbulence and higher equivalence ratio to more wrinkled, larger curvature, but a thicker structure at a higher level of turbulence and lower equivalence ratio.

Actuator grid method for turbulence generation applied to yawed wind turbines

The aerodynamics of yawed wind turbine wakes remains a major investigation topic, especially for the use of yaw angle in control strategies. Large-Eddy Simulations are employed here to study the influence of yaw and inflow conditions on the prediction of the wind turbine wake structure. A single wind turbine setup with different yaw angles and three different inflow conditions is investigated and discussed with respect to experimental data. The wind turbine blades are modeled using the actuator line method (ALM) while tower and nacelle are represented with a body-fitted unstructured mesh. The high levels of upstream turbulence, experimentally generated by turbulence grids, are emulated here with oscillating ALM. Such approach demonstrates to be highly predictive on the turbulent flow characteristics compared to the emptied wind tunnel experimental data. Results with turbine show good agreement to the experiment data with only a slight overestimation on the magnitude of the wake deflection due to yaw. When compared to a deflection model, the confinement of the wind tunnel is highlighted. The radial and azimuthal time-averaged angle of attack exhibits a high probability of dynamic stall near the hub for a yawed turbine. These results show large discrepancies on the blade loading, highlighting the need for improved and specific models.

Turbulence-level dependence of cosmic ray parallel diffusion

ABSTRACT Understanding the transport of energetic cosmic rays belongs to the most challenging topics in astrophysics. Diffusion due to scattering by electromagnetic fluctuations is a key process in cosmic ray transport. The transition from a ballistic to a diffusive-propagation regime is presented in direct numerical calculations of diffusion coefficients for homogeneous magnetic field lines subject to turbulent perturbations. Simulation results are compared with theoretical derivations of the parallel diffusion coefficient’s dependences on the energy and the fluctuation amplitudes in the limit of weak turbulence. The present study shows that the widely used extrapolation of the energy scaling for the parallel diffusion coefficient to high turbulence levels predicted by quasi-linear theory does not provide a universally accurate description in the resonant-scattering regime. It is highlighted here that the numerically calculated diffusion coefficients can be polluted for low energies due to missing resonant interaction possibilities of the particles with the turbulence. Five reduced-rigidity regimes are established, which are separated by analytical boundaries derived in this work. Consequently, a proper description of cosmic ray propagation can only be achieved by using a turbulence-level-dependent diffusion coefficient and can contribute to solving the Galactic cosmic ray gradient problem.

Balance measurements on a frigate type ship model

Balance measurements performed by testing sub-scaled ship models determine the global forces and moments acting on the ship, which allows knowing the power required for the ship's movement and provides insight to be applied in the design of the control systems used to steer the ship and to avoid instabilities while sailing.The ship superstructure may produce large separated regions and high air wake turbulence levels resulting in a set of fluctuations of the flow parameters usually determined by measuring velocity or pressure. This paper presents the balance measurement of the aerodynamic forces acting on the ship hull. Aerodynamic forces and moments produced on the ship can be interpreted as an integration of the flow parameters (velocity and pressure distributions) over the ship surface wetted by the air. Balance method provides averaged values and fluctuations of forces coefficients.Aerodynamic environment in the vicinity of a ship is influenced by a large number of factors (atmospheric wind, sea state, ship superstructure, masts, stacks, antennas …) affecting helicopter operations on board ships and their safety during the take-off and landing manoeuvres.

One year of measurements in Alderney Race: preliminary results from database analysis.

The Alderney Race is assumed to have the largest tidal-stream energy potential in the north-western European coastal seas. Interaction of the powerful tidal stream with strong wind, high waves and irregular bathymetry creates hydrodynamic conditions of extreme complexity, with high levels of turbulence. A comprehensive dataset has been created to improve the understanding of physical processes, turbulence, tidal stream and resource variability at the site. The database contains a large amount of oceanographic and meteorological measurements acquired in Alderney Race in 2017-2018. This exceptionally long period of observations (nearly one year) became possible due to modern tools and strategies of data acquisition. The paper presents some significant results from the database analysis. Among many results, we would like to underline the following: (i) a wide range of variability of mean flow and sea state parameters was documented; (ii) exceptionally large values of current velocity (7 m s-1) and significant wave height (8 m) were measured during extreme meteorological conditions; (iii) high-frequency variability of current speed during storm events was also found to be very large, with the standard deviation of velocity reaching 0.3 m s-1 in the bottom boundary layer, and 0.6 m s-1 in the surface layer; and (iv) predominant wind and wave direction relative to the flow impacts the wave height and significantly increases the turbulence kinetic energy of the flow. To our knowledge, this is the largest multi-variable database available on potential tidal energy sites. The results of database analysis can represent a significant advance in environmental conditions and resource characterization and provide advanced information to turbine developers. This article is part of the theme issue 'New insights on tidal dynamics and tidal energy harvesting in the Alderney Race'.

Development of a Time–Height Profile Assimilation Technique for Large-Eddy Simulation

Mesoscale-to-microscale coupling (MMC) aims to address the limited scope of traditional large-eddy simulations by driving the microscale flow with information concerning large-scale weather patterns provided by mesoscale models. We present a new offline MMC technique for horizontally homogeneous microscale flow conditions, in which internal forcing terms are computed based on mesoscale time–height profiles of mean-flow quantities. The advantage of such an approach is that it can be used to drive a microscale simulation with either mesoscale or observational data, and that it does not rely on specific terms in the mesoscale budget equations, which are typically not part of the default output of a mesoscale solver. The performance of the proposed profile assimilation technique is assessed based on the simulation of a typical diurnal cycle over the Scaled Wind Farm Technology site in west Texas. Results indicate that simple data assimilation techniques lead to unphysically high levels of shear and turbulence caused by the algorithm’s inability to cope with inaccuracies in the mesoscale time–height profiles. Modifying the algorithm to account for vertical coherence in the mesoscale source terms gives the microscale solver a greater ability to correct the provided mesoscale time–height profiles, leading to improved predictions of shear and turbulence statistics. The resulting turbulence statistics are in good agreement with meteorological tower observations and simulation results obtained with state-of-the-art coupling techniques using mesoscale budget components.

On the turbulence amplification in shock-wave/turbulent boundary layer interaction

The mechanism of turbulence amplification in shock-wave/boundary layer interactions is reviewed, and a new turbulence amplification mechanism is proposed based on the analysis of data from direct numerical simulation of an oblique shock-wave/flat-plate boundary layer interaction at Mach 2.25. In the upstream part of the interaction zone, the amplification of turbulence is not essentially shear driven, but induced by the interaction of the deceleration of mean flow with streamwise velocity fluctuations, which causes a rapid increase of turbulence intensity in the near-wall region. In the downstream part of the interaction zone, the high turbulence intensity is mainly due to the free shear layer generated in the interaction zone. During the initial stage of turbulence amplification, the characteristics of wall turbulence, including compact velocity streaks, streamwise vortices and an anisotropic Reynolds stress, are well preserved. The mechanism proposed explains the high level of turbulence in the near-wall region observed in some experiments and numerical simulations.

Understanding the Aerodynamic Behavior and Energy Conversion Capability of Small Darrieus Vertical Axis Wind Turbines in Turbulent Flows

Small Darrieus vertical-axis wind turbines (VAWTs) have recently been proposed as a possible solution for adoption in the built environment as their performance degrades less in complex and highly-turbulent flows. Some recent analyses have even shown an increase of the power coefficient for the large turbulence intensities and length scales typical of such environments. Starting from these insights, this study presents a combined numerical and experimental analysis aimed at assessing the physical phenomena that take place during the operation of a Darrieus VAWT in turbulent flows. Wind tunnel experiments provided a quantification of the performance variation of a two-blade VAWT rotor for different levels of turbulence intensity and length scale. Furthermore, detailed experiments on an individual airfoil provided an estimation of the aerodynamics at high turbulence levels and low Reynolds numbers. Computational fluid dynamics (CFD) simulations were used to extend the experimental results and to quantify the variation in the energy content of turbulent wind. Finally, the numerical and experimental inputs were synthetized into an engineering simulation tool, which can nicely predict the performance of a VAWT rotor under turbulent conditions.