Effects Of Turbulence Research Articles

SRANS Simulation on a Helical-Segmented Finned Tube Bank

A numerical simulation on a six-row helical-segmented finned tube in a staggered layout is performed with the Reynolds Averaged Navier-Stokes (RANS) approach. The turbulence effect is represented with the k-ε RNG model, and a fixed temperature in each tube row considers the inside fluid temperature. The tube bank is represented by a small finned tube bank and a single interacted module. The small finned tube bank considers symmetry conditions on the sides of the computational domain. The single interacted module considers periodic boundary conditions for velocity, pressure, and temperature. Both simulations are compared to develop the basis of complex simulations such as repetitive finned tube modules. Average values on parallel planes to the streamwise direction for velocities, pressures, and temperatures are compared in both simulations. The comparative analyses show deviations lower than 11% for the velocity field, 12% for the pressure field, and 10% single for the temperature field. Therefore, results show an appropriate flow representation with periodic boundary conditions.

EDFA based adaptive RZ-OOK transmission for FSO communication

This paper proposes an erbium-doped fiber amplifier (EDFA) based adaptive return-to-zero on-off keying (RZ-OOK) transmission for free-space optical (FSO) communication. EDFA has the characteristics of average power limitation. Therefore, the transmitted RZ-OOK duty cycle is varied for accomplishing pulse power variations relying on determined average signal-to-noise ratio of received signal. Thus, the atmospheric turbulent effects can be compensated by this type of adaptive RZ-OOK transmission. The channel models with different turbulence effects are established, and the proposed technique is validated in simulation. The simulation results illustrate that the communication quality of system is considerably enhanced by the proposed technique compared with fixed threshold decision and adaptive threshold decision.

Performance Analysis of DP‐QAM‐Based Optical OFDM PON Employing Optical Multicarrier

This work proposes a new hybrid bidirectional standard single mode fiber (SSMF)/free space optical (FSO) dual polarization quadrature amplitude modulation (DP‐QAM) employing optical orthogonal frequency division multiplexing (OFDM). An optical multicarrier generator (OMG) source is utilized. The suggested model is simulated using VPI photonics software. Thirty‐two optical OFDM channels are used; each carrier’s 30 Gbps information data with DP‐QAM are employed to improve the performance of the fifth‐generation passive optical network (5G‐PON) for downstream and upstream transmission. In addition, 16‐QAM signals are used in DP. The impact of continuous wavelength (CW) lasers and high data rates in the proposed 5G‐PON has been investigated for DP‐QAM‐OFDM. The results reveal 0.96 Tb/s and 0.1 Tb/s for the downstream and upstream directions, respectively, under different turbulence effects. Furthermore, the simulation results show that the proposed model can achieve an SSMF length and FSO propagation ranges of 20 km and 2 km, respectively, with the symbol error rate (SER) (<10−9) and error vector magnitude (EVM) (<0.186).

Investigating Extreme Scattering Events by Volumetric Ray-tracing

Extreme scattering events (ESEs) are observed as dramatic (>50%) drops in flux density that occur over an extended period of weeks to months. Discrete plasma lensing structures are theorized to scatter the radio waves produced by distant sources such as pulsars, causing the signature decrease in flux density and characteristic caustic spikes in ESE light curves. While plasma lens models in the extant literature have reproduced key features of ESE light curves, they have all faced the problem of being highly overdense and overpressured relative to the surrounding interstellar medium by orders of magnitude. We model ESEs by numerically ray tracing through analytic, volumetric plasma lens models by solving the eikonal equation. Delaunay triangulation connecting the rays approximates the wave front, generating a mapping from the observer plane to the source plane to account for multiple imaging. This eikonal method of ray tracing is tested against known analytic solutions and is then applied to a three-dimensional Gaussian-distributed electron volume density lens and a filament model inspired by Grafton et al. We find convergence of our numerical results with established analytic solutions, validating our numerical method, and reproduce ESE-like light curves. Our numerical ray-tracing method lends itself well to exploring the lensing effects of volumetric turbulence as well as sheet-like lenses, which is currently in progress.

Design AMT, Manufacturing AMT, and Product Innovation Performance (PIP)—The Contingent Effect of Organizational Culture and Environmental Turbulence

Design AMT, Manufacturing AMT, and Product Innovation Performance (PIP)—The Contingent Effect of Organizational Culture and Environmental Turbulence

Experimental investigation of flame morphology and instabilities in turbulent wind-driven flames

This paper investigates the effect of freestream turbulence on flame profile parameters and streak behavior for boundary layer flames stabilized over a liquid fuel-soaked wick. In contrast to conventional gaseous burners, the study utilized a condensed fuel surface to better emulate realistic burning scenarios, thereby increasing the reliability of the obtained results. This study explores the impact of crossflow velocity (ranging from 1 m/s to 2 m/s) and turbulent intensity (varying between 1.8 % and 16 %) on the dynamic evolution of flame geometry and streak profile. For a given flow velocity, the results indicate a reduction in flame length and flame attachment length with turbulence intensity; in contrast, an increase in tilt angle was observed with increasing turbulence intensity. However, the rate of their increase or decrease was found to vary across different ranges of freestream turbulence intensity. Also, the flame exhibits a curling action in higher turbulence intensity scenarios that influence the global flame structure. The average mass burning rate was also found to be positively related to the flow turbulence in all considered cases. Freestream turbulence induces significant disturbances that amplify and destabilize coherent structures, accelerating the transition to turbulence in the overall flow and resulting in a shorter flame attachment length. Streak spacing experiences accelerated growth with higher turbulence levels, with a sharp surge in the initial region due to the blowing effect caused by the condensed fuel surface. The influence of flow turbulence was also observed by plotting lognormal distribution curves representing streak spacing data. The impact of elevated freestream turbulence on the dispersion of the distribution curve was found to be more pronounced than that of the flow velocity, emphasizing its crucial role in governing the streak dynamics.

Background vapor effect on droplet evaporation in a turbulent flow at elevated pressure

Investigation of the evaporation of single droplets holds a significant importance as it emulates the diluted spray region and helps improve the accuracy of numerical modelling. In the present study, a fan-stirred spherical chamber was employed to generate a homogeneous and isotropic turbulent flow field with negligible mean velocity. This setup was used to investigate the effect of background vapor on the vaporization of a 500 µm pentane droplet in a turbulent flow at elevated pressure and room temperature. Turbulence intensity and pressure were varied, respectively, from stagnant to 1.5 m/s and from 1 to 5 bar. Additionally, the background pentane vapor was altered (on molar basis) from 0 % up to 40 %. The results show that the d2-law is valid throughout all explored test conditions, including in the presence of background vapor. The findings indicated that the presence of background vapor around the evaporating droplet diminishes the droplet vaporization rate under both stagnant and turbulent flow scenarios, with a more pronounced impact at elevated pressure conditions. Furthermore, turbulence is observed to enhance the vaporization rate in the presence of background vapor, with its influence being more substantial at elevated pressure levels and higher concentrations of background vapor. Finally, the experimental data was used to correlate the coupled effect of ambient background vapor, turbulence, and pressure on the vaporization of a pentane droplet in terms of the flow turbulent Reynolds number.

Effects of Unstable Stratified Ocean Turbulence on Hermite-Gaussian Optical Communication System

Effects of Unstable Stratified Ocean Turbulence on Hermite-Gaussian Optical Communication System

К теории спиральной турбулентности немагнитного астрофизического диска. Образование крупномасштабных вихревых структур

A closed system of three-dimensional hydrodynamic equations of the mean motion scale is presented for modeling spiral turbulence in a rotating astrophysical disk. The diffusion equations for the averaged vortex and the integral vortex spirality transport equation are derived. A general concept of the emergence of energy-consuming mezoscale coherent vortex structures in a thermodynamically open subsystem of turbulent chaos, associated with the realization of the inverse cascade of kine-tic energy in mirror-nonsymmetric disk turbulence, is formulated. It is shown that negative viscosity in the rotating disk three-dimensional system is apparently a manifestation of cascade processes in spiral turbulence, when an inverse energy transfer from small vortices to larger ones is realized. It is also shown that a relatively long decay of turbulence in the disk is associated with the absence of reflective symmetry of the anisotropic field of turbulent velocities relative to its equatorial plane. The work is of a review character, made with the aim of improving new models of astrophysical nonmagnetic disks for which the effects of spiral turbulence play a determining role.

Solar Plasma Structures and Solar Wind Plasma Turbulences in Relation with Geomagnetic Disturbances during Decline Phase of Solar Cycle 24 and Rising Phase of Solar Cycle 25

We have studied the effect of solar structures and turbulences in solar wind plasma on the Earth's magnetosphere during the decline phase of solar cycle 24 and the rising phase of solar cycle 25. Most CMEs are geoeffective, cause geomagnetic storms, and are associated with C-class and M-class solar flares (S.F.s). It is also observed that these geomagnetic storms are associated with disturbances in solar wind plasma parameters. We have determined a large positive correlation with a correlation coefficient of 0.68, 0.58, 0.65, and 0.63 between the magnitude of the extensive geomagnetic storms and the peak value of IMF Btotal, the magnitude of IMF Btotal, the peak value of disturbances in the southward component of interplanetary magnetic fields (IMF Bz), and magnitude of disturbances in the southward component of interplanetary magnetic fields (IMF Bz). The correlation coefficient of 0.61 is higher between the magnitude of Dst and the magnitude of SWPT, as compared with the magnitude of flow pressure (0.39). From the present work, we have concluded that solar plasma structures and solar wind turbulences are mainly responsible for generating extensive geomagnetic storms.

Effect of atmospheric turbulence on imaging quality of high-resolution remote sensing satellites

Effect of atmospheric turbulence on imaging quality of high-resolution remote sensing satellites

A review and reassessment of the aerodynamics of cricket ball swing

This paper reviews 70 years of cricket ball swing literature and reassesses the results in light of new measurements. A comparison of ball tracking data with published experimental results shows that current understanding does not explain the behaviour observed in professional cricket matches. Descriptions of cricket ball boundary layer aerodynamics are updated with new results which show that the seam acts more like a series of vortex generators than a trip and that there is a laminar separation bubble on the seam side of the ball. Previous results for reverse swing are consolidated and compared with new data, showing that different magnitudes of swing occur depending on the condition of both sides of the ball. Variation in pressure and temperature should be included when the Reynolds number, a non-dimensional ball speed, is calculated, while humidity can be neglected. Studies on the effect of wind speed and direction are summarised and results considering the effect of free-stream turbulence are compared with new measurements. Throughout the paper, recommendations for future work are suggested. These include quantitative study of the effect of surface defects and roughness, an assessment of whether atmospheric turbulence can affect swing and investigation into the effect of backspin on swing.

On the Relative Effect of Underwater Optical Turbulence in Different Channel Conditions

On the Relative Effect of Underwater Optical Turbulence in Different Channel Conditions

Local statistics of turbulent spherical expanding flames for NH3/CH4/H2/air measured by 10 kHz PIV

In this paper, we present high-speed particle image velocimetry measurements to quantify combustion-induced turbulence and turbulent stretch rate effects on flame propagation in a fan-stirred constant-volume combustion vessel. Three stoichiometric fuel/air mixtures including methane/air, methane/hydrogen/air and ammonia/methane/air were tested. The expansion rate of flame radius is found to scale with a half-power law of the mean-radius based Reynolds number. Measurements of velocity in the reactant zone show that the gas mean and rms velocity increase within the flame brush then gradually decrease when approaching the detection edge. Combustion enhances the measured non-reacting flow turbulence by ∼64 % within the reactant side of the flame brush of this expanding spherical flame. The evolution of rms velocity is found to be insensitive to pressure but sensitive to initial turbulent intensity and laminar flame speed, scaling with Reynolds number in a similar manner as the rate of expansion of the flame radius. Hydrogen blending shows the largest absolute value of rms velocity increase, but ammonia blending leads to the largest increase in rms velocity relative to laminar flame speed. The pdfs of 2D curvature are broadened as u′ and p increase but gradually saturate at high u′ for all mixtures, those of stretch rate are widened by flame speed. The temporal evolution of statistical flame stretch rates suggests the tangential stretch rate is larger than and anti-correlated with the normal stretch, and that there is a strong positive correlation between instantaneous flame propagation rate and the rms strain rate. Finally, the mean flame curvature and mean stretch rate are found to be near zero as flame propagates outwardly.

Parametric Investigation of Pelton Turbine Injector under Hydro-abrasive Erosion Conditions

In high-head Pelton turbines, the injector faces severe erosion due to suspended sediment leading to a reduction in turbine efficiency and higher maintenance costs. Here, the effects of design parameters such as the bend angle of the nozzle pipe, nozzle angle, and needle angle along with an operating parameter stroke ratio on hydro-abrasive erosion of Pelton turbine injector are numerically investigated. The Volume of Fluid (VOF) model was implemented for capturing the interphase between air and water; whereas, the SST k-ω model is used for modelling the turbulence effect. For tracking the discrete phase, a Eulerian-Lagrangian based Discrete Phase Model (DPM) is considered. The bend angles led to flow circulations in the nozzle pipe causing the non-uniform distribution of sediment concentration and uneven erosion patterns. Irrespective of the bend angle, the erosion hotspot in the needle is observed toward the bend side. Further, for larger sediment particles, higher bend angles lead to more erosion rate in both the nozzle and needle and must be avoided to prevent excessive damage. As the needle angle increases from 40° to 60° for a constant nozzle angle, the nozzle erosion rate increases by 70% and the needle erosion rate decreases by 99%. Hence, an injector design can be optimized in hydro-abrasive erosion conditions by selecting a needle angle between 40° and 60°. Further, the operation of the injector at too high and low a stroke ratio results in excessive erosion of the nozzle and needle, respectively. In this study, a stroke ratio of 0.45 is found to be the most suitable for hydro-abrasive erosion conditions. Moreover, the asymmetricity in the erosion pattern of the needle increases with needle angle and stroke ratio resulting in jet quality degradation, one major reason for efficiency reduction in Pelton turbines.

The impact of 180° return bend inclination on pressure drop characteristics and phase distribution during oil-water flow

The present work aims to capture the influence of the inclination of the return bend on flow patterns and pressure drop during oil-water flow. The experiments were carried out for different inclinations (0°, 15°, 30°, and 45°) of return bend for various superficial velocity combinations of oil (kerosene) and water ranging from 0.07 to 0.66 m/s. The experiments showed that pressure drop increases with the increase in inclination. However, the pressure drop at a fixed inclination (say 15°) decreases with the increase in the superficial velocity of the water. Distinct flow patterns observed in the return bend were droplet flow, film inversion, slug flow, plug flow and large slug flow. Droplet flow dominates at the lower range of kerosene (i.e., Usk = 0.07–0.2 m/s) and higher range of water superficial velocity (i.e., Usw = 0.40–0.66 m/s) at all the inclinations considered in this study. Additionally, comparisons between the experimental and numerical simulation results were made. The numerical solution utilized the Euler-Euler approach, considering the different phases as interpenetrating continua. The Volume of Fluid (VOF) model was used within this approach, monitoring the volume fraction of each phase over the domain while calculating one set of momentum equations for each phase. To capture the turbulent effects accurately, the k-ε turbulence model was incorporated. It happened to be found that the numerical findings showed remarkable agreement with the experimental data.

LES of pilot n-heptane ignition and its interaction with the lean premixed methane–air mixture in a dual-fuel combustion engine

Large eddy simulations of pilot fuel ignited, lean premixed, natural gas engines are performed to study the pilot-ignition process and its subsequent interaction with the premixed charge. The injection pressure (Pinj) and injection duration (Δinj) are varied (i.e. 800 bar/760μs, 800 bar/500μs, and 400bar/760μs) to study the impact of the injection process on the subsequent combustion evolution. Open-cycle simulations considering the full engine geometry are used to predict the in-cylinder flows, while combustion is modeled using a finite-rate chemistry model. In-cylinder methane (CH4) is shown to delay the low-temperature ignition of the pilot fuel, regardless of the pilot injection setting, which subsequently prolongs the overall pilot fuel ignition delay. Moreover, all simulated cases show the occurrence of back-supported combustion (BSC), where the burning of CH4-air mixture is “back-supported” by pilot fuel radicals. Despite both the 800 bar/500μs and 400 bar/760μs cases having the same injected pilot fuel mass, the peak in-cylinder pressure and burning rate of the premixed CH4-air mixture in the former case are higher. Higher Pinj and shorter Δinj lead to better mixing between the pilot fuel and the premixed CH4-air charge. Subsequently, this forms a larger volume of regions with elevated equivalence ratio due to the presence of pilot fuel (ϕ>ϕCH4) which, consequently promotes the formation of BSC. The impact of in-cylinder flow fields on the dual-fuel combustion process is investigated by performing two closed-cycle 800 bar/500μs cases with one assuming solid-body rotation and another without solid-body rotation (i.e. zero velocity field). In-cylinder flow field is shown to have a visible impact on the transition stage between the pilot ignition stage and the premixed flame propagation stage, but have an insignificant effect on the pilot fuel ignition process. In the transition stage, slower flame propagation is observed in the zero-velocity case. The results show that this is not only due to the turbulence effect on premixed flame but also due to differences in the volume and distribution of pilot fuels that impacts BSC.

Free space optical communication system in the presence of atmospheric losses

<span>Free space optical communication is gaining importance in the field of optical communication due to its high speed and high bandwidth applications. Free space optical communication system (FSOCS) provides many benefits as compared to traditional wireless communication system and fiber optic cables. This makes this technology the reasonable extension of metropolitan area network and also provides the quick recovery during natural disaster. This system performance is limited due to the atmospheric turbulence effect and various atmospheric losses such as rain, and fog. Gamma gamma atmospheric turbulent model is used to analyze the system performance in the presence of moderate to strong atmospheric turbulence. We have designed the FSO gamma <span>gamma turbulent model with non-return to zero (NRZ) modulation format employing wavelength division multiplexing (WDM), spatial diversity multiple input multiple output (MIMO) (8×8) at various atmospheric turbulence levels and attenuation loss of 10 dB/km at the distance of 2-4 km. Using the proposed model, the link distance is enhanced up to 4km in the presence of turbulence and atmospheric losses with minimum laser transmitted power.</span></span>

The Turbulent Effects of COVID-19 Policy on Australian Federalism and National Uniform Legislation

The Turbulent Effects of COVID-19 Policy on Australian Federalism and National Uniform Legislation

THEORETICAL STUDY OF A THREE-DIMENSIONAL TURBULENT BOUNDARY LAYER OVER A ROTATING CONE OR DISK

In this study, we investigated turbulent boundary layer flow due to a rotating cone or disk. A comprehensive theoretical analysis was performed and the results for several physical quantities of significant interest are presented. The problem was modeled such that both cone and disk cases could be achieved under the same formulation. In most situations, comparisons were made with the experimental results available in the literature. A primary objective of this study was to present reliable results of a rotating cone or disk with respect to several physical quantities of significant interest in order to facilitate future studies. The results were obtained at 45° and 60° angles in the cone case and at a 90° angle in the disk case. The Keller-box method was applied to numerically solve the Reynolds-averaged Navier-Stokes equations. The mixing-length turbulent model was implemented to incorporate the turbulence effects. The validity of this method was first confirmed by comparing the obtained solution for the disk case with the existing literature produced by different researchers. Circumferential and radial velocity distributions and polar plots in the cone and disk cases are presented for Reynolds numbers up to Re<i><sub>r</sub></i> = 1 × 10<sup>6</sup>. However, the circumferential and radial skin-friction coefficients, combined friction coefficient, moment coefficient, boundary layer thickness, displacement thickness, velocity shape factor, flow rate, and flow swirl angle are provided up to Re<i><sub>r</sub></i> = 1 × 10<sup>8</sup>. We also noted that the asymptotic nature of velocities can be achieved by setting the value of grid parameter <i>K</i>.