High Turbulence Intensity Research Articles

The impact of upstream turbulence on a plane jet

The influence of upstream turbulence on the flow produced by a plane jet is investigated experimentally with hot-wire anemometry and smoke flow visualisation. An innovative active grid, where each wing can be independently controlled, is used to change the upstream turbulence conditions. Three cases are investigated: a canonical reference case, a case with the same integral scale as the reference case but an order of magnitude increase in turbulence intensity, and a case with both an order of magnitude increase in turbulence intensity and an order of magnitude increase in integral scale compared to the reference case. It is demonstrated that the wake width increases with turbulence intensity, but decreases with integral scale for constant turbulence intensity. In addition, the positional variability of the wake width is highest with high turbulence intensity and a short integral scale. Along the jet centreline, the potential core region is shorter with elevated upstream turbulence intensity; this is reflected in both the mean velocity and the variance. The decay of the centreline mean velocity is also retarded by incoming turbulence. In all, increased incoming turbulence results in increased jet spreading, and a shorter integral scale further increases the spreading.Graphic abstract

Numerical investigation of stall characteristics for winglet blade of a horizontal axis wind turbine

Wind energy is one of the renewable energy resources which is clean and sustainable energy and the wind turbine is used for harnessing energy from the wind. The blades are the key components of a wind turbine to convert wind energy into rotational energy. Recently, wingtip devices are used in the blades of horizontal axis wind turbine (HAWT), which decreases the vortex and drag, while increases the lift and thereby improve the performance of the turbine. In the present study, a winglet is used at the tip of an NREL phase VI wind turbine blade. Solidworks, Pointwise, and Ansys-Fluent are used for geometric modeling, computational grid generation, and CFD simulation, respectively. The computational result obtained using SST k-ω turbulence modeling is well validated with the experimental data of NREL at 5 and 7 m/s of wind speeds. Numerical investigation of stall characteristics is carried out for wingleted blade at higher turbulence intensity (21% and 25%) and angle of attack (00 to 300 at 50 intervals) at 7 m/s wind speed. The result found that wingletd blade delay stall to 150 for both the cases of turbulence intensity. Increasing the turbulence intensity increases the lift coefficient at stall angle but drag coefficient also increases and thus a lower aerodynamic performance (CL/CD ratio = 13) is obtained. Wingleted blade improves the performance as the intensity of vortices is smaller compared to baseline blade

Rotational characteristics of Savonius wind turbine in high turbulent intensity field

Rotational characteristics of Savonius wind turbine in high turbulent intensity field

Bipolar Precharger for Hybrid Electrostatic Filtration Systems

ABSTRACT Both hybrid electrostatic filtration and agglomeration technologies can effectively improve the efficiency of fine particle collection. A hybrid electrostatic filtration system with a wire-tube bipolar precharger was developed on the basis of bipolar transverse plate electrostatic precipitation technology. Theoretical analyses of electric field and flow field distribution showed that a uniform electric field and high turbulence intensity improve ion migration and particle agglomeration in the novel wire-tube bipolar precharger. The influence of unipolar and bipolar prechargers on dust removal was investigated. In the hybrid electrostatic filtration system with the bipolar precharger, the corona current was 30% higher, the particles deposited on the surface of the filter bag were 19% larger, and the pressure drop was 30% smaller than a filtration system with a unipolar precharger. When silicon powder with a median diameter of 1.7 µm was used, the mean penetration of the experimental device with the bipolar precharger was 45% lower than that of the experimental device with the unipolar precharger.

Numerical study of turbulent flow past a rotating axial-flow pump based on a level-set immersed boundary method

Large-eddy simulation is performed to study the turbulence statistics and flow structures of the water past a rotating axial-flow pump under different flow-rate working conditions. A novel sharp-interface level-set based immersed boundary method is applied to capture the complex geometry of the pump. An unstructured triangular mesh is used to discretize the complex surface geometry of the pump, and a ray-tracing method is employed to classify the computational domain into fluid and solid regions. Turbulence statistics, including the mean velocity, turbulent kinetic energy (TKE), turbulence production, and turbulence dissipation, are analyzed under five different flow-rate working conditions around the designed condition. The results show that unsteady wake, tip leakage flow, and flow separation are accompanied by a high TKE magnitude. For the high turbulence intensity under off-designed working conditions, the tip leakage flow plays a leading role at low flow-rates, and flow separation dominates at high flow-rates.

Intermittency, Moments, and Friction Coefficient during the Subcritical Transition of Channel Flow.

The intermittent distribution of localized turbulent structures is a key feature of the subcritical transitions in channel flows, which are studied in this paper with a wind channel and theoretical modeling. Entrance disturbances are introduced by small beads, and localized turbulent patches can be triggered at low Reynolds numbers (Re). High turbulence intensity represents strong ability of perturbation spread, and a maximum turbulence intensity is found for every test case as Re ≥ 950, where the turbulence fraction increases abruptly with Re. Skewness can reflect the velocity defects of localized turbulent patches and is revealed to become negative when Re is as low as about 660. It is shown that the third-order moments of the midplane streamwise velocities have minima, while the corresponding forth-order moments have maxima during the transition. These kinematic extremes and different variation scenarios of the friction coefficient during the transition are explained with an intermittent structure model, where the robust localized turbulent structure is simplified as a turbulence unit, a structure whose statistical properties are only weak functions of the Reynolds number.

Evaluation of mass and photon transfer enhancement by an impinging jet atomization photoreactor for photocatalytic degradation of p-nitrophenol

In this study, the photocatalytic degradation of p-nitrophenol (PNP) was investigated in a slurry impinging jet atomization reactor and TiO2 nanoparticles have been applied as the photo-catalyst. Formation of the thin leaf-like liquid sheet with high mixing and turbulence intensity due to the collision of two jets, brings a perfect mass transfer. Moreover, the low thickness of the sheet will enhance photon transfer which is an integral factor in UV/TiO2 processes. The impact of different parameters consisting of initial PNP concentration (30−70 mg/L), catalyst loading (0.2−0.8 gr/L) and Reynolds number (15000–31000) on the PNP conversion were investigated in the operating time of 180 min. Based on the results, the optimum experimental condition for the PNP initial concentration of 50 mg/L was 0.5 g/L catalyst loading and Reynolds number of 31,000 with the removal percentage of 55.78 % during 180 min that has been enhanced to 71.91 % by increasing the operation time to 360 min. DLS**Dynamic light scattering analysis has also been conducted for investigation of TiO2 loading effect on particles agglomeration. A pseudo-first-order kinetic model, which is the modified form of Langmuir-Hinshelwood kinetic model, was applied for prediction of p-nitrophenol photocatalytic degradation. A good compatibility between the predicted and experimental data has been observed.

The relation between flame surface area and turbulent burning velocity in statistically planar turbulent stratified flames

The relation between turbulent burning velocity and flame surface area for turbulent stratified flames has been analyzed using a direct numerical simulation database. The simulations have been carried out for different initial turbulence intensities and initial integral length scales of equivalence ratio fluctuation for a given root-mean-square value in a globally stoichiometric mixture. Additionally, statistically similar turbulent premixed flames have been considered for comparison. The turbulent burning velocity and flame surface area for stratified flames are found to be significantly smaller than in premixed flames for slow mixing cases (large scalar integral length scale and low turbulence intensity), although this trend weakens in fast mixing cases (low scalar integral length scale and high turbulence intensity). In slow mixing cases, the increase in the burning rate occurs at a smaller proportion than the increase in the flame surface area. The contributions of the components arising from tangential diffusion of displacement speed and cross-scalar dissipation rate to the turbulent burning velocity are found to be negligible in comparison to that arising from the combined reaction and normal diffusion component for all cases. The increased probability of finding fuel-lean and fuel-rich mixture affects the contribution of turbulent burning velocity arising from the combined reaction and normal diffusion component of displacement speed for slow mixing cases. Front-supported flame elements dominate over back-supported flame elements for large scalar length scale cases, but this behavior reverses for small scalar length scale cases. These findings suggest that increases in the burning rate and flame surface area do not take place in the same proportions for turbulent stratified combustion.

Modelling the Wake Flow Behind a Model of Horizontal Axis Wind Turbine using RANS Approach: A Comparative Study

Steady CFD simulations, which is based on Reynolds Average Navier-Stokes (RANS) approach, were conducted for a model of Horizontal Axis Wind Turbine (HAWT) and compared with experimental data in order to assess the accuracy of these models in simulating the wake flow of HAWTs. The models can be categorized into two categories: fully developed models (Standard k-e model, Realizable k-e model, and SST k-ω model) and transitional models (K-kl-ω transition model and Transition SST Model). In this paper, ANSYS FLUENT 19.2 was being used to execute the simulations of the model turbine standing on a closed-loop wind tunnel at Norwegian University of Sciences and Technology (NTNU). Experiments were operating on a high turbulence intensity uniform inflow. Velocity, turbulence intensity, and turbulent kinetic energy profiles are illustrated at two downstream cross sections. Furthermore, contours of these parameters are set to investigate the developing of wake flow behind the turbine. It is demonstrated that models reasonably predict the velocity profile at the wake region. Transitional models are more accurate in predicting the power of the turbine. Turbulent kinetic energy and turbulence intensity were underestimated for all models.

Cold flow characteristics of a novel high-hydrogen Micromix model burner based on multiple confluent turbulent round jets

As a promising commercial hydrogen-rich gas turbine combustion technology, micro-mixing combustion has been characterized for its excellent performance with low NOx emissions. New flame stabilization mechanism of micro-mixing flames may produce new design criteria. In order to explore that, cold flow characteristics of a novel Micromix model burner based on multiple confluent round jets has been studied experimentally and numerically, which is considered to be the basis for the exploration. A three-dimensional laser Doppler velocimetry system (3D-LDV) was used to measure the flow field of the model burner. It was found that the cold flow characteristics of the burner were different from the twin plane jets, the twin round jets, and the low Reynolds number confluent round jets. Compared to which, the interior of the micro-mixing nozzle is at a very high turbulence intensity level, and the jets merging point of the burner moved upstream; however, the position of the combined point of the burner was close to the confluent round jets. There is no recirculation region between jets near the burner outlet when the nozzle spacing was equal to 3 times the nozzle diameter and the Reynolds number was less than 16,702. The steady computational Reynolds averaged equations (RANS) model results were used to compare with the experimental results. It was found that the RANS results can match the experimental results well, and the three RANS models predict the spatial mixing deficiency less than 1% at the outlet, indicating that the fuel and air were almost completely premixed uniformly.

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.

A direct numerical simulation of Jet A flame kernel quenching

Abstract The safe operation of aeronautical engines requires an understanding of flame ignition, propagation and extinction. In this study, direct numerical simulations are performed using a 29 species reduced chemical mechanism for jet fuel surrogate Jet A to understand the flame quenching process. Initially laminar spherical flames of varying sizes and equivalence ratios are subject to an identical periodic domain of decaying and isotropic high intensity turbulence with a turbulent Reynolds number of 2400. All cases become quenched, except for the larger kernel with lower Karlovitz number. An analysis of the flame structure shows broadened preheat zone, flame shortening on the product side, differential species diffusion and partial fuel pyrolysis in the fresh mixture. Two extinction mechanisms are identified arising from flame shortening and high flame stretch. Flame shortening occurs due to turbulence-chemistry interactions that resemble the flame–flame interaction in a laminar counterflow reactant-to-reactant configuration, which contorts and breaks up the ignition kernel. Flame stretch is a local effect that attenuates the heat release rate and causes the flame to retreat towards the product mixtures, similar to what has been observed for reactant-to-product laminar counterflow flames. Chemical explosive mode analysis was also performed to quantify the flame structure and local combustion mode. The diffusion–reaction balance in pinched-off flame islands favors extinction of these smaller structures, while auto-ignition modes are observed within the flame kernel after fresh mixture is engulfed and preheated in the product kernel. Statistics of the density-weighted displacement speed conditional on local combustion mode indicates strong correlation between the local extinction mode and negative displacement speed. The local balance between diffusion and reaction ultimately determines the propensity for local extinction in both laminar and turbulent flames, the extent of which has an impact on global flame propagation.

Aerodynamic Optimization over Frigate Helicopter Flight Deck by Hangar Shape Modifications

Helicopter operations on frigates imply complex maneuvers for pilots. During the aircraft approach to the frigate, the helicopter rotor is immersed in a changing aerodynamic flow, with high speed and turbulence intensity gradients. This occurs as a consequence of the nonaerodynamic surfaces that compose the frigates and generate detached flow on their decks. Typically, the helicopter operations on frigates are located at the stern. That is, after the superstructure of the ship and just behind the hangar where the helicopter can be hosted. Using a simplified frigate shape model, tests have been carried out at the National Institute of Aerospace Technology’s low-speed wind tunnel by modifying the hangar geometry in a bubble-shaped way to optimize it aerodynamically and improve the flow over the frigate aftdeck. Roof and side walls have been modified by linear , circular , and elliptical geometries. A particle image velocimetry (PIV) technique has been used for obtaining the flow velocity field above and behind the hangars proposed to evaluate the effectiveness of each geometry modification proposed. Finally, a numerical comparison with the PIV results and parameters such as the storage capacity reduction of the hangar due to its geometry change has been carried out.

Characteristics of self-oscillating jets in a confined cavity

Jets emanating into a confined cavity exhibit self-oscillating behavior. This study is focused on evaluating characteristics of oscillating square and round jets. The jet exits from a submerged square or round nozzle of the same hydraulic diameter into a thin rectangular cavity at a Reynolds number of 54 000 based on the nozzle hydraulic diameter and average jet exit velocity. An investigation of the three-dimensional self-oscillatory flow structures is conducted using the unsteady Reynolds-averaged Navier–Stokes equations with the Reynolds stress turbulence model. Vortex identification using the λ2-criterion is used to investigate the flow dynamics. For the oscillating square jet, vortex rings initially have a square shape near the nozzle exit, before axis-switching and transforming into a circular ring. Upon impact on the walls, two tornado-like vortices are produced. The decay rate of oscillating square and round jets initially shows a trend traditionally noted in the corresponding free jets but changes significantly with distance from the nozzle as the effects of oscillation and confinement begin to dominate. Reynolds stress profiles for both types of jets are qualitatively similar and show two peaks on either side of the centerline, which convert to mild peaks farther downstream. Spread and decay rates of oscillating square jets are higher, while oscillating round jets have higher turbulence intensities near the jet center. Compared to free jets, more uniform Reynolds stresses at farther distances from the jet centerline in oscillating jets will enhance heat transfer over a larger area, making oscillating jets suitable in many cooling applications.

Applicability of Two-Parameter Turbulence Models to Simulation of the Interaction of Near-Wall Flows with Blowing and Suction on Permeable Surfaces

The applicability of two-parameter turbulence models to investigation of the impact of free-stream high-intensity turbulence on the characteristics of near-wall flow and heat- and mass-transfer in the steady-state and unsteady boundary layers under the conditions of blowing and suction through permeable surface sections is analyzed. The calculated results obtained in specifying a constant relative mass flow rate parameter in the cases of blowing and suction through permeable sections are compared with the experimental data. The mechanisms of the interaction of blowing and suction of a gas of homogeneous constitution with the flow on two permeable sections upstream and downstream of these sections in the near-wall zone of turbulent boundary layer are studied on the basis of numerical solutions. The calculated results obtained in specifying a time-dependent flow-rate parameter on two permeable surface sections are given.

On the validity of Damköhler's second hypothesis in statistically planar turbulent premixed flames in the thin reaction zones regime

The validity of Damköhler's second hypothesis, which was originally proposed for premixed combustion for conditions where the large scale turbulent timescale remains smaller than the chemical timescale and the integral length scale remains smaller than the flame thickness, has been assessed for a range of Damköhler and Karlovitz numbers (i.e. 0.2≤Da≤3.0 and 0.58≤Ka≤33.34) using a three-dimensional DNS database of statistically planar turbulent premixed flames subjected to forced unburned gas turbulence. It has been found that Damköhler's first hypothesis remains valid for all cases considered in this work irrespective of the combustion regime. By contrast, Damköhler's second hypothesis does not hold in the strict sense for the thin reaction zones regime flames with small values of Damköhler number. However, the ratio of the turbulent flame speed normalised by laminar burning velocity and the square root of the ratio of turbulent diffusivity to mass diffusivity converges to a value of the order of unity for the flames with high turbulence intensity and low Damköhler number in the thin reaction zones regime. Under this condition, Damköhler's second hypothesis, in an order of magnitude sense, can be considered to be valid alongside Damköhler's first hypothesis. A scaling analysis has been carried out using the equilibrium of the tangential strain rate and curvature terms in the Flame Surface Density transport equation to demonstrate that Damköhler's second hypothesis can be expected to be valid only in an order of magnitude sense for the thin reaction zones regime flames with small values of Damköhler number.

Characterization of anisotropic turbulence behavior in pulsatile blood flow

Turbulent-like hemodynamics with prominent cycle-to-cycle flow variations have received increased attention as a potential stimulus for cardiovascular diseases. These turbulent conditions are typically evaluated in a statistical sense from single scalars extracted from ensemble-averaged tensors (such as the Reynolds stress tensor), limiting the amount of information that can be used for physical interpretations and quality assessments of numerical models. In this study, barycentric anisotropy invariant mapping was used to demonstrate an efficient and comprehensive approach to characterize turbulence-related tensor fields in patient-specific cardiovascular flows, obtained from scale-resolving large eddy simulations. These techniques were also used to analyze some common modeling compromises as well as MRI turbulence measurements through an idealized constriction. The proposed method found explicit sites of elevated turbulence anisotropy, including a broad but time-varying spectrum of characteristics over the flow deceleration phase, which was different for both the steady inflow and Reynolds-averaged Navier–Stokes modeling assumptions. Qualitatively, the MRI results showed overall expected post-stenotic turbulence characteristics, however, also with apparent regions of unrealizable or conceivably physically unrealistic conditions, including the highest turbulence intensity ranges. These findings suggest that more detailed studies of MRI-measured turbulence fields are needed, which hopefully can be assisted by more comprehensive evaluation tools such as the once described herein.

Turbulent flame propagation limits of ammonia/methane/air premixed mixture in a constant volume vessel

Ammonia is one of promising energy carriers that can be directly used as carbon-neutral fuel for combustion applications. However, because of the low-burning velocity of ammonia, it is challenging to introduce ammonia to practical combustors those are designed for general hydrocarbon fuels. One of ways to enhance the combustibility of ammonia is by mixing it with other hydrocarbon fuels, such as methane, with a burning velocity is much higher than the burning velocity of ammonia. In this study, we conducted flame propagation experiments of ammonia/methane/air using a fan-stirred constant volume vessel to clarify the effect of methane addition to ammonia on the turbulent flame propagation limit. From experimental results, we constructed the flame propagation maps and clarified the flame propagation limits. The results show that the flame propagation limits were extended with an increase in mixing a fraction of methane to ammonia. Additionally, ammonia/methane/air mixtures with the equivalence ration of 0.9 can propagate at the highest turbulent intensity, even though the peak of the laminar burning velocity is the fuel-rich side because of the diffusional-thermal instability of the flame surface. Furthermore, the Markstein number of the mixture obtained in this research successfully expressed the strength of the diffusional-thermal instability effect on the flame propagation capability. The turbulence Karlovitz number at the flame propagation limit monotonically increases with the decreasing Markstein number.

Study of an effusion-cooled plate with high level of upstream fluctuation

The flow field and surface adiabatic coolant-film effectiveness (ACE) distribution of a combustor representative effusion cooling array with cylindrical cooling holes has been studied both experimentally and numerically. Both studies focus on the influence of inflow turbulence, especially the high inflow turbulence which is always present in the combustor environment but rarely studied in the literature. A fluctuating inflow at roughly 20% intensity level is generated in the wind tunnel, and distributions of ACE measured for blowing ratios (BR) between 1.8 and 4.1. For comparison, ACE distributions are also measured at a low inflow turbulence intensity of 5%. For further investigation on the mechanism of inflow turbulence effects, hybrid large eddy simulations (LES) are carried out at a BR of around 1.8 under both low and high inflow turbulence intensities. The fluctuating inflow is generated using the Synthetic Eddy Method (SEM) with similar turbulence intensity. The predicted surface ACE distributions of the 2 cases are compared with the measurements. More detailed studies of the flow field are carried out based on the numerical results. The effects of inflow fluctuation levels are studied by comparing various flow statistics between the low and high fluctuation cases. The formation of the coolant film is also studied based on the development of the coolant film thickness. The interaction between the upstream and downstream coolant jets is investigated by visualising the coolant jet centre trajectory, as well as analysing the turbulence structures, spectra and coherence at selected positions. These analyses clearly show that the highly fluctuating inflow results in an enhanced mixing of the coolant and mainstream. In the high turbulence intensity case, this leads to wider span-wise and shorter stream-wise film coverage over the first few rows of the array. These effects diminish as soon as a thick coolant film is formed in the downstream, especially at high BR conditions.

Turbulent flame-vortex dynamics of bluff-body premixed flames

Abstract To better understand the coupled dynamics of turbulence and premixed flames, this study experimentally investigates the effects of turbulence on reacting vorticity dynamics. In this investigation, a bluff-body stabilized, turbulent, premixed flame is analyzed in a high-speed combustor. A custom-designed turbulence generator is used to vary the inflow turbulence conditions and understand the subsequent impacts on the flame-flow field. The turbulent reacting flow-field is measured using simultaneous high-speed particle imaging velocimetry (PIV) and CH* chemiluminescence. A Lagrangian tracking method is implemented to study flame-vortex dynamics. Lagrangian fluid elements are tagged from the experimental data and are tracked as they convect through the flame, from reactants to products. The components of the vorticity transport equation are decomposed along the Lagrangian trajectories to determine their relative balance under various turbulence conditions. The results confirm that altering the turbulence intensity influences flame-vortex interactions and the relative balance of the vorticity components. At high turbulence intensities, the baroclinic torque and dilatation decrease, while the magnitude of viscous diffusion increases, and vortex stretching dominates the reacting flow field. These results suggest that increasing the turbulence intensity alters the dominant physical processes in the reacting domain as the combustion-driven phenomena become less significant.