Range Of Turbulence Intensities Research Articles

Experimental investigation of preferential concentration in zooplankton swimming in turbulence.

Turbulence can cause particles to accumulate within specific regions of the flow. One mechanism responsible for this phenomenon, called preferential concentration, consists in particle-fluid interactions yielding inhomogeneous spatial distribution of particles into clusters or depleted regions due to density difference or finite-size effects. In the case of living particles such as plankton, clustering may also originate from their motility or from their behavioral response to turbulent forcing. Preferential concentration of plankton has attracted much attention, because it is a key determinant of encounter rates and therefore relevant for a wide range of ecological processes. However, most studies have focused on microscopic cells, and consequently the case of larger organisms remains poorly studied. Here, we use high-performance particle tracking and three-dimensional Voronoï analysis to test for the emergence of clustering in the spatial distribution of calanoid copepods, the most important metazoans in the oceans in terms of biomass. We found that neither inertia nor motility resulted in significant departure from a random Poisson process over a range of turbulence intensity from very strong to moderate. However, we observed weak clustering in calm water, which may originate from hydrodynamic and olfactory interactions between organisms. Our results improve our understanding of fluid-particle interactions in the zooplankton and have important implications for the modeling of their encounter rates in turbulence.

Modeling the displacement speed in the flame surface density method for turbulent premixed flames at high pressures

We propose a model of the local displacement speed for the large-eddy simulation (LES) of turbulent premixed combustion with the flame surface density (FSD) method. This model accounts for flame stretch and curvature effects in order to improve the prediction of LES–FSD for turbulent premixed flames with the non-unity Lewis number at high pressures. The proposed model is validated by a priori and a posteriori tests, with the direct numerical simulation (DNS) and LES–FSD of statistically planar turbulent premixed flames of lean hydrogen at 1 and 10 atm and a range of turbulence intensities. The a priori test confirms that the proposed model accurately estimates the negative correlation of the displacement speed and the flame curvature. In the a posteriori test, the comparison of DNS and LES–FSD results demonstrates that the proposed model improves the prediction on the turbulent burning velocity and the turbulent flame thickness, compared with those from the existing displacement–speed model. The stretch effect in the present model leads to the rise of the turbulent burning velocity and the suppression of the “bending” phenomenon in LES–FSD, consistent with the DNS result.

Life and death of inertial particle clusters in turbulence

Abstract

On the Influence of High Turbulence on the Convective Heat Flux on the High-Pressure Turbine Vane LS89

High-pressure turbine vanes and blades are subjected to a turbulent combustor flow affecting the heat transfer and boundary layer transition, hence, the temperature distribution. The accurate prediction of the temperature distribution is crucial for a reliable design and cooling implementation. Engine-representative measurements are hence mandatory for improving design tools. Recently, convective heat transfer measurements were conducted on a high-pressure turbine inlet guide vane (VKI LS89 airfoil) in the Isentropic Compression Tube (CT-2) facility at the von Karman Institute. This contribution focuses on the effect of high freestream turbulence generated by a new turbulence grid allowing a range of turbulence intensities in excess of 10% with representative length scales of the order of 1–2 cm. Three cases with varying turbulence levels are discussed in this paper. The different flow conditions are exit isentropic Mach numbers of 0.70–0.97, Reynolds numbers of 0.53∙× 106 and 1.15∙× 106 and a constant temperature ratio equal to 1.36. The heat transfer distributions along the vane suction side indicate a clear link between boundary layer transition and the stream-wise pressure gradients even at high levels of freestream turbulence intensity. Differences are put in evidence in the dynamics of the transition development. Future developments will focus also on the contribution of the other flow parameters under high turbulence. Heat transfer predictions from the boundary layer code TEXSTAN and Reynolds-Averaged Navier–Stokes code elsA (ensemble logiciel pour la simulation en Aérodynamique) are additionally compared to the experiments. Inherent difficulties associated with high turbulence modelling are clear from this early numerical work.

Turbulent combustion evolution of stoichiometric H2/CH4/air mixtures within a spherical space

Turbulent combustion evolutions of stoichiometric H2/CH4/air mixtures were experimentally studied within a spherical constant-volume combustion vessel. A series of initial turbulent ambience (with the range of turbulence intensity from 0 to 1.309 m/s) and a series of hydrogen volumetric fraction (with the range from 0.3 to 0.9) were taken as the variables to studied the influences of turbulence intensity and the fuel composition on the turbulent combustion evolutions. The evolutions of explosion overpressure were studied upon the variations of maximal pressure, the influences of turbulence intensity mainly located at heat loss while the influences of fuel composition mainly located at adiabatic explosion. Subsequently, the evolutions of burnt mass were discussed, the competition between pressure rising and temperature rising induced by the heat release during combustion was considered as major influence mechanism. Then, the nexus between burning velocity and the related burnt mass rate were discussed, the variations regulations of maximal burning velocity brought by turbulence intensity and hydrogen volumetric fraction were analysed. Finally, the nexus between maximum burning velocity and heat loss was discussed.

Turbulence exerts nutrients uptake and assimilation of bloom-forming Dolichospermum through modulating morphological traits: Field and chemostat culture studies

Hydrodynamic conditions are closely related to the development and dissipation of cyanobacterial blooms. The morphological features of Dolichospermum under different hydrodynamic conditions were analysed during three blooms in Gaoyang Lake, which is part of the backwater area of the China Three Gorges Reservoir, from 2007 to 2010. The results showed that the length of filaments and the morphology of cells were different in relation to the turbulence caused by the difference in hydraulic retention times. Thus, it was hypothesized that turbulence could shape the morphology and physiology of cyanobacteria. To answer the question regarding what the morphological and physiological responses of cyanobacteria to turbulent mixing mean for these organisms, laboratory experiments in continuous cultivation under different dilution rates were conducted to analyse the effects of specific turbulence intensity on the growth, nutrient uptake and morphology of Dolichospermum flos-aquae. Increasing the turbulence intensity caused synchronous increases in the ratio of the cellular length to the width, in the specific surface area of the filament and the cell and in the nutrient uptake rate; at the same time, the average filament length decreased. These indicated that the turbulence, within the range of our experimental design, could stimulate the growth of Dolichospermum by increasing its nutrient uptake. Additionally, at a high specific growth rate, the nutrient uptake rate of Dolichospermum changed more noticeably with the increasing morphological indicators, indicating that the rapidly growing Dolichospermum was more sensitive to turbulence. These findings explain the role of morphological strategies in the dominance of Dolichospermum within a certain range of turbulence intensity, especially in the early growth stage of blooms. The results also facilitate a greater understanding of the hydrodynamic effects on cyanobacteria and will be instrumental in developing flow regulation to control cyanobacterial blooms in reservoirs.

Stretch Rate and Displacement Speed Correlations for Increasingly-Turbulent Premixed Flames

Probability density functions of the components of stretch rate are investigated using a previously-published Direct Numerical Simulation dataset spanning a range of turbulence intensities in the Thin Reaction Zones (TRZ) regime. The dataset was generated by varying the turbulence intensity across five different simulations while maintaining fixed the remaining physico-chemical input parameters such as integral length scale and laminar flame thickness and speed. Across the entire dataset, the joint probability density function of stretch rate and displacement speed displays a distinctive shape with two branches consistent with previous studies at low turbulence intensities. This joint probability density function is analysed further by extracting individual contributions of stretch rate components to determine their relative importance across the branches. The curvature dependence of displacement speed appears to play an important role in shaping these branches. Implications of this result with regard to evaluation of the components of stretch rate in the TRZ regime are discussed.

A quasi-steady model to account for the effects of upstream turbulence characteristics on pressure fluctuations on a low-rise building

The effects of upstream turbulence on roof pressure fluctuations of a low-rise building are investigated via the quasi-steady (QS) vector model. Atmospheric boundary layer (ABL) turbulence, with intensities ranging from 13% to 27% and integral length scales from 6 to 13 times the building height, is simulated in a boundary layer wind tunnel. The model building surface pressures are measured synchronously with the velocity at a point one building height above the leading edge. The QS model is found to accurately explain the effects of the ABL turbulence with scales larger than about 5 building heights. Furthermore, a QS model established in one terrain can explain the pressure fluctuations in the other terrains based on the fact that the model functions are similar over the range of observed upstream terrain conditions. This finding is important to the Partial Turbulence Simulation approach since there may be a range of turbulence intensity – integral scale combinations which can yield the same aerodynamic behavior, allowing more flexibility for choosing the model length scales. However, further work is required for modelling the effects of small-scale turbulence on the peak pressures and in defining appropriate bounds when the precise turbulence simulation can be relaxed.

Scalar transport and the validity of Damköhler’s hypotheses for flame propagation in intense turbulence

The turbulent burning velocity of premixed flames is sensitive to the turbulence intensity of the unburned mixture. Premixed flame propagation models that incorporate these effects of turbulence rest on either of the two hypotheses proposed by Damköhler. The first hypothesis applies to low-intensity turbulence that acts mainly to increase the turbulent burning velocity by increasing the flame surface area. The second hypothesis states that, at sufficiently high intensities of turbulence, the turbulent burning velocity is governed mainly by enhanced diffusivity. Most studies to date have examined the validity of the first hypothesis under increasingly high intensities of turbulence. In the present study, the validity of Damköhler’s second hypothesis is investigated. A range of turbulence intensities is addressed by means of direct numerical simulations spanning the “flamelet” and “broken reaction zones” regimes. The validity of Damköhler’s second hypothesis is found to be strongly linked to the behaviour of turbulent transport within the flame.

Measurement and scaling of minimum ignition energy transition for spark ignition in intense isotropic turbulence from 1 to 5 atm

This paper presents high-pressure minimum ignition energies (MIE) and their scaling from measurements on spark discharges in lean methane/air mixtures at the equivalence ratio ϕ = 0.6 by cylindrical electrodes with flat ends in near-isotropic turbulence over a range of turbulent intensities (u′/SL = 0–50), from 1 to 5 atm, using a large dual-chamber, constant-pressure, fan-stirred explosion facility, where SL is the laminar burning velocity. Voltage and current waveforms of spark discharges with nearly square profiles are carefully generated for accurate determination of MIE, commonly defined as the 50% successful ignitability. Applying high-speed schlieren imaging, we observe a drastic change of kernel development from turbulent flamelet to distributed like with island formation and local quench even at 5 atm, when u′/SL is greater than some critical values depending on p. It is found that the scaling slopes of MIET/MIEL versus u′/SL change abruptly from a linear increase to an exponential increase when u′/SL > (u′/SL)c, showing ignition transition. The subscripts T and L represent turbulent and laminar properties, MIEL ≈ 6.84 mJ (1 atm), 2.81 (3 atm), and 2.11 (5 atm), and the transition occurs at (u′/SL)c ≈ 12 (1 atm), 24 (3 atm), and 34 (5 atm). It is also found that the above scattering MIET/MIEL data at different u′/SL and p can be merged together into a single curve when scaled with a pressure-corrected kernel (reaction zone, RZ) Péclet number, Pe* = PeRZ(p/p0)−1/4, showing the first and fourth power dependence before and after MIE transition at a critical Pe* ≈ 3.6. PeRZ = u′ηk/αRZ, ηk is the Kolmogorov length scale of turbulence, αRZ is the thermal diffusivity estimated at the instant of kernel formation, and p0 = 1 atm. These results reveal a self-similar spark ignition phenomenon.

The influence of gas phase velocity fluctuations on primary atomization and droplet deformation

The effects of grid-generated velocity fluctuations on the primary atomization and subsequent droplet deformation of a range of laminar liquid jets are examined using microscopic high-speed backlit imaging of the break-up zone and laser Doppler anemometry of the gas phase separately. This is done for fixed gas mean flow conditions in a miniature wind tunnel experiment utilizing a selection of fuels, turbulence-generating grids and two syringe sizes. The constant mean flow allows for an isolated study of velocity fluctuation effects on primary atomization in a close approximation to homogeneous decaying turbulence. The qualitative morphology of the primary break-up region is examined over a range of turbulence intensities, and spectral analysis is performed in order to ascertain the break-up frequency which, for a case of no grid, compares well with the existing literature. The addition of velocity fluctuations tends to randomize the break-up process. Slightly downstream of the break-up region, image processing is conducted in order to extract a number of metrics, which do not depend on droplet sphericity, and these include droplet aspect ratio and orientation, the latter quantity being somewhat unconventional in spray characterization. A turbulent Weber number \(We^{\prime}\) which takes into account gas phase fluctuations is utilized to characterize the resulting droplet shapes, in addition to a mean Weber number . Above a \(We^{\prime}>0.05\) a clear positive relationship exists between the mean aspect ratio of droplets and the turbulent Weber number where \(We^{\prime}\) is varied by altering all relevant variables including the velocity root mean square, the initial droplet diameter, the surface tension and the density.

Intermittency in premixed turbulent reacting flows

Intermittency in premixed reacting flows is studied using numerical simulations of premixed flames at a range of turbulence intensities. The flames are modeled using a simplified reaction mechanism that represents a stoichiometric H2-air mixture. Intermittency is associated with high probabilities of large fluctuations in flow quantities, and these fluctuations can have substantial effects on the evolution and structure of premixed flames. Intermittency is characterized here using probability density functions (pdfs) and moments of the local enstrophy, pseudo-dissipation rate (strain rate magnitude), and scalar (reactant mass fraction) dissipation rate. Simulations of homogeneous isotropic turbulence with a nonreacting passive scalar are also carried out in order to provide a baseline for analyzing the reacting flow results. In the reacting flow simulations, conditional analyses based on local, instantaneous values of the scalar are used to study variations in the pdfs, moments, and intermittency through the flame. For low intensities, pdfs of the local enstrophy vary substantially through the flame, with greater intermittency near the products. Changes in the pseudo-dissipation pdfs are, however, less pronounced. As the intensity increases, both the enstrophy and pseudo-dissipation pdfs become increasingly independent of position in the flame and are similar to results from the nonreacting simulations. The scalar dissipation intermittency is largest near the reactants and increases at all flame locations with increasing turbulence intensity. For low intensities and in the reaction zone, however, scalar dissipation pdfs approximately follow a Gaussian distribution, indicative of substantially reduced intermittency. Deviations from log-normality are observed in the pdfs of all quantities, even for intensities and flame locations characterized by strong intermittency. The implications of these results for the internal structure of the flame are discussed, and we also propose a connection between reacting flow intermittency and anisotropic vorticity suppression by the flame.

Heat release rate correlation and combustion noise in premixed flames

The sound emission from open turbulent flames is dictated by the two-point spatial correlation of the rate of change of the fluctuating heat release rate. This correlation in premixed flames can be represented well using Gaussian-type functions and unstrained laminar flame thermal thickness can be used to scale the correlation length scale, which is about a quarter of the planar laminar flame thermal thickness. This correlation and its length scale are observed to be less influenced by the fuel type or stoichiometry or turbulence Reynolds and Damkohler numbers. The time scale for fluctuating heat release rate is deduced to be about τc/34 on an average, where τc is the planar laminar flame time scale, using direct numerical simulation (DNS) data. These results and the spatial distribution of mean reaction rate obtained from Reynolds-averaged Navier–Stokes (RANS) calculations of open turbulent premixed flames employing the standard model and an algebraic reaction rate closure, involving a recently developed scalar dissipation rate model, are used to obtain the far-field sound pressure level from open flames. The calculated values agree well with measured values for flames of different stoichiometry and fuel types, having a range of turbulence intensities and heat output. Detailed analyses of RANS results clearly suggest that the noise level from turbulent premixed flames having an extensive and uniform spatial distribution of heat release rate is low.

More on Minimum Ignition Energy Transition for Lean Premixed Turbulent Methane Combustion in Flamelet and Distributed Regimes

At the 31st Combustion Symposium, Shy et al. found a transition on minimum ignition energy (MIE) of methane-air mixtures at the equivalence ratio φ = 0.6 in intense isotropic turbulence, where ignition energies of a spark-electrode was quantitatively measured by an energy-adjustable high-power pulse ignition system. Using the same methodology, this paper presents for the first time two new MIE data sets at φ = 0.7 and 0.8 over a wide range of turbulent intensities. It is found that MIE transition due to different modes of turbulent combustion depends on a turbulent Karlovitz number (Ka) indicating the time ratio between chemical reaction and turbulence, for which MIE first increases gradually with Ka and then increases drastically when Ka > K a c ≈ 4 ∼ 9 depending also on φ. The effect of the electrode gap on ignition energies and turbulence influence to centrally-ignited, outwardly propagating flames are also discussed.

Experimental investigation into the routes to bypass transition and the shear-sheltering phenomenon

The objective of this investigation is to give experimental support to recent direct numerical simulation (DNS) results which demonstrated that in bypass transition the flow first breaks down to turbulence on the low-speed streaks (or so-called negative jets) that are lifted up towards the boundary-layer edge region. In order to do this, wall-normal profiles of the streamwise fluctuation velocity are presented in terms of maximum positive and negative values over a range of turbulence intensities (1.3–6%) and Reynolds numbers for zero pressure gradient flow upstream of, and including, transition onset. For all turbulence intensities considered, it was found that the peak negative fluctuation velocity increased in magnitude above the peak positive fluctuations and their positions relative to the wall shifted as transition onset approached; the peak negative value moved towards the boundary-layer edge and the peak positive value moved toward the wall. An experimental measure of the penetration depth (PD) of free-stream disturbances into the boundary layer has been gained through the use of the skewness function. The penetration depth (measured from the boundary-layer edge) scales as PD ∝ (ω Rexτw)−0.3), where ω is the frequency of the largest eddies in the free stream, Rex is the Reynolds number of the flow based on the streamwise distance from the leading edge and τw is the wall shear stress. The parameter dependence demonstrated by this scaling compares favourably with recent solutions to the Orr–Sommerfeld equation on the penetration depth of disturbances into the boundary layer. The findings illustrate the importance of negative fluctuation velocities in the transition process, giving experimental support to suggestions from recent DNS predictions that the breakdown to turbulence is initiated on the low-speed regions of the flow in the upper portion of the boundary layer. The representation of pre-transitional disturbances in time-averaged form is shown to be inadequate in elucidating which disturbances grow to cause the breakdown to turbulence.

Performance of Flare Flames in a Crosswind With Nitrogen Dilution

Abstract Flares are used for the safe, clean, and economical disposal of waste gases, whether in upstream oil production (solution gas flares), refineries, gas plants, or other chemical processing facilities. Elevated flares are exposed to a range of weather patterns; perhaps the most important being the crosswind. The CETC Flare Test Facility (FTF) was constructed to address the question of performance of solution gas flares, in response to limited field trials that indicated the possibility of very low combustion efficiencies under certain conditions. The FTF produces crosswind speeds up to 45 km/h with very low turbulence intensity, as designed. Model solution gases are produced from natural gas, propane, inert diluents (carbon dioxide and nitrogen), and volatile liquids. Atmospheric wind is a turbulent shear flow with an intensity of around 7﹪. To more closely replicate the turbulence properties of atmospheric wind, a range of turbulence intensities and integral length scales were imposed in the FTF by using grids. The flare flame efficiency is measured by the conversion of carbon in the solution gas to carbon dioxide. Efficiency is lowest for pure natural gas, and increases with the amount of propane. Conversion efficiency decreases significantly with the increase of turbulence intensity of the crosswind, which has implications for existing and novel strategies to improve flare performance. The primary product of incomplete combustion is methane, a significant greenhouse gas. The combined effect of inert diluents and turbulence intensity on flame stability shows that turbulence amplifies the destabilizing effect of fuel dilution. Introduction Flares are used for the safe, clean, and economical disposal of waste gases, whether in upstream oil production (solution gas flares), refineries, gas plants or other chemical processing facilities. Elevated flares are exposed to all the weather patterns; perhaps the most important is the crosswind. The flaring of solution gas (also called associated gas) was a common practice in upstream oil extraction. This gas is a mixture of light hydrocarbons, mostly methane but including possibly significant amounts of ethane, propane, butane, hydrogen, perhaps some inert gases (nitrogen and carbon dioxide), and sometimes hydrogen sulphide (sour gas). The composition and amount of solution gas can vary a great deal between production sites. Over 800 million m3 of gas was flared in Alberta in 2000, which is almost a 50﹪ reduction from 1996(1). The current global flaring rate is estimated between 100 and 126 billion m3 of solution gas each year(2). Strosher(3) conducted laboratory-scale investigations on an enclosed flare and an open-air flare, and on two commercial flares in the field. He found in the field investigations that combustion efficiency is typically about 70%, sometimes as low as 62﹪. The unburned portion consisted of methane, unsaturated hydrocarbons, light aromatics such as benzene, and polycyclic aromatic hydrocarbons (PAH). This work has led to much of the current interest in flare research by industry, government, academia, and the public. An important parameter in the flare flame behaviour is the ratio of the fuel jet momentum flux to the crosswind momentum flux.

Anomalous transport in plasmas

AbstractThe application of stochastic methods to the problem of particle and energy transport in turbulent plasmas is briefly reviewed. The “classical” Corrsin approximation is shown to be valid only in the limit of weak turbulence. The recently developed method of decorrelation trajectories is applicable over the whole range of turbulence intensities and yields the correct asymptotic behavior in the limit of very strong turbulence (subdiffusion). © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Wind Shear and Turbulence Effects on Rotor Fatigue and Loads Control

The effects of wind shear and turbulence on rotor fatigue and loads control are explored for a large horizontal axis wind turbine in variable speed operation at wind speeds from 4 to 20 m/s. Two- and three-blade rigid rotors are considered over a range of wind shear exponents up to 1.25 and a range of turbulence intensities up to 17%. RMS blade root flatwise moments are predicted to be very substantially increased at higher wind shear, and resultant fatigue damage is increased by many orders of magnitude. Smaller but similar trends occur with increasing turbulence levels. In-plane fatigue damage is driven by 1P gravity loads and exacerbated by turbulence level at higher wind speeds. This damage is higher by one to two orders of magnitude at the roots of the three-blade rotor compared with the two-blade rotor. Individual blade pitch control of fluctuating flatwise moments markedly reduces flatwise fatigue damage due to this source, and, to a lesser degree, the in-plane damage due to turbulence. The same is true of fluctuating rotor torque moments driven by turbulence and transmitted to the drive train. Blade root moments out of the plane of rotation aggregate to create rotor pitching and yawing moments transmitted to the turbine structure through the drive train to the yaw drive system and the tower. These moments are predicted to be relatively insensitive to turbulence level and essentially proportional to the wind shear exponent for the two-blade rotor. Fluctuating moments are substantially reduced with individual blade pitch control, and addition of a teeter degree-of-freedom should further contribute to this end. Fluctuating pitching and yawing moments of the three-blade rotor are substantially less sensitive to wind shear, more sensitive to turbulence level, and substantially lower than those for the two-blade rotor. Mean rotor torque and, hence, power are essentially the same for both rotors, independent of wind shear, and are somewhat reduced with individual blade pitch control of fluctuating flatwise moments. The same is true of mean rotor thrust; however fluctuations in rotor thrust are substantially reduced with individual blade pitch control.

Scaling and efficiency of prism in adaptive simulations of turbulent premixed flames

The dominant computational cost in modeling turbulent combustion phenomena numerically with high-fidelity chemical mechanisms is the time required to solve the ordinary differential equations associated with chemical kinetics. One technique applied toward reducing computational cost develops an inexpensive surrogate model that accurately represents the evolution of chemical kinetics. One such approach, PRISM, constructs a polynomial representation of the chemistry evolution in a local region of chemical composition space, to be stored for later use. Within this region the representation provides a good approximation to the kinetics. As the computation proceeds, the chemistry evolution for other points within the same region are computed by evaluating the polynomials instead of calling an ordinary differential equation solver. If chemistry evolution is required for a region for which a polynomial has not been defined, the methodology dynamically samples that region and constructs a new representation for it. In this paper we assess the PRISM methodology in the context of a turbulent premixed flame in two diniensions. The practicality is influenced by the size and number of regions necessary to model the subset of composition space that is active during a simulation. The number required grows with decreasing size in a manner that scales exponentially with the dimensionality of active composition space. We considered a range of turbulent intensities, ranging from weak turbulence that had little effect on the flame to strong turbulence that tore pockets of burning fluid from the main flame: for each case we explored the scaling behavior as a function of region size and turbulent intensity.

Turbulence effects on surface pressures of rectangular cylinders

This paper presents the effects of free-stream turbulence on streamwise surface pressure fluctuations on two-dimensional rectangular cylinders. Particular attention is given to possible effects of turbulence integral scale on fluctuation and peak pressures. The mean, standard deviation, peak pressure coefficients, spectra and cross-correlation of fluctuating pressures were measured to investigate the nature of the separation and reattachment phenomenon in turbulent flows over a wide range of turbulence intensity and integral scale.