Turbulent Mass Research Articles

Large‐eddy simulation and low‐order modeling of sediment‐oxygen uptake in a transitional oscillatory flow

We have tested a dissolved oxygen (DO) transport model based on large‐eddy simulation (LES) of a transitional oscillatory flow observed in the bottom boundary layer of Lake Alpnach, Switzerland. The transition from a quasi‐laminar to a fully turbulent state makes this flow difficult to study with a Reynolds‐averaged Navier‐Stokes equation (RANSE) model. By resolving the full range of governing transport processes, LES provides a reliable prediction of the sediment‐oxygen uptake (SOU). The model biogeochemical and flow parameters have been calibrated against DO and velocity measurements from published in situ data at the earliest phase available in the cycle. The fully developed flow thus obtained is used as an initial condition for the imposed oscillatory forcing. Numerical predictions show that transport in the outer layer is in equilibrium with the main current throughout most of the cycle and that nonequilibrium effects are limited to the diffusive sublayer response to the external forcing. During flow deceleration, the concentration boundary layer slowly expands as turbulence decays; later, during re‐transition, mixing is restored by rapid and intense turbulent production events enhancing the SOU with a well‐defined time lag. An algebraic model for the SOU is proposed for eventual inclusion in RANSE biogeochemical management‐type models developed based on parameterizations used in turbulent mass transfer and with the support of published numerical data and the present simulation. The only input parameters required are the sediment oxidation rate, bulk temperature and DO concentration, and friction velocity.

Turbulent mass transfer of Al2O3 and TiO2 electrolyte nanofluids in circular tube

Experimental study was performed to investigate turbulent mass transfer in straight circular tube. Electrochemical limiting diffusion current technique was used to measure the mass transfer coefficient in fully developed hydrodynamics and under developed mass transfer region. TiO2 and γ-Al2O3 nanoparticles were added into the electrolyte solution (ES) to make electrolyte nanofluids (ENF). Measurements revealed that enhancement in mass transfer reaches 10 % in a 0.01 vol% γ-Al2O3/electrolyte nanofluid while 18 % in a 0.015 vol% TiO2/electrolyte nanofluid relative to the base ES. Mass transfer coefficients increased with nanoparticles concentration up to an optimum concentration (0.01 % in γ-Al2O3/electrolyte nanofluid and 0.015 % in TiO2/electrolyte nanofluid) while decreased by increasing nanoparticles concentration further. Enhancement ratio which is the ratio of the mass transfer coefficient of nanofluid to that of the base fluid was a function of nanoparticle concentration and was independent of Reynolds number. The mechanisms of nanoparticles Brownian motion and nanoparticles clustering were used to describe the behavior of the enhancement ratio in ENF.

Numerical up-scaling approach for the simulation of heat-transfer in randomly packed beds

The objectives of this paper is to report the development and validation of a computational fluid dynamics (CFD) modeling approach to predict heat transfers in arbitrary randomly packed tubular reactors. The capacity of the packed beds to supply or remove heat from the core to the tube walls is indeed critical for the performance of reactors involved in various chemical applications of catalytic reactions. The understanding of the physical mechanisms and the modeling of turbulent heat and mass transfer is thus essential for the design, optimization and scale-up of fixed-bed reactors.For this objective, the heterogeneous catalyst bed can be represented by a macroscopic or homogeneous one where turbulence does exist at the small scales. In this study, an up-scaling approach is considered where the volume averaging procedure is used to derive the macroscopic equations. The flow and turbulent variables are decomposed according to the double decomposition concept within the framework of the volume averaging method. In particular, the dispersive kinetic energy is considered for the modeling of the thermal dispersion term appearing in the macroscopic temperature equation with a k–ε approach. The model is extended for the prediction of the channeling effects near the tube walls. The closure approximations are validated with microscopic simulations performed at the scale of representative elementary volume and comparisons with macroscopic simulations. Several packed bed configurations are considered, and the model predictions for the core heat transfer and the wall heat transfer are also compared to usual correlations when available.The microscopic simulations performed for various particle shapes are also confirming the predominant contribution of the mechanical dispersion to the convective heat transfer.This study is demonstrating the feasibility to predict the global heat transfer inside arbitrary tubular reactors with simple macroscopic simulations as an alternative to complex and computationally expensive microscopic simulations, out of the range of usual engineering tasks.

A rapid and reliable method for the quantitation of hydroxychloroquine in serum using turbulent flow liquid chromatography-tandem mass spectrometry

BackgroundHydroxychloroquine is routinely used in managing systemic lupus erythematosus and rheumatoid arthritis. Whole blood levels are currently measured in the laboratory but at least one study suggests that serum levels may be equally useful. Moreover, serum samples are the preferred matrix type in the clinical laboratory as a result of their reduced complexity compared to whole blood. These observations suggest that the clinical utility of serum hydroxychloroquine levels needs to be reevaluated using larger studies and more robust assays. We report a turbulent flow LC-MS/MS method we developed for this purpose. MethodsAfter protein precipitation from serum with 0.33mol/l perchloric acid, hydroxychloroquine and its deuterated analog were injected onto a Cyclone turbulent flow column for sample cleanup. Analytical separation was accomplished on a HypersilGold C8 column with a gradient of water and methanol, each containing 0.1% formic acid and 10mmol/l ammonium formate. Analytes were ionized and detected by electrospray ionization mass spectrometry with multiple reaction monitoring. ResultsOur method was linear from 15.7 to 2000ng/ml. Total imprecision at multiple levels was <5% and accuracy was within ±15%. The method showed minimal carryover. Our extraction efficiency was 103% and the matrix factor was 101%. Comparison with a reference laboratory method identified constant bias but good correlation between the 2 methods. ConclusionsWe present a novel turbulent flow liquid chromatography-tandem mass spectrometry method for quantification of hydroxychloroquine in serum. Our method has comparable sensitivity, selectivity, precision, accuracy, and linearity to previously reported methods. However, it offers simpler sample processing, shorter overall analysis time, and minimal carryover. These characteristics make our method well-suited for efficient analysis of the large number of samples necessary for studies on the clinical utility of serum HQ levels.

Turbulent flame and mass burning rate in a spark ignition engine

Turbulent burning rates were measured in an spark-ignition engine with extensive optical access for a fairly wide range of operating conditions. Skip-fired operation was employed to eliminate the effects caused by trapped or recirculated exhaust gases. Three main stages of turbulent flame propagation in SI engines were discerned: an initial acceleration, propagation with approximately constant speed and final deceleration caused by the proximity of walls. The rate of the initial flame acceleration showed little sensitivity to the engine operating parameters if the burning rate was expressed in terms of dimensionless flame radius. Propagation at a constant burning rate occurred over most of the charge volume but only for a minor fraction of its mass; the burning rate in this regime was proportional to the turbulence root-mean-square velocity while the influence of all other relevant factors could be encapsulated in either Karlovitz stretch factor or Damkohler number. The final stage of combustion, where the burnt gas radius was within one integral length scale from the walls, was responsible for more than two-thirds of the charge mass consumption and occurred at decreasing burning rate and flame brush thickness.

Convective mass transfer and pressure drop correlations for cross-flow structured hollow fiber membrane bundles under low Reynolds numbers but with turbulent flow behaviors

Cross-flow structured hollow fiber membrane bundles where the fibers are arranged either in inline or in staggered arrays are investigated for their bundle side convective mass transfer and pressure drop characteristics. A high speed hot wire anemometry is used to validate the turbulent flow behavior in the bundle at low Reynolds numbers. A mathematical model for the turbulent fluid flow and convective mass transfer across the bundle under uniform mass boundary conditions is set up, which is then validated by an air humidification test. With the validated model, the Sherwood numbers and friction factors across the bundle are obtained for different fiber arrangements, pitch to diameter ratios, and Reynolds numbers. Correlations are then proposed to estimate the friction factors and Sherwood numbers across the bundles with various parameters. It is found that compared to the previous researches and the available correlations, the present results are more appropriate for the cross flow hollow fiber membrane bundles that are usually operated under low Reynolds numbers from 100 to 500, but have turbulent flow behaviors.

Large eddy simulation of turbulent flow and mass transfer in far-field of swirl diffusers

Herein, the turbulent flow structures, mass transfer and ventilations characteristics in the far-field of swirl diffusers are numerically studied at different swirl angles (45°≤α≤65°). The results are obtained via a 2nd order finite difference scheme and large eddy simulation (LES) with the dynamic Smagorinsky procedure in sub-grid-scale modeling. Accordingly, the instantaneous and average velocity distributions, transient evolution of the mass transfer, and the overall and local air qualities are investigated in this paper. Also, three main secondary-flow structures are observed in the simulations: a bubble vortex breakdown (α≤45°), a toroidal recirculation zone on the top corners of the room (α≤50°), and a toroidal central recirculation zone attached to the floor (far-field). However, the jet type alternates from the open swirl to Coanda jet in the range 50°≤α≤55°, where both types exhibit enhanced ventilation characteristics. In this range, the open swirl jet with the highest swirl angle is more effective in discharging the aged air and also shows improved local air quality in the central region of the room (at α=50°). On the other hand, the Coanda jet with the least swirl angle is more effective in mixing the fresh air throughout the domain and improves the average air quality (at α=55°).

Air entrainment and multiphase turbulence in the bubbly wake of a transom stern

Accurate prediction of the highly-mixed flow in the near field of a surface ship is a challenging and active research topic in Computational Ship Hydrodynamics. The disparity in the time and length scales and the scales of entrainment dictates the use of bubble source and mixed-phase flow models in which the current state of the art models are ad hoc. This paper presents the air entrainment characteristics and multiphase turbulence modeling of the near-field flow of a canonical stern with the inclusion of simple geometry effects. Using state of the art Cartesian-grid numerical methods with the full field equations, high-resolution two-phase flow data sets of a canonical stern with three different half-beam to draft ratios are simulated down to the scales of bubble entrainment. These data sets are used as the foundation for: (1) characterization of wake structure and near-wake air entrainment of the stern; (2) analysis of turbulent mass flux in the wake of the stern; and (3) a priori testing of multiphase turbulence models for turbulent mass flux. Results are obtained to show that these techniques enable analysis and physics-based parameterization of near-field air entrainment about surface ships for use in Computational Ship Hydrodynamics.

Eddy Diffusivity and Turbulent Schmidt Number in a Turbulent Diffusion Field with a Chemical Reaction

Eddy diffusivity Dtα and turbulent Schmidt number Sctα (α = A,B,R) in a planar liquid jet with a second-order chemical reaction (A+B → R) are experimentally investigated. Species A is premixed in the jet flow and species B is premixed in the main flow. The concentrations of reactive species and the streamwise velocity are simultaneously measured by combining I-type hot-film anemometry with an optical fiber probe based on the light absorption spectrometry. Eddy diffusivity and turbulent Schmidt number on the jet centerline are estimated from the results of the simultaneous measurements of concentration and velocity. The results show that the chemical reaction makes SctA large, and SctB has a negative value in the region where the mean concentration of species B decreases in the downstream direction whereas SctB for the nonreactive case has a positive value in the whole region on the jet centerline. It is also shown that SctR has a positive value in the upstream region whereas it has a negative value in the downstream region. The difference in SctB between the reactive and nonreactive cases becomes small in the downstream region. These results imply that the gradient diffusion model using the global constant turbulent Schmidt number poorly predicts the turbulent mass flux for reactive flows.

Some approaches to improve ventilation system in underground coal mines environment – A computational fluid dynamic study

This study addresses gas control in a typical “room-and-pillar” structure in an underground coal mine via mathematical model utilizing commercial computational fluid dynamic software. In essence, the model considers conservation of turbulent mass, momentum, species, and energy. Several turbulence models are tested and compared with published experimental data; good agreement between model prediction and experimental data is obtained for Spallart–Almaras turbulence model. Various design features were evaluated, e.g., flow stopping designs for improved quality (e.g., removal of contaminants) and quantity control (e.g., magnitude and direction of airflow). We further focus on the cross-cut region where rapid mining development take place, several ventilation scenarios with and/or without additional auxiliary ventilation are investigated for effective removal of methane. Two different cutting scenarios in rapid mining development are also evaluated. The advantages and limitations of each design are discussed and compared not only in terms of quality and quantity, but also in terms of the overall pressure drop which represent the associated cost of ventilation system.

Evolution of axisymmetric weakly turbulent mixtures interacting with shock or rarefaction waves

This paper deals with the interaction of a shock or a rarefaction wave with a weakly turbulent mixture of perfect gases. Assuming weak density-velocity fluctuations, Kovasznay decomposition applies and linear theories can be used to predict the evolution of the joint spectrum of density and velocity during the interaction. In this work, the upstream spectrum is restricted to axisymmetric fields free of acoustic perturbations, in order to comply with shock tube experimental conditions. Besides, spectral anisotropy is limited to a first order spherical harmonic decomposition. With these assumptions, transfer matrices can be obtained which relate the Reynolds stresses, turbulent mass flux and density variance after interaction to their counterparts before interaction. Results are given for both shock waves and rarefaction or compression waves; they are intended to help improve one-point statistical turbulence models applied to shock tube experiments.

난류 파이프 유동 내 물질전달에 대한 레이놀즈 수 영향: Part I. 평균 농도장 및 저차 난류통계치

Large Eddy Simulation(LES) of turbulent mass transfer in fully developed turbulent pipe flow has been performed to study the effect of Reynolds number on the concentration fields at <TEX>$Re_{\tau}=180$</TEX>, 395, 590 based on friction velocity and pipe radius. Dynamic subgrid-scale models for the turbulent subgrid-scale stresses and mass fluxes were employed to close the governing equations. Fully developed turbulent pipe flows with constant mass flux imposed at the wall are studied for Sc=0.71. The mean concentration profiles and turbulent intensities obtained from the present LES are in good agreement with the previous numerical and experimental results currently available. To show the effects of Reynolds number on the turbulent mass transfer, the mean concentration profile, root-mean-square of concentration fluctuations, turbulent mass fluxes, cross-correlation coefficient, turbulent diffusivity and turbulent Schmidt number are presented.

난류 파이프 유동 내 물질전달에 대한 레이놀즈 수 영향: Part II. 순간농도장, 고차 난류통계치 및 물질전달수지

Large Eddy Simulation(LES) of turbulent mass transfer in fully developed turbulent pipe flow has been performed to study the effect of Reynolds number on the concentration fields at $Re_{\tau}

입구 와류발생과 질량분사가 있는 연소실 내부유동의 LES 해석

하이브리드 로켓은 축 방향의 산화제 유동과 고체 연료의 기화로 인한 벽면 분출 유동사이의 상호 간섭에 의해 복잡한 형태의 혼합 전단층이 존재한다. 연소실 입구에 링이 설치되어 있고 질량분사가 있는 실린더 유동에 대하여 압축성 효과를 고려한 LES(Large Eddy Simulation) 기법을 적용하여 수치계산을 수행하였다. 최근의 실험에 의하면 연료 중간에 링과 같은 다이아프램이 설치된 경우, 연소율의 증가가 관찰되었다. 계산 결과에 따르면, 축방향 유동과 벽면 분출 유동이 상호 간섭하여 발생하는 벽면 와류가 국부적인 연료 표면으로의 열전달을 증가시켜 실험에서 관찰되는 딤플이 생성되는 것을 확인하였다. 또한 연소실 입구에 설치된 링에 의하여 발생되는 와류는 벽면 와류가 보다 활발하게 생성되고 열전달과 혼합을 향상시키는 역할을 하며 이 때문에 연소율이 증가되는 것으로 보인다. Hybrid rocket shows interesting characteristics of complicated mixing layers developed by interactions between turbulent oxidizer flow and mass flow from surface due to fuel vaporization. In this study, compressible LES with a ring structure attached at the entrance of the combustor are performed. According to one recent report, adding a ring structure in the middle of the combustor helps increasing regression rate. From the numerical results, it is seen that vortex structures near the wall becomes stronger due to the interaction with surface mass injection, and the local heat flux increases due to the vortices. This phenomenon is obviously related to the generation of dimple structures which are seen in the number of experiments. Also, the ring structure at the entrance induces strong vortex flow which enhances heat transfer to the wall surface and mixing between fuel and oxidizer as well as reaction efficiency.

Simultaneous measurements of reactive scalar and velocity in a planar liquid jet with a second-order chemical reaction

This paper presents a new experimental approach for simultaneous measurements of velocity and concentration in a turbulent liquid flow with a chemical reaction. For the simultaneous measurements, we developed a combined probe consisting of an I-type hot-film probe and an optical fiber probe based on the light absorption spectrometric method. In a turbulent planar liquid jet with a second-order chemical reaction $$(\mathrm{A}+\mathrm{B}\rightarrow \mathrm{R})$$ , streamwise velocity and concentrations of all reactive species are measured by the combined probe. The turbulent mass fluxes of the reactive species are estimated from the simultaneous measurements. The results show that the influence of the chemical reaction on the turbulent mass flux of the reactant species near the jet exit is different from its influence in other regions, and the turbulent mass flux of the product species has a negative value near the jet exit and a positive value in other regions.

Molecular gas in galaxies: much more than just the fuel of star formation

AbstractIt is well known that shocks can heat molecular gas, but we are only starting to understand what the consequences might be for galaxy evolution. Observations are now revealing a growing number of galaxies undergoing phases of strong injection of kinetic energy through galaxy interactions or feedback from powerful AGN, which have bright line emission from shock-heated molecular gas that is likely powered by the dissipation of turbulent energy. These observations demonstrate that turbulence – now recognized as an important mechanism regulating star formation – may also be key in regulating galaxy growth. We report on our on-going efforts to quantify the role of molecular gas for the dissipation of kinetic energy through observations on two very different examples, the Antennae galaxy merger and the radio galaxy 3C326 N, a site of strong mechanical AGN feedback. Both galaxies show signatures of strong dissipation of kinetic energy through shocks. We will illustrate how key parameters of these processes like bulk and turbulent velocities, gas masses, and dissipation timescales can be measured with multi-wavelength data, and how this furthers our understanding of how galaxy evolution and star formation in galaxies depend on each other.

Comparative study of turbulent mass transfer in the viscous sublayer using electrochemical method and direct numerical simulations

In this study we present, analyze and compare the power spectral density of the wall shear stress in a turbulent plane channel flow obtained with different techniques. Experimentally the instantaneous wall shear stress was measured with the electrochemical technique using different probes, which give approximately the same results after applying the transfer function for correction of the probe’s inertia. Numerically, the time evolution of the wall shear stress has been determined using direct numerical simulations (DNS) and large eddy simulations (LES). The results of DNS are in a good agreement with the electrochemical flow measurements. However the power spectra of the wall shear stress obtained with LES shows deviations with respect to DNS at high frequencies because of the spatial filtering inherent to the LES technique.

THE DEPENDENCE OF PRESTELLAR CORE MASS DISTRIBUTIONS ON THE STRUCTURE OF THE PARENTAL CLOUD

The mass distribution of prestellar cores is obtained for clouds with arbitrary internal mass distributions using a selection criterion based on the thermal and turbulent Jeans mass and applied hierarchically from small to large scales. We have checked this methodology comparing our results for a lognormal density PDF with the theoretical CMF derived by Hennebelle and Chabrier, namely a power-law at large scales and a log-normal cutoff at low scales, but our method can be applied to any mass distributions representing a star-forming cloud. This methodology enables us to connect the parental cloud structure with the mass distribution of the cores and their spatial distribution, providing an efficient tool for investigating the physical properties of the molecular clouds that give rise to the prestellar core distributions observed. Simulated fBm clouds with the Hurst exponent close to the value H=1/3 give the best agreement with the theoretical CMF derived by Hennebelle and Chabrier and Chabrier's system IMF. Likewise, the spatial distribution of the cores derived from our methodology show a surface density of companions compatible with those observed in Trapezium and Ophiucus star-forming regions. This method also allows us to analyze the properties of the mass distribution of cores for different realizations. We found that the variations in the number of cores formed in different realizations of fBm clouds (with the same Hurst exponent) are much larger than the expected root N statistical fluctuations, increasing with H.

Asymptotic solutions for turbulent mass transfer augmented by a first order chemical reaction

We present asymptotic solutions for turbulent mass transfer in a smooth conduit at high Schmidt number in the presence of a first-order chemical reaction in the fluid. Far-field solutions are derived for a case dominated by (1) mass transfer at the wall and (2) the first-order chemical reaction. An approximate solution is derived for the regime where both are important. The solutions are in good agreement with numerical solutions and with the literature. At high Damköhler numbers the system is governed by a reaction–diffusion equation and the observed increase in mass transfer coefficient is caused by thinning of the mass transfer boundary layer due to the fast chemical reaction in the fluid. We present closed-form far-field solutions for Dirichlet, Neumann and Robin boundary conditions and comment on grid resolution requirements to accurately resolve the mass transfer boundary layer. The solution strategy presented can be straightforwardly extended to non-linear wall- and bulk-reactions.

A New Model for the Simulation of CO&lt;sub&gt;2&lt;/sub&gt; Absorption in an Industrial-Scale Packed Column

A new computational mass transfer model is proposed for simulating the chemical absorption process with heat effect by solving the average fluctuating mass flux in turbulent mass conservation equation and the average fluctuating heat flux in turbulent heat conservation equation, so that the concentration profile and the temperature profile of column can be obtained. The feather of the proposed model is to abandon the conventional way of introducing the unknown turbulent mass transfer diffusivity Dtand the turbulent thermal diffusivity αtin the mass and heat conservation equations. By using the proposed model, the simulated results of CO2absorption by aqueous monoethanolamine (MEA) solution in an industrial scale column is presented, including MEA concentration, CO2loading and liquid phase temperature. The simulations are in agreement with the published experiment data.