Laminar Conditions Research Articles

Hydrodynamic controls on muddy sedimentary-fabric development on the Southwest Louisiana subaqueous delta

This study examines the sedimentary products of current-wave-enhanced sediment gravity flows (CWESGFs) on the muddy Southwest Louisiana (SWLA) subaqueous delta where the combined effects of gravity, currents, and wave orbital velocity exert a strong control on sedimentary-fabric development, and contribute to development of a muddy inner-shelf clinothem. CWESGF microstratigraphy from the SWLA subaqueous delta displays an overall structure similar that is similar to the tripartite layering of WESGFs previously documented on the Eel Shelf, where wave-enhanced sediment gravity flow deposits were first described (basal crossbeds, intermediate planar interlaminations of clay and silt; topmost clay-rich drape). However, on the SWLA subaqueous delta, a shorter wave period appears to impart more rapid alternations in turbulent and laminar conditions than for the Eel shelf, reducing the prominence of the intermediate planar laminations that are thought to develop during laminar flow conditions only. A novel framework for relating the sedimentary products of CWESGFs to interacting unidirectional, wave-driven, and gravity-driven flow is used to place our results in a global context, with respect to other modern examples. This is an important contribution as it may provide insights into paleo-depositional conditions represented by sedimentary structures in ancient strata, and provide the basis for developing additional recognition criteria for CWESGFs, an important and diverse class of sediment gravity flows.

Aggregation and breakup of acrylic latex particles inside millimetric scale reactors

Aggregation of acrylic latex is investigated inside tubular millireactors working under laminar hydrodynamic conditions. The size distribution and fractal dimension of aggregates are measured using light scattering. Results show that the equilibrium between rupture and aggregation is achieved quickly, allowing the study of cluster size distribution and shape at the aggregation/rupture steady state. Both laminar hydrodynamic conditions and high shear rate are suspected to promote the formation of aggregates with a high fractal dimension, which means that the particles are almost spherical, thereby offering an interesting alternative to conventional batch processes. These results can provide useful information for industries aiming at producing aggregates at specified size and quality.

Direct numerical simulation of particle laden flow in a human airway bifurcation model

During the delivery of inhaled medicines, and depending on the size distribution of the particles in the formulation, airway bifurcations are areas of preferential deposition. Previous studies of laminar flow through airway bifurcations point to an interplay of inertial and centrifugal forces that leads to rich flow phenomena and controls particle deposition patterns. However, recent computational studies have shown that the airflow in the upper human airways is turbulent during much of the respiratory cycle. The question of how the presence of turbulence modifies these effects remains open. In this study, we perform for the first time Direct Numerical Simulations (DNS) of fully developed turbulent flow through a single human airway bifurcation model, emulating steady prolonged inspiration and expiration. We use the rich information obtained from the DNS in order to identify key structures in the flow field and scrutinize their role in determining deposition patterns in the bifurcation. We find that the vortical structures present in the bifurcation during expiration differ from those identified during inspiration. While Dean vortices are present in both cases, a set of three dimensional “carinal vortices” are identified only during expiration. A set of laminar simulations in the same geometries, but at lower Reynolds numbers, allow us to identify key differences in aerosol deposition patterns between laminar and turbulent respiration. We also report deposition fractions for representative Stokes numbers for both laminar and turbulent conditions. Given the suspected role of external mechanical stress on the airway epithelium in determining mucus clearance and chronic disease development, here we report wall shear stress distributions for both the turbulent and laminar cases. Finally, we also perform Large Eddy Simulations (LES) and Reynolds-Averaged Navier-Stokes (RANS) simulations for the same configuration in order to asses their performance as compared to DNS. We find that LES and RANS perform well and that they are able to capture the key characteristics of the flow field. The agreement between DNS and RANS holds true only for the mean flow field, which is primarily influenced by curvature effects.

Use of a Smoothed Model for Pipe Friction Loss

This note examines a globally smooth approximation for friction loss in pipelines suggested by Burgschweiger et al. The approximation contains five parameters, two of which are open to selection. A method is provided for choosing these based on laminar conditions near zero flow. Using the proposed approach, approximation accuracy is reported in terms of relative roughness and Reynolds number.

An unstable truth: how massive stars get their mass

The pressure exerted by massive stars' radiation fields is an important mechanism regulating their formation. Detailed simulation of massive star formation therefore requires an accurate treatment of radiation. However, all published simulations have either used a diffusion approximation of limited validity; have only been able to simulate a single star fixed in space, thereby suppressing potentially-important instabilities; or did not provide adequate resolution at locations where instabilities may develop. To remedy this we have developed a new, highly accurate radiation algorithm that properly treats the absorption of the direct radiation field from stars and the re-emission and processing by interstellar dust. We use our new tool to perform three-dimensional radiation-hydrodynamic simulations of the collapse of massive pre-stellar cores with laminar and turbulent initial conditions and properly resolve regions where we expect instabilities to grow. We find that mass is channeled to the stellar system via gravitational and Rayleigh-Taylor (RT) instabilities, in agreement with previous results using stars capable of moving, but in disagreement with methods where the star is held fixed or with simulations that do not adequately resolve the development of RT instabilities. For laminar initial conditions, proper treatment of the direct radiation field produces later onset of instability, but does not suppress it entirely provided the edges of radiation-dominated bubbles are adequately resolved. Instabilities arise immediately for turbulent pre-stellar cores because the initial turbulence seeds the instabilities. Our results suggest that RT features are significant and should be present around accreting massive stars throughout their formation.

Taylor flow in intermediate diameter channels: Simulation and hydrodynamic models

The two-phase Taylor flow pattern has been studied extensively. However, limited information is available for flows in intermediate diameter channels (5≲Bo≲40, or 6mm≲D≲27mm for ambient gas–water flows), as found in air-lift and bubble pumps. Previous investigations have primarily evaluated Taylor flow models in terms of overall pressure drop, which incorporates hydrostatic and multiple hydrodynamic components. Thus, individual sub-models and sources of error could not be directly assessed. In this investigation, volume-of-fluid (VOF) based Taylor flow simulations are performed over a wide range of laminar and turbulent conditions in the intermediate Bond number regime (5<Bo<20, 250<Nf<1000, and 20<Rej<8100). Results are applied to individually evaluate hydrodynamic sub-models for bubble-region frictional pressure drop gradient (∇pf,b), slug frictional pressure drop gradient (∇pf,s), and flow transition pressure loss (Δptrans). Based on these results, recommendations are provided for selection of hydrodynamic sub-models. These hydrodynamic closure models are integrated with kinematic flow models to yield a complete intermediate Bond number Taylor flow formulation for which all submodels were independently validated. The resulting model achieves improved accuracy for predicting experimental liquid flow rates compared with previous Taylor flow models (81% of cases within 50% of measured flows rates).

Implementation of ‘chaotic’ advection for viscous fluids in heat exchanger/reactors

When viscous fluids are involved, laminar hydraulic conditions and heat and mass transfer intensification are conflicting phenomena. A channel geometry based on Split-And-Recombine (SAR) patterns is experimentally investigated. The principle implements the Baker’s transformation and ‘chaotic’ structures are generated to promote heat and mass transfer. This work assesses the energy efficiency of different heat exchanger/reactors integrating these SAR patterns.The heat transfer capacity is assessed and compared with the energy consumption of each mock-up. It is sensitive to the cooling mode and to the number of SAR patterns per length unit as well.The continuous oxidation of sodium thiosulfate with hydrogen peroxide has been implemented. Conversions up to 99% are reached according to the utility fluid temperature and the residence time.Finally, the whole performances of the SAR geometries are compared to a plate-type heat exchanger/reactor with a corrugated pattern. The more viscous the fluid, the more the energy efficiency of the SAR design increases compared to the corrugated design because of the balance between advection and diffusion mechanisms. The interest in terms of energy efficiency in working with SAR heat exchanger/reactor appears from Reynolds numbers below 50.

Erratum to: Gas Flow Dynamics in Inlet Capillaries: Evidence for non Laminar Conditions.

Erratum to : J. Am. Soc. Mass Spectrom (2016) DOI: 10.1007/s13361-016-1415-z In the preceding article BGas Flow Dynamics in Inlet Capillaries: Evidence for non Laminar Conditions^ by Wisdorf et al., the errors as listed below were not appropriately corrected during the final galley proofing stage. Errors number 1–4were present in the submitted revised manuscript, errors 5 and 6 appeared additionally in the galley proofs. The authors are indebted to Dr. Martin F. Jerrold, Chemistry Department, Indiana University, who found inconsistencies caused by errors number 2–4 when analyzing our data using the online version of the article and reported these immediately to us. We sincerely apologize for this mishap. It is emphasized though that all calculations presented in the preceding articlewere computed with the correct equations and the appropriate units as given below. None of the data, figures, and conclusions drawn in the article are thus affected.

Computational Study of NOx Formation at Conditions Relevant to Gas Turbine Operation: Part 1

Along with other nitrogen oxides, nitric oxide (NO) is a regulated air pollutant that is primarily produced as a result of combustion processes. This pollutant is produced in every combustion system that uses oxidizer mixtures containing N2 and/or fuels that contain organically bound nitrogen. To design combustors to minimize NO emissions, high-fidelity computational models for NO production and NO/NO2 interconversion are required. To improve model performance and understanding of NO production pathways, a computational parametric study is performed to investigate the effects of fuel chemistry, reaction temperature history, and inert gas dilution on NO production in methane and ethylene combustion. Predictions using popular models from the literature are compared for premixed laminar flame conditions as well as against previously reported stirred reactor measurements. Differences in the predictions and their relationships to the NOx submodel and hydrocarbon chemistry are investigated. We find that signifi...

Dynamic characterization of external and internal mass transport in heterotrophic biofilms from microsensors measurements

Knowledge of mass transport mechanisms in biofilm-based technologies such as biofilters is essential to improve bioreactors performance by preventing mass transport limitation. External and internal mass transport in biofilms was characterized in heterotrophic biofilms grown on a flat plate bioreactor. Mass transport resistance through the liquid-biofilm interphase and diffusion within biofilms were quantified by in situ measurements using microsensors with a high spatial resolution (<50 μm). Experimental conditions were selected using a mathematical procedure based on the Fisher Information Matrix to increase the reliability of experimental data and minimize confidence intervals of estimated mass transport coefficients. The sensitivity of external and internal mass transport resistances to flow conditions within the range of typical fluid velocities over biofilms (Reynolds numbers between 0.5 and 7) was assessed. Estimated external mass transfer coefficients at different liquid phase flow velocities showed discrepancies with studies considering laminar conditions in the diffusive boundary layer near the liquid-biofilm interphase. The correlation of effective diffusivity with flow velocities showed that the heterogeneous structure of biofilms defines the transport mechanisms inside biofilms. Internal mass transport was driven by diffusion through cell clusters and aggregates at Re below 2.8. Conversely, mass transport was driven by advection within pores, voids and water channels at Re above 5.6. Between both flow velocities, mass transport occurred by a combination of advection and diffusion. Effective diffusivities estimated at different biofilm densities showed a linear increase of mass transport resistance due to a porosity decrease up to biofilm densities of 50 g VSS·L−1. Mass transport was strongly limited at higher biofilm densities. Internal mass transport results were used to propose an empirical correlation to assess the effective diffusivity within biofilms considering the influence of hydrodynamics and biofilm density.

Gas Flow Dynamics in Inlet Capillaries: Evidence for non Laminar Conditions.

In this work, the characteristics of gas flow in inlet capillaries are examined. Such inlet capillaries are widely used as a first flow restriction stage in commercial atmospheric pressure ionization mass spectrometers. Contrary to the common assumption, we consider the gas flow in typical glass inlet capillaries with 0.5 to 0.6 mm inner diameters and lengths about 20 cm as transitional or turbulent. The measured volume flow of the choked turbulent gas stream in such capillaries is 0.8 L·min(-1) to 1.6 L·min(-1) under typical operation conditions, which is in good agreement to theoretically calculated values. Likewise, the change of the volume flow in dependence of the pressure difference along the capillary agrees well with a theoretical model for turbulent conditions as well as with exemplary measurements of the static pressure inside the capillary channel. However, the results for the volume flow of heated glass and metal inlet capillaries are neither in agreement with turbulent nor with laminar models. The velocity profile of the neutral gas in a quartz capillary with an inner diameter similar to commercial inlet capillaries was experimentally determined with spatially resolved ion transfer time measurements. The determined gas velocity profiles do not contradict the turbulent character of the flow. Finally, inducing disturbances of the gas flow by placing obstacles in the capillary channel is found to not change the flow characteristics significantly. In combination the findings suggest that laminar conditions inside inlet capillaries are not a valid primary explanation for the observed high ion transparency of inlet capillaries under common operation conditions. Graphical Abstract ᅟ.

Enhancing mixing at a very low Reynolds number by a heaving square cylinder

Mixing fluids in a channel under laminar conditions is a challenge for which many options have been proposed to enhance the resulting mixing. Here, an active mixing technique based on the interaction between the fluids and a structure, a square cylinder heaving in the middle of a channel, is studied numerically by means of Finite Volume Methods. For a given cylinder-to-channel width ratio and for a very small Reynolds number, it is shown how the amplitude and the frequency of the heaving oscillation affect the efficiency of the resulting mixing at the outlet section of the channel. After studying 7 different frequencies and 5 different amplitudes, an optimal combination has been found for which the mixing efficiency is the highest, being more than 10 times higher than when the mixing takes place in the same channel but with the cylinder steady. Besides of this, an optimal oscillation frequency has been found for each oscillation amplitude for which the mixing efficiency is the highest. Additionally, the input works needed both to heave the cylinder and to run the channel are also characterised as a function of the heaving frequency and amplitude. Surprisingly, the highest mixing efficiency configuration does not require either the highest input work for heaving the cylinder or for running the channel. Finally, we shall show that the optimum mixing, quantified in terms of a required input energy-efficiency ratio, is achieved with low heaving oscillation frequencies and high heaving amplitudes.

Using the fully developed concentration profile to determine particle penetration in a laminar flow tube

A previously found analytical expression for the fully developed concentration profile of aerosol particles flowing through a circular tube under laminar conditions has been used to derived a simple analytical expression to determine particle penetration. The resulting expression, P=0.846exp(−3.659β) where β is the dimensionless diffusion coefficient of the particle, has been compared satisfactorily with the well-known Gormley–Kennedy equation even for values of β as low as 0.04 in spite that, in principle, the approximate expression for the fully developed concentration profile is only valid for β>0.2.

Experimental investigation on the thermal performance of a coiled heat exchanger using a new hybrid nanofluid

In this work the thermal performance of hybrid and single type nanofluid is investigated in a coiled heat exchanger at constant wall temperature and laminar flow operating conditions. The nanoparticle concentration is in the range of 0.1–0.4vol% and the composition of the synthesized nanoparticle regarding hybrid nanofluid experiments is 97.5% alumina and 2.5% Ag. The maximum rate of heat transfer can be obtained by using the hybrid nanofluid at a concentration of 0.4vol% which is 31.58% higher than that of the distilled water. Overall, the maximum thermal performance factor for hybrid nanofluid is about 2.55 suggesting the superior performance of the presented approach for energy intensification in heat exchangers.

Experimental investigation of the solar FPC performance using graphene oxide nanofluid under forced circulation

Experiments were conducted on a solar flat plate collector (FPC) with graphene oxide (GO) nanofluid as the working fluid, circulated forcibly with the aid of a pump. The FPC had a collector area of 2m2, a concentric tube heat exchanger (CTHX) and a storage tank. GO nanoparticles were synthesized from graphite by the modified Hummer’s method. The morphology of the GO was estimated by XRD, UV–vis spectrometry and SEM imaging. The GO nanofluid was prepared by ultrasonication of the GO nanoparticles with de-ionized (DI) water as the base fluid, without adding any surfactant. Homogeneous and stable GO nanofluid for three different mass concentrations, (φ)=0.005, 0.01 and 0.02, were prepared. The nanofluids were found to be stable for 60days without any sedimentation issues. The thermo-physical properties of the GO nanofluids, such as the thermal conductivity, viscosity, density and specific heat, were estimated. For the flow supplied by the pump, the maximum value of the FPC inlet pipe and fluid property-based Reynolds number was found to be 430. The FPC’s overall heat transfer co-efficient (h), friction factor and collector efficiency were investigated for GO nanofluids under laminar conditions. It was observed that for GO nanofluid with φ=0.02 and a flow rate of 0.0167kg/s, the enhancement in the collector efficiency was 7.3% over that of the base fluid (DI water). The collector efficiency was found to increase with increasing φ and flow rate. The increases in h for the GO nanofluid with φ=0.005, 0.01 and 0.02 were 8.03%, 10.93% and 11.5%, respectively.

Microscopic Study of Wax Deposition: Mass Transfer Boundary Layer and Deposit Morphology

This study presents a pioneering effort to enable visualization of the in situ wax deposition process in a microscopic scale with an emphasis on mass transfer boundary layer and deposit morphology observations. The mass transfer boundary layer study validates the existence of partial wax supersaturation near the wax deposit and oil interface. The discrepancy between the actual mass transfer boundary layer thickness and the heat–mass transfer analogy prediction expands as the Reynolds number increases and the ΔT (temperature difference between the bulk and cold interface) decreases. This conclusion is in agreement with several flow loop data with the same oil and similar flowing conditions. The morphology study reveals that, under laminar conditions, the distribution of the deposit crystal aspect ratio resembles a log-normal pattern with a distribution mode between 2 and 3. This characteristic exhibits similarity with wax crystals observed under static cooling conditions. Deposits formed under laminar cond...

Experimental study on EHD heat transfer enhancement from flush-mounted ribbons with different arrangements of wire electrodes in a channel

In the present study, the heat transfer enhancement of a bundle of flush-mounted ribbons placed on the floor of a rectangular duct was investigated experimentally. The flush-mounted ribbons act as heat sources and the cooling happens with air. The air flow was three-dimensional, steady, viscous and incompressible under both laminar and turbulent conditions ( $$500 \le \text{Re}_{{D_{h} }} \le 4500$$ ). The hydrodynamics and heat transfer behavior of the air flow was studied by means of an active method with application of corona wind. The state of the art of this work revolves around an experimental investigation of an electrohydrodynamics (EHD) active method and heat transfer enhancement from the surfaces of the flush-mounted ribbons. Due to the intricacies of the required experiment, a special apparatus needed to be designed and constructed. The aim of this work is application of EHD active method for convective heat transfer enhancement. In this method the different arrangement of wire electrodes has been achieved. The results show that in same Reynolds numbers and voltages of wires, the heat transfer enhancement was increase in arrangement 1 than other 4 arrangements.

On streamwise vortices in Large Eddy Simulations of initially laminar plane mixing layers

This paper details the influence of the nature of imposed inflow fluctuations on Large Eddy Simulations of a spatially developing turbulent mixing layer originating from laminar boundary layers. A simulation with imposed white-noise random fluctuations, commonly used in numerical simulations, produces mean-flow statistics that agree well with reference experimental data. Whilst flow visualisation images show evidence for streamwise vorticity in this simulation, quantitative statistics do not reveal the presence of statistically stationary streamwise vortices. A further simulation that uses physically-correlated inflow fluctuations also produces good mean-flow statistical agreement with reference data. From secondary shear stress contours it can be inferred that this simulation does, however, predict the presence of statistically stationary streamwise vortices. The properties of the streamwise vortices are in good agreement with experimental data. The data presented here indicate that, even for initially laminar conditions, plane mixing layer simulations require accurate physically correlated inflow conditions in order to reproduce the flow features found experimentally.

Fuel performances in Spark-Ignition (SI) engines: Impact of flame stretch

In a context of decreasing pollutant emissions, the transport sector has to tackle improvements to the engine concept as well as fuel diversification. The use of these different fuels often has an impact on the combustion performance itself. In the case of Spark Ignition (SI) engines, efficiency is a function of the combustion speed, i.e. the speed at which the fresh air–fuel mixture is consumed by the flame front. Every expanding flame is subject to flame curvature and strain rate, which both contribute to flame stretch. As each air–fuel mixture responds differently to flame stretch, this paper focuses on understanding the impact of flame stretch on fuel performances in SI engines. Different air–fuel mixtures (different fuels or equivalence ratios) with similar unstretched laminar burning speeds and thermodynamic properties but different responses to stretch were selected. The mixtures were studied in a turbulent spherical vessel and in an optical engine using Mie-Scattering tomography. The combustion phasing was also investigated in both optical and all-metal single cylinder engines. Results show that flame stretch sensitivity properties such as Markstein length and Lewis number, determined in laminar combustion conditions, are relevant parameters that need to be taken into consideration to predict the global performance of fuels, either experimentally or for modeling simulation.

Bumblebee Flight in Heavy Turbulence.

High-resolution numerical simulations of a tethered model bumblebee in forward flight are performed superimposing homogeneous isotropic turbulent fluctuations to the uniform inflow. Despite tremendous variation in turbulence intensity, between 17% and 99% with respect to the mean flow, we do not find significant changes in cycle-averaged aerodynamic forces, moments, or flight power when averaged over realizations, compared to laminar inflow conditions. The variance of aerodynamic measures, however, significantly increases with increasing turbulence intensity, which may explain flight instabilities observed in freely flying bees.