Investigation of the onset of bioconvection in a suspension of oxytactic microorganisms subjected to high-frequency vertical vibration

This paper investigates the effect of vertical vibration on the stability of a dilute suspension of oxytactic microorganisms in a shallow horizontal fluid layer. For the case of high-frequency vibration, an averaging method is utilized to derive the equations describing the mean flow by decomposing the solutions of governing equations into two components: one that varies slowly with time, and a second that varies rapidly with time. Linear stability analysis is used to investigate the stability of the obtained averaged equations. It is predicted that high-frequency, low-amplitude vertical vibration has a stabilizing effect on a suspension of oxytactic microorganisms confined in a shallow horizontal layer. PACS 47.27 Te; 68.60 Dv     

Investigation of the onset of thermo-bioconvection in a suspension of oxytactic microorganisms in a shallow fluid layer heated from below

This paper investigates the combined effect of density stratification due to oxytactic upswimming and heating from below on the stability of a suspension of motile oxytactic microorganisms in a shallow fluid layer. Different from traditional bioconvection, thermo-bioconvection has two destabilizing mechanisms that contribute to creating the unstable density stratification. This problem may be relevant to a number of geophysical applications, such as the investigation of the dynamics of some species of thermophiles (heat loving microorganisms) living in hot springs. By performing a linear stability analysis, we obtained a correlation between the critical value of the bioconvection Rayleigh number and the traditional, “thermal” Rayleigh number. It is established that heating from below makes the system more unstable and helps the development of bioconvection.     

The onset of thermo-bioconvection in a shallow fluid saturated porous layer heated from below in a suspension of oxytactic microorganisms

The aim of this paper is to present a continuum model for thermo-bioconvection of oxytactic bacteria in a porous medium and investigate the combined effects of microorganisms' upswimming and heating from below on the stability of bioconvection in a horizontal layer filled with a fluid saturated porous medium. Different from traditional bioconvection, thermo-bioconvection has two destabilizing mechanisms that contribute to creating the unstable density stratification. This problem may be relevant to a number of geophysical applications, such as the investigation of the dynamics of oxytactic species of thermophiles (heat loving microorganisms) living in hot springs, microbial-enhanced oil recovery, and modeling oil- and gas-bearing sedimentary basins. The utilization of the Galerkin method to solve a linear stability problem leads to a correlation between the critical value of the bioconvection Rayleigh number and the traditional “thermal” Rayleigh number.     

水基MWCNTs/MoS2复合纳米流体的摩擦学性能研究

使用离子液体[EMIm]BF₄分散多壁碳纳米管(MWCNTs),再以[EMIm]BF₄-阿拉伯树胶(GA)为添加剂分散二硫化钼(MoS₂),二者的水溶液复配得到复合纳米流体. 采用拉曼光谱分析了MWCNTs的改性度,通过吸光度和粒度对复合纳米流体的分散与悬浮稳定性进行了表征. 对不同纳米颗粒配比的复合纳米流体润湿性能和摩擦学性能进行测试,结果表明:MWCNTs和MoS₂质量分数为0.6%、1.2%时复合纳米流体的铺展成膜能力最好,其接触角约为63.04°,相比于去离子水降低了23.55%. 摩擦磨损测试结果也表明此配比下的减摩抗磨性能最佳,平均摩擦系数为0.073,比去离子水降低了61.98%,同时体积磨损率降低了67.87%. 磨痕形貌观测表明,最优配比下磨痕浅,且表面光滑、无犁沟. X射线光电子能谱(XPS)表明MWCNTs和MoS₂共同参与摩擦并在基底成膜,由此协同实现了高效润滑.      

Superparamagnetic nanoparticle assisted hyperthermia and cooling protocol for optimum damage of internal carcinoma using computational predictive model

A predictive model has been developed for superparamagnetic nanoparticle assisted hyperthermia in presence of external electromagnetic field to affect necrosis of internal carcinoma. Effect of different parameters on tissue temperature rise is tested and finally damage is analyzed to get a definitive measure of tissue destruction. A cooling protocol has been predicted to obtain maximum tumor destruction while maintaining minimum damage of the healthy tissue. We propose the comprehensive modeling strategy outlined here as the precursor to the development of efficient therapeutic planning tools for cancer patients.     

Anomalous rheology of polypyrrole nanoparticle/alginate suspensions: effect of solids volume fraction, particle size, and electronic state

The rheology of dispersions of polypyrrole (PPY) nanoparticles (nPPY) is compared to that of micron-sized PPY particles (CPPY), each suspended in aqueous sodium alginate. With increasing PPY volume fraction, the Newtonian viscosity of the CPPY/alginate suspensions exhibits a ??normal?? increase, whereas that of the nPPY/alginate suspensions decreases to a minimum and then increases again. Enhanced elasticity, indicative of agglomerate formation via bridging interactions with the alginate, is observed only in the CPPY rheology. By comparing doped versus dedoped nPPY particles, and investigating the effect of nPPY particle size, we conclude that the negative viscosity change of the nPPY dispersions is due to adsorption of a dense layer of alginate, resulting in a decrease in bulk alginate concentration. The viscosity upturn at higher nPPY volume fractions indicates the onset of particle agglomeration via bridging interactions with alginate. The results demonstrate improved dispersability of both doped and dedoped nPPY over CPPY particles.     

Fluid dynamics of self-propelled microorganisms,from individuals to concentrated populations

Nearly close-packed populations of the swimming bacterium Bacillus subtilis form a collective phase, the “Zooming BioNematic” (ZBN). This state exhibits large-scale orientational coherence, analogous to the molecular alignment of nematic liquid crystals, coupled with remarkable spatial and temporal correlations of velocity and vorticity, as measured by both novel and standard applications of particle imaging velocimetry. The appearance of turbulent dynamics in a system which is nominally in the regime of Stokes flow can be understood by accounting for the local energy input by the swimmers, with a new dimensionless ratio analogous to the Reynolds number. The interaction between organisms and boundaries, and with one another, is modeled by application of the methods of regularized Stokeslets.     

Influence of interphase and filler distribution on the elastic properties of nanoparticle filled polymers

The present work presents a preliminary study on the effect of the filler distribution on the elastic modulus of a nanoparticle filled polymer. To this end, two different theoretical approaches are implemented and compared. Both of them account for an interphase layer embedding the nanoparticle, with mechanical properties different from those of the matrix. Conversely, only one of them accounts for the variation of the interparticle distance. The comparison between these models allows to draw some conclusions on the effect of the filler distribution.     

Experimental time-resolved study of the interaction between a pulsating injectant and a steady cross-flow: aerodynamics of film cooling

A test rig incorporating the injection from a single cylindrical hole with an inclination of 30° to a thermally uniform mainstream flow was used for determining variations in flow structures due to injectant pulsation. The average blowing ratios ([`(M)] \overline{M} ) were 0.65, 1, and 1.25. The periodic variations in injectant flow were rendered by a loudspeaker-based pulsation system to nondimensionalized excitation frequency (St St ) of 0, 0.2, 0.3, and 0.5. Pulsation resulting in a close-wall orientation of injectant fluid compared with steady blowing bearing outward orientation was only observed in few cases. At [`(M)] \overline{M}  = 0.65, jet fluid remains aligned and covers a significant part of the wall under steady blowing. At higher blowing ratios, pulsation induces large spatial variations in the jet trajectory, collapsing of the jet body, and the shedding of wake structures due to the periodic variation of injection flow rate. It was found that the pulsation improves wall coverage of the injectant fluid under low frequency excitation as the separation of the jet from the wall becomes evident ([`(M)] \overline{M}  = 1 and 1.25).     

Analytical considerations of flow/thermal coupling of nanofluids in foam metals with local thermal non-equilibrium (LTNE) phenomena and inhomogeneous nanoparticle distribution

Foam metals with micro pores own excellent thermal performance, however, poor heat conductive ability of most heat-transfer fluids restricts further heat transfer improvement. Combination of foam metal and nanofluid with highly conductive nanoparticles is a promising solution. Convective thermal characteristics of nanofluids in porous foams are theoretically investigated in this work. Effects of Brownian motion and thermophoretic diffusion of nanoparticles in the base fluid on thermal performance are considered. The nanoparticle and the base-fluid are considered to be in thermal equilibrium and the temperature difference between the nanofluid and foam ligaments is especially considered. Compared with the base-fluid flow in a duct, the velocity distribution for the nanofluid flow in a porous foam is more uniform with a decreased dimensionless temperature. The pressure drop of the nanofluid increases with an increase in the concentration of the nanoparticles. By employing foam metals and nanofluid, the cross-sectional temperature becomes closer to the wall temperature. Simultaneously, notable difference between solid and fluid temperatures can be observed, revealing the LTNE effect of the nanofluid on the porous foam. It is found that the Nusselt number first increases and then decreases with an increase in nanoparticle concentration. Furthermore, the Nusselt number decreases with an increase in the foam porosity. It is found that the thermal performance of a nanofluid in a plain tube is different from that in the foam metals.     

Vehicle-wet snow interaction: Testing,modeling and validation

For a vehicle interacting with snow, whether dry or wet, uncertainties exist in the mechanical properties of snow, and in the interfacial properties between the tires of the vehicle and snow. For dry snow, these uncertainties have been studied recently using methods within a statistical framework employing a simple stochastic tire-snow interaction model and several validation metrics. Wet snow is more complicated and much less studied than dry snow, especially for tire-snow interaction. In this paper, the authors used a physical tire-snow interaction model and a similar statistical framework as was used to analyze dry snow, and presented results of calibration and validation of the interaction model for wet snow in conjunction with new test data based on a single test run with the assumption that it would provide needed sampling points for statistical analysis. Four local and global statistical validation metrics were used to assess the physical and statistical models with good results. Comparison between wet and dry snow, based on a single test run, shows that the former has a lower interfacial coefficient of friction, and a higher drawbar pull than the latter.     

The influence of cooling rates on the distribution of the structure and phases in hardened cylindrical elements

Summary The paper presents an analysis of the temperature field and phase transformation kinetics for cylindrical steel elements with arbitrary shaped cross sections. The influence of different cooling rates as well as different time intervals to achieve the half of the full austenite into pearlite transformation τ_0.5 and weight fractions of pearlite and martensite are taken into account. The analysis is based on Lomakin's theory [2, 3, 4] and its modification [5] as well as the laws of phase transformation kinetics of [9]. The elaborated program of numerical calculations refers to steel grades characterized by C-shaped T-T-T curves and with carbon content close to that of an eutectoidal steel. The analysis is made for a two-dimensional region. As an example, a spline shaft with arbitrary shaped cross section is investigated. It is assumed that the heat exchange occurs on the boundary of the cross section alone. The solution of the problem is based on the variational difference method, being a combination of the finite element method and the finite difference method. Accepted for publication 6 August 1996     

Film cooling and heat transfer from a combination of two rows of simple and/or compound angle holes in inline and/or staggered configuration

This paper describes the results of an experimental investigation into the film cooling effectiveness and heat transfer characteristics of two staggered injection rows of either a combination of one row of simple angle holes with another row of compound angle holes or with both rows of compound angle holes. The effect of using various injections holes arrangements as well as the relative location of the compound angle holes row to the simple angle holes row have been investigated for different blowing rates. Using combination of one row of downstream compound angle holes with another upstream simple injection holes row provides a significant increase in the film cooling protection over a flat plate surface, over that obtained from either two rows of only simple injections holes or compound angle holes. Received on 17 July 1998     

The thermoconvective instability of plane poiseuille flow heated from below: A proposed benchmark solution for open boundary flows

The transient two-dimensional Navier-Stokes and energy equations have been solved numerically for flow in a horizontal channel heated from below in the Boussinesq limit. For the set of dimensionless parameters chosen, the flow consists of periodic transverse travelling waves resulting from a convective instability. The solution is proposed as a benchmark for the application of outflow boundary conditions (OBC) in time-dependent flows with strong buoyancy effects. Richardson extrapolation in both time and space is used in obtaining the solution. Field plots and profiles of velocity, temperature, vorticity and streamfunction at selected axial positions and times are also presented from the finest grid and smallest time step calculation. The calculations have been made on an extended domain so that the effects of OBC used in the present study would be negligible in the test region.     

3-D analysis of temperature distribution in the material during pulsed laser and material interaction

Recently, lasers are being increasingly used in the industry owing to their precision and low cost. Material is heated and evaporated during laser and material interaction due to the absorption of laser beams by the material. In this study, a 3-D Laser heating model including evaporation has been solved using the electron- kinetic theory approach. The basis in examining the problem using the kinetic theory approach is to describe the heat conduction through electron-phonon and molecule-phonon collisions. The problem is solved by using the electron-kinetic theory approach in such a way that heat conduction is taken into account until the material is heated to its melting temperature and non-conduction limited heat transfer is considered after the melting temperature is reached. Non-conduction limited heat transfer through the phase change process is resulted from vacancy-molecule collisions. A numerical scheme is introduced to solve the governing equation, owing to the fact that the energy equation resulted is in the form of integro-differential equation. Four different materials, namely iron, nickel, tantalum and titanium are chosen in this study determine the material response to laser pulse heating. For each material, time dependent temperature distribution through the depth of the material and on the surface of the material is computed and analyzed for four different materials.     

Film cooling on a convex surface: influence of external pressure gradient and Mach number on film cooling performance

 The film cooling performance on a convex surface subjected to zero and favourable pressure gradient free-stream flow was investigated. Adiabatic film cooling effectiveness values were obtained for five different injection geometries, three with cylindrical holes and two with shaped holes. Heat transfer coefficients were derived for selected injection configurations. CO₂ was used as coolant to simulate density ratios between coolant and free-stream close to gas turbine engine conditions. The film cooling effectiveness results indicate a strong dependency on the free-stream Mach number level. Results obtained at the higher free-stream Mach number show for cylindrical holes generally and for shaped holes at moderate blowing rates significant higher film cooling effectiveness values compared to the lower free-stream Mach number data. Free-stream acceleration generally reduced adiabatic film cooling effectiveness relative to constant free-stream flow conditions. The different free-stream conditions investigated indicate no significant effects on the corresponding heat transfer increase due to film injection. The determined heat flux ratios or film cooling performance indicated that coolant injection with shaped film cooling holes is much more efficient than with cylindrical holes especially at higher blowing rates. Heat flux penalties can occur at high blowing rates when using cylindrical holes. Received on 29 May 2000     

Suspensions of titania nanoparticle networks in nematic liquid crystals: rheology and microstructure

We study the influence of confinement on the rheology and structure of nematic liquid crystals (NLCs). NLCs get confined in networks of titania (TiO₂, primary particle size = 21 nm) nanoparticles in suspensions of TiO₂ and NLC, N-(4-methoxybenzylidene)-4-butylaniline (MBBA). Suspensions with TiO₂ nanoparticle volume fraction (?) of 0.006–0.017, form viscoelastic solids with low elastic modulus (G′) of 10¹ Pa–10² Pa and short relaxation times. Increase in TiO₂ nanoparticle ? leads to a rise in G′ with TiO₂ nanoparticles forming a percolating network at a critical volume fraction (? _c) = 0.023, and G′ of ~10³ Pa. TiO₂/MBBA NLC suspensions at and above ? _c = 0.023 show G′ ~ ω ^x?1 scaling, where ω is the angular frequency and the minimum in loss modulus (G′′) with ω. The effective noise temperature, x decreases and approaches 1 with the increase in the TiO₂ nanoparticle ? from 0.023–0.035, is indicative of an increase in the glassy dynamics. Through the polarized light microscopy and differential scanning calorimetry experiments, we propose that the progressive addition of TiO₂ nanoparticles introduces a quenched random disorder (QRD) in the NLC medium which disturbs the nematic order. This results in metastable TiO₂/MBBA NLC suspensions in which NLC domains get confined in the network of flocs of TiO₂ nanoparticles. We also show that the salient rheological signatures of soft glassy rheology develop only in the presence of NLC MBBA and are absent in the isotropic phase of MBBA.