Generation of chiral asymmetry via helical magnetic fields

Fluid flow, heat transfer and entropy generation characteristics of micro-pipes are investigated computationally by considering the simultaneous effects of pipe diameter, wall heat flux and Reynolds number in detail. Variable fluid property continuity, Navier-Stokes and energy equations are numerically handled for wide ranges of pipe diameter (d = 0.50–1.00 mm), wall heat flux (q''= 1000–2000 W/m2) and Reynolds number (Re = 1 – 2000), where the relative roughness is kept constant at e/d = 0.001 in the complete set of the scenarios considered. Computations indicated slight shifts in velocity profiles from the laminar character at Re = 500 with the corresponding shape factor (H) and intermittency values (γ) of H = 3.293→3.275 and γ = 0.041→0.051 (d = 1.00→0.50 mm). Moreover, the onset of transition was determined to move down to Retra = 1,656, 1,607, 1,491, 1,341 and 1,272 at d = 1.00, 0.90, 0.75, 0.60 and 0.50 mm, respectively. The impacts of pipe diameter on friction mechanism and heat transfer rates are evaluated to become more significant at high Reynolds numbers, resulting in the rise of energy loss data at the identical conditions as well. In cases with low pipe diameter and high Reynolds number, wall heat flux is determined to promote the magnitude of local thermal entropy generation rates. Local Bejan numbers are inspected to rise with wall heat flux at high Reynolds numbers, indicating that the elevating role of wall heat flux on local thermal entropy generation is dominant to the suppressing function of Reynolds number on local thermal entropy generation. Cross-sectional total entropy generation is computed to be most influenced by pipe diameter at high wall heat flux and low Reynolds numbers.

Magnetohydrodynamic (MHD) systems can be strongly nonlinear (turbulent) when their kinetic and magnetic Reynolds numbers are high, as is the case in many astrophysical and space plasma flows. Unfortunately these high Reynolds numbers are typically much greater than those currently attainable in numerical simulations of MHD turbulence. A natural question to ask is how can researchers be sure that their simulations have reproduced all of the most influential physics of the flows and magnetic fields? In this paper, a metric is defined to indicate whether the necessary physics of interest has been captured. It is found that current computing resources will typically not be sufficient to achieve this minimum state metric.

Two spectral regimes of magnetic field amplification in magnetohydrodynamic (MHD) flows can be distinguished by the scale on which fields are amplified relative to the primary forcing scale of the turbulence. For field amplification at or below the forcing scale, the amplification can be called a “small-scale dynamo.” For amplification at and above the forcing scale the process can be called a “large-scale dynamo.” Non - local (in wave number) effects play a key role in both the growth of the small-scale field in non-helical turbulence and the growth of large and smallscale fields in helical turbulence. Mean field dynamo (MFD) theory represents a simple semi-analytic way to get a handle on large-scale field amplification in MHD turbulence. Helicity has long been known to be important for large scale, flux generating, externally forced MFDs. The extent to which such MFDs operate “slow” or “fast” (dependent or independent on magnetic Reynolds number) has been controversial, but there has been recent progress. Simulations of α2 dynamos in a periodic box dynamo and their quenching can now be largely understood within a simplified dynamical non-linear paradigm in which the MFD growth equation is supplemented by the total magnetic helicity evolution equation. For α2 dynamos, the large-scale field growth is directly related to the large-scale magnetic helicity growth. Magnetic helicity conservation then implies that growth of the large-scale magnetic helicity induces growth of small-scale magnetic (and current) helicity of the opposite sign, which eventually suppresses the α effect driving the MFD growth. Although the α2 MFD then becomes slow in the long time limit, substantial large-scale field growth proceeds in a kinematic, “fast” phase before non-linear asymptotic quenching of the “slow” phase applies. Ultimately, the MFD emerges as a process that transfers magnetic helicity between small and large scales. How these concepts apply to more general dynamos with shear, and open boundary dynamos is a topic of ongoing research. Some unresolved issues are identified. Overall, the following summarizes the most recent progress in mean-field dynamo theory: For a closed turbulent flow, the non-linear mean field dynamo, is first fast and kinematic, then slow and dynamic, and magnetic helicity transfer makes it so.KeywordsMagnetic EnergyMagnetic HelicityMagnetic Reynolds NumberDynamo TheoryForce ScaleThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Local and global effects of cylindrical vortex generators on the mass-transfer distributions over the four active walls of a square, rib-roughened rotating duct with a sharp 180-deg bend are investigated. Cylindrical vortex generators (rods) are placed above, and parallel to, every other rib on the leading and trailing walls of the duct so that their wake can interact with the shear layer and recirculation region formed behind the ribs, as well as the rotation-generated secondary e ows. Local increases in near-wall turbulence intensity resulting from these interactions give rise to local enhancement of mass (heat)transfer. Measurements are presented for duct Reynolds numbersReintherange5 £ 10 3‐3 £ 10 4 andforrotationnumbersin therange0 ‐0.3. Therib height-to-hydraulic diameter ratio e/Dh is e xed at 0.1, and the rib pitch-to-rib height ratio P/e is 10.5. The vortex generator rods have a diameter-to-rib height ratio d/e of 0.78, and the distance separating them from the ribs relative to the rib height s/e is 0.55. Mass-transfer measurements of naphthalene sublimation have been carried out using an automated acquisition system and are correlated with heat transfer using the heat/mass transfer analogy. The results indicate that the vortex generators tend to enhance overall mass transfer in the duct, compared to the case where only ribs are present, both before and after the bend at high Reynolds and rotation numbers. Local enhancements of up to 30%are observed on all fourwalls oftheduct. At low Reynolds numbers, forexample, 5 ££ 10 3 , the insertion of the rods often leads to mass-transfer degradation. At high Reynolds numbers, for example, 3 ££ 10 4 , the enhancement due to the rods occurs on the surfaces stabilized by rotation (trailing edge on the inlet pass and leading edge on the outlet pass )and the side walls. The enhancement is more pronounced as the rotation number is increased. The detailed measurements in a ribbed duct with vortex-generator rods clearly show localized regions of enhanced mass (heat) transfer at Reynolds and rotation numbers within the envelope of practical interest for gas-turbine blade cooling applications.

The application of the Kutta condition to unsteady flows has been controversial over the years, with increased research activities over the 1970s and 1980s. This dissatisfaction with the Kutta condition has been recently rejuvenated with the increased interest in low-Reynolds-number, high-frequency bio-inspired flight. However, there is no convincing alternative to the Kutta condition, even though it is not mathematically derived. Realizing that the lift generation and vorticity production are essentially viscous processes, we provide a viscous extension of the classical theory of unsteady aerodynamics by relaxing the Kutta condition. We introduce a trailing-edge singularity term in the pressure distribution and determine its strength by using the triple-deck viscous boundary layer theory. Based on the extended theory, we develop (for the first time) a theoretical viscous (Reynolds-number-dependent) extension of the Theodorsen lift frequency response function. It is found that viscosity induces more phase lag to the Theodorsen function particularly at high frequencies and low Reynolds numbers. The obtained theoretical results are validated against numerical laminar simulations of Navier–Stokes equations over a sinusoidally pitching NACA 0012 at low Reynolds numbers and using Reynolds-averaged Navier–Stokes equations at relatively high Reynolds numbers. The physics behind the observed viscosity-induced lag is discussed in relation to wake viscous damping, circulation development and the Kutta condition. Also, the viscous contribution to the lift is shown to significantly decrease the virtual mass, particularly at high frequencies and Reynolds numbers.

Introduction I wing design it is very important to determine the maximum lift coefficient as accurately as possible, since this lift coefficient corresponds to the stall speed that is the minimum speed at which controllable flight can be maintained. Any further increase in angle of incidence will increase flow separation on the wing upper surface, resulting in a loss of lift and a large increase in drag. Despite the significant advances in computational fluid dynamics, our ability to predict the maximum lift coefficient and poststall behavior of single airfoils, wings, multielement airfoils, or wings is still not satisfactory. The chief culprit is the lack of an accurate turbulence model to represent flows with extensive separation. Inaccuracies of numerical solutions of the conservation equations at these flow conditions, as well as difficulties in modeling of flow near the trailing edge of an airfoil or wing, add to this dilemma. Recently Cebeci et al. described an interactive boundary-layer method, together with the e approach to the calculation of transition, for predicting stall and poststall behavior of airfoils at low and high Reynolds numbers. The turbulence model was based on the Cebeci-Smith algebraic eddy-viscosity formulation with improvements for strong pressure gradient effects and transitional flows at low Reynolds numbers. Comparison of calculated results for incompressible flows indicated good agreement with experiment for a wide range of Reynolds numbers. Preliminary calculations for low-Mach-number flows with this interactive method that applies compressibility corrections to the panel method also indicated good agreement with data and showed that at a Mach number of 0.30, the compressibility effect on (Q)max was not negligible. The ability of the calculation method of Ref. 1 to predict the effect of ice on airfoil stall at high Reynolds numbers is examined here, and the results are presented in the following section. The interactive boundary-layer method previously developed for iced airfoils and coupled to the LEWICE code for predicting ice shapes' is replaced with the calculation method of Ref. 1. Appropriate changes are made to this method, including roughness effects due to ice, and the results are presented in the following section.

We study laminar forced convection mass transfer to single layer arrays of equidistantly and non-equidistantly spaced micro-spheres. We report average Sherwood numbers as a function of geometry and flow conditions, for open frontal area fractions between 0.04 and 0.95, Schmidt numbers between 0.7 and 10, and Reynolds numbers (based on micro-sphere diameter and the free stream velocity) between 0.1 and 100. For equidistantly spaced arrays of micro-spheres we propose a general analytical expression for the average Sherwood number as a function of the Reynolds number, Schmidt number and the open frontal area fraction, as well as asymptotic scaling rules for small and large Reynolds. For all studied Schmidt numbers, equidistant arrays exhibit decreasing average Sherwood numbers for decreasing open frontal area fractions at low Reynolds numbers. For high Reynolds numbers, the Sherwood number approaches that of a single spheres in cross-flow, independent of the open frontal area fraction. For equal open frontal area fractions, the Sherwood number in non-equidistant arrays is lower than in equidistant arrays for intermediate Reynolds numbers. For very low and high Reynolds numbers, non-uniformity does not influence mass transfer.

Low Reynolds number flow through nozzle-diffuser elements in valveless micropumps

This paper incorporates concepts of unimodal and bimodal sedimentation to develop a model that accurately predicts bed expansion during particulate fluidization. During bed expansion a particle is considered to be fluidized not by the pure fluid, but by a slurry consisting of the pure fluid and other surrounding particles. The contributions of the other surrounding particles to the additional buoyant and drag forces are accounted for with the use of effective fluid or slurry properties, density and viscosity. As bed expansion proceeds, influences of the surrounding particles decrease; therefore, these effective properties are functions of the changing void fraction of the suspension. Furthermore, the expansion index, which empirically represents the degree to which viscous and inertial forces are present, is traditionally a function of a constant terminal Reynold's number. Because the effective fluid properties are considered to be changing as fluidization proceeds, the degree to which viscous and inertial forces also changes; therefore, the expansion index is written as a function of a local or intermediate Reynold's number. These concepts are further extended to bimodal fluidization in which small or light particles aid in the fluidization of the large or heavy particles. The results indicate that the proposed model more accurately predicts particulate bed expansion for a wider range of systems (gas -- liquid, low Reynold's number -- high Reynold's number) than other analytical or empirical models.

Spray drying is a widely used unit operation for producing particulate products directly from a liquid feed. Important processes that occur inside the spray dryer are droplet formation, droplet drying and interactions between droplets and recycled fines. Various studies have looked at the first two processes, but the latter phenomenon has received less attention. Literature on droplet-particle interaction which aims at quantitatively describing agglomeration in spray drying is scarce and mainly qualitative. For product quality the formation of agglomerates is often desirable. This thesis models and investigates the collisions of individual particles with single droplets. The surface tack of drying droplets has been identified as an important variable for the formation of agglomerates. In this thesis a novel method for measuring tack from the liquid phase has been further improved. The improvements are a more accurate load measurement, an automated control of the tack probe and an improved layout of the sample holder and probe. The key feature of the device is its ability to measure tack of drying droplets, whereas other devices measure tack by wetting a powder. Using our method the tack of a commonly spray dried product, yeast extract, has been measured. From these experiments it was found that with decreasing average moisture content the surface tack increases to a maximum. Below a critical average moisture content the surface of the droplet is dry and the tack rapidly decreases upon further drying. Another important parameter in determining the degree of agglomeration is the degree of penetration. If the particle penetrates the droplet too deeply, the agglomerate structure becomes too dense. To predict the penetration depth, a non-dimensional model has been developed. It describes the penetration of a particle into a liquid droplet during a head-on collision. It is based on a force balance and incorporates surface tension force, viscous force and capillary pressure force. The important parameters determining the collision outcome are the contact angle, the size of the droplet relative to the particle, the Reynolds and Weber numbers. For each contact angle an equilibrium penetration position exists, at this point the surface tension force vector is perpendicular to the penetration direction. Five different penetrations regimes are identified. At low Reynolds numbers, viscous forces dominate and the particle asymptotically travels towards the equilibrium position. Reducing the viscous drag force by increasing the Reynolds number results in initially overshooting the equilibrium position, but the surface tension force pulls the particle back, to attain the equilibrium in an oscillating motion. At even higher Reynolds numbers the particle fully penetrates the droplet, and reaches the centre of the droplet for even higher values for the Reynolds number. The ejection regime is found at high Reynolds number and low Weber numbers and the liquid should be non-wetting. Using the regime maps one is able to identify in which region a spray dryer is operating. Although the full penetration regimes are useful for capturing fines, it should be avoided when agglomeration is desired. The ejection regime should be avoided as well. To validate the model, impact experiments were carried out by dropping glass spheres on the surface of different liquids. These validation experiments were the first attempt to experimentally validate the collision of a single particle with a liquid surface. Besides yeast extract, which has non-Newtonian rheological properties, silicone oils with constant viscosities of 100 mPa•s and 1 Pa•s have been tested. The penetration over time for different impact velocities was determined by analysing high speed camera recordings. The typical penetration times ranged from 0.2 s to 2 s. To obtain accurate location data was recorded at frame rates up to 38 000 frames per second. Glass spheres, with a size of 2 mm were used to allow the visual tracking. Modelling the impacts showed that the model consistently predicted faster penetration times than were observed experimentally. The relative difference increased with increasing viscosity. A parameter fitting exercise showed that better agreement could be obtained by using a higher viscosity and a higher contact angle in the model. With this knowledge the most likely factor influencing the model-experiment mismatch was identified as being the dynamics of wetting of the particle surface. It was also found that using the dynamic contact angle in the model would improve its results. The non-Newtonian characteristics of the yeast extract resulted in the particle rebound and the formation of an air cavity upon impact. The tack measurement technique and penetration model presented in this thesis will be useful tools for the design of spray dryers. Recommendations are made for further model improvement. The experimental validation is the first attempt to validate the presented model. Future improvements are recommended and suggestions are presented.

Effect of employing a new biological nanofluid containing functionalized graphene nanoplatelets on thermal and hydraulic characteristics of a spiral heat exchanger

It is widely acknowledged that the spacer grid has great effect on heat transfer downstream of it. The conventional correlations to predict the augmentation of the spacer were carried out on high Reynolds numbers. However, recent studies have shown that Reynolds number on the heat transfer enhancement is not negligible when the Reynolds number is lower than about 10000. An experiment to investigate the single-phase convective heat transfer downstream of the spacer grid at low flow rate has been performed in a 5×5 rod bundle. The test section was uniformly heated by a DC power and cooled by water. The Reynolds number covered from about 2000 to 10000. The experiment showed that the existing correlations for heat transfer enhancement by a spacer grid underestimated the maximum enhancement at the grid exit of the spacer grid at low Reynolds numbers. As the Reynolds number decreases, the maximum enhancement increases, nevertheless, when Reynolds number decreases to about 4300, the maximum enhancement tend to converge at a certain value. A new correlation has been proposed to account for the Reynolds number effect on heat transfer enhancement downstream of the spacer grid at low Reynolds numbers and which gave good predictions.

Two sets of model tests were conducted at the facilities of the Offshore Technology Corporation (OTC) to determine the hydrodynamic forces and moments exerted on ships stationed in a current. The first set of tests which were executed in 1981, involved a large drill ship in deep water [l]. The tests were conducted in a conventional towing basin. A wide range of current speeds and yaw angles was investigated. A Reynold's Number based upon ship breadth of 5 × 105 was achieved at 1arge yaw angles. In 1983, the development of a unique current generating system in the OTC Wave and Current Basin was completed. At the first opportunity after calibration of this system, a second set of internal R & D tests were executed on a VLCC in shallow water only and only at a yaw angle of 90 degrees with respect to the direction of the current. Current speed was varied and? Reynold's Number based upon ship breadth of 7 × 105 was achieved. The side force coefficients and yaw moment coefficients derived from these tests were compared with the results of previous similar tests on a model of the drill ship WIMPEY SEALAB conducted at the Nationa1 Physica1 Laboratory in U.K. and reported by English and Wise in [2]. The more recent results were also compared with the results of tests in restricted water depths by the Netherlands Ship Model Basin between 1968 and 1976 on tanker hulls and which are reported in 1976 by the Oil Companies Internatiana1 Marine Forum [3]. The tests reported in [2] and [3] were conducted at Reynolds Numbers (based upon vessel breadth) below 4 × 10' The side force coefficients for these tests were found to be 75% higher than those derived from the more recent tests at the higher Reynold's Number. Furthermore on the basis of limited data, it appears that the strong dependence of side force coefficient upon the ratio of draft to water depth found in the low Reynold's Number, N.S.M.B. tests reported in [3] is not found at higher Reynold's Number. For example, the side force coefficient presented in the OCIMF report for a typical tanker with a bulbous bow at a yaw angle of 90 degrees and with a water depth to draft ratio of 1.67 was 1.5. Under nearly similar conditions at a Reynold's Number of 6.6 × 105, that coefficient was 0.95. The ratio of side force coefficient for very deep water to that for a depth to draft ratio of 1.67 was 1.2. The results of this initial investigation suggest a strong impact of the Reynold's Number on the validity of estimates of current induced forces on highly yawed ship shaped bodies derived from model experiments. Since these coefficients are vital to the development of moored and dynamically positioned vessel response further investigations appear to be justified.

The configuration and the performance of louvered fins for compact automotive radiators is a subject matter widely studied [1–7]. The effect of buoyancy on the performance of these fins, however, is assumed to be negligible because fanned radiators normally operate at higher Reynolds numbers. This paper explores the possibility of radiators operating with acceptable performance at lower Reynolds numbers and presents a series of scaling simulations to measure the effect of buoyancy force on the performance of plain and louvered fins. The simulations are generated from a finite element model of a single fin with attached base surfaces and periodic boundary conditions. Transient calculations are performed until the model reaches steady or quasi-steady state, at which point the effect of buoyancy force is quantified for Reynolds numbers between 45 and 2,179 and Froude numbers between 0.4 and 18.9 by intentionally varying the inlet velocity and the gravity constant. At higher Reynolds numbers, the result is consistent with the published reports in the literature, validating the model to an extent. At lower Reynolds numbers, the result shows that the Stanton number scales with Froude numbers, indicating that natural convection is a significant force of the heat transferred. Natural convection is observed coming from the bottom base surface of the fin rather than the fin itself. As the fin is vertical while the base surface horizontal, this effect depends on the orientation of the radiator. On the amount of heat transferred, Nusselt number has a local saddle point at the transition between natural convection and force convection (Fr = 3 ~ 7). The performance at this point is lower because airflow pattern for each mode of convection differs and, at the transition, neither mode of convection dominates the airflow.

Numerical study on the momentum and heat transfer of porous spheroids under laminar flow