Search

Blood-borne bacteria disseminate in tissue through microvasculature or capillaries. Capillary size, presence of red blood cells (RBCs), and bacteria motility affect bacteria intracapillary transport, an important yet largely unexplored phenomenon. Computational description of the system comprising interactions between plasma, RBCs, and motile bacteria in 5–10 μ m diameter capillaries pose several challenges. The Immersed Boundary Method (IBM) was used to resolve the capillary, deformed RBCs, and bacteria. The challenge of disparate coupled time scales of flow and bacteria motion are reconciled by a temporal multiscale simulation method. Bacterium-wall and bacterium-RBC collisions were detected using a hierarchical contact- detection algorithm. Motile bacteria showed a net outward radial velocity of 2.8 µm/s compared to −0.5 µm/s inward for non-motile bacteria; thus, exhibiting a greater propensity to escape the bolus flow region between RBCs and marginate for potential extravasation, suggesting motility enhances extravasation of bacteria from capillaries. [ABSTRACT FROM AUTHOR]

Heat transfer characteristics of random suspensions of 0.25 aspect ratio (AR) cylinders are investigated for Reynolds numbers (Re) between 10 and 300 and solid fraction (φ) ranging from 0.1 to 0.3 using particle resolved simulations. The effect of particle inclination with respect to flow and particle clustering on heat transfer is investigated. The Nusselt number decreases with an increase in inclination angle and the dependence becomes stronger as φ and Re increase. On the other hand, while prolate ellipsoid suspensions of AR 2.5 follow the same trend, the Nusselt number increases with inclination angle as AR increases to 5 and 10 and as φ increases. Local particle clustering nominally decreases the Nusselt number because of the dominance of thermal wakes. At low φ⁠, this effect is felt only at low Re⁠, but as φ increases, the effect spreads to higher Re⁠. Similar but weaker trends are also found in suspensions of prolate ellipsoids of AR 2.5, 5, and 10. High AR, low Re prolate ellipsoids exhibit the greatest dependence of Nusselt number on local solid fraction. Implementation of two independent definitions of reference length, i.e., volume equivalent sphere diameter deq for ellipsoids and diameter dp of the cylindrical particle in the correlation of Tavassoli et al. (2015, "Direct Numerical Simulation of Fluid-Particle Heat Transfer in Fixed Random Arrays of Non-Spherical Particles," Chem. Eng. Sci., 129, pp. 42-48) provides good estimates of the respective suspension mean Nusselt numbers. [ABSTRACT FROM AUTHOR]

The response of a rigid flapping wing in forward flight, at Re = 10,000, subjected to frontal gusts has been investigated. The phasing and duration of the gusts and their impact on the various unsteady mechanisms are analyzed within a single flapping cycle. The gust is characterized by a step function with integral length scale much larger than that of the physical dimension of the micro air vehicle and with time scale much smaller than the flapping time period. The instantaneous lift and thrust profiles were observed to be influenced by a combination of the effective angle of attack, wing rotation, and the leading-edge vortex structures existing in the flow at any given time, with the leading-edge vortices themselves being influenced by the duration and magnitude of the change in effective angle of attack. Frontal gusts applied during the downstroke accelerated the development of the flow, resulting in the formation and detachment of multiple leading-edge vortices on the wing surface that increased the lift and thrust, illustrating the importance of the leading-edge vortex dynamics to force production. The effect of the gust is observed to be diminished when it occurs during rapid supination of the wing. The lift and thrust profiles are found to react in a similar fashion for gusts applied during the downstroke, whereas they experienced opposite effects during the upstroke. During the upstroke, force characteristics are shown to primarily react to effective angle-of-attack changes more than to changes in flow structures. DOI: 10.2514/1.J050263

In bird and insect flight, wing deformation plays an important role in aerodynamic performance. The wing deformation is produced by neuromuscular control and/or by aeroelastic effects. The focus of the current study is to evaluate the effects of wing deformation by coupling a large-eddy simulation solver with a linear elastic membrane model. Different membrane prestresses are investigated to give a desired camber in response to the aerodynamic pressure. All simulations are carried out at Re = 10,000 for forward flight with an advance ratio of 0.5. The results show that the camber introduced by a flexible wing increases the thrust and lift production considerably. An analysis of flow structure reveals that, for flexible wings, the leading-edge vortex stays attached on the top surface of the wing and glides along the camber and covers a major part of the wing, which results in high force production. On the other hand, for rigid wings, the leading-edge vortex lifts off from the surface resulting in lower force production. Further, introduction of the camber also increases the force component contributing to thrust, leading to a high thrust-to-lift ratio. In comparison to a rigid wing, a 40% increase in thrust is observed for the low-prestress case, which results in a camber of 0.25 chord. Further, the results also show that the wing with high spanwise prestress and low chordwise prestress offers better performance both in terms of force production and uniformity in the force-induced cambering. DOI: 10.2514/1.39957

Unsteady aerodynamics of a rigid flapping wing at a Reynolds number of 10,000 for forward flight with an advance ratio of 0.5 is analyzed. A spiral leading-edge vortex with a strong spanwise flow along its core is formed during the downstroke, resulting in a peak lift and thrust. A negative spanwise flow formed due to the tip vortex prevents the removal of vorticity from the leading-edge vortex, leading to instability and separation of the leading-edge vortex. Analysis of different rotation timings shows that supination results in the leading-edge vortex formation near the base and its strength depends on the flapping velocity. A stronger vortex is formed for advanced rotation and it generates high lift. Delayed rotation affects thrust production during translation and results in low propulsive efficiency. Analysis of rotation duration shows that shorter rotation results in high instantaneous lift values, whereas continuous long-duration rotation results in high thrust and propulsive efficiency. Analysis of different angles of attack show that a moderate angle of attack, which results in a high thrust-to-lift ratio and complete shedding of the leading-edge vortex at the end of translation, is required for high propulsive efficiency. DOI: 10.2514/1.37540

Large eddy simulation calculations are conducted for flow in a channel with dimples and protrusions on opposite walls with both surfaces heated at three Reynolds numbers, [Re.sub.H] = 220, 940, and 9300, ranging from laminar, weakly turbulent, to fully turbulent, respectively. Turbulence generated by the separated shear layer in the dimple and along the downstream rim of the dimple is primarily responsible for heat transfer augmentation on the dimple surface. On the other hand, augmentation on the protrusion surface is mostly driven by flow impingement and flow acceleration between protrusions, while the turbulence generated in the wake has a secondary effect. Heat transfer augmentation ratios of 0.99 at [Re.sub.H] = 220, 2.9 at [Re.sub.H] = 940, and 2.5 at [Re.sub.H] = 9300 are obtained. Both skin friction and form losses contribute to pressure drop in the channel. Form losses increase from 45% to 80% with increasing Reynolds number Friction coefficient augmentation ratios of 1.67, 4.82, and 6.37 are obtained at [Re.sub.H] = 220, 940, and 9300, respectively. Based on the geometry studied, it is found that dimples and protrusions may not be viable heat transfer augmentation surfaces when the flow is steady and laminar

Detailed investigation of film cooling for a cylindrical leading edge is carried out using large eddy simulation (LES). The paper focuses on the effects of coolant to mainstream blowing ratio on flow features and, consequently, on the adiabatic effectiveness and heat transfer coefficient. With the advantage of obtaining unique, accurate, and dynamic results from LES, the influential coherent structures in the flow are identified. Describing the mechanism of jet-mainstream interaction, it is shown that as the blowing ratio increases, a more turbulent shear layer and stronger mainstream entrainment occur. The combined effects lead to a lower adiabatic effectiveness and higher heat transfer coefficient. Surface distribution and span-averaged profiles are shown for both adiabatic effectiveness and heat transfer (presented by Frossling number). Results are in good agreement with the experimental data of Ekkad et al. [1998, 'Detailed Film Cooling Measurement on a Cylindrical Leading Edge Model: Effect of Free-Steam Turbulence and Coolant Density,' ASME J. Turbomach., 120, pp. 799-807].

The problem of accurately predicting the flow and heat transfer in the ribbed internal cooling duct of a rotating gas turbine blade is addressed with the use of large eddy simulations (LES). Four calculations of the developing flow region of a rotating duct with ribs on opposite walls are used to study changes in the buoyancy parameter at a constant rotation rate. The Reynolds number is 20,000, the rotation number is 0.3, and the buoyancy parameter is varied between 0.00, 0.25, 0.45, and 0.65. Previous experimental studies have noted that leading wall heat transfer augmentation decreases as the buoyancy parameter increases with low buoyancy, but heat transfer then increases with high buoyancy. However, no consistent physical explanation has been given in the literature. The LES results from this study show that the initial decrease in augmentation with buoyancy is a result of larger separated regions at the leading wall However, as the separated region spans the full pitch between ribs with an increase in buoyancy parameter, it leads to increased turbulence and increased entrainment of mainstream fluid, which is redirected toward the leading wall by the presence of a rib. The impinging mainstream fluid results in heat transfer augmentation in the region immediately upstream of a rib. The results obtained from this study are in very good agreement with previous experimental results. [DOI: 10.1115/1.2437779]

A ribbed square duct (P/e= 10, e/[D.sub.h]=0.10) subjected to sand ingestion is studied using large-eddy simulations (LES). Particle sizes of 10 [micro]m, 50 [micro]m, and 100 [micro]m with nondimensional response times (normalized by friction velocity and hydraulic diameter) of 0.06875, 1.71875, and 6.875, respectively are considered. The calculations are performed for a nominal bulk Reynolds number of 20,O00 under fully-developed conditions. Distributions of impingement density, impingement velocities and angles, together with fractional energy transfer are presented for each surface. It is found that about 40% of the total number of 10 micron particles are concentrated in the vicinity (within 0.05 Oh) of the duct surfaces, compared to 25-30% of the 50 and 100 micron particles. The 10 micron particles are more sensitive to the primary and secondary flow velocities than the larger particles. While the 10 micron particles exhibit high energy transfer to the surface near the rib side-wall junction and immediately upstream of the rib, the larger particles exhibit more uniform distributions. The largest fraction of incoming particulate energy is transferred to the front face of the rib and is between one to two orders of magnitude larger than the other surfaces. As particle size increases, substantial particle energy is also transferred to the back face of the rib by particles bouncing off the front face and carrying enough momentum to impinge on the back face of the preceding rib. [DOI: 10.1115/1.2720509]

A nonlocal pressure equation is derived from mean-field free energy theory for calculating liquid-vapor systems. The proposed equation is validated analytically by showing that it reduces to van der Waals' square-gradient approximation under the assumption of slow density variations. The proposed nonlocal pressure is implemented in the mean-field free energy lattice Boltzmann method (LBM). The LBM is applied to simulate equilibrium liquid-vapor interface properties and interface dynamics of capillary waves and oscillating droplets in vapor. Computed results are validated with Maxwell constructions of liquid-vapor coexistence densities, theoretical relationship of variation of surface tension with temperature, theoretical planar interface density profiles, Laplace's law of capillarity, dispersion relationship between frequency and wave number of capillary waves, and the relationship between radius and the oscillating frequency of droplets in vapor. It is shown that the nonlocal pressure formulation gives excellent agreement with theory.

The capabilities of the detached eddy simulation (DES) and the unsteady Reynolds averaged Navier-Stokes (URANS) versions of the 1988 k-[omega] model in predicting the turbulent flow field in a two-pass internal cooling duct with normal ribs is presented. The flow is dominated by the separation and reattachment of shear layers; unsteady vorticity induced secondary flows and strong streamline curvature. The techniques are evaluated in predicting the developing flow at the entrance to the duct and downstream of the 180 deg bend, fully developed regime in the first pass, and in the 180 deg bend. Results of mean flow quantities, secondary flows, and the average friction factor are compared to experiments and large-eddy simulations (LES). DES predicts a slower flow development than LES, whereas URANS predicts it much earlier than LES computations and experiments. However, it is observed that as fully developed conditions are established, the capability of the base model in predicting the flow is enhanced by the DES formulation. DES accurately predicts the flow both in the fully developed region as well as the 180 deg bend of the duct. URANS fails to predict the secondary flows in the fully developed region of the duct and is clearly inferior to DES in the 180 deg bend. [DOI: 10.1115/1.2353279]

Large eddy simulation of the 180 deg bend in a stationary ribbed duct is presented. The domain studied includes three ribs upstream of the bend region and three ribs downstream of the bend with an outflow extension added to the end, using a total of 8.4 million cells. Two cases are compared to each other: one includes a rib in the bend and the other does not. The friction factor, mean flow, turbulence, and heat transfer are compared in the two cases to help explain the benefits and disadvantages of the wide number of flow effects seen in the bend, including flow separation at the tip of the dividing wall, counter-rotating Dean vortices, high heat transfer at areas of flow impingement, and flow separation at the upstream and downstream corners of the bend. Mean flow results show a region of separated flow at the tip of the dividing region in the case with no rib in the bend, but no separation region is observed in the case with a rib. A pair of counter-rotating Dean vortices in the middle of the bend is observed in both cases. Turbulent kinetic energy profiles show a 30% increase in the midplane of the bend when the rib is added. High gradients of heat transfer augmentation are observed on the back wall and downstream outside wall, where mean flow impingement occurs. This heat transfer is increased with the presence of a rib. Including a rib in the bend increases the friction factor in the bend by 80%, and it increases the heat transfer augmentation by approximately 20%, resulting in a trade-off between pressure drop and heat transfer.

Detached Eddy Simulation (DES) of a hydrodynamic and thermally developed turbulent flow is presented for a stationary duct with square ribs aligned normal to the main flow direction. The rib height to channel hydraulic diameter (e/[D.sub.h]) is 0.1, the rib pitch to rib height (P/e) is 10 and the calculations have been carried out for a bulk Reynolds number of 20,000. DES calculations are carried out on a [96.sup.3] grid, a [64.sup.3] grid, and a [48.sup.3] grid to study the effect of grid resolution. Based on the agreement with earlier LES computations, the [64.sup.3] grid is observed to be suitable for the DES computation. DES and RANS calculations carried out on the [64.sup.3] grid are compared to LES calculations on [96.sup.3]/ [128.sup.3] grids and experimental measurements. The flow and heat transfer characteristics for the DES cases compare well with the LES results and the experiments. The average friction and the augmentation ratios are consistent with experimental results, predicting values within 10% of the measured quantities, at a cost lower than the LES calculations. RANS fails to capture some key features of the flow. [DOI: 10.1115/1.2033010]

Results from large eddy simulations (LES) of fully developed flow in a 90 deg ribbed duct are presented with rib pitch-to-height ratio P/e=10 and a rib height-to-hydraulic-diameter ratio e/[D.sub.h] = 0.1. Three rotation numbers Ro = 0.18, 0.36, and 0.68 are studied at a nominal Reynolds number based on bulk velocity of 20 000. Centrifugal buoyancy effects are included at two Richardson numbers of Ri=12, 28 (Buoyancy parameter, Bo =0.12 and 0.30) for each rotation case. Heat transfer augmentation on the trailing side of the duct due to the action of Coriolis forces alone asymptotes to a value of 3.7 [+ or -] 5% by Ro = 0.2. On the other hand, augmentation ratios on the leading surface keep decreasing with an increase in rotation number with values ranging from 1.7 at Ro=0.18 to 1.2 at Ro=0.67. Secondary flow cells augment the heat transfer coefficient on the smooth walls by 20% to 30% over a stationary duct. Centrifugal buoyancy further strengthens the secondary flow cells in the duct cross-section which leads to an additional increase of 10% to 15%. Buoyancy also accentuates the augmentation of turbulence near the trailing wall of the duct and increases the heat transfer augmentation ratio 10% to 20% over the action of Coriolis forces alone. However, it does not have any significant effect at the leading side of the duct. The overall effect of buoyancy on heat transfer augmentation for the ribbed duct is found to be less than 10% over the effect of Coriolis forces alone. Friction on the other hand is augmented 15% to 20% at the highest buoyancy number studied. Comparison with available experiments in the literature show excellent agreement.

The paper presents numerical simulations and analysis of electrokinetic induced mixing in a microchamber in the presence of a fluctuating electric field. Two microchamber geometries are investigated; one plain and the other with strategically placed microbaffles. Both geometries are tested for two extreme surface conditions: a charged surface with induced electrokinesis and another with a neutral or passive surface. Through order of magnitude analysis and numerical experiments it is found that there is an optimal choice of nondimensional frequency and driving potential which leads to the best mixing characteristics. This is given by the relationship [Re.sub.eof]/f* < 5 and the condition that f* = O(1), where [Re.sub.eof] is the ratio of electrokinetic forces to viscous forces and f* is the nondimensional frequency. Optimal mixing is shown to occur at [Re.sub.eof] 100 and f* = 30. In all cases, best mixing is found to occur when conditions are favorable for the establishment and sustainence of a rotational cell in the chamber driven by the fluctuating ac current. It is shown that the plain microchamber performs better under conditions of surface neutrality while microbaffles enhance mixing substantially in a charged microchamber. In the presence of a rotational cell, the characteristic time scale for mixing is reduced by 2-3 orders of magnitude compared to plain diffusion and is calculated to be between 5 and 10 s for aqueous buffers.

Three-dimensional numerical simulations are conducted to calculate the surface heat transfer coefficient (HTC) of the water jet impinging on a TA2/Q235B clad plate. The accuracy of the simulation model is validated by experiment. The simulation results are basically in agreement with the experimental results. The effect of clad plate temperature and thickness, jet height, orifice diameter, and orifice velocity on the HTC is investigated. The numerical results show that the average surface HTC of TA2/Q235B clad plate is greatly affected by plate temperature, Reynolds number (orifice velocity and orifice diameter), and plate thickness, while it is less affected by jet height. The surface HTC decreases with the increase of clad plate temperature and thickness. The surface HTC increases dramatically with the increase of jet Reynolds number. At the same Reynolds number (more than 14500), the cooling effect of increasing the orifice velocity is stronger than increasing the orifice diameter, while at the same Reynolds number (less than 14500), the orifice diameter has the larger impact on the surface HTC than the orifice velocity. [ABSTRACT FROM AUTHOR]