Numerical investigation of gaseous pollutant cross-transmission for single-sided natural ventilation driven by buoyancy and wind.

Single-sided natural ventilation was numerically investigated to determine the impact of buoyancy and wind on the cross-transmission of pollution by considering six window types commonly found in multistory buildings. The goal of this study was to predict the gaseous pollutant transmission using computational fluid dynamics based on the Reynolds-averaged Navier-Stokes equations and baseline k-ω turbulence equations. The results indicated that ventilation rates generally increased with increasing wind speeds if the effects of buoyancy and wind were not suppressed; however, the re-entry ratio representing the proportion of expelled air re-entering other floors and the corresponding risk of infection decreased. If the source of the virus was on a central floor, the risk of infection was the highest on the floors closest to the source. Different window types were also considered for determining their effectiveness in controlling cross-transmission and infection risk, depending on the source location and driving force (e.g., buoyancy and wind).

Effects of fish caudal fin sweep angle and kinematics on thrust production during low-speed thunniform swimming.

Scombrid fish lunate caudal fins are characterized by a wide range of sweep angles. Scombrid that have small sweep-angle caudal fins move at higher swimming speeds, suggesting that smaller angles produce more thrust. Furthermore, scombrids occasionally use high angles of attack (AoA) suggesting this also has some thrust benefit. This work examined the hypothesis that a smaller sweep angle and higher AoA improved thrust in swimmers by experimentally analyzing a robophysical model. The robophysical model was tested in a water tunnel at speeds between 0.35 and 0.7 body lengths per second. Three swept caudal fins were analyzed at three different AoA, three different freestream velocities, and four different Strouhal numbers, for a total of 108 cases. Results demonstrated that the fin with the largest sweep angle of 50° resulted in lower thrust production than the 40° and 30° fins, especially at higher Strouhal numbers. Larger AoA up to 25° increased thrust production at the higher Strouhal numbers, but at lower Strouhal numbers, produced less thrust. Differences in thrust production due to fin sweep angle and AoA were attributed to the variation in spanwise flow and leading edge vortex dynamics.

Characterization of bubble column photobioreactors for shear-sensitive microalgae culture.

The shear-sensitive marine algal dinoflagellate Karlodinium veneficum was grown in a cylindrical bubble column photobioreactor with an internal diameter of 0.044 m. Initial liquid height varied from 0.5 to 1.75 m, superficial gas velocities from 0.0014 to 0.0057 ms-1, and nozzle diameter from 1 to 2.5 mm. Computational fluid dynamics was used to characterize the flow hydrodynamics and energy dissipation rates. Experimental gas holdup and volumetric mass transfer coefficient strongly depended on the liquid height and correlated well with the Froude number. Energy dissipation near the head space (EDtop) was one order of magnitude higher than the average energy dissipation in the whole reactor (EDwhole), and the value in the sparger zone (EDspar) was one order of magnitude higher than EDtop. Cultures of K. veneficum were limited by CO2 transfer at low EDwhole and severely stressed above a critical value of EDwhole.

A computational study on the influence of insect wing geometry on bee flight mechanics.

Two-dimensional computational fluid dynamics (CFD) is applied to better understand the effects of wing cross-sectional morphology on flow field and force production. This study investigates the influence of wing cross-section on insect scale flapping flight performance, for the first time, using a morphologically representative model of a bee (Bombus pensylvanicus) wing. The bee wing cross-section was determined using a micro-computed tomography scanner. The results of the bee wing are compared with flat and elliptical cross-sections, representative of those used in modern literature, to determine the impact of profile variation on aerodynamic performance. The flow field surrounding each cross-section and the resulting forces are resolved using CFD for a flight speed range of 1 to 5 m/s. A significant variation in vortex formation is found when comparing the ellipse and flat plate with the true bee wing. During the upstroke, the bee and approximate wing cross-sections have a much shorter wake structure than the flat plate or ellipse. During the downstroke, the flat plate and elliptical cross-sections generate a single leading edge vortex, while the approximate and bee wings generate numerous, smaller structures that are shed throughout the stroke. Comparing the instantaneous aerodynamic forces on the wing, the ellipse and flat plate sections deviate progressively with velocity from the true bee wing. Based on the present findings, a simplified cross-section of an insect wing can misrepresent the flow field and force production. We present the first aerodynamic study using a true insect wing cross-section and show that the wing corrugation increases the leading edge vortex formation frequency for a given set of kinematics.

Gaseous pollutant transmission through windows between vertical floors in a multistory building with natural ventilation.

Natural ventilation is an effective strategy to control thermal comfort in buildings, and can be enhanced depending on the window style. The combination of natural ventilation and window can also facilitate the removal or dilution of gaseous pollutants from indoor sources in newly decorated buildings. However, the windows on the same facade may cause gaseous pollutant cross-transmission during single-sided natural ventilation between households on different floors close to the source. Although some research has focused on the pollutant cross-transmission in buildings, the simplification of windows into rectangular openings often affects accurate knowledge of pollutant transmission characteristics. Therefore, this investigation explored gaseous pollutant cross-transmission through real windows during single-sided, buoyancy-driven ventilation in a multistory building. Six types of windows were modeled for the indoor pollutant of gaseous formaldehyde (HCHO). Computational fluid dynamics (CFD) was utilized to solve characteristics of pollutant transmission inside and outside the multistory building. The results indicated that the ventilation rates, thermal profiles and pollutant transmission inside and outside the building varied for each window type, although the open window areas were identical. The re-entry ratio of exhausted air entering upper floors and the infection risk of epidemic viruses caused by airborne cross-transmission was sensitive to ventilation rates and window configurations, while the sensitivities for window configurations varied case by case. The comparisons also revealed that the specification of ambient temperature and pollutant release rate ultimately did not affect the evaluation of pollutant cross-transmission using CFD.

Theoretical study of the pyrolysis of methyltrichlorosilane in the gas phase. 3. Reaction rate constant calculations.

The rate constants for the gas-phase reactions in the silicon carbide chemical vapor deposition of methyltrichlorosilane (Ge, Y. B.; Gordon, M. S.; Battaglia, F.; Fox, R. O. J. Phys. Chem. A 2007, 111, 1462.) were calculated. Transition state theory was applied to the reactions with a well-defined transition state; canonical variational transition state theory was applied to the barrierless reactions by finding the generalized transition state with the maximum Gibbs free energy along the reaction path. Geometry optimizations were carried out with second-order perturbation theory (MP2) and the cc-pVDZ basis set. The partition functions were calculated within the harmonic oscillator and rigid rotor approximations. The final potential energy surfaces were obtained using the left-eigenstate coupled-cluster theory, CR-CC(2,3) with the cc-pVTZ basis set. The high-pressure approximation was applied to the unimolecular reactions. The predicted rate constants for more than 50 reactions were compared with the experimental ones at various temperatures and pressures; the deviations are generally less than 1 order of magnitude. Theory is found to be in reasonable agreement with the experiments.

Theoretical study of the pyrolysis of methyltrichlorosilane in the gas phase. 1. Thermodynamics.

Structures and energies of the gas-phase species produced during and after the various unimolecular decomposition reactions of methyltrichlorosilane (MTS) with the presence of H2 carrier gas were determined using second-order perturbation theory (MP2). Single point energies were obtained using singles + doubles coupled cluster theory, augmented by perturbative triples, CCSD(T). Partition functions were obtained using the harmonic oscillator-rigid rotor approximation. A 114-reaction mechanism is proposed to account for the gas-phase chemistry of MTS decompositions. Reaction enthalpies, entropies, and Gibbs free energies for these reactions were obtained at 11 temperatures ranging from 0 to 2000 K including room temperature and typical chemical vapor deposition (CVD) temperatures. Calculated and experimental thermodynamic properties such as heat capacities and entropies of various species and reaction enthalpies are compared, and theory is found to provide good agreement with experiment.

Theoretical study of the pyrolysis of methyltrichlorosilane in the gas phase. 2. Reaction paths and transition states.

The kinetics for the previously proposed 114-reaction mechanism for the chemical vapor deposition (CVD) process that leads from methyltrichlorosilane (MTS) to silicon carbide (SiC) are examined. Among the 114 reactions, 41 are predicted to proceed with no intervening barrier. For the remaining 73 reactions, transition states and their corresponding barrier heights have been explored using second-order perturbation theory (MP2) with the aug-cc-pVDZ basis set. Final energies for the reaction barriers were obtained using both MP2 with the aug-cc-pVTZ basis set and coupled cluster theory (CCSD(T)) with the aug-cc-pVDZ basis set. CCSD(T)/aug-cc-pVTZ energies were estimated by assuming additivity of basis set and correlation effects. Partition functions for the computation of thermodynamic properties of the transition states were calculated with MP2/aug-cc-pVDZ. Forward and reverse Gibbs free energy barriers were obtained at 11 temperatures ranging from 0 to 2000 K. Important reaction pathways are illustrated at 0 and 1400 K.

Dynamics and structures of segregation in a dense, vibrating granular bed.

We study the rise dynamics of a large particle in a granular bed under vertical vibration using molecular dynamics simulations. Systematic variation of the particle properties and external wall friction in the simulations shows that the large particle rising is very sensitive to external wall friction. The dynamical response of the granular bed with wall friction is shown to include an expansion stage and a compression stage within one cycle. With wall friction, large-scale force networks bearing larger-than-average forces are found in the compression stage. However, without wall friction, large strong force networks do not exist. The distribution of normal contact forces in the force networks is found to have an exponential tail similar to those in packing experiments. Numerical estimation of the two-point spatial correlation of normal contact force reveals predominantly short-range force correlation persisting over only 2-3 particle diameters. The structural properties of the force network are analyzed using a graph-theoretic approach, which is a modification of a minimum spanning tree (MST) constructed on the particles in physical space. The modified MST algorithm, which identifies local structures such as nearest neighbors, asymptotically recovers all particle contacts, the force distribution, and spatial force correlation of the force network. This indicates that although wall friction strongly affects the rising dynamics of the Brazil nut over the device scale (tens of particle diameters) through the force network, it does so through local short-range interactions. Thus this study affirms the basis for local constitutive models in continuum descriptions of segregation.