Direct numerical simulation of turbulent scalar transport across a flat surface

To elucidate the physical mechanisms that play a role in the interfacial transfer of atmospheric gases into water, a series of direct numerical simulations of mass transfer across the air-water interface driven by isotropic turbulence diffusing from below has been carried out for various turbulent Reynolds numbers ( RT=84,195,507). To allow a direct (unbiased) comparison of the instantaneous effects of scalar diffusivity, in each of the DNS up to six scalar advection-diffusion equations with different Schmidt numbers were solved simultaneously. As far as the authors are aware this is the first simulation that is capable to accurately resolve the realistic Schmidt number, Sc=500, that is typical for the transport of atmospheric gases such as oxygen in water. For the range of turbulent Reynolds numbers and Schmidt numbers considered, the normalized transfer velocity KL was found to scale with RT-1/2 and Sc-1/2, which indicates that the largest eddies present in the isotropic turbulent flow introduced at the bottom of the computational domain tend to determine the mass transfer. The KL results were also found to be in good agreement with the surface divergence model of McCready, Vassiliadou & Hanratty (AIChE J., vol. 32, 1986, pp. 1108-1115) when using a constant of proportionality of 0.525. Although close to the surface large eddies are responsible for the bulk of the gas transfer, it was also observed that for higher $R-T$ the influence of smaller eddies becomes more important. © 2014 Cambridge University Press

Turbulent kinetic energy production in the vane of a low-pressure linear turbine cascade with incoming wakes

Copyright © 2015 V. Michelassi and J. G. Wissink. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Incompressible large eddy simulation and direct numerical simulation of a low-pressure turbine at R e = 5.18 × 10 4 and 1.48 × 10 5 with discrete incoming wakes are analyzed to identify the turbulent kinetic energy generation mechanism outside of the blade boundary layer. The results highlight the growth of turbulent kinetic energy at the bow apex of the wake and correlate it to the stress-strain tensors relative orientation. The production rate is analytically split according to the principal axes, and then terms are computed by using the simulation results. The analysis of the turbulent kinetic energy is followed both along the discrete incoming wakes and in the stationary frame of reference. Both direct numerical and large eddy simulation concur in identifying the same production mechanism that is driven by both a growth of strain rate in the wake, first, followed by the growth of turbulent shear stress after. The peak of turbulent kinetic energy diffuses and can eventually reach the suction side boundary layer for the largest Reynolds number investigated here with higher incidence angle. As a consequence, the local turbulence intensity outside the boundary layer can grow significantly above the free-stream level with a potential impact on the suction side boundary layer transition mechanism.The German Research Foundation (DFG) within the joint Project “Periodic Unsteady Flow in Turbomachinery.

Travelling waves in two-dimensional plane Poiseuille flow

The asymptotic structure of laminar modulated travelling waves in two-dimensional high-Reynolds-number plane Poiseuille flow is investigated on the upper-energy branch. A finite set of independent slowly varying parameters are identified which parameterize the solution of the Navier–Stokes equations in this subset of the phase space. Our parameterization of the weakly stable modes describes an attracting manifold of maximum-entropy configurations. The complementary modes, which have been neglected in this parameterization, are strongly damped. In order to seek a closure, a countably infinite number of modulation equations are derived on the long viscous time scale: a single equation for averaged kinetic energy and momentum; and the remaining equations for averaged powers of vorticity. Only a finite number of these vorticity modulation equations are required to determine the finite number of unknowns. The new results show that the evolution of the slowly varying amplitude parameters is determined by the vorticity field and that the phase velocity responds to these changes in the amplitude in accordance with the kinetic energy and momentum. The new results also show that the most crucial physical mechanism in the production of vorticity is the interaction between vorticity and kinetic energy, this interaction being responsible for the existence of the attractor

Parameterization of travelling waves in plane Poiseuille flow

© The authors 2012. Published by Oxford University Press on behalf of the Institute of Mathematics and its Applications. All rights reserved. This is a pre-copyedited, author-produced PDF of an article accepted for publication in [IMA Journal of Applied Mathematics ] following peer review. The version of record [ IMA Journal of Applied Mathematics (2014) 79(1): 22-32.] is available online at: http://imamat.oxfordjournals.org/content/79/1/22The first finite-dimensional parameterization of a subset of the phase space of the Navier-Stokes equations is presented. Travelling waves in two-dimensional plane Poiseuille flow are numerically shown to approximate maximum-entropy configurations. In a coordinate system moving with the phase velocity, the enclosed body of the flow exhibits a hyperbolic sinusoidal relationship between the vorticity and stream function. The phase velocity and two-amplitude parameters describe the stable manifold on the slow viscous time scale. This original parameterization provides a valuable visualization of this subset of the phase space of the Navier-Stokes equations. These new results provide physical insight into an important intermediate stage in the instability process of plane Poiseuille flow

Simulation of air-water interfacial mass transfer driven by high-intensity isotropic turbulence

The data used in various plots can be found in http://www.ifh.kit.edu/dns data/gas transfer.The authors would like to thank the German Research Foundation (DFG) for funding this project and the Gauss Centre for Supercomputing e.V. (www.gauss-centre.eu) for providing computing time on the GCS Supercomputer SuperMUC at Leibniz Supercomputing Centre (LRZ, www.lrz.de).https://publikationen.bibliothek.kit.edu/100008909

Exploiting the misalignment of the serrated trailing edges for improved aerofoil broadband noise reduction

This paper presents the experimental results of adding variable flap angles to the serrated trailing edges and their effects on the self-noise radiation of an aerofoil. The investigation included aeroacoustics and wake flow measurements on an NACA 65-(12)10 aerofoil. This paper explores further combinations of positive and negative serration flap angles, U and L respectively, across the spanwise extent of the trailing edge. A new parameter, the serration oscillation wavelength (λ), was also included. The study investigates five individual cases, Baseline (B), Straight-Serration (SS), sIngle-Flapped-Serration (IFS), Multi-FlappedSerration (MFS), Split-Flapped-Serration (SFS) and Spanwise-Wavy-Serration (SWS). The serration amplitude and wavelength remained consistent as 30 mm and 3.3 mm respectively. It is clear that all the “misaligned” trailing edges produce different noise characteristics compared to the conventional SS trailing edge. At low-to-mid frequency range, the conventional SS trailing edge still outperforms both the MFS and SFS trailing edges. However, from mid-to-high frequency, both the MFS and SFS trailing edges can achieve higher noise reduction than the SS trailing edge considerably. At very high frequency, where the SS trailing edge would otherwise experience noise increase, there is no noise increase for both the MFS and SFS trailing edges because the periodic misalignment reduces the crossflow component in the space between adjacent members of the serration. The SWS trailing edge produces the same level of noise reduction as the SS trailing edge across the low-to-mid frequency range. From mid-to-high frequency range, the SWS trailing edge outperforms the SS trailing edge, but the level of further noise reduction it can achieve is less than those of MFS and SFS trailing edges. At very high frequency, the SWS begins to experience slightly noise increase over the baseline, B, trailing edge. However, the increase in noise level by the SWS trailing edge at this high frequency range is still less than that by the SS trailing edge. The near-wake flow measurement results provide some explanations for the mechanisms underpinning the broadband noise reduction and high frequency noise suppression by these trailing edge devices

Airflow in Supermarkets and Similar Retail Stores: a rapid survey on infection transmission

Purpose: to assess the literature relating to ventilation in supermarkets and similar retail premises and whether or how to alter ventilation (HVAC system) operation to minimise the risk of cross-infection in the light of the size of aerosols exhaled by customers from normal activity. Objectives: 1) ascertain the typical size range of particles and droplets exhaled; 2) examine current system design and operation guidelines; 3) report on advice from professional bodies about feasible changes to current practice. Summary: 1) Breathing expels a wide range of droplet sizes with some studies showing 80– 90% of particles are smaller than 1 μm (non-settling particles). 2) Typical HVAC configuration gives a well-mixed air flow through most of a store. 3) Air flow rates in supermarkets are large compared with exhalation volumes from customers; exhaled non-settling particles will be carried in the airflow. 4) Increased air changes per hour further reduces the risk of cross-infection. Assumptions: 1) UK stores of mainstream food and other retailers. 2) Customers are not displaying obvious symptoms of SARS-Cov2 (i.e. not coughing) as they are already instructed to self-isolate. Customers are either free from the virus or are asymptomatic. 3) People breathe normally and speak at normal conversation levels

Azimuthal anisotropy in Au+Au collisions at sqrtsNN = 200 GeV

The results from the STAR Collaboration on directed flow (v_1), elliptic flow (v_2), and the fourth harmonic (v_4) in the anisotropic azimuthal distribution of particles from Au+Au collisions at sqrtsNN = 200 GeV are summarized and compared with results from other experiments and theoretical models. Results for identified particles are presented and fit with a Blast Wave model. Different anisotropic flow analysis methods are compared and nonflow effects are extracted from the data. For v_2, scaling with the number of constituent quarks and parton coalescence is discussed. For v_4, scaling with v_2^2 and quark coalescence is discussed.Comment: 26 pages. As accepted by Phys. Rev. C. Text rearranged, figures modified, but data the same. However, in Fig. 35 the hydro calculations are corrected in this version. The data tables are available at http://www.star.bnl.gov/central/publications/ by searching for "flow" and then this pape

Rapidity and Centrality Dependence of Proton and Anti-proton Production from Au+Au Collisions at sqrt(sNN) = 130GeV

We report on the rapidity and centrality dependence of proton and anti-proton transverse mass distributions from Au+Au collisions at sqrt(sNN) = 130GeV as measured by the STAR experiment at RHIC. Our results are from the rapidity and transverse momentum range of |y|<0.5 and 0.35 <p_t<1.00GeV/c. For both protons and anti-protons, transverse mass distributions become more convex from peripheral to central collisions demonstrating characteristics of collective expansion. The measured rapidity distributions and the mean transverse momenta versus rapidity are flat within |y|<0.5. Comparisons of our data with results from model calculations indicate that in order to obtain a consistent picture of the proton(anti-proton) yields and transverse mass distributions the possibility of pre-hadronic collective expansion may have to be taken into account.Comment: 4 pages, 3 figures, 1 table, submitted to PR

The M/GP5 Glycoprotein Complex of Porcine Reproductive and Respiratory Syndrome Virus Binds the Sialoadhesin Receptor in a Sialic Acid-Dependent Manner

The porcine reproductive and respiratory syndrome virus (PRRSV) is a major threat to swine health worldwide and is considered the most significant viral disease in the swine industry today. In past years, studies on the entry of the virus into its host cell have led to the identification of a number of essential virus receptors and entry mediators. However, viral counterparts for these molecules have remained elusive and this has made rational development of new generation vaccines impossible. The main objective of this study was to identify the viral counterparts for sialoadhesin, a crucial PRRSV receptor on macrophages. For this purpose, a soluble form of sialoadhesin was constructed and validated. The soluble sialoadhesin could bind PRRSV in a sialic acid-dependent manner and could neutralize PRRSV infection of macrophages, thereby confirming the role of sialoadhesin as an essential PRRSV receptor on macrophages. Although sialic acids are present on the GP3, GP4 and GP5 envelope glycoproteins, only the M/GP5 glycoprotein complex of PRRSV was identified as a ligand for sialoadhesin. The interaction was found to be dependent on the sialic acid binding capacity of sialoadhesin and on the presence of sialic acids on GP5. These findings not only contribute to a better understanding of PRRSV biology, but the knowledge and tools generated in this study also hold the key to the development of a new generation of PRRSV vaccines