Rearrangement of secondary flow over spanwise heterogeneous roughness

Turbulent flow over a surface with streamwise-elongated rough and smooth stripes is studied by means of direct numerical simulation (DNS) in a periodic plane open channel with fully resolved roughness. The goal is to understand how the mean height of roughness affects the characteristics of the secondary flow formed above a spanwise heterogeneous rough surface. To this end, while the statistical properties of roughness texture as well as the width and spacing of the rough stripes are kept constant, the elevation of the smooth stripes is systematically varied in different simulation cases. Utilizing this variation, three configurations – representing protruding, recessed and an intermediate type of roughness – are analysed. In all cases, secondary flows are present and the skin friction coefficients calculated for all the heterogeneous rough surfaces are meaningfully larger than what would result from the area-weighted average of those of homogeneous smooth and rough surfaces. This drag increase appears to be linked to the strength of the secondary flow. The rotational direction of the secondary motion is shown to depend on the relative surface elevation. The present results suggest that this rearrangement of the secondary flow is linked to the spatial distribution of the spanwise-wall-normal Reynolds stress component, which carries opposing signs for protruding and recessed roughness

Modelling spanwise heterogeneous roughness through a parametric forcing approach

Inhomogeneous rough surfaces in which strips of roughness alternate with smooth-wall strips are known to generate large-scale secondary motions. Those secondary motions are strongest if the strip width is of the order of the half-channel height and they generate a spatial wall shear stress distribution whose mean value can significantly exceed the area-averaged mean value of a homogeneously smooth and rough surface. In the present paper it is shown that a parametric forcing approach (Busse & Sandham, J. Fluid Mech., vol. 712, 2012, pp. 169–202; Forooghi et al., Intl J. Heat Fluid Flow, vol. 71, 2018, pp. 200–209), calibrated with data from turbulent channel flows over homogeneous roughness, can capture the topological features of the secondary motion over protruding and recessed roughness strips (Stroh et al., J. Fluid Mech., vol. 885, 2020, R5). However, the results suggest that the parametric forcing approach roughness model induces a slightly larger wall offset when applied to the present heterogeneous rough-wall conditions. Contrary to roughness-resolving simulations, where a significantly higher resolution is required to capture roughness geometry, the parametric forcing approach can be applied with usual smooth-wall direct numerical simulation resolution resulting in less computationally expensive simulations for the study of localized roughness effects. Such roughness model simulations are employed to systematically investigate the effect of the relative roughness protrusion on the physical mechanism of secondary flow formation and the related drag increase. It is found that strong secondary motions present over spanwise heterogeneous roughness with geometrical height difference generally lead to a drag increase. However, the physical mechanism guiding the secondary flow formation, and the resulting secondary flow topology, is different for protruding roughness strips and recessed roughness strips separated by protruding smooth surface strips

DNS of Turbulent Heat Transfer in Impinging Jets at Different Reynolds and Prandtl Numbers

The heat transfer between an impinging circular jet and a flat plate is studied by means of direct numerical simulations (DNS) for different Prandtl numbers of the fluid. The thermal field is resolved for Pr= 1, 0.72, 0.025, and 0.01. The flow is incompressible and the temperature is treated as a passive scalar field. The jet originates from a fully developed turbulent pipe flow and impinges perpendicularly on a smooth solid heated plate placed at two pipe diameters distance from the jet exit section. The values of Reynolds numbers based on the pipe diameter and bulk mean velocity in the pipe are set to Re= 5300 and Re= 10000. Inflow boundary conditions are enforced using a precursor simulation. Heat transfer at the wall is addressed through the Nusselt number distribution and main flow field statistics. At fixed Reynolds number it is shown that the Prandtl number influences the intensity of the Nusselt number at a given radial location, and that the Nusselt number distribution along the plate exhibit similar features at different Prandtl numbers. The characteristic secondary peak in the Nusselt number distribution is found for both Reynolds numbers for Pr= 0.025 and Pr = 0.01. All the simulations presented in this study were performed with the high order spectral element code Nek5000. Generated flow field statistics are available in the open access repository KITOpen

Comparison of Different Solvers and Geometry Representation Strategies for Dns of Rough Wall Channel Flow

In the present study we investigate an incompressible turbulent channel flow with heat transfer at Reτ = 180 with a deterministic surface topography consisting of truncated cones. Two solvers for each of the two boundary handling strategies are considered. With Nek5000 and OpenFOAM the influence of the roughness elements is directly accounted for by an unstructured body fitted mesh, whereas Xcompact3d and SIMSON utilize the immersed boundary method (IBM) to deal with the 3D geometry. The main focus of this work is on an evaluation of the usability of the IBM and a comparison of the parallel performance of the different solvers. Since usability is an ambiguous definition, various quantities are compared: global statistics like Nusselt number and friction coefficient, one-dimensional wall-normal profiles for first and second order statistics, as well as three-dimensional averages over roughness sections. In addition, the computational effort for each method is documented

Secondary flow impact on velocity and thermal fields of a structured turbulent channel flow

Surface structuring in form of streamwise elongated triangular ridges is investigated in a framework of a fully developed turbulent channel flow at Reb = 18000. The ridges generate a strong large-scale secondary motion and hence alter momentum and heat transfer in the channel. A comparison of the obtained skin friction and heat transfer coefficients reveals a 30% enhancement for both quantities relatively to the smooth channel. Tripe decomposition analysis of the total stress and heat flux results in an observation of different mechanisms behind this enhancements: while the skin friction coefficient is mainly increased by the secondary-motion-related contribution, the heat flux is similarly enhanced by the secondary motion and modification of turbulent properties of the flow

Immune boosting by B.1.1.529 (Omicron) depends on previous SARS-CoV-2 exposure

The Omicron, or Pango lineage B.1.1.529, variant of SARS-CoV-2 carries multiple spike mutations with high transmissibility and partial neutralizing antibody (nAb) escape. Vaccinated individuals show protection from severe disease, often attributed to primed cellular immunity. We investigated T and B cell immunity against B.1.1.529 in triple mRNA vaccinated healthcare workers (HCW) with different SARS-CoV-2 infection histories. B and T cell immunity against previous variants of concern was enhanced in triple vaccinated individuals, but magnitude of T and B cell responses against B.1.1.529 spike protein was reduced. Immune imprinting by infection with the earlier B.1.1.7 (Alpha) variant resulted in less durable binding antibody against B.1.1.529. Previously infection-naïve HCW who became infected during the B.1.1.529 wave showed enhanced immunity against earlier variants, but reduced nAb potency and T cell responses against B.1.1.529 itself. Previous Wuhan Hu-1 infection abrogated T cell recognition and any enhanced cross-reactive neutralizing immunity on infection with B.1.1.529

Blood transcriptional biomarkers of acute viral infection for detection of pre-symptomatic SARS-CoV-2 infection: a nested, case-control diagnostic accuracy study

Background We hypothesised that host-response biomarkers of viral infections might contribute to early identification of individuals infected with SARS-CoV-2, which is critical to breaking the chains of transmission. We aimed to evaluate the diagnostic accuracy of existing candidate whole-blood transcriptomic signatures for viral infection to predict positivity of nasopharyngeal SARS-CoV-2 PCR testing.Methods We did a nested case-control diagnostic accuracy study among a prospective cohort of health-care workers (aged ≥18 years) at St Bartholomew’s Hospital (London, UK) undergoing weekly blood and nasopharyngeal swab sampling for whole-blood RNA sequencing and SARS-CoV-2 PCR testing, when fit to attend work. We identified candidate blood transcriptomic signatures for viral infection through a systematic literature search. We searched MEDLINE for articles published between database inception and Oct 12, 2020, using comprehensive MeSH and keyword terms for “viral infection”, “transcriptome”, “biomarker”, and “blood”. We reconstructed signature scores in blood RNA sequencing data and evaluated their diagnostic accuracy for contemporaneous SARS-CoV-2 infection, compared with the gold standard of SARS-CoV-2 PCR testing, by quantifying the area under the receiver operating characteristic curve (AUROC), sensitivities, and specificities at a standardised Z score of at least 2 based on the distribution of signature scores in test-negative controls. We used pairwise DeLong tests compared with the most discriminating signature to identify the subset of best performing biomarkers. We evaluated associations between signature expression, viral load (using PCR cycle thresholds), and symptom status visually and using Spearman rank correlation. The primary outcome was the AUROC for discriminating between samples from participants who tested negative throughout the study (test-negative controls) and samples from participants with PCR-confirmed SARS-CoV-2 infection (test-positive participants) during their first week of PCR positivity.Findings We identified 20 candidate blood transcriptomic signatures of viral infection from 18 studies and evaluated their accuracy among 169 blood RNA samples from 96 participants over 24 weeks. Participants were recruited between March 23 and March 31, 2020. 114 samples were from 41 participants with SARS-CoV-2 infection, and 55 samples were from 55 test-negative controls. The median age of participants was 36 years (IQR 27–47) and 69 (72%) of 96 were women. Signatures had little overlap of component genes, but were mostly correlated as components of type I interferon responses. A single blood transcript for IFI27 provided the highest accuracy for discriminating between test-negative controls and test-positive individuals at the time of their first positive SARS-CoV-2 PCR result, with AUROC of 0·95 (95% CI 0·91–0·99), sensitivity 0·84 (0·70–0·93), and specificity 0·95 (0·85–0·98) at a predefined threshold (Z score >2). The transcript performed equally well in individuals with and without symptoms. Three other candidate signatures (including two to 48 transcripts) had statistically equivalent discrimination to IFI27 (AUROCs 0·91–0·95).Interpretation Our findings support further urgent evaluation and development of blood IFI27 transcripts as a biomarker for early phase SARS-CoV-2 infection for screening individuals at high risk of infection, such as contacts of index cases, to facilitate early case isolation and early use of antiviral treatments as they emerge

Quantitative, multiplexed, targeted proteomics for ascertaining variant specific SARS-CoV-2 antibody response

Determining the protection an individual has to severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) variants of concern (VoCs) is crucial for future immune surveillance, vaccine development, and understanding of the changing immune response. We devised an informative assay to current ELISA-based serology using multiplexed, baited, targeted proteomics for direct detection of multiple proteins in the SARS-CoV-2 anti-spike antibody immunocomplex. Serum from individuals collected after infection or first- and second-dose vaccination demonstrates this approach and shows concordance with existing serology and neutralization. Our assays show altered responses of both immunoglobulins and complement to the Alpha (B.1.1.7), Beta (B.1.351), and Delta (B.1.617.1) VoCs and a reduced response to Omicron (B1.1.1529). We were able to identify individuals who had prior infection, and observed that C1q is closely associated with IgG1 (r > 0.82) and may better reflect neutralization to VoCs. Analyzing additional immunoproteins beyond immunoglobulin (Ig) G, provides important information about our understanding of the response to infection and vaccination

Comprehensive investigation of an inverse geometry problem in heat conduction via iterative regularization method

An inverse geometry problem in heat conduction is solved using different versions of an iterative regularization method. The algorithm consists of direct and inverse problems, which aims to modification of geometry. The direct problem is solved using a finite-element method. The employed iterative regularization method is constructed using the adjoint and sensitivity equations that are used to calculate the gradient of the objective function and the optimal step size, respectively. Results show that the Powel-Beale version has the best convergence rate compared to the Fletcher-Reeves and Polak-Ribiere versions of the conjugate gradient method. Effects of geometric parameters, location and number of sensors, heat flux value, error of sensors, and size of meshes are studied. Results show that as the sensors get closer to the unknown boundary, both accuracy and the convergence rate of the algorithm improve. Increasing the number of sensors has a positive effect on accuracy and the convergence rate, only when it is smaller than a certain number. The presence of a measurement error leads to inaccurate estimation of the geometry shape. A proper size of mesh has the best convergence and accuracy in the shape identification problem

Assessment of thermal non-equilibrium condition on heat transfer through a channel lined with porous media- constant wall temperature

In this paper, forced convection in a channel lined with a porous layer is investigated. The main goal is to assess the effect of local thermal non-equilibrium condition on overall heat transfer in the channel. The effects of thermal conductivity of solid and thickness of porous layer are also studied. Flow assumed to be laminar and fully developed. The Brinkman-Forchheimer model for flow as well as the two equation energy model is used. The results showed that when the problem tends to local thermal equilibrium condition, heat transfer is enhanced due to heat conduction through solid phase. Another factor, which can facilitate the heat transfer, is the increase of the thermal conductivity of solid material. This trend is sensitive to the thickness of porous layer and modified Biot number, which is a measure (criterion) of local fluid to solid heat transfer. As thickness and modified Biot number increase, the Nusselt number becomes more sensitive to the thermal conductivity ratio