Large Eddy Simulations and modal reconstruction of laminar transonic buffet

Transonic buffet refers to the self-sustained periodic motion of shock waves observed in transonic flows over wings and limits the flight envelope of aircraft. Based on the boundary layer characteristics at the shock foot, buffet has been classified as laminar or turbulent and the mechanisms underlying the two have been proposed to be different (Dandois et al., 2018, J. Fluid Mech., vol. 18, pp. 156-178). The effect of various flow parameters (freestream Mach and Reynolds numbers and sweep and incidence angles) on laminar transonic buffet on an infinite wing (Dassault Aviation's supercritical V2C aerofoil) is reported here by performing Large-Eddy Simulations (LES) for a wide range of parameters. A spectral proper orthogonal decomposition identified the presence of a low-frequency mode associated with buffet and high-frequency wake modes related to vortex shedding. A flow reconstruction based only on the former shows periodic boundary-layer separation and reattachment accompanying shock wave motion. A modal reconstruction based only on the wake mode suggests that the separation bubble breathing phenomenon reported by Dandois et al. is due to this mode. Together, these results indicate that the physical mechanisms governing laminar and turbulent buffet are the same. Buffet was also simulated at zero incidence. Shock waves appear on both aerofoil surfaces and oscillate out of phase with each other indicating the occurrence of a Type I buffet (Giannelis et al., 2018, Aerosp. Sci. Technol., vol. 18, pp. 89-101) on a supercritical aerofoil. These results suggest that the mechanisms underlying different buffet types are the same.Comment: 40 pages, 31 figures, submitted to Journal of Fluid Mechanic

On the co-existence of transonic buffet and separation-bubble modes for the OALT25 laminar-flow wing section

Transonic buffet is an unsteady flow phenomenon that limits the safe flight envelope of modern aircraft. Scale-resolving simulations with span-periodic boundary conditions are capable of providing new insights into its flow physics. The present contribution shows the co-existence of multiple modes of flow unsteadiness over an unswept laminar-flow wing section, appearing in the following order of increasing frequency: (a) a low-frequency transonic buffet mode, (b) an intermediate-frequency separation bubble mode, and (c) high-frequency wake modes associated with vortex shedding. Simulations are run over a range of Reynolds and Mach numbers to connect the lower frequency modes from moderate to high Reynolds numbers and from pre-buffet to established buffet conditions. The intermediate frequency mode is found to be more sensitive to Reynolds-number effects compared to those of Mach number, which is the opposite trend to that observed for transonic buffet. Spectral proper orthogonal decomposition is used to extract the spatial structure of the modes. The buffet mode involves coherent oscillations of the suction-side shock structure, consistent with previous studies including global mode analysis. The laminar separation-bubble mode at intermediate frequency is fundamentally different, with a phase relationship between separation and reattachment that does not correspond to a simple `breathing' mode and is not at the same Strouhal number observed for shock-induced separation bubbles. Instead, a Strouhal number based on separation bubble length and reverse flow magnitude is found to be independent of Reynolds number within the range of cases studied

Connecting transonic buffet with incompressible low-frequency oscillations on aerofoils

Self-sustained low-frequency flow unsteadiness over rigid aerofoils in the transonic regime is referred to as transonic buffet. Although the exact physical mechanisms underlying this phenomenon are unclear, it is generally assumed to be unique to the transonic regime. This assumption is shown to be incorrect here by performing large-eddy simulations of flow over a NACA0012 profile for a wide range of flow conditions. At zero incidence and sufficiently high freestream Mach numbers, M, transonic buffet occurs with shock waves present in the flow. However, self-sustained oscillations that occur at similar frequencies are observed at lower M for which shock waves are absent and the entire flow field remains subsonic at all times. At higher incidences, the oscillations are sustained at progressively lower M. Oscillations were observed for M as low as 0.3, where compressibility effects are small. A spectral proper orthogonal decomposition shows that the spatial structure of these oscillations (i.e., mode shapes) are essentially the same for all cases. These results indicate that buffet on aerofoils does not necessarily require the presence of shock waves. Furthermore, the trend seen with increasing incidence angles suggests that transonic buffet on aerofoils and low-frequency oscillations reported in the incompressible regime (Zaman et al., 1989, J. Fluid Mech., vol. 202, pp. 403--442) have similar origins. Thus, models which rely specifically on shock waves to explain transonic buffet are incorrect. These insights could be useful in understanding the origins of ``transonic" buffet and reformulating mitigation strategies by shifting the focus away from shock waves.Comment: 28 pages, 20 figure

TPU positioning in the global research and educational environment

Томск вошел в число инновационных городов мира по результатам авторитетного рейтинга "Innovation Cities Global Index 2012-2013". Всего в рейтинг попали 445из 1542 городов мира, где проводились исследования. Российских городов - 20. Большой вклад в инновационную составляющую города вносит Томский консор-циум научно-образовательных и научных организаций, возглавляемый ректором ТПУ П.С. Чубиком. Университеты становятся активными игроками не только в производстве новых знаний, но и в их распространении и использовании посредствоминновационной деятельности.Tomsk is rated as one of the global innovative cities by the reputable ranking Innovation Cities Global Index 2012-2013. The survey covered 1542 cities of the world, with 445 ofthem having been included in the ranked list. There were 20 Russian cities rated. A great contribution to the Tomsk innovative component is provided by Tomsk Consortium ofResearch and Educational Organizations headed by P.S. Chubik, TPU Rector. Universities are becoming actively involved not only in the generation of new knowledge, but also in its dissemination and application by means of innovative activities

Damage evolution in wood: synchrotron radiation micro-computed tomography (SRμCT) as a complementary tool for interpreting acoustic emission (AE) behavior

Tensile tests of miniature spruce wood specimens have been performed to investigate the damage evolution in wood at the microscopic scale. For this purpose, the samples were stepwise tensile loaded in the longitudinal (L) and radial (R) directions and the damage evolution was monitored in real-time by acoustic emission (AE) and synchrotron radiation micro-computed tomography (SRμCT). This combination is of outstanding benefit as SRμCT monitoring provides an insight on the crack evolution and the final fracture at microscopic scale, whereas AE permits the detection of the associated accumulation and interaction of single damage events on all length scales with high time resolution. A significant drawback of the AE testing of wood has been overcome by means of calibrating the AE amplitudes with the underlying crack length development. Thus, a setup-dependent and wood species-dependent calibration value was estimated, which associates 1 μm2 crack area generating of 0.0038 mV in the detected AE amplitude. Furthermore, for both L and R specimens, AE signals were classified into two clusters by using a frequency-based approach of unsupervised pattern recognition. The shares of AE signals of both clusters correlate with the ratio of the relative crack area of the interwall and transwall cracks gained from the fractographic analysis of SRμCT scans

Clinical relevance of lung transplantation for COVID-19 ARDS: a nationwide study

BACKGROUND: Although the number of lung transplantations (LTx) performed worldwide for COVID-19 induced acute respiratory distress syndrome (ARDS) is still low, there is general agreement that this treatment can save a subgroup of most severly ill patients with irreversible lung damage. However, the true proportion of patients eligible for LTx, the overall outcome and the impact of LTx to the pandemic are unknown. METHODS: A retrospective analysis was performed using a nationwide registry of hospitalised patients with confirmed severe acute respiratory syndrome coronavirus type 2 (SARS-Cov-2) infection admitted between January 1, 2020 and May 30, 2021 in Austria. Patients referred to one of the two Austrian LTx centers were analyzed and grouped into patients accepted and rejected for LTx. Detailed outcome analysis was performed for all patients who received a LTx for post-COVID-19 ARDS and compared to patients who underwent LTx for other indications. RESULTS: Between January 1, 2020 and May 30, 2021, 39.485 patients were hospitalised for COVID-19 in Austria. 2323 required mechanical ventilation, 183 received extra-corporeal membrane oxygenation (ECMO) support. 106 patients with severe COVID-19 ARDS were referred for LTx. Of these, 19 (18%) underwent LTx. 30-day mortality after LTx was 0% for COVID-19 ARDS transplant recipients. With a median follow-up of 134 (47–450) days, 14/19 patients are alive. CONCLUSIONS: Early referral of ECMO patients to a LTx center is pivotal in order to select patients eligible for LTx. Transplantation offers excellent midterm outcomes and should be incorporated in the treatment algorithm of post-COVID-19 ARDS

Survival of newly formed particles in haze conditions

Intense new particle formation events are regularly observed under highly polluted conditions, despite the high loss rates of nucleated clusters. Higher than expected cluster survival probability implies either ineffective scavenging by pre-existing particles or missing growth mechanisms. Here we present experiments performed in the CLOUD chamber at CERN showing particle formation from a mixture of anthropogenic vapours, under condensation sinks typical of haze conditions, up to 0.1 s(-1). We find that new particle formation rates substantially decrease at higher concentrations of pre-existing particles, demonstrating experimentally for the first time that molecular clusters are efficiently scavenged by larger sized particles. Additionally, we demonstrate that in the presence of supersaturated gas-phase nitric acid (HNO3) and ammonia (NH3), freshly nucleated particles can grow extremely rapidly, maintaining a high particle number concentration, even in the presence of a high condensation sink. Such high growth rates may explain the high survival probability of freshly formed particles under haze conditions. We identify under what typical urban conditions HNO3 and NH3 can be expected to contribute to particle survival during haze.Peer reviewe

Survival of newly formed particles in haze conditions

Intense new particle formation events are regularly observed under highly polluted conditions, despite the high loss rates of nucleated clusters. Higher than expected cluster survival probability implies either ineffective scavenging by pre-existing particles or missing growth mechanisms. Here we present experiments performed in the CLOUD chamber at CERN showing particle formation from a mixture of anthropogenic vapours, under condensation sinks typical of haze conditions, up to 0.1 s(-1). We find that new particle formation rates substantially decrease at higher concentrations of pre-existing particles, demonstrating experimentally for the first time that molecular clusters are efficiently scavenged by larger sized particles. Additionally, we demonstrate that in the presence of supersaturated gas-phase nitric acid (HNO3) and ammonia (NH3), freshly nucleated particles can grow extremely rapidly, maintaining a high particle number concentration, even in the presence of a high condensation sink. Such high growth rates may explain the high survival probability of freshly formed particles under haze conditions. We identify under what typical urban conditions HNO3 and NH3 can be expected to contribute to particle survival during haze.Peer reviewe

An intercomparison study of four different techniques for measuring the chemical composition of nanoparticles

Currently, the complete chemical characterization of nanoparticles (< 100 nm) represents an analytical challenge, since these particles are abundant in number but have negligible mass. Several methods for particle-phase characterization have been recently developed to better detect and infer more accurately the sources and fates of sub-100 nm particles, but a detailed comparison of different approaches is missing. Here we report on the chemical composition of secondary organic aerosol (SOA) nanoparticles from experimental studies of α-pinene ozonolysis at −50, −30, and −10 ∘C and intercompare the results measured by different techniques. The experiments were performed at the Cosmics Leaving OUtdoor Droplets (CLOUD) chamber at the European Organization for Nuclear Research (CERN). The chemical composition was measured simultaneously by four different techniques: (1) thermal desorption–differential mobility analyzer (TD–DMA) coupled to a NO$^-_3$ chemical ionization–atmospheric-pressure-interface–time-of-flight (CI–APi–TOF) mass spectrometer, (2) filter inlet for gases and aerosols (FIGAERO) coupled to an I$^−$ high-resolution time-of-flight chemical ionization mass spectrometer (HRToF-CIMS), (3) extractive electrospray Na$^+$ ionization time-of-flight mass spectrometer (EESI-TOF), and (4) offline analysis of filters (FILTER) using ultra-high-performance liquid chromatography (UHPLC) and heated electrospray ionization (HESI) coupled to an Orbitrap high-resolution mass spectrometer (HRMS). Intercomparison was performed by contrasting the observed chemical composition as a function of oxidation state and carbon number, by estimating the volatility and comparing the fraction of volatility classes, and by comparing the thermal desorption behavior (for the thermal desorption techniques: TD–DMA and FIGAERO) and performing positive matrix factorization (PMF) analysis for the thermograms. We found that the methods generally agree on the most important compounds that are found in the nanoparticles. However, they do see different parts of the organic spectrum. We suggest potential explanations for these differences: thermal decomposition, aging, sampling artifacts, etc. We applied PMF analysis and found insights of thermal decomposition in the TD–DMA and the FIGAERO