Experimental Investigation and Large-Eddy Simulation of the Turbulent Flow past a Smooth and Rigid Hemisphere

Computations carried out on the German Federal Top-Level Computer SuperMUC at LRZ Munich under the contract number pr84na.International audienceThe objective of the present paper is to provide a detailed experimental and numerical investigation on the turbulent flow past a hemispherical obstacle (diameter D). For this purpose, the bluff body is exposed to a thick turbulent boundary layer of the thickness δ = D/2 at Re = 50,000. In the experiment this boundary layer thickness is achieved by specific fences placed in the upstream region of the wind tunnel. A detailed measurement of the upstream flow conditions by laser-Doppler and hot-film probes allows to mimic the inflow conditions for the complementary large-eddy simulation of the flow field using a synthetic turbulence inflow generator. These clearly defined boundary and operating conditions are the prerequisites for a combined experimental and numerical investigation of the flow field relying on the laser-Doppler anemometry and a finite-volume Navier-Stokes solver for block-structured curvilinear grids. The results comprise an analysis on the unsteady flow features observed in the vicinity of the hemisphere as well as a detailed discussion of the time-averaged flow field. The latter includes the mean velocity field as well as the Reynolds stresses. Owing to the proper description of the oncoming flow and supplementary numerical studies guaranteeing the choice of an appropriate grid and subgrid-scale model, the results of the measurements and the prediction are found to be in close agreement

Analysis of shared heritability in common disorders of the brain

ience, this issue p. eaap8757 Structured Abstract INTRODUCTION Brain disorders may exhibit shared symptoms and substantial epidemiological comorbidity, inciting debate about their etiologic overlap. However, detailed study of phenotypes with different ages of onset, severity, and presentation poses a considerable challenge. Recently developed heritability methods allow us to accurately measure correlation of genome-wide common variant risk between two phenotypes from pools of different individuals and assess how connected they, or at least their genetic risks, are on the genomic level. We used genome-wide association data for 265,218 patients and 784,643 control participants, as well as 17 phenotypes from a total of 1,191,588 individuals, to quantify the degree of overlap for genetic risk factors of 25 common brain disorders. RATIONALE Over the past century, the classification of brain disorders has evolved to reflect the medical and scientific communities' assessments of the presumed root causes of clinical phenomena such as behavioral change, loss of motor function, or alterations of consciousness. Directly observable phenomena (such as the presence of emboli, protein tangles, or unusual electrical activity patterns) generally define and separate neurological disorders from psychiatric disorders. Understanding the genetic underpinnings and categorical distinctions for brain disorders and related phenotypes may inform the search for their biological mechanisms. RESULTS Common variant risk for psychiatric disorders was shown to correlate significantly, especially among attention deficit hyperactivity disorder (ADHD), bipolar disorder, major depressive disorder (MDD), and schizophrenia. By contrast, neurological disorders appear more distinct from one another and from the psychiatric disorders, except for migraine, which was significantly correlated to ADHD, MDD, and Tourette syndrome. We demonstrate that, in the general population, the personality trait neuroticism is significantly correlated with almost every psychiatric disorder and migraine. We also identify significant genetic sharing between disorders and early life cognitive measures (e.g., years of education and college attainment) in the general population, demonstrating positive correlation with several psychiatric disorders (e.g., anorexia nervosa and bipolar disorder) and negative correlation with several neurological phenotypes (e.g., Alzheimer's disease and ischemic stroke), even though the latter are considered to result from specific processes that occur later in life. Extensive simulations were also performed to inform how statistical power, diagnostic misclassification, and phenotypic heterogeneity influence genetic correlations. CONCLUSION The high degree of genetic correlation among many of the psychiatric disorders adds further evidence that their current clinical boundaries do not reflect distinct underlying pathogenic processes, at least on the genetic level. This suggests a deeply interconnected nature for psychiatric disorders, in contrast to neurological disorders, and underscores the need to refine psychiatric diagnostics. Genetically informed analyses may provide important "scaffolding" to support such restructuring of psychiatric nosology, which likely requires incorporating many levels of information. By contrast, we find limited evidence for widespread common genetic risk sharing among neurological disorders or across neurological and psychiatric disorders. We show that both psychiatric and neurological disorders have robust correlations with cognitive and personality measures. Further study is needed to evaluate whether overlapping genetic contributions to psychiatric pathology may influence treatment choices. Ultimately, such developments may pave the way toward reduced heterogeneity and improved diagnosis and treatment of psychiatric disorders

Experimental studies on the instantaneous fluid–structure interaction of an air-inflated flexible membrane in turbulent flow

International audienceThe present paper investigates the interaction between a turbulent fluid flow and a flexible membrane structure. Such flexible structures are of increasing interest for modern engineering applications due to their adaptable utilization. Highly flexible membranes under turbulent flow conditions still bare fundamental challenges such as the structural response to fluid loads leading to the motivation of the present study. It investigates the fluid-structure interaction of a flexible membranous structure in the shape of a hemisphere. The air-inflated structure is placed in the test section of a wind tunnel and is exposed to a turbulent boundary layer flow. The properties of the turbulent boundary layer are clearly defined so that the test case is reproducible by numerical simulations. Three Reynolds numbers (50,000, 75,000 and 100,000) are chosen to examine the interaction between the turbulent flow and the pressurized membrane. Special emphasis is put on the instantaneous effects. Furthermore, the flow field around an equally sized rigid hemisphere is measured under identical conditions serving as a reference for the flexible case. The experiments are conducted by combining particle-image-velocimetry for the flow field and high-speed digital-image correlation measurements for the deformation of the oscillating membrane. Furthermore, a constant-temperature anemometer is used for evaluating the velocity spectra at locations close to the wall to connect the independently performed fluid and structure measurements. A thorough analysis of the comprehensive data sets for the fluid flow and the displacements of the structure leads to the characterization of the behavior of the flexible structure under changing flow conditions

Numerical investigations on the dynamic behavior of a 2-DOF airfoil in the transitional Re number regime based on fully coupled simulations relying on an eddy-resolving technique

International audienceThe paper is the numerical counterpart of the experimental investigation on the fluid-structure interaction (FSI) of a wing with two degrees of freedom (DOF), i.e., pitch and heave. Wood et al. (2020) has provided the experimental basis by studying the flutter stability of an elastically mounted straight wing (NACA 0012 airfoil) in a wind tunnel considering the transitional Reynolds number regime. Three different configurations with varying distances between the fixed elastic axis and the variable center of gravity were considered. Additional free-oscillation tests in still air were carried out in order to make the mechanical properties of the setup available for the simulations. The present contribution describes the numerical methodology applied consisting of a partitioned coupled solver combining eddy-resolving large-eddy simulations on the fluid side with a solver for the governing equations of the translation and rotation of the rigid wing. In order to prove the parameters provided by the experiment and to determine the pure material damping coefficients not available from the measurements , simulations of 1-DOF free-oscillation tests in still air are carried out and analyzed. For validation purposes the corresponding 2-DOF free-oscillation tests in still air are assessed and a good agreement with the experimental data is achieved. Finally, the wing exposed to a constant free-stream of varying strength is analyzed leading to the characterization of complex instantaneous FSI phenomena such as limit-cycle oscillations and flutter. Under full utilization of the supplementary measurements the predictions are evaluated in detail. Contrary to the experiments the simulations provide the entire fluid data and unique data for the translatory and rotatory movement allowing to investigate the causes of the observed phenomena. Both limit-cycle oscillations and flutter can be reproduced by the coupled FSI predictions

Experimental investigations on the dynamic behavior of a 2-DOF airfoil in the transitional Re number regime based on digital-image correlation measurements

International audienceThe present paper investigates the fluid-structure interaction (FSI) of a wing with two degrees of freedom (DOF), i.e., pitch and heave, in the transitional Reynolds number regime. This 2-DOF setup marks a classic configuration in aeroelasticity to demonstrate flutter stability of wings. In the past, mainly analytic approaches have been developed to investigate this challenging problem under simplifying assumptions such as potential flow. Although the classical theory offers satisfying results for certain cases, modern numerical simulations based on fully coupled approaches, which are more generally applicable and powerful, are still rarely found. Thus, the aim of this paper is to provide appropriate experimental reference data for well-defined configurations under clear operating conditions. In a follow-up contribution these will be used to demonstrate the capability of modern simulation techniques to capture instantaneous physical phenomena such as flutter. The measurements in a wind tunnel are carried out based on digital-image correlation (DIC). The investigated setup consists of a straight wing using a symmetric NACA 0012 airfoil. For the experiments the model is mounted into a frame by means of bending and torsional springs imitating the elastic behavior of the wing. Three different configurations of the wing possessing a fixed elastic axis are considered. For this purpose, the center of gravity is shifted along the chord line of the airfoil influencing the flutter stability of the setup. Still air free-oscillation tests are used to determine characteristic properties of the unloaded system (e.g. mass moment of inertia and damping ratios) for one (pitch or heave) and two degrees (pitch and heave) of freedom. The investigations on the coupled 2-DOF system in the wind tunnel are performed in an overall chord Reynolds number range of 9.66 × 10 3 ≤ Re ≤ 8.77 × 10 4. The effect of the fluid-load induced damping is studied for the three configurations. Furthermore, the cases of limit-cycle oscillation (LCO) as well as diverging flutter motion of the wing are characterized in detail. In addition to the DIC measurements, hot-film measurements of the wake flow for the rigid and the oscillating airfoil are presented in order to distinguish effects originating from the flow and the structure

Enhanced injection method for synthetically generated turbulence within the flow domain of eddy-resolving simulations

International audienceThe quality of eddy-resolving turbulence simulations strongly depends on appropriate inflow conditions. In most cases they have to be time-dependent and satisfy certain conditions for the first (mean velocities) and second-order moments (Reynolds stresses) as well as concerning suitable length scales. To mimic a physically realistic incoming flow, synthetically generated turbulent velocity fluctuations superimposed on the mean velocity field are a valuable solution. However, the resolution of the grid near the inlet has to be sufficiently fine to avoid excessive damping of the turbulence intensity. In order to circumvent this problem, the injection of synthetically generated inflow data not at the inlet itself but inside the flow domain near the area of interest, where the grid is typically much finer, is an elegant loophole. In the present study two different injection techniques based on a source term formulation are analyzed and evaluated. In addition to these techniques the injected data are weighted by a Gaussian distribution defining the influence area. In the recent work the definition of the influence area is enhanced compared to the initial version of Schmidt and Breuer (2017) extending the application range. The case of a rather small influence area in comparison with the grid cell size is now tackled which is often relevant for industrial applications. The flow past a wall-mounted hemisphere is chosen as test case. The bluff body is exposed to a thick turbulent boundary layer at Re = 50,000. The generation of the turbulent velocity fluctuations in the present investigation relies on the digital filter concept, but the injection techniques evaluated are not restricted to this inflow generator. The synthetic turbulent velocity fluctuations are injected about one diameter upstream of the hemisphere. Wall-resolved large-eddy simulations are carried out for two grid resolutions and the corresponding results are analyzed and compared with the reference measurements by Wood et al. (2016). Finally, one injection technique is found to be clearly superior to the other, since it guarantees the correct level of the velocity fluctuations and the reproduction of the autocorrelations

Test case on QNET ERCOFTAC database (Underlying Flow Regime 3-33): Turbulent flow past a wall-mounted hemisphere.

https://kbwiki.ercoftac.org/w/index.php/Abstr:UFR_3-33New complementary experimental and numerical CFD test case.The objective of the present contribution is to provide a detailed experimental and numerical investigation on the turbulent flow past a smooth and rigid wall-mounted hemispherical obstacle. For this purpose, the hemisphere (diameter D) is exposed to a thick turbulent boundary layer of the thickness δ = D/2 at Re = 50,000. In order to generate the desired boundary layer in the experiment, a combination of specific fences is placed in the upstream region of the wind tunnel. Detailed measurements of the inflow conditions are realized using laser-Doppler and hot-film anemometry. Furthermore, the experimental data are utilized to generate inflow conditions for the numerical simulation that match the experimental inflow conditions. These clearly defined boundary and operating conditions are the prerequisites for a combined experimental and numerical investigation of the flow field relying on laser-Doppler anemometry measurements and on large-eddy simulations.The numerical results are produced by a finite-volume Navier-Stokes solver for block-structured curvilinear grids. A fine wall-resolving mesh is applied resulting from a preliminary study. An additional analysis is conducted to select a suitable subgrid-scale model.The final investigation includes a profound analysis on the unsteady flow features observed in the vicinity of the hemisphere like the horseshoe vortex, the recirculation area, the hairpin structure or the vortex shedding processes. A detailed discussion of the time-averaged flow field comprising the mean velocity field as well as the Reynolds stresses is provided. Owing to the proper description of the oncoming flow and the additional numerical studies guaranteeing the choice of an appropriate grid and subgrid-scale model, the experimental and numerical results are found to be in close agreement

Numerical studies on the instantaneous fluid–structure interaction of an air-inflated flexible membrane in turbulent flow

International audienceThe present paper is the numerical counterpart of a recently published experimental investigation by Wood et al. (2018). Both studies aim at the investigation of instantaneous fluid-structure interaction (FSI) phenomena observed for an air-inflated flexible membrane exposed to a turbulent boundary layer, but looking at the coupled system based on different methodologies. The objective of the numerical studies is to supplement the experimental investigations by additional insights, which were impossible to achieve in the experiments. Relying on the large-eddy simulation technique for the predictions of the turbulent flow, non-linear membrane elements for the structure and a partitioned algorithm for the FSI coupling, three cases with different Reynolds numbers (Re = 50,000, 75,000 and 100,000) are simulated. The time-averaged first and second-order moments of the flow are presented as well as the time-averaged deformations and standard deviations. The predictions are compared with the experimental references data solely available for 2D planes. In order to better comprehend the three-dimensionality of the problem, the data analysis of the predictions is extended to 3D time-averaged flow and structure data. Despite minor discrepancies an overall satisfying agreement concerning the time-averaged data is reached between experimental data in the symmetry plane and the simulations. Thus for an in-depth analysis, the numerical results are used to characterize the transient FSI phenomena of the present cases either related to the flow or to the structure. Particular attention is paid to depict the different vortex shedding types occurring at the top, on the side and in the wake of the flexible hemispherical membrane. Since the fluid flow plays a significant role in the FSI phenomena, but at the same the flexible membrane with its eigenmodes also impacts the deformations, the analysis is based on the frequencies and Strouhal numbers found allowing to categorize the different observations accordingly

Analysis of Shared Heritability in Common Disorders of the Brain

Disorders of the brain can exhibit considerable epidemiological comorbidity and often share symptoms, provoking debate about their etiologic overlap. We quantified the genetic sharing of 25 brain disorders from genome-wide association studies of 265,218 patients and 784,643 control participants and assessed their relationship to 17 phenotypes from 1,191,588 individuals. Psychiatric disorders share common variant risk, whereas neurological disorders appear more distinct from one another and from the psychiatric disorders. We also identified significant sharing between disorders and a number of brain phenotypes, including cognitive measures. Further, we conducted simulations to explore how statistical power, diagnostic misclassification, and phenotypic heterogeneity affect genetic correlations. These results highlight the importance of common genetic variation as a risk factor for brain disorders and the value of heritability-based methods in understanding their etiology