Variational assimilation of sparse time-averaged data for efficient adjoint-based optimization of unsteady RANS simulations

Data assimilation (DA) plays a crucial role in extracting valuable information from flow measurements in fluid dynamics problems. Often only time-averaged data is available, which poses challenges for DA in the context of unsteady flow problems. Recent works have shown promising results in optimizing Reynolds-averaged Navier-Stokes (RANS) simulations of stationary flows using sparse data through variational data assimilation, enabling the reconstruction of mean flow profiles. In this study we perform three-dimensional variational data assimilation of sparse time-averaged data into an unsteady RANS (URANS) simulation by means of a stationary divergence-free forcing term in the URANS equations. Efficiency and speed of our method are enhanced by employing coarse URANS simulations and leveraging the stationary discrete adjoint method for the time-averaged URANS equations. The data assimilation codes were developed in-house using OpenFOAM for the URANS simulations as well as for the solution of the adjoint problem, and Python for the gradient-based optimization. Our results demonstrate that data assimilation of sparse time-averaged velocity measurements not only enables accurate mean flow reconstruction, but also improves the flow dynamics, specifically the vortex shedding frequency. To validate the efficacy of our approach, we applied it to turbulent flows around cylinders of various shapes at Reynolds numbers ranging from 3000 to 22000. Our findings indicate that data points near the cylinder play a crucial role in improving the vortex shedding frequency, while additional data points further downstream are necessary to also reconstruct the time-averaged velocity field in the wake region

Magnetic circular dichroism in X-ray fluorescence of Heusler alloys at threshold excitation

The results of fluorescence measurements of magnetic circular dichroism (MCD) in Mn L_2,L_3 X-ray emission and absorption for Heusler alloys NiMnSb and Co2MnSb are presented. Very intense resonance Mn L_3 emission is found at the Mn 2p_3/2 threshold and is attributed to a peculiarity of the threshold excitation in materials with the half-metallic character of the electronic structure. A theoretical model for the description of resonance scattering of polarized x-rays is suggested.Comment: 3 pages, 2 figures. Discussed at conferences, submitting process in progres

Measuring Liquidity in Agricultural Land Markets

This paper contributes to the sparse empirical literature on measuring liquidity in agricultural land markets. Using data from Lower Saxony (Germany), we inspect the spatial and temporal variability of various liquidity indicators. We apply a panel vector autoregression (VAR) and Granger causality tests to examine the relationship between liquidity and prices and to identify further determinants of land market liquidity, such as supply shocks and clientele effects. Unlike in housing markets, no positive relationship between prices and market liquidity exists. We conclude that in agricultural land markets, a high demand from expanding farms absorbs supply shocks regardless of prevailing prices

Robust variational data assimilation of sparse velocity reference data in RANS simulations through a divergence-free forcing term

The Reynolds-averaged Navier-Stokes (RANS) equations offer a computationally efficient way of solving fluid flow problems in engineering applications. However, the use of closure models to represent turbulence effects can compromise their accuracy. In order to address this issue, recent research has explored data-driven techniques like data assimilation and machine learning. We present an efficient variational data assimilation (DA) approach to enhance steady-state RANS simulations based on eddy viscosity closure models. Our method introduces a corrective forcing term based on a potential field that is divergence-free and enhances simulation accuracy. The DA implementation relies on the discrete adjoint method and approximations for efficient gradient evaluation. The implementation is built on a two-dimensional coupled RANS solver in \emph{OpenFOAM}, which is extended to allow the computation of the adjoint velocity and pressure as well as the adjoint gradient. A gradient-based optimizer is employed to minimize the difference between the simulation results and the reference data. To assess this approach, it is compared with alternative data assimilation methods for canonical stationary two-dimensional turbulent flow problems. For the DA, sparsely distributed data from averaged high-fidelity simulation results are used. The findings indicate that the proposed method achieves the optimization goal more efficiently compared to applying data assimilation for obtaining the eddy viscosity, or a field modifying the eddy viscosity, directly. It is sturdy with respect to varying the regularization parameters and the number of reference data points, and runs efficiently by leveraging coarse meshes.Comment: 23 pages, 13 figure

Eine Fundstelle der Glockenbecherzeit in Wetzikon ZH – Kempten, Tösstalstrasse 32–36

Geplante Bauarbeiten zur Erweiterung der Berufswahl- und Weiterbildungsschule Zürcher Oberland erforderten vorgängige archäologische Sondierungen. Abgesehen von römischen Funden wurde eine Strate mit glockenbecherzeitlichen Scherben entdeckt. Die darauf eingeleitete Rettungsgrabung führte zur Freilegung eines verlandeten Bacharmes. In dieser natürlichen Senke war eine Kulturschicht der Glockenbecherzeit erhalten geblieben. Bauliche Strukturen wurden nicht festgestellt. Das umfangreiche Fundmaterial und die C14-Daten lassen eine über längere Zeit dauernde Besiedlung vermuten. Von grossem Interesse sind zudem die Ergebnisse der Archäobiologie. An Tierknochen liessen sich ausschliesslich Haustiere bestimmen. Unter den Kulturpflanzen war Gerste, gefolgt von Emmer am stärksten vertreten. Der deutliche Nachweis von Grünlandpflanzen weist auf die Verwendung von Heu. Dies - zusammen mit weiteren Umweltdaten - deutet auf eine Öffnung der Landschaft im Endneolithikum

Effect of structure on tribomechanical properties of sintered (Al-12Si)-Sn composites

The structure, mechanical and tribological properties of sintered (Al-12Si)-xSn composites were investigated in the present study. The composites were obtained by sintering of the briquettes made of aluminum (Al-12Si) and tin powders mixture. It was found that the hot densification substantially increases the strength of sintered composites and also makes much higher their wear resistance under the dry friction against a steel counterbody. The mechanical properties of the obtained materials slowly decrease with an increasing of tin content up to 40 wt. %, while their wear resistance increases

Magnetic phase separation in ordered alloys

We present a lattice model to study the equilibrium phase diagram of ordered alloys with one magnetic component that exhibits a low temperature phase separation between paramagnetic and ferromagnetic phases. The model is constructed from the experimental facts observed in Cu$_{3-x}$AlMn$_{x}$ and it includes coupling between configurational and magnetic degrees of freedom which are appropriated for reproducing the low temperature miscibility gap. The essential ingredient for the occurrence of such a coexistence region is the development of ferromagnetic order induced by the long-range atomic order of the magnetic component. A comparative study of both mean-field and Monte Carlo solutions is presented. Moreover, the model may enable the study of the structure of the ferromagnetic domains embedded in the non-magnetic matrix. This is relevant in relation to phenomena such as magnetoresistance and paramagnetism.Comment: 12 pages, 11 figures, accepted in Phys. Rev.