Direct numerical simulation of an unsteady wall-bounded turbulent flow configuration for the assessment of large-eddy simulation models.

A new benchmark case for the evaluation of direct numerical simulation (DNS) and large-eddy simulation (LES) models and methods is presented in this study. The known Taylor-Green vortex is modified by replacing the periodic boundary conditions in one direction with a no-slip boundary. A passive scalar is added and transported from the wall into the fluid. The addition of walls allows for the study of transient-instationary flows in a simple geometry with clean boundary and initial conditions, which is a key requirement for the assessment of LES modeling strategies. The added scalar mimics heat transfer through the wall. The case features reasonable computational cost for highly-resolved LES and DNS calculations. Simulations of the wall-bounded Taylor-Green vortex are easy to setup and do not require additional modeling. The proposed modification of the case is compared to the default Taylor-Green vortex and the difference in flow-physics is discussed. A detailed convergence study with four meshes, each of them refined by a factor of 2, has been conducted. The results reveal that converged second-order statistics can be obtained up to a dimensionless time of [Formula: see text]. Beyond that, due to the unsteady chaotic nature of the flow, some uncertainties remain. The results show that the case features challenging (near-wall) flow dynamics, which cannot be covered using the default Taylor-Green vortex and hence, justify the proposed case as a useful benchmark.

An experimental/numerical investigation of non-reacting turbulent flow in a piloted premixed Bunsen burner.

ABSTRACT: In this paper, an experimental study of the non-reacting turbulent flow field characteristics of a piloted premixed Bunsen burner designed for operational at elevated pressure conditions is presented. The generated turbulent flow fields were experimentally investigated at atmospheric and elevated pressure by means of high-speed particle image velocimetry (PIV). The in-nozzle flow through the burner was computed using large-eddy simulation (LES), and the turbulent flow field predicted at the burner exit was compared against the experimental results. The findings show that the burner yields a reasonably homogeneous, nearly isotropic turbulence at the nozzle exit with highly reproducible boundary conditions that can be well predicted by numerical simulations. Similar levels of turbulence intensities and turbulent length scales were obtained at varied pressures and bulk velocities with turbulent Reynolds numbers up to 5300. This work demonstrates the burner's potential for the study of premixed flames subject to intermediate and extreme turbulence at the elevated pressure conditions found in gas turbine combustors.

Gas-phase aluminium acetylacetonate decomposition: revision of the current mechanism by VUV synchrotron radiation.

Although aluminium acetylacetonate, Al(C5H7O2)3, is a common precursor for chemical vapor deposition (CVD) of aluminium oxide, its gas-phase decomposition is not well-known. Here, we studied its thermal decomposition in a microreactor by double imaging photoelectron photoion coincidence spectroscopy (i2PEPICO) between 325 and 1273 K. The reactor flow field was characterized by CFD. Quantum chemical calculations were used for the assignment of certain species. The dissociative ionization of the room temperature precursor molecule starts at a photon energy of 8.5 eV by the rupture of the bond to an acetylacetonate ligand leading to the formation of the Al(C5H7O2)2+ ion. In pyrolysis experiments, up to 49 species were detected and identified in the gas-phase, including reactive intermediates and isomeric/isobaric hydrocarbons, oxygenated species as well as aluminium containing molecules. We detected aluminium bis(diketo)acetylacetonate-H, Al(C5H7O2)C5H6O2, at m/z 224 together with acetylacetone (C5H8O2) as the major initial products formed at temperatures above 600 K. A second decomposition channel affords Al(OH)2(C5H7O2) along with the formation of a substituted pentalene ring species (C10H12O2) as assigned by Franck-Condon simulations and quantum chemical calculations. Acetylallene (C5H6O), acetone (C3H6O) and ketene (C2H2O) were major secondary decomposition products, formed upon decomposition of the primary products. Three gas-phase aromatic hydrocarbons were also detected and partially assigned for the first time: m/z 210, m/z 186 (C14H18 or C12H10O2) and m/z 146 (C11H14 or C9H6O2) and their formation mechanism is discussed. Finally, Arrhenius parameters are presented on the gas-phase decomposition kinetics of Al(C5H7O2)3, aided by numerical simulation of the flow field.

Instantaneous 3D imaging of highly turbulent flames using computed tomography of chemiluminescence.

The computed tomography of chemiluminescence (CTC) technique was applied for the first time to a real highly turbulent swirl flame setup, using a large number of CCD cameras (Nq=24 views), to directly reconstruct the three-dimensional instantaneous and time-averaged chemiluminescence fields. The views were obtained from a 172.5° region (in one plane) around the flame, and the CTC algorithm [Floyd et al., Combust. Flame158, 376 (2011)CBFMAO0010-2180] was used to reconstruct the flame by discretizing the domain into voxels. We investigated how the reconstructions are affected by the views' arrangement and the settings of the algorithm, and considered how the quality of reconstructions should be assessed to ensure a realistic description of the capabilities of the technique. Reconstructions using Nq≤12 were generally better when the cameras were distributed more equiangularly. When Nq was severely low (e.g., 3), the reconstruction could be improved by using fewer voxels. The paper concludes with a summary of the strengths and weaknesses of the CTC technique for examining a real turbulent flame geometry and provides guidance on best practice.

Comparison of visual and computerized estrous detection and evaluation of influencing factors.

The aim of this study was to compare the automatic estrous detection system (AED) Heatime® with visual estrous detection (VED). The study was conducted on 139 Holstein Friesian cows in one dairy herd in Northern Germany. The cows were fitted with activity collars from d 21 postpartum until d 40 of gestation and a 30-min visual estrous observation was conducted three times a day. In addition, as a separate part of the VED, estrous detection by exclusive consideration of standing estrus (SE) was investigated. Ovulation detected by regular trans-rectal ultrasonography and serum progesterone analyses served as gold standard to calculate estrous detection rate (EDR) and reliability rate (RR) for each of the three estrous detection systems (AED, VED and SE). Change in body condition antepartum and postpartum, lameness, milk yield and milk fat- milk protein- ratio (FPR) on the expression and detection of estrus were investigated. Estrus was more precisely detected by the AED (EDR: 85.1%) than by VED (EDR: 52.2%) and SE (EDR: 22.3%) (P<0.05). The RR when using the three methods did not differ (P>0.05). Changes in body condition, lameness, milk yield or the FPR were not associated with the estrous detection rate by the AED. The estrous detection rate by VED in lame animals (EDR: 24.2%) was, however, less than in cows without any lameness (EDR: 52.7%; P<0.05). In conclusion, the AED Heatime® system can be effectively used for estrous detection and can be used to more precisely detect estrus than with VED.

Multi-directional 3D flame chemiluminescence tomography based on lens imaging.

Flame chemiluminescence tomography (FCT) has been widely used in flame diagnostics for three-dimensional (3D), spatially resolved measurements of instantaneous flame geometry and, to some extent, of species concentrations. However, in most studies, tomographic reconstructions are based on a traditional parallel projection model. Due to the light collection characteristics of a lens, a parallel projection model is not appropriate for the practical optical setups that are used for emission imaging, particularly at small F-numbers. Taking the light collection effect of the lens into account, this Letter establishes a complete and novel tomographic theory for a multi-directional tomography system consisting of a lens and CCD cameras. A modified camera calibration method is presented first. It determines the exact spatial locations and intrinsic parameters of the cameras. A 3D projection model based on the lens imaging theory is then proposed and integrated into the multiplicative algebraic reconstruction technique (MART). The new approach is demonstrated with a 12-camera system that is used to reconstruct the emission field of a propane flame, thereby resolving space and time.

Freeprocessing: Transparent in situ visualization via data interception.

In situ visualization has become a popular method for avoiding the slowest component of many visualization pipelines: reading data from disk. Most previous in situ work has focused on achieving visualization scalability on par with simulation codes, or on the data movement concerns that become prevalent at extreme scales. In this work, we consider in situ analysis with respect to ease of use and programmability. We describe an abstraction that opens up new applications for in situ visualization, and demonstrate that this abstraction and an expanded set of use cases can be realized without a performance cost.

Simultaneous temperature, mixture fraction and velocity imaging in turbulent flows using thermographic phosphor tracer particles.

This paper presents an optical diagnostic technique based on seeded thermographic phosphor particles, which allows the simultaneous two-dimensional measurement of gas temperature, velocity and mixture fraction in turbulent flows. The particle Mie scattering signal is recorded to determine the velocity using a conventional PIV approach and the phosphorescence emission is detected to determine the tracer temperature using a two-color method. Theoretical models presented in this work show that the temperature of small tracer particles matches the gas temperature. In addition, by seeding phosphorescent particles to one stream and non-luminescent particles to the other stream, the mixture fraction can also be determined using the phosphorescence emission intensity after conditioning for temperature. The experimental technique is described in detail and a suitable phosphor is identified based on spectroscopic investigations. The joint diagnostics are demonstrated by simultaneously measuring temperature, velocity and mixture fraction in a turbulent jet heated up to 700 K. Correlated single shots are presented with a precision of 2 to 5% and an accuracy of 2%.