Incorporating unsteady flow-field effects in flamelet-generated manifolds

In general, simulating combustion can be a very costly job. This is caused by the large number of chemical reacting species that are strongly coupled. Moreover, all the (chemical) time-scales that are present, span a multitude of orders, which results in a very large, stiff system of strongly coupled, nonlinear equations and solving such a system is very CPU-intensive. Fortunately, it appears that many combustion systems are dominated by a handful of (slow) processes only. This is due to the fact that the fastest processes rapidly become exhausted and therefore are often neglected. This has subsequently led to a number of reduction techniques that take advantage of the observation that combustion can often be predicted reasonably accurate by taking only a small number of time-scales into account. In this thesis a reduction technique that was introduced by van Oijen [61], i.e., Flamelet- Generated Manifolds (FGM), is expanded upon. The main goal of this thesis is to study whether unsteady flow effects can be captured within the Flamelet-Generated Manifolds concept. The flames that are studied are one-dimensional, non-premixed, stagnation flames and although FGMwas initially developed for and successfully applied to premixed flames but in principle it can also be applied to non-premixed flames. To that end, first a unified one-dimensional flame model is presented, which can be used to describe (partially) premixed and non-premixed flames. Such a one-dimensional flame model is often referred to as flameletmodel. The first step is to decompose the combustion process into three distinct sub-problems, i.e., 1) fluid motion and mixing of enthalpy and elements, 2) the flame front dynamics and 3) the dynamics of the internal flame structure embedded within this flame front. When flames are considered, it is often useful to use a so-called flame adapted coordinate system, where coordinate surfaces correspond to flame surfaces. The flame front dynamics can be described by the evolution of these flame surfaces, which correspond to iso-surfaces of a so-called principal controlling variable Y, for which a conservation equation can be solved. Applying such a coordinate transformation, leads to a set of quasi-one-dimensional combustion equations, which serve as the basis of the FGM method. Generally it is assumed that perturbations from one-dimensional flame behavior are small, and can therefore be neglected. In order to numerically assess these assumptions, the species conservation equation is subdivided into several individual contributions, i.e., an unsteady term, normal transport, flame stretch, curvature, tangential diffusion and the chemical production and consumption terms, respectively. Three different twodimensional flames are simulated, one unsteady premixed flame, one unsteady nonpremixed flame and one steady non-premixed flame. Using the numerical results from these detailed flame simulations, the individual contributions of the species conservation equations are computed and compared to each other. The results show that besides normal transport and chemistry, flame stretch rate and curvature can also be important in both premixed and non-premixed flame simulations. From the two unsteady flame simulations it also follows that the unsteady contribution can be significant. Furthermore, for the steady non-premixed flame, two different principal controlling variables were chosen, resulting in two different coordinate transformations, i.e., a typical non-premixed flame-adapted coordinate system and a typical premixed one. This is possible due to the fact that the flamelet model derived in this thesis is a unified flamelet model, which is able to describe both (partially) premixed flames as well as non-premixed flames. To study whether the effect of transient, local flow fluctuations can be captured by the FGM approach, both steady and unsteady non-premixed flamelet simulations with a detailed chemistry model are studied. Two different situations are studied, 1) a flame which is significantly strained but still far away from the steady extinction limit and 2) a flame where the applied strain-rate is near or even beyond the steady extinction limit. For both situations, two different Flamelet-Generated Manifolds are constructed, i.e., one based on a set steady flamelet simulations and one based on a set of unsteady flamelet simulations. The chemical compositions found during the steady flamelet simulations form a two-dimensional manifold in composition space. On the other hand, a detailed analysis of the chemical compositions found during the unsteady flamelet simulations shows that the unsteady flamelet simulations form a three-dimensional manifold in composition space. Both manifolds are applied to simulate one-dimensional flames that are subjected to sinusoidally varying strain-rate. The results of both FGM simulations are compared to an unsteady simulation with a detailed chemistry model. Both local observables, like species mass fractions and temperature for example, as well as flamesurface area properties like the integral source-term, are represented well with both manifolds. However, for species that are related to the slowest time-scales it is shown that a three-dimensional manifold may result in less accurate predictions, and more controlling variables may be needed

Induced work function changes at Mg-doped MgO/Ag(001) interfaces: Combined Auger electron diffraction and density functional study

The properties of MgO/Ag(001) ultrathin films with substitutional Mg atoms in the interface metal layer have been investigated by means of Auger electron diffraction experiments, ultraviolet photoemission spectroscopy, and density functional theory (DFT) calculations. Exploiting the layer-by-layer resolution of the MgKL23L23 Auger spectra and using multiple scattering calculations, we first determine the interlayer distances as well as the morphological parameters of the MgO/Ag(001) system with and without Mg atoms incorporated at the interface. We find that the Mg atom incorporation drives a strong distortion of the interface layers and that its impact on the metal/oxide electronic structure is an important reduction of the work function (0.5 eV) related to band-offset variations at the interface. These experimental observations are in very good agreement with our DFT calculations which reproduce the induced lattice distortion and which reveal (through a Bader analysis) that the increase of the interface Mg concentration results in an electron transfer from Mg to Ag atoms of the metallic interface layer. Although the local lattice distortion appears as a consequence of the attractive (repulsive) Coulomb interaction between O2− ions of the MgO interface layer and the nearest positively (negatively) charged Mg (Ag) neighbors of the metallic interface layer, its effect on the work function reduction is only limited. Finally, an analysis of the induced work function changes in terms of charge transfer, rumpling, and electrostatic compression contributions is attempted and reveals that the metal/oxide work function changes induced by interface Mg atoms incorporation are essentially driven by the increase of the electrostatic compression effect

Band bending in Mg-colored and O₂-activated ultrathin MgO(001) films

Ultrathin MgO films grown on Ag(001) have been investigated using X-ray and ultraviolet photoemission spectroscopies for oxide films successively exposed to Mg and O₂ flux. Studying work functions and layer-resolved Auger shifts allows us to keep track of band profiles from the oxide surface to the interface and reveal the charge- transfer mechanisms underlying the controlled creation of Mg-induced surface color centers and the catalytic enhancement of O₂ activation. Our results demonstrate that one can intimately probe the catalytic properties of metal-supported ultrathin oxide films by studying the electronic band alignment at interfaces

Ultrasound- Versus Fluoroscopy-Guided Strategy for Transfemoral Transcatheter Aortic Valve Replacement Access: A Systematic Review and Meta-Analysis

Background:Access site vascular and bleeding complications remain problematic for patients undergoing transcatheter aortic valve replacement (TAVR). Ultrasound-guided transfemoral access approach has been suggested as a technique to reduce access site complications, but there is wide variation in adoption in TAVR. We performed a systematic review and meta-analysis to compare access site vascular and bleeding complications according to the Valve Academic Research Consortium-2 classification following the use of either ultrasound- or conventional fluoroscopy-guided transfemoral TAVR access.Methods:Medline, Embase, Web of Science, and The Cochrane Library were searched to November 2020 for studies comparing ultrasound- and fluoroscopy-guided access for transfemoral TAVR. A priori defined primary outcomes were extracted: (1) major, (2) minor, and (3) major and minor (total) access site vascular complications and (4) life-threatening/major, (5) minor, and (6) life-threatening, major, and minor (total) access site bleeding complications.Results:Eight observational studies (n=3875) were included, with a mean participant age of 82.8 years, STS score 5.81, and peripheral vascular disease in 23.5%. An ultrasound-guided approach was significantly associated with a reduced risk of total (Mantel-Haenszel odds ratio [MH-OR], 0.50 [95% CI, 0.35–0.73]), major (MH-OR, 0.51 [95% CI, 0.35–0.74]), and minor (MH-OR, 0.59 [95% CI, 0.38–0.91]) access site vascular complications. Ultrasound guidance was also significantly associated with total access site bleeding complications (MH-OR, 0.59 [95% CI, 0.39–0.90]). The association remained significant in sensitivity analyses of maximally adjusted minor and total vascular access site complications (MH-OR, 0.51 [95% CI, 0.29–0.90]; MH-OR, 0.44 [95% CI, 0.20–0.99], respectively).Conclusions:In the absence of randomized studies, our data suggests a potential benefit for ultrasound guidance to obtain percutaneous femoral access in TAVR

Temperature measurement and stabilization in a birefringent whispering gallery resonator

Temperature measurement with nano-Kelvin resolution is demonstrated at room temperature, based on the thermal dependence of an optical crystal anisotropy in a high quality whispering gallery resonator. As the resonator's TE and TM modes frequencies have different temperature coefficients, their differential shift provides a sensitive measurement of the temperature variation, which is used for active stabilization of the temperature

Rev-erb-alpha modulates skeletal muscle oxidative capacity by regulating mitochondrial biogenesis and autophagy

The nuclear receptor Rev-erb-α modulates hepatic lipid and glucose metabolism, adipogenesis and the inflammatory response in macrophages. We show here that Rev-erb-α is highly expressed in oxidative skeletal muscle and plays a role in mitochondrial biogenesis and oxidative function, in gain- and loss-of function studies. Rev-erb-α-deficiency in skeletal muscle leads to reduced mitochondrial content and oxidative function, resulting in compromised exercise capacity. This phenotype was recapitulated in isolated fibers and in muscle cells upon Rev-erbα knock-down, while Rev-erb-α over-expression increased the number of mitochondria with improved respiratory capacity. Rev-erb-α-deficiency resulted in deactivation of the Stk11–Ampk–Sirt1–Ppargc1-α signaling pathway, whereas autophagy was up-regulated, resulting in both impaired mitochondrial biogenesis and increased clearance. Muscle over-expression or pharmacological activation of Rev-erb-α increased respiration and exercise capacity. This study identifies Rev-erb-α as a pharmacological target which improves muscle oxidative function by modulating gene networks controlling mitochondrial number and function

Acceleration of generalized hypergeometric functions through precise remainder asymptotics

We express the asymptotics of the remainders of the partial sums {s_n} of the generalized hypergeometric function q+1_F_q through an inverse power series z^n n^l \sum_k c_k/n^k, where the exponent l and the asymptotic coefficients {c_k} may be recursively computed to any desired order from the hypergeometric parameters and argument. From this we derive a new series acceleration technique that can be applied to any such function, even with complex parameters and at the branch point z=1. For moderate parameters (up to approximately ten) a C implementation at fixed precision is very effective at computing these functions; for larger parameters an implementation in higher than machine precision would be needed. Even for larger parameters, however, our C implementation is able to correctly determine whether or not it has converged; and when it converges, its estimate of its error is accurate.Comment: 36 pages, 6 figures, LaTeX2e. Fixed sign error in Eq. (2.28), added several references, added comparison to other methods, and added discussion of recursion stabilit