A Coursebook in a Trilingual Classroom: To Use or not to Use

Coursebook use has proved to be a controversial issue in methodology. In this paper we refer to different writing on this subject, and take the argument a step further. As the way out, we look at an aspect of teaching English as a foreign language (EFL) that sooner or later every teacher comes up against – a need to write his/her materials. As our research has shown, this becomes of major importance in a trilingual classroom. Here, we also refer to some of the theoretical positions underlying third language acquisition (TLA)

Large eddy simulation of a low-pressure turbine cascade with turbulent end wall boundary layers

We present results of implicit large eddy simulation (LES) and different Reynolds-averaged Navier-Stokes (RANS) models of the MTU 161 low pressure turbine at an exit Reynolds number of 90,000 and exit Mach number of 0.6. The LES results are based on a high order discontinuous Galerkin method and the RANS is computed using a classical finite-volume approach. The paper discusses the steps taken to create realistic inflow boundary conditions in terms of end wall boundary layer thickness and free stream turbulence intensity. This is achieved by tailoring the input distribution of total pressure and temperature, Reynolds stresses and turbulent length scale to a Fourier series based synthetic turbulence generator. With this procedure, excellent agreement with the experiment can be achieved in terms of blade loading at midspan and wake total pressure losses at midspan and over the channel height. Based on the validated setup, we focus on the discussion of secondary flow structures emerging due to the interaction of the incoming boundary layer and the turbine blade and compare the LES to two commonly used RANS models. Since we are able to create consistent setups for both LES and RANS, all discrepancies can be directly attributed to physical modelling problems. We show that both a linear eddy viscosity model and a differential Reynolds stress model coupled with a state-of-the-art correlation-based transition model fail, in this case, to predict the separation induced transition process around midspan. Moreover, their prediction of secondary flow losses leaves room for improvement as shown by a detailed discussion turbulence kinetic energy and anisotropy fields.Comment: invited submission to Flow, Turbulence and Combustion Special Issue: 13th ERCOFTAC Workshop on Direct and Large Eddy Simulation - DLES1

Statistical Error Estimation Methods for Engineering-Relevant Quantities From Scale-Resolving Simulations

Scale-resolving simulations, such as large eddy simulations, have become affordable tools to investigate the flow in turbomachinery components. The resulting time-resolved flow field is typically analyzed using first- and second-order statistical moments. However, two sources of uncertainty are present when statistical moments from scale-resolving simulations are computed: the influence of initial transients and statistical errors due to the finite number of samples. In this paper, both are systematically analyzed for several quantities of engineering interest using time series from a long-time large eddy simulation of the low-pressure turbine cascade T106C. A set of statistical tools to either remove or quantify these sources of uncertainty is assessed. First, the Marginal Standard Error Rule is used to detect the end of the initial transient. The method is validated for integral and local quantities and guidelines on how to handle spatially varying initial transients are formulated. With the initial transient reliably removed, the statistical error is estimated based on standard error relations considering correlations in the time series. The resulting confidence intervals are carefully verified for quantities of engineering interest utilizing cumulative and simple moving averages. Furthermore, the influence of periodic content from large scale vortex shedding on the error estimation is studied. Based on the confidence intervals, the required averaging interval to reduce the statistical uncertainty to a specific level is indicated for each considered quantity

Low Mach preconditioning for turbomachinery flow simulations with cavities and variable gas compositions

The optimization of turbomachines increasingly relies on highly accurate numerical performance predictions. Loss predictions require the cavities of the machine to be included in numerical simulations. Commonly, in cavities, the velocity of the simulated fluid is small. For density-based solvers, this results in slow convergence and inaccurate computations. Further, the fluid in cavities is often composed of several gases. This paper presents the low Mach preconditioning method for multi-component thermally perfect gas of DLR’s inhouse solver TRACE. Two low Mach academic test cases, a lid driven cavity and an air and exhaust gas mixing layer, are computed to validate the preconditioner. Both test cases show an accelarated convergence and an improved accuracy, when preconditioning is used. A 1.5 stage low-pressure turbine rig with a labyrinth seal is computed with thermally perfect air. The result shows a good agreement with the experimental reference. The fluid is then changed to exhaust gas, and two air inflows are added in the labyrinth seal, to analyze the effect of low Mach preconditioning on the mixing of the two gases. The preconditioned computation shows an improved convergence in the cavity. Moreover, the wall temperature and the gas distribution in the cavity differ, when preconditioning is applied

Simulation of Indexing and Clocking with a New Multidimensional Time Harmonic Balance Approach

Alongside the capability to simulate rotor-stator interactions, a central aspect within the development of frequencydomain methods for turbomachinery flows, is the ability of the method to accurately predict rotor-rotor and stator-stator interactions on a single-passage domain. To simulate such interactions, state-of-the-art frequency-domain approaches require one fundamental interblade phase angle, and therefore it can be necessary to resort to multi-passage configurations. Other approaches neglect the cross-coupling of different harmonics. As a consequence, the influence of indexing on the propagation of the unsteady disturbances is not captured. To overcome these issues, the harmonic balance approach based on multidimensional Fourier transforms in time, recently introduced by the authors, is extended in this work to account for arbitrary interblade phase angle ratios on a single-passage domain. To assess the ability of the approach to simulate the influence of indexing on the steady, as well as on the unsteady, part of the flow, the proposed extension is applied to a modern low-pressure fan stage of a civil aero engine under the influence of an inhomogeneous inflow condition. The results are compared to unsteady simulations in the time-domain and to state-of-the-art frequency-domain methods based on one-dimensional discrete Fourier transforms

A New Harmonic Balance Approach Using Multidimensional Time

Over the past years, nonlinear frequency-domain methods have become a state-of-the-art technique for the numerical simulation of unsteady flow fields within multistage turbomachinery as they are capable of fully exploiting the given spatial and temporal periodicities, as well as modelling flow nonlinearities in a computationally efficient manner. Despite this success, it still remains a significant challenge to capture nonlinear interaction effects within the context of configurations with multiple fundamental frequencies. If all frequencies are integer multiples of a common fundamental frequency, the interval spanned by the sampling points typically resolves the period of the common base frequency. For configurations in which the common frequency is very low in relation to the frequencies of primary interest, many sampling points are required to resolve the highest harmonic of the common fundamental frequency and the method becomes inefficient. In addition when a problem can no longer be described by harmonic perturbations that are integer multiples of one fundamental frequency, as it may occur in two-shaft configurations or when simulating the nonlinear interaction in the context of forced response or flutter, then the standard discrete Fourier transform is no longer suitable and the basic harmonic balance method requires extension. One possible approach is to use almost periodic Fourier transforms with equidistant or non-equidistant time sampling. However, the definition of suitable sampling points that lead to well-conditioned Fourier transform matrices and small aliasing errors is an intricate issue and far from straightforward. To overcome the issues regarding multi-frequency problems described above, a new harmonic balance approach based on multidimensional Fourier transforms in time is presented. The basic idea of the approach is that, instead of defining common sampling points in a common time period, separate time domains, one for each base frequency, are spanned and the sampling points are computed equidistantly within each base frequency's period. Since the sampling domain is now extended to a multidimensional time-domain, all time instant combinations covering the whole multidimensional domain are computed as the Cartesian product of the sampling points on the axes. In a similar fashion the frequency-domain is extended to a multidimensional frequency-domain by the Cartesian product of the harmonics of each base frequency, so that every point defined by the Cartesian product is an integer linear combination of the occurring frequencies. In this way the proposed method is capable of fully integrating the nonlinear coupling effects between higher harmonics of different fundamental frequencies by using multidimensional discrete Fourier transforms within the harmonic balance solution procedure. The aim of this paper is to introduce the multidimensional harmonic balance method in detail and demonstrate the capability of the approach to simultaneously capture unsteady disturbances with arbitrary excitation frequencies. Therefore the well established aeroelasticity testcase standard configuration 10 in the presence of an artificial inflow disturbance, that mimics an upstream blade wake, is investigated. The crucial aspect of the proposed testcase is that a small ratio of the frequency of the inflow disturbance and the blades vibration frequency is chosen. To demonstrate the advantages of the newly proposed multidimensional harmonic balance approach, the results are compared to unsteady simulations in the time-domain and to state-of-the-art frequency-domain methods based on one-dimensional discrete Fourier transforms

Разработка алгоритма поэтапного совмещения перекрывающихся изображений для сравнительного анализа методов обнаружения ключевых точек

Memristive switches are promising devices for future nonvolatile nanocrossbar memory devices. In particular, complementary resistive switches (CRSs) are the key enabler for passive crossbar array implementation solving the sneak path obstacle. To provide logic along with memory functionality, "material implication" (IMP) was suggested as the basic logic operation for bipolar resistive switches. Here, we show that every bipolar resistive switch as well as CRSs can be considered as an elementary IMP logic unit and can systematically be understood in terms of finite-state machines, i.e., either a Moore or a Mealy machine. We prove our assumptions by measurements, which make the IMP capability evident. Local fusion of logic and memory functions in crossbar arrays becomes feasible for CRS arrays, particularly for the suggested stacked topology, which offers even more common Boolean logic operations such as AND and NOR

A Numerical Test Rig for Turbomachinery Flows Based on Large Eddy Simulations With a High-Order Discontinuous Galerkin Scheme - Part 3: Secondary Flow Effects

In this final paper of a three-part series, we apply the numerical test rig based on a high-order Discontinuous Galerkin scheme to the MTU T161 low pressure turbine with diverging end walls at off-design Reynolds number of 90,000, Mach number of 0.6 and inflow angle of 41'. The inflow end wall boundary layers are prescribed in accordance with the experiment. Validation of the setup is shown against recent numerical references and the corresponding experimental data. Additionally, we propose and conduct a purely numerical experiment with upstream bar wake generators at a Strouhal number of 1.25, which is well above what was possible in the experiment. We discuss the flow physics at midspan and in the end wall region and highlight the influence of the wakes from the upstream row on the complex secondary flow system using instantaneous flow visualization, phase averages and modal decomposition techniques