A Transfer Function Approach to Structural Vibrations Induced by Thermoacoustic Sources

To decrease NOx emissions from a combustion system, lean premixed combustion in combination with an annular combustor is used. One of the disadvantages is an increase in sound pressure levels in the combustion system, resulting in an increased excitation of the surrounding structure, the liner. This causes fatigue, which limits the life time of the combustor. To model the interaction between flame, acoustics and structure, a transfer function approach is used. In this approach, the components are represented by the frequency dependent linear transfer between their inputs and outputs. For the flame a low pass filter with convective time delay is used as transfer function between velocity perturbations at the burner outlet and the flame as acoustic volume source. The acoustic transfer from volume source to velocity perturbation at the burner outlet is obtained from a harmonic finite element analysis, in which a temperature field from CFD calculations is used. The calculated response is subsequently curve-fitted using a pole-zero model to allow for fast calculations. The finite element model includes the two way coupling between structural vibrations and acoustics, which allows extraction of the vibration levels. The different transfers are finally coupled in one model. Results show frequencies of high acoustic response which are susceptible to thermoacoustic instability. Damping mechanisms and the phase relation between the different components determine stable or unstable behavior and the amplitude of the resulting perturbations. Furthermore there are also frequencies of high structural response. Especially when the two coincide, the risk of structural damage is high, whereas when they move away from each other, the risk decreases

Vibration of the liner in an industrial combustion system due to an acoustic field

The subject of this paper is a numerical study of the properties of the liner of a test rig to be built in the future. The test-rig consists of a flexible tube of square crosssection surrounded by a pressure vessel, also with a square cross-section. At first instance, a two dimensional structural analytical model of the cross-section is made. The influence of the air between liner and pressure vessel and that within the liner on the vibration of the liner is studied using a coupled 2D finite element model. Furthermore the influence of the vibration of the liner on the acoustics of the setup is studied. After this the problem is extended to three dimensions and again the influence of the cavity surrounding the liner is analyzed. Both 2D and 3D results are compared. The cavities are found to substantially influence the structural behavior and therefore they cannot be neglected in predicting the behavior of the liner

Migrant Women on the Labour Market

The behaviour of migrant women on the labour market is influenced by a variety of factors, among which the culture of the home and the host country. Part of the literature investigates the role of home-country culture. This study extends the literature by including a measure for the influence of host-country culture as an additional determinant of the participation of migrant women. The empirical model explains participation from demographics and educational attainment, and uses home- and host-country female participation as proxies for culture. Evidence on the basis of the Dutch Labour Force Survey 1996 – 2007 suggests that both differences in home-country female participation and the trend in native female participation, as a measure for host-country culture, affect the participation of migrant women. The results suggest that host-country participation is at least as important as home-country participation. Keywords: female labour force participation, immigration, cultural transmission

Migrant Women on the Labour Market: On the Role of Home- and Host-Country Participation

The behaviour of migrant women on the labour market is influenced by a variety of factors, among which the culture of the home and host country. Part of the literature investigates the role of home-country culture. This study extends the literature by including a measure for the influence of host-country culture as an additional determinant of the participation of migrant women. The empirical model explains participation from demographics and educational attainment, and uses home- and host-country female participation as proxies for culture. Evidence on the basis of the Dutch Labour Force Survey 1996-2007 suggests that both differences in home-country female participation and the trend in native female participation, as a measure for host-country culture, affect the participation of migrant women. The results suggest that host-country participation is at least as important as home-country participation.female labour force participation, immigration, cultural transmission

Experimental validation of the interaction between combustion and structural vibration

To decrease NOx emissions from combustion systems, lean premixed combustion is used. A disadvantage is the increase in sound pressure levels in the combustor, resulting in an increased excitation of the surrounding structure: the liner. This causes fatigue, which limits the life time of the combustor. To study this problem experimentally, a test setup has been built consisting of a single burner, 500kW, 5 bar combustion system. The thin structure (liner) is contained in a thick pressure vessel with optical access for a traversing laser vibrometer system to measure the vibration levels and mode shapes of the liner. The acoustic excitation of the liner is measured using pressure sensors measuring the acoustic pressures inside the combustion chamber and in the cooling passage between the liner and the pressure vessel. To validate models, measurements were performed in steps of increasing complexity. Firstly, the structural properties, obtained by modal analysis of the liner outside the pressure vessel, have been compared with a finite element model. Subsequently, results of an acoustic finite element model of the setup have been compared to acoustic measurements on the test rig to validate the acoustic properties of the model, which are made by mounting a well defined acoustic source to the rig. Finally, measured pressures and vibration levels in the presence of combustion are shown

A state space model for exponential smoothing with group seasonality

We present an approach to improve forecast accuracy by simultaneously forecasting a group of products that exhibit similar seasonal demand patterns. Better seasonality estimates can be made by using information on all products in a group, and using these improved estimates when forecasting at the individual product level. This approach is called the group seasonal indices (GSI) approach, and is a generalization of the classical Holt-Winters procedure. This article describes an underlying state space model for this method and presents simulation results that show when it yields more accurate forecasts than Holt-Winters.Common seasonality; demand forecasting; exponential smoothing; Holt-Winters; state space model.

Hybrid contractual landscapes of governance: Generation of fragmented regimes of public accountability through urban regeneration

In this article we explore the idea of public accountability in the contemporary entrepreneurial governance of cities, which are influenced by market dependency and private sector involvement. We specifically focus on the fragmentation of public accountability through hybrid contractual landscapes of governance, in which the public and private sector actors interactively produce a diversity of instruments to ensure performance in service. This is in sharp contrast to the traditional vague norms and values appealed to by urban planning institutions, to safeguard the public interest. We argue that within these complex contractual governance environments public accountability is produced by public and private sector actors, through highly diverse sets of contractual relations and diverse control instruments that define responsibilities of diverse actors who are involved in a project within a market-dependent planning and policy making environment, which contains context-specific characteristics set by the specific rules of public-private collaboration. These complexities mean public accountability has become fragmented and largely reduced to performance control. Moreover, our understanding of contractual urban governance remains vague and unclear due to very limited empirical studies focusing on the actual technologies of contractual urban development. By deciphering the complex hybrid landscapes of contractual governance, with comparative empirical evidence from The Netherlands, UK and Brazil, we demonstrate how public accountability is assuming a more ‘contractual’ and unpredictable meaning in policy and plan implementation process

When the frame fits the social picture:The effects of framed social norm messages on healthy and unhealthy food consumption

We investigated the influence of framed norm messages about food consumption on motivation to consume, and actual consumption of, healthy and unhealthy foods. We proposed that the effects of positive and negative message frames would vary by the type of underlying norms (i.e., injunctive, descriptive). More specifically, based on information processing theories, it was expected that injunctive norms would be more effective when framed negatively compared with positively, while the opposite was expected for descriptive norms. In both experiments, participants were randomly assigned to one of four framed social norm conditions or a no-norm control condition. In experiment 1, motivation to consume healthy and unhealthy foods was assessed by means of both indirect and self-report measures. In experiment 2, actual food consumption was assessed. In both experiments, the predicted interaction was found. Results show that injunctive norms benefit from a negative (vs. Positive) frame, while preliminary evidence suggests the opposite for descriptive norms