A correction to the enhanced bottom drag parameterisation of tidal turbines

Hydrodynamic modelling is an important tool for the development of tidal stream energy projects. Many hydrodynamic models incorporate the effect of tidal turbines through an enhanced bottom drag. In this paper we show that although for coarse grid resolutions (kilometre scale) the resulting force exerted on the flow agrees well with the theoretical value, the force starts decreasing with decreasing grid sizes when these become smaller than the length scale of the wake recovery. This is because the assumption that the upstream velocity can be approximated by the local model velocity, is no longer valid. Using linear momentum actuator disc theory however, we derive a relationship between these two velocities and formulate a correction to the enhanced bottom drag formulation that consistently applies a force that remains closed to the theoretical value, for all grid sizes down to the turbine scale. In addition, a better understanding of the relation between the model, upstream, and actual turbine velocity, as predicted by actuator disc theory, leads to an improved estimate of the usefully extractable energy. We show how the corrections can be applied (demonstrated here for the models MIKE 21 and Fluidity) by a simple modification of the drag coefficient

Performance evaluations and control system design

The accuracy of information available to managers about an employee's performance, combined with the transparency of performance evaluations based on that information, can help to motivate managers to reward good performance and highlight poor performance

Advanced numerical methods for mantle convection models

Numerical modelling of Earth's mantle is a complex, and computationally demanding task due to, amongst others, the broad spectrum of temporal and spatial scales playing a role in mantle flow, large uncertainties in the physical properties of mantle material, with large and localised transitions in viscosity and density. This thesis introduces and analyses a number of numerical techniques that may bring a significant contribution in meeting some of these challenges. Firstly, we introduce a novel time integration scheme for free surface movement in mantle convection models that is more accurate and stable for large time steps. Secondly, we extend the capabilities of anisotropic mesh optimisation, which allows efficient focussing of mesh resolution, to handle cylindrical and spherical shell domains and demonstrate that a significant reduction in the required number of degrees of freedom is possible while maintaing accuracy. Finally, to verify correctness, and evaluate and compare properties of various numerical schemes, we derive an extensive suite of analytical solutions to the Stokes equations governing mantle flow in cylindrical and spherical shell domains, with physically relevant boundary conditions. As a numerical benchmark they also serve to facilitate comparisons of different geodynamical models, and the further development of numerical techniques to improve these.Open Acces

PERFORMANCE EVALUATION UNDER UNCERTAINTY: THE IMPACT OF INFORMATION SYSTEM INTEGRATION ON PROCEDURAL JUSTICE PERCEPTIONS

The design and functioning of organizational control systems that allow for the monitoring of employee performance is at the core of management accounting research. With the emergence of enterprise resource planning (ERP) systems and the recent developments towards information system integration, the traditional structures of organizational control mechanisms have significantly changed. Only sparse evidence exists, however, regarding the role that the integration of information technology plays in performance measurement and evaluation. As a response, this paper investigates the impact that information system integration has on managers’ attitudes towards their performance evaluation processes. Based on organizational justice theory we hypothesize that integrated information systems positively impact managers’ perceptions of procedural justice, as these systems can improve the comprehensiveness and objectivity of performance data as well as the traceability of managers’ decisions. We further hypothesize that this effect is amplified with an increasing environmental uncertainty the managers perceive. Survey evidence from 132 managers of companies in Germany confirms the hypotheses and highlights the importance of integrated information systems in the performance evaluation process