Robustness to Estimation Errors for Size-Aware Scheduling

When scheduling size-aware single server systems, Shortest Remaining Processing Time (SRPT) has strong optimality properties - it minimizes the number of jobs at the server and as a consequence, the mean response time. A major caveat of SRPT is that it requires job sizes a priori. This thesis examines a scenario that is likely to occur in practice, where only estimates of job sizes are available. A single server model and a multi-server model using SRPT are compared to a Class-Based policy that is designed to increase robustness to estimation errors. In the single server model we observe from simulations that such robustness is crucial to achieve good performance. In contrast, we observe that a multi-server system is inherently more robust than a single server system. Both policies work well with the estimation errors in the multi-server system.ThesisMaster of Applied Science (MASc

GT2006-90214 UNSTEADY AERODYNAMIC BLADE EXCITATION AT THE STABILITY LIMIT AND DURING ROTATING STALL IN AN AXIAL COMPRESSOR

ABSTRACT The stable operating range of axial compressors is limited by the onset of rotating stall and surge. These flow conditions endanger the reliability of operation and have definitely to be avoided in compressors of gas turbines. However, there is still a need to improve the physical understanding of these flow phenomena to prevent them while utilizing the maximum available working potential of the compressor. This paper discusses detailed experimental investigations of the rotating stall onset with the main emphasis on the aerodynamic blade excitation in the Dresden four-stage LowSpeed Research Compressor. The stall inception, which is triggered by modal waves, as well as the main flow features during rotating stall operation are discussed. To investigate the unsteady pressure distributions, both the rotor and the stator blades of the first stage were equipped with piezoresistive pressure transducers. Based on these measurements the unsteady blade pressure forces are calculated. Time-resolved results at the stability limit as well as during rotating stall are presented. For all operating conditions rotor-stator-interactions play an important role on the blade force excitation. Furthermore the role of the inertia driven momentum exchange at the stall cell boundaries on the aerodynamic blade force excitation is pointed out

Pilotierung von Teilhabe an der praktischen Lehre durch Einsatz von Telepräsenzrobotern an der Medizinischen Fakultät Dresden

Das Medizinische Interprofessionelle Trainingszentrum (MITZ) ist ein Skills Lab, in welchem Studierende (ST) praktische und kommunikative Kompetenzen für ihren späteren Berufsalltag erlernen. Bedingt durch die COVID-19-Pandemie musste sich das MITZ mit neuen Möglichkeiten der Teilhabe an der Lehre auseinandersetzen (Röhle, 2021) und pilotierte dies mit der Anschaffung von zwei Telepräsenzrobotern (TPR). TPR sind mobile Remote-Präsenz-Systeme, welche neben der klassischen Videokonferenz den ST zusätzlich die Möglichkeit geben, sich autonom zu bewegen (Wolff & Möller, 2021). Dadurch wird die informelle Kommunikation und der Austausch unterstützt (Lee & Takayama, 2011), deren Fehlen ST während des Studiums in Zeiten der COVID-19-Pandemie als besonders herausfordernd empfunden haben (Traus et al., 2020). Im Folgenden wird die Pilotierung des Einsatzes von Telepräsenzrobotern in der medizinischen Lehre vorgestellt. [Aus: Hintergrund

Near-Wall Flow in Turbomachinery Cascades—Results of a German Collaborative Project

This article provides a summarizing account of the results obtained in the current collabora-tive work of four research institutes concerning near-wall flow in turbomachinery. Specific questions regarding the influences of boundary layer development on blades and endwalls as well as loss mech-anisms due to secondary flow are investigated. These address skewness, periodical distortion, wake interaction and heat transfer, among others. Several test rigs with modifiable configurations are used for the experimental investigations including an axial low speed compressor, an axial high-speed wind tunnel, and an axial low-speed turbine. Approved stationary and time resolving measurements techniques are applied in combination with custom hot-film sensor-arrays. The experiments are complemented by URANS simulations, and one group focusses on turbulence-resolving simulations to elucidate the specific impact of rotation. Juxtaposing and interlacing their results the four groups provide a broad picture of the underlying phenomena, ranging from compressors to turbines, from isothermal to non-adiabatic, and from incompressible to compressible flows.The investigations reported in this article were conducted within the framework of the joint research project “Near-Wall Flow in Turbomachinery Cascades” which was funded and supported by the Deutsche Forschungsgemeinschaft (DFG) under grant number PAK 948. The responsibility for the contents of this publication lies entirely by the authors.Peer ReviewedPostprint (published version

MEE-DBD Plasma Actuator Effect on Aerodynamics of a NACA0015 Aerofoil: Separation and 3D Wake

© 2020, Springer Nature Switzerland AG. Dielectric barrier discharge (DBD) plasma actuators have received considerable attention by many researchers for various flow control applications. Having no moving parts, being light-weight, easily manufacturable, and their ability to respond almost instantly are amongst the advantages which has made them a popular flow control device especially for application on aircraft wings. The new configuration of DBDs which uses multiple encapsulated electrodes (MEE) has been shown to produce a superior and more desirable performance over the standard actuator design. The objective of the current study is to examine the effect of this new actuator configuration on the aerodynamic performance of an aerofoil under leading edge separation and wake interaction conditions. The plasma actuator is placed at the leading edge of a symmetric NACA 0015 aerofoil which corresponds to the location of the leading edge slat. The aerofoil is operated in a chord Reynolds number of 0.2×106. Surface pressure measurements along with the mean velocity profile of the wake using pitot measurements are used to determine the lift and drag coefficients, respectively. Particle image velocimetry (PIV) is also utilised to visualise and quantify the induced flow field. The results show improvement in aerodynamic performances of aerofoil under leading edge separation and also facing the wake region

Fan casing contouring under consideration of aeroacoustics, mechanics, aeroelasticity and whole engine performance

Aircraft propulsion will continue to rely on gas turbine technology for the next decades to come. Thus, to achieve environmental agreements, ensure engine safety and retain economic competitiveness, ongoing development with a multidisciplinary design approach is indispensable. In the present study the multi criteria analysis of the fan, a decisive component in modern aero engines, is examined. In particular the interaction of the fan blades with the fan casing is analyzed and an appropriate design approach including automatic optimization is used. As one part of the disciplines conjunction an automated aeroacoustic approach is realized. The aerodynamic and acoustic fitness functions and constraints are based on Reynolds-Averaged Navier-Stokes (RANS) simulations of the fan stage. A fast analytical prediction tool for fan noise, PropNoise, which has been under development since recent years and already validated on several test cases, is used. Preliminary studies have shown that the flow in the rotor tip region is a major contributor to the broadband noise emission. Based on this, the optimization process focuses on the variation of the casing contour around the fan blades. The impact of the modified flow field in the rotor tip region concerning the aeroelastic behavior is also investigated. As aeroelastic evaluation requires a high level of know-how and is very time consuming, it is linked to the optimization process chain by a discrete evaluation of selected members. This allows a simultaneous adjustment of the design in case of aeroelastic issues. Furthermore, the impact of the fan modifications regarding the overall engine performance is evaluated. Off-design cycle calculations allow incorporating such detailed studies in a global engine optimization