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The aerodynamic drag of a body for instance of an air plane mainly depends on the conditions of the flow in the near-wall region, called boundary layer. Classically, one distinguishes between laminar and turbulent boundary layer, with the later one causing higher drag. The change of boundary layer condition from laminar to turbulent is called transition. The position of as well as the process of transition strongly depends on body shape, steady-state flow characteristics and especially on flow fluctuations. The fluctuations appear either inside the boundary layer or outside. Outer fluctuations have to be coupled inside the boundary layer flow and increase in magnitude while moving downstream to finally lead to transition. Wing or blade/vane trailing edges are sources of acoustic pressure wave generation. The pressure waves propagate in all direction from this source. The upstream moving part of the pressure wave propagates against the main flow direction leading to small wave velocities relative to the airfoil surface. Dependent on pressure wave amplitude and frequency initially harmonic waves steepen non-linear to shocks. A self-organization process of these weak shocks is observed during their further motion. Namely, the frequency and amplitude of the pressure waves that is random and chaotic at the trailing edge due to their generation by turbulent vortices become ordered in the mid-chord region. Stronger shock waves that are moving faster catch up the weaker ones and merge with them. At the end, the frequency is dramatically reduced towards the position of airfoil maximum thickness and the wave fronts are oriented nearly orthogonal to the airfoil surface. In technical literature strong supposition that weak upstream moving shock waves influence the transition process is expressed. In this work the interaction of weak shocks and transitional boundary layer are assessed with means of CFD under geometrically simplified conditions. This simplification allows a systematic variation of pressure wave parameters like amplitude and frequency. The unsteady flow field and especially the boundary layer condition are analyzed in detail.

This book deals with the development and testing of high-response piezo-servovalves for improved performance of electrohydraulic cylinder drives. Performance improvement means in the context of this work an enhancement of reference and disturbance response of the drive in a closed-loop pressure and position control. Reference and disturbance responses determine the precision of the drive which influences product quality and motion accuracy in stationary and mobile applications. This thesis shows by means of linearised mathematical models that the drive performance can be significantly improved by high dynamics of the control valve. The increase in valve dynamics can be achieved by the use of piezoelectric actuators, which outmatch the response of common electromechanical valve actuators. Within the scope of this work two different novel high-response piezo-servovalves are prototyped. These are four-way directional flow control valves with linear spools. The book presents different steps of the design and development process of the valves such as conceptual design, calculation, dimensioning, manufacturing, design of internal valve control, testing according to the internationally standardized procedures and comparison to the performance of common servovalves. The achieved dynamics of the piezo-servovalves surpasses the response of the conventional valves. Exemplary investigations of an electrohydraulic cylinder drive show an improved reference response of pressure control by 50% and an enhanced disturbance response of position control by 40% due to the use of the piezo-servovalve at the drive. This work demonstrates that the commercially available piezo-actuators can be successfully implemented in industrial hydraulic valves, bringing advantages for the overall drive performance. The high reliability and endurance of the piezo-actuators have already been proven by the use in common rail fuel injectors in the automobile industry since 2002. Taking into account falling costs of the actuators due to the mass production for the automobile industry they can become a very attractive alternative or supplement for hydraulic valve technology.

This thesis deals at first with the development of a consistent mixing model for the interactive coupling (two-way-coupling) of a CFD code and a multi-zone code based on multiple zero-dimensional reactors. The interactive coupling allows for a computationally efficient modeling of HCCI or PCCI combustion, respectively. The physical domain in the CFD code is subdivided into multiple zones based on three phase variables (fuel mixture fraction, dilution, and total enthalpy). These phase variables are sufficient for the description of the thermodynamic state of each zone, assuming that each zone is at the same pressure. Each zone in the CFD code is represented by a corresponding one in the zero-dimensional code. The zero-dimensional code solves the chemistry for each zone, and the heat release is fed back into the CFD code. The difficulty in facing this kind of methodology is to keep the thermodynamic state of each zone consistent between the CFD code and the zero-dimensional code after the initialization of the zones in the multi-zone code has taken place. The thermodynamic state of each zone (and thereby the phase variables) changes in time due to mixing and source terms (e.g., vaporization of fuel, wall heat transfer). The focus of this work lies on a consistent description of the mixing between the zones in phase space in the zero-dimensional code, based on the solution of the CFD code. Two mixing models with different degrees of accuracy, complexity, and numerical effort are described. The most elaborate mixing model (and an appropriate treatment of the source terms) keeps the thermodynamic state of the zones in the CFD code and the zero-dimensional code identical. The models are applied to a test case of HCCI combustion in a gasoline single-cylinder research engine. Following from there, a simulation model for PCCI combustion is derived that can be used in closed-loop control development. For the high-pressure part of the engine cycle, the interactively coupled CFD-multi-zone approach is systematically reduced to a stand-alone multi-zone chemistry model. This multi-zone chemistry model is extended by a mean value model accounting for the gas exchange losses. The resulting model is capable of describing PCCI combustion with stationary exactness, and is at the same time very economic with respect to computational costs. The model is further extended by identified system dynamics influencing the stationary inputs. For this purpose, a Wiener model is set up that uses the stationary model as a nonlinear system representation. In this way, a dynamic nonlinear model for the representation of the controlled plant Diesel engine is created.

Today, numerical flow models are widely used tools in hydraulic engineering, as for the design of turbine intakes. In this field, problems are often closely linked to the question of an optimal solution depending on various influences. With the connection of an optimization algorithm and a flow model the tool of automatic optimization is created, which can be used to find the optimal solution. The intention of this thesis is to answer the question, if automatic optimization is a trustworthy tool and to what extend it can contribute to the design process in hydraulic engineering. The various types of problems in hydraulic engineering demand a flexible tool, which can be adapted to a specific application. A concept was developed, which easily allows the exchange of the modules of the automatic optimization. A comprehensive error analysis shows, how errors can impede a successful termination of the optimization process. In order to ensure a validated optimization, a stepwise process of error control was developed. The separation pier of a small water power plant was optimized to show the practical applicability of the new tool. The horizontal and the vertical shape of the pier were subject to optimization. In comparison to other, well known pier geometries the optimized contours offer an additional reduction of flow losses at the trash-rack. With the help of the automatic optimization a pier contour was identified, which also shows good properties regarding constructional demands.