Martin Böhle

Grundlagen der Strömungsmechanik

Ein Kraftfahrzeug wird von einer reibungsfreien Parallelstromung der Geschwindigkeit U ∞ angestromt. Abbildung 2.1.1a zeigt das Kraftfahrzeug und die Parallelstromung im Mittelschnitt der (x, z)-Ebene. Unter Vernachlassigung von Bodeneinflussen lasst sich die Umstromung des Kraftfahrzeug-Mittelschnittes in drei unterschiedliche Bereiche einteilen.

A Systematical Study of the Influence of Blade Length, Blade Width, and Side Channel Height on the Performance of a Side Channel Pump

Two modular side channel pump models have been investigated both numerically and experimentally. For both modular designs, different side channels and impellers could be studied, with the aim to get information about the influence of the different geometries on the performance and the inner flow phenomena of the pump. By understanding the geometry influences, statements about the design process of the pump are possible. Changes of the geometry of the side channel or the impeller affect the flow in both components. This means that the geometrical dimensions must always be related to each other, in order to make statements about influences of the geometry on the characteristics. Thus, various geometrical configurations are setup, their sizes in industrial pumps are indicated and their influence is investigated by simulations. To evaluate the gained numerical data, it is important to understand the influence of mesh and simulation setup on the results. Therefore, a grid study was conducted and additionally the turbulence model was varied. In this paper, two parameters are focused on: these are the side channel height to the blade length (h/l) and the depth of the side channel in relation to the width of the blade (t/w).

Numerical and Experimental Investigations of the Three-Dimensional-Flow Structure of Tandem Cascades in the Sidewall Region

Tandem blades can be superior to single blades, particularly when large turning angles are required. This is well documented in the open literature and many investigations have been performed on the 2D-flow of tandem cascades to date. However, much less information on the flow near the sidewalls is available. Thus, the question arises as to how the geometry of a tandem cascade should be chosen near the sidewall in order to minimize the flow losses for large turning angles. The present work examines the 3D-flow field in the region of the sidewall of two high turning tandem cascades. A large spacing ratio was chosen for the forward blade of the first tandem cascade ((t/l)1=1.92). The second tandem cascade possessed a smaller spacing ratio for the forward blades ((t/l)1=1.0). Both cascades had the same total spacing ratio of t/l=0.6. Flow phenomena, such as the corner stall of the 3D boundary layer near the sidewall, are examined using both numerical and experimental methods. The empirical correlations of Lieblein and Lei are applied. The flow topology of both tandem cascades is explained and the locations of loss onset are identified. In addition, oil pictures from experiments and streamline pictures of the numerical simulations are shown and discussed for the flow close to the sidewalls. Finally, design rules such as the aerodynamic load splitting and the spacing ratio of the forward- and aft-blades, etc. are taken into account. The examinations are performed for tandem cascades designed for flow turning of approximately 50 deg at a Reynolds number of 8×105. The tandem cascades consist of NACA65 blades with circular camber lines and an aspect ratio of b/l=1.0.

Multidisciplinary Design Optimization of a Mixed Flow Turbine Wheel

Designing turbine wheels for automotive turbochargers one is faced with a multidisciplinary design problem with many input and output parameters. Especially in the automotive industry short development cycles for high quality products in a competitive environment are daily routine.For meeting these requirements optimization algorithms can be a powerful tool in the design process. This paper presents the multidisciplinary optimization of an automotive mixed flow turbine wheel used in a 4 cylinder 1.6 l spark ignition engine. Before describing the optimization workflow in detail, the requirements for turbines operating in an automotive environment under pulsating flow conditions and during an engine load step are discussed. From there objectives for a multidisciplinary optimization are derived.The turbine wheel is optimized with respect to maximizing efficiency in two design points and minimizing its moment of inertia. For the optimization process, an algorithm based on evolution theory is used. As constraints, the operating points are fixed and the natural frequencies are limited to ensure the mechanical strength of the turbine. To speed up the optimization process meta models based on neural networks are applied. Three designs of the Pareto frontier are chosen and their characteristics are discussed. Using statistical methods, the interaction of the input variables and their impact on turbine performance are presented.Copyright

Effect of suction side blade profile on the performance of a side channel pump

Due to the centrifugal effect of the radial impeller, side channel pumps are a kind of regenerative pumps that provide high head at low flow rate. The geometry of the impeller affects flow patterns and energy conversion directly, greatly influencing the performance of side channel pumps. To investigate the effect of blade profile for suction side on the performance of a side channel pump, three different base angles of 10°, 20°, and 30°, respectively, on the blade suction side were discussed and analyzed both with numerical and experimental methods. The hydraulic performance, exchange mass flow, and velocity vectors were discussed in detail. The numerical work was validated by the comparison between the simulated result and tested result. The results show that the hydraulic performance of the impeller with 30° angle is the best one of the three impellers, especially for head performance. The evaluation method based on exchanged mass flow also confirms that the performance of the side channel pump can be improved by increasing the angle on the suction side of the blade. In addition, the radial vortex on the impeller flow passage has negative effect on the performance of the side channel pump. However, the axial vortex among the impeller and side channel directly affects the energy conversion and has a beneficial effect on the performance of the pump. The results could be used to modify the design models and conclude the effect of blade shapes on the performance of a side channel pump.

Influence of Degree of Reaction on Turbine Performance for Pulsating Flow Conditions

This paper presents a study on the influence of the degree of reaction (DoR) on turbine performance under highly pulsating inflow. A reference test turbine wheel is designed and scaled to three different wheel diameters while an identical flow capacity of all three turbines is provided by adjusting the volute size. Hence, the three turbines differ by their DoR, inertia and efficiency characteristic. The investigation is done completely numerically using highly validated models. Naturally, the pulsating flow character of a 4-cylinder gasoline engine requires unsteady CFD. In addition steady-state turbine maps were calculated beforehand as a reference base.The results of the steady state calculation show that for the combination of the bigger turbine wheel with the smaller turbine volute the peak efficiency is smaller but is shifted towards higher pressure ratios respectively to lower blade speed ratios. This is fundamentally beneficial for turbines in automotive turbochargers for gasoline engines characterized by highly pulsating flow conditions, in particular at lower engine speeds. For the transient flow calculations with pulsating turbine inflow, the hysteresis loop and the turbine power generation was investigated. It is shown that the smallest volute compared to the biggest one causes a more contracted hysteresis loop combined with increased power output within one pulse cycle.In order to include the influence of moment of inertia, the turbines with varying DoR but same flow capacity were analytically compared with a 1D code simulating engine load step operation. Thus, the paper shows the effect of turbine DoR on both, steady-state turbine performance under pulsating inflow and the capability for optimum engine load step operation.Copyright

A viscosity adaption method for Lattice Boltzmann simulations

Abstract In this work, we consider the limited fitness for practical use of the Lattice Boltzmann Method for non-Newtonian fluid flows. Several authors have shown that the LBM is capable of correctly simulating those fluids. However, due to stability reasons the modeled viscosity range has to be truncated. The resulting viscosity boundaries are chosen arbitrarily, because the correct simulation Mach number for the physical problem is unknown a priori. This easily leads to corrupt simulation results. A viscosity adaption method (VAM) is derived which drastically improves the applicability of LBM for non-Newtonian fluid flows by adaption of the modeled viscosity range to the actual physical problem. This is done through tuning of the global Mach number to the solution-dependent shear rate. We demonstrate that the VAM can be used to accelerate LBM simulations and improve their accuracy, for both steady state and transient cases.

Accuracy of non-Newtonian Lattice Boltzmann simulations

This work deals with the accuracy of non-Newtonian Lattice Boltzmann simulations. Previous work for Newtonian fluids indicate that, depending on the numerical value of the dimensionless collision frequency ?, additional artificial viscosity is introduced, which negatively influences the accuracy. Since the non-Newtonian fluid behavior is incorporated through appropriate modeling of the dimensionless collision frequency, a ? dependent error E ? is introduced and its influence on the overall error is investigated. Here, simulations with the SRT and the MRT model are carried out for power-law fluids in order to numerically investigate the accuracy of non-Newtonian Lattice Boltzmann simulations. A goal of this accuracy analysis is to derive a recommendation for an optimal choice of the time step size and the simulation Mach number, respectively. For the non-Newtonian case, an error estimate for E ? in the form of a functional is derived on the basis of a series expansion of the Lattice Boltzmann equation. This functional can be solved analytically for the case of the Hagen-Poiseuille channel flow of non-Newtonian fluids. With the help of the error functional, the prediction of the global error minimum of the velocity field is excellent in regions where the E ? error is the dominant source of error. With an optimal simulation Mach number, the simulation is about one order of magnitude more accurate. Additionally, for both collision models a detailed study of the convergence behavior of the method in the non-Newtonian case is conducted. The results show that the simulation Mach number has a major impact on the convergence rate and second order accuracy is not preserved for every choice of the simulation Mach number.

Theoretische und experimentelle Untersuchungen an ungestaffelten Gittern aus Profilen mit mechanischen Klappen

ZusammenfassungTheoretische und experimentelle Untersuchungen an vier Verstell-Eintrittsleitrad-Gittern werden beschrieben. Von diesen Gittern ist ein Gitter ein konventionelles Umstaffelungsgitter (variabler Gittergeometrie); die drei anderen Gitter sind neuartige Klappengitter (variabler Profilgeometrie)—bestehend aus Halbkörperprofilen mit mechanischen Klappen (xK/l=1/3; 1/2; 2/3).Auf theoretischem Gebiet werden die wesentlichen Beziehungen einer geschlossenen Lösung für ungestaffelte Gitter aus ebenen Platten mit angelenkten Klappen in reibungsloser inkompressibler Strömung mitgeteilt. An Hand von Ergebnissen zahlreicher Beispielrechnungen werden Umlenkeigenschaften und Druckverteilungen dieser Gitter erläutert.Auf experimentellem Gebiet werden die Ergebnisse von Nachlaufmessungen, die mit und ohne Turbulenzfäden auf den Profilen bei einer Reynoldszahl von Re2th=3,5·105 durchgefürht wurden, mitgeteilt und analysiert (dmax/l=0,08; t/l=1,0). Wichtigstes Ergebnis: Mit den neuartigen Klappengittern lassen sich die Verstellbereiche geringer Profilverluste ganz erheblich vergrößern. Die hier maximal erreichte Vergrößerung wurde zwischen den Verlustkurven des NACA-0008-Umstaffelungsgitters ohne Turbulenzfäden und des EHK1/3-Klappengitters mit Turbulenzfäden gemessen. Ein auf gleichem Verlustniveau, ζv1 ≈ 0,035, vorgenommener Vergleich ergab eine Vergrößerung des Verstellbereichs von λ=15° auf η=32°, d.h. um 17°. Die damit erreichte Verbesserung gegenüber dem Stand der Technik—repräsentiert durch das Umstaffelungsgitter—beträgt somit mehr als 100%.

Lattice Boltzmann Simulation of the Flow Field in Pump Intakes—A New Approach

In recent years, lattice Boltzmann methods (LBMs) have become popular for solving fluid flow problems of engineering interest. Reasons for this popularity are due to the advantages of this method, which are, for example, the simplicity to handle complex geometries and the high efficiency in calculating transient flows. For the operational reliability and efficiency of pumps and pump systems, the incoming flow conditions are crucial. Since the efficiency and reliability requirements of pumps are rising and must be guaranteed by the pump and plant manufacturer, the flow conditions in pump intakes need to be evaluated during plant design. Recent trends show that pump intakes are built more and more compact, what makes the flow in the intake even more complex and holds a higher risk for unacceptable pump inflow conditions. In this contribution, a numerical scheme for the simulation of pump intake flows based on a lattice Boltzmann-large eddy simulation (LES) approach is presented and the ability of the method to capture the flow phenomena in intake flows is analyzed. Special attention is turned to the potential of the numerical scheme to reproduce the transient vortex behavior of intake flows, which results in a very complex flow structure and is challenging to model numerically.