Franco Magagnato

Optimum dimple diameter for friction reduction with laser surface texturing: the effect of velocity gradient

The morphological texturing of surfaces has demonstrated high potential to reduce friction and wear. In order to understand the effect of different velocity gradients over the textured area on the optimum dimple diameter, we textured brass pins with round dimples having diameters between 20 and 200 μm. The dimple depth and packing density were kept constant. The samples were tested in a pin-on-disc fashion against sapphire discs and experiments were conducted under mixed lubrication and for two different sliding radii. Our results show that larger velocity gradients favor smaller dimples, whereas for the smaller velocity gradients, larger dimple diameters were beneficial. The effect of there being an influence of the velocity gradient was also found in computational fluid dynamics (CFD) simulations. Experimentally, friction forces could be reduced by up to 80%, demonstrating the tremendous potential of laser surface texturing (LST) to lower friction forces and reduce CO2 emissions.

Prediction of the Resonance Characteristics of Combustion Chambers on the Basis of Large-Eddy Simulation

In the last few years intensive experimental investigations were performed at the University of Karlsruhe to develop an analytical model for the Helmholtz resonator-type combustion system. In the present work the resonance characteristics of a Helmholtz resonator-type combustion chamber were investigated using large-eddy simulations (LES), to understand better the flow effects in the chamber and to localize the dissipation. In this paper the results of the LES are presented, which show good agreement with the experiments. The comparison of the LES study with the experiments sheds light on the significant role of the wall roughness in the exhaust gas pipe.

Stochastic-field cavitation model

Nonlinear phenomena can often be well described using probability density functions (pdf) and pdf transport models. Traditionally, the simulation of pdf transport requires Monte-Carlo codes based on Lagrangian “particles” or prescribed pdf assumptions including binning techniques. Recently, in the field of combustion, a novel formulation called the stochastic-field method solving pdf transport based on Eulerian fields has been proposed which eliminates the necessity to mix Eulerian and Lagrangian techniques or prescribed pdf assumptions. In the present work, for the first time the stochastic-field method is applied to multi-phase flow and, in particular, to cavitating flow. To validate the proposed stochastic-field cavitation model, two applications are considered. First, sheet cavitation is simulated in a Venturi-type nozzle. The second application is an innovative fluidic diode which exhibits coolant flashing. Agreement with experimental results is obtained for both applications with a fixed set of model constants. The stochastic-field cavitation model captures the wide range of pdf shapes present at different locations.

Investigation of the Effect of Surface Roughness on the Pulsating Flow in Combustion Chambers with LES

Self-excited oscillations often occur in combustion systems due to the combustion instabilities. The high pressure oscillations can lead to higher emissions and structural damage of the chamber. In the last years intensive experimental investigations were performed at the University of Karlsruhe to develop an analytical model for the Helmholtz resonator-type combustion systems [1]. In order to better understand the flow effects in the chamber and to localize the dissipation, Large Eddy Simulations (LES) were carried out. Magagnato et al. [2] describe the investigation of a simplified combustion system where the LES were carried out exclusively with a hydraulic smooth wall. The comparison of the results with experimental data shows the important influence of the surface roughness in the resonator neck on the resonant characteristics of the system. In order to catch this effect with CFD as well, the modeling of surface roughness is needed. In this paper the Discrete Element Method has been implemented into our research code and extended for LES. The simulation of the combustion chamber with roughness agrees well with the experimental results.

Large Eddy Simulation (LES) with Moving Meshes on a Rapid Compression Machine: Part 2: Numerical Investigations Using Euler–Lagrange-Technique

The flow inside a simplified one-stroke engine with squared cross section has been calculated with compressible Large Eddy Simulation (LES) using our code SPARC and compared with the measurements on the same geometry. The one-stroke engine has a turbulence generator, which can ether generate a tumble or homogenous turbulence depending on the configuration. By waiting different amount of time after the turbulence generation process a variable turbulence level can be achieved. During the up going motion of the piston the turbulent fuel mixture is compressed and ignited by a row of spark plugs. The simulation has been using more then 8 million points for the space discretization. A space conservation law was used to calculate the grid motion with Euler-Lagrange technique. The mesh was refined in the shear layers and close to the wall so that y+ < 1 results almost everywhere. A comparison between Miles (monotonically integrated large eddy simulation) approach and conventional subgrid scale modelling (dynamic Smagorinsky) showed very similar solutions. Mean and fluctuating velocities at TDC are compared with available experimental findings.

Comparison of DES and LES on the Transitional Flow of Turbine Blades

The prediction of the laminar to turbulence transition is essential in the calculation of turbine blades, compressor blades or airfoils of airplanes since a non negligible part of the flow field is laminar or transitional. In this paper we compare the prediction capability of the Detached Eddy Simulation (DES) with the Large Eddy Simulation (LES) using the high-pass filtered (HPF) Smagorinsky model (Stolz et al., 2003) when applied to the calculation of transitional flows on turbine blades. Detailed measurements from (Canepa et al, 2003) of the well known VKI-turbine blade served to compare our results with the experiments. The calculations have been made on a fraction of the blade (10%) using non-reflective boundary conditions of Freund at the inlet and outlet plane extended to internal flows by (Magagnato et al., 2006) in combination with the Synthetic Eddy Method (SEM) proposed by (Jarrin et al., 2005). The SEM has also been extended by (Pritz et al., 2006) for compressible flows. It has been repeatedly shown that hybrid approaches can satisfactory predict flows of engineering relevance. In this work we wanted to investigate if they can also be used successfully in this difficult test case.

Numerical Investigation of the VKI Turbine Blade by Large Eddy Simulation

In the frame of this work, the numerical investigations of the flow through the VKI turbine cascade were performed by means of Large Eddy Simulation.

The Buffer Layer Technique Applied to Transonic Flow Calculations

The main goal of numerical simulations is to predict a flow field as close to the real one as possible. All disturbances which travel down to an inlet/outlet boundary of the domain should pass through the boundary. Unfortunately, the standard numerical boundary conditions produce unphysical reflections of the disturbances. This is especially undesirable for unsteady calculations. To avoid this effect, a non-reflecting boundary condition must be applied. According to recent papers, one can find some different approaches to avoid the unphysical reflections, called non-reflecting boundary conditions. One of those is the buffer layer technique. The buffer layer non-reflecting boundary, proposed by Freund3, has been implemented into our code10 and tested for various test cases.

Thermo-hydraulic flow in a sudden expansion

The paper deals with the turbulent flow of liquid metal directed upwards in a vertical channel featuring a backward-facing step. The vertical wall behind the step is heated at various rates thereby inducing forced and mixed convection. Due to the low Prandtl number of liquid metal flow a data basis for this technically relevant flow type did not exist so far. Here, DNS and LES results are presented to provide detailed information about the statistics of the turbulent motion, budgets of turbulent kinetic energy and other quantities. This information is then further used to develop suitable statistical turbulence models capable of properly covering this flow and similar ones, i.e. forced, mixed and free convection of liquid metals. Finally, the paper reports on the construction of an experiment conceived for exactly the same configuration as simulated, with the purpose of close cross validation between the different approaches.

Simulation of a Centrifugal Pump by Using the Harmonic Balance Method

The harmonic balance method was used for the flow simulation in a centrifugal pump. Independence studies have been done to choose proper number of harmonic modes and inlet eddy viscosity ratio value. The results from harmonic balance method show good agreements with PIV experiments and unsteady calculation results (which is based on the dual time stepping method) for the predicted head and the phase-averaged velocity. A detailed analysis of the flow fields at different flow rates shows that the flow rate has an evident influence on the flow fields. At 0.6, some vortices begin to appear in the impeller, and at 0.4 some vortices have blocked the flow passage. The flow fields at different positions at 0.6 and 0.4 show how the complicated flow phenomena are forming, developing, and even disappearing. The harmonic balance method can be used for the flow simulation in pumps, showing the same accuracy as unsteady methods, but is considerably faster.