Hermann Lienhart

Low-Reynolds-number flow around an oscillating circular cylinder at low Keulegan-Carpenter numbers

Time-averaged LDA measurements and time-resolved numerical flow predictions were performed to investigate the laminar flow induced by the harmonic in-line oscillation of a circular cylinder in water at rest. The key parameters, Reynolds number Re and Keulegan–Carpenter number KC , were varied to study three parameter combinations in detail. Good agreement was observed for Re =100 and KC =5 between measurements and predictions comparing phase-averaged velocity vectors. For Re =200 and KC =10 weakly stable and non-periodic flow patterns occurred, which made repeatable time-averaged measurements impossible. Nevertheless, the experimentally visualized vortex dynamics was reproduced by the two-dimensional computations. For the third combination, Re =210 and KC =6, which refers to a totally different flow regime, the computations again resulted in the correct fluid behaviour. Applying the widely used model of Morison et al . (1950) to the computed in-line force history, the drag and the added-mass coefficients were calculated and compared for different grid levels and time steps. Using these to reproduce the force functions revealed deviations from those originally computed as already noted in previous studies. They were found to be much higher than the deviations for the coarsest computational grid or the largest time step. The comparison of several in-line force coefficients with results obtained experimentally by Kuhtz (1996) for β=35 confirmed that force predictions could also be reliably obtained by the computations.

Flow and Turbulence Structure in the Wake of a Simplified Car Model

A comprehensive study using LDA (Laser Doppler Anemometry), HWA (Hot-Wire Anemometry) and static pressure measurements was performed in order to investigate the flow and turbulence structure around a simplified car model.The aim was to supply a detailed data set acquired under well defined boundary conditions to be used as reference data for numerical simulations in general and the validation and verification of refined turbulence models in particular. Because of the fact that the losses in the detached wake region make the major contribution to the aerodynamic drag and the prediction accuracy of the wake is a quite selective criteria for the turbulence models the study focused to the wake behind a simplified car model.

Flow and Turbulence Structures in the Wake of a Simplified Car Model (Ahmed Modell)

The aim of the “Models for Vehicle Aerodynamics” (MOVA) Project is to develop, refine, and validate the latest generation of turbulence models for selected examples encountered in vehicle aerodynamics. The validation of turbulence models requires the availability of detailed experimental data. These quantitative data should cover the most critical flow regions around a bluff car-shaped body and they should give physical quantities that can directly be correlated to the results of numerical simulations. Such experimental data were measured in the LSTM low speed wind tunnel using a 2-component laser-Doppler anemometer (LDA) mounted on a traversing system and a simplified model of a car (Ahmed model). Measurements were made for two rear vehicle body slant angles (25° and 35°) at a bulk air velocity of 40 m/s. This paper serves as a synopsis of the major results of this experimental investigation.

Flow around three-dimensional obstacles in boundary layers

Abstract An experimental investigation was carried out to study the structure of the flow field around three-dimensional obstacles of different aspect ratios, in two different types of boundary layers. The dimensions of the rectangular block obstacles were chosen to represent generic shapes of buildings. In order to study the flow field around a building structure in a wind tunnel test, the simulation of a boundary layer similar to the atmospheric boundary layer was a crucial issue. For the oncoming flow, a boundary-layer profile representing a suburban site was adopted in the present investigation. The studies were performed using different kinds of flow visualisation techniques and a laser Doppler anemometry (LDA) system, specially developed for wind tunnel applications and designed to provide a high spatial resolution. The results of the experiments showed the dependence of the flow structure around the obstacle on its aspect ratio, the angle of attack, the Reynolds number, and the type of boundary layer. Detailed quantitative information of turbulence parameters in the vicinity of the obstacle was attained. In addition, all data were compiled to schematic sketches of the flow topology around three-dimensional obstacles.

Homogeneity of turbulence generated by static-grid structures

Homogeneity of turbulence generated by static grids is investigated with the help of hot-wire measurements in a wind-tunnel and direct numerical simulations based on the Lattice Bolztmann method. It is shown experimentally that Reynolds stresses and their anisotropy do not become homogeneous downstream of the grid, independent of the mesh Reynolds number for a grid porosity of 64 %, which is higher than the lowest porosities suggested in the literature to realize homogeneous turbulence downstream of the grid. In order to validate the experimental observations and elucidate possible reasons for the inhomogeneity, direct numerical simulations have been performed over a wide range of grid porosity at a constant mesh Reynolds number. It is found from the simulations that the turbulence wake behind the symmetric grids is only homogeneous in its mean velocity but is inhomogeneous when turbulence quantities are considered, whereas the mean velocity field becomes inhomogeneous in the wake of a slightly non-uniform grid. The simulations are further analysed by evaluating the terms in the transport equation of the kinetic energy of turbulence to provide an explanation for the persistence of the inhomogeneity of Reynolds stresses far downstream of the grid. It is shown that the early homogenization of the mean velocity field hinders the homogenization of the turbulence field.

Experimental Benchmark: Self-Excited Fluid-Structure Interaction Test Cases

The swivelling motion of a flexible structure immersed in a flow can become self-excited as a result of different fluid-structure interaction mechanisms. The accurate simulation of these mechanisms still constitutes a challenge with respect to mathematical modelling, numerical discretization, solution techniques, and implementation as software tools on modern computer architectures. Thus, to support the development of numerical codes for fluid structure interaction computations, in the present work an experimental investigation on the two-dimensional self-excited periodic swivelling motion of flexible structures in both laminar and turbulent uniform flows was performed. The investigated structural model consisted of a stainless-steel flexible sheet attached to a cylindrical front body. At the trailing edge of the flexible sheet, a rectangular mass was considered. The entire structure model was free to rotate around an axle located in the central point of the front body. During the experimental investigation, the general character of the elastic-dynamic response of the structure model was studied first. The tests in laminar flows were performed in a polyglycol syrup (dynamic viscosity: 1.64 ×10−4 m{ 2}/s) for a Reynolds number smaller than 270, whereas the tests in turbulent flows were conducted in water for Reynolds numbers up to 44000. In both cases, the maximum incoming velocity tested was about 2 m/s. Subsequently, three specific test cases were selected and characterized in more detail as far as the flow velocity field and structure mechanical behavior are concerned. Thus, the present contribution presents the detailed results obtained at 1.07 m/s and at 1.45 m/s in laminar and at 0.68 m/s in turbulent flows. It also compares the experimental data with numerical results obtained for the same conditions using different simulating approaches. They revealed very good agreement in some of the fluid-structure interaction modes whereas in others deficiencies were observed that need to be analyzed in more detail.

Flow–Induced Oscillation of a Flat Plate – A Fluid–Structure–Interaction Study Using Experiment and LES

The research unit FOR 493 tackles the challenging task of fluid-structure interaction (FSI). On the one hand, the objective is to provide reliable experimental reference data for all groups which are developing numerical methodologies to predict such coupled FSI problems. Since measurements on fluid-structure interaction setups using welldefined boundary and operating conditions are rare, the present Particle Image Velocimetry and excitation measurements for a swiveling flat plate support to fill a gap by providing phase-resolved data of this useful test case. On the other hand, the objective is to develop numerical methods for coupled FSI investigations which involve turbulent flows. Thus, an especially designed coupling scheme to be used in combination with the large-eddy simulation technique was set up and applied to study the flow around the hinged flat plate and the development of self-excited periodic processes with rotary oscillation of the flat plate.

Wind Monitoring System for Full Scale Tests on Trains

The present paper summarises work carried out at the Lehrstuhl fur Stromungsmechanik (LSTM) of the University of Erlangen-Nurnberg within the EU-project TRANSAERO. The task performed was to lay out, construct, calibrate and operate an onboard wind monitoring system for full scale side wind tests. The tests were aimed to measure the aerodynamic effects caused by cross winds on high speed trains. In order to correlate the dynamic response of the train’s leading coach to the actual wind input, continuous wind data measured onboard the train were needed. The wind monitoring system described in the present paper recorded the relative wind speed in magnitude and direction as well as the ambient air temperature and pressure. The mounting and positioning of the measuring devices was chosen in such a way that their readings were independent of the particular train adopted for the test runs.

Total Pressure Measurements Behind an Axial Ventilator Using a Kiel Probe Array

This work suggests a new arrangement for measuring the total pressure inside ducts or pipes with non-standard flow field distributions. The device consists of an array of 81 Kiel probes distributed over the duct’s cross section. It facilitates the quick measurement of the integral total pressure e.g. behind an axial ventilator without the need to scan the duct section with a traversing or integrating additional substantial flow straightening devices. Wind tunnel tests have proven a tolerance towards deviating on-flow conditions of more than \(\pm 35^\circ \). The application of the Kiel probe array is shown in a wind tunnel fan test rig.

Evolution of transitional structures from puff to slug through multiple splitting in a pipe flow at low Reynolds number

During laminar-to-turbulent transition in low Reynolds pipe flows, three main types of flow structures occur: traveling waves and the turbulent flow structures, namely puffs and slugs. In the present work, detailed experiments on the probability of occurrence and propagation speed of puffs, splitting puffs and slugs were conducted with the transition pipe-flow facility of LSTM-Erlangen. During the investigations, fully developed laminar pipe flow was triggered by an iris diaphragm with a pre-defined amplitude and lapse time. Different types of single and multiple puffs are classified and the probability of their occurrence as well as their propagation speed at the end of pipes with different lengths are evaluated.