Christian Kähler

Instantaneous flow field measurements for propeller aircraft and rotorcraft research

This paper illustrates results of instantaneous flow field measurements which were performed by the PIV group of DLR Gottingen during test campaigns in three different wind tunnel facilities of the Dutch-German Wind Tunnel foundation (DNW). The test results described first were obtained in the low speed tunnel LST in order to obtain detailed flow field information about the wake of a new high speed propeller model. The second test was performed in the transonic wind tunnel TWG in the frame of a German helicopter research project. Here the flow field above a pitching airfoil equipped with a flap driven by an integrated piezoelectric actuator has been observed with high spatial resolution. During the third test, stereoscopic flow field measurements were performed in the wake of a rotorcraft model in order to characterize the blade tip vortices at different flight conditions. The test was conducted in the 6 x 8 m2 open jet test section of the large low speed facility LLF which required observation distances exceeding 9 meters between camera and light sheet.

Flow Investigations in a Stalling Nacelle Inlet Under Disturbed Inflow

A generic, cold flow-through nacelle under atmospheric disturbed inflow was experimentally investigated. The boundary layer development of the nacelle resembles that of a powered engine at take-off conditions and high mass flow rates. A vortex generator was positioned ({{l}/{c_text {n}}=3.3}) in front of the nacelle’s leading edge in order to generate an inhomogeneous inflow. This vortex generator is a motor driven airfoil, which pitches from angles of attack ({alpha _{text {vg}}=-11^{circ }}) to ({alpha _{text {vg}}=11^{circ }}) within approximately ({58,text {ms}}). This way, vortices are generated in the wind tunnel. These vortices induce an upward velocity component which is comparable to a realistic gust. The investigations are performed for attached, as well as separated, flow in the nacelle. It is shown that for the attached flow the low speed near-wall area and the turbulent fluctuation velocities are increased. For the higher angle of attack of the nacelle, the interaction with the generic gust increases the size of the turbulent separation bubble as well as the amplitude of the fluctuation velocities. In both cases the changes might lead to problems on the compressor of a powered jet engine. The results indicate crucial distortions due to the interaction of the relatively small generic gust with the flow through the nacelle.

PARTICLE IMAGE VELOCIMETRY – AN ADVANCED EXPERIMENTAL TOOL FOR THE INVESTIGATION OF TURBULENT FLOW FIELDS

For the characterization of turbulent flows it is often required to measure both, the temporal behavior of the velocity fluctuations along with their spectral contents and the spatial flow structure. Conventional point-wise measurement techniques such as hot-wire or laser Doppler velocimetry have been applied for a long time in order to obtain the temporal information at a single point. Since a few years the Particle Image Velocimetry (PIV) technique, which for the first time allows to measure instantaneous velocity vector maps, enables quantitative insight into the development of the spatial structures as well. It will be shown that by employing more complex multi-laser, multi-camera set-ups for PIV, it becomes possible to measure other relevant quantities required for the understanding of turbulent flows such as the complete velocity gradient tensor or the components of the spatio-temporal correlation tensor for example.

On the reliable estimation of heat transfer coefficients for nanofluids in a microchannel

Nanofluids (base fluid and nanoparticles) can enhance the heat transfer coefficient h in comparison to the base fluid. This open the door for the design of efficient cooling system for microelectronics component for instance. Since theoretical Nusselt number correlations for microchannels are not available, the direct method using an energy balance has to be applied to determine h. However, for low nanoparticle concentrations the absolute numbers are small and hard to measure. Therefore, the study examines the laminar convective heat transfer of Al2O3-water nanofluids in a square microchannel with a cross section of 0.5 × 0.5 mm2 and a length of 30 mm under constant wall temperature. The Al2O3 nanoparticles have a diameter size distribution of 30-60 nm. A sensitivity analysis with error propagation was done to reduce the error for a reliable heat transfer coefficient estimation. An enhancement of heat transfer coefficient with increasing nanoparticles volume concentration was confirmed. A maximum enhancement of 6.9% and 21% were realized for 0.6% Al2O3-water and 1% Al2O3-water nanofluids.

A new experiment of a turbulent boundary layer flow at adverse pressure gradient for validation and improvement of RANS turbulence models

We present a turbulent boundary layer flow experiment at a significant adverse pressure gradient and at a high Reynolds number. We describe the design of the test case and the set-up in the wind-tunnel so that the flow is suitable for the validation of RANS turbulence models. We present RANS simulations using the SST k-(omega ) model, and the SSG/LRR-(omega ) and the JHh-v2 Reynolds stress model. We show that the predictive accuraccy in the adverse pressure gradient region is significantly effected by the predictive accuracy in the upstream located region of a favourable pressure gradient, where the mean flow follows a curved surface. The effects of flow history have to be taken into account when assessing turbulence models in the adverse pressure gradient region. We study in detail the role of the cross-diffusion term in the (omega )-equation for favourable and adverse pressure gradients.

Applications: Micro PIV

This section discusses various applications and techniques used in (mu text {PIV}). The applications include very high spatial resolution measurements of pressure-driven flow in a rectangular capillary and measurements in a challenging toroidal vortex.

Techniques for 3D-PIV

This chapter initially provides the reader with an extensive survey of the many methods available to measure the flow velocity in three-dimensional problems. Afterwards, the chapter devotes three main sections to the most common or emerging methods: Tomographic PIV, 3D-PTV techniques and Shake-the-Box. Tomographic PIV is considered the technique of choice at present, and is extensively discussed. Aspects cover hardware components, requirements for illumination of a volume and techniques to increase particles visibility. A discussion is given of the effects of number of cameras and their configuration to deal with densely seeded experiments, where the phenomenon of ghost particles dominates the experimental errors. The theoretical background is given about the techniques used for tomographic reconstruction of 3D particle intensity distribution, followed by a critical evaluation of the accuracy of reconstruction. The discussion of Tomographic PIV closes with the description of the most recent algorithms based on multi-exposure reconstruction and time-resolved data analysis. The second part of the chapter is a new addition to the book and it deals with the fundamental principles of 3D-PTV. Particle detection, triangulation and pairing are the most important operations to perform a successful 3D-PTV evaluation. Hybrid methods based on tomographic reconstruction and individual particle tracking are discussed. The section concludes suggesting the working range of PTV techniques in 3D experiments. The Shake-the-Box technique is an emerging method that performs Lagrangian particle tracking with high potential in terms of computation speed and for the high accuracy of particle motion estimation. Its rapid diffusion among research laboratories justifies a detailed description of its working principles, main features and characteristic performance. The section elaborates on the concepts of Iterative Particle Reconstruction, Optical Transfer Function calibration and Data Assimilation to restore the results on a Cartesian mesh. Algorithms for image sequences as well as for four-frame recordings are detailed.

Applications: Flows at Different Temperatures

These experimental investigations of flows by means of PIV have been carried out in 1996 by DLR in cooperation with the Center of Applied Space Technology and Microgravity (ZARM), University of Bremen, in order to complete their numerical simulations and LDV measurements.

Applications: Stereo PIV and Multiplane Stereo PIV

The various methods of image reconstruction and calibration as described in Sect. 8.1 were applied in the measurement of the unsteady vortex ring flow field in 1995. Figure 11.125 outlines a vortex ring generator having a simple construction with very reproducible flow characteristics. The vortex ring is generated by discharging a bank of electrolytic capacitors ((60,000,upmu text {F})) through a pair of loudspeakers which are mounted facing inward on to two sides of a wooden box. By forcing the loudspeaker membranes inward, air is impulsively forced out of a cylindrical, sharpened nozzle (inner diameter (=34.7,text {mm})) on the top of the box. The shear layer formed at the tip of the nozzle then rolls up into a vortex ring and separates from the nozzle as the membranes move back to their equilibrium positions due to the decay in supply voltage. As long as the charging voltage is kept constant, the formation of the vortex ring will be very reproducible. The generator also has a seeding pipe with a check valve allowing the interior of the box and ultimately the core of the vortex ring to be seeded.

Applications: Volumetric Flow Measurements

The transition to turbulence in circular jets is a complex three-dimensional process that requires the use of 3D-PIV and temporal resolution for the full understanding the dynamical behavior of coherent vortices. Jets are used in a multitude of engineering systems and their study often regards the process heat and mass transfer.