Research Group Prof. Rösgen (Emeritus)
The research in Prof. Rösgen’s group had primarily experimental character. It was centered on the development of novel flow diagnostics methods (with special emphasis on imaging techniques) and the investigation of applied flow problems utilizing these techniques.
The imaging activities aimed at the development of computer-enhanced visualization and sensing techniques to probe complex three-dimensional flow fields.
The applied research projects covered a wide range of topics, including environmental and industrial flows, biomedical problems, aerospace applications and studies of fundamental fluid physics phenomena.
In accordance with the experimental focus, Prof. Rösgen’s group offered education not only in fundamental aspects of fluid mechanics and engineering experimentation in general, but also in several advanced topics like imaging techniques, compressible flows and aerodynamics. Practical training and thesis work utilizing the research groups’ infrastructure and expertise was offered both at the undergraduate and graduate levels, exposing students to many aspects of modern experimentation in fluid mechanics.
Research Areas and Projects
Streak-Based Volumetric Flow Reconstruction
Volumetric velocimetry techniques for fluid flows, such as 3D Particle Tracking Velocimetry (3D-PTV) and Tomographic Particle Image Velocimetry (Tomo-PIV), use the projected image of tracer particles in two or more camera views to reconstruct the tracers’ positions in 3D space, measure their displacement over time and retrieve the flow velocity. The 3D-reconstruction step can result in so-called “ghost particles,” which are reconstructed tracer particles for which it remains uncertain whether they are real or artifacts of the reconstruction process. While 3D-PTV relies on the reconstruction of points that represent “frozen” particles, a related but less commonly used technique, 3D Particle Streak Velocimetry (3D-PSV), uses long-exposure images where the signature of each particle is its pathline over the exposure time, a “streak.”
This work leverages the topological information provided by long-exposure imaging in 3D-PSV to reduce the number of ambiguous particle reconstructions. The use of information about the pathline shape allows the reduction of the required camera views and frame rate, while it can be beneficial in analyzing flows with large dynamic velocity range.
Flow-Induced Nanoparticle Rotation with Luminescence Anisotropy
Flow-induced microrotation, also known as vorticity, is a fundamental characteristic of turbulent flows. Despite ongoing research in the field, adequately measuring this fundamental property remains a significant challenge. Various methods have been developed with specific advantages and disadvantages. They should be evaluated on their cost, complexity, and performance while achieving an adequate spatial and temporal resolution.
This work examines the utility of luminescence anisotropy for the measurement of fluid-particle rotations. The basic concept rests on the idea of depolarization of an initially polarized excited population of dye molecules. Analyzing the rate of depolarization it should become should be possible to measure changes in in molecular orientation due to fluid rotations.
Motion Capture of Manually Operated Flow Probes
In certain application areas popular field-based imaging techniques such as Particle Image Velocimetry (PIV) are not optimal because of the long setup times and restricted optical access. In contrast, human-operated probes can be quickly deployed and operated without lengthy teach-in or alignment periods. In the “ProCap” project, modern motion capture technologies are combined with established flow sensing techniques (e.g. multi-hole probes) and real-time flow visualization hardware and software to provide a fast, intuitive measurement environment for wind tunnel applications.
Real-time 3D Particle Tracking Using Dynamic Vision Sensors
One of the main drawbacks of modern flow imaging techniques is the requirement for very large data transfer rates in order to store the acquired image data. A typical example is the analysis of tracer motion in “Particle Tracking Velocimetry” (PTV), where high resolution images have to be acquired rapidly to follow the motion of a comparatively small number of isolated image features. An efficient alternative may be the use of so-called “Dynamic Vision Sensors” (DVS) which sense and transmit only the coordinates and time instants of contrast changes / motion events in a scene. An ongoing project at IFD aims at the development of a real-time capable 3D particle tracking system for large wind tunnels. Neutrally buoyant soap bubbles are tracked and their paths are reconstructed using an array of 3 DVS sensors.
Speckle-based Background Oriented Schlieren
The so-called “Background Oriented Schlieren” methods (BOS) has recently attracted considerable interest because of its capability to provide quantitative density and/or temperature field measurements. At IFD a new variant of BOS is being developed which uses laser speckles as the image background pattern. This allows to keep the test article in focus while adjusting the overall system sensitivity, two features unavailable in conventional BOS.
Doppler Imaging Velocimetry
“Doppler Global Velocimetry” (DGV) is an alternative to PIV methods for image Velocimetry, particularly also in large facilities because it eliminates the requirement for resolving individual tracer particles. An undesirable feature remains the encoding of flow velocities as intensity variations because of the susceptibility to various noise sources. In order to alleviate this problem, another approach is being followed at IFD. Using suitable laser illumination geometries and high speed cameras together with customized processing algorithms, the measurement of a velocity field can be translated into the direct analysis of Doppler modulation frequencies, similar to the processing principles underlying the functioning of classical point-wise laser Doppler probes.
MEMS Based Pressure Sensors
Surface pressure measurements using pressure taps are routinely used in aerodynamic model investigations. The cost of installation is often a limiting factor because of the pneumatic harness required. At IFD, a project was initiated to develop a completely passive and wireless pressure sensing technique. Small micromechanical (MEMS) resonators were developed whose resonance frequency changed in response to the applied aerodynamic pressure. Readout of the sensing elements was achieved via a frequency-sensitive camera which acted essentially as an imaging vibrometer.
Study of Backlayering in Tunnel Fires
The aim of this research activity is to study the dynamic development and structure of the so-called “backlayering zone” which occurs in tunnel fires. The understanding and control of this phenomenon can be critical in the design of emergency ventilation systems.
Speckle BOS imaging will be introduced to the dedicated fire simulation facility operated at the Institute to provide new insights into this near-wall, variable density flow. The specific advantage of this new technique developed at IFD is its capability to image density variations in flows close to walls, where other schlieren type techniques are ineffective.
Infrared Pyrometry for Reentry Aerothermodynamics
The European Experimental Reentry Testbed (EXPERT) is a capsule designed to re-enter earth’s atmosphere after a suborbital ballistic flight. The IFD contribution to this multi-national experiment platform is a near-infrared camera with special high-temperature sapphire optics to monitor the surface temperature distribution on one of the vehicle’s stabilizing flaps. The next evolutionary step in ESA’s reentry research program is the Intermediate Experimental Vehicle (IXV), a spacecraft which will orbit earth and then perform a controlled reentry while executing aerodynamic maneuvers. Based on the EXPERT module, IFD has designed the hardware and software for a self-calibrating, multispectral infrared camera. A further ongoing IFD project is the development of a camera system to record the breakup dynamics of the Automated Transfer Vehicle (ATV) while it is de-orbited at the end of its service flight to the International Space Station.
Turbulence Modification in Magnetic Fluids
A common interpretation of turbulence dynamics is the so-called “energy cascade” where large scale forcing of the flow leads to turbulent fluctuations at ever decreasing time and length scales down to the level where viscosity finally dissipates the fluctuation energy. This model description has been confirmed in many circumstances but may not represent the complete picture. In particular, there is also evidence for the existence of a “reverse cascade” where the very small fluctuations may have an impact on the larger scales. This phenomenon is not only of physical interest but becomes important in the design of numerical codes (“large eddy simulations” where the small scale dynamics of turbulent flows is not fully resolved but rather approximated by phenomenological models.
The present project is to provide much needed experimental data on the turbulence dynamics at the dissipation scale. A candidate flow scenario is generated in a paramagnetic colloidal suspension, where an external magnetic field leads to configurational changes at the molecular scale, resulting in an anisotropic viscosity.
Unsteady Flow Phenomena in the Wake of Bioprosthetic Heart Valves
The design of aortic heart valve prostheses is affected by various limitations. While mechanical heart valves yield long life cycles, they can cause blood damage and typically require life-long anticoagulation medication. In contrast, bioprosthetic heart valves do not lead to blood damage, but have a very limited life span due to structural failure.
It has been suggested that this failure is related to the local blood flow. In order to better understand this phenomenon, fully space- and time-resolved flow data sets for bioprosthetic aortic valves are being acquired and analyzed by means of tomographic Particle Image Velocimetry (TomoPIV).