Daniel Haufe

Scattering effects at near-wall flow measurements using Doppler global velocimetry

Doppler global velocimetry (DGV) is considered to be a useful optical measurement tool for acquiring flow velocity fields. Often near-wall measurements are required, which is still challenging due to errors resulting from background scattering and multiple-particle scattering. Since the magnitudes of both errors are unknown so far, they are investigated by scattering simulations and experiments. Multiple-particle scattering mainly causes a stochastic error, which can be reduced by averaging. Contrary to this, background scattering results in a relative systematic error, which is directly proportional to the ratio of the background scattered light power to the total scattered light power. After applying a correction method and optimizing the measurement arrangement, a subsonic flat plate boundary layer was successfully measured achieving a minimum wall distance of 100 μm with a maximum relative error of 6%. The investigations reveal the current capabilities and perspectives of DGV for near-wall measurements.

Optical multi-point measurements of the acoustic particle velocity with frequency modulated Doppler global velocimetry

To reduce the noise of machines such as aircraft engines, the development and propagation of sound has to be investigated. Since the applicability of microphones is limited due to their intrusiveness, contactless measurement techniques are required. For this reason, the present study describes an optical method based on the Doppler effect and its application for acoustic particle velocity (APV) measurements. While former APV measurements with Doppler techniques are point measurements, the applied system is capable of simultaneous measurements at multiple points. In its current state, the system provides linear array measurements of one component of the APV demonstrated by multi-tone experiments with tones up to 17 kHz for the first time.

Lock-in spectroscopy employing a high-speed camera and a micro-scanner for volumetric investigations of unsteady flows.

Spectroscopic methods are established tools for nonintrusive measurements of flow velocity. However, those methods are either restricted by measuring pointwise or with low measurement rates of several hertz. To investigate fast unsteady phenomena, e.g., in sprays, volumetric (3D) measurement techniques with kHz rate are required. For this purpose, a spectroscopic technique is realized with a power amplified, frequency modulated laser and an Mfps high-speed camera. This allows fast continuous planar measurements of the velocity. Volumetric data is finally obtained by slewing the laser light sheet in depth with an oscillating microelectromechanical systems (MEMS) scanner. As a result, volumetric velocity measurements are obtained for 256×128×25 voxels over 14.4  mm×7.2  mm×6.5  mm with a repetition rate of 1 kHz, which allows the investigation of unsteady phenomena in sprays such as transients and local velocity oscillations. The respective measurement capabilities are demonstrated by experiments. Hence, a significant progress regarding the data rate was achieved in spectroscopy by using the Mfps high-speed camera, which enables new application fields such as the analysis of fast unsteady phenomena.

Transmission of independent signals through a multimode fiber using digital optical phase conjugation

Multimode fibers are attractive for a variety of applications such as communication engineering and biophotonics. However, a major hurdle for the optical transmission through multimode fibers is the inherent mode mixing. Although an image transmission was successfully accomplished using wavefront shaping, the image information was not transmitted individually for each of the independent pixels. We demonstrate a transmission of independent signals using individually shaped wavefronts employing a single segmented spatial light modulator for optical phase conjugation regarding each light signal. Our findings pave the way towards transferring independent signals through strongly scattering media.

Spectral Analysis of Velocity Fluctuations in the Vicinity of a Bias Flow Liner With Respect to the Damping Efficiency

Bias flow liners are perforated walls with an additional bias flow through the perforation. A common application is their integration in walls of hot gas areas, e.g. in combustion chambers of aero engines, for cooling. It is wellknown for a long time that the bias flow additionally provides a substantial amount of acoustic damping. However, until now the detailed damping mechanisms are not fully understood. Therefore, this study takes a closer look at the energy transfer from a propagating acoustic wave to the flow velocity field, which can be measured in the vicinity of the perforated wall. Two laser-optical measurement techniques, Doppler global velocimetry with sinusoidal laser frequency modulation (FM-DGV) and acoustic particle image velocimetry (A-PIV), were applied to a bias flow liner. The time-continuous velocity signal measured by FM-DGV was used to perform a spectral analysis of the fluctuating velocity components above the liner surface. This was done for different acousticexcitation frequencies, where the liner exhibits different dissipation rates. After removal of the acoustic Content from the power spectral density, the hydrodynamic velocity components feature a spectral augmentation in the vicinity of the excitation frequency. The effect appears downstream of the holes. The amount of spectral Augmentation correlates to the dissipation rate of the liner at the different excitation frequencies. Consequently, a certain transfer of acoustic energy into turbulent velocity fluctuations occurs in different frequency regimes. This energy transfer is, as a matter of fact, a function of the dissipation rate of the liner. Moreover, it has a strong dependence on the spatial location with respect to the orifice of the bias flow liner.

Aeroacoustic near-field measurements with microscale resolution

For the investigation of sound-flow interaction in near-fields, like aeroacoustic damping or acoustic streaming, measurements of the acoustic particle velocity (APV) and the flow velocity field with a micrometer resolution are required. In addition, a high working distance is needed for contactless measurement. For this task, the laser Doppler velocity profile sensor is shown to be a predestined tool. First, the APV measurement is successfully validated in an aeroacoustic duct using a microphone-based measurement method as a reference. Here, a minimum APV amplitude of 4 mm/s was resolved in agreement with the reference measurements. Then, the profile sensor was applied for measurements at a perforated acoustic liner with bias flow. Acoustically induced flow vortex structures were resolved with a spatial resolution of 10μm with a minimum distance of 350μm to the liner perforation. A comparison to frequency modulated Doppler global velocimetry (FM-DGV) demonstrated the advantage of the profile sensor for spatially resolved measurements of small scale structures. In contrast, FM-DGV is beneficial due to its high measurement rate which enables the spectral analysis of the velocity in order to better understand the energy transfer from sound to flow.

Transmission of Multiple Signals through an Optical Fiber Using Wavefront Shaping

The transmission of multiple independent optical signals through a multimode fiber is accomplished using wavefront shaping in order to compensate for the light distortion during the propagation within the fiber. Our methodology is based on digital optical phase conjugation employing only a single spatial light modulator, where the optical wavefront is individually modulated at different regions of the modulator, one region per light signal. Digital optical phase conjugation approaches are considered to be faster than other wavefront shaping approaches, where (for example) a complete determination of the wave propagation behavior of the fiber is performed. In contrast, the presented approach is time-efficient since it only requires one calibration per light signal. The proposed method is potentially appropriate for spatial division multiplexing in communications engineering. Further application fields are endoscopic light delivery in biophotonics, especially in optogenetics, where single cells in biological tissue have to be selectively illuminated with high spatial and temporal resolution.

High Dynamic Range Measurements of Acoustic Particle Velocity and Flow Velocity for the Application at Liners

Today, acoustic liners contribute significantly to the reduction of aircraft engine noise. To maximize the damping performance of so-called bias flow liners, a deeper understanding of the interaction between the acoustic wave and the bias flow is demanded. Therefore, contactless field measurements of the acoustic particle velocity and the flow velocity at a bias flow liner are necessary. Since the acoustic particle velocity is several orders of magnitude lower than the flow velocity, a low measurement uncertainty and a high dynamic range are required. In order to analyze the transfer from acoustic energy to flow turbulence, the velocity spectrum has to be resolved, which necessitates a high measurement rate. Doppler global velocimetry with sinusoidal frequency modulation fulfills these requirements, which is demonstrated in this paper. The application of the measurement method at a bias flow liner in a flow duct at a Mach number of 0.1 is presented for the first time. The measurement uncertainty of the acoustic particle velocity amplitude is 5 mm/s and a dynamic range of 7000 is achieved. The velocity measurement rate of 50 kHz allows to resolve the turbulence spectrum up to 25 kHz. The measurement method is validated by reference measurements with established microphone-technique and a comparison with results of acoustic particle image velocimetry measurements is shown.

Non-invasive seedingless measurements of the flame transfer function using high-speed camera-based laser vibrometry

The characterization of modern jet engines or stationary gas turbines running with lean combustion by means of swirl-stabilized flames necessitates seedingless optical field measurements of the flame transfer function, i.e. the ratio of the fluctuating heat release rate inside the flame volume, the instationary flow velocity at the combustor outlet and the time average of both quantities. For this reason, a high-speed camera-based laser interferometric vibrometer is proposed for spatio-temporally resolved measurements of the flame transfer function inside a swirl-stabilized technically premixed flame. Each pixel provides line-of-sight measurements of the heat release rate due to the linear coupling to fluctuations of the refractive index along the laser beam, which are based on density fluctuations inside the flame volume. Additionally, field measurements of the instationary flow velocity are possible due to correlation of simultaneously measured pixel signals and the known distance between the measurement positions. Thus, the new system enables the spatially resolved detection of the flame transfer function and instationary flow behavior with a single measurement for the first time. The presented setup offers single pixel resolution with measurement rates up to 40 kHz at an maximum image resolution of 256 px x 128 px. Based on a comparison with reference measurements using a standard pointwise laser interferometric vibrometer, the new system is validated and a discussion of the measurement uncertainty is presented. Finally, the measurement of refractive index fluctuations inside a flame volume is demonstrated.

Perspectives of multimode fibers and digital holography for optogenetics

Optogenetic approaches allow the activation or inhibition of genetically prescribed populations of neurons by light. In principle, optogenetics offers not only the ability to elucidate the functions of neural circuitry, but also new approaches to a treatment of neurodegenerative diseases and recovery of vision and auditory perception. Optogenetics already has revolutionized research in neuroscience. However, new methods for delivering light to three-dimensionally distributed structures e.g. in the brain are necessary. A major hurdle for focusing light through biological tissue is the occurring scattering and scrambling of the light. We demonstrate the correction of the scrambling in a multimode fiber by digital optical phase conjugation with a perspective for optogenetics.