Jörg König

Precise micro flow rate measurements by a laser Doppler velocity profile sensor with time division multiplexing

This paper presents the measurement of flow rate inside a microchannel by using a laser Doppler technique. For this application a novel laser Doppler velocity profile sensor has been developed. Instead of parallel fringe systems, two superposed fan-like fringe systems with opposite gradients are employed to determine the velocity distribution inside the microchannel directly. The sensor utilizes the time division multiplexing technique to discriminate both fringe systems. A velocity uncertainty of 0.18% and a spatial resolution of 960 nm are demonstrated in the flow, which is the highest spatially resolved measurement by a laser Doppler technique published to date. Flow rate measurements, in the range of 30 µl min−1, with a statistical uncertainty of 5 × 10−4 are further presented. In comparison to a reference, by precise weighing, the mean deviation between both measurement principles amounts to 1%. With the advantage of high spatial resolution with simultaneous low velocity uncertainty, the laser Doppler velocity profile sensor offers a new tool for microfluidic diagnostics, e.g. in lab-on-a-chip systems or for drug delivery, which requires very small flow rates.

Speckle noise influence on measuring turbulence spectra using time-resolved Doppler global velocimetry with laser frequency modulation

A novel Doppler global velocimeter (DGV) with high temporal resolution is presented as a tool for measuring spatially resolved flow turbulence spectra for three components in order to characterize complex flows, e.g. in turbomachines. The proposed DGV technique is based on a sinusoidal laser frequency modulation. Its maximum available measurement rate equals the modulation frequency and amounts currently to 100 kHz. The harmonic analysis of the detector array signals reduces errors due to detector offset drifts, detector sensitivity changes, ambient light, camera misalignment and beam splitting errors in comparison with conventional DGV systems. The achievable statistical errors are considered by theoretical investigations and by experiments regarding detector noise as well as temporal and spatial scattered light fluctuations, e.g. due to speckles. An error propagation finally provides the determination of the noise power spectral density occurring as virtual turbulence in the measured turbulence spectra. It amounts to about 1.2 × 10−4 (m2 s−2) Hz−2 for mean flow velocities up to 40 m s−1 and 1 nW mean scattered light power per detector element. It rises for higher flow velocities in dependence on the flow turbulence. For the example of a nozzle flow with a mean velocity of 85 m s−1, which is disturbed by a cylinder, the final uncertainty is demonstrated to result in an effective bandwidth of the acquired turbulence spectra of 10 kHz and is thus sufficiently high for flow turbulence analysis. The measured velocity spectra agree well with comparison measurements using a hot-wire anemometer.

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.

Analysis of the Electrolyte Convection inside the Concentration Boundary Layer during Structured Electrodeposition of Copper in High Magnetic Gradient Fields

To experimentally reveal the correlation between electrodeposited structure and electrolyte convection induced inside the concentration boundary layer, a highly inhomogeneous magnetic field, generated by a magnetized Fe-wire, has been applied to an electrochemical system. The influence of Lorentz and magnetic field gradient force to the local transport phenomena of copper ions has been studied using a novel two-component laser Doppler velocity profile sensor. With this sensor, the electrolyte convection within 500 μm of a horizontally aligned cathode is presented. The electrode-normal two-component velocity profiles below the electrodeposited structure show that electrolyte convection is induced and directed toward the rim of the Fe-wire. The measured deposited structure directly correlates to the observed boundary layer flow. As the local concentration of Cu(2+) ions is enhanced due to the induced convection, maximum deposit thicknesses can be found at the rim of the Fe-wire. Furthermore, a complex boundary layer flow structure was determined, indicating that electrolyte convection of second order is induced. Moreover, the Lorentz force-driven convection rapidly vanishes, while the electrolyte convection induced by the magnetic field gradient force is preserved much longer. The progress for research is the first direct experimental proof of the electrolyte convection inside the concentration boundary layer that correlates to the deposited structure and reveals that the magnetic field gradient force is responsible for the observed structuring effect.

Wavefront shaping for imaging-based flow velocity measurements through distortions using a Fresnel guide star

Imaging-based flow measurement techniques, like particle image velocimetry (PIV), are vulnerable to time-varying distortions like refractive index inhomogeneities or fluctuating phase boundaries. Such distortions strongly increase the velocity error, as the position assignment of the tracer particles and the decrease of image contrast exhibit significant uncertainties. We demonstrate that wavefront shaping based on spatially distributed guide stars has the potential to significantly reduce the measurement uncertainty. Proof of concept experiments show an improvement by more than one order of magnitude. Possible applications for the wavefront shaping PIV range from measurements in jets and film flows to biomedical applications.

Flow Investigations in the Tip Gap of Rotor Blade Tips With Squealer Cavity

The tip leakage flow in turbines is considered to be responsible/or significant machine losses. An efficient reduction of these losses by e. g. squealer cavities at rotor blade tips requires a detailed physical and quantitative understanding of the tip leakage flow. For this purpose, numerical flow simulations are a valuable tool, but they have to be validated by measurements. However, non-intrusive, optical flow measurements in a rotating machine are challenging due to the small tip gap dimensions.Using an optimized optical setup, all three velocity components of the tip gap flow field were resolved while the turbine (1.5 stage low Mach number turbine test rig) was running with 930Hz blade passing frequency at the design point. The measurement results are in good qualitative agreement with numerical flow simulations. The gap flow above the squealer cavity is not homogeneous, but has several flow gradients, which mainly result from the blade tip geometry and the continuity of the flow. Furthermore, the flow structure between two successive rotor blades was resolved yielding the size and shape of the tip leakage vortex downstream at the suction side of the rotor blade in the measurement plane. Consequently, the capabilities of the applied measurement approach opens promising perspectives toward the development of optimum blade tip designs with minimized tip leakage.Copyright

Laser Doppler Field Sensor for Two Dimensional Flow Measurements in Three Velocity Components

The laser Doppler field sensor presented in this paper is suited for high resolution flow velocity field imaging on the micro scale without use of a camera. The field sensor is constructed by the superposition of two laser Doppler velocity profile sensors. The profile sensor is based on the principle of Laser Doppler Anemometry (LDA). However, instead of one parallel fringe system one divergent and one convergent fringe system are employed. Utilizing a total number of four fringe systems the field sensor can determine the tracer particle position two-dimensionally in the measurement volume. The spatial resolution and velocity accuracy are increased compared to conventional LDA by more than one order of magnitude. Since no camera is used, the resolution of the sensor is not influenced by pixel size effects. Two-dimensional velocity field measurements of an injection nozzle flow and of a micro channel flow with quadratic cross section are presented.

Messunsicherheitsschranke der Doppler-Global-Velozimetrie mit LaserfrequenzmodulationFundamental Measurement Uncertainty Limits of Doppler Global Velocimetry with Laser Frequency Modulation

Zusammenfassung Es wird ein Doppler-Global-Velozimeter mit sinusförmiger Laserfrequenzmodulation (FM-DGV) vorgestellt, mit welchem Geschwindigkeitsfelder in Strömungen berührungslos gemessen werden können. Eingesetzt wird dabei ein Diodenlaser (852,36 nm), eine Cäsium-Absorptionszelle und ein fasergekoppeltes Lawinenphotodiodenarray. Die minimal erreichbare Messunsicherheit wird anhand einer Berechnung der Cramér-Rao-Schranke abgeschätzt und mit der der konventionellen DGV-Messtechnik verglichen. Die betrachteten Rauschquellen sind das Quantenschrotrauschen sowie das thermische und Dunkelstromrauschen der Photodetektoren. Die Berechnungen zeigen in Übereinstimmung mit Messungen, dass minimal 0,03 m/s bei einer Streulichtleistung von 3nW, einer Zeitauflösung von 16 ms und bei einem minimalen NEP-Wert des Detektors von 35 fW/√Hz erreicht werden können. Zwar wird für die betrachteten Störquellen die resultierende Unsicherheit bei konventionellen DGV-Systemen als ähnlich hoch abgeschätzt, jedoch entfallen beim FM-DGV die u. U. hohen Bildausrichtungs- und Strahlteilungsfehler, da keine Referenzkamera benötigt wird. Es ergeben sich somit vielversprechende Perspektiven in der Anwendung z.B. bei der präzisen Vermessung von 2D3K-Strömungsgeschwindigkeiten in Turbomaschinen und, aufgrund der möglichen hohen Zeitauflösung von bis zu 10 μs, bei der simultanen ortsaufgelösten Messung von Geschwindigkeitsspektren im Rahmen einer Turbulenzanalyse.

Adaptive flow-field measurements using digital holography

Variations of the optical detection path-length in image correlation based flow-field measurements result in strong errors in position allocation and thus lead to a strong enhancement of the measurement uncertainty of the velocity. In this contribution we use digital holography to measure the wavefront distortion induced by fluctuating phase boundary, employing spatially extended guide stars. The measured phase information is used to correct the influence of the phase boundary in the detection path employing a spatial light modulator. We analyze the potential of guide stars that are reflected by the phase boundary, i.e. the Fresnel reflex, and transmitted. Our results show, that the usage of wavefront shaping enables to strongly reduce the measurement uncertainty and to strongly improve the quality of image correlation based flow-field measurements. The approaches presented here are not limited to application in flow measurement, but could be useful for a variety of applications.

Smart laser sensor with measurement uncertainties beyond the diffraction limit

Optical measurement methods exhibit several potential advantages such as non-intrusiveness, high sensitivity, stability and selectivity. On the other hand the diffraction of light limits their measurement properties. At laser Doppler measurements the diffraction limit results in a complementarity relation between the velocity and the distance of moving scattering objects. In this contribution a novel laser Doppler sensor is presented providing velocity measurements beyond the diffraction limit. This smart laser sensor employs multi sensor data fusion, adaptive signal validation and digital frequency processing close to Cramér Rao lower bound. The benefits of the smart sensor will be demonstrated by simultaneous velocity and position measurements of complex and microfluidic flows, and high-speed rotating objects.