J. Czarske

Two-dimensional ultrasound Doppler velocimeter for flow mapping of unsteady liquid metal flows.

We present a novel pulsed-wave ultrasound Doppler system for fluid flow investigations being able to determine two-dimensional vector fields of flow velocities. Electromagnetically-driven liquid metal flows appear as an attractive application field for such a measurement system. Two linear ultrasound transducer arrays each equipped with 25 transducer elements are used to measure the flow field in a square plane of 67×67 mm(2). The application of advanced processing methods as a multi-beam operation, an interlaced echo signal acquisition and a segmental array technique enable high data acquisition rates and concurrently a high spatial resolution, which have not been obtained so far for flow measurements in liquid metals. The extended pulsing strategy and essential operation principles such as the multiplexing electronic concept will be presented within this paper. The capabilities of the measuring system make it suitable for investigations of non-transparent, turbulent flows. Here, we present measurements of liquid metal flows driven by a rotating magnetic field for demonstration purposes. The measuring setup realized here reveals details of the swirling fluid motion in a horizontal section of a cube. Frame acquisition rates up to 30 fps were achieved for a complete two-dimensional flow mapping.

Effects of axial scanning in confocal microscopy employing adaptive lenses (CAL)

We analyze axial scanning in Confocal microscopy based on Adaptive Lenses (CAL). A tunable lens located in the illumination path of a confocal setup enables scanning the focus position by applying an electrical voltage. This opens up the possibility to replace mechanical axial scanning which is commonly used. In our proof-of-principle experiment, we demonstrate a tuning range of about 380 μm. The range can easily be extended by using the whole possible tuning range. During the scan the axial resolution degrades by a factor of about 2.3. The deterioration is introduced by aberrations that strongly depend on the scanning process. Therefore a second lens is located in the detection path of the CAL setup to balance the aberration effects. Both experiments and simulations show that this approach allows creating a homogeneous axial resolution throughout the scan. This is at the cost of tuning range which halves to about 200 μm. The lateral resolution is not noticeably affected and amounts to 500 nm.

Adjustment and verification of macroscopic melt flow during solidification by means of various AC magnetic fields

We present an experimental study concerning the solidification of AlSi alloys exposed to a pulsed rotating magnetic field. Isothermal flow measurements were carried out in order to understand the flow structures resulting from the application of time-modulated magnetic fields. These investigations revealed transient flow regimes showing distinct inertial oscillations and coherent vortex structures. An intense melt flow with periodic reversals of the flow direction at the solidification front can be created by a suitable choice of the magnetic field parameters. Such resonant states of the flow pattern have been proven to provide beneficial conditions for solidification processes. Optimised flow conditions realized in a solidifying melt result in a significant grain refinement without provoking the formation of harmful segregation freckles.

Multimode Laser Doppler Anemometer for Turbulence Measurements With High Spatial Resolution

The advantageous employment of multimode fibres for beam delivery in laser Doppler anemometers (LDA) is presented. Turbulent boundary layers can be investigated with high precision and high spatial resolution. For measurements of turbulence data usually hot wire anemometers are employed which, however, do not fulfil the requirement of non-intrusiveness. Therefore laser Doppler anemometers are employed which are limited by the size of their measurement volume of usually about 50 μm × 100 mm. The spatial resolution can be improved by a stronger focusing or by using a side receiver, which restricts the detection area. Furthermore, the measurement of turbulence data is limited by the varying fringe spacing, which pretends a non-existing degree of turbulence (“virtual turbulence”) and is caused by the wave-front curvature of the employed Gaussian laser mode. In this contribution it is demonstrated, that the employment of multimode-light with beam quality factors M2 >> 1 the length of the measurement volume is reduced to a few percent compared to the intersection volume length of the two laser beams because of the low spatial coherence of the multimode light. The uniformity of the fringe spacing is significantly improved. The variation of fringe spacing (“virtual turbulence”) is less than 0.05%. The multimode-fibre LDA (MMF-LDA) combines the advantages of both a short measurement volume guaranteeing a high spatial resolution as well as low virtual turbulence in one device. It is therefore well suited for high accurate determination of velocity gradients in laminar or turbulent boundary layers. A MMF-LDA with about 100 fringes and 5·10−4 fringe spacing variation within a measurement volume of length 80 μm was used to perform fluid measurements in a wind tunnel. The remaining turbulence intensity of the free wind tunnel stream was determined to 0.3%. Boundary layer measurements on a well-known laminar velocity profile, the Blasius boundary layer, were performed and the wall shear stress was determined. All results are in excellent agreement with the theory. Measurements of turbulent boundary layers are presented. Multimode fibres allow the transfer of significantly higher power into the LDA measurement volume and need lower alignment effort compared to the usually employed single-mode fibres. Powerful laser diodes can now be applied for LDA set-ups, enabling sensitive velocity measurements of fluid flows.Copyright

3D shape measurements with a single interferometric sensor for in-situ lathe monitoring

Temperature drifts, tool deterioration, unknown vibrations as well as spindle play are major effects which decrease the achievable precision of computerized numerically controlled (CNC) lathes and lead to shape deviations between the processed work pieces. Since currently no measurement system exist for fast, precise and in-situ 3d shape monitoring with keyhole access, much effort has to be made to simulate and compensate these effects. Therefore we introduce an optical interferometric sensor for absolute 3d shape measurements, which was integrated into a working lathe. According to the spindle rotational speed, a measurement rate of 2,500 Hz was achieved. In-situ absolute shape, surface profile and vibration measurements are presented. While thermal drifts of the sensor led to errors of several mµm for the absolute shape, reference measurements with a coordinate machine show, that the surface profile could be measured with an uncertainty below one micron. Additionally, the spindle play of 0.8 µm was measured with the sensor.

A Laser-Optical Sensor System for Simultaneous Tip Clearance and Vibration Monitoring of Compressor Rotor Blades

In order to improve the efficiency and increase the life-time of compressors and turbines, online monitoring of operating parameters is an essential tool. One aim is to predict critical events like stall or flutter by observing blade vibrations and deformations. Due to superior material properties ceramic, carbon-fiber and glass-fiber reinforced composite blades become more and more popular. Hence, conventional measurement systems like capacitive probes are not able to deliver the necessary precision or, in the worst case, are not able to measure at all. Therefore, we developed a fiber-coupled, laser-optical sensor, named laser Doppler distance sensor (LDDS), which overcomes this drawback. The sensor is able to resolve the circumferential blade tip velocity as well as the radial expansion of each blade. Moreover, conventional blade tip timing measurements are possible as well.Our aim is to provide a universal stall prediction and monitoring sensor system that is applicable to all types of blades.Copyright

Novel Dynamic Rotor and Blade Deformation and Vibration Monitoring Technique

Monitoring rotor deformations and vibrations dynamically is an important task for improving both the safety and the lifetime as well as the energy efficiency of motors and turbo machines. However, due to the high rotor speed encountered in particular at turbo machines, this requires concurrently high measurement rate and high accuracy, which is hardly possible to achieve with currently available measurement techniques. To solve this problem, in this paper, we present a novel non-incremental interferometric optical sensor that measures simultaneously the in-plane velocity and the out-of-plane position of laterally moving objects with micrometer precision and concurrently with microsecond temporal resolution. It will be shown that this sensor exhibits the outstanding feature that its measurement uncertainty is generally independent of the object velocity, which enables precise deformation and vibration measurements also at high rotor speed. Moreover, this sensor does not require an in situ calibration and it allows a direct measurement of blade velocity variations in contrast to BTT systems. For application under harsh environmental conditions such as high temperatures, a robust and miniaturized fiber-optic sensor setup was developed. To demonstrate the capability of this sensor, measurements of tip clearance changes and rotor blade vibrations at varying operating conditions of a transonic centrifugal compressor test rig at blade tip velocities up to 600 m/s are presented amongst others.Copyright

Investigation of the Tip Clearance Flow in a Compressor Cascade Using a Novel Laser Measurement Technique With High Temporal Resolution

The understanding of the tip clearance flow in axial compressors is a key issue for developing new compressors with enhanced efficiency and reduced noise for instance. However, necessary flow measurements in the blade tip region and within the tip clearance are challenging due to the small gap width. The application of a novel optical measurement technique named Doppler global velocimetry with laser frequency modulation is presented, which provides velocity field measurements of all three velocity components non-intrusively in the tip clearance flow of a linear cascade at near stall conditions. These array measurements have a high temporal resolution enabling turbulence analysis such as the evaluation of velocity standard deviations and turbulence spectra up to several kilohertz. Conventional pneumatic and hot-wire measurements in planes at the inlet and the outlet as well as on the blade surface were taken to complete the flow pattern and validate the data of the Doppler global velocimetry. Wake measurements identified a strong flow separation in the rear suction side dominating the transient character of the cascade flow. Towards the endwall this high loss region is reduced by the clearance flow and the resulting vortex, which is obviously not affected by the profile separation and the pulsating blockage frequency. Inside the blade passage and the tip clearance the Doppler global velocimetry measurements allowed a spatial assignment of the origin of the tip leakage flow and the downstream developing vortex. In addition, the tip clearance vortex could be resolved and identified successfully as the most dominant turbulence generating effect in the near endwall region at this high loading operating point of the blading.Copyright

Investigations of fast-rotating bodies using an interferometric laser Doppler distance sensor system

One challenge in micrometrology is to measure precisely the shape of fast moved objects with high temporal resolution. Deformation measurements of lightweight composite materials are of importance to guarantee its robustness e.g. against impacts. In a high-speed rotor test rig their elastic and plastic deformations due to centrifugal forces can be evaluated. Non-contact inspection techniques with micron resolution under vacuum conditions are necessary. For the first time, we present high-speed deformation measurements of a cylindrical rotor by a non-incremental laser Doppler distance sensor system using fiber and diffractive optics. Besides the determination of the radial enlargement also wobbling of the rotor was monitored.

Velocity Measurements of Shear Flows by a Novel Velocity Profile Sensor With Micrometer Spatial Resolution

A measuring system based on a differential laser-Doppler velocimeter has been extended to determine one-dimensional velocity profiles with a spatial resolution inside the measurement volume. The principle of the velocity profile sensor is based on the generation of two fringe systems with different gradients of the fringe spacings. The determination of the corresponding two Doppler frequencies yields the position as well as the velocity component of individual tracer particles, which results in the velocity profile for detecting several particles. The sensor was used to determine velocity profiles of flat-plate laminar boundary layers for varying Reynolds numbers. A precise determination of the wall shear stress was accomplished. All results are in good agreement with the theory. The main application of the velocity profile sensor is the spatial high-resolved investigation of turbulent boundary layers.Copyright