17.10.2014
The spatial resolution of commercial LDA systems for the measurement of local flow velocities in gases and liquids is in the range of 1 mm to 10 mm and is given by the length of the intersection volume of the crossing laser beams. In the context of the cooperation project "Zentrales Innovationsprogramm Mittelstand" (ZIM) a measurement principle developed at PTB was implemented in a compact profile sensor upgrade module for existing LDA systems which enables the measurement of velocity profiles with a spatial resolution of 0.3 % of the length of the measurement volume.
The Laser Doppler Anemometry (LDA) enables the non-contact measurement of local flow velocities in gases and liquids by detecting light scattered by tracer particles carried within the flow. The spatial resolution of commercial LDA systems is thereby given by the ellipsoidal intersection volume of two crossing laser beams. A fundamental down-scaling of this measurement volume, which is in the range of 1 to 10 mm, is limited by physical and application specific constraints. In the context of a cooperation project of the "Zentrales Innovationsprogramm Mittelstand" (ZIM), a cost-efficient profile sensor upgrade module for existing commercial LDA systems was implemented which is based on a measurement principle developed at PTB that – additionally to the velocity of the tracer particle – determines its transit position through the measurement volume. This principle enables a spatial resolution of velocity profiles of 0.3 % of the measurement volume.
Many applications, i.e. the investigation of flow at boundary layers, the formation of turbulences in the vicinity of surfaces or the determination of flow velocities in pipes, demand a higher spatial resolution than can be provided by current commercial LDA systems. A down-scaling of the measurement volume – and thereby an enhancement in spatial resolution – by changing the geometry or the optical setup of the measurement is – from an application point of view – often not possible since the access to the measurement position is insufficiently close. Alternative measurement techniques like pitot pressure probes or hot-wire anemometers have the disadvantages that they are not contactless, they complicate the access to the measurement position and need intricate calibration.
Hence, in collaboration with the Technische Universität Dresden and the companies ILA GmbH and OPTOLUTION Messtechnik at the start of the year a project of the “Zentrales Innovationsprogramm Mittelstand” (ZIM) funded by the BMWi began. The objective of this project is the development of sensor types for simultaneous, high resolution position and velocity measurement (profile sensors) for Laser Doppler Anemometers (LDA).
Within this collaboration project, PTB is responsible for the development of a cost-efficient and easy-to-adapt sensor upgrade module which is compatible with industry (Figure 1) – including the necessary signal processing. Besides the functional characteristics, emphasis is placed on flexibility in application and the simple integration of the upgrade module in already existing cross-beam LDA.
Figure 1: Receiver module with integrated duo-photodiode and the two low-noise preamplifiers, dimensions: 3.8 cm × 3.0 cm × 7.5 cm (left) and front view of the duo-PIN photodiode (right).
The determination of the velocity of the tracer particles carried within the flow with LDA systems – usually containing one receiver diode – takes place by measuring the Doppler frequency generated. The simultaneous detection of the transit position can be achieved by utilizing a second detector which is positioned beyond the measurement volume. The transit position of the particle is given by the phase difference ∆φ between both Doppler signals recorded by the two detectors.
References:
[1] M. Borys, V. Strunck, H. Müller, D. Dopheide: „Interferometrische Ortsauflösung der Strömungsgeschwindigkeit innerhalb des Messvolumens eines Referenzstrahl-LDA“, Lasermethoden in der Strömungsmesstechnik, GALA e.V. Aachen: Shaker (2000), ISBN 3-8265-7809-0, Beitrag 2.1-2.8 (in German)
Contact person:
Stefan Oertel, Dept. 1.4, WG 1.41, e-mail: stefan.oertel@ptb.de