Flow characterization using a laser Doppler vibrometer

Abstract

This paper shows how a scanning laser Doppler vibrometer (LDV), an instrument designed to measure vibrations of structures or objects, can be used in a non-traditional fashion to measure the flow around a cylinder. In particular the von Karmann vortex street which appears in the cylinder wake will be visualized. This is achieved by measuring the changes in the optical path induced by local fluctuation of air refraction index to which the laser vibrometer is sensitive.

The measurements obtained with the LDV will be compared visually to measurements done with particle image velocimetry and also with CFD computations for one test case. The specific frequency of the Von Karmann street predicted by the vibrometer, will be compared numerically to the other techniques for two different sized cylinders at three different velocities.

Introduction

Measuring physical phenomena in most cases means perturbing that exact phenomenon. This is no different in the fluid dynamic world. Measurement techniques are ever evolving minimizing the interaction between measurement equipment and the physical event itself [1], [2]. In the fluid dynamic world this started out with relatively simple devices such as the constant temperature anemometer (CTA) and time resolved pressure recordings. These techniques, however, suffer from a number of disadvantages mostly due to their intrusive nature, which for instance makes it quite difficult for them to resolve high level turbulence or work at extreme temperatures. The next step came with the possibility to use optical measurement techniques such as the laser Doppler anemometer (LDA) and particle image velocimetry (PIV). Both techniques require the introduction of so-called tracer particles to the flow. The displacement of these tracers is then measured by means of an optical system and a laser. LDA offers a high spatial resolution measurement (by scanning) but at only one position at a time, whereas the PIV offers full imagery of the flow at reduced resolution. The latter gives good results for resolving turbulence and with the correct tracer particles can operate in a wide range of temperatures.

However, if one is only interested in obtaining a qualitative visualization of the flow relatively quickly, choosing these correct tracer particles and tuning the test set-up could be overzealous. Using a laser Doppler vibrometer (LDV) it is possible to visualize flows without any intrusion whatsoever [3], [4], [5]. A LDV, which is traditionally used to measure vibrations is also sensitive to changes in refractive index of the medium, in casu density variations of the measurement volume along the line-of-sight. Therefore it is possible to measure e.g. flows or even acoustic phenomena. Moreover it is possible to retrieve this spectral information simultaneously, without hampering measurement time or needing a different test set-up. Now, it is well known that the signals acquired by interferometric techniques are line integrals over the laser beam optical path, so therefore images are often taken at different angles to derive local density distribution, which in turn implies needing tomographic reconstruction algorithms [6]. In this paper we are only interested in a 2-dimensional view of the flow and because of the specific nature of the test object no reconstructive tomography is necessary as will be shown in the following paragraphs.

In this paper the flow in a cylinder wake is visualized with an LDV, at different free stream velocities and for different sizes of cylinders. The frequency information, characterizing the von Karmann street in the wake of the cylinder, gained from the LDV measurements will be compared numerically to calculations done from the Strouhal number, computational fluid dynamics (CFD) calculations as well as a PIV measurement. As the technique does not give direct quantitative information on velocities, the measured density variations will be compared visually to the density calculated from CFD simulations.