Using conventional LDV systems in strong velocity gradients is difficult, because the measured velocity is an average over the length of the measuring volume. Even the use of front lenses with short focal length and measuring volumes of less than 1mm is not a solution because the spatial resolution required for measurements in boundary layers is about 1-10 µm. ILA GmbH developed in cooperation with OPTOLUTION and the Technical University of Dresden (Prof. Jürgen Czarske*, Dr. Lars Büttner*) a new LDV-profile sensor that offers a spatial resolution of 1% of the length of the measurement volume.
The basic idea of the developed LDV-profile sensor is to detect the position of the particle inside the measuring volume. This is realized by the overlap of two measuring volumes with different wavelength, one with a divergent fringe system the other with a convergent one (Fig.3). The ratio of the detected Doppler frequency of both fringe systems fD1/fD2 is used to calculate the particle position z inside the measuring volume. The particle position and velocity can be calculated with the known fringe distance under consideration of the deviation of the fringe distances.
Fig. 2 shows the comparison of the measurement results inside a free jet of a conventional LDV (red dots) and a LDV-profile sensor (small blue dots). The free jet has a diameter of 1mm. The length of the measurement volume of the conventional LDV (f=160mm) is about 500µm. The LDV profile sensor offers a length of the measuring volume of 1mm with a spatial resolution of 1% (10µm). It is obvious that the conventional LDV is not able to resolve the high velocity gradient in the shear stress region of the free jet.
*„Czarske, J., Büttner, L., Razik, T., & Müller, H. (2002). Boundary layer velocity measurements by a laser Doppler profile sensor with micrometre spatial resolution. Measurement Science and Technology, 13(12), 1979-1989.”
Fig. 1: Measuring volume of a profile sensor inside a free jet
Fig. 2: comparison of the measurement in a jet flow with a profile sensor and a conventional LDV