A novel laser doppler array sensor for measurements of micro-scale velocity correlations in turbulent flows

One of the aims of turbulence research is the resolution of very small flow structures which occur at high Reynolds numbers. Correlation functions and the instantaneous velocity gradient are often used to characterize these flows and their vortices. In order to evaluate the mentioned measures, e.g. hot wire anemometry can be used. However, it is limited due to the size of the probe which makes measurements with very small separation distances impossible. Furthermore, it is intrusive. To overcome these difficulties, an optical Doppler sensor system with array detection is presented in this paper. The detection array enables parallel processing of measurement signals. Hence, simultaneous velocity information can be evaluated at two different positions inside the flow. The measurement system is applied for correlation measurements in turbulent flows and a resolution of 15 μm could be achieved for the spatial correlation function. An uncertainty evaluation for the estimate of the Taylor length scale λ is carried out. The progress for fluid mechanics lies in the ability to precisely evaluate correlation functions of highly turbulent flows.