Measurement of turbulence production in the cylinder separated shear-layer using event-triggered Laser-Doppler anemometry

In the transitional cylinder wake as well as the primary vortex street, the separated shear layer becomes unstable and sequences of small-scale vortex structures are produced intermittently. This paper describes how LDA data can be collected exclusively for these intermittent events. A real-time digital signal processing algorithm is developed in order to detect the instability of the shear layer from the characteristic hot-wire signal. Simultaneously, LDA data is selected so that the turbulence production in the unstable separated shear-layer can be derived. This trigger technique can be used for any kind of intermittent signal and measurement process.