Speaker: Daniele Fiscaletti (Delft University of Technology)

Room: SISSA - Santorio A - room 133

Abstract:

The investigations of flows at high Reynolds number is of great interest for the theory of turbulence, as the large and the small scales of turbulence show a clear separation. But, being the Kolmogorov length scale almost inversely proportional to the Reynolds number, the investigation of the small scales in these flows requires a spatial resolution that can not be achieved by a traditional PIV system. In the present work, a long-range μPIV system was applied to investigate the small-smale motions in the central region of a self-similar round jet at high Reynolds number (Reλ ≈ 350). The application of a long-range μPIV system allowed to achieve a vector spacing of 1.5η, where the Kolmogorov length scale was estimated to be 55 μm. The main critical aspects in the implementation of the measurement technique, such as the low seeding concentration and the illumination of out-of-focus particles, were successfully solved. The resulting velocity fields were used to characterize the small-scale flow structures in this jet. The autocorrelation maps of vorticity and λci (the imaginary part of the eigenvalue of the reduced velocity gradient tensor) reveal that the structures of intense vorticity have a characteristic diameter of approximately 10η. From the autocorrelation map of the reduced (2D) rate of dissipation, it is obtained that the regions of intense dissipation organize preferentially in the form of sheets, with a characteristic thickness of approximately 10η. The regions of intense dissipation have the tendency to be found in proximity of intense vortices. Furthermore, the joint pdf of the two invariants of the reduced velocity gradient tensor presents the characteristic teapot-shape. These results, based on a statistical analysis of the data, are in agreement with previous numerical and experimental studies at lower Reynolds number, which validates the suitability of long-range μPIV for characterizing turbulent flow structures at high Reynolds number. In addition, a tomographic long-range μPIV system is currently under development, with the aim of retrieving the full velocity gradient tensor. The experimental approach is to use a single lens and a diagphram in each camera, instead of a long-range microscope, which permits to respect the Scheimpflug condition when positioning the optics.