Daniele Fiscaletti's seminar at SISSA

[Emanuele Tuillier Illingworth]

SEMINAR ANNOUNCEMENT


Date: Wednesday, 9 September, 2015 - 14:00

Room: SISSA - Santorio A - room 133

Title: Long-range μPIV to resolve the fine scales of turbulence in a jet 
at high Reynolds number

Speaker: Daniele Fiscaletti (Delft University of Technology)

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.