Daniele Fiscaletti's seminar at SISSA
Emanuele Tuillier Illingworth
tuillier at sissa.it
Tue Sep 8 13:16:42 CEST 2015
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.
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