Abstracts - KTH Mechanics

Assessment of a wavelet based coherent structure eduction
technique in turbulent wall flows.
R. Camussi a, W. Da Riz a
The aim of the work is to present an assessment of a wavelet based method for
identifying coherent structures (CS) from 2D PIV data in turbulent wall flows.
The database used is provided by Adrian and co-workers
(http://ltcf.tam.uiuc.edu/) and previously used in Adrian et al. 1 for high and low
Reynolds number flows (Re = 1015/7705, = 8.84/10.35 mm).
The algorithm is based on wavelet analysis of 2D PIV velocity field (Camussi 2) and
validated against a method based on the swirling strength (Zhou et al. 3). An algorithm
for the computation of the velocity gradient tensor at each point of the 2D PIV
velocity field has been implemented: the CS has been identified as to be the area
where the velocity gradient tensor has a pair of conjugate complex eigenvalues with
squared imaginary part ci 2 overcoming a fixed threshold equals to std(ci 2). The
algorithm for CS identification via wavelet analysis is based on the LIM indicator
(Farge 4, Camussi and Guj 5); here only the velocity component normal to the wall has
been used. The CS has been identified as the point where the maximum value of LIM
over all wavelet scales shows a relative peak. Statistics of CS identified by means of the
two methods revealed no substantial differences for negative-vorticity structure, and a
relationship between wavelet wavelength and vortex size has been carried out (Fig. 1a).
For positive-vorticity structures the analysis of results shown striking differences
arising from the fact that the LIM based algorithm tends to identify negative-vorticity
vortex packets (Fig. 1-b circled), which form a zone of local positive vorticity (Fig. 1-b
red) along with positive-vorticity structures. Similarities and differences between high
and low Re flows are also shown.
a Mechanical and Industrial Engineering Department, University “ROMA TRE”, Rome, Italy.
1 Adrian, R. J.,Meinhart C. D., Tomkins C., J. Fluid Mech. 422, 1–53 (2000).
2 Camussi R., Exps. Fluids, 32, 76-86 (2002).
3 Zhou, J., Adrian, R.J., Balachandar S., Kendall T.M., J. Fluid Mech. 387, 353-396 (1999).
4 Farge M., Annual Review of Fluid Mechanics, 24, 395-457 (1992).
5 Camussi R., Guj G., J. Fluid Mech. 348, 177-199 (1997).
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Fig. 1: (a) r0 vs. wavelength. (b) Mean velocity field vorticity (wavelength = 26.6 mm)
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