Cost Action FP1005 – Nancy 13th October 2011
Experimental investigations on
secondary structures
in a fully developed turbulent jet
and extension to fibre-laden case
Alessandro Capone, Alfredo Soldati
Department of Energetics, University of Udine,
Italy
Giampaolo Romano
Department of Mechanics and Aeronautics,
University of Rome La Sapienza, Italy
Main aim
Investigation on streamwise vorticity
developing in a round jet and its effect on
mixing processes
Why secondary structures?
• Turbulence develops in 3D giving rise to
so-called secondary structures
• Main feature is streamwise vorticity
secondary
structures
Courtesy Prof. A.Soldati
Courtesy Prof. GP Romano
Why secondary structures?
• Mixing and efficiency at entraining fluid is
decisive in many industrial applications
• Streamwise vorticity takes part to this
process
Experimental setup
• Water, fully developed turbulent round jet.
• Streamwise
(up to x/D=7)
and crosswise
acquisitions
• Reynolds
number:
3000-30000
Results: average fields
• Mean velocity field at x/D=1.5 and Reynolds
number 4800 and 19300
Instantaneous fields
• Time evolution
of secondary
structures at
x/D=4.5 and
Reynolds
number 4800
Coherency spectra
• Re=4800
• For x/D>2.5
peak vanishes
Spatial correlation functions
• Size decreases
• Asymptotic
behaviour for
Re>14500
Entrainment Rate
• At low x/D
(<1.5) vortex
rings effect on
entrainment is
relevant
Entrainment Rate
• At x/D=4.5 sheer
entrainment rate
increase
• Steady state
shifts towards
higher radii
Fibre-laden case
• Rod orientation contains information about
velocity gradients tensor
• Parsa et al.
Rotation and alignment
of rods in twodimensional chaotic flow
Phys. Fluids 23 (2011)
Synthetic plastic fibres (nylon)
Fibre-laden case
• Phase discrimination and orientation estimate
Preprocessing
Object
identification
Discriminate
objects
Fitting rod
to an ellipse
Fibre-laden case
• Experimental results from pipe flow: near wall
mean orientation
Conclusion and developments
• Time-resolved PIV allowed statistical and spectral
analysis
• Streamwise vorticity take control of entrainment
as x/D and Re increase
• The size of secondary vortical structures appears
to be strictly connected to entrainment
• Rod rotation calculations bring useful info about
vorticity
Questions
Thank you for your
attention!