105 research outputs found
Preferential concentration of inertial sub-kolmogorov particles. The roles of mass loading of particles, Stokes and Reynolds numbers
Turbulent flows laden with inertial particles present multiple open questions
and are a subject of great interest in current research. Due to their higher
density compared to the carrier fluid, inertial particles tend to form high
concentration regions, i.e. clusters, and low concentration regions, i.e.
voids, due to the interaction with the turbulence. In this work, we present an
experimental investigation of the clustering phenomenon of heavy sub-Kolmogorov
particles in homogeneous isotropic turbulent flows. Three control parameters
have been varied over significant ranges: ,
and volume fraction . The scaling of clustering characteristics, such as the distribution
of Vorono\"i areas and the dimensions of cluster and void regions, with the
three parameters are discussed. In particular, for the polydispersed size
distributions considered here, clustering is found to be enhanced strongly
(quasi-linearly) by and noticeably (with a square-root
dependency) with , while the cluster and void sizes, scaled with the
Kolmogorov lengthscale , are driven primarily by . Cluster
length scales up to , measured
at the highest , while void length
scaled also with is typically two times larger ().
The lack of sensitivity of the above characteristics to the Stokes number lends
support to the "sweep-stick" particle accumulation scenario. The non-negligible
influence of the volume fraction, however, is not considered by that model and
can be connected with collective effects
An experimental study on the settling velocity of inertial particles in different homogeneous isotropic turbulent flows
We propose an experimental study on the gravitational settling velocity of
dense, sub-Kolmogorov inertial particles under different background turbulent
flows. We report Phase Doppler Particle Analyzer measurements in a low-speed
wind tunnel uniformly seeded with micrometer scale water droplets. Turbulence
is generated with three different grids (two consisting on different
active-grid protocols while the third is a regular static grid), allowing us to
cover a very wide range of turbulence conditions in terms of Taylor-scale based
Reynolds numbers (), Rouse numbers ()
and volume fractions (). We
find, in agreement with previous works, that enhancement of the settling
velocity occurs at low Rouse number, while hindering of the settling occurs at
higher Rouse number for decreasing turbulence energy levels. The wide range of
flow parameters explored allowed us to observe that enhancement decreases
significantly with the Taylor Reynolds number and is significantly affected by
the volume fraction . We also studied the effect of large-scale forcing
on settling velocity modification. The possibility of changing the inflow
conditions by using different grids allowed us to test cases with fixed
and turbulent intensity but different integral length scale.
Finally, we assess the existence of secondary flows in the wind tunnel and
their role on particle settling. This is achieved by characterising the
settling velocity at two different positions, the centreline and close to the
wall, with the same streamwise coordinate.Comment: 21 pages, 11 figures, submitted to the Journal of Fluid Mechanic
Momentum Injection via Dielectric Barrier Discharge Actuators in Low-Speed External Flow
Dielectric barrier discharge (DBD) plasma actuators can generate a wall jet
without moving parts through interaction between ionized and neutral molecules
in an electric field. The coupling between electro-hydrodynamic, turbulence,
and viscous effects in the flow boundary layer remains unclear and deserves
careful investigation. We present an experimental investigation of momentum
injection by DBD actuators in a U_external = 5 m/s and U_external = 11 m/s
co-flow and counter-flow configuration over a range of VAC = 12 kV - 19.5 kV
peak-to-peak at a frequency of 2 kHz. In the co-flow configuration, the DBD
actuator adds momentum to the boundary layer, similar to an electrohydrodynamic
(EHD) jet in quiescent conditions. In the counter-flow configuration, flow
separation is observed at free stream velocity U_external = 5 m/s. The momentum
displacement in the counter-flow configuration is ~ 6x greater than EHD jet
momentum in a quiescent environment. Both co-flow and counter-flow momentum
injections show diminishing effects with increased external flow speed. This
work highlights that the resulting flow pattern is not a simple superposition
of the EHD jet and the free stream but is determined by a balance between the
inertial, viscous and Coulombic forces of the EHD and the external flow. The
velocity profiles and momentum characteristics can be used to validate
numerical models and inform the design of DBD actuators for active flow
control.Comment: 14 Pages, 13 figure
Dynamics of large turbulent structures in a steady breaker
The flow near the leading edge of a steady breaker has been studied experimentally using Bubble Image Velocimetry (BIV) with the aim of characterizing the dynamics of the large eddies responsible for air entrainment. It is well reported in the literature, and confirmed by our measurements of the instantaneous velocity field, that this flow shares some important features with the turbulent shear-layer formed between two parallel semi-infinite streams with different velocities. Namely, the formation of a periodic array of coherent vortices, the constant convective velocity of those vortices, the linear relation between their size and their downstream position and the self-similar structure of both mean velocity profiles and Reynolds shear stresses. Nonetheless, important differences exists between the dynamics of the large eddies in a steady breaker and those in a free shear-layer. Particularly, the convective velocity of these large structures is slower in a steady breaker and, consistent with this, their growth rates are larger. A physical interpretation of these differences is provided together with a discussion of their implications. To support our measurements and conclusions, we present a careful analysis of the accuracy of the BIV technique in turbulent flows with large bubblesThe authors wish to thank Professor Emil J. Hopfinger for his valuable suggestions on the interpretation of the experimental data. This work was supported by the ONR through Grant N00014-05-1-0121 and by the Spanish Ministry of Science (MICINN) through Grant DPI2008-06369Publicad
Measurement of Liquid Core Length of a Coaxial Two-fluid Spray
Shadowgraphs, tube-source X-ray radiographs, and synchrotron X-ray radiographs from a coaxial two-fluid spray are analyzed to measure the liquid core length of the spray. Two flow conditions: Rel = 1,100, Reg = 21,300, We = 40, and Rel = 1,100, Reg = 46,700, We = 196 are investigated. The standard deviation of the fluctuating intensity values are calculated and analyzed to estimate the liquid core length. Additionally, the largest connected domain is used to find an instantaneous breakup position of the spray. The results show that the high standard deviation region is related to the ligament development region, and the instantaneous position identifies ligament formation in the spray
Statistical analysis of focused beam radiographs taken from a coaxial airblast spray
Studying the near-field region of sprays is particularly challenging because it is optically dense. However, energy in the X-ray range is capable of penetrating this dense region and obtaining information that would otherwise be unavailable. Through time-resolved X-ray radiography, a better understanding of the near-field region is currently being developed. The 7-BM beamline at the Advanced Photon Source at Argonne National Lab was focused down to a 5 x 6 μm cross-sectional area. The attenuation in the beam, which is used to calculate the effective path length of liquid, was then collected at an effective rate of 270 kHz for 10 seconds. Various statistical measures were applied to the X-ray focused beam radiographs including average, standard deviation, skewness, and kurtosis, to quantify the spray from a canonical coaxial airblast nozzle. Results show that the average effective path length is useful in determining the intact length and spray angle. The capabilities of additional statistical measures in determining important spray characteristics are also discussed
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