Abstract
We have developed and validated a new adaptive method, particle tracking velocimetry and accelerometry (PTVA), to measure velocity and acceleration from the post-processing of particle tracking (PT) data. This method is shown to be more accurate than non-adaptive methods based on PT: errors are about six times smaller on velocity measurements and about four times smaller on acceleration ones. We apply this method to a turbulent-like flow generated and controlled in the laboratory. Taking advantage of the Eulerian repeatability of our multi-scale laminar flow, we are able to extract the acceleration field, a, and all terms of Navier–Stokes equation. To complete this we extract u·a and ∇·a fields. We finally compare the probability density function of the acceleration components of our turbulent-like flow with one of the highly turbulent flows and show that they are similar. The quality of these PTVA results and their robustness (in particular to local convection) are extremely encouraging. This method allows access to a deeper insight into the physic of turbulent-like flows and its high accuracy may apply to a broader range of flows.
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Notes
The PTVA method is based on a post-processing algorithm using existing tracked particle positions. The tracking of the particles is performed separately.
If ϕsmall is unknown, as a first iteration, the initial ϕsmall could be estimated via the resolution of the measurement (e.g. ±1 px) and the diameter of the particles so as to determine Δtarget (taking \(\Updelta_{min} = \frac{\phi_{\rm small}}{2}\) and Δmax = 2Δtarget). The measurements and post-processing can then be adjusted according to this first iteration.
Starting from an initial value of N t (keeping the value of N t used in the previous time step for a same trajectory), if Δ s < Δtarget then N t is increased (else N t is decreased), until |Δ s − Δtarget| ≤ ε, with ε small compared to Δtarget.
The best accuracy is obtained with 21 positions (orders 2 and 4) for the velocity whilst it is obtained with 233 positions (order 4) and 75 positions (order 2) for the acceleration. 9, 21, 47, 75, 127, 181, 233, 287 and 339 positions have been tested for the non-adaptive methods. The discrepancy between the best number of positions for the measures of velocity and acceleration could be noted. The average number of positions used by the PTVA algorithm is 123. The errors obtained for the non-adaptive methods using this number of positions are 19.6 (order 2) and 7.3 (order 4) for the velocity and 23.7 (order 2) and 32.3 (order 4) for the acceleration.
The best accuracy is obtained with 153 positions for the velocity and 199 positions for acceleration using order 4, whilst it is obtained with 35 positions for velocity and with 153 positions for acceleration using order 2. 17, 35, 63, 93, 153, 199, 243, 289 and 335 positions have been tested for the non-adaptive methods. The average number of positions used by the PTVA algorithm is 115. The errors obtained for the non-adaptive methods using this number of positions are 61.3 (order 2) and 24.2 (order 4) for the velocity and 93.1 (order 2) and 53.2 (order 4) for the acceleration.
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Acknowledgments
We acknowledge the Marie Curie Multi-Partner European Training Site on Environmental Turbulence, the Royal Society, The Leverhulme Trust, the EPSRC, S. Dalziel and Digiflow’s group, G. Querzoli, and J.C. Vassilicos.
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Ferrari, S., Rossi, L. Particle tracking velocimetry and accelerometry (PTVA) measurements applied to quasi-two-dimensional multi-scale flows. Exp Fluids 44, 873–886 (2008). https://doi.org/10.1007/s00348-007-0443-7
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DOI: https://doi.org/10.1007/s00348-007-0443-7