On the uncertainty of digital PIV and PTV near walls

Abstract

The reliable measurement of mean flow properties near walls and interfaces between different fluids or fluid and gas phases is a very important task, as well as a challenging problem, in many fields of science and technology. Due to the decreasing concentration of tracer particles and the strong flow gradients, these velocity measurements are usually biased. To investigate the reason and the effect of the bias errors systematically, a detailed theoretical analysis was performed using window-correlation, singe-pixel ensemble-correlation and particle tracking evaluation methods. The different findings were validated experimentally for microscopic, long-range microscopic and large field imaging conditions. It is shown that for constant flow gradients and homogeneous particle image density, the bias errors are usually averaged out. This legitimates the use of these techniques far away from walls or interfaces. However, for inhomogeneous seeding and/or nonconstant flow gradients, only PTV image analysis techniques give reliable results. This implies that for wall distances below half an interrogation window dimension, the singe-pixel ensemble-correlation or PTV evaluation should always be applied. For distances smaller than the particle image diameter, only PTV yields reliable results.

Main results of the 4th International PIV Challenge

“…Unfortunately, the definition of the boundary location is associated with large uncertain- ties, as already discussed, and therefore, this modelbased approach is associated with large uncertainties. Therefore, it would be more reliable to make use of evaluation techniques which enhance the spatial resolution without making any model assumption such as single-pixel PIV or PTV evaluation techniques (Kähler et al 2012b). However, as the uncertainty of these evaluation techniques is not always better than spatial correlation approaches, it is recommended to evaluate the measurements in a zonal-like fashion as done in numerical flow simulations, where RANS simulations are coupled with LES or even DNS simulations to resolve the flow unsteadiness at relevant locations.…”

“…While synthetic images are often used for this purpose (Kähler et al 2012b), images of real particles are physically more accurate if the particle displacement field is precisely predetermined. For Case F images are obtained by recording the particles in a liquid column rotating at a steady uniform angular velocity.…”

Load-estimation techniques for unsteady incompressible flows

“…In some cases, the limitations with regard to spatial resolution in terms of discretization in regions of high shear or near walls poses a significant challenge, as described by Westerweel (2008) and Kähler et al (2012b), respectively. The following describes a prospective outlook towards the implementation of Lagrangian-based measurement techniques and analyses going forward.…”

Imaging Flow Velocimetry with Laser Mie Scattering

“…Boundary layer measurements close to walls with DGV and PTV are reported in [139,180,181], respectively. Under optimal conditions, PTV measurements allow flow velocity measurements with a minimal distance that is the radius of the scattering particle.…”