Parallax correction for precise near-wall flow investigations using particle imaging

Abstract

For the basic understanding of turbulence generation in wall-bounded flows, precise measurements of the mean velocity profile and the mean velocity fluctuations very close to the wall are essential. Therefore, three techniques are established for high-resolution velocity profile measurements close to solid surfaces: (1) the nanoprobe sensor developed at Princeton University, which is a miniaturization of a classical hot-wire probe [Exp. Fluids 51, 1521 (2011)]; (2) the laser Doppler velocimetry (LDV) profile sensor, which allows measurement of the location of the particles inside the probe volume using a superposition of two fringe systems [Exp. Fluids 40, 473 (2006)]; and (3) the combination of particle image velocimetry and tracking techniques (PIV/PTV), which identify the location and velocity of submicrometer particles within the flow with digital imaging techniques [Exp. Fluids 52, 1641 (2006)]. The last technique is usually considered less accurate and precise than the other two. However, in addition to the measurement precision, the effect of the probe size, the position error, and errors due to vibrations of the model, test facility, or measurement equipment have to be considered. Taking these into account, the overall accuracy of the PTV technique can be superior, as all these effects can be compensated for. However, for very accurate PTV measurements close to walls, it is necessary to compensate the perspective error, which occurs for particles not located on the optical axis. In this paper, we outline a detailed analysis for this bias error and procedures for its compensation. To demonstrate the capability of the approach, we measured a turbulent boundary layer at Re(δ)=0.4×10(6) and applied the proposed methods.

Near-wall measurements of the bubble- and Lorentz-force-driven convection at gas-evolving electrodes

“…Further experiments with a higher optical magnification and a more powerful laser will considerably enhance the detectability of particles located in the light-absorbing bubble curtain and thus enable measurements closer to the electrode. In this context, simultaneous measurements with two cameras and different optical magnifications as shown by Cierpka et al (2013b) could be used to resolve the entire wall jet.…”

Combined experimental and numerical investigation of a transonic space launcher wake

“…7 showing a comparison of the velocity pro¦les of the incoming boundary layers for PIV and LES at x/D = −0.1. First, the nonuniform grid of LES allows for the resolution of all relevant scales down to the viscous sublayer, whereas the PIV resolution cannot be re¦ned in regions with strong gradients and the resolution in the near-wall region is limited by the particle image size [17,18]. Thus, PIV resolves the pro¦le only down to the logarithmic layer.…”

Investigation of a transonic separating/reattaching shear layer by means of PIV

“…The connection between cone and main body is smooth to avoid leading edge separation. The model was made of aluminium and the surface is polished to avoid diffuse reflections at the wall, which would bias the near wall PIV measurements 20,21 . A rear sting, 21.5 mm in diameter, in the base of the cylinder was used for mounting the model in the middle of the test section of the wind tunnel.…”