Assessment and Application of Wavelet-based Optical Flow Velocimetry (wOFV) to Wall-Bounded Turbulent Flows

TL;DR

This paper evaluates a wavelet-based optical flow velocimetry (wOFV) technique for high-resolution velocity field measurement in wall-bounded turbulence, demonstrating its advantages over traditional PIV methods especially near walls.

Contribution

The study introduces and assesses a wavelet-based optical flow method that improves accuracy and resolution of velocity measurements near walls in turbulent flows, outperforming PIV in key aspects.

Findings

wOFV modestly outperforms PIV in vector accuracy across lambda range

wOFV effectively resolves viscous sublayer and wall shear stress

wOFV captures small-scale turbulent motions more accurately near boundaries

Abstract

The performance of a wavelet-based optical flow velocimetry (wOFV) algorithm to extract high accuracy and high resolution velocity fields from particle images in wall-bounded turbulent flows is assessed. wOFV is first evaluated using synthetic particle images generated from a channel flow DNS of a turbulent boundary layer. The sensitivity of wOFV to the regularization parameter (lambda) is quantified and results are compared to PIV. Results on synthetic particle images indicated different sensitivity to under-regularization or over-regularization depending on which region of the boundary layer is analyzed. Synthetic data revealed that wOFV can modestly outperform PIV in vector accuracy across a broad lambda range. wOFV showed clear advantages over PIV in resolving the viscous sublayer and obtaining highly accurate estimates of the wall shear stress. wOFV was also applied to experimental data of a developing turbulent boundary layer. Overall, wOFV revealed good agreement with both PIV and PIV + PTV. However, wOFV was able to successfully resolve the wall shear stress and correctly normalize the boundary layer streamwise velocity to wall units where PIV and PIV + PTV showed larger deviations. Analysis of the turbulent velocity fluctuations revealed spurious results for PIV in close proximity to the wall, leading to significantly exaggerated and non-physical turbulence intensity. PIV + PTV showed a minor improvement in this aspect. wOFV did not exhibit this same effect, revealing that it is more accurate in capturing small-scale turbulent motion in the vicinity of boundaries. The enhanced vector resolution of wOFV enabled improved estimation of instantaneous derivative quantities and intricate flow structure both closer to the wall. These aspects show that, within a reasonable lambda range, wOFV can improve resolving the turbulent motion occurring in the vicinity of physical boundaries.