Forcing large-scale coherent streaks in a zero pressure gradient turbulent boundary layer

TL;DR

This study demonstrates the artificial forcing of large-scale coherent streaks in a turbulent boundary layer, revealing their growth, scale, and agreement with transient growth theory, advancing understanding of turbulence structures.

Contribution

The paper experimentally shows large-scale streaks in turbulent boundary layers and confirms their behavior aligns with recent transient growth predictions.

Findings

Maximum streak amplitude is proportional to spanwise wavelength.

Most amplified streaks have a very large scale of approximately 6 times boundary layer thickness.

Results agree with optimal transient growth analysis predictions.

Abstract

Large scale coherent streaks are artificially forced in a well developed turbulent boundary layer at $\Redstar \approx 1000$ using an array of cylindrical roughness elements. Measures of the velocity field with particle image velocimetry reveal the presence of well reproducible, streamwise oriented, steady coherent streaks. We show that the amplitude of these coherent streaks transiently grows in space. The position of the maximum amplitude is proportional to the spanwise wavelength of the streaks and the most amplified spanwise wavelength is of very large scale $\lz \approx 6 \dblo$. These results are in good agreement with the recent predictions based on the optimal transient growth analysis of turbulent mean flows.