Turbulent spots in channel flow: an experimental study: large-scale flow, inner structure and low-order model

Area of Science:

• Fluid Dynamics
• Turbulence Research

Background:

• Turbulent spots are key to understanding subcritical transition to turbulence in channel flows.
• The internal structure and surrounding flow dynamics of these spots require detailed investigation.

Purpose of the Study:

• To experimentally measure the internal structure of turbulent spots in channel flow.
• To characterize the large-scale flow surrounding turbulent spots.
• To compare experimental observations with predictions from a nonlinear reduced order model.

Main Methods:

• Utilizing Time Resolved Stereoscopic Particle Image Velocimetry (TR-SPIV) for high-resolution flow measurements.
• Analyzing the spatial and temporal evolution of turbulent fluctuations.
• Quantifying mean flow distortions and their relationship to turbulent structures.

Main Results:

• Observation of traveling-wave-like structures at the trailing edge of turbulent spots.
• Identification of an asymmetric quadrupole as the dominant large-scale flow structure centered on the spot.
• Detailed measurements of turbulent fluctuations and mean flow distortions.

Conclusions:

• Experimental data provides new insights into the internal dynamics of turbulent spots.
• The observed large-scale quadrupole flow is a significant feature of turbulent spot development.
• The results validate the predictive capabilities of nonlinear reduced order models for subcritical transition phenomena.