湍流边界层结构 Turbulent Boundary Layer Structure

Streak breakdown
Flow visualisation (1)
Streak breakdown
Flow visualisation (2)
Streak breakdown
Flow visualisation (3)
Streak breakdown
Flow visualisation (4)
Streak breakdown
Flow visualisation (5)
Streak breakdown
Flow visualisation (6)
Streak breakdown
DNS results
Jimenez and Pinelli (1999)
Near-wall structure
Introduction
Near-wall structure

There are many different turbulence events in turbulent boundary layers, such as Ejection Sweep Burst Inrush…
Introduction
Introduction
Turbulent boundary layers
Kline et al. (1967)
Introduction
Turbulent boundary layers
Kline et al. (1967)
Introduction
Near-wall structure
Robinson et al. (1989)
Hairpin structure (5)
Hutchins (2003)
Near-wall structure
Conceptual model
Stage 1: Deformation of spanwise vortices Stage 2: Development of hairpins Stage 3: Production of counter-rotating longitudinal vortices
Ejection event
Choi et al. (2003)
Large-scale turbulence
Hairpin structure (1)
Head and Bandyopadhyay (1981)
Large-scale turbulence
Hairpin structure (2)
Zhou et al. (1999)
Conclusions

Near-wall structures are vitally important for energy production in the turbulent boundary layers. There is also a growing evidence to suggest that the hairpin structures contribute much to the self-sustaining mechanisms of turbulence.
Near-wall structure
Characterisation of sweeps
Near-wall structure
Ejection event
The streak breakdown is preceded by sweep events on both flanks of lowspeed streaks, suggesting that the sweep events act as main agents for local disturbance. This leads to an ejection of low-speed fluids.

Introduction
Coherent structure
Brown & Roshko (1974)
Introduction
Coherent structure
Bradshaw et al. (1964)
Introduction
Near-wall structure



Recent results revealed the existence of coherent structure in turbulent boundary layers, where the sequence of events is quasi-periodic. It is believed that there is a self-sustaining mechanism for the turbulence structure. Energy production during the turbulence events accounts for 80% of the total energy production.
Turbulent Boundary Layer Structures
Kwing-So Choi University of Nottingham
Introduction
Coherent Structure



Turbulent shear flows look random and chaotic, but there are identifiable structures within. The coherent structure in turbulent shear flows is dynamically significant, contributing much to the turbulent energy production. It is, therefore, important to understand the coherent structure for flow control purpose.
Introduction
Near-wall structure
Soldati (2001)
Near-wall structure
Sweep events



Take place in the near-wall region (y+ < 15) Induced by the quasi-longitudinal vortices Splash high-momentum fluid onto the wall Increase local skin-friction Contribute to the Reynolds stress production Q4 events (+u and -v)
Large-scale turbulence
Hairpin structure (3)
Adrian et al. (2000)
Large-scale turbulence
Hairpin structure (4)
Robinson et al. (1988)
Large-scale turbuNear-wall structure

There are many different types of structure in turbulent boundary layer, including Low speed streaks Quasi-streamwise vortices Hairpin vortices Banana eddies…
Turbulent boundary layers
Kasagi and Hirata (1980)
Introduction
Turbulent boundary layers
Kline et al. (1967)
Introduction
Turbulent boundary layers
Kline et al. (1967)