Boundary Layer Flow 16 - Turbulent Boundary Layer Flashcards
Differences in laminar and turbulent mean velocity profiles
Through a pipe - laminar is parabolic curve, turbulent is shallower curve at end of profile
Over a plate - laminar is steeper, turbulent is lower and more curved
Why use mean quantities?
Instantaneous fluctuations vary ‘randomly’ in space and time
Reynolds decomposition
Mean + fluctuations
Streamwise mean momentum balance
Substitute Reynolds decomposition into NS and take mean
Total shear stress
Viscous stress + Reynolds stress
Turbulent BL velocity profile
Viscous wall layer - viscous shear dominates
Overlap layer - viscous and turbulent shear forces equally important
Outer layer - fully turbulent region, turbulent shear dominates, no direct effect of viscosity on mean flow
Important parameters close to wall
Viscosity
Wall shear stress
Density
Local Reynolds number
Measure of the relative importance of viscous and turbulent processes
Viscous wall layer
Local Reynolds number < 50
Velocity must be dependent on vertical distance, wall shear stress, density and viscosity
Viscous sublayer
Local Reynolds number < 5
Very close to wall and very thin
Assume velocity changes nearly linearly with distance from the wall, so velocity gradient is constant
Outer (velocity defect) layer
Local Reynolds number > 50
Velocity defect is independent of kinematic viscosity but depends on distance from wall relative to thickness, shear stress at interface with viscous sublayer and fluid density
Define velocity defect
Local reduction in velocity from free stream value
Overlap layer
30 < local Reynolds number < 0.1 x outer region Reynolds number
Log law
Approximates nearly entire velocity profile
Law of the wall
Used for viscous sublayer
