Boundary Layer Flow 15 - Boundary Layer Flow Transition Flashcards
Laminar-turbulent transition
As Re increases (i.e. further from start of boundary layer), ratio between the inertial and viscous forces increases, so small perturbations become unstable, grow into vortices and eventually turbulence
Transition to turbulence -> higher rate of momentum transfer -> boundary layer thicker -> flow losses (e.g. in a gas turbine or aircraft wing)
Define stability
Immunity to (small) disturbances (Criminale 1967)
Factors affecting transition
Free stream disturbances (kinematic, acoustic, thermal) can cause earlier transition
Roughness will generally cause earlier transition
Adverse pressure gradient causes earlier transition
Mach number
Wall cooling/heating
Suction/blowing
Morkovin’s transition roadmap (1984)
Natural transition - Tu < 1% via Tollmien-Schlichting (TS) waves
Bypass transition - Tu > 1%, TS waves ‘bypassed’ (e.g. turbo machinery)
What is Tu?
Free stream turbulence intensity
Natural transition process over a smooth surface
Starts with unstable 2D waves, TS waves, convected downstream slower than outer velocity
Their amplitude is amplified exponentially downstream, and develops 3D waves and eddies
Vortex breakdowns at regions of high shear
Formation of turbulence spots (i.e. localised turbulent regions)
Coalescence of spots leads to an ‘explosion’ to turbulence
Bypass transition process
Development of streaks (span wise modulation of BL), bypassing first three stages of natural transition
Streak breakdown, incipient spots
Formation, interaction and merging of turbulent spots
Consequences of BL transition
More efficient transfer of momentum/energy/heat in BL to/from wall region
Sudden increase in BL thickness
More uniform velocity profile in the BL
More resilient to the adverse pressure gradients
Increased friction drag
General properties of turbulence
Irregular, diffusive, dissipative
Rotational and 3D
Comprising vortex tubes (eddies) of different sizes or scales superimposed on each other in a highly complex manner
Range of eddies depends on Re of mean flow
Points about turbulent flows being irregular
Velocity taken at fixed point in space
Irregular fluctuation both with time and space, but not statistically random
Points about turbulent flows being diffusive
Fluctuation leads to enhanced diffusivity which results in much greater mixing of properties
Rate of mass/heat transfer can be several orders of magnitudes larger than that in laminar flow
Points about turbulent flows being dissipative
Within vortices, viscous shear stresses perform work which converts kinetic energy of turbulence into internal energy of the fluid
Rate of energy dissipation is much greater than corresponding viscous dissipation in laminar flow
If there is no continuous external energy source for the continuous generation of turbulent motions, the turbulent motion will decay
In most cases, turbulence comes from high strain rate region (e.g. wall BL)
Turbulence control
Delay/promote transition
Reduce friction drag
Vanish turbulence
Suppressing turbulence in a pipe
Vanish turbulence by momentarily making flow even more turbulent
