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Fully Developed Turbulent Flow
There are two distinct layers
Detail of the laminar sub layer
The sub layer is characterized by alternate growth and bursting of the boundary layer
The boundary layer grows until it becomes unstable (local Re becomes supercritical)
It breaks down with a burst of "turbulence"
Additional observations
- The bursts are frequent in space and time
- The bust is 3D
- It injects a large amount of relatively slow moving fluid into faster moving fluid (eddies)
- This amount of fluid is replaced by faster moving fluid from the turbulent region
- The elements moving into the turbulent region provides a source of turbulent kinetic energy
- Fluid viscosity attempts to reduce this production of eddies through transformation into thermal energy
- A new state of stability is established
Solving for Turbulence
Numerical
There are four established ways to solve for turbulence:
- Reynolds Averaged Navier-Stokes Equation (RANS). This is the most widely used technique in the industry. Can be applied on personal workstations. The Navier-Stokes equations are time averaged and solved using additional models for handling turbulence parameters
- Reynolds stress models. The additional time perturbation terms is solved directly through additional transport equations
- Large Eddy Simulation (LES). The large eddies are solved directly, while the small eddies are estimated using an extrapolation scheme. Requires significant increase in computational resources (several networked/parallel workstations) and still in final state of research.
- Direct Numerical Simulation (DNS). The turbulence problem is solved directly with no time averaging. Both large and small eddies are solved. Incredible amount of resources needed. Networked super computers. Leading edge research.
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