1. Flow Control
By Dana Elam

2. What is flow control?
“Any mechanism or process through which the boundary layer of a fluid flow is caused to behave differently than it normally would were the flow developing naturally along a smooth straight surface” [Flatt, 1961]

3. Turbulent Boundary layers:
Low-speed streaks
Ejections of low-speed fluid outward from the wall
Sweeps of high-speed fluid toward the fall
Vortical structures of several proposed forms
Strong internal shear layers in the wall zone
Near wall pockets, observed as areas clear of marked fluid
Backs: surfaces across which the streamwise velocity changes abruptly
Large scale motions in the outer layers (superlayers and deep valleys of free stream fluid) [Kline and Robinson, 1990]

4. What is the benefit? Skin Friction drag accounts for:
80% of total drag for ships
25-40% of total drag for commercial aircraft
Also has implication in other industries due to skin friction in pipe and channel flows
Increasing requirement with legislation, diminishing resources and oil now at $100 a barrel
Ships and aircraft pose completely different problems:
Aircraft high velocity flow, temperature implications re = 105
Ships much lower speeds

5. Passive v active control
Passive flow control is where no energy is expended - added advantage of reduced maintenance requirements
Active control is where energy is expended in controlling the flow - greater reductions in skin friction are possible

6. Passive control Riblets - stabilising of vortices
LEBU - break up of vortices
Compliant surfaces
Seen in nature, shark skin!

7. Active methods- Open loop
Suction:
Removal of flow through holes or slots
Heating/cooling of boundary layer
Cooling lowers viscosity of air, heating for water
Surface additives
polymers, bubbles, ultrasound

8. Active Methods-Closed loop
Lorentz Force
Electromagnetic field not viable in air, inefficient
Active blowing/sucking
-25% reduction possible [Choi,1994]
Wall deformation
- MEMs acutation and sensors

9. Issues Common issues for closed loop systems: Sensor placement
Computational requirement - linear system
Actuator density, size
Time delay
System survival / ease of implementation
Lack of research at higher Reynolds, effectiveness of control schemes may be different.