Flow Control By Dana Elam

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]

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]

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

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

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

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

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

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.