Flow Control By Dana Elam What is flow

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.