Boundary layer control refers to methods of controlling the behaviour of fluid flow

boundary layer In physics and fluid mechanics, a boundary layer is the thin layer of fluid in the immediate vicinity of a bounding surface formed by the fluid flowing along the surface. The fluid's interaction with the wall induces a no-slip boundary cond ...

s. It may be desirable to reduce flow separation on fast vehicles to reduce the size of the wake (streamlining), which may reduce drag. Boundary layer separation is generally undesirable in aircraft high lift coefficient systems and jet engine intakes. Laminar flow produces less skin friction than turbulent but a turbulent boundary layer transfers heat better. Turbulent boundary layers are more resistant to separation. The energy in a boundary layer may need to be increased to keep it attached to its surface. Fresh air can be introduced through slots or mixed in from above. The low momentum layer at the surface can be sucked away through a perforated surface or bled away when it is in a high pressure duct. It can be scooped off completely by a diverter or internal bleed ducting. Its energy can be increased above that of the free stream by introducing high velocity air.

Nature

Frank E. Fish states that dolphins appear to have a turbulent boundary layer to reduce the likelihood of separation and minimize drag, and that mechanisms for maintaining a laminar boundary layer to reduce skin friction have not been demonstrated for dolphins. The wings of birds have a leading edge feature called the

Alula The alula , or bastard wing, (plural ''alulae'') is a small projection on the anterior edge of the wing of modern birds and a few non-avian dinosaurs. The word is Latin and means "winglet"; it is the diminutive of ''ala'', meaning "wing". The al ...

which delays wing stalling at low speeds in a similar manner to the leading edge slat on an aircraft wing. Thin membrane wings found on bats and insects have features which appear to cause favourable roughening at the Reynolds numbers involved, thereby enabling these creatures to fly better than would otherwise be the case.

Sports

Balls may be given features which roughen the surface and extend the hit or throw distance. Roughening causes the boundary layer to become turbulent and remain attached farther round the back before breaking away with a smaller wake than would otherwise be the case. Balls may be struck in different ways to give them spin which makes them follow a curved path. The spin causes boundary layer separation to be biased to one side which produces a side force. BL control (roughening) was applied to golf balls in the 19th century. The stitching on cricket balls and baseballs acts as a boundary layer control structure.

On a cylinder

In the case of a freestream flow past a cylinder, three methods may be employed to control the

boundary layer separation In fluid dynamics, flow separation or boundary layer separation is the detachment of a boundary layer from a surface into a wake. A boundary layer exists whenever there is relative movement between a fluid and a solid surface with viscous f ...

that occurs due to the adverse pressure gradient. Rotation of the cylinder can reduce or eliminate the boundary layer that is formed on the side which is moving in the same direction as the freestream. The side moving against the flow also exhibits only partial separation of the boundary layer. Suction applied through a slit in the cylinder near a separation point can also delay the onset of separation by removing fluid particles that have been slowed in the boundary layer. Alternatively, fluid can be blown from a faired slit such that the slowed fluid is accelerated and thus the point of separation is delayed.

Maintaining a laminar boundary layer on aircraft

Laminar flow airfoils were developed in the 1930s by shaping to maintain a favourable pressure gradient to prevent them becoming turbulent. Their low-drag wind tunnel results led to them being used on aircraft such as the P-51 and B-24 but maintaining laminar flow required low levels of surface roughness and waviness not routinely found in service."Understanding Aerodynamics Arguing from the Real Physics"McLean Doug, John Wiley & Sons Ltd. Chichester, , p.339 Krag states that tests on the P-51 airfoil done in the high speed DVL wind tunnel in Berlin showed the laminar flow effect completely disappeared at real flight Reynolds numbers. Implementing laminar flow in high-Reynolds-number applications generally requires very smooth, wave-free surfaces, which can be difficult to produce and maintain. Maintaining laminar flow by controlling the pressure distribution on an airfoil is called Natural laminar flow (NLF) and has been achieved by sailplane designers with great success. On swept wings a favorable pressure gradient becomes destabilizing due to cross flow and suction is necessary to control cross flow. Supplementing the effect of airfoil shaping with

boundary layer suction Boundary layer suction is a boundary layer control technique in which an air pump is used to extract the boundary layer at the wing or the inlet of an aircraft. Improving the air flow can reduce drag. Improvements in fuel efficiency have been est ...

is known as laminar flow control (LFC) The particular control method required for laminar control depends on Reynolds-number and wing leading edge sweep. Hybrid laminar flow control (HLFC) refers to swept wing technology in which LFC is applied only to the leading edge region of a swept wing and NLF aft of that. NASA-sponsored activities include NLF on engine nacelles and HLFC on wing upper surfaces and tail horizontal and vertical surfaces.

Aircraft design

In aeronautical engineering, boundary layer control may be used to reduce

parasitic drag Parasitic drag, also known as profile drag, is a type of aerodynamic drag that acts on any object when the object is moving through a fluid. Parasitic drag is a combination of form drag and skin friction drag. It affects all objects regardless of ...

and increase usable angle of attack. Fuselage-mounted engine intakes are sometimes equipped with a splitter plate. Much research was conducted to study the lift performance enhancement due to suction for aerofoils in the 1920s and 1930s at the Aerodynamische Versuchsanstalt in

Göttingen Göttingen (, , ; nds, Chöttingen) is a university city in Lower Saxony, central Germany, the capital of the eponymous district. The River Leine runs through it. At the end of 2019, the population was 118,911. General information The ori ...

. An example of an aircraft with active boundary layer control is the Japanese

sea plane A seaplane is a powered fixed-wing aircraft capable of taking off and landing (alighting) on water.Gunston, "The Cambridge Aerospace Dictionary", 2009. Seaplanes are usually divided into two categories based on their technological characteris ...

ShinMaywa US-1. This large, four-engined aircraft was used for

anti-submarine warfare Anti-submarine warfare (ASW, or in older form A/S) is a branch of underwater warfare that uses surface warships, aircraft, submarines, or other platforms, to find, track, and deter, damage, or destroy enemy submarines. Such operations are t ...

(ASW) and search and rescue (SAR). It was capable of STOL operation and very low air speeds. Its replacement in the SAR role, the ShinMaywa US-2, uses a similar system for its capability to fly at 50 knots.Explanation and data on the website of ShinMaywa, retrieved Dec. 12, 2020
/ref> This feature is also used in Boeing's 787-9 Dreamliner aircraft.

References