Boundary layer control
Methods to control fluid flow boundary layers for reduced drag and improved lift.
Boundary layer control is a set of engineering techniques for managing how fluid flows behave in the thin region next to a surface. On fast vehicles, controlling this flow can reduce the size of the wake behind them, which cuts drag. In aircraft, separation of the boundary layer is especially unwanted in high-lift systems and jet engine intakes. To keep the boundary layer attached to the surface, its energy often needs to be boosted. This can be done by adding fresh air through slots, mixing in air from above, sucking the slow-moving layer away through a perforated surface, bleeding it off inside a high-pressure duct, or scooping it away with a diverter or internal bleed ducting. Another option is to inject high-velocity air to raise the boundary layer's energy above that of the free stream.
In nature, British zoologist Sir James Gray noted that dolphins appear to have a turbulent boundary layer, which reduces the chance of separation and minimizes drag. He also stated that no mechanisms for maintaining a laminar boundary layer to lower skin friction have been shown in dolphins—a puzzle known as Gray's Paradox. Birds have a leading-edge feather called the alula that delays wing stalling at low speeds, much like a leading-edge slat on an aircraft wing. Bats and insects have thin membrane wings with features that seem to create favorable roughening at the Reynolds numbers they fly at, helping them perform better than they otherwise would.
In sports, balls are often given rough surfaces to extend how far they can be hit or thrown. Roughening makes the boundary layer turbulent, which keeps it attached longer around the back of the ball before separating, leaving a smaller wake. Spin applied to a ball causes boundary layer separation to shift to one side, creating a side force that curves the ball's path. Golf balls have used this roughening since the 19th century, and the stitching on cricket and baseballs acts as a boundary layer control feature.
For a cylinder in a freestream flow, three methods can control the separation caused by an adverse pressure gradient. Rotating the cylinder can reduce or eliminate the boundary layer on the side moving with the flow, while the side moving against it shows only partial separation. Suction through a slit near the separation point can delay separation by removing slowed fluid particles.
- Notable aircraft with active control
- ShinMaywa US-1, ShinMaywa US-2, Boeing 787-9 Dreamliner
- Early research location
- Aerodynamische Versuchsanstalt in Göttingen
- Natural laminar flow airfoil examples
- P-51, B-24
- Sailplane application
- Natural laminar flow achieved by sailplane designers with great success
Lore & Background
British zoologist Sir James Gray stated that dolphins appeared 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. This became known as Gray's Paradox. The wings of birds have a leading edge feature called the Alula 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.
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. 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 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.
Reader's Guide
Boundary layer control is significant in aeronautical engineering for reducing parasitic drag and increasing usable angle of attack. 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. An example of an aircraft with active boundary layer control is the Japanese sea plane ShinMaywa US-1, which 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. This feature is also used in Boeing's 787-9 Dreamliner aircraft. NASA-sponsored activities include NLF on engine nacelles and HLFC on wing upper surfaces and tail horizontal and vertical surfaces. The legacy of boundary layer control extends to sports, where 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. BL control (roughening) was applied to golf balls in the 19th century, and the stitching on cricket balls and baseballs acts as a boundary layer control structure.
Did You Know?
- Gray's Paradox refers to British zoologist Sir James Gray's statement that dolphins appeared to have a turbulent boundary layer to reduce separation and minimize drag.
- The Alula on bird wings delays wing stalling at low speeds, similar to a leading edge slat on an aircraft wing.
- The ShinMaywa US-2 uses active boundary layer control to fly at 50 knots.
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