Effects of Surface-Roughness Geometry on Separation-Bubble Transition

Roberts, S. K.; Yaras, M. I.

doi:10.1115/1.2101852

Cited by 46 publications

(9 citation statements)

References 46 publications

(7 reference statements)

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“…30,31 Examples for the application of specifically adapted surfaces structures can also be found in nature. Sharks possess riblet structures on their skin which are assumed to damp the spanwise and the wall-normal velocity fluctuations and, thus, the momentum transport, reducing the turbulent drag.…”

Section: Transition Control (Velvet)mentioning

confidence: 99%

Owl Inspired Silent Flight

Bachmann¹,

Winzen²

2014

Handbook of Biomimetics and Bioinspiration

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Urbanization and the constant upgrade of airports and wind power plants have led to the need of innovative wing-designs and applications to reduce noise of aircraft and rotors. How efficiently noise can be reduced during flight is shown by owls. Owls evolved unique wing geometries, surface-and edge-features to reduce flight noise to a minimum. Wings of owls are huge in comparison to the body mass. This in combination with a specific profiling leads to a wing geometry that produces much lift even at low flight-speeds. A velvety surface texture reduces friction noise of feathers and influences the boundary layer of the wing to delay separation. Serrations at the leading edge and fringes at the trailing edge of each remex influence both lift control and noise production by reducing turbulent eddies. Further, fringes merge neighboring feathers at the spread wing to create a smooth surface. Finally, the dense and porous plumage which covers the whole body acts as an acoustic absorber that dampens noise at the point of production. The complete knowledge of the physical mechanisms underlying silent flight of owls could lead to a future wing design and to applications that reduce noise pollution for humans and nature.

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Section: Transition Control (Velvet)mentioning

confidence: 99%

Owl Inspired Silent Flight

Bachmann¹,

Winzen²

2014

Handbook of Biomimetics and Bioinspiration

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“…In the case of steady forcing, three main principles are applicable; namely, to place rather large elements on the surface which are two-dimensional in the spanwise direction, to put smaller three-dimensional elements on the surface, or to increase the overall roughness finish of the turbine blade [11]. The twodimensional elements normally induce separation of the flow upstream of the unforced separation point, which is highly unstable and will cause the flow to transition to turbulence [12], thus preventing separation.…”

Section: Introductionmentioning

confidence: 99%

“…This perturbation will not cause a transition to turbulence upstream of the separation point, but will cause the separation bubble to transition faster [13]. An overall increase in the roughness of the turbine blade will have much the same effect in terms of accelerating transition as the two former methods, depending on the average height of the roughness [11].…”

Section: Introductionmentioning

confidence: 99%

The Effect of Surface Roughness on Laminar Separated Boundary Layers

Simens

Gungor

2013

Volume 6C: Turbomachinery

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Roughness effects on a laminar separation bubble, formed on a flat plate boundary layer due to a strong adverse pressure gradient similar to those encountered on the suction side of typical low-pressure turbine blades, are studied by direct numerical simulation. The discrete roughness elements that have a uniform height in the spanwise direction and ones that have a height that is a function of the spanwise coordinate are modeled using the immersed boundary method. The location and the size of the roughness element are varied to study the effects on boundary development and turbulent transition, and it was found that the size of the separation bubble can be controlled by positioning the roughness element away from the separation bubble. Roughnesses that have a height that varies in a periodic manner in the spanwise direction have a big influence on the separation bubble. The separation point is moved downstream due to the accelerated flow in the openings in the roughness element, which also prevents the formation of the recirculation region after the roughness element. The reattachment point is moved upstream, while the height of the separation bubble is reduced. These numerical experiments indicate that laminar separation and turbulent transition, are mainly affected by the type, the height, and the location of the roughness element. Finally a comparison between the individual influence of wakes and roughness on the separation is made. It is found that the transition of the separated boundary layer with wakes occurs at almost the same streamwise location as that induced by the three-dimensional roughness element.

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“…The length of the transition region is given according to Roberts and Yaras [11] by the relation t 2 t t 0.55 2.2 1 0.63 0.14…”

Section: Algebraic Transition Modelmentioning

confidence: 99%