Sharks, Dolphins and Butterflies: Micro-Sized Surfaces Have Macro Effects

2017 ◽  
Author(s):  
Amy Lang ◽  
Farhana Afroz ◽  
Philip Motta ◽  
Jacob Wilroy ◽  
Redha Wahidi ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Leading Edge ◽  
Separation Control ◽  
Friction Drag ◽  
Local Skin ◽  
Significant Control ◽  
Shortfin Mako ◽  
Microscopic Features ◽  
Transverse Grooves

Sharks, dolphins and butterflies swim and fly in different flow regimes, yet the structure of their surfaces interacting with the surrounding fluid all appear to contain very important microscopic features that lead to reduced drag and increased flying or swimming efficiency. Sharks have moveable scales (approximately 200 microns in size) that act as a passive, flow-actuated dynamic roughness for separation control. Water tunnel experiments with real shortfin mako shark skin samples mounted to models have shown significant control of flow separation in both laminar and turbulent boundary layer scenarios. Dolphins have sinusoidal-shaped millimeter-sized transverse grooves covering a large percentage of their body. Experiments show that similar geometries embedded in a turbulent boundary layer can lead to separation control at the slight expense of increased friction drag. Alternatively, butterfly scales (100 microns in size covering the wings in a roof shingle pattern) appear to fundamentally alter the local skin friction drag depending on flow orientation for what is dominantly a laminar boundary layer interacting with the wings. However, in this case the surface may also slow the growth and formation of the leading-edge vortex and these effects shown in experiments may help explain a mean decrease in climbing efficiency (joules per flap) of 37.8% for live butterflies once their scales were removed. An overview of these results is discussed for these three cases, bringing out the importance of finding solutions in nature for essential engineering problems.

Passive Separation Control of Shortfin Mako Shark Skin in a Turbulent Boundary Layer

2021 ◽  
pp. 110433
Author(s):  
Leonardo M. Santos ◽  
Amy Lang ◽  
Redha Wahidi ◽  
Andrew Bonacci ◽  
Sashank Gautam ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Separation Control ◽  
Shark Skin ◽  
Mako Shark ◽  
Shortfin Mako ◽  
Passive Separation

Active control of a turbulent boundary layer based on local surface perturbation

2014 ◽  
Vol 750 ◽  
pp. 316-354 ◽  
Author(s):  
H. L. Bai ◽  
Y. Zhou ◽  
W. G. Zhang ◽  
S. J. Xu ◽  
Y. Wang ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Drag Reduction ◽  
Skin Friction ◽  
Active Control ◽  
Valuable Insight ◽  
Friction Drag ◽  
Local Skin ◽  
Insight Into ◽  
Near Wall

AbstractActive control of a turbulent boundary layer has been experimentally investigated with a view to reducing the skin-friction drag and gaining some insight into the mechanism that leads to drag reduction. A spanwise-aligned array of piezo-ceramic actuators was employed to generate a transverse travelling wave along the wall surface, with a specified phase shift between adjacent actuators. Local skin-friction drag exhibits a strong dependence on control parameters, including the wavelength, amplitude and frequency of the oscillation. A maximum drag reduction of 50 % has been achieved at 17 wall units downstream of the actuators. The near-wall flow structure under control, measured using smoke–wire flow visualization, hot-wire and particle image velocimetry techniques, is compared with that without control. The data have been carefully analysed using techniques such as streak detection, power spectra and conditional averaging based on the variable-interval time-average detection. All the results point to a pronounced change in the organization of the perturbed boundary layer. It is proposed that the actuation-induced wave generates a layer of highly regularized streamwise vortices, which acts as a barrier between the large-scale coherent structures and the wall, thus interfering with the turbulence production cycle and contributing partially to the drag reduction. Associated with the generation of regularized vortices is a significant increase, in the near-wall region, of the mean energy dissipation rate, as inferred from a substantial decrease in the Taylor microscale. This increase also contributes to the drag reduction. The scaling of the drag reduction is also examined empirically, providing valuable insight into the active control of drag reduction.

scholarly journals Global effect of local skin friction drag reduction in spatially developing turbulent boundary layer

2016 ◽  
Vol 805 ◽  
pp. 303-321 ◽  
Author(s):  
A. Stroh ◽  
Y. Hasegawa ◽  
P. Schlatter ◽  
B. Frohnapfel
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Drag Reduction ◽  
Control Region ◽  
Reduction Rate ◽  
Wall Turbulence ◽  
Local Skin ◽  
Control Schemes ◽  
Local Skin Friction ◽  
Friction Drag Reduction

A numerical investigation of two locally applied drag-reducing control schemes is carried out in the configuration of a spatially developing turbulent boundary layer (TBL). One control is designed to damp near-wall turbulence and the other induces constant mass flux in the wall-normal direction. Both control schemes yield similar local drag reduction rates within the control region. However, the flow development downstream of the control significantly differs: persistent drag reduction is found for the uniform blowing case, whereas drag increase is found for the turbulence damping case. In order to account for this difference, the formulation of a global drag reduction rate is suggested. It represents the reduction of the streamwise force exerted by the fluid on a plate of finite length. Furthermore, it is shown that the far-downstream development of the TBL after the control region can be described by a single quantity, namely a streamwise shift of the uncontrolled boundary layer, i.e. a changed virtual origin. Based on this result, a simple model is developed that allows the local drag reduction rate to be related to the global one without the need to conduct expensive simulations or measurements far downstream of the control region.

Experience of the Using of Cascade Method for Turbulent Boundary-Layer Control Through Air Blowing

2014 ◽  
Vol 9 (1) ◽  
pp. 49-61
Author(s):  
Vladimir Kornilov ◽  
Ivan Kavun ◽  
Anatoliy Popkov
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Wind Tunnel Test ◽  
Barometric Pressure ◽  
Local Skin ◽  
Air Blowing ◽  
Ground Conditions ◽  
Local Skin Friction ◽  
The Difference ◽  
Cascade Method

The possibilities of turbulent drag reduction in an incompressible turbulent boundary layer of a flat plate with air blowing through a microperforated surface which consists of sequentially arranged one behind the other self-contained permeable areas were studied. Mass flow rate of air blowing per unit area Q was increased with increasing distance downstream, but in total was not more than 0.0768 kg/s/m2 . A consistent reduction of the local skin friction values along the length of the model, up to 70% at the end of the last active blowing area was shown. The experimental data characterizing the ability to manage a turbulent boundary layer in the ground conditions by passive air overflow generated by the difference between the barometric pressure and the pressure in the wind tunnel test section were obtained

scholarly journals Evaluation of Skin Friction Drag Reduction in the Turbulent Boundary Layer Using Riblets

2019 ◽  
Vol 9 (23) ◽  
pp. 5199
Author(s):  
Hidemi Takahashi ◽  
Hidetoshi Iijima ◽  
Mitsuru Kurita ◽  
Seigo Koga
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Skin Friction ◽  
Three Dimensional ◽  
Hot Wire ◽  
Friction Drag ◽  
Freestream Velocity ◽  
Turbulent Statistics ◽  
Friction Drag Reduction ◽  
Riblet Surface

A unique approach to evaluate the reduction of skin friction drag by riblets was applied to boundary layer profiles measured in wind tunnel experiments. The proposed approach emphasized the turbulent scales based on hot-wire anemometry data obtained at a sampling frequency of 20 kHz in the turbulent boundary layer to evaluate the skin friction drag reduction. Three-dimensional riblet surfaces were fabricated using aviation paint and were applied to a flat-plate model surface. The turbulent statistics, such as the turbulent scales and intensities, in the boundary layer were identified based on the freestream velocity data obtained from the hot-wire anemometry. Those turbulent statistics obtained for the riblet surface were compared to those obtained for a smooth flat plate without riblets. Results indicated that the riblet surface increased the integral scales and decreased the turbulence intensity, which indicated that the turbulent structure became favorable for reducing skin friction drag. The proposed method showed that the current three-dimensional riblet surface reduced skin friction drag by about 2.8% at a chord length of 67% downstream of the model’s leading edge and at a freestream velocity of 41.7 m/s (Mach 0.12). This result is consistent with that obtained by the momentum integration method based on the pitot-rake measurement, which provided a reference dataset of the boundary layer profile.

The turbulent boundary layer over transverse square cavities

1999 ◽  
Vol 395 ◽  
pp. 271-294 ◽  
Author(s):  
L. DJENIDI ◽  
R. ELAVARASAN ◽  
R. A. ANTONIA
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Flow Direction ◽  
Reynolds Shear Stress ◽  
Local Maximum ◽  
Turbulent Energy ◽  
Skin Friction Coefficient ◽  
Reynolds Stresses ◽  
Smooth Wall ◽  
Local Skin

Laser-induced uorescence (LIF) and laser Doppler velocimetry (LDV) are used to explore the structure of a turbulent boundary layer over a wall made up of two-dimensional square cavities placed transversely to the flow direction. There is strong evidence of occurrence of outflows of fluid from the cavities as well as inflows into the cavities. These events occur in a pseudo-random manner and are closely associated with the passage of near-wall quasi-streamwise vortices. These vortices and the associated low-speed streaks are similar to those found in a turbulent boundary layer over a smooth wall. It is conjectured that outflows play an important role in maintaining the level of turbulent energy in the layer and enhancing the approach towards self-preservation. Relative to a smooth wall layer, there is a discernible increase in the magnitudes of all the Reynolds stresses and a smaller streamwise variation of the local skin friction coefficient. A local maximum in the Reynolds shear stress is observed in the shear layers over the cavities.

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