A Whale of an Idea in Wing Design
Figure 1: Tubercule wing variants for the NASA CRM configuration.
1875 words / 9 minutes read
Hit a Roadblock
Engineers for over 100 years have been in the lookout for improving the performance of the lifting device –blades used in aircraft, wind turbines, compressors, axial pumps, fans, etc. Modifications in airfoil contours, add-on devices like flaps, slats, vortex-generators, fences, winglets, shrouds, etc., have improved the blade performances in different conditions. And yet, there hasn’t been one single device which can deliver all the goodies without asking for compromises on other essential performance characteristics. It’s a no-surprise that design of blades is considered as an amalgamation of compromises.
Hitting roadblock in making further significant improvements, engineers are now turning to nature for inspiration and in particular to the gentle giants of the ocean – the humpback whales.
Figure 2: Humpback whale’s flippers with wavy leading edge formed by tubercles.
Is Whale a Gymnast?
Humpback whales are one of the largest living animals on earth. Growing in size up to 12-16 m in length and weighing 25-30 tons, these mammals are surprisingly very agile. They perform underwater acrobatic feats like rolls, loops and flips while catching their prey. Whale researchers believe these maneuvering abilities are primarily due to the bumps on the leading edge of their flippers called tubercles.
Because of their large size and weight, they can’t use their bodies to swim like dolphins or porpoises. Instead, they rely on their tails to propel forward and flippers to turn and flip. Like aircraft, they risk stalling and sinking if they try to move fast or turn quickly at high angles.
To overcome this fluid flow problem, nature through millions of years of evolution has come-up with tubercles, which aid the flippers in generating continuous lift and maintain stability at all times. These leading-edge humps regulate the pressure distribution on the flippers to ensure that different parts of the flipper stall at different pitch and avoid abrupt catastrophic stall. This evolutionary modification, increases the whales maneuvering abilities, allowing it to attack at higher angles and yet maintain stability.
Inspired by these findings in nature, modern engineers are working to replicate humpback’s tubercles in designing the next generation of aerodynamic and hydrodynamic vehicles.
A Novel Flow
Wind tunnel and CFD simulations have confirmed that the tubercles are indeed a revolutionary design in fluid flow management. It is observed that tubercle wings generate higher lift with reduced drag, always stall gradually, achieve higher stall angles than conventional wings, attenuate span-wise flows and tip stalling, and induce lower noise and vibration.
These performance characteristic makes tubercle wings very quiet, very efficient, hyper-stable and a smooth gradual stalling device.
Figure 3: Comparision of aerodynamic forces between a conventional wing and a tubercular wing. Image source – reference 9.
Figure 3 shows the wind tunnel experimental results comparing the performance of a tubercle wing with a conventional wing. Stall, which typically occurs at around 12 degrees on straight leading edge blade gets delayed until it reaches 18 degrees on tubercule wing. That’s an amazing delay in stall angle by 40 percent. At conventional blade stall angle of 12 degrees, tubercle wing shows a 32 percent lower drag value. Further, the leading edge bumps increase the Cl_max by 8 percent.
What these performance characteristics mean is that tubercle wings can operate at a significantly larger pitch angle than normal wings, thereby aiding the wing to generate more lift. More lift, reduced drag and a gradual stall is the ideal blade performance any aerodynamicist looks for and tubercle blades perfectly deliver it on dot.
Figure 4: Pressure contours and streamlines at α = 10 for NACA 63–021 with straight leading edge and with tubercles. a. Shows mid-chord separation line on the upper part of the conventional wing. b. Shows large vortices down-stream of the troughs along the leading edge and laminar attached flow downstream of the tubercles without separation. Images source – reference 9.
Let’s Take The Lid Off..
On a closer look at the flow physics, researchers have observed that tubercles appear to function in a manner similar to vortex generators. Like vortex generators, they too generate counter-rotating vortices which energize the boundary layer flow, compelling the fluid to hug the wing surface and thereby aiding the wing to generate lift at higher alphas.
However, there is a fundamental difference: Tubercle blades increase lift and lower drag whereas, vortex generators increase lift and drag simultaneously. This implies, vortex generators at best can offer limited gains in aerodynamic efficiency, while tubercles paying a substantially lower drag penalty, easily outperforms vortex generators.
Figure 5: Shows the dye enriched flow pattern developed by the leading edge hump for a hydrofoil. Image credit – Dr. Jamid Johari.
Another physical reason for the better performance of tubercle blades is due to the larger suction pressure than in a conventional straight leading- edge blades when the blade goes into stall. In a way, tubercles play the dual role of a flow management system as well as that of a stall-control system. Figure 5, shows counter-rotating spirals originating from the leading edge humps of a hydrofoil in a water tank.
Figure 6: Video showing the surface mesh for varying blade shapes created using one topology multi-geometry gridding approach for CFD based optimization studies.
The 3D Effect
Interestingly, the superior performance of tubercles is observed only in truly 3D semi-span wings. On infinite 2D wings, laminar straight leading edge wings outperform the tubercles. However, even though semi-span tubercle wings attain lesser Cl_max, they tend to generate substantial lift even after stall, resulting in a gradual stall.
This difference in 2D and 3D planform performance arises because, in 2D, the vortical flows reduce the effectiveness of the wing by triggering early separation, but on 3D semi-span wings, they enhance the effectiveness by inhibiting span-wise stall progression.
In pre-stall alphas, both show similar trend i.e. decrease in lift and increase in drag. However, in post-stall, they show opposite trends – the 3D semi-span wings show increasing lift and decreasing drag while 2D wing shows decreasing lift with increasing drag. In a way, the development of vortices by tubercles are truly beneficial only in 3D planforms.
Attenuation of Span-wise Flow
Another remarkable aspect of tubercle leading edges is, they enhance the effectiveness of the semi-span wings by preventing span-wise stall progression. This is important because aerodynamic efficiency reduces considerably with span-wise pumping and tip stall.
Usually, on laminar flow blades, span-wise flow sets in when air detaches from the surface and starts to flow towards the tip. This change in flow pattern disrupts the attached flows along the span and increase the drag. This damaging effect becomes even more severe when it leads to tip-stall. Tip-stall literally wastes the power-generating potential in the air, when it pushes the air stream off the edge of the blade tip.
In aircraft, this phenomenon not only reduces the aerodynamic efficiency but also increase fuel consumption. It’s for this reason, modern wings have winglets at their tips and rotating aerodynamic blades like compressors, gas turbines, axial pumps, fans have shrouds to block air flowing off the tips.
The damages due to tip stall doesn’t end there. Sudden, abrupt separation of the vortices at the tip generates blade vibrations along with audible and very low-frequency subsonic noises. It is reported that nearly 85 percent of the noise in wind turbine blades is caused by tip-stall. If left unattended, the powerful vibrations will trigger blade degradation, laminate de-bonding and in the worst case– break the blade itself.
Tubercles without the need for any physical structural modifications at the wing tips very effectively attenuates the span-wise flow and tip-stall. The chord-wise channular flow which gets set up around the humps creates virtual fences, which breaks up the separation regions and halt the span-wise movement of flow.
Figure 7: Tubercular wind turbine blade with tapes and streamers attached on the upper surface of the wing in a wind tunnel experimental setup. a. Shows the spanwise flow near to the root region where the straight leading edge is present (40 percent of semi-span). b. Attenuation of spanwise flow and onset of pre-dominant chordwise flow in the region with wavy leading edge ( 60 percent of semi-span). Image courtesy – whalepower corporation.
Figure 7, shows this phenomenon in action for a wing with tubercles at the leading edge for the outer 60 percent of the blade span. The green chord-wise tape shows the location from where the tubercles begin. On a normal straight blade, span-wise flow with tip-stall is observed for a pitch angle of 22 degrees rotating at a speed of 55 rpm. On this tubercle blade, when the span-wise flow reaches the green tape line, it gets halted abruptly, forced to change direction and made to flow chord-wise. Figure 7b shows the continuation of this corrected flow pattern all the way to the blade tip.
Figure 8: New surfboard skeg design based on tubercule technology. Image courtesy – H. Swales and Fluid Earth.
Research studies in understanding the flow-physics around tubercle wings have been going on for the last 10 years and it is only now that this technology is garnering acceptance and making its way into the design of various control surfaces like rudders, wings, fans, propellers, tails, sailboat keels, fins, skegs, etc.
Figure 9: Wavy leading edge of a wind turbine blade using tubercular technology. Image courtesy – whalepower corporation.
One of the primary areas where tubercles perfectly fits in is wind turbine blade design. It is reported that tubercles respond better to rapid changes in wind speed. Unlike the conventional blades, tubercles have the unique ability to eke out power even from low-speed wind. They easily increase the power generation capacity of the windmill by 20 percent and greatly reduce the noise due to tip chatter.
Figure 10: Envira-North Systems Ltd. industrial fans with tubercule modifications. Image source – whalepower corporation.
When introduced in industrial ceiling fans used in large buildings like dairy barns, factories, warehouses, etc., tubercles help to increase air-circulation by 25 percent while consuming 20 percent less power than a conventional blade. In addition, these tubercles fans are 20 percent less noisy, as they suppress the tonal noise.
Figure 11: Tubercule micro-fan designs for computers and other electronic cooling applications. Image source whalepower corporation.
On a smaller scale, these humps show their performance-enhancing abilities in cooling computer components, where they are found to be 20 percent more efficient than regular blades.
Several aerospace companies are also actively researching on this device to build next-generation safer, stable, highly maneuvering vehicles. Since tubercles delay stall at high alphas, they can potentially replace boundary-layer control devices like flaps and slots which are used during landing and takeoff of conventional aircrafts. Elimination of these devices will significantly reduce aircraft weight and thereby increase fuel economy.
Figure 12: Model of a commercial transport aircraft with tubercular wings and horizontal tails. Image source – reference 1.
There is a lot to learn from nature. The field of biomimicry does this by learning, appreciating different aspects of nature and figuring out ways to mimic it in human endeavours. In many cases, a straight forward transition of understanding to human needs is not possible as the scales, conditions and requirements of human applications are much more varied and complex. However, in this case, the tubercle technology from humpback whales can be directly used to various hydrodynamic and aerodynamic applications without any modifications.
Odd as it may seem, we humans have been rampantly killing these species for decades, pushing them to the brink of extinction. However, when we have hit saturation in our developmental progress, these gentle giants standup to offer solutions.