Effects Of Tubercles Research Articles

Investigation on the effect of leading edge tubercles of sweptback wing at low reynolds number

Aerodynamic efficiency of an airplane wing can be improved either by increasing its lift generation tendency or by reducing the drag. Recently, Bio-inspired designs have been received greater attention for the geometric modifications of airplane wings. One of the bio-inspired designs contains sinusoidal Humpback Whale (HW) tubercles, i.e., protuberances exist at the wing leading edge (LE). The tubercles have excellent flow control characteristics at low Reynolds numbers. The present work describes about the effect of tubercles on swept back wing performance at various Angle of Attack (AoA). NACA 0015 and NACA 4415 airfoils are used for swept back wing design with sweep angle about 30°. The modified wings (HUMP 0015 A, HUMP 0015 B, HUMP 4415 A, HUMP 4415 B) are designed with two amplitude to wavelength ratios (η) of 0.1 & 0.24 for the performance analysis. It is a novel effort to analyze the tubercle vortices along the span that induce additional flow energy especially, behind the tubercles peak and trough region. Subsequently, Co-efficient of Lift (CL), Co-efficient of Drag (CD) and boundary layer pressure gradients also predicted for modified and baseline (smooth LE) models in the pre & post-stall regimes. It was observed that the tubercles increase the performance of swept back wings by the enhanced CL/CD ratio in the pre-stall AoA region. Interestingly, the flow separation region behind the centerline of tubercles and formation of Laminar Separation Bubbles (LSB) were asymmetric because of the sweep.

Effect of Leading-Edge Tubercles on Compressor Cascade Performance

Tubercles are modifications to the leading edge of an airfoil in the form of blunt wave-like serrations. Several studies on the effect of tubercles on isolated airfoils have shown a beneficial effect in the poststall regime as reduced drag and increased lift, leading to a delay of stall. The prospect of delaying stall is particularly attractive to designers of axial compressors in gas turbines, because this leads to designs with higher loading and therefore higher pressure rise with fewer number of stages. In the present study, experiments were performed on a cascade of airfoils with a NACA 65209 profile with different tubercle geometries. The measurements were made over an exit plane using a five-hole probe to compare the cascade performance parameters. Additionally, hot-wire measurements were taken near the blade surface to understand the nature of the flow in the region close to the tubercles. Oil-flow visualization on the cascade end wall reveals the flow through the passage of blades with and without tubercles. For the cascade considered, the estimated stall angle for the best performing set of blades is found to increase up to 8.6 deg from that of the unmodified blade of 6.0 deg. Application of such structures in axial compressor blades may well lead to suppression of stall in axial compressors and extend the operating range.

Tubercles and Their Applications

AbstractThe implementation of tubercles on foils has demonstrated significant benefits, with the most evident occurring during post-stall. However, the flow mechanism(s) responsible for these benefits is currently unknown, and several possibilities have been proposed. These include compartmentalization, vortex lift, varying effective angle of attack, and boundary layer momentum exchange. Currently, it is only known that tubercles create pairs of streamwise, counter-rotating vortices. By determining how tubercles work, the effects of their addition to untested foils in untested conditions can be hypothesized. This paper reviews the current status of the field of tubercles, comparing hypotheses with published results. The effects of tubercles on the principal components of drag are conjectured from consideration of similar flow control devices. Current applications of tubercles are detailed, and potential applications are suggested.

NACA 4415 Wing Modification Using Tubercles - A Numerical Analysis

In this work, the characteristic design of the humpback whale flippers is incorporated and investigated on NACA 4415 airfoil at very low Mach number. The effect of Tubercle Leading Edge on NACA4415 airfoil has been studied. This novel study attempts to mimic the effect of tubercles on the airfoil wing to improve lift and delay stall. The results showed significant improvement in aerodynamic performance of TLE when compared to CW. TLE, in comparison to wing with vortex generators, performed better. An improvement in lift by about 13.6% was obtained contrary to only 6.3% increase in case of VG under same Reynolds number. In addition, it was also observed that incorporation of tubercles further delayed stall and continued to produce lift at high angle of attacks.

HUMPBACKS' BUMPY FLIPPERS

Humpback whales' lumpy flippers don't look like anything a human engineer would design – unlike airplane wings, bumps called tubercles cover the fins' leading edges, giving them an almost serrated appearance. But humpbacks are some of the most agile of the large whales. Maybe their strange flippers actually give them an advantage in maneuvering, one that engineers haven't explored.After pondering humpback whale flippers for many years, Frank Fish decided to test this idea. Reporting in Physics of Fluids, he and his colleagues from the US Naval Academy and Duke University showed that the tubercles do increase hydrodynamic performance – they delay stall,increase lift, and decrease drag.The group built two model humpback whale flippers, one with a smooth leading edge and one with a wavy edge approximating the usual spacing of tubercles. They tested the scaled-down fins in the Naval Academy's wind tunnel by matching the Reynolds number of a swimming humpback, around 500,000, and measured the steady lift and drag forces on the fins at a variety of angles to the oncoming flow, ranging from –2° to 20°.The smooth-edged flipper behaved much like a standard airplane wing,although its shape gave it a few advantages over a normal wing. As the angle of attack increased, the lift force increased until the flipper stalled out at around 12° when the lift dropped and the drag increased substantially. This performance is very similar to a standard wing, but the lift did not drop as much as the group expected, possibly because fins are tapered towards the tip. The taper might let different parts of the fin stall later than others,leading to a more gradual drop off in the lift force.The really impressive results, however, came from the flipper with tubercles. The fin did not stall until it reached an angle of 16° and produced up to 6% higher lift and as much as 32% lower drag than the smooth fin. Over nearly the entire operating range of angles the bumpy flipper performed better.Summarizing the performance of the flipper models as an aerodynamic efficiency – the ratio of lift force to drag force – they found that tubercles increased efficiency at almost any angle, and particularly augmented the efficiency at high angles. Maximum efficiency jumped from 22.5 to 23.5.The group hypothesizes that the tubercles function like vortex generators,speeding up the flow in the gaps between bumps. The energized flow stays attached to the flipper better, helping to prevent stall at high angles and increasing the lift force. These increased forces probably contribute to humpbacks' surprising agility.While the effect of tubercles may not seem particularly huge, remember that engineers have been tinkering with the shape of airplane wings for more than 50 years. At this point, a change in force or efficiency of a few percent or so is considered large. But humpback flippers reveal a simple, passive way to increase lift by 6%, decrease drag by 32%, and delay stall by 40% –pretty astounding results in the engineering world. Simply adding tubercles– bumpy modified knuckles – to the leading edge does the trick.