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Figure 1.   Dr. Sumon Sinha (left) and employees
conducting tests at Global Aircraft, Starkville, MS, 2002.

Dr. Sinha has researched airfoil flow control issues for over 10 years. His early work involved active surfaces to control turbulence (2,3,4). Dr. Sinha is now working on a flexible composite surface deturbulator (FCSD) that operates passively to simultaneously reduce turbulence and drag (4,5) and to increase lift. Wind tunnel and flight tests have demonstrated large drag reductions and lift increases.

This device should not be confused with turbulators. Although it is applied to the wing like turbulator tape it does not work the same way. Whereas, turbulators add to skin friction as the price for decreasing form drag from separation bubbles, Sinha Deturbulators reduce both skin friction. Physically, Sinha Deturbulator is thinner than turbulator tape and works by means of surface-flow interactions, rather than edge effects to achieve sophisticated control of boundary layer flow.

These claims have been demonstrated repeatedly in flight tests that demonstrated (mid-span) profile drag reductions up to 27% and a 7% increase in total aircraft lift/drag ratio.

Technical overview:
A flexible composite surface has been developed (patent pending) which can be affixed to selected regions on aircraft wings to reduce the combined pressure and viscous drag under cruise conditions. This is a passive (non-powered) device.

The Sinha Deturbulator concept originated from earlier research by Dr. Sinha on Active Flexible Walls (AFW) for aerodynamic flow separation control. The AFW, which he patented in 1999, uses an array of electrodes under a flexible membrane to sense oncoming flow separation (sensor mode) and an external AC-signal to vibrate selected regions of the membrane (actuator mode) to re-attach the separated flow. The FCSD can be seen as a non-powered version of the AFW since many similarities exist in the interaction mechanisms between the sub-micron level (typically 10-100nm amplitude) oscillatory motion on the surface of both devices and the flow (for details, please refer to his paper on the AFW in the May-June 2001 issue of the Journal of Aircraft published by the American Society of Aeronautics and Astronautics). However, the principal difference is that the passive FCSD, unlike the electrically powered AFW, does not add any disturbances to the flow. It simply acts as a spectral filter for turbulence which helps sustain a thin layer of slow moving air next to the wall. Such a flow structure lowers the loss of energy by reducing the wall shear stresses (or skin friction), thereby reducing drag. The slow moving layer is extremely thin. Typical boundary layer velocity profiles are based on measurements above this layer. Due to the lowered skin friction these appear fuller and similar to turbulent boundary layer profiles. Such velocity profiles also offer superior resistance to flow separation near the trailing edge of high-performance airfoils, thus reducing the wake width and resulting form (or pressure) drag.

Unlike turbulators, which energize the near-wall layer by increasing turbulent velocity fluctuations, the Deturbulator reduces overall turbulence levels, making the flow more laminar (hence the name Deturbulator). Residual turbulent fluctuations are present only around a narrow frequency band. Since deturbulation minimizes skin friction, the total profile drag can be lowered compared to turbulation (i.e., reducing form drag by increasing skin friction).

What all of this means to you is by using the FCSD properly on the wing of almost any aircraft, the wing profile drag can be reduced significantly over the entire operational range, in spite of unintended roughening of the wing leading edge (e.g., bug hits). The drag reduction benefit also contributes to a large increase in the wing lift coefficient.

Since a combination of conditions has to exist for sub-micron level surface vibrations to appreciably change a 30- 100 m/s flow over the wing, the location and mounting details of the FCSD are critical. We have found that an incorrectly mounted device can degrade performance. However, a correctly mounted device can retain its performance enhancement in spite of degradation due to normal use.

Some operational advantages:
This device can be applied to the surface without requiring structural modifications or subsurface machining.

Figure 2.   Sinha-Deturbulator mounted on the bottom of the wing
Preliminary verification of aircraft wing drag reduction:
Flight tests were conducted with the Sinha Deturbulator mounted on the top and bottom surfaces of an advanced 1.24-m chord, NLF-0414F natural laminar flow airfoil wing of a Global-GT3 trainer aircraft (manufactured by Global Aircraft Inc., Starkville, MS). Fig 2 is a photograph of a 300-mm (12") spanwise and 50-mm (2") chordwise section of Sinha Deturbulator (shinny area in photograph) mounted on the bottom of the wing at mid-span, along with the boundary layer mouse used to measure boundary layer velocity profiles. The aircraft was flown at about 3000 ft pressure altitude at its level cruising speed of 106-kt. This corresponded to a chord-based Reynolds number Rec @ 4.8 x 106, flight Mach number M @ 0.22 and a section angle of attack a @ - 1 .

Several sets of data were acquired both for the clean airplane without the Deturbulator and with it. Fig 3 shows a typical comparison of clean wing vs Deturbulator velocity profiles for the top wing surface. The two measurements of the clean wing show a measurement uncertainty that is most obvious in the freestream region.

These data indicate a total wing section drag reduction of 18%.


Figure 3. Measured Suction-side Boundary layer Velocity Profiles
(The difference between Clean-Wing-1 and Clean-Wing-2 profiles shows test uncertainties.)


Other wing sections:
The large drag reduction indicated in the aforementioned tests may be partly due to the geometry of the advanced NLF-0414F wing section (Fig 4).
Figure 4. NLF-0414F Wing Section

However, similar drag reductions have been measured on a very different airfoil in the Wortmann FX series (Fig 5). In this case, tests were performed on a Standard Cirrus sailplane.

Figure 5. Wortmann FX S 02-196 Wing Section

Fig 6 illustrates the results. For an explanation of this graph and details on the testing methodology, go to the SinhaFCSD Progress page.

Figure 6. Drag vs airspeed with and without Sinha Deturbulator
on lower surface of Standard Cirrus wing


Potential commercial and military uses:
For large commercial transport aircraft, a reduction in overall drag by as small as 1% can generate significant fuel cost savings. For such aircraft, a 1-3% reduction in overall drag would require about 3-10% reduction in wing drag, which is believed to be feasible with the Deturbulator.

For the military, the Deturbulator can be used to increase the range and endurance of high-altitude long-endurance unmanned aerial vehicles by an estimated 10-15%. In the near term, it can be used as a retrofit for existing UAVs, such as the Predator. In particular, loss in range and endurance brought about by an increase in wing loading due to weapons can be compensated by using the Deturbulator.

The Deturbulator can play a role in reducing drag on wings optimized for reduced radar cross section. It can also be used to increase the range of missiles, and improve the aerodynamics of engine inlets.


Funding:
Funding for this work has been provided for 2004 by NASA/Langley Research Center and the National Science Foundation.


References:

1  McGhee, R.J., Viken, J.K., Pfenninger, W., and Beasley, W.D., 1984 Experimental Results for a Flapped Natural Laminar Flow Airfoil with High Lift/Drag Ratio, NASA TM-85788, NASA Langley Research Center, 1984.

2  Sinha, S.K., 1999, System for Efficient Control of Separation using a Driven Flexible Wall, U.S. Patent No. 5,961,080, awarded October 5, 1999.

3  Sinha, S.K., 2001a., "Flow Separation Control with Microflexural Wall Vibrations, " Journal of Aircraft, Special Issue on Flow Control (Vol.38, No.3., May-June-2001) pp. 496-503.

4  Sinha, S.K., 2001b., Exploring Separating Boundary Layers With a Flexible Wall Transducer Array, Proc. ASME FEDSM-01, 2001 ASME Fluids Eng Summer Meet, New Orleans, LA, May 29-June 1, 2001.

5  Sinha, S.K., 2002, A Flexible Composite Surface for Enhancing Heat Transfer in Heat Exchanger Passages without Increasing Flow Pressure Drop U.S. Provisional Patent Application 60/354,702, filed Feb 4, 2002.

6  Sinha, S.K., 2003, System and Method for Using a Flexible Composite Surface for Pressure-Drop Free Heat Transfer Enhancement and Flow Drag Reduction, U.S. Patent Applied for Jan 29, 2003.


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