This study examines the flow patterns around flat plate wings with and without an innovative flap attached, at a Reynolds number of 2,000 and angle of attack of 30 degrees. Flow visualization shows that on the wing with the flap, the leading edge vortex travels slower down the wing and is trapped by the flap to form a bound vortex, improving lift characteristics. However, both vortexes breakdown after a similar distance of travel, suggesting the flap is more effective at maintaining peak lift than delaying stall. The goal is to apply this innovative flap concept to micro air vehicles to improve their flight performance at ultra-low Reynolds numbers.

1. Motor
Wing
Camera
Perspex tank
Water with ⌀30 microns
polycrystalline particles
Lasers
Ball screw linear guide
Group Members :
Zhang Nuda, River Valley High School
Raimie Farid Tang Yee Ming, River Valley High School
Mentor:
Doctor Cui Yongdong, Temasek Laboratories@National University of Singapore
Co-mentor:
Professor Lim Hock, Temasek Laboratories@National University of Singapore
Introduction:
• Ornithopter micro air vehicles are tiny flapping-wing aircrafts that
operate at ultra-low Re below the order of 104, a regime which is still
little understood, and current designs are largely inefficient.
• Following are observations made by previous studies:
• Hypothesis: By attaching an innovative flap that mimics the behaviour
of bird’s feathers to the suction side of a model wing, flow separation
will be delayed at high α, and the LEV will remain attached to the
surface for distinctly longer durations, thereby improving lift
characteristics. This, when applied to MAVs, can improve their flight
performance.
EXPERIMENTAL STUDY OF INNOVATIVE FLAPS ON
SLIDING FLAT PLATE AT ULTRA-LOW REYNOLDS NUMBER
Abstract:
An innovative flap fixed to the suction side of an aerofoil is proposed to
improve high angle of attack performance at ultra-low Reynolds number
(Re). Flow patterns around two similar flat plate wings, one with an
innovative flap attached, were compared at Re = 2,000, α = 30° using flow
visualisation. The leading edge vortex (LEV) on the wing with flap attached
is observed to travel slower down the wing than that without, and forms a
bound vortex. Results show that although the flap is effective in sustaining
peak lift, it may less so in delaying stall.
Objective:
• To use flow visualization to qualitatively study flow pattern over a flat
plate wing at Re 2,000 and α = 30° (beyond static stall), with
particular attention to the LEV (this is the control test).
• To study flow pattern over the same wing with an innovative flap
attachment under the same conditions. This is compared with the
control test, to identify the effects of the flap on the flow.
Methodology:
Experimental stage 1 (control):
• Model wing is translated
through water (Re 2,000, α =
30°)
• Video camera records flow.
Analysis stage 1:
• Videos from stage 1 analyzed using
Tracker software.
• Flow pattern and vortex formation
noted.
Experimental stage 2:
• Similar wing but with innovative flap
tested under same conditions.
• Video camera records flow.
• Several designs tested to produce
optimum effect.
Analysis stage 2:
• Videos from stage 2
analyzed using Tracker
software.
• Best flap design chosen.
Analysis stage 3:
• Results from stage 2 compared with control test to form conclusion.
Conclusion:
• At Re 2,000 and α = 30°, innovative flaps are found to improve lift
characteristics delaying LEV separation from the wing and trapping the
LEV to form a bound vortex. LEV is also prevented from interacting with
trailing edge vortex.
• However, as LEV on both wings break down after approximately the
same travel, it suggests that although flap can be effective in
maintaining peak CL , it may be less so in delaying stall.
Wing
Travel:
Control Test [1]: Flap Attached [2]: Remarks:
0.5c [1, 2]: LEV forms over leading
edge. (arrow in 1st frame
indicates direction of flow)
1.5c [1]: LEV reaches midchord and
begins to detach.
[2]: LEV is 2/5 down the chord.
2.5c [1]: LEV reaches trailing edge.
[2]: Flap deploys and LEV is
trapped by it at midcord to
resemble bound vortex.
3.0c [1]: LEV interacts with trailing
edge vortex and begins to break
down.
[2]: LEV is maintained.
3.5c [1, 2]: LEV dissipates. Thick
turbulent boundary layer
formed.
Observation Explanation
For an impulsively started wing at
low Re, a pronounced peak in CL
occurs at approx. 0.6c of travel,
followed by a steep drop and a
recovery to intermediate levels.
The initial peak in lift is attributed to
the formation of a LEV, and sudden
drop in lift is in turn caused by the
detachment of the LEV from the
wing surface.
It is known that in landing (low
speed, high α), light feathers
covering the upper surface of the
bird’s wings may pop up.
Feathers act as a boundary layer
fences to delay flow separation, and
also as high lift devices.
Results:
• Best flap design is of paper (61×25mm) fixed near trailing edge, with limiter
strings to prevent it from being washed over by reverse flow.
• LEV trapped by flap to form bound vortex, which contributes greatly to lift
generation. Flap also blocks reverse flow to prevent spread of separation.