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1. Introduction
Laminar flow control(LFC) has been known to have high potential to improve aircraft
performance. The aircrafts fuel consumption and hence the range and endurance can be
improved using LFC which is the only known single aeronautical technology that offers such
an improvement in performance. For transport airplanes the amount of fuel consumed could be
reduced by 30% 1. Laminar flow over most part of the wing due to low drag BLS can lead
to improvement in L/D ratio and when L/D ratio is improved especially in long range aircrafts
there is always a reduction in specific fuel consumption and inturn will increase the range of
the aircraft at altitudes above 20,000 feet2, 3. LFC can be categorized into two, passive LFC
and active LFC, passive LFC is beyond the scope of this paper. A combination of passive LFC
which is known as the Natural Laminar Flow Control (NLFC) and active LFC is called Hybrid
Laminar Flow Control (HLFC). Currently two principles of active LFC are under research which
are surface cooling and boundary layer suction (BLS). It has been understood from previous
research and literature that HLFC increases CL, decreases CD, reduces the boundary layer
thickness and delays the transition of laminar to turbulent flow. BLS is an active type LFC
where small amounts of energy input helps in keeping the boundary layer attached. Though
BLS is extremely successful in delaying transition, increasing lift and decreasing drag, it has not
been implemented on aircrafts due to its high maintenance in keeping the BLS holes clean. It
is also true that once the flow is turbulent no combination of suction helps in reducing drag4.
It was also observed that if BLS failed, then the max lift coefficient of the aerofoil drops below
the max lift coefficient of the original airfoil without BLS5.
BLS has also been very effective in reducing viscous drag. Two types of suction are generally
used, dicrete suction and distributed suction6. This paper surveys and reviews only BLS based
LFC. LFC used in compressor blades would make it an exhaustive survey and hence has been
neglected. BLS on compressor blades are out of the scope of this paper.
1.1. Historical Background
The very initial idea of suction being used as tool to control boundary layer dates back to
the inception of boundary layer theory itself, Pradtl in his article The Mechanics of Viscous
Fluids in Aerodynamic Theory, Vol. III in 1935 makes a mention of boundary layer control
using suction7. Work on LFC began in the early 1930’s with United Kingdom, Germany, and
the United states using wind tunnels to test its effect. Initial tests using multiple suctions slots
resulted in laminar flow upto a Reynolds number of 7 million which was considered a phenomenon
achievement during that period of time1. The very first flight experimentation began in the
early part of 1940’s where researchers placed suction holes between 20 and 60 percent of the
chord on a B18 airplane. Research gained importance and peaked in the 1940’s with the NACA
6 series airfoils. With the introduction of sweep in the 1950’s due to high subsonic speed ranges
the problem of cross flow came into effect 8. High subsonic and supersonic speeds require
sweeping of wings and it was found that only suction can control the cross flow disturbances
that helps in promoting boundary layer transition from laminar to turbulent flow 1 The work
on suction based LFC further culminated in the 1960’s with experimentation on aircrafts built
by Britain and America. The application of suction based LFC had a new area of research in
the 1970’s which were related to supercritical wings. The present survey is to focus attention on
the suction based LFC and to understand the mechanism of boundary layer suction. To gain a
deeper insight into the developments of BLS during its early years of development the reader is
referred to references 1 and 8.
1.2. Flight Tests
In the 1960’s Northrop X-21 which was an experimental aircraft with 300
swept wings was
equipped with BLS slots along the entire span of the wing under the supervision of Werner
Pfenninger who was one of the pioneers in BLS. Experiments have been then performed on
various aircrafts in the past which includes Dassault Aviation, Jetstar and Airbus to name a
few, the results of these flight tests have been reviewed in this subsection.
Flight tests done on the German made aircraft DO-27 have shown that by using BLS the net
drag during cruise can be reduced. The test results obtained on the DO-27 also accounted for
the power requirement for the BLS 9. The values of maximum lift by using BLS during flight
test were similar to that of using leading edge slats. At higher angle of attacks where the flow
over the fuselage had already separated the aircraft was still maneuverable when BLS was on9.
Flight tests early in 1941 were performed on a Douglas B-18 with BLS slots mounted on the left
wing with the NACA 35-215 airfoil, the BLS slots were placed on the upper surface between 20
and 60 percent of the chord10. The Douglas B-18 was a twin engine mid wing monoplane with
slot spacing of 5 percent of the chord.
2. Study on the Effect of Boundary Layer Suction on Performance
In this section, the effects of BLS on curved surfaces(airfoils) will be reviewed and not flat plates.
As the fluid passes around the airfoil the flow is accelerated from the stagnation point onwards
and at the trailing edge the flow is decelerated, if the flow is decelerated over a large part of the
chord the adverse pressure gradient is not so severe enough to separate the boudary layer, by
using BLS this limitation could be extended11. The total kinetic energy absorbed by the fluid
during acceleration is required to overcome the pressure gradient during decceleration in order
for the fluid to come to rest at the trailing edge after accelerating from the stagnation point12.
But in the case of air which is a viscous fluid the kinetic energy is lost due to friction, due to
this loss the kinetic energy remaining is insufficient to overcome the pressure gradient and hence
the flow does not come to rest at the trailing edge and there is present a velocity component in
the direction of the motion of the wing.
Initial theoretical studies of BLS were developed for flat plates where at very low Reynolds
number no suction is required to keep the flow attached and stable, but in the case of airfoils the
presence of adverse pressure gradient at the rear portion requires BLS at all Reynolds number to
keep the flow laminar until the trailing edge13. Suction could be discrete or distributed over the
chord and span of the airfoil and wing respectively to obtain best reduction in drag, increase in
lift and delay in transition. Anderson et al.14 studied suction distribution and defined optimum
suction distribution as the one in which minimum amount of suction is required at each suction
point to delay/prevent transition.
Various factors affects the efficiency of boundary layer suction. The suction pressure, suction
hole position, the size of the suction hole, Reynolds number, boundary layer thickness are a
few of the factors than can influence the efficiency of the boundary layer suction in increasing
lift reducing drag and delaying transition of laminar to turbulent flow. Reynolds number and
suction rate influence the boundary layer thickness which in turn could play a vital role in
reducing drag15.
2.1. Effect of Suction Coefficient and Pressure
It was found that by increasing the suction coefficient(Cq) the CL increased and CD decreased,
which also includes skin friction drag and pressure drag. But when Cq reaches its critical value
drag remains unaffected and also increases slowly16. It was also noted by Glauert et al.17
that the airfoil with suction at low angle of attacks adhered to the quantity of suction required
as per theoretical calculations but as the angle of attack increased and the flow separated the
quantity of suction required to re-attach the flow was three to four times higher. Azim et al.18
studied the effect of suction pressure on trailing edge separation, it was found that the separation
point moves towards the trailing edge with the decrease in the suction pressure. There is drastic
improvement in the L/D ratio as the suction pressure is dropped. Kianoosh et al.19 studied the
effect of suction coefficient on stall angle increase, and observed an increase in stall angle when
there is an increase in the suction coefficient. The increase in suction coefficient also caused an
decrease in the vortex formation behind the airfoil.
2.2. Effect of Suction Hole Width and Angle
Suction hole width also known and called as suction jet length has an effect on the performance
of BLS. Shi et al.16 studied the impact of hole width during BLS and its in influence on
performance and transition. It is a general understanding that suction hole width directly
represents the suction mass, and hence by increasing the suction hole width it was noted that
the boundary layer thickness grows thinner and, as a result of the decreasing boundary layer
thickness CL is increased while skin friction drag initially decreased but later began to increase.
As the boundary layer is thin and Reynolds number increases, there could be a posibility that
the critical Reynolds number for roughness could exceed and induce large disturbances at the
boundary layer13. On the contrary pressure drag had a continuous decreasing effect with
increase in suction hole width16. But a very large suction slot tends to have a adverse effect on
performance11. E. D. Poppleton20 concluded based on his study of boundary layer suction
on a 40 degree swept back wing that the size of the slot is function of choking conditions, the
best slot size would be the one which is just not choking when Clmax is reached.
Kianoosh et al.21 observed that when a suction perpendicular to the flow is used, not only
does the L/D ratio improve but the stall angle also increases, in the case of a NACA 0012 airfoil
the stall angle improved from 140
to 220
. The hole width also had a direct effect on the L/D
ratio, where the increase in the hole width augmented the L/D ratio and delayed the separation
further downstream21. It was also observed that the L/D ratio increased as the hole width
increased to 2.5% of the chord and then insignificantly decreased. Kianoosh et al.19 observed
that when the suction jet length increased till 2.5% of the chord length the lift increased and
drag decreased and as the suction jet length increased between 2.5% to 3% of the chord there
was insignificant improvement.
BLS can be performed at various angle Huang et al.22 studied the effect of varying suction
angle and concluded that perpendicular suction (suction at 90 degrees to flow) has the largest
impact to increase lift.
2.3. Impact of Suction Hole Position
2.3.1. Impact of Suction Hole Position on the Chord It is important to understand that there
is a point on the airfoils chord which is the point of transition without any LFC. This point is
called the natural transition point. Shi et al.16 studied the effect of position of the BLS hole
as a function of the chord length. It was noted that as the position of the hole moved towards
the trailing edge CL increased and CD decreased. But a very important observation was made
that after the BLS point moved past the natural transition point towards the trailing edge, the
CL decreased and CD increased16. There is no possibility of a benefit of transition delay if
the point is placed after the natural transition point as the transition from laminar to turbulent
has already occurred and it has an adverse effect on aerodynamic properties16. Azim et al.18
found that on the NACA 4412 airfoil the trailing edge separation began at 0.7c from the leading
edge and a BLS slot when placed at 0.68c moved the trailing separation to 0.88c. When a
slot is placed nearer to the leading edge at 0.56c and 0.48c the BLS decreased performance by
decreasing lift and increased turbulence which causes an increase in drag 6 times to that of
without suction. Azim et al.18 concluded that placing the BLS slot only near to the separation
point helps in reducing drag. Tutty et al.23 also suggested that placing a BLS slot before the
transition point makes BLS ineffective and placing the suction point after the transition point
has no effect in delaying the transition. It has also been documented by flight tests that leading
edge suction helps in delaying transition and is a very effective means of increasing maximum
lift, it has a similar effect to that of a leading edge slat9, 22. Kianoosh and Reza24 observed
that when the suction slot is placed near the leading edge at 10% of the chord there is a negligible
affect below stall angle (14 degrees). But had a phenomenal impact on the L/D ratio beyond
the stall angle. A. T. Piperas 25 observed that when the transition point is downstream the
suction location a delay in transition can be achieved but when the transition point has moved
upstream of the suction point, the suction becomes ineffective. By placing the suction slot close
to the leading edge one can improve the pressure difference between the upper and lower surfaces
and the aerodynamic behavior25. Millard J Bamber12 found that the best BLS slot location
would depend on the angle of attack. After thorough wind tunnel test it was found that the best
slot position for small angle of attacks is near the trailing edge and as the angle of attacks keeps
increasing the BLS slot should move further upstream towards the leading edge12. Suction
slot spacing when multiple slots are present is another area of concern, Dale et al.26 concluded
that when the slot spacing is very large the suction power required is very large and smaller
suction slot spacing would cause a manufacturing challenge. It is a challenge to optimize the
slot spacing when multiple BLS slots are used.
The position of suction hole is a problem without a conclusion as different authors conclude
differently. There could be a possibility that different airfoils(t/c ratio), mach number, angle of
attack, suction pressure, the application and the conditions at which boundary layer suction is
used to improve performance could have different positions. Clearly there seems to be a need
for further research in this area to sort out the discrepancies and to further understand the
mechanisms of boundary layer suction and the positioning of BLS slots.
2.3.2. Impact of Suction Position on the Span of the Wing Kianoosh and Reza24 studied the
effect of suction area on a 3-D wing by placing the suction area along the span of the wing. Two
configurations were studied, tip suction and center suction where the width of the suction area
was 2.5% of the chord. The location of the suction area was set to 10% of the chord from the
leading edge.
When considering the overall effect, center suction was considered to be a better choice, where
the L/D ratio increased better with center suction than tip suction24. When the length of the
suction slot is greater than half the wing span center suction is better and when the suction
length is less than half the wing span tip suction is better24. Leading edge contamination is
often seen in a 3-D swept wing when the swept wing is attached to the fuselage or to the wall
while performing a wind tunnel test. By placing the BLS slot near the leading edge of a swept
wing along the attachment line it is possible to prevent the attachment line contamination27.
H. J. B. van de Wal28 as a part of his master thesis in Aerospace Engineering at TU Delft
attempted to design a wing with boundary layer suction by redesigning the wing of the EuroENAER
EE10 Eaglet, a research aircraft of TU Delft. The most critical region where most
suction was needed was at the wing tip and the least needed was at the root. Also when flaps
were deployed the suction required was less. H. J. B. van de Wal28 also concluded that there
was no need of suction at the wing root due to the roughness which would trigger a turbulent
boundary layer immediately and another reason for turbulent flow in that region is due to wake
generated from the propellers. The flight parameters also had an improvement due to the Eaglets
new wing with BLS installed. The total drag reduction was very small as the wing profile drag
is a relatively small portion of the aircrafts total drag. The aircraft had a steeper lift curve when
compared to the original28.
2.4. Effect of Suction Amplitude and Suction Velocity
When a 2-D airfoil with BLS is investigated there is no cross-flow(CF) effect, flows with the
absence of CF leads to different flow physics. 3-D wings especially swept wings have the effect
of cross flow which should be considered while analyzing boundary layer suction on wings29.
Kianoosh et al.19 defined suction amplitude as the ratio of suction velocity to free stream
velocity. Three suction amplitudes were considered, 0.1, 0.3 and 0.5 and the increase of suction
amplitude from 0.3 to 0.5 had a greater effect in increases lift and reducing drag and hence an
improvement in the L/D ratio. The maximum L/D ratio was obtained at a suction amplitude
of 0.519. Below a suction amplitude of 0.01 there seemed to be no significant effect due to
BLS, but above the suction amplitude of 0.01 lift increased as suction amplitude increased22.
It was noted by Robert et al.30 that below the critical value of suction velocity which is the
minimum suction velocity the drag increased with a decrease in the suction velocity and above
the critical suction velocity the drag remained constant30.
2.5. Effect of Slats and Flaps
The effect of high lift devices on BLS have been rarely studied, a few researchers attempted
to understand the influence of high lift devices on BLS and this has been reviewed in this
subsection. The deflection of flaps also tends to influence the efficiency of BLS. It has been
found experimentally that there is an increase in Clmax from 1.9 to 2.2 when BLS was applied
for a 0.2 chord split flap which was deflected by 600
. But higher suction pressure may be required
to be able to balance the addition pressure difference caused due to the deflection of the flaps31.
Extension of leading edge slats tend to delay leading edge separation and it was found that a
suction slot closer to the leading edge has a more favorable influence on the maximum lift of the
airfoil without slat32.
3. Effect of Boundary Layer Suction on Aerodynamic Parameters
Boundary layer suction as we know improves aerodynamic parameters like increase in Cl
,
decrease in Cd, delaying stall and delaying the onset of turbulent boundary. A few of these
parameters have been reviewed in the previous sections. In this section the effect of BLS on a
few other aerodynamic parameters will be reviewed.
Stall delay is one of the advantages of BLS and stall in a plain wing without suction appears
to be because of leading edge stall but whereas the stall in a wing with BLS present is a
result of boundary layer separation at the trailing edge30. Also the stagnation point moves
forward further for a wing with BLS in comparison to a plain wing as the angle of attack is
increased. With the introduction of BLS airfoils with higher thickness to chord ratios(t/c) can
be used without the high drag expectation due to separation11. Though it has been found
experimentally that no significant decrease in drag is possible for airfoils with normal thickness
on which separation does not occur33.
Another possible feature of BLS is that it could improve the lateral control of the aircraft.
As lift could be increased with BLS, a rolling moment could be produced by varying the wing
pressure in the outermost region of the wing12.
4. Mathematical Models Used in Numerical Analysis to Predict the Effects of
Boundary Layer Suction
Flow over an 2-D airfoil can exhibit different complex phenomenons such as wakes, flow
separation, boundary layers etc. Computational Fluid Dynamics(CFD) developing at such a
rapid pace is being used to predict not the 2-D airfoil but also the finite wing which is a 3-
D body. The 3-D effects such as downwash, induced drag, trailing edge vortex can also be
predicted using CFD. Experimental work is very important to produce data that is required to
analyze suction based flow control. Obtaining very fine and sensitive detailing requires repetitive
experimentation which could be an expensive affair. Hence numerical methods/analysis could
be used to capture and predict the effects. Various techniques and mathematical models are
available to capture the physics. Different transition and turbulence models used to predict the
transition from laminar to turbulent flow have been reviewed in this section.
RANS(Reynolds-averaged NavierStokes) is an cost effective method compares to DNS(Direct
Numerical Simulation) or LES(Large Eddy Simulation)34. Shi et al.16 took a numerical
approach to analyze hybrid laminar flow control (HFLC) using the Menter and Langtry’s
transition model. Computational results which were obtained were validated using experimental
results. It was found that the Menter and Langtry’s transition can predict the transition of
boundary layer from laminar to transition when boundary layer suction was present. Sun et
al.35 studied boundary layer suction on a linear compressor cascade using RMS (Reynolds
Stress Model) which was described in literature as the most suitable for complex threedimensional
separations. Azim et al.18 investigated the effect of BLS using the Spalart Allmaras
turbulence model. The Spalart Allmaras turbulence model is a single equation linear eddy
viscosity model which was initially designed for aerodynamic flows. Kianoosh and Reza24
studied the effect of 3-D suction on a NACA 0012 wing using RANS with the K-? SST turbulence
model which can predict flows with separation very accurately24. Kianoosh et al.21 used
RANS equations in conjunction with the Menters shear stress turbulent model which is two
equation model(K-? SST) which is as mentioned capable of predicting flows with separation.
A. T. Piperas 25 concluded that the SST Gamma Theta turbulence model predicts the effects
of transition better than the K-? SST model.
Different turbulent and transition models were studied to verify the equations ability to
predict turbulence and transition by Serdar et al.34 and it was found that the K-? SST
transition and turbulent model tends to under-predict turbulence and the K- RNG overpredicted
the stall. The K ?Kl ? ? model was found to be relatively better in agreement
with available experimental data.
5. Summary
There is a significant improvement in aerodynamic performance due to laminar flow control by
BLS and hence an improvement in the overall performance of an aircraft with active BLS. With
the increase in suction coefficient lift and stall angle increased with the decrease in the drag
and vortex formation behind the airfoil. A conclusion on the most effective BLS slot position
could not be drawn, further research into optimization of slot position is a requirement. There
was a decrease in the boundary layer thickness and pressure drag reduction with the increase in
suction hole width. When suction is applied at center of the wing span it improves the L/D ratio
better than when the suction is applied at the tip of the wing span. BLS near the leading edge
if the wing can prevent attachment line contamination. Higher suction amplitude influences
improvement in lift and reduction in drag. Higher t/c ratio airfoils can be used when BLS is
used without the increase in drag due to boundary layer separation. There is also a possibility
of better lateral control of an aircraft when BLS is present and active. The K ? Kt ? ? model
was considered to be the best turbulence model to predict the effect of BLS and flow separation
while using computational fluid dynamics to analyze BLS.

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