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OCR for page 698
Analysis of a Jet-Controlled High-Lift Hydrofoil with a Flap
Shin Hyung Rhee, Sung-Eun Kim (Fluent Inc., USA),
Haeseong Ahn, Jungkeun Oh, Hyochu] Kim
(Res. Tnst. of Marine Systems Eng., Seoul Nat'] Univ., Korea)
ABSTRACT
A jet-controlled high-lift hydrofoil with a flap is
investigated using both experimental and
computational methods. Experiments are being
carried out in a cavitation tunnel to measure forces
and moment acting on the hydrofoil, and surface
pressure distribution. The measured data show the
feasibility of such a device for marine applications.
Computational studies have also been carried out in
parallel with the measurements. The computational
results are analyzed in terms of global and local
quantities using available experimental data. The
present computational results compare well with the
well-known experimental data for circulation control
flows. The results for flow around a hydrofoil with a
blown flap further validate the concept behind the
proposed device. The results of the study demonstrate
the applicability of the technology to the design of
practical control surfaces.
INTRODUCTION
=, _ ~
It is widely acknowledged that one of the
most efficient ways to increase lift is boundary layer
and flow control. The technology has been
investigated both experimentally and numerically.
However, most of the studies focused on aerospace
applications, and efforts targeted for marine
applications are rare. With the growing interest in
low-soeed maneuverability which has become
increasingly Important In design process or surface
ships and under-water vehicles, studies of high-lift
devices for marine applications are warranted.
Besides mechanical flaps or slats or spoilers,
internal blowing systems for boundary layer control
(BLC) is known to be most efficient for lift
production. Internally blowing BLC systems are
characterized by the use of fluid jets ducted from
within the control surface. BLC utilizes the added
momentum due to the injected fluid to delay flow
separation at critical locations on a lifting surface,
augmenting lift much beyond what is possible
without the injection.
For application to marine vehicle control,
the concept of a blown flap has several advantages
over other power devices (Wilson and von Kerczek,
1979~; (1) it is mechanically relatively simple, (2) the
flap size could be chosen to provide adequate
performance without blowing at high speed, (3)
powered operation at low speeds could provide a
wide range of available control force, (4) the flap
could be used as a plain device without power, if
fluid pumping system fails, and vice versa, and (5)
water exhausted from the ship could be ducted into a
high-pressure plenum that would act as a supply for
blowing. Also in terms of hydrodynamic
performance, the blown flap is the most attractive
high-lift scheme of all those considered in Wilson
and van Kerczek (1979~.
Many experimental studies have been done
for BLC airfoil with a blown flap in the
aerodynamics field, where substantial lift-
augmentation was demonstrated (e.g., Attinello,
1961~. Similar studies with marine applications in
mind are, however, hardly found. Studies of
circulation control (CC) wings also have been carried
out, and some of them were intended for application
to marine vehicle control surfaces (e.g., Englar and
Williams, 1971~. Yet most studies focused on short
take-off and landing (STOL) and vertical take-off and
landing (VTOL) capabilities and lift augmentation
for transport aircraft (Abramson and Rogers, 1983;
Englar and Huson, 1983; McLachlan, 1989; Englar et
al., 1993~.
A number of computational studies for CC
flows are available in literature. Dvorak and his
colleagues used interactive approaches of potential
and viscous flow solvers (Dvorak and Kind, 1979;
Dvorak and Choi, 1983) in early days. Reynolds-
Averaged Navier-Stokes (RANS) equations were
solved later with algebraic and two-equation
turbulence models for jets of small and intermediate
momentum (Shrewsbury, 1985; Berman, 1985;
Pulliam et al., 1985; Linton, 1994~. All of them
OCR for page 699
showed reasonable and promising results, although
limited to relatively mild conditions. Quite recently,
computational studies for unsteady flows around
more practical geometry (Liu et al., 2001) and for jets
of higher momentum were reported using
sophisticated turbulence models (Slomski et al.,
2002~. These studies provide valuable insights into
the physical and numerical aspects of the flows.
Studies for BLC hydrofoil using a blown flap,
however, have not been reported in the literature, to
the authors' knowledge.
The present study is concerned with
experimental and computational investigation of a
jet-controlled high-lift hydrofoil with a flap, i.e., BLC
hydrofoil using a blown flap. The primary objective
is to understand the lift increase phenomena by such
a device, which can be implemented in many of
already deployed surface ships and under-water
vehicles. To this end, an extensive experimental
study is underway at the Research Institute of Marine
Systems Engineering (RIMSE) of Seoul National
University (SNU). In parallel, a concurrent and
complementary computational investigation has been
conducted at Fluent Inc. The global quantities, such
as lift, drag and momentum, and local flow
measurements, such as surface pressure, are
measured for a range of flow conditions.
Computational fluid dynamics (CFD) study
reinforces the validity of the technology and provides
insights for optimum design of such devices. CFD
study of CC flows for two types of geometry and
conditions were also carried out to verify the
selection of appropriate numerical schemes and
turbulence models.
The present paper is organized as follows.
The experimental study is described in the next
section along with the experimental facility,
equipments, set-up, and conditions, followed by
computational method employed for the present
study. The CFD validation for CC flows is presented
first in the section for comparison and validation, and
the experimental and CFD study results and
comparison for BLC flow follow. Lastly, some
concluding remarks are made.
EXPERIMENTAL METHOD AND
VERIFICATION
The experiments are being carried out in the
cavitation tunnel of RIMSE. The geometry of two-
dimensional (2D) hydrofoil section was taken from
NACA 0021 airfoil, which is close to a typical rudder
section, and a 0.25c flap, where c is the chord length,
was installed in a way that the jet slot maintains its
height at 0.0088c. The blowing system is
accommodated inside the morel. Figure 1 shows the
hydrofoil section with a flap. The jet flow is supplied
by a non-contact type Labyrinth connector and kept
at a constant rate using a pressurized chamber outside
the tunnel. The air in the chamber was compressed,
so that fluctuations due to water supplied are
suppressed.
A three-component load cell was
manufactured and installed inside the hydrofoil for
force measurements. Prior to being assembled, the
load cell was calibrated statically for each component
of force. Calibrations were also carried out after the
assembly of the load cell. The calibration results
showed good linearity, negligibly small hysteresis,
and reasonably small interference between force
components. The voltage signals for each force
component are digitized sequentially through an A/D
converter and stored. One thousand data points per
second were recorded for 20 seconds. Each data set
was divided into several subsets and only the ones
with the standard deviation less than 10-3 were taken.
For surface pressure measurements, 28
pressure taps were placed on the hydrofoil surface
with a cosine spacing distribution. A scannivalve was
connected to the pressure taps by vinyl tubes and
pressure transducers were used to get the total head
corresponding to surface pressure.
~ Her Id- -S~n~s~r
,,, i
'~5_
N
,;E, 1 I/
_
Figure 1. Foil section with a flap (dimension in mm).
1.2'
no`
n7`
.,'_ _ _ _ _ L _ _ _ _ ___ _ _ I _
0 5 10 15 20 25
a
Figure 2. Lift coefficient vs. angle of attack for
NACA 0021 without a flap.
In order to verify the appropriateness of the
experimental set-up and measurement accuracy, a
OCR for page 700
NACA 0021 airfoil without a flap was built, and then
forces and surface pressure were measured. Figure 2
shows sectional lift coefficient, Car, at angle of attack,
cz, between 5 and 30 degrees. Although the present
measurements show a delayed stall, which is
attributed to possible three-dimensional (3D) effects,
the agreement is fairly good up to a=20 degrees.
Figure 3 shows the surface pressure coefficients at
cx=0 degree. Also shown for comparison are Euler
and RANS solutions from FLUENT. The comparison
is commendable and the experiment is shown to
capture the boundary layer effect near the trailing
edge correctly, which verifies that the present
pressure measurement system is reliable.
The non-dimensionalized momentum of jet,
i.e., jet momentum coefficient, Cal, is defined as
C =mVj/] pV2c (1)
where m is jet mass flow rate, Vje, is averaged jet
velocity through the slot, and pa and Van is free
stream density and velocity, respectively.
0.5
c) 0
1< D ' '
O Present ~ '
, Euler solution (FLUENT)
~ , ~
, - - - - -, RANS k-m poludon (FLUENT)
_~
-1 - -—1
0 0.05
U.1 U.1) U.2 U.~)
x (m)
Figure 3. Surface pressure coefficients vs. chord
length for NACA 0021 without a flap at a=0 degree.
Based on the experience obtained through
this preliminary study, the experimental set-up and
measurement techniques were refined, and
experiments are being carried out for BLC hydrofoil
using a blown flap. Some of the results for global
quantities were presented and demonstrated the
feasibility and potential of the device (Akin et al.,
2000~. It was evident there that the Coanda effect
works favorably, i.e., the flow speed increases and
pressure decreases on the suction side, and
consequently separation is suppressed, resulting in
lift increase.
COMPUTATIONAL METHOD
CFD study was carried out using FLUENT.
FLUENT solves the Reynolds-averaged Navier-
Stokes equations. The k-co SST (Menter, 1994), k-w
hereafter, and Reynolds stress transport (Kim, 2001),
RSTM hereafter, turbulence models are used for
turbulence closure in the present study. The k-co
model is one of the most widely used turbulence
models for external aero- and hydrodynamics. RSTM
is the most advanced turbulence models for
engineering applications and has shown better
potential to predict the key features of the present
flow than other models. FLUENT employs a cell-
centered finite-volume method along with a linear
reconstruction scheme that allows use of
computational elements with arbitrary polyhedral
shape. Convection terms are discretized using a
second order accurate upwind scheme, while
diffusion terms are discretized using a second order
accurate central differencing scheme. For transient
flow calculations, time derivative terms are
discretized using a first order backward implicit
scheme. The velocity-pressure coupling and overall
solution procedure are based on a SIMPLE type
segregated algorithm adapted to unstructured grid.
The discretized equations are solved using pointwise
Gauss-Seidel iterations, and algebraic multi-grid
method accelerates the solution convergence. More
detailed description of numerical method is available
in Kim et al. (1998~.
CFD RESULTS AND COMPARISON DATA
Both CC and BLC flows are challenging to
any CFD codes: different Reynolds number flow
regimes, due to the difference in length and velocity
scales between the foil and jet slot, should be
considered in a single flow field; a turbulence model
that can properly take wall jet effects into account is
required, and; stagnation point and pressure gradient
near the jet, which are largely correlated with surface
pressure, should be accurately predicted.
In the present section, CFD results for both
CC and BLC flows are presented. CC flow
simulations serve as verification tests of the present
physical modeling and numerical schemes. CC flow
results are validated against the well-known
experimental data for elliptic foil geometry (Kind and
Maul, 1968; Englar, 19711. BLC flows are
considered for a range of parameters and the results
are compared with data obtained from the present
experimental study.
Circulation Control Flows around Elliptic Airfoils
First, CC flow around a 15-percent pure
elliptic airfoil, Englar case hereafter, is considered
(Englar, 1971). An upper surface tangential slot with
a height of 0.00125c is placed at 0.924c from the
OCR for page 701
leading edge. The computational domain is oval-
shaped with extent -2 < x/c < 6 and
-2.5 < y/c < 2.5 . The computational mesh shown in
Figure 4 consists of 50,984 quadrilateral cells and the
first cell spacing off the solid surface is
approximately I in terms of wall y+. Computational
conditions are set to reproduce one of the
experimental conditions in Englar (1971), i.e., chord
based Reynolds number Re = V~c/v = 5.48xlO ,
where v is the kinematic viscosity, cx=3 degrees and
COO. 138. Incompressible air flow was assumed.
Based on c of 0.2032 m, m of 0.084 kg/s is supplied
at the end of the plenum shown in Figure 4, while
Vet =39.4 m/s is imposed on the front and side
boundaries. No-slip condition is imposed on solid
surfaces, and zero-static-pressure condition on exit
boundary.
it.
~ _
a_ _
Figure 4. Computational mesh for Englar case.
reproduced. Two CFD results are so close to each
other and hard to discern one from the other,
suggesting that k-a) model performs sufficiently well
for this type of flows, i.e., boundary layer with
clearly detaching wall jets.
Aeon tic; ec~loo~
on -2
l
-4 ~
. _~.
:! , , . 1
6` f . .
r ~ Presents k-m
-- ~ -- Present,,RSTM
O Englar 61971)
-v , ., . I,,,, I,,,, I,,, _ _
0 o.os 0.1 0.15 0.2
x (m)
Figure 5. Foil surface pressure coefficient vs. chord
length for Englar case.
~ ~42
1~87e+Ce
7~
14~
I..'. 1~
: S.: T:
, j .
1.
9~1
.8,~1 .
B.~,
5~g1
4.~1 ,
2~:
7.3300i:
1 :
~~ aY~ty ~6 em $, Cat za =2
Table 1. Cl for Englar case. Figure 6. Velocity magnitude contours for Englar
k-co RSTM Measured case: k-m solution.
C, 1.986 1.946 1.944
Both the global, i.e., sectional lift coefficient
Cl = lip/ 2 REV 2 ~ and local, i.e., pressure
coefficient Cp = pressure/2 pV2, quantities are
considered for validation. No adjustments were done
to either ~ or Car, since two-dimensionality was
already assured in the measured data. Table 1
presents predicted Car obtained using both k-m and
RSTM. Also presented is the measured value (Englar,
19711. Both results agree well, less than 2.2%
difference, with the experimental data.
Cp predictions also compare well with
experimental data as shown in Figure 5. Peak
pressures and slopes of the Cp curves are well
: ~
. . .
~ Path Lines =~ Dy P+sure Bert
Mar 26 ~ 12
E!JI 6.~(Z~, set—led, sst~
Figure 7. Pathlines colored by Cp for Englar case: k-m
solution.
OCR for page 702
Figure 6 shows contours of velocity
magnitude, q = ,/u2 +v2, where u and v are x- and
y-components of velocity in the Cartesian coordinate
system, around the trailing edge. The jet flow
detaches smoothly and clearly from the trailing edge,
which is not round enough to produce strong Coanda
effect, and the flow field resembles that of BLC
flows. This is the reason why rounded trailing edges
produce larger lift and are preferred for CC airfoil
studies (Kind and Maul, 1968; Englar, 1971;
Abramson and Rogers, 1983; Englar et al., 1993~.
Pathlines colored by Cp around the foil are shown in
Figure 7. The Coanda effects, i.e., relocated
stagnation point and increased pressure difference
between suction and pressure sides, are clearly
displayed.
Figure 8. Computational mesh for K&M case.
Table 2. C/ for K&M case.
. . . . .
k-co RSTM Measured
C, 2.27 2.60 2.71
The second case for CC flow is turbulent
flow around a 20-percent elliptic foil with circular
trailing edge that creates 0.00067c high slots on the
upper and lower surfaces at 0.962c from the leading
edge, K&M case hereafter (Kind and Maul, 1968~.
The computational domain is larger than that of
Englar case with extent ~ < x/c < 11 and
-5 < y/c < 5, to accommodate thicker airfoil and
encompass larger jet influenced region. The
computational mesh shown in Figure 8 consists of
48,600 quadrilateral cells and the first cell spacing off
the solid surface is approximately I in terms of wall
y+. Computational conditions are set to the
experimental conditions, i.e., Re = 7.5x 10 , oc=-0.6
degree and C~=0.094 from the upper slot only. As for
Englar case, incompressible air flow was assumed.
Based on c of 0.372 m, m of 0.076 kg/s is supplied at
the end of the plenum, while VOO =29.45 m/s is
imposed on the front and side boundaries. No-slip
condition is imposed on solid surfaces, and zero-
static-pressure condition on exit boundary.
-
~G
-
-
-8 . . . . . . . . . . . . . . . . I . . . . . . . . . . .
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
x (m)
Figure 9. Foil surface pressure coefficient vs. chord
length for K&M case.
A.
-1A15~9`
_~.~t~
_1~
_~*K
_~`q:
_~*=
'_~.~4-0C"''''''
—
4.~x
- .85 - QC'''''
; _~-.~ . ..
~ P.IbLiO65~P~ - =~8~
.,, ~ ~ , `54', 2~,,~ .~
~ "~'r2i°.~_.
..................................
^pt0, ~
~_ _
Figure 10. Pathlines colored by Cp for K&M case:
k-m solution.
_~.~x
-1~
_1.5~X
=~X
~X
=.~X ~
-3~,—... .^ - -
_~X
_~x ...~,,,,....~...
_4~0C
. .
- ~X
[' ~
/' '\
~ : ~
Pbthtm8$~P~ - ~8tl ~t01
FLLIENT6.~ ~j R~
Figure 11. Pathlines colored by Cp for K&M case:
RSTM solution.
OCR for page 703
Again Car and Cp are considered for
validation. No adjustments were done to either ~ or
Cat. Table 2 presents predicted Car obtained using both
k-w and RSTM. Also presented is the measured value
(Kind and Maul, 19681.
The RSTM prediction is again quite good,
i.e., 4.2% difference, although not as close to the
measured value as for the Englar case. Of interest
here is, however, the fact that, unlike the Englar case,
k-6o model prediction is much lower than the
experimental data. Similar behavior was also
observed by Slomski et al. (2002) with a family of k-
£ models. They found that isotropic, eddy-viscosity
based turbulence models are inadequate for flows
with curved wall jets and separation. Although k-m
models generally show better performance than k-£
models for boundary layer with crossflow and
streamwise vortices (Kim and Rhee, 2002), they
suffer from the lack of fidelity as far as curved wall
jets are concerned. This is supported by Figure 9,
which shows Cp comparison on the hydrofoil surface.
The pressure side Cp is low and flat after x=0.3m and
it contributes to the low Car prediction. The reason of
this low Cp can be found in the behavior of the
pathlines around the foil shown in Figures 10 and 1 1.
In the k-co model solution, the jet remains attached to
the foil surface further around the round trailing edge,
and this suction reduces Cp in this region and
eventually C', although not as severe as in the k-£
model results reported by Slomski et al. (2002~. This
behavior warrants a special care when CC flows with
rounded trailing edges are to be simulated. However,
as observed by the Englar case, the k-m model seems
to perform quite reasonably, as long as there is a clear
jet detachment physically at the trailing edge.
Boundary Layer Control Flow around a Jet-
Controlled Hydrofoil with a Flap
Experimental and computational conditions,
measured and computed Car and Cp, and flow field
analysis using CFD results for BLC hydrofoil using a
blown flap are described and discussed in this
section.
Based on the authors' experience and related
study (Kerwin et al., 1972), and considering marine
vehicles' operating conditions, two As, i.e., 0- and
10 degrees, and three C,u's' i.e., 0, 0.16 and 0.64, are
considered with one flap angle, &20 degrees.
Computational conditions are indicated by
combinations of numbers for ~x, 3, and C, hereafter,
i.e., (10,20,0.64) case indicates that oc=10 degrees,
~20 degrees, and CJU=0.64. Re based on the original
chord length, 0.25m, is l.91xlO5. Two non-zero C,~`'s
corresponding to the chord length can be reproduced
with mof 6.6 kg/s and 13.2 kg/s at the end of the
plenum, and Van =l m/s is imposed on the front and
side boundaries. No-slip condition is imposed on
solid surfaces, and zero-static-pressure condition on
exit boundary. The computational domain is C-
shaped with extent -2 < x/c < 5 and -4 < y/c < 4 .
. . . ~ _ . . .
Figure 12. Computational mesh for (10,20,0.64) case.
The computational mesh shown in Figure 12
is for (10,20,0.64) case and consists of 198,892
quadrilateral and triangle cells. Boundary layers are
placed around the solid surface with the first cell
spacing equal to approximately l in terms of wall y+.
Circular sub-domain around the hydrofoil is
generated and triangle cells fill the remaining region
of the sub-domain. The region outside this circular
sub-domain is also filled with appropriately growing
triangle cells using the sizing function capability
available in GAMBIT, a Fluent pre-processor.
Computational meshes for other cases, i.e., other As,
are generated simply by rotating the circular sub-
domain and remeshing the region outside the sub-
domain. In this way, mesh quality around the
hydrofoil is maintained even with different As.
-7.
0 0.0s of. 0.~5 0.2 0.2s
x (m)
Figure 13. Foil surface pressure coefficient for
(10,20,0.64) case obtained by k-m and RSTM models.
OCR for page 704
In order to have confidence in the turbulence
model selected and mesh quality, the influence of
turbulence models on the solution and mesh
dependence were investigated with (10,20,0.64) case.
As expected from the already discussed CC flow
results, both k-m and RSTM models perform equally
well for this type of flows. Car values show 2%
difference from each other, i.e., 4.33 (k-Go) and 4.24
(RSTM). Cp's on the hydrofoil surface are also close
to each other as shown in Figure 13. For BLC
hydrofoil using a blown flap, which is discussed in
the present section, therefore, only k-w model is
employed.
-1
-7.5 _
_ _ ~
//
lo/
I_
·,` . . . . . . . . . . . . . . . . .
-1V 0.05 0.1 0.15 0.2 0.25
x (m)
Figure 14. Foil surface pressure coefficients for
(10,20,0.64' case obtained on coarse and fine meshes.
Mesh dependence test was done using two
meshes, and computed results on the meshes were
compared. The fine mesh is shown and discussed
above. The coarse mesh is generated in the same
domain, but with larger first cell spacing, y+~30, and
less cell numbers, 46,002 cells. Figure 14 shows one
of the comparisons, i.e., Cp on the hydrofoil surface
for (10,20,0.64J case, where two curves show close
agreement. Car comparison between two solutions also
shows less than 3.7% difference, i.e., 4.33 (fine) and
4.17 (coarse). The fine mesh was used for
computations and presentations.
Table 3. Car for BLC cases.
.
Computed Measured
0.56
1.20
2.09
1.48
2.20
(0,20,0) 0.62+0.022
(0,20,0.16) 1.97
(0,20,0.64) 2.96
(10,20,0) 1.47+0.008
(10,20,0.16) 3.08
(10,20,0.64) 4.33 3.33
As for CC flow cases, both global and local
quantities are considered for analysis and discussion.
Cl's obtained from experiments and computations are
compared in Table 3. Note that computed C`'s for
(0,20,0J and (10,20,0) cases display unsteady
oscillations, which are unavoidable with 20 degree
flap angle, and are presented by mean values
plus/minus fluctuations. Measured Cats for the same
cases show mean values only. Unlike CC flow cases,
differences between computed and measured values
are large. Reasons of these large differences seem to
be (a) increased three-dimensional flow and tunnel
wall effect with jet flow and flapped geometry, and
(b) inconsistency in C,u due to inadequate jet ejection
system. Nevertheless, it is interesting to see that (a)
lift augmentation is more efficient at ~0 degree than
at o`=lO degrees, i.e., 4.8 VS. 2.95 times from C~=0 to
C~=0.64, (b) lift augmentation is larger between
COO and CFO.I6 than between C,~O.16 and
C'O.64, and (c) unsteady oscillations in lift are
removed by BLC.
{;ofiou~s clt T~t~ YE (~4 -I ~~ ~ ~ =
fLU~r6.~[,= ~~81" 8~. ~,
Figure 15. Turbulent eddy viscosity contours for
(0,20,0) case at a certain instant.
o
-2
4
-6
0 0.05 0.1 0.15 0.2 0.25
x (m)
l
l l l l
0~ ' I I LO O ~ O
_ ~ ~ it;, <) ~ ~ ~ ~
l l l l
1 1 1 1
t00006~ i' ,
r-----~---------~----------~---- ~
1 , Ill
, , ,
1 1 1 1
l l l Ill
1 1 1 1'
I 1 1 1 1
, , , 1 1
I I 1 1 1
_ 1 , I ~ —
O ' Masured ' ' | I
~ Computed ~ , l
l l ~ 1 l
1 1 1 \ l
,,,.i.,.,i,,,,i,,,~ i,...
Figure 16. Surface pressure coefficients for
(0,20,0.16) case.
The unsteadiness in (0,20,0) and (10,20,0)
cases are confirmed by transient mode computations
with time step size 0.001. Figure 15 shows turbulent
eddy viscosity contours for (0,20,0) case at a certain
instant where shed vortices are clearly displayed.
OCR for page 705
Removing unsteadiness in the flow field is one of the
advantages of using BLC for a flapped hydrofoil,
because high frequency unsteadiness is a main cause
of vibration, noise, and control problems.
2.5
Or
_ r
~7.5
. ~00000 100 ~
~ I I
IU ,,,,, ~ .,, . i,,,, 6,,, . ~ · .,,
0.05 0.1 0.15 0.2 0.25
x (m)
Figure 17. Surface pressure coefficients for
¢0,20,0.64) case.
2 _
o
-2
-6
1 ~ 1 1
~0 ' ~- 'I ' O O J
~ ;
. ;,0 ~ r I ~ T
1 1
1 1
1 1
u~
1
_ _ _ _ J _ _ _ _ _ _ _ _ _ _ _ _ .
i~ ~ M=
/ ~ Computed
. - , ,
,...~....~....~....~....
-8( 0.05 0.1 0.15 0.2 0. 25
x (m)
Figure 18. Surface pressure coefficient for
(10,20,0.16> case.
2.5
o
-2.
Sac
5
7 ~
1 ~ ~ 1
W00g 0 O.' '= 0~0
' ~ , o04
O Q 0 , oo ,/
,° / !
. ~ , ,
, , ~
. - i- ~ ~ M88~ j
/ , Computed ~
I
10 /,,,, i, ... ~ ., .. I ..........
~ 0 0.05 0.1 0.15 0.2 0. 25
x (m)
Figure 19. Surface pressure coefficient for
(10,20,0.64) case.
'
Cp comparisons on the hydrofoil surface are
presented in Figures 16 through 19. As for Car,
differences are larger than that shown in the
validations for CC flow cases. Suction side pressure
peaks and the overall pressure difference between
suction and pressure side are smaller in the measured
data. The differences between measured and
computed Cp's increase consistently with increasing
ax and Cal. Having seen the good agreement shown for
verification measurements and CC flow validations,
there appears to be non-negligible amount of
uncertainties and errors in the measurements,
especially the jet ejection system. More rigorous
verification and improvement of the measurement
system are under way.
Pa Lass =~ by Pr~ssuls coor~aort pir~g-7.4~ol ~ ~ Mer 29, 20132
FLUENTLY t~ `~t~ ache ~~
Figure 20. Pathlines colored by Cp for (0,10,0) case.
| Path Lines Id by Pressum (;O~QIlt led Mix) 20
~ FLU~T ~ ~ ~3. se~t" ,
Figure 21. Pathlines colored by Cp for (0,109O.16)
case.
Flow field alteration is another advantage of
employing BLC hydrofoil using a blown flap.
Figures 20 through 22 show the pathlines colored by
Cp around the hydrofoil. The undulating and
recirculating flow field due to the 20 degree deflected
flap is removed by BLC. Increasing circulation, i.e.,
shifting down jet flow in the wake, and larger
difference in suction and pressure side CptS7 which
augment lift, are evident. This also can help the wake
flow straighten up and eventually reduce noise and
signature.
OCR for page 706
n~cines=~ tyP~m =~8~ M8t29.~
. ___ ~T ~ O (= ~~ eta
Figure 22. Pathlines colored by Cp for (0, 20, 0.64J
case.
CONCLUDING REMARKS
A jet-controlled high-lift hydrofoil with a
flap is investigated using both experimental and
computational methods. The experiments are being
carried out in a cavitation tunnel to measure forces
and moment and surface pressure distribution on the
hydrofoil. The measured data, although not final,
prove the feasibility of such a device operating in
water flow. In addition to that, a substantial amount
of know-how has been accumulated in the course of
the experimental program, which would improve the
on-going experiments.
CFD studies have been carried out
simultaneously for the CC flow around a trailing
edge, and the present problem, i.e., BLC flow around
a hydrofoil with a blown flap. The computational
results are analyzed with global and local quantities
and validated using available experimental data. The
present computational method reproduces the well-
known experimental data for CC flow quite well. The
results for BLC hydrofoil flow using a blown flap
reinforce the validity of the technology. Also the
CFD study identifies the high frequency and small
amplitude unsteadiness in the cases of deflected flap
without blowing, closely capturing the flow field in
the wake modified due to the flap. Another important
finding from the present CFD study is that k-m model
performance is largely comparable to that of RSTM's
for some of the less severe cases.
The results of the study demonstrate the
applicability of the technology to the design of
practical control surfaces. Also it is found that the
device functions more efficiently at smaller ~ and Cal,
which is desirable especially for low-speed
maneuvering. However, some improvements are
needed. Jet ejection system needs to be improved so
that more reliable data can be obtained. Rigorous
uncertainty analysis is warranted for both
experimental and computational studies.
As for the future work, it would be interesting to
consider unsteady fluctuating jet ejection with less jet
momentum. The effects on propeller wake, where
usually control surfaces are located, and hull/control-
surface juncture flow should also be taken into
account.
ACKNOWLEDGMENT
The experimental portion of the present
study is being supported by RIMSE and the Brain
Korea 21 project.
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Representative terms from entire chapter:
aiaa paper
