Effects of Rib Turbulator on Distortion Index of a
Serpentine Inlet Duct
S. Pradeep1, H. K. Narahari2, Shiva Kumar 3
1- (Engg) student, 2-Professor and Dean, 3-Ph.D student (AAE)
Aircraft and Automotive Engineering
M.S.Ramaiah School of Advanced Studies, Bangalore
Abstract
Stealth capabilities are a major requirement of modern combat aircraft both manned and unmanned. The major contributor
to radar cross section is reflection from the engine face. The fan face can be hidden from radar by developing an inlet duct that
is offset so that there is no direct reflection of radar beam from engine face. However, this complex flow path leads to secondary
flows in the duct, which in turn affect flow field at engine face. Engine performance is adversely by such heavily distorted flow
field. Hence it is essential to understand the flow behaviour and adopt different flow control (active and passive) methods to
reduce distortion. Current study investigates the effects of Rib Turbulator inside the Serpentine Duct and their effect on
Distortion Index. The computational study has been carried out on baseline serpentine duct to know the flow behaviour and then
Rib Turbulator is added to the structure to investigate its effects on the flow behaviour. With the Rib Turbulator installed in the
duct, an improvement in static pressure recovery was observed with decreased pressure loss by 21.67% and the reduction in
distortion values by 58.69% were observed. Performance at AOA of 2o and 4o are also calculated where pressure loss shows a
downtrend by increasing to 265% and 100% respectively and distortion values shows an uptrend by decreasing to 63.58% and
53.56% respectively.
Nomenclature
Symbol
Cp
P
T

Abbreviations
AOA
CAD
CFD
Pt, inf
Pt,ef
qinf
P,min 60
60 deg wedge
q ef,avg
engine face
SST
UAV
VG
RT
-
Description
Unit
Co-eff of Pressure
Pressure
Temperature
Density
-Pa
K
kg/m3
Angle of Attack
Computer Aided Design
Computational Fluid Dynamics
Total Pressure at the inlet
Total Pressure at the engine face
Dynamic Pressure entering the duct
Min Averaged Total Pressure at any
depends on the quality of air delivered by the duct. At
higher speeds, it is necessary to deliver the air with
minimum pressure loss and less distortion at the engine
face and at low velocity and high pressure, for which
various types of inlet ducts are available. An important
function of inlet duct is to deliver air to compressor in
entire flight envelope with minimum amount of
pressure drop. The stealth capabilities can be achieved
by different methods like shaping the aircraft outer
contours, radar absorbing paints, small and canted
vertical tail etc. A major contribution to RCS is from
reflections coming from engine face. Reduction of this
can be achieved by suitably modifying the inlet duct. A
‘serpentine’ inlet duct ensures that radar signals are not
reflected back to the source.
Area Averaged Dynamic Pressure at
Shear stress transport
Unmanned Aerial Vehicle
Vortex Generators
Rib Turbulator
1. INTRODUCTION
Inlet duct of the gas turbine engine is one of its
main components is normally considered as the part of
the airframe. The task of an intake is to transport the air
mass flow required by the engine with as little total
pressure losses as possible to the engine inlet. They are
mainly used to diffuse the flow with less distortion at
the engine face. The performance of the engine also
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.
Fig 1.Serpentine Duct [2]
Thus the stealth requirements in modern day
aircraft and UAVs have led to compact S-shaped inlet
ducts commonly called as serpentine ducts [1-7], which
often have variations in cross section [8] and curved
centrelines, these complex shaped ducts will lead to
strong secondary flow structures, which in turn led to
the reduced pressure recovery and increased distortion
at the engine face. This can mean detrimental effects on
the engine performance as well as the engine life.
Figure 1 shows a schematic diagram of serpentine duct.
The serpentine ducts or S-ducts having complex
mean line, offsets the intake plane from the engine face
which leads to complex three dimensional flows
developed in the duct, such as flow separation and
swirl. The secondary flows developed in the duct
will adversely affect the performance of aircraft engine
and much more critical during different manoeuvres.
Therefore the compromise has to be made between
the low radar visibility and other performance
attributes.
Different flow control techniques such as
using passive devices like different types of Vortex
Generators (VG), Rib Turbulator (RT), or by proper
contouring of the duct surface have been explored to
minimise the secondary flow. Use of vortex generators,
which mix the high energy core flow with the low
energy boundary flow, have been studied [6, 7].
However, Vortex Generators showed limited
effectiveness, hence in this thesis an attempt has been
made to use RT in place of vortex generators and to
investigate its effect on the flow and distortion index at
the face of engine. RT are protrusions placed in a
controlled way along the walls and are commonly used
to improve heat transfer in gas turbine engine blades
[5].
In a serpentine duct complex shape of mean line
generates two counter rotating vortices at engine face
and increase the flow distortion with pressure loss. An
effort is also made to understand the flow details at
different angles in order to look for any adverse effect
of RTs.
2. GEOMETRIC MODEL
The entire geometry as shown in figure 2 was
reconstructed in CATIA V5 R14 software as shown in
Fig.3 and the geometric specifications are mentioned in
the Table 1.
The flow path of interest in this study is an ultracompact serpentine inlet duct designed by Lockheed
Martin Aeronautics Company. The inlet is identical to
the test article of the research conducted by [1, 4]. The
duct features a length-to diameter ratio of 2.5 and
consists of a 4:1 aspect ratio, bi-convex entrance section
followed by a dual-turn offset section and a diffusing,
elliptical-to-circular exit section and a length of 63.5 cm
and an exit diameter of 25.4 cm.
2
Fig 2. Three views of the Inlet duct
Specifications
Length
Inlet Minor Diameter
Inlet Major Diameter
Outlet diameter
0.63 m
0.124 m
0.49 m
0.254 m
Table 1. Geometric Details of the Serpentine Inlet
Duct
Fig 3. CAD model of Serpentine Inlet Duct
RTs were modelled inside the duct at a distance of
one radii of the minor axis of the inlet from the flow
separation point, the minor axis radius is used for the
height ratio of the RT.
Fig 4. Rib Turbulator modelling in the
Serpentine Duct
Specifications
Rib width
Rib Thickness
0.096 m
0.0186 m
Table 2. Geometric Details of the Serpentine Inlet
Duct
2.1 Discretization of the Model
A multi block grid was created by ANSYS ICEM
CFD Software. Figure 5 shows the discretised model of
the serpentine duct, the mesh was kept fine at the first
and 2nd bend and near the walls regions which were the
critical regions were the study of boundary layer
separation and generation of vortices was of the main
interest. The inlet duct with the RT was also discretised
in the same manner as of the baseline duct, but the block
was split near each of the RT to have a grid control on
the RT.
This SST k– ω turbulence model is a two-equation
eddy-viscosity model. The use of a k-ω formulation in
the inner parts of the boundary layer makes the model
directly usable all the way down to the wall through the
viscous sub-layer, hence the SST k-ω model can be
used as a Low-Re turbulence model without any extra
damping functions. The SST formulation also switches
to a k-ε behaviour in the free-stream and thereby avoids
the common k-ω problem that the model is too sensitive
to the inlet free-stream turbulence properties. In
addition, SST k-ω model often shows good behaviour
in adverse pressure gradients and separating flow [9].
Table 3 shows the parameters at the boundary
conditions.
Parameters
Inlet Total Pressure, Pa
Inlet Static Pressure, Pa
Inlet Total Temperature, K
Exit Static Pressure, Pa
Exit Static Temperature, K
Value
99991.78
97313.02
298.15
97889.49
296.37
Table 3. Parameters at Boundary Conditions and
their values
3. GRID INDEPENDENCE STUDY
Fig 5. Discretized model of a Serpentine Inlet
Duct
2.2 Boundary Conditions
The SST k– ω turbulence model was used along
with the boundary conditions shown in figure 6. Also a
stationary no-slip wall is considered for calculations.
The results from numerical analysis are sensitive to
number of grid points, so great care needs to be taken
in preventing the results dependent on number of grids.
For the current problem, initially a coarse mesh with
1032952 cells was used to do the flow analysis. The cell
size was subsequently increased to 1491658 and
2024365 cells. Figure 7 shows the grid independence
with respect to Cp loss.
Figure 6 shows different part of the duct where boundary
conditions are applied.
Fig 7. Grid independence study
The distortion value and the pressure loss values
for 1491658 and 2024365 did not differ significantly,
so considering the available resources with the accuracy
of the results required, the G2 grid is considered as final
mesh for the further computations.
To ensure the results grid independence study has
been carried out for the duct with RT.
Fig 6. Different part of duct where Boundary
Conditions are applied
3
Fig 9. Contours of Velocity near 1st and 2nd
Separation Point and at Rib Turbulators
Fig 8. Comparison of Cp contours obtained on three
grids G1, G2 and G3
Where in the duct with the RT, there are no reverse
flows either near the first bend or at the second bend,
indicating that there are no flow separations inside the
duct.
Figure 8 shows the Cp Loss comparison. From the
comparison we can find that the counter rotating
vortices have almost disappeared in all 3 grids and the
incorporation of RT successfully managed to suppress
the counter rotating vortices at the engine inlet.
4. RESULTS AND DISCUSSIONS
4. 1 Effect of RT
The computational results obtained for the duct
with RT is compared with the baseline results to know
the effects of RT on the flow structure. Figure 9 shows
the comparison for velocity along the axial direction. In
the figure 9 the flow separation near the second bend
can be seen for the baseline duct, where there are
reverse flows leading to flow separation which are
shown as the path lines in the Figure 9(a).
Fig 10. Contours of Vorticity Magnitude for Baseline,
and with the RT
Figure 10 compares Vorticity magnitude of
Baseline duct and duct with RT
Figure 11 shows the contour plots of pressure loss
at the engine face, in figure 11 (a) pair of large counter
rotating vortices can be seen at the top of the duct
indicating the flow separation, whereas in figure 11 (b)
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the duct with RT does not show any such vortices,
which is a clear indication that there is no flow
separation in the duct, the vortices which were
generated in the duct because of the bend are counter
acted by the RT and the reduction in Cp loss and
distortion values at the engine face in terms of
percentage compared to baseline are listed below.
Fig 12. Static Pressure Plots for Baseline, and with
the RT for Lower and upper points
Table 4. Listing of the Reduction in the Cp and
Distortion Compared to Baseline
4.2 Performance Comparisons at different Angle of
Attack
A well designed inlet duct straightens the flow at
the engine face irrespective of angle of attack of the
aircraft. Computations were done at moderate angles of
attack in order to verify if RT had any adverse effect.
AOA values chosen are 2o and 4o. Simulations are
carried out for these cases by incorporating changes in
velocity components due to different AOA.
AOA = 2o
Fig 11. Pressure Loss Contours at the Engine Face
The performance of the duct was compared in
terms of Cp loss, distortion and static pressure plots
which are defined as follows.
 Cp loss = ((Pt,inf - pt, ef)/qinf)*100
 Distortion (DC60) = DC 60 =((Pt,ef - Pmin
60,avg)/qef,avg)*100
Figure 12 shows static pressure plots on chosen
lower and upper points. Here plots of baseline duct and
duct with RT are compared. We can observe that there
are fluctuations in pressure value near the placement of
Ribs for duct with RT.
Figure 13 shows comparison of Total Pressure at
Engine Face for Baseline duct and Duct with RT at
angle of attack 2deg. It can be seen that there is a very
good decrease in the distortion values in case of Duct
with RT.
Fig 13. Total Pressure at Engine Face, AOA=2o
For angle of attack of 2deg, the distortion value at
Engine Face in percentage are 70.24 and 25.58 for
Baseline duct and Duct with RT respectively. Thus the
net improvement calculated is 63.58%.
AOA = 4o
Figure 14 shows comparison of Total Pressure at
Engine Face for Baseline duct and Duct with RT at
angle of attack 4deg. It can be seen that there is a very
good decrease in the distortion values in case of Duct
with RT.
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Fig 14. Total Pressure at Engine Face, AOA=4deg
Fig 16. Static Pressure Comparison for Baseline and
with RT at different AOA (Upper Points)
5. CONCLUSIONS
The RT were incorporated in the duct to know
its effect on the flow properties such as distortion,,Cp
loss and at various angles of attack.

The baseline results for distortion and Cp loss show
the same trend as of the experimental results and
results obtained from literature as shown in Table 4.
However, there were some differences in the Cp
Loss and distortion index values as compared to
experimental results

The RT were effective in preventing the flow
separation.

They were effective in controlling the pressure
distortion, a 58.69% reduction at engine face is
observed with the RT installed with no AOA.

They were also effective in controlling the pressure
loss at the engine face, about 21.67% of reduction
in Cp pressure loss was observed with the RT
installed with no AOA.

It was observed that the RT installations tend to
distribute the low energy flow uniformly around the
inside periphery of the engine face, leaving a high
energy core flow.

It can be seen that RT have negligible effect on
distortion at engine face at small AOA of 2deg and
4deg.
Fig 15. Static Pressure Comparison for Baseline and
with RT at different AOA (Lower Points)
Figures 15and 16 shows static pressure plots on
chosen lower and upper points. Here plots of baseline
duct at AOA=0o, 2o, 4o and duct with RT at AOA=2o,
4o are compared. We can observe that these plots
closely match for all chosen AOA.
REFERENCES
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'Numerical and experimental investigation of a
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2009
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