Influence of Rotor Ingestion on Performance of S-shaped Inlet in Distributed Propulsion System

Journal Title Volume XX Issue XX, Month Year Journal Website

1

Influence of Rotor Ingestion on

Performance of S-shaped Inlet in

Distributed Propulsion System

School of Energy and Power Engineering, Beijing University of Aeronautics and Astronautics

XueYuan Road No.37, HaiDian District, BeiJing, China

Abstract

The distributed propulsion is a new type of aircraft

power system with fuselage/propulsion integration. In

order to study the influence of fan ingestion on

aerodynamic performance of the S-shaped inlet, a three

dimension model with fan ingestion and static pressure

outlet, was studied to discuss the influence of fan

ingestion on the aerodynamic performance of the inlet.

Unsteady simulation method was adopted to get the full

flow field of the test models. Results showed that as the

mass flow rate decreases, the distortion intensity

decreases. Contrast to the case with a static pressure

outlet condition, the case with fan ingestion could turn

the “swirl vortex” into “bulk vortex” as a result of the

difference of pressure in different regions. The response

of the fan to the distortion introduced the difference. So

the distortion intensity by fan ingestion is bigger than

that with a static pressure outlet condition.

Keywords

Distributed Propulsion; S-shaped inlet; Fan Ingestion;

Computational Fluid Dynamics; Aerodynamic Performance

Introduction

With the development of society, energy shortage

and pollution have been the problems the world

concerns. Aircrafts contribute to a lot for this by

fuel burns, exhaust emissions and noise

productions. Traditional propulsion system has

been limited in performance improvement such as

lift-drag ratio enhancement and fuel consumption

reduction. It is urgent to find a new way to meet the

increasingly needs of development. As a new

aircraft configuration, Blended Wing Body has been

introduced. In this concept, highly integrated

boundary layer ingesting(BLI), offset or S-shaped

inlet have the potential benefits of reducing weight,

size, drag by eliminating the boundary layer

diverter and shortening the inlet duct.

By using S-shaped inlet and boundary layer

ingestion, the flow is distortion when it gets the

Aerodynamic Interface Plane (hereinafter AIP).

Boundary layer near the low surface would be

transferred to the central area of the inlet as a result

of adverse pressure gradient along radial direction

[1-3]. And adverse pressure gradient along the flow

direction would cause flow separation close to the

low surface. As a result, there will be a “Swirl

vortex” on the AIP plane [4-5]. This is a big

challenge for the design of the Fan stage.

A search of open literatures revealed a lot of

simulations and experimental information on BLI S-

shaped inlet performance for inlets with large

amounts of BLI operating at realistic conditions.

Most reported in the literature by either numerical

simulation or experimental method, the direct

influence of the fan ingestion was not taken into

consideration. When numerical simulation method

is conducted, static pressure is placed at the outlet

location of the test schedules. The response of the

rotor to distortion will give different effects from

the case with a back pressure condition [6-9]. So

there is a need to make a study of the influence of

fan ingestion on the performance of S-shaped inlet.

The objective of the work is to make a better

understand of the interaction of the fan ingestion

and S-shaped inlet performance. To achieve this

goal, a single transonic fan rotor and S-shaped inlet

have been used. Unsteady simulation method is

used for the calculation. After analyzing the results

Journal Website Journal Title Volume * Issue *, Month Year

of computations, the mechanism of the interaction

can be obtained.

Test Schedules

The test schedules include an S-shaped inlet and a

transonic rotor. Their configurations are

demonstrated in FIG.1 and FIG.2. When operating

at 100% speed, the total pressure ratio of rotor is

1.42. Other key rotor parameters are presented in

Table 1.

Table.1 Key design parameters of the rotor

Number of rotor blades 23

Height to chord ratio 1.2

Solidity at hub 2.5

Nominal tip clearance of rotor blades 0.4

Tip rotating speed of the rotor blade 300

Load coefficient factor 0.37

FIG.1 S-SHAPED INLET MODEL

FIG.2 FAN ROTOR MODEL

Computational Approaches

Simulation model

The CFD solver employed in this work is code CFX

5.0. The turbulence model selected in the simulation

is two-equation k-e model. Near end-wall area,

scalable wall function was used. For unsteady

calculation, the time-step was one-tenth of the rotor

blade passing time, and 10 inner iterations were

performed each time to obtain converged solutions.

FIG.3 gives a detail description of the

computational domain.

FIG.3 COMPUTATIONAL DOMAIN

Total pressure and temperature are imposed at inlet

which are set to be Pin=P∞*PR, Tin=T∞ respectively

with P∞=36400 Pa and T∞=248 K. Factor PR is short

for total pressure recovery. The walls were

specified to be adiabatic with no-slip condition. The

stagnatione pressure at the outlet of the rotor was

set to the average pressure. In next section, we will

discuss the inlet boundary condition.

A. Definition of the inlet boundary condition

When flow develops along the up surface of the

BWB aircrafts, the boundary layer is assumed to be

turbulent. From the CFD results of the vehicle, the

boundary layer is about 26% of the inlet height. Its

total pressure is distributed as follows:

36400; 1.13917 0.09804( )

27812.5 33273; 0.09804 0.224796

m x mPt Pa

x m x m

Where number -0.09804 m stands for boundary line

point of the clean and distortion section. FIG.4

illustrates the plot of the total pressure distribution

at the inlet plane.

-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.426000

28000

30000

32000

34000

36000

38000

To

tal

Pre

ss

ure

(P

a)

Span (m)

Data From Team

FIG.4 DSTRIBUTION OF TOTAP PRESSURE AT INLET PLANE

Results and Discussion

In order to describe the intensity of distortion in S-

shaped inlet, the factor DC was introduced into the

analysis. The definition of distortion intensity was

shown as follows:


3

* *

*

360

*

0

*

,

1( ) ( ),

360

1( ) ( )

i

AV D

AV

AV i

D i

P PDc

P

P P d

P P d

Where the PAV* stands for area-average total

pressure of the whole plane while PD* stands for the

area-average total pressure of the distortion section.

In this paper, if the total pressure is below 32000 Pa,

it belongs to distortion section [8].

Overall performance at different mass flow rate

In order to compare the difference of performance

of S-shaped inlet under different outlet condition,

different cases with static pressure and fan

ingestion are calculated by CFX code. In this part of

the paper, Case 1 stands for the case with static

pressure and Case 2 stands for the case with fan

ingestion. Results are listed in FIG.5.

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.20.140

0.145

0.150

0.155

0.160

0.165

0.170

0.175

0.180

DC

Axial Position Normal

Case1-156kg/s

Case2-156kg/s

Case1-152kg/s

Case2-152kg/s

Case1-141kg/s

Case2-141kg/s

FIG.5 DC DISTRIBUTION ALONG FLOW DIRECTION AT

DIFFERENT MASS FLOW RATE

It can be seen clearly from the FIG.5, as mass flow

rates decrease, the DC decreases. DC of Case 1 is

lower than that of Case 2 at the same mass flow rate,

indicating that the flow distribution at the outlet

face of S-shaped inlet is serious when a fan rotor is

placed rear of the inlet. Also along the axial

direction of the inlet, the trends of DC factor are not

in linear change. From the curve in the plot, it can

be obtained that the curve is like a parabolic curve

with its opening down. Before 60% axial position,

the DC increases with the increment of axial

distance. After 60% axial position, the DC factor

value decreases contrarily. At the point 60% axial

position, the DC factor achieves the maximum

value. Next section, the difference of flow field will

be discussed under different outlet condition. Two

operating points are selected for analysis: high and

low mass flow rate.

Performance at high mass flow rate

In this part, high mass flow (mass=156 kg/s Ma=0.64)

was selected for analysis. FIG.6-FIG.8 show the

streamline distribution at three different axial

positions: 20%, 60% and 100% (AIP). The left

picture stands for inlet with stagnation pressure

(Case 1) and wright picture stands for inlet with fan

ingestion (Case 2). Results show that due to the

duct curvature, a cross-stream pressure gradient

with sub-atmosphere pressure exits along the low

wall of the inlet. At the 20% axial position, Case 1 is

familiar with Case 2. At the 60% axial position,

streamline of Case 1 is symmetry along the vertical

diameter, but for Case 2, the streamline is not

symmetry. At the AIP plane, it can be seen clearly

that in Case 1, a “Swirl Vortex” comes into being. In

Case 2, the “Swirl vortex” is transported into a

“bulk vortex”.

FIG.6 COMPARISON OF STREAMLINE AT 20% AXIAL

POSITION

FIG.7 COMPARISON OF STREAMLINE AT 60% AXIAL

POSITION


FIG.8 COMPARISON OF STREAMLINE AT AIP

Fig.9 and Fig.10 illustrate the limit streamlines on

the low surface of inlet. Contrast to Case 1, along

the rotating direction, one of the “Swirl Vortex” is

stretched and the other is compressed in Case 2.

The median line is not the symmetry line of the

swirl vortexes any more.

FIG.9 LIMIT STREAMLINE ON LOW SURFACE (CASE 1)

FIG.10 LIMIT STREAMLINE ON LOW SURFACE (CASE 2)

B. Performance at low mass flow rate

Like the analysis at high mass flow rate, the

streamlines distribution at three different axial

positions is demonstrated at low mass flow rate,

just as the pictures FIG.11-FIG.13 show.

FIG.11 COMPARISON OF STREAMLINE AT 20%AXIAL

POSITION

FIG.12 COMPARISON OF STREAMLINE AT 60%AXIAL

POSITION

FIG.13 COMPARISON OF STREAMLINE AT AIP

Just like high mass flow rate condition, along the

axial direction, the streamline is different. Contrast

to high mass rate condition, the “Swirl Vortex” is

not as serious as above in Case 1. The centre of the

vortexes is closer to the low surface of inlet,

meaning that the area and strength of the vortexes

at low mass flow rate condition are weaker than

that at high mass flow rate condition. This

phenomenon corresponds to the Dc distribution in

FIG.5. For Case 2, the centre of “bulk vortex” is

further away from the symmetry line (the yellow

line in FIG.13) at low mass flow condition than that

at high mass flow condition.

FIG.14 Limit streamline on low surface (Case 1)


5

FIG.15 Limit streamline on low surface (Case 2)

FIG.14-15 show the distribution of limit stream line

on the low wall of the inlet. In Case 2, along the

rotation direction, the level of stretching and

compression is more serious than that in Case1 and

Case 2 at high mass flow rate condition.

From above, a conclusion can be made that fan

interaction with the S-shaped inlet can make the

parameters more non-uniform at AIP plane. For

Case 1 and Case 2, the redistribution of the flow is

main factor that causes the different. When fan is

placed at the outlet of the S-shaped inlet, central

line of the vortexes is further away from the vertical

symmetry diameter of the AIP plane. The response

of the rotor to the distortion makes this difference.

In next section, the mechanism will be discussed.

Mechanism of Influence

FIG.16 and FIG.17 demonstrate the distribution of

the Vu normal along the vertical symmetry line at

AIP plane.

max

normal

VuVu

Vu

The Vumax above stands for the maximum value of

the Vu at the AIP plane at the low mass flow rate

condition.

Evidently, for Case 1, the parameters are symmetry

along the vertical diameter, so the Vu normal here

is almost 0. The distribution of the Vu at low mass

flow rate condition is familiar with that at high

mass flow rate. For Case 2, under the 35% height,

the Vu normal becomes different. As the low rate

decrease, the Vu normal close to low wall become

bigger compared to Case 1. This change induces the

deviation of vortex. Next we will analyse the

mechanism of the increase in Vu Normal.

-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-0.10.00.10.20.30.40.50.60.70.80.91.01.1

Sp

an

No

rma

l

Vu Normal

Case1_156kg/s

Case2_156kg/s

FIG.16 THE VU NORMAL DISTRIBUTION ALONG

SYMMETRY LINE (HIGH MASS FLOW RATE)

-0.10.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1-0.10.00.10.20.30.40.50.60.70.80.91.01.1

Sp

an

No

rma

l

Vu Normal

Case1_141kg/s

Case2_141kg/s

FIG.17 THE VU NORMAL DISTRIBUTION ALONG

SYMMETRY LINE (LOW MASS FLOW RATE)

FIG.18 demonstrates Vu contours at AIP plane from

the unsteady numerical computations at high mass

flow rate condition. Close to low wall section, right

side of plane has a high Vu while left side has low

Vu. The distribution of the Vu will promote the

vortex on the left side to be stretched. Vortex on the

right side will be compressed. As a result, the

centre of vortex is transferred to right section.

Contrast to Case 1, the value of Vu is bigger. So

some force may be produced by the response to the

distortion. What is the cause of the force? The

difference of static pressure on the plane may be the

factor of the change.


FIG.18 VU DISTRIBUTION AT AIP (LEFT: CASE1; RIGHT:

CASE 2)

FIG.19 shows the pressure contour at AIP plane. It

is obvious that flow structure is different. The high

pressure section in Case 2 is transferred to the left

side of the AIP plane contrast to that in Case 1. So

by the rotation of fan, the response of parts of the

circumferential blades to the distortion is different.

High pressure section will move in the adverse

orientation of the rotation. And in Case2, the

difference of the pressure in high section and low

section is bigger than Case 1. An extra force in

circumferential direction will produce. The flow

will be accelerated. But in left side of the plane, the

acceleration is bigger than that of right side. As a

result of this, the symmetry line move towards the

right. The “swirl vortex” turns into a “bulk vortex”.

FIG.19 PRESSURE DISTRIBUTION AT AIP(LEFT: CASE1;

RIGHT: CASE 2)

Conclusions

The purpose of this work was to explore the

influence of fan ingestion on the performance of S-

Shaped inlet. To achieve this goal, unsteady

numerical computations have been used to capture

the fundamental characteristics of S-inlet flows like

the separation etc. By analysing the flow field, we

can draw the conclusions as follows:

1. The Dc (distortion intensity) decreases with

mass flow rate decreases and Dc with fan

ingestion is more serious than that with

stagnation pressure at the outlet AIP plane;

2. Under the ingestion of fan, at AIP plane, the

“Swirl vortex” is transformed into “bulk

vortex”. The symmetry line is transferred

along the rotation direction;

3. Change of the stagnation pressure gradient

along circumferential direction is the cause of

transform of the vortex structure at AIP plane.

Fan response to distortion is different when

rotor blades enter and leave the distortion

section.

References

[1] Anabtawi, Amer J.; Blackwelder, Ron; Liebeck,

Robert; and Lissaman, Peter: Experimental Investigation

of Boundary Layer Ingesting Diffusers of a Semi-Circular

Cross Section. AIAA-98-0945, 1998.

[2] Anabtawi, Amer J.; Blackwelder, Ron F.; Lissaman, Peter

B.S.; and Liebeck, Robert H.: An Experimental

Investigation of Boundary Layer Ingestion in a Diffusing

S-Duct With and Without Passive Flow Control. AIAA

99-0739, 1999.

[3] A. P. Plas, M. A. Sargeant, V. Madani, D. Crichton, E. M.

Greitzer,T. P. Hynes, C. A. Hall**; Performance of a

Boundary Layer Ingesting (BLI) Propulsion System,

AIAA 2007-450.

[4] Gorton, S. A., Owens, L. R., Jenkins, L. N., Allan, B. G.,

and Schuster, E. P., \Active Flow Control on a Boundary-

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[5] Lie beck, R.H.; Page, M.A.; and Rawdon, B.K.: Blended-

Wing-Body Subsonic Commercial Transport. AIAA 98-

0438, Jan. 1998.

[6] Roman,D.; Allen,J.B., Liebeck, R.H., Aerodynamic

Design Challenges of the Blended-Wing-Body Subsonic

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[7] Steven R. Wellborn, etc. Study of the compressible Flow

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[8] Yanfeng Zhang, Wuli Chu, Xingen Lu, Numerical


7

Simulation of the Effect of Inlet Distortion to the

Subsonic Axial-Flow Compressor, AIAA 2008-77.

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2009-04.(In chinese)

Influence of Rotor Ingestion on Performance of S-shaped Inlet in Distributed Propulsion System

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