1

ISABE-2015-20020

Influence of Secondary Flow within Integrated Engine Inlets on the Performance and Stability of a Jet Engine

Rudolf P. M. Rademakers, Stefan Bindl, Reinhard Niehuis

University of the German Federal Armed Forces Munich, Germany

Institute of Jet Propulsion

ruud.rademakers@unibw.de

Abstract

Engine inlet distortion can have a significant influence

on the performance, stability, and durability of a jet en-

gine. Total pressure distortion has been under investi-

gation for decades. The consideration of inlet swirl dis-

tortion is relatively rare within this field of research, but

especially the interactions between both pressure and

swirl distortion and subsequent influences on the pro-

pulsion system are not truly understood.

Direct-connect experiments were conducted at the en-

gine test bed of the Institute of Jet Propulsion. Distor-

tion screens and a delta-wing were integrated within the

engine inlet system to generate distortion patterns as

they typically occur within s-duct engine inlets. The in-

fluences of pressure, twin-swirl, and combined pres-

sure-swirl distortions on both the performance and the

stability of a jet engine were analyzed. In this paper,

performance and stability are mainly assessed by means

of specific fuel consumption and surge margin of the

low pressure compressor, respectively.

A clear linear relation between inlet distortion and both

the engine’s performance and stability is recognizable

if solely the pressure distortion is considered. Depend-

ing on the operating point a twin-swirl distortion can

have a positive or negative influence on the stability,

however, does not have a significant influence on the

performance of the propulsion system. Assessment of

combined pressure-swirl distortion is only possible to a

limited extent since dedicated distortion descriptors are

not available. An additional twin-swirl enhances the

stability margin by improving the total pressure distri-

bution within the engine intake plane. On the other

hand engine performance is not significantly influenced

by adding twin-swirl to an existing pressure distortion.

Nomenclature

Symbols

𝐴𝑒𝑓𝑓 [−] effective cross-sectional intake area

𝐴𝑂𝐴 [°] angle of attack

𝑑 [𝑚] diameter of the engine’s intake plane

�̇� [ / 𝑘𝑔

𝑠] corrected engine mass flow

𝑁𝐿𝑐 [%] corrected relative spool speed (LPC)

p𝑇(1,21) [−] inlet-compressor pressure ratio

Δp𝑇,𝐴𝐼𝑃 [%] total pressure loss

𝑆𝐹𝐶 [ / 𝑔

𝑘𝑁𝑠] specific fuel consumption

𝑆𝑀𝐿𝑃𝐶 [%] surge margin of the LPC

Abbreviations

AIP Aerodynamic Interface Plane

AIR Aerospace Information Report

ARP Aerospace Recommended Practice

EOP Engine Operating Point

LPC Low Pressure Compressor

SAE Society of Automotive Engineers

1. Introduction

Pressure and swirl distortion typically occur within a

bent inlet of an integrated airframe-propulsion system.

Such distortions influence the performance, stability,

and durability of the propulsion system.

A lot of knowledge was gained during the last decades

with respect to inlet total pressure distortion. The SAE

Aerospace Information Report (AIR) 1419 (2013) is a

well-known document summarizing this knowledge,

which finally led to the Aerospace Recommended Prac-

tices (ARP) 1420 (2002) being universally accepted for

the assessment of inlet pressure distortion. The consid-

eration of swirl distortion is becoming more important

due to increasing demands on integrated airframe-pro-

pulsion systems. The SAE addressed this by the intro-

duction of the AIR 5686 (2010) document. It summa-

rizes the current knowledge of different kinds of swirl

distortion and their influence on the stability of the

downstream propulsion system. The interactions be-

tween both pressure and swirl distortion and the result-

ant influence on the propulsion system is not suffi-

ciently understood and accordingly indicated in the

SAE AIR 5686 (2010) as a major subject for future re-

search. Much work has yet to be conducted before an

ARP, based on the AIR 5686, can be issued.

2

Figure 1. Larzac 04 test vehicle

Engine tests with twin-swirl inlet distortion were pre-

sented by e.g. Pazur and Fottner (1991) and experi-

ments with a modified JT15D-1 engine and a distortion

screen by e.g. Lucas et al. (2014). Nevertheless, to the

authors knowledge this is the first publication present-

ing experimental investigations with a wide range of

pressure, twin-swirl, and combined pressure-swirl dis-

tortion patterns and an assessment of their influence on

both the performance and stability of a jet engine. The

results are meant to contribute to the establishment of

an ARP, which relates to the AIR5686.

2. Experimental set-up

2.1 Test vehicle

The Larzac 04 C5 jet engine (see Fig. 1) has an exten-

sive instrumentation in its two-stage low pressure com-

pressor (LPC) and is thus well-suited for investigations

regarding inlet flow distortions. The experimental set-

up is schematically displayed in Fig. 2 and described in

the following.

2.2 Airmeter

Probes to measure total (𝑝𝑇1) and static pressure (𝑝1) as

well as total temperature (𝑇𝑇1) are installed in the air-

meter (see Fig. 2, no. 1) to determine the engine mass

flow. The probes are installed in a sufficient distance

upstream of the distortion generators such that up-

stream propagating flow phenomena cannot influence

the engine mass flow measurement.

2.3 Distortion generators

A housing for the installation of arbitrary distortion

screens (see Fig. 2, no. 2) is positioned about 3 ∙ 𝑑 up-

stream of the compressor system to generate a pressure

Figure 2. Experimental Test set-up

distortion. This position is adequate according to Bailey

and O’Brien (2013) since a damping or mixture of the

distortion is not expected to occur. Six different screen

configurations (see Fig. 4) were designed for the exper-

iments presented here. The screens were designed to

evoke different kinds of pressure distortions, which

typically occur in bent engine inlet configurations. Ta-

ble 1 gives more detailed information about the geom-

etry of the screens.

A delta-wing (see Fig. 2, no. 3) generates a counter ro-

tating twin-swirl distortion as it typically occurs in en-

gine inlet ducts with an s-bend in the vertical plane.

This device is installed about 2.5 ∙ 𝑑 upstream of the

compressor system. The wing has a leading edge sweep

of 60° and a wingspan of approximately 0.8 ∙ 𝑑. The

angle of attack (𝐴𝑂𝐴) of the wing can be repositioned

to alter the intensity of the vortices.

Beale et al. (2002) described and recommended both

devices, which have been utilized for current experi-

mental investigations. Nevertheless, the assessment of

phenomena such as side-winds, which can turn a twin-

swirl within an s-duct into a bulk swirl, are not covered

with the described set-up. If swirl is mentioned in the

following it always concerns a twin-swirl as it is evoked

by the delta-wing.

2.4 Traversable measurement rake

A measurement rake (see Fig. 2 no. 4) is installed be-

tween the distortion generators and the compressor sys-

tem. The rake consists of eight five-hole probes in-

stalled in equal distance to each other along the rake

and moreover, the rake can be displaced in radial and

circumferential direction. Hence, flow data can be ob-

tained for a large distribution of measurement positions

by displacing the rake. This measurement plane is po-

sitioned about 1.0 ∙ 𝑑 upstream of the LPC and is called

Aerodynamic Interface Plane (AIP) in the following.

3

(1)

Figure 3. Positioning of the five-hole probes

The traversing procedure for an optimal pressure as

well as swirl distortion evaluation was defined using an

in-house tool. It was decided to measure 144 positions

within the AIP as it is schematically shown in Fig. 3.

According to Rademakers et al. (2014) this results in

acceptable errors for the evaluated distortion de-

scriptors while the engine operating time is limited to a

minimum.

2.5 Engine instrumentation

Extensive instrumentation is integrated in the Larzac 04

test vehicle. In the following it is solely the instrumen-

tation within the LPC (see Fig. 2 no. 5) outlined, which

was applied during current investigations.

Three vanes within the first stator stage and two vanes

within the first row of the second stator stage are

equipped with pitot probes. The probes are integrated

in the leading edge of the stator (seven probes at the

vanes of the first stator stage and five probes at the

vanes of the first row of the second stator) and tangen-

tially aligned with the chord of the stator’s profile at the

leading edge. The position of the probes in radial direc-

tion was set in such way that they cover circular rings

with equal areas. A rake with six pitot probes is in-

stalled behind the second row of the second stator stage

for the determination of the total pressure at the exit of

the LPC (𝑝𝑇21). The latter can be resolved with a single

rake. This was assured after a comparison of pressure

measurements with the stator-instrumentation and the

𝑝𝑇21-probe, which proved that any inlet distortion is

evenly spread over the cross-sectional area of the sec-

ond stator stage.

2.6 Bypass nozzle aperture

The unmixed nozzle configuration is an important fea-

ture of the Larzac 04 jet engine for the investigations

presented here. It enables an independent throttling of

both the core and the bypass flow. The bypass nozzle

aperture (see Fig. 2 no. 6) can reduce the cross-sectional

area of the nozzle till 10% of the design area to force

the LPC into stall. In case that surge occurs an emer-

gency shut-off can open the throttling device instantly.

Further details of the controlling and operating mode of

this bypass throttle are specified by Höss et al. (1998).

3. Definitions

3.1 Distortion descriptors

The pressure loss coefficient

∆𝑝𝑇,𝐴𝐼𝑃 = (𝑝𝑇1 − 𝑝𝑇,𝐴𝐼𝑃̅̅ ̅̅ ̅̅ ̅

𝑝𝑇1

) ∙ 100%

is used to illustrate the distortion patterns. It indicates

the percentage of surface averaged total pressure loss

within the AIP related to the total pressure being meas-

ured in the airmeter. This parameter is often used for a

preliminary characterization of an inlet flow distortion.

A broad spectrum of distortion descriptors can be found

in the open literature. The descriptors being described

in SAE ARP 1420 (2002) are well-established for the

evaluation of pressure distortion and their robustness

has been verified during numerous experimental inves-

tigations, which are summarized in SAE AIR 1419

(2013).

Bouldin and Sheoran (2002) proposed descriptors for

the evaluation of a pure swirl distortion. They use a

similar approach as it was used for the pressure distor-

tion descriptors in the SAE ARP1420. These swirl de-

scriptors are applicable to characterize a wide range of

swirl patterns.

Nevertheless, none of the previously mentioned para-

meters is truly dedicated to evaluate the flow within

highly bent engine inlet systems because complex in-

teractions between pressure and swirl distortion occur

in such ducts. It is not sufficient to analyze both pres-

sure and swirl distortion separately. In the AIR5686

document the SAE calls on the industry to deepen

knowledge for the establishment of an ARP with re-

spect to swirl and pressure-swirl distortion assessment.

4

(2)

(4)

(5)

(3)

Case Screen

size

Screen

blockage

𝑨𝒆𝒇𝒇

[-]

𝑨𝑶𝑨

[°]

Sym-

bol

0 n/a n/a 1 0°

0s1 n/a n/a 1 12°

0s2 n/a n/a 1 24°

1 120° (r) 42% 0.895 0°

2 80° (r) 63% 0.895 0°

3 135° 42% 0.843 0°

3s1 135° 42% 0.843 12°

4 120° (r) 63% 0.843 0°

4s1 120° (r) 63% 0.843 12°

5 90° 63% 0.843 0°

5s1 90° 63% 0.843 12°

5s1 90° 63% 0.843 24°

6 135° 63% 0.764 0°

6s1 135° 63% 0.764 12°

6s2 135° 63% 0.764 24° Table 1. Overview of all test cases

With the investigations presented in this paper it is

meant to contribute to the establishment of an ARP,

which is associated with to the AIR5686.

3.2 Engine parameters

Direct-connect experiments with a jet engine enable an

assessment of the engine’s behaviour without putting

the focus on solely the distortion pattern. Several pa-

rameters are introduced to evaluate both stability and

performance of the test vehicle in the following.

The determined engine mass flow is corrected with the

inlet conditions in the airmeter and furthermore the

pressure and temperature at International Standard At-

mosphere conditions. In the following this corrected

engine mass flow

�̇�𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 = �̇�𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 ∙ √𝑇𝑇1

𝑝𝑇1

∙𝑝𝐼𝑆𝐴

√𝑇𝐼𝑆𝐴

is denoted as �̇� in the following.

The inlet-compressor pressure ratio

𝑝𝑇(1,21) = 𝑝𝑇21

𝑝𝑇1

puts the pressure downstream of the LPC (𝑝𝑇,21) in re-

lation to 𝑝𝑇1 and thus considers the inlet and the LPC

Figure 4. Distortion screens

as a single system to enable a comparison of all test

cases in a single compressor performance map (see e.g.

Fig. 6 or 7). The respective inlet-compressor system is

indicated with a black dashed box in Fig. 2.

The surge margin

𝑆𝑀𝐿𝑃𝐶 =

√[(𝑝𝑇(1,21))𝑆𝑀 − (𝑝𝑇(1,21))𝑂𝐿]2

+ [�̇�𝑂𝐿 − �̇�𝑆𝑀]2

describes the stability of the LPC as function of both

𝑝𝑇(1,21) and �̇� at both the operating line (OL) and the

surge margin (SM). This is best practice since the in-

vestigated compressor maps encompass relatively flat

speed lines (see e.g. Fig. 6 or 7) and secondly, differing

gradients of the speed lines are included in the perfor-

mance assessment. The surge margin of the reference

case (Case 0) is set to 𝑆𝑀𝐿𝑃𝐶 = 100% and all other test

cases were set in relation to this reference case.

The specific fuel consumption

𝑆𝐹𝐶 = �̇�𝑓𝑢𝑒𝑙

𝐹𝑇ℎ𝑟𝑢𝑠𝑡

is applied to evaluate the efficiency of the entire pro-

pulsion system.

4. Test cases

In Tab. 1 all 15 test cases under consideration here are

summarized. For the reference case (Case 0) a distor-

tion screen was not installed and the delta-wing was set

to 𝐴𝑂𝐴 = 0°. Six different screens were designed to

5

Figure 5. Pressure distortion patterns within the

AIP for all test cases at NLc = 90%

simulate distortion patterns as they are likely to occur

in highly bent engine inlet ducts. The screens are sche-

matically displayed in Fig. 4 and geometrical properties

are summarized in Tab. 1. The screens in Cases 1, 2,

Figure 6. Pressure distortion configurations within

the LPC performance map at 𝑵𝑳𝒄 = 𝟕𝟔%

Figure 7. Pressure distortion configurations within

the LPC performance map at 𝑵𝑳𝒄 = 𝟗𝟎%

and 4 cover a reduced area in radial direction, which is

denoted with (r) in Tab. 1. The delta-wing was utilized

to add a twin-swirl to the inlet flow. According to

Genssler et al. (1987) this delta-wing at 𝐴𝑂𝐴 = 12°

evokes a moderate twin-swirl representing the swirl

within an s-duct with modest bends. A strong swirl rep-

resenting the upper maximum of swirl distortion, which

could occur in a highly bent s-duct is simulated by the

delta-wing at 𝐴𝑂𝐴 = 24°.

Experiments were conducted at the spool speed settings

𝑁𝐿𝑐 = 76% and 90%. 𝑁𝐿𝑐 = 90% is the design oper-

ating point of the LPC as it was depicted by Stößel et

al. (2013) and thus this engine operating point (EOP)

was preferred for current investigations instead of the

engine’s full thrust setting.

6

Figure 8. Compressor stability with pressure

distortion at 𝑵𝑳𝒄 = 𝟕𝟔%

Figure 9. Compressor stability with pressure

distortion at 𝑵𝑳𝒄 = 𝟗𝟎%

5. Results

5.1 Visualization of the test cases

The distortion pattern of each test case at 𝑁𝐿𝑐 = 90%

is displayed in Fig. 5 by means of the pressure loss co-

efficient. The distortion patterns do not change qualita-

tively over the operating range of the test vehicle as it

was reported by Rademakers et al. (2015).

Seven plots on the left hand side display all cases with

the delta-wing set to 𝐴𝑂𝐴 = 0°. A slight pressure dis-

tortion due to the installation of the delta-wing is still

visible in the center of the AIP. For five cases (shown

in middle of Fig. 5) the delta-wing at 𝐴𝑂𝐴 = 12° gen-

erates a moderate twin-swirl distortion, which super-

poses the pressure distortion. These cases are indicated

with the designation „s1”. For three test cases shown on

the right hand side the delta-wing was set to 𝐴𝑂𝐴 =

24°. Latter cases are labelled with „s2”.

5.2 Influence of pressure distortion on the stability

and the performance of a jet engine

This subsection only evaluates the influence of pressure

distortion and thus all test cases with the designation

„s1” and „s2” are not considered. A first visualization

of the influences of all pressure distortion patterns on

the stability and performance of the LPC at 𝑁𝐿𝑐 =

76% and 90% is shown in the performance maps in

Fig. 6 and 7, respectively.

The colored speed lines depict the time-variant unfil-

tered data during throttling of the bypass nozzle while

the spool speed of the LPC was kept constant. The sym-

bols indicate the new surge line for the respective case.

The influence of pressure distortion on both the pres-

sure ratio and the surge margin in all six cases is clearly

visible for both EOPs. The operating point 𝑁𝐿𝑐 = 90%

is the design point of the compressor system and for that

reason more sensitive to inlet distortion. Hence, the

speed lines are spread over a larger area within the map.

Stability and performance are evaluated individually in

the following by means of 𝑆𝑀𝐿𝑃𝐶 and 𝑆𝐹𝐶, respec-

tively, for a proper assessment of both characteristics.

5.2.1 Engine stability assessment

Figure 8 and 9 show the 𝑆𝑀𝐿𝑃𝐶 as function of the mean

pressure distortion within the AIP for seven test cases

at 𝑁𝐿𝑐 = 76% and 90%, respectively.

There is a linear relation recognizable between both pa-

rameters. The same relation is distinguishable if other

pressure distortion descriptors (e.g. the descriptors

from the SAE ARP 1420) are applied for assessment,

see Rademakers et al. (2015). This states the robustness

of those distortion descriptors and enables the para-

metrization of pressure distortion patterns occurring

within s-shaped engine inlets.

While assessing both Fig. 8 and 9 it can be noticed that

the Cases 1, 2, and 4 gain in surge margin relatively to

the other cases. These test cases cover distortion pat-

terns within the tip area. At its design point the blading

near the hub of the LPC seems to be most sensitive to

inlet pressure distortion.

SM

LP

C [%

] S

ML

PC

[%

]

ΔpT,AIP,mean [%]

ΔpT,AIP,mean [%]

7

Figure 10. Engine performance with pressure

distortion at NLc = 76%

Figure 11. Engine performance with pressure

distortion at NLc = 90%

5.2.2 Engine performance assessment

Figure 10 and 11 show the 𝑆𝐹𝐶 as function of the mean

pressure distortion within the AIP for seven test set-ups

at 𝑁𝐿𝑐 = 76% and 90%, respectively. The linear rela-

tion between the jet engine performance and the distor-

tion enables a general parametrization of pressure dis-

tortion, especially because the linear relation remains

unchanged if other distortion descriptors are used for

assessment. The latter is not shown in separate graphs

for the sake of clarity. Instead it is referred to Rademak-

ers et al. (2015) for the interested reader.

As a conclusion it can be stated that the stability and

performance of a jet engine, which is exposed solely to

an inlet pressure distortion, can be predicted well as

long as the upstream total pressure distortion pattern is

known in advance.

Figure 12. Combined pressure-swirl distortion

configurations within the LPC performance map at

𝑵𝑳𝒄 = 𝟕𝟔%

Figure 13. Combined pressure-swirl distortion

configurations within the LPC performance map at

𝑵𝑳𝒄 = 𝟗𝟎%

5.3 Influence of swirl and combined pressure-swirl

distortion on the stability and the performance of a

jet engine

A dedicated coefficient to assess the results regarding

combined pressure-swirl distortion is not available.

Bouldin and Sheoran (2002) proposed descriptors for

the evaluation of swirl distortion and the SAE AIR

5686 document describes a procedure to combine sev-

eral pressure and swirl descriptors. However, every sin-

gle coefficient within this combined pressure-swirl dis-

tortion descriptor has to be weighted with an additional

sensitivity coefficient. These sensitivity coefficients are

not yet generally defined.

For a first visualization of the test cases with combined

pressure-swirl distortion the speed lines of five cases

ΔpT,AIP,mean [%]

ΔpT,AIP,mean [%]

8

(6)

are shown in Fig. 12 and 13 at 𝑁𝐿𝑐 = 76% and 90%,

respectively, in the compressor performance map.

Both Cases 3 and 6 (moderate and severe pressure dis-

tortion, respectively) were already shown in Fig. 6 and

7. An additional moderate swirl distortion (Case 3-s1)

does not displace the speed line of Case 3 into a lower

region of the compressor performance map, however,

enhances the surge margin for both investigated spool

speed settings. In the case of a severe pressure distor-

tion (Case 6) the speed line shifts into a lower region of

the LPC map with an additional swirl distortion. The

surge margin is on the other hand significantly ex-

tended, especially at 𝑁𝐿𝑐 = 90%. It seems that an ad-

ditional swirl distortion mainly influences the stability

of the compressor system in a positive way for both

cases with moderate and severe swirl.

The enhancement of the surge margin is mainly ex-

plained by the position of the distortion generators

within the engine inlet of the test set-up. The delta-wing

is positioned downstream of the distortion screen. The

vortices induced by the delta-wing spread the pressure

distortion over the AIP as it is apparent in the plots of

Fig. 5. A spreading of the pressure distortion pattern has

a greater impact than the negative effect of an addi-

tional swirl distortion by means of reducing the surge

margin. The other test cases with combined pressure-

swirl distortion are not shown for the sake of clarity.

Cases 4 and 4-s1 show the same characteristics as

Cases 3 and 3-s1. Cases 5, 5-s1, and 5-s2 are further-

more comparable to Cases 6, 6-s1, and 6-s, respectively.

For a proper performance and stability assessment both

characteristics are evaluated individually in the follow-

ing subsections.

5.3.1 Engine stability assessment

In the case of a combined pressure-swirl distortion it is

not adequate to assess the influence of both phenomena

separately. The interactions between both flow distor-

tions have to be considered as it is described in the SAE

AIR 5686 documents (see Eq. 6) meaning that the surge

margin is influenced by the pressure distortion (∆𝑆𝑀𝑝),

by the swirl distortion (∆𝑆𝑀𝜃), and the interactions be-

tween both pressure and swirl distortion (∆𝑆𝑀𝑝+𝜃).

∆𝑆𝑀 = ∆𝑆𝑀𝑝 + ∆𝑆𝑀𝜃 + ∆𝑆𝑀𝑝+𝜃

Figure 14. Stability of the compressor system with

pressure, swirl, and combined pressure-swirl

distortion at 𝑵𝑳𝒄 = 𝟕𝟔%

Figure 15. Stability of the compressor system with

pressure, swirl, and combined pressure-swirl

distortion at 𝑵𝑳𝒄 = 𝟗𝟎%

The LPC surge margin for all test cases is depicted in

Fig. 14 and 15 for both investigated spool speed set-

tings. There is a strong relation between an increasing

pressure distortion and a degrading stability margin as

already shown in Fig. 8 and 9. The coefficient ∆𝑆𝑀𝑝

within Eq. 6 is thus always negative and relatively easy

to determine.

According to the numerical investigations by Davis et

al. (2008) and Sheoran et al. (2012) the surge margin is

also reduced if it is solely the twin-swirl being consid-

ered. This suggests that ∆𝑆𝑀𝜃 in Eq. 6 is negative as

well. This agrees with the test results for 𝑁𝐿𝑐 = 90%

(Fig. 15), however, for the reduced spool speed setting

in Fig. 14 the surge margin is barely decreased by a

moderate twin-swirl distortion (Case 0-s1) and even en-

SM

LP

C [%

] S

ML

PC

[%

]

9

(7)

Figure 16. Performance of the engine with a

combined pressure-swirl distortion at 𝑵𝑳𝒄 = 𝟕𝟔%

Figure 17. Performance of the engine with a

combined pressure-swirl distortion at 𝑵𝑳𝒄 = 𝟗𝟎%

hanced in the case of a severe twin-swirl distortion

(Case 0-s2).

The surge margin increases in all test cases where an

initial pressure distortion is superposed with a swirl dis-

tortion, which yields a positive ∆𝑆𝑀𝑝+𝜃 coefficient. In

some of the cases with a reduced spool speed the

∆𝑆𝑀𝑝+𝜃 even repeals the negative influence of both

∆𝑆𝑀𝑝 and ∆𝑆𝑀𝜃 (see Case 3-s1, Case 5-s2, and Case

6-s2).

Analyzing the influence of swirl distortion and espe-

cially the interactions between pressure and swirl dis-

tortion within an engine inlet on degrading stability

margin is extremely complicated. Once again it is noted

that during the current work only twin-swirl has been

considered. Other types of swirl should be considered

for a completion of the SAE AIR 5686 and a future es-

tablishment of a corresponding ARP.

Current investigations also show potential advantages

of passive flow control in s-duct inlets. A negative ef-

fect of an additional swirl distortion is more than com-

pensated by an improved distribution of pressure losses

within the AIP for the cases investigated here. The ad-

vantages of passive flow control devices were shown

by e.g. Owens et al. (2006) and Tournier et al. (2006)

during wind-tunnels tests. The real potential of passive

flow control in a modern engine inlet can only be deter-

mined by experimental investigation with a jet engine.

5.3.2 Engine performance assessment

The influence of swirl as well as combined pressure-

swirl distortion on the performance of the propulsion

system is not particularly considered in the SAE AIR

5686 document. For an optimized inlet-propulsion sys-

tem, however, the influence on the performance has to

be considered. In this paper the performance of the en-

tire propulsion system is assessed by means of the 𝑆𝐹𝐶.

At first it is distinguished between the influence of pres-

sure, swirl, and the interaction between both types of

distortion as shown in Eq. 7 similar to the approach in

the preceding subsection.

∆𝑆𝐹𝐶 = ∆𝑆𝐹𝐶𝑝 + ∆𝑆𝐹𝐶𝜃 + ∆𝑆𝐹𝐶𝑝+𝜃

The 𝑆𝐹𝐶 as function of the mean pressure loss in the

AIP for all test cases is depicted in Fig. 16 and 17 for

𝑁𝐿𝑐 = 76% and 90%, respectively. There is a strong

relationship between an increasing pressure distortion

and a degrading stability margin as it was already

shown in Fig. 10 and 11. The coefficient ∆𝑆𝐹𝐶𝑝 within

Eq. 7 is hence negative and relatively straightforward

to identify. A strong swirl distortion (Case 0-s2) results

in an increased 𝑆𝐹𝐶. It is not reasonable to consider this

case in both Fig. 16 and 17, since there was no signifi-

cant pressure distortion and thus ∆𝑝𝑇,𝐴𝐼𝑃,𝑚𝑒𝑎𝑛 cannot be

used for an assessment here.

The same linear relationship between the 𝑆𝐹𝐶 and the

distortion exists if all test cases „s1” and „s2” are in-

cluded in the same graph. It seems that the additional

swirl distortion itself does not have any influence on the

performance of the propulsion system. In fact, the ad-

ditional swirl distortion changes the pressure distortion

ΔpT,AIP,mean [%]

ΔpT,AIP,mean [%]

10

pattern, which again has an influence on the perfor-

mance. This perceptions yields that that ∆𝑆𝐹𝐶 ≈

∆𝑆𝐹𝐶𝑝 and thus |∆𝑆𝐹𝐶𝜃| + |∆𝑆𝐹𝐶𝑝+𝜃| ≈ 0.

6. Conclusions

Great efforts were made in the last decades to assess the

influences of pressure distortion within engine-inlet

systems. However, the effects of twin-swirl distortion

on a propulsion system are only roughly known and not

generally parametrized. Especially the understanding

of interactions between pressure and swirl distortion

and the subsequent influence on the stability and per-

formance of a propulsion system is extremely limited.

In this paper pressure, twin-swirl, and combined pres-

sure-swirl distortions have been investigated experi-

mentally with the Larzac 04 jet engine. The distortion

patterns were chosen to represent flow distortions as

they typically occur within s-duct engine inlet systems.

The following conclusions were drawn from the exper-

imental results:

1) There is a clear linear relationship between s-duct

type pressure distortion and both the stability (𝑆𝑀𝐿𝑃𝐶)

and performance (𝑆𝐹𝐶) of a jet engine.

2) If it is solely twin-swirl being considered, the inlet

distortion can have a negative as well as a positive in-

fluence on the surge margin of the compressor system.

At the design point of the compressor system a twin-

swirl degrades the surge margin.

3) Is a pressure distortion superposed with a twin-swirl

distortion the surge margin is enhanced due to an redis-

tribution of the total pressure distribution in the AIP.

This indicates the major influence of pressure distortion

compared to twin-swirl by means of surge margin re-

duction and furthermore, that a separate assessment of

both phenomena is not sufficient in case of a combined

pressure-swirl distortion.

4) A twin-swirl distortion decreases the efficiency of

the compressor system and subsequently decreases the

𝑆𝐹𝐶 of the propulsion system. The performance is not

significantly influenced if a pressure distortion is super-

posed with a moderate twin-swirl distortion. An addi-

tional severe twin-swirl distortion does have an influ-

ence, but only because it changes the total pressure dis-

tortion pattern.

5) Pressure distortion is the dominating factor by means

of both engine stability and performance as long as the

swirl angles are moderate. From this it can be con-

cluded that the design focus of passive flow control de-

vices should be a reduction of critical pressure distor-

tion within the AIP and not the reduction of secondary

flow phenomena. Additional twin-swirl can be ac-

cepted when the alteration of the pressure distortion

pattern is positive. The real potential of passive flow

control in a modern engine inlet, however, can only be

determined within a dedicated testing set-up.

7. Acknowledgement

The authors would like to express their appreciation to

the members of the institute’s technical staff Heinz

Hampel, Georg Köttner, and Waldi Weigel. Their ef-

forts for the preparation of the experimental test set-up

and conducting of engine tests at the engine test facility

are acknowledged.

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