Experimental Aerodynamic Analysis of a Plug Nozzle for Supersonic Business Jet Application

John TapeeDr. John Sullivan

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CAD Model Schliere

n

Shadowgraph

Static Pressures

Dynamic Pressures

Installed Hardware

Introduction/Overview• Experimental static test of plug nozzle• Research carried out under Task 7c of the Supersonic Business Jet program (SSBJ)• Purpose:

– Characterize behavior, especially low nozzle pressure ratio (NPR) unsteady effects– Provide basis for CFD comparison & evaluation

• Test geometry derived from Gulfstream’s High-Flow Bypass concept– Designed for sonic boom mitigation– High-flow bypass region avoids thick engine nacelle that would create strong shock wave– Zero-energy-added stream; only intent is to reduce losses– Plug nozzle design chosen for easy integration with this concept

3Gulfstream High-Flow Bypass Concept [Conners, 2008]

Flow

Flow

0, upstreamNPRPP

Background• Plug nozzles are altitude-compensating• Free jet boundary expands to match

local ambient pressure– For NPRs below design, avoids

overexpansion– For NPRs at and above design, behaves

like standard C-D nozzle• Shocks/expansions are the mechanisms

that enable altitude-compensation– Consider thrust as integral of surface

pressure over projected area– Pressure plot at right shows better

performance for plug at low NPR• Can truncate plug to get net increase in

performance• Plug design has also been shown to be

less noisy [Dosanjh, 1986; Stone, 2000]

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Surface Pressures at Low NPR for Plug and C-D Nozzle

Plug Nozzle at NPR < NPRdesign [Hagemann, 1998]

[Ruf, 1997]

Background• Similar studies have been performed for conical, truncated, and contoured plugs• Images shown here taken from tests conducted by Verma at India’s National

Aerospace Labs• Both plugs are conical, not contoured• Design NPR ≈ 7.8, both images at NPR = 2.57

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[Verma, 2008][Verma, 2008]

Model Geometry

Notable Geometry Differences

• No high-flow bypass stream• Shroud wall thickness increased for

structural soundness and machinability• Subsonic convergent angle increased• Hot/cold flow split introduced• New strut design for rig compatibility and

instrumentation pass-through

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Flow

Flow

Gulfstream High-Flow Bypass Concept [Conners, 2008](reproduced with permission)

Test Geometry No geometry modifications to supersonic stream – flow behavior should be similar to full scaleDark Gray = Rig Hardware

Light Gray = Plug Nozzle Hardware

• Design nozzle pressure ratio (NPR) of 6.23• Scale = 0.155

– Based on available mass flow• Unmixed core & bypass airstreams• Constructed from stainless steel (hot parts) and aluminum (cold parts)• Swappable aluminum and glass shrouds for pressure measurement and internal flow

visualization, respectively• Hollow struts for instrumentation pass-through

Model Geometry Overview

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HOT

COLD

COLD

HOT6.811 in

11.579 in

“Shroud” “Plug” or

“Centerbody”

Facility and Instrumentation

Test Facility

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• Used new dual-stream, co-annular nozzle rig developed under SSBJ Task 7b• Simulates typical turbofan engine exhaust

– Unheated bypass stream– Vitiated core stream

• Blowdown facility capable of test times on the order of several minutes [Trebs, 2008]

• Condition-monitoring temperature and pressure measurements throughout• Total temperature & pressure rakes provide nozzle feed conditions

Mass-averaged nozzle pressure ratio (NPR) defined as:

where total pressures are averaged over space, not time

Instrumentation• Static Pressure Taps

– Focused primarily on axial resolution (best is 0.5”)• High-frequency Sensors

– Kulite Pressure Transducers– Accelerometers– Used Welch’s method to pull power spectral density from raw data

• High-speed Schlieren/Shadowgraph

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Pressure Measurements

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Angle Reference(looking upstream)

High-frequency Transducers

• Omitted taps marked (P19, P29, S15)• Primary static measurement along

+45° row (blue, 0.5” spacing)– 20 taps on plug– 12 taps on shroud

• Additional taps for azimuthal variation– 10 on plug– 9 on shroud

Flow Visualization• Schlieren

– Sensitive to 1st derivative of density in direction perpendicular to knife edge

– Most tests with vertical knife edge– Images external section of nozzle– Considerable background noise

• Shadowgraph– Sensitive to 2nd derivative of density, non-

directional– Implemented due to schlieren limitations with

glass shroud– Images almost entire nozzle (from about 1”

downstream of throat)– Image processing technique applied to reduce

effect of glass imperfections on image

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x

Original Result

Sample Full-Range Visualization

Schlieren• Playback at actual speed• NPR range: 1.0 – 5.5

Shadowgraph• Playback at 1/10th speed• NPR range: 1.0 – 2.5

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NOT SYNCHRONIZED

Tip Vibration Analysis• Two methods of analysis:

1. Track intensity of individual pixel near centerbody edge• Pixel choice important• Not that sensitive to noise in schlieren images

2. Track tip location• Provides amplitude as well as frequency• Quite sensitive to noise in schlieren images

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Method #1

Track intensity value from chosen pixel

Method #2

Tip (track x,y coordinates)

Centerline

Vertical Centers

Tip Vibration Results

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• Sample case: Hot fire, NPR 2.03, shadowgraph imaging

• Tip deflection plots show maximum amplitude of roughly 0.015 in– Less than one pixel

• Dominant frequency at 18 Hz• Similar power spectra for each method

Method #2: Tip TrackingMethod #1: Pixel Intensity

y de

flect

ion

(in)

x de

flect

ion

(in)

Tip Vibration Results• Schlieren images of high-NPR hot fire tests show

thermal growth of centerbody (corresponds to ΔT of about 75°F over 5 seconds)

• Vibration magnitude (and frequency) independent of NPR

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Tip is difficult to detect in some schlieren images

Hot, 4.47

y de

flect

ion

(in)

y de

flect

ion

(in)

Hot, 6.12Cold, 2.56

Cold, 1.74 (Shadowgraph)

Selected Test Cases

Tests• Total of 57 successful tests

– Conducted from Jan. 28, 2009 through Feb. 23, 2009– 22 steady-state hot, NPRs of 1.77 to 6.12– 30 steady-state cold, NPRs of 1.26 to 5.75– 5 cold flow sweeps

• For hot fires, temperature varied between 600 °F and 1200 °F– Insufficient control of fuel flow to reduce this variation

• Concentrated on low NPRs ( < 3.0 )– Region of concern for unsteady characteristics

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Case Type NPR Core Temp (°F)

1 Cold 3.73 n/a

2 Cold 1.59 n/a

3 Hot 2.50 660

4 Hot 6.12 870

Selected Cases

Case #1:Cold Flow, NPR 3.73

• Flow fairly steady• No substantial peaks in power spectrum• Little asymmetry in pressure distribution

– x/L = 0.52• Black/yellow diamonds indicate suspect data

– Possible leak or geometry error

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Playback at 1/10th speed

Case #2:Cold Flow, NPR 1.59

• Shock location very unsteady• Unsteadiness shown in dynamic transducers

– Peak oscillation at 200 Hz• Almost no asymmetry in pressure distribution

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Playback at 1/10th speed

Image at NPR 1.74

Case #3:Hot Fire, NPR 2.50

• Axial position reasonably steady• No specific frequency peaks, just broadband

oscillations below 200 Hz• Somewhat asymmetric at shock (x/L = 0.38)

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• Shadowgraph: Cold, NPR 2.45• Playback (both) at 1/10th speed

Case #4:Hot Fire, Cruise Condition

• Actual NPR = 6.12 (cruise target is 6.23)• Difficult to capture hot fire schlieren due to

refractive index gradients• Fairly steady – frequency range > 1kHz

shows combustion frequencies• Little asymmetry (x/L = 0.52 again)

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Playback at 1/10th speed

Detailed Data Comparisons & Additional Analysis

Cold 3.70 (approx.)

Hor

izon

tal K

nife

-Edg

e

Cold 3.73

Vert

ical

Kni

fe-E

dge

Cold 2.45 Shadowgraph

Shock Structure

• Classic lambda shock forms on both plug and shroud wall

• Large region of separation on plug• Higher NPRs setup classic diamond-shock pattern in

exhaust• Schlieren and Shadowgraph techniques integrate

along the optical path – for axisymmetric flow, this results in “phantom” shock patterns (dotted lines)

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LEGEND

Cold Flow Schlieren

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NPR 1.40 NPR 1.88* NPR 2.23

NPR 3.06 NPR 3.73 NPR 5.75

*discussed in detail later

Hot Fire Schlieren

• At cruise, shroud trailing edge shock lies right at theoretical plug tip

• Quality schlieren images harder to obtain during hot fires due to combustion products and temperature gradients

• Mixing layer between hot core and cold bypass streams not very clear due to light path integration issue

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NPR 2.50 NPR 3.48

NPR 4.47 NPR 6.12

[Rossmann, 2001]

v1, ρ1

v2, ρ2

Sample Expected Mixing Layer Image

Mixing Layer Edge

Low-NPR Shadowgraph

• Optical properties of glass shroud prevented schlieren use

• More detail visible in cold flow• After 1st hot fire, some

condensed liquid accumulated on the shroud’s inner surface, forming a visualization of shock location at the wall typical of oil flow

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Hot, NPR 2.03 Hot, NPR 2.39 Hot, NPR 2.93

Cold, NPR 2.14 Cold, NPR 2.45

CFD Schlieren Comparison

• Phantom normal shock train– No shock reflection or separation on plug– “Wavy” shape of apparent shocks is uncharacteristic of normal shocks

• Axisymmetric CFD shows that a large separation region does exist and that the faint normal shocks do not extend all the way to the plug surface

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Cold Flow, NPR 1.88Hot Fire, NPR 2.03 (courtesy Dheeraj Kapilavai)

High-Speed Schlieren

• Cold Flow 2.11• Recorded at 6000 fps• Playback at 10 fps (1/600th speed)

• Shock oscillation visually observed– Period of 17 frames– Equivalent to 353 Hz

• Very good match with dynamic pressure transducers

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Steady Pressures: Experimental Comparisons

• Can clearly see shock locations, separation regions• Black/yellow diamonds indicate suspect data points• Primary difference between hot & cold operation is position of shock along

plug– Caused by temperature difference of roughly 800-1000°F

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*only viewing 45° taps

Separation

Shocks

Steady Pressures: CFD Comparisons

• Fluent analysis with coarse unstructured grid for sizing– No dedicated boundary layer gridding

• Most glaring difference is axial shock location• Conclusion: flow is dominated by boundary layer and separation

– This CFD does not resolve the boundary layer well• Comparison near throat shows why two pressure taps are suspect

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CFD – solid linesExperimental – discrete data points

Cold Flow Kulites• Substantial pressure

oscillations at NPRs ≤ 2.25 due to shock movement

• Dominant frequency = 200-400 Hz

• Unsteady behavior beyond dynamic transducers for NPR ≥ 2.59

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Hot Fire Kulites• Frequency peaks in same

200-400 Hz range, not as evident as with cold flow

• Spectrum for f > 1000 Hz caused by combustion instabilities

• 60 Hz noise seen in many hot fire tests

• Near NPR 2.50, wide band of pressure oscillation frequencies, but no distinct peaks (repeatable during 3 distinct tests)

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Flow Structure Jump atNPR ≈ 2.05

• Abrupt change in flow behavior & shock train at NPR of roughly 2.05• Shows distinct asymmetry in 2nd structure• (Recorded at 100 fps, playback at 1/10th speed)

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Flow Structure Jump atNPR ≈ 2.05

• Pressure distributions show this structure change• Shock abruptly moves aft along plug (and forward along

shroud) as NPR slowly increases past 2.05

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• Flow structure change visible in pressure histories as well– Each line reflects pressure at tap

indicated by similarly colored diamond• Overall, shows decrease in pressure

fluctuations in 2nd structure• NPR plot below shows no abrupt

change in feed conditions

Flow Structure Jump atNPR ≈ 2.05

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Time window for pressure histories

Conclusions

Summary & Conclusions

Summary• A derivative of Gulfstream’s high-flow bypass nozzle has been designed and manufactured to fit

the new nozzle rig at HPL.• A series of experimental tests of this nozzle has been successfully conducted in close

partnership with our sponsors.• Instrumentation suite allowed collection of steady-state pressure profiles and unsteady flow

characteristics• Used a combination of schlieren and shadowgraph methods to enhance understanding of

nozzle flow physics

Conclusions• At cruise (NPR = 6.23), nozzle performs well• At NPRs between 2.5 and 6, flow is relatively steady

– Shock/boundary layer interaction and separation along plug and shroud• At NPRs between 1.0 and 2.5, flow is unsteady

– Dominated by boundary layer and separation characteristics on plug

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• Variable geometry will be necessary for a production system (this was expected but is still worth noting)

• Truncate Plug– Literature indicates that additional compressions and/or expansions that would occur would

remain within the plume and have little effect on the external flow field– Reduce/eliminate any tip vibration and reduce manufacturing complexity– Weight savings → possible net performance gain

• Shorten Shroud– Incorporate better altitude-compensating behavior– Reduce/eliminate massive separation along plug– Simple modification to existing geometry

• Use focused schlieren– Combat the “phantom” shocks created by the light path integration issue– Would also reduce sensitivity to background noise

Recommendations

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Gulfstream Shroud Extension

[Hagemann, 1998]

Questions?

References1) Tim Conners. Gulfstream Aerospace Corporation. Presentation to Rolls-Royce, 23 January

2008. Images reproduced with permission.2) Adam Trebs. Biannular Airbreathing Nozzle Rig Facility Development. Master’s thesis, Purdue

University, August 2008.3) S.B. Verma. Performance Characteristics of an Annular Conical Aerospike Nozzle with

Freestream Effect. In 44th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, July 2008.

4) G. Hagemann, H. Immich, and M. Terhardt. Flow Phenomena in Advanced Rocket Nozzles – the Plug Nozzle. In 34th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, July 1998.

5) J. H. Ruf and P. K. McConnaughey. The Plume Physics Behind Aerospike Nozzle Altitude Compensation and Slipstream Effect. In 33rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference, July 1997.

6) D.S. Dosanjh and I.S. Das. Aeroacoustics of Contoured Plug-Nozzle Supersonic Jet Flows. In AIAA 10th Aeroacoustics Conference, July 1986.

7) James R. Stone, Eugene A. Krejsa, Ian Halliwell, and Bruce J. Clark. Noise Suppression Nozzles for Supersonic Business Jet. In 36th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, July 2000.

8) Tobias Rossmann, M. Godfrey Mungal, and Ronald K. Hanson. Acetone PLIF and Schlieren Imaging of High Compressibility Mixing Layers. In 39th AIAA Aerospace Sciences Meeting and Exhibit, January 2001.

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