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

1. Experimental Aerodynamic Analysis of a Plug Nozzle for Supersonic Business Jet Application John Tapee Dr. John Sullivan

2. CAD Model Schlieren Installed Hardware Static Pressures Shadowgraph Dynamic Pressures

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

4. 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] Plug Nozzle at NPR < NPRdesign[Hagemann, 1998] Surface Pressures at Low NPR for Plug and C-D Nozzle [Ruf, 1997]

5. 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 [Verma, 2008] [Verma, 2008]

6. Model Geometry

7. 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 Gulfstream High-Flow Bypass Concept[Conners, 2008] (reproduced with permission) Flow Flow No geometry modifications to supersonic stream – flow behavior should be similar to full scale Test Geometry Dark Gray = Rig Hardware Light Gray = Plug Nozzle Hardware

8. Model Geometry Overview • 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 “Shroud” “Plug” or “Centerbody” 6.811 in HOT HOT 11.579 in COLD COLD

9. Facility and Instrumentation

10. Test Facility • 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

11. 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

12. Pressure Measurements • 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 Angle Reference (looking upstream) High-frequency Transducers

13. 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 Original Result

14. 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 NOT SYNCHRONIZED

15. Tip Vibration Analysis Method #2 • Two methods of analysis: • Track intensity of individual pixel near centerbody edge • Pixel choice important • Not that sensitive to noise in schlieren images • Track tip location • Provides amplitude as well as frequency • Quite sensitive to noise in schlieren images Vertical Centers Centerline Method #1 Tip (track x,y coordinates) Track intensity value from chosen pixel

16. Tip Vibration Results • 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 #1: Pixel Intensity Method #2: Tip Tracking

17. 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 Tip is difficult to detect in some schlieren images Cold, 1.74 (Shadowgraph) y deflection (in) Hot, 4.47 x deflection (in) y deflection (in) y deflection (in) Cold, 2.56 Hot, 6.12

18. Selected Test Cases

19. 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 Selected Cases

20. 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 Playback at 1/10th speed

21. 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 Playback at 1/10th speed Image at NPR 1.74

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

23. 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) Playback at 1/10th speed

24. Detailed Data Comparisons & Additional Analysis

25. 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) LEGEND Horizontal Knife-Edge Vertical Knife-Edge Cold 3.70 (approx.) Cold 3.73 Cold 2.45 Shadowgraph

26. Cold Flow Schlieren NPR 1.40 NPR 1.88* NPR 2.23 *discussed in detail later NPR 3.06 NPR 3.73 NPR 5.75

27. 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 NPR 2.50 NPR 3.48 Mixing Layer Edge Sample Expected Mixing Layer Image v1, ρ1 NPR 4.47 NPR 6.12 v2, ρ2 [Rossmann, 2001]

28. 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 Cold, NPR 2.45 Cold, NPR 2.14 Hot, NPR 2.03 Hot, NPR 2.39 Hot, NPR 2.93

29. 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 Cold Flow, NPR 1.88 Hot Fire, NPR 2.03 (courtesy DheerajKapilavai)

30. 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

31. 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 Separation Shocks *only viewing 45° taps

32. 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 CFD – solid lines Experimental – discrete data points

33. 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

34. 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)

35. 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)

36. 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 Placement in time shown here 75 s 60 s

37. Flow Structure Jump atNPR ≈ 2.05 • 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 Time window for pressure histories

38. Conclusions

39. 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

40. Recommendations • 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 Gulfstream Shroud Extension [Hagemann, 1998]

41. Questions?

42. References • Tim Conners. Gulfstream Aerospace Corporation. Presentation to Rolls-Royce, 23 January 2008. Images reproduced with permission. • Adam Trebs. BiannularAirbreathing Nozzle Rig Facility Development. Master’s thesis, Purdue University, August 2008. • 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. • 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. • 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. • D.S. Dosanjh and I.S. Das. Aeroacoustics of Contoured Plug-Nozzle Supersonic Jet Flows. In AIAA 10thAeroacoustics Conference, July 1986. • 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. • 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.