Flow and Acoustics of Jets from Practical Nozzles for High-Performance Military Aircraft

Flow and Acoustics of Jets from Practical Nozzles for High-Performance Military Aircraft Ph.D. Dissertation submitted to the Department of Aerospace Engineering by David E. Munday University of Cincinnati 29 October 2010 Cincinnati, Ohio

Outline • Introduction • Background • Methodology • Results • Baseline • Chevrons • Microjets • Summary • Recommendations • Acknowledgements

The problem • Military aircraft noise effects communities around military bases • $34.4 Million settlement from one law suit for one base • Political pressure limits training and testing • Noise leads to health issues for personnel who work around military aircraft • More then $750 Million in hearing-loss disability payments in ‘05 35° 100° 150°

From Subsonic to Supersonic Jets • Lots of work has already been done on subsonic jets • Several noise reduction techniques have been explored • Supersonic jets bring additional physics (shocks) • Additional noise production mechanisms (shock related) • Application of noise control to shock containing jets is a relatively new area Supersonic, underexpanded

Practical Nozzle Geometry • Modern high-performance aircraft have variable geometry nozzles to adapt to different operating conditions • The “practical nozzle” in the title refers to these • They differ from traditional C-D nozzles in that they have sharp throats and they are divergent all the way to the exit • There is almost nothing published about this kind of nozzle

Baseline nozzle Md = 1.5 Chevron cap Shroud Cases Md= 1.65 Md= 1.50 Md= 1.50 Md= 1.30 Md= 1.56 Md= 1.56 • Practical nozzles simplified to conic C-D • Chevrons and blowing applied

Three components of Jet Noise • (Meyer, 1908, Pack, 1950, Lighthill, 1952 & 1954, Ffowcs-Williams, 1963, Lilley, 1974, Crow & Champagne, 1971, Brown & Roshko, 1974, Tam, Golebiowski and Seiner, 1996, Tam, Viswanathan, Ahuja and Panda, 1998, Crow & Champagne, 1971, Brown & Roshko, 1974, Michalke, 1965, Zaman and Hussain , 1984, Yule,1978, Lepicovsky, Ahuja, Brown & Burrin, 1987, Norum & Seiner , 1982, Powell, 1953, Yu & Seiner, 1983, Norum, 1983, Yu & Seiner, 1983, Harper-Bourne & Fisher, 1974, Pao & Salas, 1981, & Seiner, 1983, Seiner & Yu, 1984, Tam & Tanna, 1982, Tam, Seiner and Yu, 1986, Norum and Shearin, 1986, Bechert, 1975, Jubelin, 1980, Seiner & Norum, 1979, Long and Martens, 2009, Martens and Spyropoulos, 2010, PSU) • Mixing noise (fine scale and large scale) are common to subsonic and supersonic jets. (peak source location at end of potential core, broad band) • Broad-Band Shock-Associated noise (BBSN) arises from interaction between large-scale structures and the shocks in the jet. (Peak frequency is a function of observer angle, peak source location near later shock cells) • Screech is a feed-back loop between upstream running BBSN and the large scale structures it induces at nozzle exit (narrow peak, multiple apparent source locations at shock reflections) Overexpanded Perfectly expanded Underexpanded

Before-Chevrons Figures from Smith (1989) • Tabs (Bradbury and Khadem,1975, Tanna, 1977, Ahuja, Manes, Massey and Calloway, 1990, Samimy, Zaman, Reeder, 1993) • Corrugate jet cross section • Eliminate screech • Reduce mixing noise • The mechanism for noise reduction are streamwise vortices • Lobes → Tabs → Delta Tabs → Chevrons Figure from Samimy, Zaman and Reader (1993) Figures from Saiyed, Mikkelsen and Bridges (2003)

Chevrons on subsonic jets • Delta tabs and Chevrons (Saiyed, Mikkelsen and Bridges, 2000 and Bridges and Wernet, 2002, Callender, Gutmark, Martens, 2004 and 2008, Bridges and Brown, 2004, Opalski, Wernet and Bridges, 2005, Alkislar, Krothapalli and Butler, 2007) • Similar effects and trends at tabs • Reduced centerline velocities • Produced radial velocity (inward at tips, outward in valleys) • Reduced TKE where it had been highest • Introduce new TKE near the nozzle • Require some penetration to work • Low frequency benefit, high frequency penalty • Effectiveness increases with penetration and shear velocity • Same trends for hot jets • Chevron length is relatively unimportant • Velocity is important, not temperature or Mach number • Each source location is moved upstream

Chevrons on supersonic jets • Rask, Gutmark and Martens (2006, 2007) • commercial separate-flow exhaust nozzle with centerbody (convergent, Md = 1) • Slightly underexpanded jet, Mj = 1.18. With and without M2 (results for M2 = 0 here) • Shortened shock cells • BBSN increased, and shifter to higher freq • Increased OASPL • Reduced TKE downstream, increased near-nozzle • Long and Martens (2009) • Faceted C-D nozzle, Md = 1.3, 1.5, 1.65 • Far field 1/3rd octave band, relative amplitudes only • Near field along a single line, no spectral information • Reduced forward propagating and aft propagating sound • Increased high freq near exit reduced low freq downstream

Chevrons on supersonic jets (full scale) • Martens and Spyropoulos (2010) • Full scale F404 engine test (engines don’t screech) • Far field 1/3rd octave band, relative amplitudes only • Jet conditions not shown, but all cases overexpanded • OASPL reduced, • small impact on forward propagating • larger on aft propagating • Length of chevron is important

Microjets on subsonic jets • Introduce streamwise vorticity, but can be turned off • Air microjets have been studied by (Chauvet, Deck and Jacquin,2007, Alkislar, Krothapalli and Butler, 2007; Arakeri, Krothapalli, Siddavaram, Alklislar and Louranco, 2003; Laurendeau, Bonnet and Delville, 2006; Aberg, Szasz, Fuchs, Gutmark, 2007, Alklislar with Krothapalli and Butlerr, 2007, Callender, Gutmark and Martens, 2007, Camussi, Guj,Tomassi and Sisto, 2008, Zaman, 2007, Castelain, T., Sunyach, M., Juve, D., Bera, J.-C., 2006 and 2008, Krothapalli, Greska and Arakeri, 2002) • Mutual induction drives vortex pairs in, out (switched) • Sometimes reduce mixing and lengthen potential core • Sometimes reduce sometimes increase peak TKE • Injection angle has influence like penetration • Microjet self noise contributes to high frequency penalty • Too many microjets (too close together) spoils effects

Microjets on supersonic jets • Krothapalli, Greska and Arakeri (2002) • Convergent, Md = 1.0, Mj = 1.38, hot, 500psi, 1% • Shock cell length shortened • OASPL reduced in aft quadrant • Screech suppressed • BBSN minimally affected at 90° • Greska, Krothapalli and Arakeri (2003) • Smooth C-D nozzle, Md = 1.8, Mj = 1.63, 1.8, 1.96, hot, 250 psi • OASPL reduced • No screech to eliminate • Microjet effectiveness reduces if moved downstream • Henderson and Norum (2007, 2008) • commercial separate-flow exhaust nozzle with centerbody (convergent, Md = 1) • Mj up to 1.16 • With and without azimuthal variation, 1.2% • Mj < 1.06 microjets increased noise • BBSN reduced

Microjets on supersonic jets (full scale) • Greska, Krothapalli, Arakeri (2003), Greska, Krothapalli, Burnside and Horne (2004) • J79 full scale with convergent nozzle, Mj = 1.3, 115-600psi, 0.3-1% • OASPL reduced • BBSN reduced

Flow Measurement • Shadowgraph with an Oriel arc lamp and a pair of 12” parabolic mirrors is used with one of the PIV cameras fitted with a telephoto lens • Centerline pressure was measured by a cone probe, a United Sensor model SDF-15-6-15-600 • LaVision PIV suite • Flow seeded with 1μm droplets of olive oil • 500 mJ nd:YAG laser formed into a sheet containing the jet axis • 2 LaVision 1376x1040 12-bit PIV cameras acquiring simultaneously • Laser, sheet optics and cameras translate together to 4 streamwise locations for 8 image panes

Adapted from our coaxial flow nozzle model. 24x25 anechoic chamber good to 500Hz T0 at Station 0 is uncontrolled, but is around room temperature 8 B&K ¼” free-field mics from ψ=35° to 150° measured from upstream The arc is 47 exit diameters from the nozzle exit Sampled at 200KHz, good data to 80kHz EXHAUST WALL Acoustic Measurement ARRAY OF FAR-FIELD MICS 35° NEAR FIELD MIC RAKE: SOURCE LOCATION 150° UC ACOUSTIC NOZZLE RIG

Uncertainty Estimation • Jet velocity is held to within 4.5 m/s (95%) or 1% of velocity so 8% of Prms or 0.7 dB • Same day acoustic repeatability is 0.6 dB (95%). Agreement with other facilities is good • PIV seed is around 0.7 μm diameter Stoke’s number ranges from 0.01 to 0.56. Ut/g = 3.0 x 10-4s so a 1000g acceleration will give a terminal velocity 3 orders below Uj • PIV pressure and shadowgraph compare well with one another and with LES. For x/D < 4 PIV uncertainty is 11 m/s • PIV quality degrades as one moves downstream.

No shock-free condition near design point • Sharp throat creates a wave regardless of Mj > 1 • Non-parallel exit causes an inward turn if pressures match • Lip wave is a shock despite the underexpanded condition • Parameters defined • Turbulence at exit is fine. Scale increases downstream • Bright line near exit is first throat reflection • Only first cell apparent, later cells obscured by turbulence • Downstream shocks fuzzy due to reflection from moving shear layer, more reflections more movement, fuzzier • Slip line shows • PIV domain begins at x/D ≈ 0.25 • u < 50 m/s is black, can see low speed flow engulfed • Random fluctuation inside jet same order Δu over shocks • Averaging brings out details of double diamond Double Diamond Structure • No shock-free condition • Sharp throat • Non-parallel exit • Mj = 1.56 has lip shock • Later cells unsteady • Low speed engulfed • Double diamond previously unreported LES Figure from NRL

Influence of Mj Mj=1.47 Mj=1.56 Mj=1.50 Mj=1.64 Mj=1.71 Mj=1.22 Mj=1.36 • Double diamonds in all cases including design condition • For overexpanded cases the two diamonds grow closer to one another as Mach disk forms, by Mj = 1.22 they coalesce by first reflection • For underexpanded cases the P-M fan from the lip widens until it envelops the throat wave entirely

Shock Cell length Increasing Md • Prandtl-Pack equation predicts Ls/Dj only a function of Mj, but it reduces with Md also • Ls for conic C-D nozzles are in line with traditional nozzles

Md = 1.50 , Mj = 1.50 Far-field Acoustics ψ = 35° Md = 1.3 ψ = 35° Md = 1.5 ψ = 35° Md = 1.65 • There is a reduction in screech and a mode switch near Mj = Md • There is no reduction in BBSN however so shocks must be present.

Shock Noise Peak Frequencies • The frequencies for conical C-D nozzles are indistinguishable from those for traditional C-D nozzles. • The dependence of frequencies on Md is likely due to the dependence on Ls • Tam’s equations under-predict, but do better if experimental values for Ls are used in place of Prandtl-Pack Screech BBSN at 90°

Flow Structure • Md = 1.50, Mj= 1.56 • Turbulence from first • Enhanced spreading • Diagonal lines in shadowgraph not in PIV so they’re on surface

Influence of Mj Mj=1.47 Mj=1.56 Mj=1.50 Mj=1.64 Mj=1.71 Mj=1.22 Mj=1.36 • Outer angle changes with Mj, chevron angle does not, effective penetration changes • Smearing due to shock cell unsteadiness is greater for chevrons for greater Mj • Vortex shed closer to root and merges earlier for increased Mj (nil for 1.22) • The “extra” diagonal lines become stronger with increasing Mj by 1.71 they dominate image

40 a b e 490 520 480 c d f 460 510 550 500 40 510 520 500 470 550 40 510 510 470 470 550 Md = Mj = 1.56 x/De 1.0 x/De = 0.5 x/De 2.0 Tip x/De = 0.5 x/De 4.0 Val • Gross changes in shear layer • Changes in wave angles in shock cells • Increased TKE near the nozzle • Axisymmetric by x/De = 4

40 a b e 490 520 480 c d f 460 510 550 500 40 510 520 500 470 550 40 510 510 470 470 550 • Lip wave is shortened by chevrons (c, d, e) • Throat wave unchanged even after reflection (f) • There is no significant difference in the shock structure between tip and valley planes • Lip Shock at (c) strengthened 5% in tip plane, weakened 4% in valley plane r/De= 0.0 c f d Tip r/De= 0.1 a b e Val r/De= 0.4

Far-field mixing noise ψ = 35° ψ = 90° ψ = 150° • For Mj = 1.22 effect is nil. Benefit increases with Mj and effective penetration • Chevrons kill screech (as do tabs) by breaking symmetry • BBSN substantially reduced for underexpanded cases (2.3 to 9.1 dB at 90°) • Peak frequencies shifted higher due to reduction in sonic diameter • Mixing noise reduction increases with Mj and effective penetration • Peak reductions of 3.2 to 5.0 dB in mixing noise at 150° • At forward and side angles reduction occurs below screech frequency • High frequencies are the only region where we see consistent increases in far-field sound with chevrons

Low f Mixing noise(1000 Hz) • Mj = 1.36 shows a reduction in mixing noise • from x/De = 5 to 10 • Mj = 1.47 it moves downstream (6 to limit) • By Mj = 1.56 we start to see a lobe of increased noise • from x/De = 1.75 to 4 • This lobe does not move, but increases in intensity Mj=1.36 Mj=1.50 Mj=1.64

Screech freq • Baseline measurements show strong screech signature • For Mj below 1.50 there is noise reduction everywhere • For higher Mj we see an increase • from x/De = 1.75 to 4

Broadband shockassociated noise • Frequencies selected based on 90° Far-field peaks • Mj = 1.36 baseline shows a large lobe centered at x/De = 5.5 • This lobe shifts to higher f with Mj increase • Chevron peak is lower in intensity, farther downstream • We do see a lobe of noise building • at x/De = 1.5 to 3

HF noise(30,000Hz) • This frequency does not discriminate any particular mechanism • The high frequencies are the only ones to show consistent noise increase with chevrons • The dominant feature is the lobe of noise near the nozzle. • This is counterbalanced somewhat by decreases downstream in most cases

Microjets • Md = 1.50, Mj = 1.56, arrangement after Alklislar, 1.4% mass to microjets • Increased spreading due to Microjets, though not as much as chevrons • Average of 100 images shows shock cells blurry downstream of microjets • Upstream of microjets is clear, so this is due to unsteadiness in cell structure • Like chevrons, numerous diagonal features are induced by the microjets

Microjet self noise • Over most of the frequency range the microjet self noise is 20 dB lower • This is negligible, so no correction is made for microjet self noise As measured, 60° by 0° Microjets, Mj = 1.56 ψ = 35° ψ = 90° ψ = 150°

Influence of microjet tubes Lossless, nondimensional, R/Dj = 100 , Mj = 1.56, cold • Presence of external tubes does have a significant influence on base acoustics • Screech is suppressed and this suppresses broadband amplification • There is little to be done about this, but to be careful in interpreting the results ψ = 90° ψ = 150° ψ = 35°

Microjet Acoustics Lossless, nondimensional, R/Dj = 100 , Mj = 1.56, cold, 1.4% • The Microjets do produce a benefit beyond that produced by the flow-off case • At 35° both BBSN and screech are reduced beyond the no-flow case • The 90° spectrum shows significant reduction which is important to fly-by or fly-over extrapolation • BBSN is lowered and shifted to higher frequency like with chevrons • Mixing noise shows low frequency benefit and high frequency penalty as chevrons do ψ = 90° ψ = 150° ψ = 35°

Microjet OASPL Full Scale, R = 8m, Mj = 1.56, cold, 1.4% • Microjets provide around 0.5 dB additional benefit beyond what tubes alone provide • Compared to the baseline without tubes, Microjets provide almost 5dB in the forward direction, more than 1 dB at nearly all angles

Microjets Compared to Chevrons Lossless, nondimensional, R/Dj = 100 , Mj = 1.56, cold, 1.4% • The chevrons remove screech as the bare tubes do • Microjet BBSN reduction is less than chevrons, especially at the 90º angle • Mixing noise low frequency benefit is less with microjets that it is with chevrons • Microjets have a greater high-frequency penalty at aft angles than chevrons • LES suggests that higher mass flows would bring microjet benefits into line with chevrons ψ = 90° ψ = 150° ψ = 35°

Microjet OASPL compared to chevrons Full Scale, R = 8m, Mj = 1.56, cold, 1.4% • Chevrons provide 4dB at the forward angle and 2.25 or 2.5 dB at aft angles • Chevrons beat microjets at every angle except 35º where they are about equal

Summary • Four areas of contribution • Expanded understanding of the function of practical C-D nozzles and how they differ from traditional C-D nozzles • Extended study of chevrons to shock-containing jets and shock-associated noise mechanisms • Extended study of Microjets to shock-containing jets and shock associated noise mechanisms • Provided high-quality reference data for CFD

Practical C-D Nozzles • Practical C-D nozzles of this type produce no shock-free condition at the exit. This is due to the non-parallel exit flow. • The sharp throat produces a second set of shock diamonds which are of comparable strength to the lip shock cells near the design condition • The presence of shocks at or near the design condition causes shock-associated noise to be present even at the design condition, making further study of shock-containing jet noise important for military engines • Practical C-D nozzles are like traditional C-D nozzles in several respects • The average shock cell length, Ls compares well with other published values for traditional C-D nozzles, but not so well to the Prandtl-Pack equation. • This leads the BBSN and screech peak frequencies to be in-line with those of traditional nozzles, but substituting actual values of Ls improve prediction over Prandtl-Pack

Chevrons applied to shock-containing jets • Chevrons applied to supersonic shock-containing jets behave in many ways like chevrons have previously been found to behave with subsonic jets • They introduce streamwise vorticity and produce a lobed or Corrugated jet cross-section • They enhance bulk mixing, spreading the jet • They reduce low frequency mixing noise downstream, but increase High frequency mixing noise near nozzle • Increase in Mj produces a more outward flow angle in the undisturbed jet. Since chevrons do not change their angle, the angle between the undisturbed jet and the chevrons changes with Mj leading to an increased effective penetration as Mj increases • Thickening of the shear layer reduces potential core radius and sonic radius so the shock cells become shorter

Chevrons applied to shock-containing jets • The shock cell structure is not Corrugated, though shock strengths vary circumferentially • The initial throat waves are not altered by the presence of chevrons in position or strength • Chevrons kill screech as tabs do, but breaking symmetry • Chevrons reduce BBSN and shift the peak to higher frequencies.

Microjets applied to shock-containing jets • Microjets applied to supersonic shock-containing jets behave like chevrons in some ways, and like microjets applied to subsonic jets • They produce a lobed or Corrugated jet cross-section • They enhance bulk mixing, spreading the jet • They reduce low frequency mixing noise downstream, but increase High frequency mixing noise near nozzle • They reduce BBSN and shift peak frequencies higher • The tubes themselves suppress screech, but blowing further reduces it • The particular arrangement and mass flow rate used here performs less well than chevrons, but LES simulations suggest that higher mass flow will match chevron results

Flow and Acoustics of Jets from Practical Nozzles for High-Performance Military Aircraft