ARCHIVE COpy DO NOT LOAN

Transcription

1 AEDC TR 8 35 c:,\ ARCHIVE COpy DO NOT LOAN Investigatins f Free-Jet Test Requirements and Techniques with Emphasis n the Adaptable Jet Stretcher R. J. Matz and E. M. Kraft ARO Inc. April 1981 Final Reprt fr Perid Octber 1, September 3, 1979 TECrllICl'\L REPORTS.FILE COP'Y' Apprved fr public release; distributin unlimited. PROPERTY OF U.S. AIR FORCE 3 AEDe TECHNICAL LIBRARY ",. ARNOLD ENGINEERING DEVELOPMENT CENTER ARNOLD AIR FORCE STATION, TENNESSEE AIR FORCE SYSTEMS COMMAND UNITED STATES AIR FORCE

2 ... ;.:.:_.. -,... :"'"ct" '.'1.,.. ' ;',,::T' t;\-}/i-"':"":::-:-::'::-':""';"':""-7:-"':""':':;'':''''';;;';;;:';;+::-:-:-:-::=::''"i===.., r;k.:.:b Qi, '<';X, ;;,,/:;.,. i. I. i.:, r. \." I.. '.

3 I REPORT NUMBER r AEDC-TR-8-35 REPORT DOCUMENTATION PAGE UNCLASSIFIED GOVT ACC ESSION NO. 4 TITLE r."d Subtitle) 5 TyPE OF READ INSTRUCTIONS BEFORE COMPLETING FORM ] REC,PIEN'''S CATALOG NUMBER REPORT & PERIOD COVERED INVESTIGATIONS OF FREE-JET TEST REQUIRE- Final Reprt - Oct. 1, MENTS AND TECHNIQUES WITH EMPHASIS ON Sent. 3, PERFORMING ORG. REPORT NUMBER THE ADAPTABLE JET STRETCHER 7. AUTHOR!,) B. CONTRACT OR GRANT R.J. Matz and E.M. Kraft, ARO, Inc. (a Sverdrup Crpratin Cmpany) NUMBER!,) 9 PERFORMING ORGANIZATION NAME AND ADDRESS 1. PROGRAM ELEMENT. PROJECT. TASt< AREA & WORt< UNIT NUMBERS Engineering Develpment Center/DOT Arnld Air Frce Systems Cmmand Arnld AF Statin, TN II CONTROLLING OFFICE NAME AND ADDRESS 12. REPORT Prgram Element Arnld Engineering Develpment Center/DOS April Air Frce Systems Cmmand I J. NUMBER OF Arnld AF Statin, TN DATE 1981 PAGES 14 MONITORING AGENCY NAME &: ADDRESS(1f dlfteumt frm CntrJ"n Olliee) 15. SECURITY CLASS. (f thi reprt) UNCLASSIFIED 6587F 16 DISTRIBUTION STATEMENT (f thjs Reprt) 15. DECL ASSI FI C ATI ON SCHEDULE Apprved fr public release; distributin unlimited. N/A DOWNGRADING 17 DISTRIBUTION STATEMENT (f the abstract entered In BleM. 1, Jf different frm Reprt) lb SUPPL EMEN T ARy NOTES Available in Defense Technical Infrmatin Center (DTIC). 19. KEY WORDS (Cntinue n reverse ;de If n e c e e e erv lind Identify by blck number) inlets test facilities test methds supersnic flw flw fields subsnic flw 2 A85 T R AC T (Cntinue n rever"e ;de If nece.s.sary and IdentIty by blck number) Preliminary investigatins f appraches that will permit relatively large air-breathing engine/inlet/frebdy systems t be tested in free-jet test facilities were accmplished. Cnnumber clusins were based upn near field flw prperties (Mach and flw angle) btained frm inviscid flw cmputatins fr slender, axisymmetric bdies at zer incidence using current AEDC flw quality design gals as an acceptance criterin. DD FORM 1 JAN EDITION OF 1 NOV 65 IS OBSOLETE UNCLASSIFIED

4 UNCLASSIFIED 2. ABSTRACT, Cncluded. Results btained with subsnic and supersnic free-flight bundary cnditins indicate that shrtened frebdies can be used t reduce the verall length f test installatins required fr vehicles equipped with aft-munted inlets. Bdies immersed in a supersnic free jet experience unacceptable flw distrtins as a result f (1) bw shck reflectins frm the cnstant pressure free-jet bundary and (2) waves emanating frm the nzzle lip because f exit plume static pressure mismatch. The adaptable jet stretcher can ptentially eliminate these disturbances ver a range f test cnditins. A rigrus mathematical prf f jet stretcher cnvergence t the desired interference-free gemetry was develped fr supersnic flw. Cnvergence was als demnstrated by a cmputer experiment fr a slender axisymmetric bdy in an ff-design supersnic jet stretcher. Near field flw disturbances were reduced t an acceptable level after tw readjustments f the jet stretcher gemetry. I\.FSC "fn:>ld I'I.FS T... UNCLASSIFIED

5 AEDC-TR-8-35 PREFACE The wrk reprted herein was cnducted by the Arnld Engineering Develpment Center (AEDC) f the Air Frce Systems Cmmand (AFSC). The results f the research were btained by ARO, Inc., AEDC Grup (a Sverdrup Crpratin Cmpany), perating cntractr fr the engine test facilities at AEDC, AFSC, Arnld Air Frce Statin, Tennessee, under Prject Number E35A-OIA. The authrs gratefully acknwledge the assistance f W. J. Armstrng and G. W. Lewis, ARO, Inc., in the cmputatinal activities. The Air Frce prject manager was M. L. Laster. The manuscript was released fr publicatin n May 28, 198.

6 A E DC-TR CONTENTS Page 1. INTRODUCTION ANALYSIS 2.1 Backgrund Free-Jet Test Requirements Flw-Field Cmputatins Assessment f Jet Stretcher Feasibility and Implementatin Prblems CONCLUDING REMARKS 19 REFERENCES 21 ILLUSTRATIONS 1. Flw-Field Characteristics abut a Supersnic Vehicle with Different Bundary Cnditins Schematic f Near Field Streamline Patterns fr a Cylinder in Crssflw with Varius Bundary Cnditins Integratin f the Test Cell and Cmputatinal Dmain Iterative Scheme fr the Adaptable Jet Stretcher Cncept Details f Circular Arc Tangent/Ogive Bdies Used in Cmputatins 3 6. Bundary Cnditins and Cmputer Cdes Used in the Flw-Field Cmputatins Flw Cnditins Near a l-eal Tangent/Ogive Bdy with Subsnic and Transnic Free-Flight Cnditins Flw Cnditins Near a 2-cal Tangem/Ogive Bdy with Subsnic and Transnic Free-Flight Cnditins Flw Cnditins Near a 3-cal Tangent/Ogive Bdy with Subsnic and Transnic Free-Flight Cnditins Flw Cnditins Near a 2-cal Tangent/Ogive Bdy with Supersnic Free-Flight Cnditins Flw Cnditins Near a 3-cal Tangem/Ogive Bdy with Supersnic Free-Flight Cnditins Cmparisn f Free-Jet Bundaries, Free-Flight Streamlines, and Cylindrical Duct Extensins Cnsidered in the Subsnic Cmputatins (3-caI Tangent/Ogive, M =.6) 42 3

7 A EDC-TR-8-35 Page 13. Flw Cnditins Near a 3-cal Tangent/Ogive Bdy with M =.6 Free-Flight and Free-Jet Operatin Flw Cnditins Near a 3-cal Tangent/Ogive Bdy with M =.6 Free-Flight and Ducted Operatin Cmparisn f Free-Jet and Free-Flight Flw Details (5-percent Blckage 3-cal Tangent/Ogive) Flw-Field Details Prduced by a 3-cal Tangent/Ogive Bdy in a Supersnic Free Jet _ Cmparisn f Flw-Field Cnditins Near a 3-cal Tangent/Ogive Bdy with Supersnic Free-Jet and Free-Flight Cnditins Extraneus Waves Intrduced by Pressure Mismatch at Free-Jet Nzzle Exit Streamline Trajectries at Varius Supersnic Free-Flight Cnditins abut a 3-cal Tangent/Ogive Bdy Cmputed Flw Cnditins Near a 3-cal Tangent/Ogive Bdy with Iterative Jet Stretcher Adjustments _ Characteristics Lines fr Secnd Internallteratin Preliminary Thughts n Pssible Prbe-Type Sensrs fr Perfrated Adaptable Supersnic Jet Stretchers 63 TABLE I. Current Free-Jet Design Gals 64 APPENDIX A. Cnvergence f the Adaptable Jet Stretcher in Supersnic Flw 65 NOMENCLATURE,.. " " 72 4

8 A EDC-TR INTRODUCTION Tests f full-scale engine/inlet systems in grund test facilities are limited at the present time because f the large air-prcessing systems required t achieve flw cnditins representative f flight. Specially cntured ducts r "jet stretchers" have been prpsed t reduce air supply requirements fr engine/inlet tests in supersnic free-jet test ficilities. Hwever, fixed-gemetry jet stretchers are unattractive because each test vehicle/test cnditin cmbinatin theretically requires a different jet stretcher. Incrpratin f variability - particularly in a feedback-cntrlled, nline, adaptive mde - wuld greatly imprve the ptential usefulness f jet stretchers. The bjective f this study was t determine the requirements fr, the alternatives t, and the feasibility f adaptable jet stretchers fr bth subsnic and supersnic free-jet testing. Feasibility was t be established in a rigrus mathematical sense and frm representative flw-field cmputatins fr an axisymmetric bdy in free-flight, free-jet, and ducted flw envirnments. Results were als t be used t btain a preliminary assessment f mechanical, instrurnentatinal, and cmputatinal requirements and t prvide a basis fr utlining additinal wrk that must be accmplished befre adaptable jet stretchers can becme a practical test tl. 2. ANALYSIS 2.1 BACKGROUND Prblems arising frm prpulsin system integratin in high-perfrmance aircraft and missiles have plagued the aerspace industry fr many years. Investigatin f these prblems with full-scale hardware has been hampered by the limited size and/r perfrmance capabilities f grund test facilities. Sme engine/inlet interactin tests have been cnducted with full-scale air-breathing missile systems in free-jet test facilities f the AEDC Engine Test Facility (ETF). Hwever, tests t date have been limited t relatively small vehicles having inlets lcated n mre than tw r three bdy diameters (r calibers) aft f the nse with simulated flight Mach numbers greater than tw and angles f attack less than 1 deg. Under these cnditins, it has been pssible t lcate the frward prtin f the vehicle, including the inlet cwl, within the test rhmbus f either planar, variable Mach number, r axisymmetric, fixed Mach number free-jet nzzles. AEDC has received inquiries abut supersnic, lw-altitude tests f ram rcket-pwered missiles with aft-munted inlets (e.g., Fig. la) at angles f attack appraching 3 deg. A large wind tunnel (Fig. lb) culd prvide the right Mach number, but the required 5

9 A E C -T R temperature and pressure cnditins are beynd the capabilities f existing facilities. A smaller free-jet facility, capable f furnishing the desired temperature and pressure cnditins, culd be cnsidered if extraneus effects such as the intersectin f the bw shck with the cnstant pressure bundary (Fig. Ic) can be avided r at least cntrlled t an acceptable degree. One scheme that has been prpsed (Ref. 1) t effectively increase the length f the interference-free test rhmbus is the s-called "jet stretcher" (Fig. Id). A jet stretcher, which in reality is a "test rhmbus stretcher," is a duct with an internal cntur that crrespnds t the prtin f a streamline surface dwnstream f the bw shck generated by the vehicle being tested. The stretcher is psitined s that the flw within it is islated frm the bw shck interactin with the free bundary. Althugh the lip f the jet stretcher intrduces a disturbance, careful design, fabricatin, and alignment shuld prduce flw cnditins appraching the inlet that very clsely apprximate free flight. Sme theretical and experimental investigatins f axisymmetric jet stretchers, designed fr use with axisymmetric test bdies, have been cnducted (Refs. 2 thrugh 5) t verify feasibility f the cncept and t identify peratinal prblems and perfrmance cnstraints. Since the internal surface f the jet stretcher crrespnds t a stream surface in the crrespnding free-flight flw field abut the test bdy, there is nly ne jet stretcher with a given inlet area that crrespnds t a given set f test cnditins (M, Re:', and attitude) fr that test bdy. Sme assessment f jet stretcher ff-design perfrmance, in terms f freestream Mach number, axial lcatin, and angular rientatin with respect t the test bdy (Refs. 3 and 4) and prsity f the jet stretcher surface (Refs. 4 and 5), have been investigated as pssible means f reducing cst and increasing flexibility f jet stretcher installatins. Test bdy surface pressures btained with free-stream Mach numbers that differed by.25 t.35 frm design cnditins were fund t be acceptable. Hwever, results btained with angular mismatch and limited prsity in the jet stretcher are incnclusive based upn investigatins t date. Furthermre, jet stretcher design and applicatin has been limited t maximum angle-f-attack testing f abut 5 t 1 deg where significant bundary-layer separatin begins t ccur (Ref. 6) n slender bdies. The relatively limited perfrmance capability f a cstly fixed-gemetry jet stretcher, tgether with limitatins f available analytical techniques required fr design purpses, has significantly delayed develpment f the cncept. Cntinued cncern abut the interpretatin f data btained with wind tunnel mdels particularly with transnic test cnditins - has arused interest in "self-crrecting" r "The internal jet stretcher cntur is mdified frm a stream surface t accunt fr bundarylayer develpment. 6

10 A EDC-TR "adaptive wall" wind tunnels (Refs. 7 and 8). The basic prblem, f curse, is that the walls f a wind tunnel impse a bundary cnditin n the flw abut a test vehicle (Fig. 2) that is nt present in free flight. Fr example, a fixed, slid wall tunnel cmpresses the streamlines abut a mdel (Fig. 2b) and artificially accelerates the lcal flw relative t cnditins btained in free flight (Fig. 2a) at the same initial Mach number cnditin. Hwever, prper mdi ficatin f the tunnel bundary cnditin by recnturing a slid wall tunnel (Fig. 2c) r by setting apprpriate suctin flws in a ventilated tunnel (Fig. 2d) will lead t streamline patterns and flw cnditins that are cmparable t free flight. The prblem is the determinatin f the prper tunnel mdificatin required, which is the bjective f the adapative wall test philsphy. T understand hw the adaptive wall cncept might be applied t an adaptable jet stretcher, cnsider the flw situatin created by an aerdynamic cnfiguratin in supersnic flight in a real fluid f unlimited extent (Fig. 3a). T simulate the flw field f Fig. 3 by a cmputatinal technique is difficult with the present state-f-the-art because f the cmplex gemetry, strng viscus effects near the bdy, and shck-bundary layer interactins. Nte, hwever, that the severe restrictins n the cmputatinal methds have t d with the regin near the bdy. Cmputatinal methds can adequately simulate the cnditins far frm the bdy. In cntrast, if a test cell is cnsidered as an analg cmputer, a gd simulatin f the flw near the bdy, which accurately accunts fr the gemetry and viscus interactins, can be prvided. Unfrtunately, because f the finite extent f the test cell it is difficult t simulate accurately the far field cnditins (unless the test article is infinitesimally small). Cnsequently, the ideal device t simulate the flw field f Fig. 3a wuld be a hybrid device using an analg simulatr (the test cell) fr the near field and a digital simulatr (cmputatinal fluid dynamics) fr the far field. The merging f the analg and digital devices can best be described by examining Fig. 3b where the infinite fluid regin is divided int tw parts, exterir (E) and interir (I) t an imaginary surface, S. First, cnsider the flw in the exterir regin. In the exterir regin there are n immersed bdies; hence, viscus effects are essentially insignificant. Cnsequently, inviscid theries (such as the methd f characteristics r, fr small perturbatins, linearized smalldisturbance thery) which are well within the realm f practical cmputatinal methds can be applied in the exterir regin. It is clear, then, at least in the inviscid apprximatin, the flw thrughut the exterir regin culd be determined by prescribing t he exist ing streamline slpes at S, fr that wuld amunt t prescribing the shape f the stream surface there. All ther flw variables thrughut the external regin (and at S) are determined by these prescribed bundary values and the cnditins at infinity. Hence, given the streamline 7

11 A E DC-TR slpes at 5, ther variables such as the static pressure are uniquely determined by the strng cnditins at in finity. Frm the fact that the flw in the exterir regin is determined by bundary values f a single variable at the surface, 5, tgether with the cnditins at in finity, it fllws that tw variables at 5 are adequate t define the cnditins fr uncnfined flw. In ther wrds, specificatin f tw flw quantities all ver 5 verdetermines the flw prblem in the exterir regin unless they have the required functinal relatinship with each ther that satisfies uncnfined flw at infinity. Turning nw t the interir regin, nte that the flw is determined by the stream parameters, the aerdynamic vehicle, and the values f the flw variables at 5, withut any apprximatins. Furthermre, if the interir regin is replaced with a test cell with an adaptable jet stretcher (Fig. 3c), then t simulate the cnditins f flight in that test cell, it is necessary and sufficient that cnditins at 5 be the same as in uncnfined flw (where 5 is any cnvenient surface within the cell). In general, these cnditins are nt met fr an arbitrary jet stretcher cnfiguratin. If tw flw quantities are measured at 5, hwever, it culd be ascertained by cnsideratin f the bundary value prblem in the regin exterir t 5 if the measured flw quantities are cnsistent with the uncnfined flw cnditins at infinity. If they are nt, then the jet stretcher gemetry wuld have t be altered in sme fashin t achieve uncnfined flw cnditins at the test article. A basic iterative scheme f measurement and calculatin fr mdifying the jet stretcher t achieve uncnfined flw is shwn in Fig. 4. Fr cncreteness, the pressure distributin (PI) and the flw angle relative t 5 (8 1 ) are selected as the tw flw variables measured at 5. First, a flw field is established in the cell, and the flw variables, PI and 8J, are measured at the given cntrl surface, 5. The exterir uncnfined regin is then evaluated by specifying 8E = 8) (subscript E designating the exterir regin) as the bundary value at 5. If the distributin at 5 f P E determined frm the exterir regin calculatins des nt agree with PJ, the flw is still cnstrained by the jet stretcher and the jet stretcher must be readjusted. The iteratin cntinues until PI and P E agree. The relaxatin factr, k, is intrduced t accelerate cnvergence f the iterative prcess. The bjective f the present wrk was t determine (1) requirements fr and feasibility adaptable jet stretchers at AEDC, (2) cmpatibility with ther free-jet hardware, and (3) additinal wrk required fr develpment f the cncept. The methd f apprach included an assessment f currently envisined free-jet test requirements at AEDC and an evaluatin f alternate test techniques. A rigrus mathematical prf that the adaptive wall cncept must cnverge t give free-flight cnditins with supersnic flw was develped t 8

12 AEDC-TR-8-35 cmplement the prf previusly develped (Ref. 9) fr subsnic flw. Cmputatins f inviscid perfect gas flw field cnditins abut simple axisymmetric shapes were used t determine differences between free-flight, free-jet, and ducted flw cnditins. Cmputatins were made fr a test bdy and a mismatched jet stretcher t demnstrate cnvergence t supersnic free-flight cnditins and t determine relative flw cnditins at intermediate iteratins. Implementatin cnsideratins and additinal wrk required t ensure that the adaptable jet stretcher is, in fact, a practical test cncept are als discussed. 2.2 FREE-JET TEST REQUIREMENTS Current free-jet test activities at AEDC are limited t the ETF Aerdynamic and Prpulsin Test Unit (APTU) where supersnic free-stream Mach number cnditins in the range frm 2 t 5 are achieved with fixed Mach number axisymmetric nzzles. Full-scale vehicles with crss-sectinal areas (i.e., blckages) ranging frm 5 t 5 percent f the nzzle exit area have been cnsidered fr test at preselected angles f attack up t 3 deg. In sme prpsed tests, the vehicle inlet is as much as 12 bdy diameters aft f the frebdy nse. The APTU supersnic free-jet test apprach assumes that nly flw cnditins in the theretical nzzle test rhmbus are acceptably representative f free-flight cnditins. The frward prtin f the vehicle t be tested, frm the nse t the inlet cwl, must therefre be psitined within the test rhmbus, and the APTU exhaust diffuser must be designed and perated t ensure that the free-jet nzzle is flwing full at all cnditins f interest. Maximum angles f attack that can be achieved depend upn the free-jet nzzle design Mach number and size and the relative size and cnfiguratin f the test vehicle. Free-jet tests accmplished t date in APTU have been limited t maximum angles f attack f abut 1 deg. N tests have been accmplished in APTU with jet stretchers. Subsnic and supersnic free-jet test capabilities t be incrprated in the Aerprpulsin Systems Test Facility (ASTF) are currently under cnsideratin. Presently perceived design gals (Table I) include bth steady-state and transient capabilities with bth subsnic and supersnic flw and pitch and yaw variatins. Tests f full-scale gas turbine engines, inlets, and any airframe surfaces that might affect flw appraching the inlets will require a large test rhmbus since blckages, even at zer attitude cnditins, will be in the 3- t 5-percent range. At maximum attitude angles, effective blckages will be even greater, and the flw prcess will be further cmplicated by impingement f the nzzle free-jet bundary n the test vehicle. Because f the limited extent f the free-jet nzzle test rhmbus, it may nt be pssible t include all f the frward airframe cmpnents that might affect flw int the engine inlet. Under these cnditins, an apprximate shrtened frebdy will be used in cnjunctin with flw-field measurements in the vicinity f the initial inlet ramp. The frebdy simulatr and test cnditins prvided in ASTF will be 9

13 AEDC-TR-BO-35 selected t prduce flw cnditins at sme measurement statin that are cmparable t thse btained frm preceding wind tunnel inlet mdel tests. Achieving the ASTF nw quality gals with all f the desired peratinal capability will be a frmidable task. 2.3 FLOW-FIELD COMPUTATIONS Apprach Cmputatins f subsnic and supersnic flw fields arund bdies representative f missile and aircraft frebdies were made t determine differences between free-flight, freejet, and ducted flw cnditins. Analysis was limited t axisymmetric bdies (Fig. 5) at zer angle f attack withut inlet thrugh-flw representatin. Six different analytical mdels were emplyed since n single existing cmputer cde has the demnstrated capability t handle this wide range f bundary cnditins. Free-flight r infinite stream cnditins (Fig. 6a) were btained in the subsnic and transnic regimes with the Suth-Jamesn finitedifference, relaxatin slutin f the full ptential equatin fr axisymmetric inviscid flw (S-J, Ref. 1) while crrespnding supersnic cnditins were btained with the Lckheed (LMOC, Ref. II) and Armstrng three-dimensinal (A3DMOC, Ref. 12) rtatinal methd-f-characteristics cdes. LMOC was used fr the supersnic free jet (Fig. 6b) calculatins and bth LMOC and A3DMOC were used fr the supersnic jet stretcher (Fig. 6c) evaluatins. The time-dependent, finite-difference Cline nzzle and plume cde (CNAP, Ref. 13) was the nly analysis available fr the subsnic free-jet (Fig. 6d) evaluatins. Bth CNAP and the Wehfer-Mger time-dependent, finite-difference analysis (W-M, Ref. 14) were used in the ducted subsnic flw (Fig. 6e) evaluatins. The Duglas-Neumann panel methd ptential flw cde (D-N, Ref. 15) was used t btain subsnic infinite stream results (Fig. 6 fr cmparisn with CNAP and W-M results. CNAP, because f its unique capability with the bundary cnditins, had t be emplyed in the subsnic free-jet cmputatins. Hwever, cnsiderable effrt was necessary t achieve reasnably stable slutins representative f steady-state cnditins. Varius cmbinatins f time step size and artifical viscsity factrs were attempted with n apparent success within the 5 t 1, time steps (5 t 1 min CPU* time with 1,6 grid pints) that were generally adequate fr typical CNAP nzzle and plume cmputatins. As a last resrt, the nse f the frebdy was mdified n the premise that the differencing scheme used in CNAP has accuracy limitatins which, in cmbinatin with the slpe discntinuity at the frebdy nse, might be aggravating the cmputatinal instabilities. The nse fairing was gradually increased in extent until reasnably stable flw cnditins "Cmputer times indicated are fr the central prcessr unit (CPU) f an 181"1 37/165 machine. 1

14 AEDC-TR-8-35 were achieved. This required a faired sting (Fig. 5) with crss-sectinal area equal t 1 percent f the bdy. With this gemetry and 8, time steps (95 min CPU time) stable, freejet slutins were finally achieved" with a free-stream Mach number f.6. CNAP and W-M (55 min CPU time fr 6 iteratins with 21 by 111 grids) were used t evaluate the mdified frebdy in a cylindrical duct under subsnic cnditins, and equivalent infinite stream results were btained with D-N** (13 sec CPU time fr 228 ndal pints). Unique capabilities and limitatins f the A3DMOC and LMOC cdes were weighed in the selectin f the apprpriate MOC analysis t use in a particular case. A3DMOC, because f its streamline tracking capability, was used t define supersnic jet stretcher gemetries. The entrpy "smearing" prblem that ccurs t sme degree in all rtatinal MOC cdes was nted in cmparisns f near field flw prperties (lcal ttal pressure and Mach number) btained with A3DMOC and LMOC (CPU times = 11.3 and 1.3 min, respectively). Because f the rerdering prcess necessary fr the mre general threedimensinal cmputatins, entrpy smearing is mre severe in A3DMOC than in LMOC. Fr this reasn and because f its general applicability t infinite stream, free-jet, and ducted bundary cnditins, LMOC was used fr the majrity f the supersnic cmputatins. AEDC mdificatins t the basic LMOC cde prvided pltting and interplating capabilities and arbitrary pressure bundary input ptins that were bth useful and necessary fr manual iteratins between internal and external adaptive jet stretcher cnditins. Because f LMOC limitatins, nly bdy shapes and free-stream Mach numbers crrespnding t attached bw shck cnditins were cnsidered in this study. All f the cmputatins made during this investigatin were preliminary in nature since they are restricted t axisymmetric gemetries and inviscid cnditins. Flw fields assciated with bdies and jet stretchers at angle f attack will be three-dimensinal in nature with significant viscus effects near the surfaces which will, in extreme cases, lead t regins f separated flw. Furthermre, an inlet lcated at sme pint alng the frebdy, (Fig. Ia) will bviusly alter the flw fields, as will the relative inlet capture/spillage cnditins. Hwever, theretical investigatins f the full viscus, cmpressible, three-dimensinal prblem this represents cannt be achieved until the apprpriate algrithms are develped fr this cmplex cmbinatin f bundary cnditins. *Cmputatinal instabilities increased as free-stream Mach number apprached unity, even with the mdified frebdy. **D-N, which includes a tangent gas apprximatin fr cmpressibility effects, had t be used instead f the mre precise S-l (3.3 min CPU time tr a 97 by 97 grid) because S-l culd nt readily handle the mdified frebdy gemetry. Hwever, flw field cmputatins made with bth S-l and D-N fr an unmdified 3-cal tangenr/give and with D-N fr the mdified frebdy with a Mach number f.6 yield substantially identical results within abut fur bdy radii frm the unmdified nse statin. I I

15 AEDC-TR Infinite Stream Results Frebdy flw-field cnditins experienced in an infinite stream were f interest fr cmparative purpses and fr assessment f the ptential fr shrt frebdy simulatrs. The ASTF flw quality gals (Table I) were used as a criterin fr determining adequacy f the test cncept t represent free-flight cnditins. Bdy surface pressure cefficients and lcal free-stream Mach number and flw angle variatins near the bdy surface where inlets wuld prbably be lcated were determined fr varius caliber circular arc tangent gives with subsnic, transnic, and supersnic flw cnditins. The subsnic results (Figs. 7 thrugh 9) indicate significant flw angle variatins in the prtin f the field near the bdy shulder and fr a distance crrespnding t I t 2 bdy radii dwnstream. Hwever, Mach number prfiles indicate relatively unifrm cnditins even ne bdy radius dwnstream f the tangent pint n the ne-caliber bdy (Fig. 7b). Therefre, fr vehicles with frebdies like the circular arc tangent give with inlets lcated mre than ne bdy radius dwnstream f the shulder, Mach number and flw angle variatins shuld be unifrm t well within the ASTF design gals f ±.5 n Mach number and ± I deg n flw angle if bundary-layer perturbatins and/r separatin are insignificant. Replacement f a lnger frebdy with a shrter versin, therefre, appears t be a viable subsnic test apprach at zer angle f attack if the test installatin des nt intrduce significant extraneus effects such as shck/bundary interactins at supercritical flw cnditins. Infinite stream supersnic flw results (Figs. 1 and 11) indicate significant variatins in flw angle, particularly near the bdy shulder, and sme Mach number variatins that result frm entrpy gradients acrss the axisymmetric bw shck wave. Hwever, dwnstream f the shulder, particularly with free-stream Mach numbers less than 3, flw angle and Mach number variatins are unifrm t within the ±.6 deg and ±.5 Mach number ASTF design gals. Therefre, use f a shrtened frebdy als appears t be a viable ptin at supersnic test cnditins Subsnic Free-Jet Results The majr cncern in all wind tunnel and free-jet tests is that the test sectin r free-jet bundary will impress r reflect extraneus flw cnditins n the test bdy that are nt representative f free flight. This was investigated with a limited series f cmputatins invlving the mdified 3-cal tangent give in free jets crrespnding t 1- and 5-percent blckage cnditins. Because f the cmputatinal stability prblems with CNAP, nly ne subcritical free-stream cnditin crrespnding t.6 Mach number culd be cnsidered during the time perid f this study. 12

16 AEDC-TR-8-35 The cnstant pressure free-jet bundaries crrespnding t bth the 1- and 5 percent blckage cnfiguratins were fund t very clsely apprximate the infinite stream 1- and 5-percent streamlines fr the case cnsidered (Fig. 12). The cnstant pressure free-jet bundary alters bdy pressure distributins upstream f the shulder regin t sme degree (Fig. 13a) and apparently causes a mre rapid return t free-stream cnditins dwnstream f the shulder than ccurs under infinite stream cnditins. Lcal free-stream Mach numbers and flw angles (Fig. 13b) are within ±.1 and ±.5 deg, respectively, f infinite stream cnditins even n the cntured prtin f the mdified frebdy where the mst significant variatins ccur in bdy surface pressure cefficients. Althugh these results indicate surprisingly gd agreement between free-jet and infinite stream cnditins even with substantial blckages, care must be exercised t avid sweeping cnclusins abut relative subsnic free-jet flw cnditins based upn this single set f idealized (mdified frebdy with inviscid, subcritical, zer angle-f-attack cnditins) results withut additinal theretical r experimental cn firmatin Subsnic Jet Stretcher Cnsideratin One f the planned wrk items f this study was t accmplish a series f iterative cmputatins t demnstrate that flw cnditins abut a bdy d, in fact, relax t freestream cnditins with an adaptable subsnic jet stretcher. Cylindrical extensins (Fig. 12) f the free-jet nzzle were selected as an initial gemetry fr the jet stretcher, and cmputatins were initiated with CNAP. Althugh the CNAP cmputatinal instability prblems precluded cmpletin, the available results d ffer sme infrmatin fr cnsideratin. Fr example, chking cnsideratins limit subsnic Mach numbers upstream f a 5-percent blckage mdel in a cylindrical duct t smething less than.3. With this cnstraint it might be cncluded that representative flw cnditins fr a free-flight Mach number f.6 culd nt be achieved. This wuld indeed be true if cmparable high quality (i.e., wind tunnel) flw cnditins were required ver the entire bdy. Hwever, in the prpsed ASTF free-jet test philsphy, representative flw cnditins will nly be required in the vicinity f the inductin system inlet. With this cncessin, it shuld be pssible t select test cnditins (i.e., verall pressure rati and/r jet stretcher gemetry) t prduce desired nminal levels and distributins f Mach number and flw angle at the inductin system inlet even thugh there are significant mismatches in ther regins f the flw field. This is illustrated mst dramatically by the CNAP results fr a mdified 3-cal tangent give in a cylindrical duct. Because f the chking prblem and the CNAP input requirements, the ducted subsnic cmputatins were made with the duct exit Mach number set at.6. Pressure cefficients (Figs. 14a and b, based n M =.6 at X/R = 16) indicate the significant verall fre-taft deviatins frm free flight that can be expected with significant blckages n bth the 13

17 A EDC-TR mdel and the uter bundary. Hwever, with matched dwnstream cnditins, the majr differences between free flight and ducted flw are frced t ccur near the upstream end f the test bdy. Even fr the extreme case represented by the 5-percent blckage mdel in a cylindrical duct, cmputed flw cnditins at the shulder (X/R a = 6) agree with free flight t within.3 n Mach number and.5 deg n flw angle (Fig. 14c). Fur bdy diameters (X/R a = 8) dwnstream f the nse statin, all cmputed results (free flight, ducted, and free jet) were fund t agree within ±.1 n lcal Mach number and ±.3 deg n lcal flw angle. Since the radial distributins are similar in shape, it appears that the agreement between ducted, free-jet, and free-flight results culd be further imprved if necessary with nly slight adjustments in bundary cnditins. Althugh these results are prbably representative fr any slender bdy f interest with subcritical flw, additinal investigatins are required t determine if they are significantly altered when supercritical cnditins ccur Supersnic Free-Jet Results The principal cause fr differences between supersnic free-flight and free-jet flw cnditins is the reflectin f bdy-generated cmpressin and rarefactin waves which significantly alter the cnstant pressure free bundary shape frm the crrespnding freeflight streamline (Fig. 15). The reflected rarefactin zne frm the bw shck/free bundary interactin (Fig. 16a) and subsequent reflectins frm the bdy and free bundary (Fig. 16b) can significantly alter near field cnditins (Fig. 17), particularly when high blckage bdies are evaluated at lw supersnic Mach numbers. Any mismatch between free-jet bundary (i.e., test cell) and free-jet nzzle lip pressure als intrduces extraneus waves that can distrt supersnic free-jet test results. An expansin fan (Fig. 18a) r a cmpressin wave (Fig. 18b) will emanate frm the free jet nzzle lip depending upn the relative magnitudes f the nzzle lip and jet bundary pressures. These waves will impinge upn and reflect frm the test bdy in a manner that culd-with a significant pressure mismatch-unacceptably distrt the flw (Fig. 18c) entering an aft-munted inlet Supersnic Jet Stretcher Results A prperly designed jet stretcher can minimize the effect f bw shck interactins with a free-jet bundary. Als, if the jet stretcher lip size and lcatin are prperly selected, the flw field f interest can be islated (Fig. l d) frm extraneus waves prduced by mismatches between the nzzle lip and test cell pressures. Hwever, a fixed-gemetry jet stretcher has bvius limitatins as indicated by a cmparisn f free-flight streamline 14

18 AE DC-T R trajectries (Fig. 19). Even with a particular bdy and blckage cmbinatin and allwance fr differences in the bw shck lcatin, free-stream Mach number alters the dwnstream streamline shape that a jet stretcher must assume. Cnsidering the additinal requirements fr angle-f-attack testing with a variety f vehicle gemetries, the need fr variability, preferably with rapid, nline feedback cntrl, is almst a necessity t ensure practicability f the jet stretcher cncept. A majr cncern in the adaptable jet stretcher philsphy is that cnvergence t freeflight cnditins can indeed be btained frm sme arbitrary initial cnditin in a reasnable number f iteratins. Fr this reasn previus mathematical prfs f the cnvergence f transnic adaptive wall wind tunnels t uncnfined r infinite stream cnditins (Ref. 9) were extended t supersnic flw and are included in the Appendix.. Cnvergence was further demnstrated in a series f cmputatins where the interir flw field cnditins between a typical bdy and an ff-design jet stretcher, which wuld nrmally be btained in a test installatin, were cmputed with LMOC. The crrespnding exterir regin cmputatins, which wuld nrmally be made nline in an actual adaptive test mde situatin, were als accmplished with LMOC. A 3-cal tangent/give was evaluated in a Mach number 2.3 airstream with an ff-design jet stretcher cnfiguratin crrespnding t an axially shifted, 5-percent blckage Mach number 5. streamline. Bdy surface pressure cefficients (Fig. 2a) and near field flw cnditins (Figs. 2b and c) btained with the shifted, mismatched jet stretcher indicate significant deviatins frm infinite stream cnditins, largely because f the 1.9-deg mismatch (Fig. 2Od) at the jet stretcher lip. Frm the initial mismatched flw cnditin, the jet stretcher cntur was varied thrugh a series f exterirlinterir regin iteratins until near field Mach numbers and flw angles agreed with infinite stream results t within ±.5 n Mach number and ±.6 deg n flw angle. Streamwise distributins f static pressure near the jet stretcher surface were used t define the uter bundary cnditins fr the interir regin cmputatins. The resulting uter bundary streamline cnturs, cmputed frm the interir slutins, were then used as a bundary cnditin in the exterir regin cmputatins. Typical LMOC CPU times were 2 and 5 sec, respectively, fr the interir and exterir cmputatins. LMOC had t be mdified t accept streamwise variatins f pressure as a functin f axial psitin and t accmplish spline fits which culd be used in the interir cmputatins. 15

19 A EDC-TR The sequence f events in the iteratins was as fllws: 1. Interir flw-field cnditins were btained with the ff-design jet stretcher cntur as the uter bundary t establish initial pressure distributins [P?(X)] alng the jet stretcher surface with M = An initial exterir regin cmputatin was made with a hypthetical axisymmetric bdy, having the ff-design jet stretcher cntur, in an infinite M = 2.3 stream t define surface pressure distributins [Px)]. 3. The initial exterir and interir pressure distributins were cmpared and a first iteratin internal pressure distributin [pi (X)] was established frm where k, the weighting factr, was arbitrarily set at A first iteratin internal slutin was accmplished with a varying pressure uter bundary represented by pi(x) t determine the first iteratin jet bundary shape. 5. A first iteratin exterir slutin was accmplished with a hypthetical axisymmetric shape crrespnding t the first iteratin jet bundary cntur t determine the crrespnding surface pressure distributin. 6. pi<x) and Pl.,<X) were used t establish a secnd iteratin pressure distributin frm Pf (\) = k [rl (X + (1 - k) f: (X 7. A secnd iteratin internal slutin was accmplished with P/<X) as the uter bundary. Changes in bdy surface pressure cefficients and near field Mach number, static pressure, and flw angle fr the tw iteratins are shwn in Fig. 2. Althugh all parameters indicate a general cnvergence t infinite stream cnditins, there is still evidence (Figs. 2d and 21) f sme residual mismatched cnditins near the jet stretcher lip that are being reflected dwnstream after the secnd iteratin. This mismatch is attributed in part t the upstream bundary cnditins used in the interir cmputatins and culd prbably be eliminated with a mre careful selectin f LMOC starting line pint cnditins. Secndary ripples in the bdy surface pressure cefficients, particularly with the initial ff-design jet 16

20 AEDC-T R stretcher gemetry, are prbably indicative f irregularities in the jet stretcher cntur that are prduced by the cmbinatin f the number f pints selected and the resulting spline fits. 2.4 ASSESSMENT OF JET STRETCHER FEASIBILITY AND IMPLEMENTATION PROBLEMS General Remarks Althugh actual evaluatin f the adaptable jet stretcher cncept was very limited and cnfined t the relatively simple case represented by an axisymmetric bdy at zer angle-fattack in axisymmetric supersnic and subcritical, subsnic inviscid jets, the results btained d indicate feasibility f the apprach. Hwever, there is bviusly a large step frm the limited feasibility demnstrated in this study t the prvisin f a practical system that can be used in either APTU r ASTF. Further investigatins, bth theretical and experimental, are bviusly required. Three-dimensinal subsnic and supersnic cmputatinal capability fr general bdies embedded in planar r axisymmetric flw fields is required t extend the feasibility studies t relevant ASTF and APTU test cnsideratins. Sme experimental mdel studies will als be.equired t evaluate cntrl systems, instrumentatin cncepts, and external regin cmputatinal algrithms. When an apprpriate cmputer cde becmes available, relative effects f flw int the engine inlet n jet stretcher aspects shuld als be cnsidered, particularly with subcritical inlet peratin. High angle-f-attack tests f large bdies that will penetrate a subsnic free-jet bundary will create unique flw prblems that may be difficult t crrect with a jet stretcher. After the planned FY 81/82 ASTF subsnic free-jet mdel tests in Prpulsin Research Cell (R-2A2) are accmplished, a better assessment can be made abut the ptential rle f adaptable jet stretchers in subsnic free-jet testing. High angle-f-attack testing f slender vehicles with aft-munted engine inlets culd als be difficult t handle with jet stretchers. Extensin t angles f attack up t abut 2 deg, where separatin ccurs n the vehicle in a fairly simple, time-wise steady manner (Ref. 6), may be pssible. Hwever, the asymmetric shedding f multiple vrtex sheets bserved (Ref. 6) with slender vehicles at higher angles f attack will pse even mre frmidable implementatin prblems. Incrpratin f adaptable jet stretchers in APTU will require a high-respnse cntrl system and a fast external flw algrithm that are cmpatible with the limited blwdwn 17

21 A E DC-TR times. Incrpratin int ASTF will depend t a large degree n cmpatibility with the planar free-jet nzzle and assciated free-jet spillage diffuser systems actually used Jet Stretcher Cntrl In actual tests, the tw independent flw parameters required fr the jet stretcher adjustments must be measured alng sme reference surface (r surfaces) in the vicinity f the jet stretcher surface. Ideal measurable flw parameters fr this purpse wuld be (I) simple t btain, (2) nninterfering, and (3) highly respnsive t jet stretcher adjustments. In current subsnic/transnic adaptive wind tunnel wall studies, flw-field measurements made depend upn the manner in which the effective wall shape is achieved. If a flexible wall is emplyed (Ref. 8), wall static pressure distributin and cntur gemetry are the cntrl parameters. In a perfrated r sltted tunnel (Ref. 7), where effective uter bundary shape is changed by air injectin r remval, static pressures and flw angles are usually measured with static pressure pipes and hemisphere/cylinder prbes at sme reference surface near the wall but utside the bundary-layer interactin regin. Static pressure measurements frm tw different reference surfaces are als being cnsidered as an alternative. Cmparable measurements culd be made in a subsnic jet stretcher. Actually recnturing the wall f a jet stretcher required fr three-dimensinal supersnic flw with temperatures greater than 2 t 3 F will be extremely difficult. If effective shaping is accmplished with air injectin r remval, cntrl parameter measurements are mre f a prblem. Optical techniques, such as laser velcimeters, wuld pse frmidable installatin and peratinal prblems. On the ther hand, insertin f prbes int the supersnic airstream will intrduce extraneus cmpressin and expansin waves. Calibrated cne prbes r duble wedge airfil sectins (Fig. 22) extending beynd the interactin zne might be used fr supersnic adjustable jet stretcher cntrl parameter measurement if the extraneus waves can be tlerated. In any case, the prbes must be carefully designed t be as small as pssible and t extend n farther beynd the bundarylayerlinbleed interactin zne than necessary. Careful cnsideratin must als be given t the number and placement f the prbes t ensure that measurements are btained at the mst critical regins with a ttal number f parameters that can be accmmdated by the available data acquisitin and cnditining equipment Online Exterir Flw Cmputatins The present studies indicate that the methd f characteristics is a reasnable algrithm t use, at least fr axisymmetric flw, in the nline exterir flw cmputatins fr supersnic flw. A new cmputer cde must be frmulated t take the specified measured 18

22 AEDC-TR-SO-35 quantities, make the apprpriate characteristics cmputatins, assess the adequacy f current cnditins, and then prvide an utput that can be cnverted int an apprpriate signal t inbleed/utbleed cntrl valves. The relatively high cmputatinal speeds achieved with a general prgram like LMOC suggest that CPU time shuld nt be a prblem fr a tailred MOC cde. Hwever, the cde shuld be cnstructed with time ptimizatin in mind since this may be crucial t satisfactry peratin with a blwdwn facility like APTU. The subsnic/transnic free-flight results presented herein suggest that elements f the ptential flw and finite-difference algrithms incrprated in D-N and S-J culd be emplyed fr external regin cmputatins with subsnic adaptable jet stretchers. The ptential flw ptin is particularly attractive because f its flexibility and speed; hwever, it is limited t subsnic cnditins n the reference surface. Algrithms being develped fr adaptive transnic wind tunnel applicatins shuld als be cnsidered fr extensin t subsnic jet stretchers. Fr example, the Prandtl-Glauert frm f the linear small disturbance equatins and a finite-difference frmulatin f the transnic small disturbance equatin have been used (Ref. 16), respectively, with subsnic and transnic flw cnditins at the reference surface. Further investigatins are required t determine advantages and disadvantages f extending these and ther adaptive wind tunnel external flw analyses t subsnic/transnic jet stretchers. 3. CONCLllDING REMARKS Appraches that will permit relatively large airbreathing engine/inlet/frebdy systems t be free-jet tested in APTU and ASTF received preliminary investigatin. Cnclusins were based upn near field flw prperties (Mach number and flw angle) btained frm inviscid flw cmputatins fr slender, axisymmetric bdies at zer incidence using ASTF flw quality design gals as an acceptance criterin. Results btained with subsnic and supersnic free flight bundary cnditins indicate that shrtened frebdies can be used t reduce the verall length f test installatins required fr vehicles equipped with aft-munted inlets. Limited axisymmetric calculatins fr slender bdies immersed in free jets at subcritical fiw cnditins indicate streamlines and lcal steady-state flw prperties that are cmparable t free-flight cnditins t well within the ASTF flw quality gals. Hwever, this agreement is favrably biased in the calculatins by an artificial upstream extensin t the frebdy which was necessary t reduce cmputatinal instabilities. Althugh the instability prblems precluded quantitative verificatin that an adaptable jet stretcher culd reduce differences between subsnic free-jet and free-flight results, the nging transnic 19

23 AEDC-TR-8-35 wind tunnel adaptive wall studies and the limited results btained n this study d tend t supprt this premise. Bdies immersed in a supersnic free jet experience unacceptable flw distrtins as a result f (1) bw shck reflectins frm the cnstant pressure free-jet bundary and (2) waves emanating frm the nzzle lip because f exit plume static pressure mismatch. The adaptable jet stretcher can ptentially eliminate these disturbances ver a range f test cnditins. A rigrus mathematical prf f jet stretcher cnvergence t the desired interference-free gemetry was develped fr supersnic flw t cmplement the previusly develped prf fr subsnic cnditins. Cnvergence was cnfirmed by a cmputer experiment fr a slender axisymmetric bdy in an ff-design supersnic jet stretcher. Near field flw disturbances were reduced t an acceptable level after tw readjustments f the jet stretcher gemetry. The methd f characteristics appears t be an acceptable algrithm fr external regin calculatins with supersnic adaptable jet stretchers. Adaptatin f linear small disturbance and finite-difference frmulatins under develpment fr adaptive transnic wind tunnels shuld be pssible fr external flw cmputatins in subsnic jet stretchers.

24 A E DC-TR-8-35 REFERENCES 1. Himrnler, E. B., et al. "A Special Technique fr Free-Jet Testing f Aft-Inlet Systems." The Marquardt Crpratin Reprt MP-1429, Nvember German, R. C. "Simulatin f Supersnic Flw Over a Bdy f Revlutin Using an Axisymmetric Jet Stretcher." AEDC-TR (AD875834), Octber Bareb, R. L. and Matkins, E. H. "Simulatin f Supersnic Flw Over a Scale Mdel Missile with Aft-Munted Inlets Using an Axisymmetric Jet Stretcher." AEDC TR (AD51489l), April Eppright, B. R. "Theretical Effects f Prsity and Angle-f-Attack n Jet Stretcher Perfrmance." University f Texas, PhD Dissertatin, May Bauer, R. C. et al. "A Theretical and Experimental Study f a Jet Stretcher System." Jurnal f Spacecraft and Rckets, Vl. 1, N.6, June 1973, pp Przirembel, C. E. A. and Shereda, D. E. "Aerdynamics f Slender Bdies at High Angles f Attack." Jurnal f Spacecraft and Rckets, Vl. 16, N. I, January February 1979, pp Sears, W. R. "Self Crrecting Wind Tunnels." Aernautical Jurnal, Vl. 78, N , February-March 1974, pp Gdyer, M. J. "The Self-Streamlining Wind Tunnel." NASA Langley Research Center Reprt NASA TMX-72699, August L, C. F. and Kraft, E. M. "Cnvergence f the Adaptive Wall Wind Tunnel." American Institute faernautics and Astrnautics Jurnal, N. I, January 1978, pp Suth, J. c., Jr. and Jamesn, A. "Relaxatin Slutins fr Inviscid Axisymmetric Transnic Flw ver Blunt r Pinted Bdies." Prceedings f the First AIAA Cmputatinal Fluid Dynamics Cnference, Palm Springs, Califrnia, July 19-2, II. Smith, S. D. and Ratliff, A. W. "Rcket Exhaust Plume Cmputer Prgram Imprvement, Vl. IV." Lckheed Missiles and Space Cmpany Reprt LMSC/HREC DI6222-1V-A, June Armstrng, W. C. and Bauer, R. C. "Analysis f Three-Dimensinal Inviscid Supersnic Flw Between a Bdy and an Outer wall (with Applicatin t a Jet Stretcher System)." AEDC-TR (AD-A29123), August