Prediction of Wing Downwash Using CFD

Transcription

1 Predcon of Wng Downwash Usng CFD Mohammed MAHDI* *Correspondng ahor Aeronacal Research Cener-Sdan P.O. Bo 334 DOI:.3/ rd Inernaonal Worshop on Nmercal Modellng n Aerospace Scences NMAS May 5 Bchares Romana (held a INCAS B-dl Il Man secor 6) Secon 5 New conceps n UAV sysems Absrac: Wng downwash sdy and esmaon of downwash effec on he al plane s an mporan as drng he arcraf desgn process alhogh a lo of papers and wors has been done b he epermenal wor s he mos mporan he progress n CFD smlaon has reached o he pon s able o redce he nmber of rns n he wnd nnel. In hs wor CFD has been lzed o calclae he downwash angle and downwash graden wh respec o he angle of aac over a hgh aspec rao of a ypcal UAV. The resls of he smlaon shall be sed n he esmaon and calclaon of he longdnal sac sably analyss of he UAV. Key Words: wng downwash CFD sac longdnal Sably UAV. INTRODUCTION Downwash s he ar forced down by he aerodynamc acon of wng or helcoper roor blade n moon as par of he process of prodcng lf (Wpeda []). Several sdes o analyze he behavor of wng downwash have been made. Jrovch [] has appled he CFD approach o he predcon of he downwash flowfeld behnd a KC-35 aner. The resls show he crcaly of caprng he ralng vore behnd he wng sng a fne mesh o no he far feld. The resls are compared o he heory vore lace resls and wnd nnel daa. Whle nmercal dsspaon s evden n he premare spreadng of he vore core hs does no affec he downwash flowfeld whch s he crcal facor n aeral refelng. Eler solons wh properly appled grddng are shown o be sffcen for caprng he aner flowfeld for aeral refelng needs. Anad & Kam 4 [3] have sded he aerodynamcs performance of wng canard confgraon sng he CFD analyss and hey smlaed he flow pah lnes and parcles flow feld over he canard-wng confgraon o esmae he bes vercal posonng of he canard wh respec o he wng. In or wor we wll sdy he flow feld properes of a ypcal UAV hgh aspec rao wng; he wng dmensons are ablaed n able. Table wng dmensons of ypcal UAV Wng Area. m Wng span 5. M Roo chord. M Tp chord.47 M Wng arfol E 396 INCAS BULLETIN Volme 7 Isse / 5 pp. 5 ISSN 66 8

2 Mohammed MAHDI 6 INCAS BULLETIN Volme 7 Isse / 5. NUMERICAL MODELING The flow feld s solved sng ncompressble Naver Soes Eqaon for he wng and whole arcraf analyss. The mass conservaon law and he energy eqaon are solved wh he Naver-Soes eqaons; he K-omega SST rblence model was sed o predc he drag accraely. For analyzng he sded wng he flow s assmed o be ncompressble de o he mamm speed s 5(m/s). So only Naver soes eqaons wh he K-Omega and conny eqaon are solved smlaneosly (Chng [4]). Mass conservaon law: () Naver-Soes Eqaons: These eqaons were employed n he followng form []: z T y S R z G y F E q () where w v q w v p E vw p v v v F p w vw w w G z z R yz yy y S zz zy z T Selecon of rblence model depends on he ype of grd.e. srcre or nsrcred grd. Accordngly for he presen smlaon K-omega SST rblen model was sed for he prpose of rblence closre. Ths model has wde spread poplary among he CFD researchers. For more nformaon abo hs model see Mener 994 [5]. He saes ha hs model s more accrae han -epslon especally near wall layers and for flows wh moderae adverse pressre gradens. He developed he SST scheme for aerospace applcaons as follows. P * (3) P (4) And snce he flow s seady ncompressble he above eqaon becomes smpler. Defnon of he eddy-vscosy:

3 7 Predcon of Wng Downwash Usng CFD Trblen sress ensor s gven by: The shear sress s calclaed as follows: (5) 3 (6) a (7) Wh he consan a =.3. On he oher hand n wo-eqaon models he shear-sress s comped from (8) The rblence nensy I s defned as he rao of he roo-mean-sqare of he velocy flcaons o he mean flow velocy avg. A rblence nensy of % or less s generally consdered low and rblence nenses hgher han % are consdered hgh. The rblence nensy n he free sream s sally avalable from he nnel characerscs. In modern low-rblence wnd nnels he free-sream rblence nensy may be as low as.5%. n hs wor he rblence nensy assmed.%. The rblen vscosy rao / s drecly proporonal o he rblen Reynolds nmber ( Re /( v) ). Re s large (on he order of o ) n hgh-reynolds-nmber bondary layers shear layers and flly-developed dc flows. However a he free-sream bondares of mos eernal flows / s farly small. Typcally he rblence parameers are se so ha / Bondary Layer Consderaon: Bondary layer s calclaed based on Reynolds nmber; he followng epresson defnes he Reynolds No. V C Re (9) Accordng o Anderson [6] he bondary layer hcness s gven by:.37 () Re / 5 Grd Generaon. Fgre () shows he srfaces mesh of wng; a fne mesh s focsed near a/c srface o smooh srfaces as well as consderng he bondary layer effec he nerval sze of he elemen s almos.5 (mm) whch s creaed nsde ANSYS worbench. Unsrcred grd wh rangles and erahedral n he srface and volme meshes appromaely mllon cells s creaed n he compaonal doman of he wng. INCAS BULLETIN Volme 7 Isse / 5

4 Mohammed MAHDI 8 Downwash Fgre wng mesh and bondary layer s clear Downwash s he ar forced down by he aerodynamc acon of a wng or helcoper roor blade n moon as par of he process of prodcng lf an arcraf prodces aerodynamc lf by deflecng ar downwards as downwash. As shown n fgre downwash generaes an eqal and oppose pwards force on he wng called lf. When he downwash force eceeds he wegh of he arcraf he arcraf wll rse snce here s a consderable pressre dfference beween he lower and pper srfaces of a wng; p vorces are prodced a he wngps [4]. Fgre Downwash Defnon The downwash s esmaed from he CFD ha rae lne s consrced along he aerodynamcs as of he HT and hen he downwash effec on he al s ploed vs. HT sem span. Accordng o Bo-Savar law he vore wll ndce downwash de o ndced velocy and gven by w w arcan () V V So; he wng downwash cold be calclaed from he formla: VZ cos V sn an () V INCAS BULLETIN Volme 7 Isse / 5

5 9 Predcon of Wng Downwash Usng CFD Resls Fgre 3 shows he flow pah lnes colored by velocy magnde a α=; he flow s aached o he pper srface and no vore s capred n he pper srface he wng p vorces s wea and a lle ws observed n he flow a he p. Fgre 3 Flow Pahlnes colored by velocy magnde a α = Fgre 4 shows he flow pah lnes over he pper srface of he wng a α = 6. Ths fgre ndcaes an esence of vore on pper srface and p vorces. The vorces zones are capred by he pencl and zoomed as shown n Fgre 4 Ths fgre reveals ha he separaon occrs near wng roo frs and sars o eend span-wsely by ncreasng he angle of aac. De o vorces near he wng roo ralng edge s predced ha he wng flap may no be effecve a α > deg. Wng p vorces also have hgh nec energy o swrl and roae whch ncrease he ndced drag as a resl; hese p vorces wll hen roll p and ge arond he local edges of a wng. Ths phenomenon wll redce he lf a he wngp saon so hey can be represened as a redcon n effecve wng span. Fgre 4 Flow vecors colored by velocy magnde a α= INCAS BULLETIN Volme 7 Isse / 5

6 Mohammed MAHDI Downwash effec on Vee al a α= s shown n Fgre 5 Fgre 5 Downwash dsrbon along Vee al qarer chord A lne s consrced along he qarer chord lne of Vee al and he downwash has been ploed along hs lne as shown n fgre 5. From hs fgre s shown ha a = he downwash s eqal.36 rad whch s eqal degree. So deg. From fgre 6 So So deg 3.86deg.3. Fgre 6 Downwash dsrbon α=6 3. CONCLUSION From he resls obaned he downwash has been calclaed effcenly sng ANSYS worbench and hs shall be sed n he developmen of he longdnal sably analyss as drng he desgn process of he UAV. The CFD shows also he flow pahlnes arond he vehcles and sall paern has been fgred o. INCAS BULLETIN Volme 7 Isse / 5

7 Predcon of Wng Downwash Usng CFD ACKNOWLEDGEMENT Ths wor s he properes of ARC; ARC s he fndng organzaon of hs wor. REFERENCES [] * * * Wpeda 5 Aprl 5. [Onlne]. Avalable: hp://en.wpeda.org/w/downwash. [] M. S. Jrovch CFD Predcon of he Flow feld behnd he KC-35R Taner 9h AIAA Appled Aerodynamcs Conference 7-3 Jne Honoll Hawa AIAA -35. [3] D. R. Anand and P. S. Kam Aerodynamcs Performance of Canard-WIng Confgraon-A CFD Sdy Bangalore 4. [4] T. J. Chng Compaonal Fld Dynamcs nd Edon ed. Cambrdge: Cambrdge Unversy Press ISBN hardbac. [5] F. R. Mener Two-Eqaon Eddy-Vscosy Trblence Models for Engneerng Applcaons AIAA vol. 3 pp [6] J. D. Anderson Fndamenals of Aerodynamcs 5h Edon ed. ISBN-: New Yor: McGraw Hll. INCAS BULLETIN Volme 7 Isse / 5