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Taşıtlarda aerodinamik direnç katsayısının taşıt parametreleri ile korelasyonu

Analysis and design of automobile forebodies using potantial flow teory and a boundary layer seperation criterion

  1. Tez No: 14381
  2. Yazar: VEDAT PEKER
  3. Danışmanlar: DOÇ.DR. METİN ERGENEMAN
  4. Tez Türü: Yüksek Lisans
  5. Konular: Makine Mühendisliği, Mechanical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1991
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 85

Özet

ÖZET Tezin genelinde bir taşıttaki aerodinamik direnç katsayısına etki yapan faktörler incelenmişdir. Bu incemele yapılırken taşıtın dış gövde detayları ve aredinamik direnç katsayısını etkileyen diğer faktörler ayrı ayrı incelenmişdir. Tezin birinci bölümünde potansiyel akış teorisi ve sınır tabaka ayrılma kriteri kullanılarak aerodinamik açıdan taşıtın ön yüzey detaylarının aerodinamik açıdan taşıtın ön yüzey detaylarının incelemesi yapılmışdır. Potansiyel akış panel metodu tanıtılmışdır. Potansiyel akış panel metodu ve rüzgar tüneli testleri kullanılarak çeşitli dizaynlar için aerodinamik direnç seviyesi araştırılmışdır. Taşıtın ön gövdesinde yer alan çeşitli köşe radyüsleri ile aerodinamik direnç katsayısının korelasyonu yapılmışdır. İkinci bölümde ise aerodinamik direnç seviyesini etkileyen diğer faktörlerin üzerinde durulmuşdur. Ford firmasının Probe IV test taşıtı üzerinde yaptığı çalışmalar ve bu çalışmaların sonucunda elde edilen gelişmeler anlatılmışdır. Yapılan test çalışmalarıyla arka cam açısı, ön cam açısı, şasenin yapısı, taşıt yüksekliği, soğutma havası gibi faktörlerin aerodinamik direnç katsayısını nasıl etkilediği incelenmişdir. Aerodinamik direnç katsayısını en düşük seviyeye indirmek için yapılan özel dizaynlar tanıtılmışdır. Tezin 3 bölümünde ise istatistik bilim dalının konuları olan regrasyon ve korelasyon analizleri anlatılmışdır. Çeşitli regrasyon ve korelasyon metodları tanıtılmışdır. En kısa tanımıyla regrasyon değişkenler arasında matematiksel bir bağıntı kurmaya korelasyon ise kurulan ilişkinin kuvvetini ölçmeye denmektedir. Tezin son bölümünde ise korelasyon yönteminin bir uygulaması yer almaktadır. Bir taşıtın çeşitli geometrik büyüklükleri ile aerodinamik direnç katsayısı arasında bir korelasyon yapılmaya çalışılmışdır. Bu çalşmada kullanılan istatiki bilgiler günümüzde kullanılan çeşitli taşıtlardan alınmışdır. -v-

Özet (Çeviri)

ANALYSIS AND DESIGN OF AUTOMDBILE FGREBDDIES USING POTANTIAL FLGUI TEQRY AND a BOUNDARY LAYER SEPERATIGN CRITERION SUMMARY An effective methot of designing Low drug, attached flow automobile forebodies has been devoloped. Based on a potantial flow panel method, a series of computer programs is used to define idealized pressure gradients at several forward corner, locations including the hood, fender windshield header and A pillar, idealized pressure gradients for several radii at each corner Location are analyzed using a turbulent boundary layer seperation criterion to determine if real-world viscous flow will remain attached. Correlation with wind tunnel test results is included. A noncircular, attached flow hood corner shape is developed based on the separation criterion and an inverse computer method. if the convective, grawitational and viscous force terms are eliminated from the novier stokes equation, The number of partial derivatives is reduced from sixty to three, The resulting single, lineer partial differential equation offers the advantage that elementary solutions can be added to produce additional solution of more complex flow fields. Three dimensional computation methods based on the linearized inviscid approximation have been termed“panel methods”since complex geometries are modelled by a large number of contiguous surface panels. The panel-method representation of a typical outomobile geometry is shown in figüre following. A generic automobile shape was modeled both numerically for potantial flow analysis and physically for wind tunnel testing. The 3/B scala wind tunnel model six interchangeable insert of various radii at each of four fqrwardcorner locations including the windshield header, fender, A-pillar, and hood. The 3B scale generik automobile tested in the university of Michigan 5ftX7 ft Low speed, low türbülance wind Tunnel Each of the 24 corner inserts was equipped with statik pressure taps and pressures were. corded during the test by photographing water- filled monometers. -Vİ-İn all cases where the flow remains attached for the speedat which the pressure measurements were taken, excellent correlation between experiment and potantial flow theory is noted. In onder to determine whether the attached f]ow, minimum forebody drag condition canbe expected to occur for real flows, a turbulent boundary layer separation criterion is used in conjunction with the static pressure information available from the inviscid potantial flow solution. For those flows that separate, the negative pressure field acting on the forward facing surface at the corner will not develop and the drug of the body will increase accordingly. Much work has been done regarding numerical solution of both the complate and simplified version of these equations. The stratford method offers the advantage of providing a quantitative definition of the degree of separation tendency. n-2 X dÇp ^ 1/10 3R f (n) dX The left hand side of equation is often referred to as the“Separation tendency factor”and the right hand side is called the“Seperation Limit”, if the value of the left hand side of equation is a function of the reynolds number, different values of the separation limit exist for different values of freestream velocity. Conclusions: Potantial flow panel methods have been shown to be an effective means of defining the miminum forebady pressure drug for a given vehicle forebady remains attached, correlation between -VI 1-measured pressures and those predicted using the potantial flow num erical methods is excellent. An integration of the pressures on a bluff vehicLe forebody for which the flow is attached has been shown to result in a thrust force which is imparted tothe vehicle. This thrust force is costant and independent of geometry for all attached flow designs applied to the forward portion of a bluff body of a given fineness ratio. The pressure gradients generated by the potantial flow numerical model can be used in conjunction with turbulent boundary layer separation criterion to determine if real-world flows can be expected to remain attached, thereby producing the inviscid- Solution. forebody thrust and devoloping a minimum forebody pressure drug. Transformation equation can be devoloped that relate 2-D potential flow velocities to their equivalent 3-D values, thus reducing the manhours necassary for numerical model building and computer run times. The Aerodynamic Development Of The Probe IV Advanced Concept Vehicle The aerodynamic development and characteristic of a four - passanger advanced concept automobile are described. An over view of the areas of the vehicle design which were dealt with to o.btain a drug coefficient value of D.153 is provided. The interior packaging philosophy is outlined which led to the potantial for packaging four to six passanger within an extremely low drag outomobile. Parametric shape studies of the mojor surface design elements are documented from the contributing devolopment testing. The particular design treatments adopted and the rationale behind to choise of design {pre examined for each of the aerodynamically- sensitive areas of the vehicle. Examinations of the unique solutions to wehicle cooling, ramp and curb clearance, front wheel skirting and vehicle a ttutude are presented. The search for advances in automotive efficiencies has been accelerated an many fronts since the 1973-1974 oil crisis. One of the most active areas of pursuit has been that of automotive aerodynamics, particularly reduction of aerodynamic drug. İt is widely belived that reductions in aerodynamic drug are the most cost effective actions to improve vehicle fuel economy. To succesfully accomplish to goal of producing a fully functional four passanger vehicle with a Cw below D.2D, a large body of previous investigations was consultedto obtain indications of the most optimal charalteristics identified in each of the areas of examination, geriarally the areas fell in to three categories Body -viii-shape parameters, internal flows and vehicle attitude. Body Shape parameters - Backlight Studies - Underbody/Chasis Studies - Windshield angle and wrap - Hood slope - Side glass offset to pillars - Wheels and wheels openings - Ranview body taper. Once initial program criteria, package dimensions and appearance guidelines were established, an aerodynamic wind tunnel test program was initiaded. An iterative process involving 3 /B scale and full size vehicles was utilized for both surface devalopment and angine cooling air flow evaluation. A general procedure involving development of surface and features in the studio, construction and test of the wind tunnel model, and repeat and incorparation of the tunnel test results was followed. This aerodynamic development program, coupled with the basic surface knowledge and packaging philosophy outlined in the general aerodynamic design approach section, produced a vehicle with basic proportions and surface treatments consistent with maximum drug and lift reduction opportunity. Specific, detailed aerodynamic solutions to each vehicle area, beyond the initial design direction mentioned earlier, were developed from this process. A fully driveable 4-passanger road vehicle with a 0° yaw Cw of 0.153 has been developed. This vehicle is the result of major aerodynamic development program involving both 3/B seal and full size vehicles. An iterative process involving major components in a task force organization facilitated the surface development process. Unique aerodynamic features have produced a drivable vehicle with the lowest Cw known in the 4-6 paosanger class including a net negative Litf coefficient. -IX-Fig. 12 - View of 3/8 scale test model including on-board active cooling system, pres sure taps, and various inlet/exhaust locations and shapes. 1. Removable Wheel Opening Covers 2. Front Wheel Fairings 3. Battery Access Cover 4. Diov.er Access Cover 5. Wiring Access Cover 6. Holes for Lifting Bars 7. Removable N.A.C.A. Air Intake Cover 8. Removable Alternate Air Intake Covers 9. Removable Air Exhaust Grilles 10. Honeycomb Air Straighteners 11. Blower Switch Access and Key 12. Battery Charger Connection 13. Ammeter 14. Pressure Taps 15. Belly Pan 16. Axle Access Covar 17. Deployable Front Spoiler 18. Front Spoiler Adjustment Crank 19. Removable Mirror 20. Battery 21. Blower 22. Plenum Chamber View of 3/8 scale test model including an board active cooling system, pressure taps, and various inlet/exhaust Location and shapes. -x-Very often in practice a relationship is found to exist between two variables for e ample: weights of adult males depend to some degree on their heights, circumferences of circle depend on their radii and pressure of given mass of gas depends on its temparature and volume. It is frequantly desirable to express this relationship mathematical form by determining an equation connecting the variables. To aid in determining an equation connecting variables, a first step is the collection of data shaving correponding values of the veriables under consideration. From the scatter diagram it is often possible to visualize a smooth curve approximating the data. Such a curve is called an approximating curve. The data appear to be appraximated well by a strain ht line we say that a linear relationship exists betwen the variables hawever although a relationship exists beetween the veriables it is not a linear relation ship and so we call it a non linear relatinship. The general problem of finding equations of approximating curves which fit given sets of data is called curve fitting. The methods of curve. fittings are following..Fittings are following. - Freehand method - Straight Line - Last squares method. Often, on the basis of sample data, we wish to estimate the value of a variable Y corrooponding to a given value of a variable x. This can be accomplished by estimating the value of Y from a least square curve which fits the sample data. The resulting curve is called a rearession curve of Y an X, since Y is estimated from X. QJe considered problem of regression or estimation of one variable from one or more related veriables. Ndw, we consider the closely related problem of correlation, or the degree of relation ship beetween variables, which seeks to determine how well a linear or other equation describes or explains the reletionship between, variables. -xi-İf all of the values of the variables satisfy an equation exatly we say that the variables are corralated or that there is perfect corralation between them. Application: İn the end of this investigation, There is a correlation application between aerodynamik druq coefficient Cw end different dimension of otomobiles which are used nowadays, for escample vehicle lengths; weights, heiphts, back light angle, deck leading edge heights, wind shield angle. İn onder to realized this aplication a computer program was done. The datas which are used in this corralation application were taken from numbers of 5D-6D diffrent otomobiles. Coi jf-alati on method was sellected as lineer-simple corr.alation. After the finding corrolotion coefficient the results were examined. -Xll-

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