Karayolu Taşıt Aerodinamiğinin Sayısal Yöntemle İncelenmesi

Abstract

Tezin genelinde bir taşıttaki aerodinamik direnç katsayısına etki yapan faktörlerden olan taşıtın arka cam açısı, taşıt yer mesafesi ve taşıt boyunda yapılan modifikasyonların etkisi irdelenmeye çalışılmaktadır. Bu arada aerodinamik direnci etkileyen diğer unsurlar üzerindede durulmaya çalışılmaktadır. Tezin birinci bölümünde bir karayol taşıtını etkileyen seyir dirençleri anlatılmaktadır. Karayol taşıtlarında yakıt ekonomisini etkileyen faktörler sıralanmaktadır. Yakıt ekonomisini geliştirmek için yapılan uygulamalardan örnekler verilmektedir. İkinci bölümde ise otomobilin başlangıcı ile günümüze kadar geçen zaman içinde taşıt aerodinamiği konusunda yapılan uygulamalardan bahsedilmeye çalışılmaktadır. Bir kara vasıtasına etki eden aerodinamik kuvvet sistemi tanıtılmaktadır. Tezin üçüncü bölümünde taşıt özellikleri ve ana parametreleri verilmektedir ve bir taşıtta uygulanan genel aerodinamik dizayn pensipleri üzerinde durulmuştur. Düşük bir aerodinamik dirence sahip olması istenen bir taşıt için temel aerodinamik dizayn prensipleri tanıtılmıştır. Deneysel aerodinamiğin temel elemanı olan rüzgar tünelleri hakkında bilgi verilmeye çalışılmaktadır. Dördüncü bölümde yapılan uygulamalarda kullanılan paket program CFD2000/SFV tanıtılmaktadır, programın çözdüğü denklemler ve k-e türbülans modeli hakkında bilgi verilmiştir. Tezin son bölümünde ise taşıt yer mesafesi, taşıt arka cam açısı ve taşıt boyunda yapılan uygulamalarla elde edilen taşıtın yerden yüksekliği ile oluşan basınç değişimi, hız vektörleri ve ağ sistemleri şekiller halinde verilmektedir.

Vehicle aerodynamics concerns the effects arising due to the air. Its importance to road vehicles became apparent when they started to achieve higher speeds. The automobile as we know it came onto the scene in the last decade of the nineteenth century. Its beginings roughly coincided with the advent of powered flight, and perhaps for this reason. It became of interest to aerodynamicists right from the start. One of the first attempts to apply aerodynamics principles to road vehicles was the streamlining given to the first holder of the land speed record, a car named Jantaud driven by Gaston Chaseloup-Laubat. The objective of this investigetion is to attempt to address a number of key questions concerning road vehicle aerodynamics. t) How does aerodynamics fit into the context of a pratical vehicle? 2) How much does aerodynamics contribute to fuel consumption? 3) How is drag generated? 4) What is the current state of the art of low-drag design? 5) What is the future outlook for low-drag design and what is the pratical lower limit of Cd for production vehicles? The motion of road vehicle is subject to three different forces - tire rolling resistance, aerodynamics drag, and gravity - which oppose the tractive propelling the vehicle. In the simple case where the vehicle is travelling on a level road at a constant speed, only the first two forces are present. The first of them, the rolling resistance, depends on the tire construction and inflation pressure, and it is proportional to the vehicle weight. Its magnitude is fairly constant, increasing only slowly with vehicle speed as well as its shape. The drag, by contrast depends strongly on the vehicle speed. Its magnitude is proportional to the vehicle frontal area an to the square of the air speed. At very low speed the drag is negligible compare to the rolling resistance, but high speeds the drag risis raridly and eventually it dominates the total resistance. XIV Afterbody Shaping: Road vehicles come in a variety of body styles, ranging from sedans through fastbacks and hatchbacks to station wagons. Going from öne style to another is a form of afterbody shaping, and it turns out that this shaping can have an important effect on drag. If öne loks at fastback, hatchbacks, and station wagons öne observes that mainly differ by the angle of the slant. The effect slant angle on which the base slant was varied över the range from 90° to 20°. The results showed that there were clearly two flow regims. in regime l the drag was slowly increasing with decreasing slant angle up to a critical angle at 43°. At that angle the flow abruptly changed to regime 2 and the drag coefficient more than doubled. Effect Of Ground Proximity : Öne of the distinquishing features of road vehicles is that they travel near the ground. The typical vehicles ground clearance, expressed in terms of an 'equilavent diameter' De,, = (4 Af/n)"2 is on the order of 0.1Deq, which is small enough to cause significant changes in the flow fıeld. The presence of the ground can have the following two effects. When the lift is negative (towards the ground), then the streamline pattern produced by the image around the real body. As a result, the negative lift force will be amplified as the distance to the ground decreased. Hovvever, if the lift is positive, the streamline pattern is just the opposite, and it will produce an effective camber bowed upward. The result in this case will be a continually increasing lift force as the ground plane is approached. Skin Friction Drag : in addition to pressure forces air flow generates tangential frictional stresses along the body surfaces. For a bluff body skin friction drag is much less than the pressure drag. Its magnitude depends on boundary layer thickness and, through it, on the Reynolds number : it decrease with increasing Reynolds nymber. By contrast, the pressure drag is pratically Reynolds number independent, and so the total body drag has only a weak Reynolds number dependence. XV Consider a typical vehicle which is box shaped with overall dimesions of length to width to height of 3 : 1.33 : l. For such a shape the ratio ör the total surface area parellel to the direction of travek to the frontal area is about 10. Taking a typical value of the skin friction coefficient to be 0.002 in the Reynolds number range of interest, we find that the friction drag contributes about 0.02 to the overall drag coefficient. This value is about 5% of the drag coefficient of a typical road vehicle. Use Of Computers For Aerodynamycs Design : So far, ali ör almost ali of the appiied and basic work done in vehicle aerodynamics has been experimental, for axample using vvind tunnel. The reason for the predominance of experiments has been that the flow fıelds involved are very complex. They are three-dimensional, with many small but signifıcant details, separeted and turbulent, ali of which combine to make analytical ör numerical work very difficult. Really important progress could be made if numerical simulations could be used to give support to and to guide the experiments. The attributes of numerical solutions are known : they permit parametric studies tobe executed rapidly, and the detailed information they provide allows investigetions of cause-and-effect relationships. The greatest problem of the simulation lies in the grid resolution. Figüre l Grid resolution The k-s Model Turbulent flows, which are of great pratical importance, are three dimensional and time dependent. Computer methods of solving the diferantial equations of fluid dynamics are well advanced even for three-dimensional time dependent flows. k-e model of a pratical turbulence model, that in which two differential equations are solved, the dependent variables of which are the turbulence energy k and dissipation rate of turbulence energy e. A form of k-s model was first proposed by Harlow and Nakayama. k s 1/2 u; u; s = v (öuj.3uj /dxk.dxk) Basic Considerations Influencing Vehicle Shape : Current production automobiles have drag coefficients an order of magnitude larger than tear-shaped bodies in free flight and several times larger than idealized shapes like Klemperer's. An important reason for this disrepancy is that vehicle shapes are subject to many pratical constraints from the point of view of intended vehicle purpose and use, vehicle safety, maintenance, and cooling and product identity. Taking these constraints in turn, the vehicle has to provide a reasonably spacious passenger compartement which dictates its overall size and means for convenient entry from outside, and it must also provide engine and language compartments. All of these combine to largely dictate the overall shape, and they also add to body details, such as door handles, which increase drag. From the safety point of view the passenger compartment must be configured to permit adequate visibility frontward, backward, and to the sides. The vehicle must provide maximum protection in accidents, which means the shape should incorporate adequate crush zones. The windshield may not be inclined below some 30° because of light refraction limits which reduce visibility. External mirrors providing rear and side view also add to overall drag. We can be ranged advantages road vehicles which have low-drag aerodynamics coefficients ; 1) low fuel consumption 2) to supply comfort reducing formed voice due to wind at high speed 3) to increase vehicle of stability with spoiler etc. XVII 4) to provide increase of vehicle performance and manoeuvres capability 5) to supply cleaning headlight and wind-screen at rainy day Aplication: in the end of this investigetion using a simulation program CFD2000/SFV, make different applications, afterbody shaping, effect of ground proximity and length of vehicle body, to observe effect of aerodynamics drag coefficient. Grid systems are done a program as easy mesh for each models, then calculated pressure and velocity values for each grid system points by CFD2000/SFV. Pressure and velocity values are given as a diagram for each model.