Taşıtlarda meydana gelen kızaklama olayının sonlu elemanlar yöntemiyle analiz edilmesi
Analysis of the hydroplaning phenomena occurs on vehicles with finite element method
Yazar: MURAT ŞAKACI
Danışman: DOÇ. DR. EMİN SÜNBÜLOĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Katı Cisimlerin Mekaniği Bilim Dalı
Derece: Yüksek Lisans, 2020
Konu: Makine Mühendisliği = Mechanical Engineering

Özet:
Araç sürüş koşullarının zorlayıcı olduğu yağmurlu havalarda, ya da su birikintisinin bulunduğu zeminlerde, araç tekerleği ile yol yüzeyi arasında su tabakasının oluşması ve bunun sonucunda yol tutuşunun kısmen ya da tamamen kaybolması durumu kızaklama olarak tanımlanmaktadır. Özellikle yağmurlu havalarda, araç sürücüden gelen yönlendirme, frenleme ve ivmeleme gibi kontrollere oluşan su tabakası sebebiyle yanıt veremez. Günümüzde trafik kazalarının büyük bir kısmı yağışlı hava koşulları altında gerçekleşmektedir.
Otomotiv firmaları ürün geliştirme süreçlerinde zaman, maliyet gibi parametrelerin yanısıra, emniyet parametresini de göz önünde bulundurmaktadırlar. Bu noktada gelişmekte olan bilgisayar destekli mühendislik araçları, birçok mühendislik sisteminin henüz üretilmeden sanal ortamda analiz edilip test edilmesini sağlar. Sonlu elemanlar yöntemi ile analiz gerçekleştiren paket programlarda bu araçlardan biridir. Günümüz de otomotiv sektöründe daha ekonomik ve operasyon anında daha emniyetli araçlar üretebilmek amacıyla sonlu elemanlar analizlerinden faydalanılmaktadır. Özellikle tekerlek üreticileri karmaşık simülasyon modelleri vasıtasıyla, tekerleğin araç karakteristiği üzerindeki etkilerini incelemektedirler. Bunun yanı sıra kızaklama olayını etkileyen faktörleri araştırmak son dönemlerde otomotiv ve lastik üreticileri tarafından oldukça önemli bir konu haline gelmiştir.
Bu kapsamda kızaklama konusu bilgisayar destekli mühendislik araçlarıyla analiz edilmeye çalışılmaktadır. Tekerlekler yapısı gereği doğrusal olmayan birçok malzeme içerdikleri için, sonlu elemanlar analiz yöntemi lastiğin yapısal karakteristiğinin incelenmesi konusunda oldukça büyük avantaj sağlamaktadır.
Kızaklamayı önlemek için, öncelikle lastik ve yol etkileşiminin incelenmesi gerekmektedir. Bilindiği kadarıyla lastik sırtının dizilimi, yükler, lastik iç basıncı kayma ve kamber açıları kızaklamayı etkileyen başlıca faktörlerdir.
Bu çalışmada ticari bir otomobil tekerinin öncelikle 2 boyutlu sonlu elemanlar modeli kurulacak ve daha sonra 3 boyutlu model elde edilecektir. Akabinde sırasıyla lastik iç basıncı ve araç yükleri uygulanarak, tekerlek, sıvı birikintisi olan bir yol üzerinde hareket ettirilecektir. Burada bulunan katı-sıvı etkileşimi problemi ABAQUS sonlu elemanlar ticari yazılımı içerisinde CEL (Birleştirilmiş Eulerian-Lagrangian) yaklaşımı ile çözülecektir. Doğruluğu sağlanmış bir model elde edildikten sonra yukarıda belirtilen lastik sırtında bulunan tırnakların dizilimi, lastik iç basıncı gibi parametrelerin, kızaklamaya olan etkileri incelenecektir

Abstract:
The production of rubber wheels dates back to the 1895s. After the large increase in the number of automobiles in the 1930s, the concept of handling emerged due to the safety requirements at the time of steering and braking. Carrying the vehicle load, torque transfer during driving and braking, generating force during cornering, minimum power consumption, low noise and vibration behaviour, long-lasting and reliable performance, high wear life is some of the general properties which is expected from a tire. Since the rubber wheel has a mixed structure containing more than one material, we can examine these components in four main groups as tread, shoulders, sidewall and heel.
Tires have independent functions such as carrying the vehicle weight, providing road holding during acceleration and braking, creating lateral forces during maneuvering, and finally damping the damage caused by the irregularities in the road structure. Therefore, all forces acting on the tire during the operation must be analyzed. The loads acting on the tire are quite complex and can occur in multiple load combinations. In this case, it is very difficult to analyze these forces accurately.
Researching the factors affecting the hydroplaning phenomenon has recently become a very important issue by automotive and tire manufacturers. In this context, the subject of aquaplaning is tried to be simulated with computer aided engineering tools. As far as is known, the arrangement of the tread, the loads, the tire inner pressure, and the camber angles are the main factors affecting the hydroplaning. Hydroplaning is defined as partial or complete lose of handling as a result of formation of a water layer between the vehicle tire and the road surface in case of rainy weathers or roads with water films where the driving conditions are challenging. Today, most of the traffic accidents occur under rainy weather conditions.
We can list the micro-textures and macro-textures as two of the most important road design criteria that affect the skidding. Micro and macro textures on the roadways greatly affect the skidding. In order to create friction force on wet surfaces, micro-textures are needed, the amount of micro-texture and the friction force generated have a linear relationship. For this reason, micro-textures have a decisive role in preventing aquaplaning, especially at low vehicle speeds. The micro-textures on the road provide the roughness required for breaking thin layers of water between the wheel and the road.
The problem occurring at low operational speeds on roads with few or no micro-texture on it is described as viscous hydroplanning. According to the researchers', viscous aquaplaning depends on factors such as tire wear, fluid viscosity and road texture. Viscous aquaplaning is not expected to occur at low speeds, as long as the tire is not completely worn and the road is not smooth. Dynamic aquaplaning occurs when
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the water layer between the moving wheel and the road creates lifting force. This happens only when the total water discharge capacity of the road and tire is insufficient. Generally, full dynamic aquaplaning takes place at high vehicle speeds in thick water layers where the depth exceeds 2.5 mm. Ongoing research is on the prevention of dynamic aquaplaning, as it is more widely observed. Heavy rains often create unfavorable conditions for dynamic aquaplaning.
Developing computer aided engineering tools enable many engineering systems to be analyzed and tested in virtual environment before they got produced. The softwares using Finite Element Method are one of these tools. Today, finite element analysis is used to produce more economical and safer vehicles in the automotive industry.
The finite element method, also known as finite element analysis, is a numerical method used to approximate the solution of partial differential equations with boundary value problems. It is also defined as finite element analysis. The finite element method divides large problems into small and simple parts called finite elements. Then, it performs an approximate solution using variational calculations to minimize the error function. In terms of mechanics, the finite element method helps us to analyze systems with complex geometries that we cannot calculate analytically and operate under different load conditions. The basic approach here is to divide the system, whose solution we do not know under various loads, into a finite number of parts for which we can obtain approximate solutions. Each of these finite number of pieces is described as "element". After obtaining the approximate solution for the displacement value of each element, the stress and strain values are calculated.
The number of elements, nodes, contact types included in the finite element model seriously affect the time required for solution. In addition to these, the solution method chosen directly affects the time required for the solution. Therefore, when separating a model into finite elements, it is important to choose the solution method to be used in analysis as well as modeling techniques that will reduce the calculation time before analysis. Implicit solution methods are mostly used in analysis types such as structural static, modal analysis, steady-state thermal analysis, while Explicit solution methods are used in analyzes where sudden deformations occur, explosion, collision, etc.
When we consider the aquaplaning problem, we see that a tire with a ribbed geometry rotates on a road covered with liquid. The geometry of the tire in contact with the road consists of rubber with hyperelastic material properties. The rotation of the tire represents a body in motion that has its own boundaries. Therefore, in this case, the boundary conditions should be defined within the model. The moving tire is in contact with the road, water and air, and the characteristics of the contacts are one of the important elements that need to be defined separately. Here the contact of air with tire and water is negligible. The weight of the vehicle will be transferred to the fluid and from there to the road through the internal energy of the tire and the contact defined in between. Since the movement of the wheel and the distribution of water are the properties we want to examine, the parts should be divided into finite elements with FEM. Here, the elements are connected to a reference region and can move with the material, or they can remain fixed to represent a certain area in space. Here, the liquid can be divided into small parts by the Finite Volume Method instead of the Finite Element Method.
During hydroplaning phenomena, transferring the load to water causes the pressure of the liquid to increase. The load transfer takes place between two finite element surfaces
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that have a solid and liquid structure. Depending on the tread structure, viscosity and density; water will drain from the surfaces of solid objects at a certain speed and in a certain direction. As can be seen, the problem mentioned under the assumptions is the FSI problem with solid-liquid interaction. In such problems, defining the solid-liquid interface and transferring the information from this interface is one of the most important issues. Various formulations can be used in the finite element analysis softwares to define the aquaplaning problem.
At the end of the day, the model will include following properties and components in order to analyse hydroplaning phenomena
- Viscoelastic and hyperelastic rubber material definition
- Composites material description
- Road-wheel-fluid contact definitions during rolling
- boundary conditions and interfaces
- Finite element model of the tire and water
In this study, a 2D finite element model of a commercial car wheel will be established first and then a 3D model will be obtained. The tire will then be moved on a road with a water layer and tire inflation pressure and vehicle loads will be applied respectively. The problem of solid-fluid interaction found here will be solved by the CEL (Coupled Eulerian-Lagrangian) approach within the ABAQUS finite element commercial software. After building-up a valid model, the effects of the parameters such as the arrangement of the grooves on the tread mentioned above and the tire inflation pressure will be examined.