Study On The Relationship Between The Air-deflector And The Resistance Of A Heavy Commercial Vehicle

With the continuous expanding of global truck market, the increasing of road transportation and the rising of fuel prices, the aerodynamic styling of heavy truck becomes an important research topic in the R&D department of truck. Cab, the key part of heavy-duty trucks, contains all performance and functionality, and is the main target for truck manufacturers. Heavy-duty truck, differing from passenger vehicles as a whole set, consists of two parts, cab and trailer. It should pay more attention to the distance, height, width between cab and trailer, because there are alternative combinations of the two parts. The differences in dimensions will cause the different drag coefficients, which determine the aerodynamic characteristic. The air deflector at the top of cab roof of heavy truck affects greatly on the performance. However, there are many factors related to the air drag. In addition, the basic dimensions have a certain scope of change as a result of restrictions and regulations and national standards, which produces many hard points during the packing of cab. The air deflector installed to cab can guide flow through the body smoothly to achieve the real effect of lowering drag. Therefore, the relationship of height and distance between cab and trailer determines not only the size of air deflector but also change of aerodynamic drag. For cab styling of heavy truck, both cab and trailer should be studied as a whole in order to get the best aerodynamic performance. A real cab styling as an example is presented here to investigate the relationship between cab and trailer specifically focusing on the parameters, such as the angle of air deflector, the height of trailer, and drag coefficient. At present, there are two methods in automotive aerodynamic study, wind tunnel experiment and CFD analysis. The former has higher accuracy and better reliability, but larger investment and longer experiment period. It contains blocking effects, and a detailed description of entire flow field cannot be given, which impedes the application in automotive design. The latter has shorter development cycle, lower cost, good reproducibility, and can be visualized. The physical value at each point in the flow field can be drawn out. In the past ten years, with the computers and the development of computing technology, CFD numerical simulation has been increasingly applied to automotive design.This thesis introduces the basic theory of CFD for the heavy vehicles' out flow area, the analysis methods and the pro-process of CFD including selection of turbulence model models, modeling, calculation area, meshing and so on. General steps can be described as follows:a. Establish the physical model of the problem, and then into the abstract mathematics and mechanical model, and then determine the flow field impact space of geometry for analysis.b. Establish the CAD model of flow field computational domain, put the outer surface of model and the entire flow field computational domain into meshes. The density of grid meshes and the shape of grid mesh cells will affect on the calculation. Different format algorithms need different meshes in order to ensure the stability and efficiency of calculation .c. Add the necessary initial conditions of solving. The general boundary conditions at the entrance and exit are velocity and pressure, and turbulent kinetic energy and dissipation rate if the turbulence is taken into account. Fluid plus wall boundary parameters should be added to flow field the computational domain. Due to viscosity on the surface, no-slip boundary conditions should be added that the speeds along all directions are zeros. Here attention should be paid to the effect of relative motion and ground, without contradiction and duplication.d. Select the appropriate method, assume the solving process and accuracy, calculate and then save the results.e. Choose a suitable post-processor to deal with the results, repeat above steps until convergence is obtained.This paper has study on the relationship between the styling of the air-deflector of a heavy commercial vehicle and its aerodynamic drag with the numerical simulation. The six 3-D models with the different angle of air-deflectors have been built in Alias AutoStudio. Through the simulations of the six models, the shapes of the front and side panels and their aerodynamic drags have been studied.Because of the styles of air-deflectors which represent the brands of manufacturers, three angles between the roof and deflector have been used for the numerical simulations: and are used with two different containers. Because heavy commercial vehicle is symmetric in shape, and the uniform flow comes from the entrance, a half model is used in order to reduce the number of meshes and save the time. The calculating ranges are set up as follows: nine times of vehicle length, five times of vehicle width, five times of vehicle height, respectively. In this paper, the tetrahedral meshes are used. FLUENT is applied to do the numerical simulation with the k ?εhigh Reynolds turbulence model.After the calculation, the aerodynamic drag coefficient, pressure distribution chart and velocity flow field chart of the six different models are shown in the paper. The aerodynamic characteristics depend on the velocity and pressure of airflow passing by the vehicles. Aerodynamic drag is related to the pressure distribution and velocity flow field. The conclusions are heavy commercial vehicles with different air-deflectors and containers. Heavy commercial vehicles in different schemes had different results and aerodynamic drag coefficients from CFD's analysis result.a. For heavy truck, the most disorder flow fields are located in the gap between the cab and the container, the back and the rear bottom of it.b. The height of container has a greater impact on the aerodynamic drag coefficient of heavy truck, and a deflector is needed to reduce air resistance.c. The deflector and the container should match each other, avoiding the energy loss or high drag.d. The deflector of a heavy truck as an example is presented for the low drag cab styling.