1 numerical simulation 1 1 basic equations and turbulence models The vehicle speed is generally much lower than the speed of sound. The vehicle aerodynamics are low-speed aerodynamics. Therefore, the flow field around the vehicle can be considered as a three-dimensional incompressible viscous flow field. Because of the complex shape of the car, the surrounding air flow is likely to cause separation, so it should be Turbulence processing. The paper uses Relizab le k turbulence model for numerical simulation. The basic control equation for turbulent flow calculation is the three-dimensional incompressible Reynolds time-averaged Navier Stokes equation. The control equation is as follows: (1) Continuous equation ui / xi = 0 (1) (2) Momentum equation (uiuj)xi = xj eff uixj + ujxi - pxj (2) where: ui, uj are the average velocity components, xi, xj are the coordinate components, p is the pressure on the fluid microelement, eff is Turbulent effective viscosity coefficient. In the process of Reynolds time homogenization for N avier Stokes equations, a new variable term u iu j (Reynolds stress term) is introduced. In order to make the system of equations closed, some assumptions about Reynolds stress are made. The standard k model may lead to negative normal stress when the average strain rate is particularly large. In order to make the flow conform to the physical law of turbulence, the normal stress is mathematically constrained. In the R elizable k model, the transport equation for k and is as follows. æ¹ The k equation of the fluid energy: (k)t +( ku i)xi = xj + t? Kxj + G k - (3) turbulent flow energy dissipation rate equation: ()t +( ui)xi = xj + t? Xj + C 1 E - C 2 k + v (4) where: is the density of the fluid and G k is the production term of the turbulent kinetic energy k due to the mean velocity gradient, G k = tuixj + ujxiuixi (5) effective viscosity coefficient of turbulence Eff is calculated by the following formula: ef = + t; t = C k 2 / ; C = 1 A 0 + A s U k /; C 1 = max 0 43, + 5;= (2E ij E ij)1 /2 k;E ij = 1 2 uixj + ujxi A s = 6cos? ;? = 1 3 arccos(6W )W = E ij E jk E kj(E ij E ij)1 /2; U = E ij E ij + % ij % ij; % ij = % ij - 2 ijk k;% ij = % ij - ijk In equation k, %ij is the time-averaged rotation rate tensor observed in a reference frame with an angular velocity of k. The literature suggests the following values: k = 1 0, ? = 1 2, C 2 = 1 9, A 0 = 4 0. In the wind resistance model, the deformation of the near wall is very large. Many swirling flows are generated in the vicinity and the pressure gradient is very large. Therefore, the use of the steady-state R elizable k. combined with the wall function is more suitable for the numerical calculation of the external flow field of the car. 1 2 Modeling The basic model being simulated is a foreign container semi-trailer. Due to the limitations of current computer hardware conditions, the model of the vehicle has been simplified to remove door handles, wipers, etc. The bottom of the trailer has been smoothed. The corresponding simplified model was used to simulate complex structures such as tread pattern, wheel hub, rear axle, frame, and rearview mirror, and a 1-3 scale model was used for numerical simulation. Simplified model As shown, according to the similar principle of fluid mechanics, the Reynolds number of the model in the numerical simulation is equal to the Reynolds number in the actual situation. According to the experimental theory, the Reynolds number has a self-mode area. When the Reynolds number of the model test exceeds a certain value, the aerodynamic force remains basically unchanged. The Reynolds number of the automobile model test recommended by the American Society of Automotive Engineers should not be less than 0 7 106, and the Reynolds number proposed by a Japanese expert is 0 5 10. The simulated wind speed used in this paper is 30m/s, and the characteristic length takes the maximum wheelbase. The Reynolds number of the model is 7 9 106. Therefore, the real vehicle can be simulated using the scaled model of this paper. 1 3 Meshing and Boundary Condition Settings The outer contour of the calculation domain is a cuboid and the car model is in a certain area of ​​the cuboid. The entrance is 3 times longer than the front of the model, the exit is 6 times longer than the rear of the model, the total height is 5 times the height of the car, and the total width is 7 times the width of the car. The OCTREE method is used to generate an unstructured spatial grid in the entire computational domain. The body surface is stretched out with a triangular prism grid parallel to it, and a density box is used in the calculation of sensitive areas to achieve local refinement and improve the accuracy of calculation. At the same time, in order to avoid the influence of the grid change caused by the local variation of the model, the same grid size is set in the same area of ​​the model in different situations. The total number of total vehicles generated by each simulation is approximately 2.8 million. In the simulation process, set the inlet boundary uniform flow rate u = 30 m/s, the turbulence intensity is 0 5%, and the outlet pressure p = 0 (relative to the atmospheric pressure); considering the influence of the bottom of the car on the air flow, set the floor to be slippery. The velocity is the same as the incoming flow velocity, u = 30m/s. The vehicle body is set as the boundary of the fixed-wall non-sliding wall, and the left and right sides of the calculation domain are set as the boundary of the sliding wall. 2 CFD Simulation Results Analysis The solution of F luent software shows that the vehicle's aerodynamic drag coefficient Cd is 0 812. From the body surface pressure cloud map, it can be seen that there is a large positive pressure area in the front face of the vehicle, and the front of the container is over the front There is also a positive pressure zone on the steps. The lower front side and side face of the front face are separated by the higher flow velocity, resulting in a negative pressure zone and a large pressure gradient. It can be seen from the velocity line and speed cloud diagram at the front of the vehicle that the air velocity behind the cab behind the container is low, and there is a large-scale vortex behind the cab, where the air flow has a large loss of energy. Due to the direct exposure of the top of the container to the incoming air, there is also a phenomenon of air separation, and there is a backflow not far from the corner of the step. As can be seen from the velocity profile and velocity cloud at the rear of the vehicle body, the pressure from the bottom stream is higher than the pressure at the top, and the air stream escaping from the bottom is curled upwards under the differential pressure to form a larger scale vortex. 3 Improvement of Aerodynamic Resistance Characteristics From the above analysis, we can see that by adding additional devices, the aerodynamic drag characteristics of the vehicle can be improved. The paper analyzes the influence of structural parameter changes such as the corner radius of the shroud R, the distance between the cab and the container on the aerodynamic drag coefficient, and finds the R and L corresponding to the minimum aerodynamic drag. Finally, consider the optimal R and L combines the effect on the aerodynamic drag coefficient. Improvement of aerodynamic drag coefficient by 3 1 R The shroud corner radius diagram. The law of the aerodynamic drag coefficient changes with the corner radius R of the shroud. As can be seen from the graph of FIG. 6 , as the radius of the fillet increases, the decrease in the drag coefficient gradually decreases. At R = 500 mm, the decrease is greatest; R is less obvious from the 1 000 2 500 mm drop; R is less from 3:00 03 500 mm. And pressure contours for the body surface and the symmetry plane after installation of the shroud. It can be seen that due to the presence of the shroud, the positive pressure zone at the upper part of the front of the original container disappears obviously, the air flow does not separate there, only a local high pressure zone is generated at the right corner of the container, and the pressure gradient is small. 9 is the symmetry plane speed flow line and speed cloud diagram of the front body. It can be seen that the airflow smoothly flows from the upper part of the shroud toward the rear of the shroud, and the wake is formed at the gap due to the shear of the air flow. However, compared with the original model (Fig. 3), the vortex has a smaller scale and the vortex center is smaller than the original model. As the models move up, the energy loss is also small. Therefore, the pressure at the gap is relatively small, which effectively reduces the differential pressure resistance, and ultimately reduces the aerodynamic drag coefficient. 0 is the velocity profile and speed cloud of the symmetry plane of the body's tail. It can be seen from 0 that the change of the wake vortex is not obvious after installing the shroud. This is because the shroud is far from the tail and has little effect on the flow field of the rear of the car. Improvement of aerodynamic drag coefficient with 3 2 L 1 and 2 are the schematic diagram of the cab and the container clearance and the influence of the spacing change on the aerodynamic drag coefficient. From 2 it can be seen that as the gap gradually increases, the drag coefficient also gradually increases. This increase is particularly evident when L increases from 1 076 mm to 1 126 mm. 1 cab and container clearance diagram 2 Cab and container spacing changes on the drag coefficient of 3 and 4 are the body surface pressure and pressure plane of the symmetry plane. It can be seen that the change in pressure distribution is not significant relative to the original model, and the reduction in spacing does not effectively reduce the pressure difference. 15 is the velocity profile and speed cloud diagram of the symmetry plane at the front of the body. As can be seen from the figure, the characteristics of the flow field at the cab and the container clearance are significantly improved, and the air flow passes through the top of the cab. Due to the influence of the container rising above the cab step, some of the air flows over the steps and flows backwards due to the high flow velocity. Strong separation of air flow is created to create a positive pressure zone on the windward side of the container. At the same time, backflow occurs at a point not far from the corner of the step, forming a vortex; the other part of the airflow flows down the gap due to the container's blockage. The reduction in spacing decomposes the original larger shear vortex into two smaller vortices. They appear above and below the cab, respectively. As a result, the air flow energy loss is relatively reduced. Therefore, the drag coefficient has been reduced to some extent. 5 The velocity of the symmetry plane at the front of the vehicle body and the speed cloud diagram (L = 876mm) 6 are the velocity lines and velocity clouds of the symmetry plane of the body's tail. As can be seen from the figure, the tail flow field does not change significantly compared with the original model. For the same reason as above, because the gap between the cab and the container is far from the tail, the change of the gap has little effect on the wake. Improvement of aerodynamic drag coefficient by combination of 3 3 R and L 6 The velocity profile of the body's tail symmetry plane and the speed cloud diagram (L = 876mm) are taken from the above simulation results to obtain the minimum resistance coefficient R (500mm) and L (876mm) values ​​for comprehensive numerical simulation, and the drag coefficient is compared with the original The amount of change in the model? The C d is - 20 59%, which is significantly better than the effect of individual changes in each parameter. 7 is a body surface pressure map. As can be seen from the figure, similar to section 31, the positive pressure zone at the front of the container disappears, effectively reducing the differential pressure resistance. 7 Body surface pressure cloud (R = 500mm, L = 876mm) 8 Body front symmetry plane Velocity flow lines and velocity cloud diagram (R = 500mm, L = 876mm) 8 is the symmetry plane of the front body velocity streamlines and velocity clouds. As can be seen from the figure, the front flow smoothly bypasses the shroud to the rear. Due to the small distance, the airflow between the driver's cab and the container gap creates air return due to the shear action of the air flow, and does not form a complete vortex to flow backward. The disappearance of the vortex reduces the energy loss of the air flow and further reduces the drag coefficient. 9 is the velocity profile and speed cloud of the symmetry plane of the body's tail. As can be seen from the figure, the tail flow field does not change significantly with respect to the original model. Because the shroud and the gap position are far from the tail, changes in various parameters have little effect on the tail flow field. 9 Body tail symmetry plane velocity streamline and speed cloud diagram (R = 500mm, L = 876mm) 4 Conclusion (1) The front of the container of the original model forms a strong retarded area. The impact on the aerodynamic drag of the vehicle is greater. (2) After installing the shroud, the aerodynamic drag decreases with the radius R of the shroud becoming smaller, with the maximum reduction of 19 34%. (3) As the distance between the cab and the container decreases, the aerodynamic drag becomes smaller. The biggest drop is 4 66%. (4) The effect of adding a shroud to improve the aerodynamic resistance is obviously better than the effect of changing the distance between the cab and the container. (5) Considering the optimal fillet radius of the shroud and the optimal distance between the cab and the container, it is better than the influence of single parameter change on the aerodynamic drag, and the reduction is 20 59%. Our company was set up in 2002.We are using the advanced cold punch technology to produce spark plug which is similar to NGK and Champion technology.With such technology, the metal shell and ceramic are better protected the quality of the spark plug are more stable to get avoid from air leaking and broken.Our spark plug products are ranged into more than 100 models with an anual out put of 6 million pcs. 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