# Aerodynamics of Trucks and Trailers

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University of HuddersfieldSchool of Computing and EngineeringAerodynamics of ground vehiclesAbstractThe pressure distribution occurring on a truck when it is traveling with trailer is investigated usingthree dimensional (3D) Computational Fluid Dynamics (CFD). The height of the trailer is variedkeeping the gap between cab and trailer constant and the gap among cab and the trailer is variedkeeping the height constant. Assuming that the comparative velocity of the vehicles has aninsignificant effect on the flow distribution over the both vehicles. The analysis is done using thesteady state with non-compressible condition. Numerical flow around the passenger car because of thetrailer variations, is the subject of present work. C. In this report, the truck is studied with the vehiclesat different variables, with steady flow conditions. Simulation outcomes have been examined andrelated in terms of detected values of aerodynamic drag coefficients.IntroductionIn the conventional meaning, reducing the aerodynamic drag is associated with appropriate shaping ofthe front and rear of a vehicle, in conjunction with change of the bluff bodied shapes to streamlinedones. In case of a truck, where a cargo box is located directly behind the cab, there is only oneoverpressure zone covering the front face of the cab and an upper part (above the cab) of the cargobox, and only one zone of underpressure located behind the truck rear face. One of the importantforces that us acting on the vehicle is drag force. This Aerodynamic drag force grow into an importantobstruction at greater speeds and the performance of the vehicle and fuel consumption are greatlyinfluenced. Since, the aerodynamic drag force always increases proportional to the square of thespeed. The weighty vehicles do cruising at high speed in intercity and take way too much over theyears. The vehicle producers invest too much in aerodynamic studies in order to escalate vehicleperformance. This important step makes the decreasing aerodynamics drag force more importantmatter for heavy vehicles which do a large portion of the transportation in and out of the city. In caseof an articulated vehicle there is an additional free clearance gap between the rear face of tractor andthe front face of semi-trailer, resulting in underpressure at rear face of the tractor, and overpressure atfront face of the trailer. In these simulations the main focus was put on determining the effect ofaerodynamic interference processes taking place between tractor, semi-trailer and a passenger car, andon the aerodynamic drag level. In this study, only primary, rectilinear geometric forms of the tractorand trailer were considered. The models did not have any roundings or aerodynamic drag reductiondevices. A numerical simulation of flow around vehicles in which truck having different height anddifferent gap between trailer and cab is achieved using commercial fluid dynamic software ANSYSFLUENT. The study concentrates on CFD based drag calculation on the car and truck body. A three-dimensional CAD model of a car and truck is used as the basic model in this report which is created inthe commercial software ANSYS WORKBENCH.Problem DefinitionA CAD geometry of the Truck (cab) with a trailer is need to be analysed for 3 different conditions oftrailer height and calculate the effect of varying height on the drag and lift forces. The design velocityshould be 56mph. The second case is to vary the distance between the cab and the trailer and see howthey reflect on the aerodynamics of the truck. The body of the cab is made separately and then
combined with the geometry of the trailer. Both were assembled using different mating features inSolidworks. Methodology1.Open Ansys Workbench 18.1 from the program window2.In the Analysis system window choose fluent as the system3.Double click on design modeller in Fluent System to create the geometry4.Go to the tools in Modeller and see that the measurement units are set to Metre
5.Now right click on the XY plane option and then click on look at6.Go to sketching option in the tree outline window and start drawing the following geometry.
The dimensions are given below7.Now go to extrude option in the tool bar select sketch 1 as a sketch to extrudeChoose the details in the extrude option as below and then click generate to create the extrudedpart8.The extruded truck will look like below
9.To create the feel of artificial atmosphere around truck we use encloser option. Go to tools andselect encloser optionFill the details in the encloser as below and click generate
The geometry created is as below10.To subtract the Truck from the encloser we will use Boolean operation. Go to Create and chooseBoolean option
Now in the detail view choose the body as following11.Now close the design modeller and open mesh sub system in Fluent.12.Expand the sizing option the mesh and write the max and min face size as below. The type ofmeshing used is tetrahedral with relevance centre as medium and Fine span angle centre. Becausethere are too many curves are present in the geometry, it is best to use the tetrahedral elements forthe meshing. The relevance centre is chosen as Medium, because Fine will create a lot of elementsand the computational time will increase exponentially
13.Now go to the front face and select it, right click on it and choose name selectionName the front face as Inlet, similarly the back face as Outlet and 4the sides of the encloser asWall. They are named because it’s easy for fluent to identify them and apply boundariesconditions on them.14.Click on mesh and then click on update in tool bar to update and generate the mesh.Meshed geometry will look like below and then close the mesh system

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