Reduction of vehicle aerodynamic drag is very serious issue in the last decade, especially after increasing of fossil fuel price and reduction of air pollution. Observation of vortex simulation flow around an ideal form of land vehicle at Reynold number until 105 using CFD at tail vehicle to reduce drag .  Observe aerodynamic drag of the vehicle ARTeC's PEC 2011 EMo-C.  Observed how to reduce drag by firing between truck and container trailer, drag reduction can be reached until 26%. Research by  observed by putting ellip flap ellip behind bus. Drag reduction about 11,1 %. Research to reduce drag by  that focus on effect of deflector to aerodynamic drag on rear side vehicle body of Ahmed model. Research the factors influence on the aerodynamics drag reduction of commercial vehicles  by add-on parts including deflector, side fenders, visor, engine compartment flow can reduce the value of Cd.  Proposed 2-D mathematical model of curve body profile shark fish to be applied at various vehicle body. Experiment results shows that aerodynamic body curve of the shark body applied to sedan-x vehicle body using CFD software shows smooth flow trajectory. Drag reduction can be found by finding value of Cd. Testing of Cd value, flow trajectory and pressure contour using Computational Fluid Dynamics (CFD). Focus in this research is to reduce drag vehicle si Pitung G4 for Shell Eco Marathon Asia 2015 by reducing aerodynamic drag coefficient.
2. Methods and Equipment
This research held at Design Laboratory of Mechanical Engineering State University of Jakarta using CFD Autodesk Flow Design Software to get drag coefficient Cd and Solid Works Flow Software for flow trajectory. The flow diagram as follows:
3D design and dimension according to Shell regulation using Autodesk Inventor then to analyzed Cd used Software Autodesk Flow Design and for flow trajectory using Solid Works Flow simulation.
Design of vehicle body must be aerodynamic, strong, light weight and cheap price and must follow Shell regulation according to Figure 4.
Data for simulation results of software reported are coefficient Cd, flow trajectory, static pressure contour, and velocity contour.
where Cd is drag coefficient, Fd is drag force (pressure and skin friction drag), ρ is specific mass of air, A is frontal area, V is speed.
Simulation results of flow trajectories
From Figure 5a, flow trajectory of fluid seems vortex around front wheel spark board at tail body of vehicle. According to Figure 5b, flow trajectory of Si Pitung G5 is quiet good, we can see from flow line color around geometry of the body.
Simulation results of static pressure
From Figures 6(a) and 6(b), it seems that distribution of high static pressure appear in front of vehicle body by red color. Low pressure and vortex appear around spark board front wheel by blue color.
Figures 7(a) and 7(b) shows that distribution of high static pressure appear in front of vehicle body by red color but vortex around spark board front wheel minimized by blue color. Reduced the vortex will reduced aerodynamic drag.
Simulation results of velocity contour
From Figures 8(a) and 8(b) seems that there is vortex around front wheel spakboard of the vehicle inside body. Vortex will increase aerodynamic drag.
From Figures 9(a) and 9(b), it seems that there is a flow improvement around the front wheel spark board body.
Reduction of aerodynamic drag can be reached by modification the form of the body to become aerodynamic form like parallel form changed to become curve form. Vortex and drop pressure on tail body must be avoided. Figure 9a and 9b are improvement points of vortex in the sparkboard vehicle body as shown in Figure 8a and 8b. From table 1 can be seen that drag coefficient will decrease when the speed is increase.
It has been found that improvement of the front wheel sparkboard vehicle body gives reduction of Cd, from 0.15 for Si pitung G4 to 0.13 for Si Pitung G5. The percentage reduction is 17.77 %.
Changing form of the front wheel sparkboard vehicle body will change pressure contour and flow trajectory. Vortex is main part to reduce aerodynamic drag.
Reduce drag automatically will reduce fuel consumption.