Hatchback Car Case Study

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Drag reduction of simplified hatchback car by underbody diffuser and two vehicle platooning
Kumar Ujjwal, Rahul Kumar Sharma, Adesh Kumar and Manimaran R.
VIT University, Chennai, India

Abstract
The study of vehicle interaction is becoming increasingly important nowadays with the use of intelligent transportation system (ITS). The aerodynamic drag of vehicle can be reduced by including underbody diffuser and reducing inter vehicular spacing. With lesser inter vehicular spacing the drag is expected to reduce. Thus, the overall fuel efficiency of vehicle will increase with reduced emission levels. In present study, Ahmed car body is used as vehicle to study the effect of underbody diffuser and inter-vehicular distance on drag and lift forces.
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Turbulence viscosity ratio is 10%.
• At the outlet the gauge pressure is set to zero with turbulence intensity of 5% and turbulence viscosity ratio of 10%.
2.4 Solver strategy and turbulence modelling
The k-ϵ turbulence model was employed in the present study using ANSYS FLUENT v14.0. The exact k-ϵ model has many unknowns and unmeasurable terms which are removed from the Launder and Spalding k-ϵ model which is also known as standard k-ϵ model. The k-ϵ turbulence model is the most common model used in computational fluid dynamics (CFD) to simulate mean flow characteristics for turbulent flow conditions. It is a two equation model which gives a general description of turbulence by means of two transport equations (PDE). The equations are as follows:
1. The equation for conservation of mass or continuity equation can be written as follows:
The general form of the mass conservation equation is valid for incompressible as well as compressible flows. The source Sm is the mass added to the continuous phase from the dispersed second phase (e.g., due to vaporization of liquid droplets) and any user-defined sources.

For 2D axi-symmetric geometries, the continuity equation is given
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Non-equilibrium wall functions was used to treat near wall conditions, as flow separation and re-attachment is major contributing factor to the drag force. Air flow before reaching the test section in wind turbine passes through settling chamber and contraction zone which makes the flow laminar and stable. Hence, turbulence intensity was kept 1% for velocity inlet with turbulent viscosity ratio of 10. While passing the test section, air faces the obstacle and flow turbulence increases. Hence, turbulence intensity was kept 5% for pressure outlet with turbulent viscosity ratio of 10. Here, discretized model was exported to ANSYS Fluent 14.0 for solving the problem. As study was performed with velocity of vehicle at 40 m/s, the flow would be incompressible with variations in pressure around the body surface without altering the density of surrounding fluid. Hence, pressure based solver was employed with absolute velocity formulation and steady state flow

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