Anwar bin Mohd SoodWaqar Asrar

International Islamic University Malaysia

Ashraf Ali OmarUniversity of Tripoli, Libya

2nd International Conference on Science and Technology(ICST), Kuala Lumpur. June 17, 2014

Introduction

Modern passenger fuel-powered cars should have good fuel economy in order to be competitive.

Most of the time a car’s fuel is used to overcome the weight of the car to move especially in the urban areas.

As the car moves at a constant high speed or cruising, it needs to overcome the air friction around its body or also known as aerodynamic drag.

A good aerodynamic performance of a car will reduce substantially the air friction and thus have a good fuel economy.

Problem Statement

The introduction of CO2 emissions legislation for European passenger cars and rising oil prices has seen the increasing focus on improving fuel efficiency through efficient engine and drive train and also reduction in weight and aerodynamics drag.

For ground vehicles above 100 km/h, 75% of the total resistance to motion is coming from the aerodynamics drag.

Therefore it is crucial to study drag reduction on passenger cars in order to reduce the fuel consumption.

This research focusing on the effectiveness of having a kick-out effect at fore of rear wheel arch (pushed-in rear door) and tapering the rear bumper sides with partially exposed rear tire surface (pushed-in rear bumper) to reduce the aerodynamic drag of the car through CFD simulation.

Motivation behind the study:BMW 3 series 2014

CD = 0.29

kick-out effect by pushing-in the rear door surface

& partially exposed rear tire surface

Pushing-in the rear bumper sides

CFD car model

kick-out effect area

& partially expose rear tire surface

Pushing-in the rear bumper sides

Model is taken from Shazele Ismail, Undergraduate thesis

CFD model case studies

Push-in by 5 mm inside

Push-in by 10 mm inside

RR door

RR bumper

View at side of body RR

CasesPushed-in rear

doorPushed-in rear

bumperBaseline case - -

Case 1 - 10 mmCase 2 5 mm 10 mm

Ahmed body 25° and 35° slant angle result

Slant angle k-Epsilon k-Omega SST

25° 3% error 1% error

35° 1% error 3% error

Fully separated flow

Attached flow to the slant

Small separation bubble

Case studies Result

Case 2 showed the lowest drag value. Case 1 gave a reduction of 3.6 drag counts while Case 2 gave the highest drag reduction by 4.3 drag counts.

Case 2 shows that a combination of kick-out effect on the rear door and pushed-in rear bumper sides will give better drag reduction when compared to Case 1 alone.

0.28

0.282

0.284

0.286

0.288

0.29

CD

Cases

Drag coefficient values

Baseline

Case 1

Case 2

-1.3%-1.5%

High pressure on RR wheel arch

Lower pressure

• The baseline case shows high pressure region on the rear tire surface and also its wheel arch. This high pressure region translates to high drag as the flow slows down due to a blockage by the rear wheel arch.

• Pushing-in the side rear bumper surface by 10 mm, the high pressure inside the wheel arch has drastically reduced by around 90%.

High pressure

Pressure plot cross sectionBaseline case

Case 1

More energized wake at the rear part of the car shown by the higher total pressure plot in Case 1 lower drag coefficient.Low pressure bubble is smaller on the RR bumper in Case 1.

Flow more attached to RR tire and the RR bumper

Comparison of flow attachment

• There is an early flow separation at the side of the rear bumper shown by the blue separation bubble in Case 1.

• The separation bubble is gone in Case 2 as the flow earlier at the rear door was energized resulting a more attached flow to the rear.

• When the flow reaches the rear bumper, the flow tends to remain attached giving more drag count reduction

Conclusion This research has discussed measures to improve the design of

the rear side profile of a car in order to reduce the overall aerodynamic drag of the car, and thus improve fuel efficiency.

Exposing the rear tire by pushing-in the side rear bumper is the easiest way to reduce the drag at the rear wheel arch.

Pushing-in the rear door adds more to the reduction of drag.

Combination of both strategies should be made carefully so that early flow separation will not occur on the side rear bumper.

The study can further be extended with a rotating wheel and an open wheel rim. Implementation to an actual size and real car geometry is expected to give more drag count reduction.

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