Grds conferences icst and icbelsh (8)
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Transcript of Grds conferences icst and icbelsh (8)
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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