effect of rear end taper on drag for

8/3/2019 Effect of Rear End Taper on Drag For

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EFFECT OF REAR

END TAPER ON DRAG

FOR A BLUFF BODY

THARANGA JAYATHUNGA

SID: 3601183


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INTRODUCTION

Aerodynamics is the analysis regarding the air flow

around a body.

Aerodynamic drag force has a direct effect on the fuel

consumption and maximum speed.

Drag coefficient (CD) is one of the governing factor on

the drag force and depends on the body shape.


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INTRODUCTION

Body shape of a vehicle is aerodynamically bluff.

Air flow separation is occurring at the rear end of thevehicle.

There is a developed adverse pressure gradient at the

back.

This is the cause for the pressure drag.


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INTRODUCTION

Degree of taper at the rear end affect the drag force.


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INTRODUCTION

Rear and taper and boat tailing are drag reductionmethods.


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RESEARCH TOPIC

Aim of the research is to generate an understanding of

interaction of side boat tailing and end taper on the

drag of a bluff body.


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METHODOLOGY

CFD simulations was performed and Star CCM+

software Version 6.04.014.

A. Ahmed model and Windsor model tested for

validation.

B. Windsor models is tested for side boat tailing and end

taper interaction.

C. Detail optimization of Windsor model.

D. Tested for theoretical best values for end taper and


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METHODOLOGY

Tapered length of the model.

Length ratio= Length of the tapered/Total length of the model

Ratio used here is 0.2 (209mm).

CAD modeling.

CATIA version 5R20 is used for the CAD modeling.

AhmedCAD.jpg

Windsor CAD.jpg
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METHODOLOGY

Star CCM+ modeling.

Number of cells is around 1 million for every cases that are tested.

Number of faces for each case kept around 300000.

Mesh type is polyhedral.

Prism layer mesh is used for the boundary layers and the thickness is

10mm.

K- model is used for the turbulence modelling.

Number of iterations limit to 1000.

Domainortho.jpg
http://var/www/apps/conversion/current/tmp/scratch25269/Windsor%20CAD.jpghttp://var/www/apps/conversion/current/tmp/scratch25269/Windsor%20CAD.jpghttp://var/www/apps/conversion/current/tmp/scratch25269/Windsor%20CAD.jpghttp://var/www/apps/conversion/current/tmp/scratch25269/Windsor%20CAD.jpghttp://var/www/apps/conversion/current/tmp/scratch25269/Windsor%20CAD.jpghttp://var/www/apps/conversion/current/tmp/scratch25269/Windsor%20CAD.jpghttp://var/www/apps/conversion/current/tmp/scratch25269/Windsor%20CAD.jpghttp://var/www/apps/conversion/current/tmp/scratch25269/Windsor%20CAD.jpghttp://var/www/apps/conversion/current/tmp/scratch25269/Windsor%20CAD.jpghttp://var/www/apps/conversion/current/tmp/scratch25269/Windsor%20CAD.jpghttp://var/www/apps/conversion/current/tmp/scratch25269/Windsor%20CAD.jpghttp://var/www/apps/conversion/current/tmp/scratch25269/Windsor%20CAD.jpghttp://var/www/apps/conversion/current/tmp/scratch25269/Windsor%20CAD.jpghttp://var/www/apps/conversion/current/tmp/scratch25269/Windsor%20CAD.jpghttp://var/www/apps/conversion/current/tmp/scratch25269/Windsor%20CAD.jpghttp://var/www/apps/conversion/current/tmp/scratch25269/Windsor%20CAD.jpghttp://var/www/apps/conversion/current/tmp/scratch25269/Windsor%20CAD.jpghttp://var/www/apps/conversion/current/tmp/scratch25269/Windsor%20CAD.jpghttp://var/www/apps/conversion/current/tmp/scratch25269/Domainortho.jpghttp://var/www/apps/conversion/current/tmp/scratch25269/Domainortho.jpg


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METHODOLOGY

Following table is showing all the combinations of side boat tailing

and end taper, that are examined.
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RESULTS

CD value obtained for the Ahmed model is 0.25.

AhmedCAD.jpg

CD value obtained for the Windsor model is 0.295.

Windsor model is in the square back configuration.
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RESULTS

CD for each configuration of the Windsor model.


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RESULTS

Largest value for CD obtained when the end taper angle

80 and side boat tail angle 80 (18% reduction).

S8T8_Vector Scene 1.jpg

Wakeregion.jpg

Windsor_Vector Scene 1.jpg

S8T8_Geometry Scene 6.jpg

stmlin2 - Copy.avi
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RESULTS

Wake region is narrowed due to the geometrychanges.

Vortices in the near wake region arecomparatively small.

Pressure loss has been reduced by around 22%due to the changes.

There is a noticeable change in the freestagnation point.

Comparatively more orderly flow is obtained in

here.
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RESULTS

Figure shows the trend of variation of CD With Sideboat tail angle for typical end taper angle.

0 2 4 6 8 10 12

0

2

4

6

8

10

12
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RESULTS

Starting from 00-80 CD is increased and after 80 CD

getting reduced up to 200.

strmlineS8T20.jpg

When the Slant angle increases strength in the 3D trailing

vortices are becoming stronger.

In the far wake region there are two separate flows.
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DETAIL OPTIMIZATION

All the edges as showing in the following figure

is rounded and CD value obtained is 92.

DEtailopt1.jpg

DetailoptCP.jpg
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RESULTS

For the tapered length ratio 0.2 (209mm), end taper

angle 12.50 and side boat tail angle 100 obtained CD

value is 49.

When the tapered length change to 222mm CD

obtained is 54.
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CONCLUSIONS

CD value for the (square back) Windsor model is 0.295.

Largest CD is obtained for the side boat tail angle 80 and

end taper angle 80. CD value is 53.

Around 22% base pressure increase compare with square

back.

18% decrease in CD.

Theoretical optimum values for side boat tail end taper given

CD value is 52.

As ect ratios of the boat tail and end ta er have an effect on
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CONCLUSIONS

Drag coefficient is influenced by after body rounding.

After body rounding at the side boat tail and end taper 80

configuration gives 31% reduction in drag coefficient.

Aspect ratios of the boat tail and end taper have an effect

on drag reduction.

Drag coefficient is influenced by after body rounding.

Body rounding at the side boat tail and end taper 80

configuration gives 31% reduction in drag coefficient.
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effect of rear end taper on drag for

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