Week 3 - External flow simulation over an Ahmed body.

Aim of the experiment :-to simulate externalflow condition over an ahmad body for refined case.To perform a grid independence study.

introduction

Ahmad body and its performance ahmad body is a simplified form an automobile body that resembles all the essential flow features around an automobile.this body allows us to capture all the characters features relevant to an automobile .Ahmad body can be usefulfor developing simulation models and validation its expermentally.

PRE-PROCESSING

GEOMETRY

Thestep file of ahmad body was importted into spaceclaim to create a geometry set up for the simulation.The ahmad body was split about the symmetry axis using split body option.Enclosure option was selected toncreate an outer enclosure around the geometry .in order to refine the regions around the ahmad body further an inner enclosure was set up near to thebody.
Interferencecheck was run to check for interference and finally sharetopology to share.

MESH

In mesher,multizone meshingmethod was used with hexadominant mesh in the outer enclosure and tetrahedaral meshon the inner enclosure.face sixing was given to the circular support pillars of the ahmad body and with y+ valueas 30 for k - epsilon model 6 inflamation layer were set up by circulating the first layer height.

Post processing

result

(Refined case)

Total number of element =479721 elements

outer enclosure element size = 0.076

inner enclosure element size =0.026

face element size = 0.0026m

first layer height = 4.445e-4

inflamatin layer =6

OBSERVATION TABLE

TOTAL NUMBER OF ELEMENT COEFFICIENT OF DRAG COEFFICIENT OF LIFT

94145 0.35 0.23

160489 0.33 0.22

330487 0.33 0.19

479421 0.30 0.25

negative pressure in the wake region

when air hits the ahmad body .it creates a high pressure point at the front and low pressure point at the rear due to air separation .This low pressure region is known as wake .This wakeis responsible for pressure drag.which reduces the performance of the vehicles.

Point of separation

The point of separation is a point where the fluid flow,due to shear stress,nolonger follows the contourof the body and grts detected from the surface of thebody.This separation gives rise to vertice and eddies.