Week 3 - External flow simulation over an Ahmed body.

External flow simulation over an Ahmed body.

AIM:
To simulate flow over ahmed body & to address the below given tasks.
1. Describe Ahmed body & its importance.
2. Explain the negative pressure in wake.
3. Explain significance of the point of seperation.

Expected results:
1. Velocity & pressure contour of the Ahmed body.
2. Cd for a refined case. (for velocity = 25m/s, Cd = 0.33)
3. Vector plot must show wake region.
4. With grid dependency test provide Cd & Cl for each case.
Note:
1. Since the ahmed body is symmetric, run the simulatoion by considering only half body. Doing this would increase the cell count and would make the mesh finer.
2. Use split body command in spaceclaim & use symmetry boundary condition.
3. Maintaining an appropriate y+ value will give correct Cd value.

THEORY:
The geometry of the vehicle is complex, the flow around the vehicle is fully three-dimensional, boundary layers are turbulent flow separation and there are large turbulent wakes in which longitudinal trailing vortices are common. Typically for the bluff body, the principal contribution to drag experienced by road vehicles is pressure drag, and an important objective of vehicle aerodynamic design is the avoidance, reduction, or control separation.  Viscous-inviscid flow interaction can be challenging to predict for any bluff body separated flow and this is particularly so when there are ground effects present.

A bluff body is a body that as a result of its shape has separated flow over a substantial part of its surface. The bluff body has a very strong interaction between viscous and inviscid regions. Consider that at the moment of starting the flow is attached. Prediction of separation position from a continuous surface is particularly difficult because it depends on both the characteristics of the upstream boundary layer and on the structure of the near wake region. Usually, bodies induce a separation region where velocity at the edge of the boundary layer is higher than free stream velocity.

Boundary layer for flow over a flat plate:

Transition of the laminar boundary layer on a flat plate into a fully turbulent boundary layer (not to scale).
The vertical scale has been greatly exaggerated, and the horizontal scale has been shortened (in reality, since Rex, transition ≅ 30 times Rex, critical, the transitional region is much longer than indicated in the figure).

Thickness of the boundary layer on a flat plate, drawn to scale. Laminar, transitional, and turbulent regions are indicated for the case of a smooth wall with calm freestream conditions.

In real-life engineering flows, transition to turbulent flow usually occurs more abruptly and much earlier (at a lower value of Rex) than the values given for a smooth flat plate with a calm free stream. Factors such as roughness along the surface, free-stream disturbances, acoustic noise, flow unsteadiness, vibrations, and curvature of the wall contribute to an earlier transition location. Because of this, an engineering critical Reynolds number of Rex , cr = 5 ⋅ 10^5 is often used to determine whether a boundary layer is most likely laminar ( Rex< Rex , cr ) or most likely turbulent ( Rex >Rex , cr ).

* Laminar values are exact and are listed to three significant digits, but turbulent values are listed to only two significant digits due to the large uncertainty
affiliated with all turbulent flow fields.
† Obtained from one-seventh-power law.
‡ Obtained from one-seventh-power law combined with empirical data for turbulent flow through smooth pipes.

INTRODUCTION :
The flow around road vehicles (car, buses, truck) under normal operating conditions is principally turbulent. It is typically characterized by large scale separation and recirculating regions, a complex wake flows long trailing vortices, and interaction of boundary layer flow on vehicle and ground. In the design of ground vehicles, the crucial part is to decide the turbulent model according to the type of vehicle. The advanced modeling techniques and solving methods require hours and days of work for CPUs to calculate at precision.

In aerodynamic flow around a vehicle, it is a three-dimensional flow and the design will have an influence on principle features such as the shape of the vehicle, aerodynamic drag, fuel consumption, noise production, and road handling.By using wind tunnel experiments the designer have an understanding of airflow around the vehicle through extensive wind tunnel testing. But wind tunnel testing for every prototype is a time-consuming and costly process. Also, the reliability of the turbulence model and its long-lasting process of calculation is another challenge.

In order to reduce time, it is necessary to use simplified computational technic and adopt a model to describe the mean effect of turbulence. Unfortunately, a simple turbulence model often fails to calculate the flow properly especially the position of flow separation on the rear slant is crucial in determining the aerodynamic drag but is an extremely difficult feature to calculate. To overcome the problem, S. R. Ahmed in 1984 described an Ahmed body with the standard results to test the various type of challenges and turbulence models. Ahmed body is a kind of baseline bluff body geometry for automobile bodies.

Since the characteristic length has been found to be 1044mm(1.044m) the Reynold's number of the flow can be calculated.
Other qualtities that are known:

Geometry description:
Ahmed body:
The Ahmed Body was first created by S.R. Ahmed in his research “Some Salient Features of the Time-Averaged Ground Vehicle Wake” in 1984. Since then, it has become a benchmark for aerodynamic simulation tools. The simple geometrical shape has a length of 1.044 meters, height of 0.288 meters, and a width of 0.389 meters. It also has 0.5-meter cylindrical legs attached to the bottom of the body and the rear surface has a slant that falls off at 40 degrees.

Selection of the configuration used in this study was governed by the requirement that it should generate the essential features of a real vehicle flow field, with the exception of that due to rotating wheels, engine and passenger compartment flow, rough underside and surface projections. It was conjectured that the model chosen should generate: a strong three-dimensional displacement flow in front, relatively uniform flow in the middle, and a large structured wake at the rear. The wind tunnel model, with an overall length of 1.044 m had a length: width: height ratio of 3.36 : 1.37 : 1. It consisted of three parts; a fore body, a mid section and a rear end.

Edges of the fore body were rounded, as indicated above to achieve a separation free flow over its surface. Middle section was a box shaped sharp edged body with a rectangular cross section.

Baseline setup:

Import geometry and cutting to half as it is symmetric about the shown plane

Preparing Outer and Inner Enclosure

Check for possible Interferance

After all checking to preparing geometry check for possible geometry import to meshing

Meshing Details- Define Inlet, Outlet, Car Wall and Symmetry with the help of Named Selection

Multizone

Face Sizing Car Wall

Face Sizing - Car Legs

Body Sizing for Inner Enclosure

Inflation Layer under consideration is only car wall- Its selected by the named selection

Setup :

Projected area: Very important to find Cd, Cl

Model setup- viscous model with k-epsilon and engery on

Boundary Conditions

Inlet

Material - Air

Outlet

Initialization of Solution

The idea is to solve for potential flow variables to get an estimate for the velocity components.

Here a value for pressure isn't possible but only the value for velocity is possible.
This provides a very good initial condition/starting point. It's not an accurate approximation, it's a very good starting point when the flow path is really complex. A PDE is obtained when you need to solve iteratively that's what happens when the solution is 'initialized'.

Create the plane as per requirement

Contour on the created plane

Create animation for required counters

Check the Reference Values which helps to calculate CD, CL

Create report file for ploting CD, CL

Report Defination

same process is followed for Lift coefficient

Solutions>Reports>Definition>Edit>Under 'Report definitions' :Select 'Lift or drag coeffecient>Compute.

Run the calculation for 1000 iteration

Velocity at Leg Leg Plane

Pressure Countor

Coefficient of Drag and Coefficient of Lift

CFD- Post- Velocity at Leg Plane

Streamlines - Velocity

NEGATIVE PRESSURE IN THE WAKE REGION -
A high-pressure region is observed in the front of Ahmed body and a drop in pressure is observed when the flow leaves the Ahmed body at the rear (As shownin Figures ). This detachment of fluid at the rear results in eddy formation and disturbance in the flow.

The region behind Ahmed body where the flow is unstable is called the wake, where pressure adversely drops.
At the inlet, fluid flows with the velocity of m/s when it encounters the Ahmed body in front of it high-velocity fluid is stagnated and reduces velocity results in high pressure. The fluid that comes in contact with the body behaves like a viscous sublayer this property of fluid remains until it is in contact with the body. At the slant of Ahmed body flows pf fluid separates the body and point of separation occur.

Boundary-Layer separation over the wing. Source: aerospaceengineeringblog.com

When fluid flows over the body, the boundary layer thickness increases. The fluid layer adjacent to the surface friction by consuming some kinetic energy. This loss of kinetic energy recovered from the adjacent fluid layer through the momentum exchange process. Thus the velocity of the layer goes on decreasing.

Along the length of the body, at a certain point, a stage may come when the boundary layer may not be able to keep sticking to a solid body. The boundary layer will be separated from the surface. This phenomenon called boundary layer separation.
Near the surface of the solid when the point of separation occurs the fluid is incapable of sticking onto the surface. This arises the point where pressure reduces to negative.

This induces extra drag force at the rear which creates a loss in the velocity of the fluid and reduces the velocity of the body. This negative pressure in the rear gives rise to vortices and eddies which creates losses. So for that reason in most cases the point of separation of the boundary layer designed in such a manner as to create the negative pressure away from the boundary.

SIGNIFICANCE OF THE POINT OF SEPARATION -
The point on the body where the boundary layer lifts off the surface is called the separation point. When the airflow separates off the surface, the pressure drag usually increases. The separation point is the point where the air stops "sticking" to an object that is moving through the air. When an object is moving through the air, the air molecules push back on it and resist the object's movement - this resistance is called pressure drag.

The position of the point of separation plays an essential role in the case where one wants less aerodynamic drag. To achieve this we can find several examples that are implemented in recent years the continuous shape of an outer layer of the body, providing wings, flaps, slats to locate the point of separation away from the body. The arrangements changes from application to application but the basic idea behind it is to reduce the drag as much as possible and make the object aerodynamically stable. After the point of separation from the figure, the formation of eddies is started.

Result