Ahmed Body Challenge
AHMED BODY:
The Ahmed body was described originally by S. R. Ahmed in 1984. Three main features were seen in the wake:
1. The A recirculation region that is formed as the flow separates at the top of the vertical back surface of the model
2. The B recirculation region is formed due to the separation at the base of the model.
3. The c-pillar vortices that form as the vorticity in the side boundary layers roll up over the slant edges.
The wake was shown to be highly dependent on the slant angle. For slant angles less than 12°, the flow remains attached over the slant. The flow is essentially two-dimensional and has low drag. Between 12° and 30°, the flow becomes much more three-dimensional as the c-pillar vortices form. These reach maximum strength at 30°. The drag increases significantly as the low-pressure cores act on the rear surfaces. Past 30° the flow separates fully off the slant. This results in a sudden decrease in drag and weaker c-pillar vortices.
Effect of Aspect Ratio: Venning et al showed that the aspect ratio of the rear slant had a significant effect on the wake. The wider bodies ceased to reattach at slant angles of 25°, suggesting that the critical angle lowers as aspect ratio increases. They also provide a vortex core location from experimental data that can be used for validation.
REFERENCES:
[1] https://www.cfd-online.com/Wiki/Ahmed_body
[2] S.R. Ahmed, G. Ramm, Some Salient Features of the Time-Averaged Ground Vehicle Wake, SAE-Paper 840300, 1984
[3] H. Lienhart, S. Becker, Flow and Turbulence Structure in the Wake of a Simplified Car Model, SAE 2003 World Congress, SAE Paper 2003-01-0656, Detroit, Michigan, USA, 2003
[4] H. Lienhart, C. Stoots, S. Becker, Flow and Turbulence Structures in the Wake of a Simplified Car Model (Ahmed Model), DGLR Fach Symp. der AG STAB, Stuttgart University, 15-17 Nov., 2000
PART B:
Q2. In order to refine the mesh around the area of interest. Create an enclosure around it and refine the mesh locally. Choose appropriate dimensions for the second enclosure and use inflation layers around the body. Perform a grid dependency test on this setup by varying the mesh appropriately (You can choose your own values). Use three different setups containing different element sizes. Try to bring the final setup as close to the academic limit as possible. Plot the velocity profiles for the three setups using a midplane cut plane
In this part, the Ahmed body described above is done Ansys Fluent simulation. The enclosure is designed as shown below for default case:
With respect to boundary conditions velocity inlet is 50m/s and outlet is zero atmospheric pressure is defined. Since velocity is 50m/s (1/6 of Mach number) density-based solver with the SIMPLE second-order upwind method is used for solving the physics of the problem.
CASE 1 [DEFAULT MESH]:
As the first case, meshing is done with default parameters (413.6mm size) without any inflation layers. Meshing for this first case is:
VELOCITY CONTOUR:
PRESSURE CONTOUR:
From the result obtained the wake region flow, in this case, is not much evident. So mesh refinement has been done using Multizone Hexahedral elements throughout the surrounding. The new enclosure has been created which surrounds the Ahmed body well and different mesh sizes are tried for the new enclosure. As well Inflation layers are too used which captures the results better around the Ahmed body.
CASE 2 [MESH REFINED 1]:
SECOND ENCLOSURE:
MESH:
VELOCITY CONTOUR:
PRESSURE CONTOUR:
VELOCITY VECTOR:
CASE 3 [MESH REFINED 2]:
VELOCITY CONTOUR:
PRESSURE CONTOUR:
CASE 4 [ FINAL]:
VELOCITY CONTOUR:
PRESSURE CONTOUR:
The NUMBER OF NODES AND ELEMENTS IN EACH CASE:
Thus from the above plots, all velocity and pressure contours seem to be similar. So for identifying how different cases yields results to velocity and pressure charts are made for four cases and they are shown below:
LINE 1:
From these two plots, we can see a significant difference in velocity and pressure contour for four cases. In the velocity profile, for default mesh (case1) velocity values are too low when compared to other cases. While refined meshes have somewhat similar profiles. Velocity at the bottom is lower and as it goes vertically velocity increases and as it approaches 0.8 units velocity is constant for all cases, which well agrees with simulation physics. Small wiggles present are because just 10 sampling points are used in this case which might affect the smoothness of the plot. In pressure profile as well as default mesh values very much than the other three profiles. At the bottom, part pressure seems to be higher when airflow hits Ahmed's body at Y=0 units pressure drops drastically and wake region forms at end of Ahmed body near Y=0.2 units than on the top part pressure value is further dropped because of vorticity developed in the flow. But still, experimental data is required to verify these profiles. Since this is run using student edition with 512K nodes and elements restriction and turbulence model issue might cause a significant difference in solution. Some of the velocity and pressure contours at different locations are shown below:
LINE 2:
LINE 3:
LINE 4:
REFERENCES: Rajesh Bhaskaran "Introduction to Engineering Analysis using ANSYS WORKBENCH" Coursera lecture series.
Just for verification of the grid dependency test, enclosure of much smaller size is used and the same IC and BC are followed. Velocity and pressure profile are shown below (Enclosure changed is a new legend):
Thus velocity profile doesn't seem to change much but pressure for new enclosure change has been slightly higher than the previous setup. It might be because of grid sizes right after the enclosure is higher which gave higher pressure values but the velocity at (3,0) and (3,2) stabilizes much there is no significant change in velocity. Thus a proper understanding of physics is much required before setting up the model.
Q3. In the post-processed results, explain the variation in pressure as to why it goes negative
In post-processing, we are obtaining static pressure as a negative value which is absolutely fine. A negative value of static pressure indicates that pressure values are lower than the gauge pressure provided at those particular points. Since in all four cases, Gauge pressure is taken as zero there might be possible of negative static pressure. But we have to make sure that absolute pressure value is positive if not there is some problem with boundary conditions and initial conditions. For instance, I ran the same setup with a gauge pressure of 1atm (101325 Pa) and corresponding static pressure is shown below:
which doesn't have a negative value in it. So depending upon the outlet pressure boundary condition pressure value changes. Please find the "outlet pressure changed" module in my Ansys simulation.
REFERENCE:
[1] https://www.cfd-online.com/Forums/fluent/28699-negative-static-pressure.html
[2] https://blog.wika.us/products/pressure-products/pressure-faqs-grip-gauge-pressure/