Evolution Of Shape Designs - Eat Design Sleep Repeat

Eat Design Sleep Repeat! That was all we did for a month straight. We had our startup shape ready but its aerodynamic efficiency although pretty good was just not good enough to satisfy some of us. So we set out to decrease the coefficient of drag to as much as we could. We found out cd values of some famous automobiles calculated in there working environments and decided not to stop till we get better values for our pod design. Although we iterated through 15 different pod design shapes, a few of them are shown below.

The first design, as presented in the Preliminary Design Briefing, had a separate body beneath the primary pod

shell for integrating several of the pod subsystems, with each body generating its own drag forces. As shown in analysis to the left, significant vorticities formed at the tail of the pod. Although the airflow over the the top of the pod was less than Mach 1, the drag created by this shape was very high due to these vortices. The Cd achieved for this shape was 1.35 at a velocity of 150m/s.

Next our team wanted to see what happens if we take the skies out from the startup shape. So, by performing the

simulation with only the primary pod shell from shape 1, a substantial improvement in the Cd of the shape was observed, encouraging exploration of designs where all systems are integrated into a single body. The Cd for this shape dropped to a staggering 0.65 at a velocity of 150m/s.

Motivated by the results of the previous iteration, shape 3 was designed with all systems integrated in a single body.

Although the coefficient of drag increased considerably, it was determined that this was due to the shape of the pod and that further refinement and analysis could reduce the Cd. The value of Cd achieved for this shape was 2.95 at a velocity of 150m/s.

For shape 4, both the nose and the tail of the pod were grounded. This shape had a lower drag than the shape 3,

however, it was much higher than the goal of our design study. The reason for the high Cd was primarily due to the large number of vorticities that formed near the tail of the pod. This was quite an eye opening result and helped us create our next test design. The value of Cd achieved for this shape was 2.13 at a velocity of 150m/s.

Learning from our previous designs, in this design we combined the shapes form 3 and 4. To create shape 5 we

used the nose from shape 4 and the tail form shape 3. The Cd of this shape dropped considerably to 1.4 from 2.13 and 2.95 at a testing speed of 150m/s, thus encouraging us further to develop more designs and test new and different shapes.

Shape 6 was our first break through. We were able to achieve a Cd of 0.21 at 150m/s. This shape was inspired from the design of an aerofoil. To keep the length of the pod within reasonable limits, we had to reduce the height

considerably. The CFD results for this design were so encouraging that we decided to take our pod speed to the next level. Although the results at 200m/s and 250m/s gave us Cd of 0.4 and 1.69, we had to get a good Cd at least at 200m/s. After 200m/s, the flow speed of the air above the pod exceeded the Kantrowitz limit, inducing a shock waves, and causing a large increase the Cd. This being our major hindrance we targeted our next design to overcome this limit.

After few more design iterations, we finally zeroed in on a refined version of the shape 6. At 200m/s, this variation of shape 6 experiences a total aerodynamic drag of 86 N with a coefficient of drag of 0.26 and a coefficient of lift of -0.01. The velocity of air over the top of the pod stays

well below Mach 1 thus keeping the Kantrowitz effect in check. The total volume of this pod shape is 1.02 m3 and

has sufficient space for integration of subsystems into the pod hull. The pressure range of air around the pod is well under safe operating limits and no significant formations of vorticities are created behind the pod. The temperature on the pod surface and of the air around the pod due to aerodynamic forces is less than 320 K. Thus, the temperatures generated by the drag are within safe functional limits.

The graph below shows the evolution of the ten shape iterations. The coefficient of drag ranged from 0.21 to 2.95 for an air velocity of 150 m/s, finally settling on design 10. The final design has a drag coefficient of 0.26 at an air velocity of 200 m/s.