Computational Fluid Dynamics Flow Over a Cylinder
Purpose: Fluid mechanics is the study of the behavior of fluids that are either at rest or in motion. In the scope of the mechanical engineer — it is heavily used in the design of pumps, compressors, turbines, process control systems, heating and air conditioning, wind turbines, and solar heating devices to name a few. Automotive manufacturer General Motors used CFD Analysis in conjunction with over 700 hours of wind tunnel testing during the design of the C7 Corvette to obtain an optimal balance between zero lift and a low drag coefficient while still maintaining enough airflow for mechanical cooling. In the history of golf, the golf balls were initially smooth. Early golfers realized the spheres travelled farther the more they were used. This lead them to intentionally scuff the golf balls. After many years and millions spent on research and development, the signature dimpled golf ball look is now able to soar almost twice as far as the similar sized smooth sphere it initially started as. It does this because the wind is better at “hugging” the golf ball as it fly’s through the air, minimizing the low-pressure zone behind it. In the smooth golf ball case, the airflow travels over the sphere and becomes detached, causing a greater low-pressure zone. In this example, I use ANSYS Fluent to simulate air flow over a cylinder at a speed of 94 m/s. A physical example of this would be a smooth golf ball travelling horizontally with zero angular velocity.
Velocity & Pressure Contour of Smooth surface
Fluent Settings — Solver: Pressure-Based, Velocity Formulation: Absolute, Time: Transient, 2D Space: Planar, Viscous Model: k-Omega SST, Boundary Condition (Cylinder): Stationary Wall/No Slip/Standard, Solution Methods: SIMPLE, Gradient: Least Squares Cell Based, Pressure: Second Order, Momentum/Turbulent Kinetic Energy/Specific Dissipation Rate/Transient Formulation: Second Order Implicit, Initialization: Hybrid
Verification & Validation:
We can verify turbulent flow from our calculations visually by inspecting the first image (Velocity Contour 1), where we can see the detached airflow and the development of wakes to form the low pressure zone (dark blue region). Minimizing this low pressure zone lowers drag and allows the ball to have more flight. In our second image (Pressure Contour 2) we can see where Bernoulli’s principle has the greatest effect (light blue region). Bernoulli’s principle states that as the velocity increases around the ball, the pressure acting on it decreases, causing lift. Dimple number, depth, shape, radius all affect both drag and lift.
Conclusion: It’s really amazing how the engineers were able to take a smooth spherical ball, keep both the same mass/diameter, and create something that travels almost twice as far. While outside the scope of my courses taken and without any formal training in computational fluid dynamics (aerospace engineers tend to do more CFD-based aerodynamics), I thought it was an interesting problem with easy to describe visuals.