Improved drag performance verification using CFD
In fluid dynamics, the drag coefficient is a dimensionless quantity that is used to quantify the drag or resistance of an object in a fluid environment, such as air or water. It is used in the drag equation in which a lower drag coefficient indicates the object will have less aerodynamic or hydrodynamic drag.
ARC Marine undertook a CFD study to understand the drag behavior of the standard Reef cube when on the seabed.
Typically, the drag area, for standard solid cuboid bodies, is the frontal geometric area presented to the oncoming flow. For the ARC team, three theories were raised for Reef Cubes® – would the drag large internal chamber surface area (great for Cephalopods) would act as a large bluff area and cause flow stagnation? Would the side ports cause turbulence, or would the portholes serve to reduce drag?
Figure: Different bluff areas tested
We wanted to determine how to calculate the drag area for this type of body – none of the literature offered any guidance on what the bluff area should be, so through a series of simulations we correlated the net drag force on the body, using on Morison’s equations.
Based on the simulated forces we back-calculated the drag area and compared the results to the series of different potential bluff areas. Using the correlation, it became clear that the key area had to be that defined by (b) because the values for the drag coefficient arising for calculations based on (a) and (c) were not feasible when compared to a solid cube. We were then able to work out the effective drag coefficient of the reef cube:
A Reef Cube® has a drag coefficient of 0.85 when suspended in a free stream. This compares to 1.05 for a solid cube in a free stream or 0.94 coefficient on the seabed.
Figure: 0.6 m full reef cube flow velocity contour [m/s]
Last but most importantly we wanted to work out what the drag coefficient was for a reef cube on the seabed in a one-sided flow.
A series of simulations were performed for different Reef Cube® sizes fixed on the seabed in a one-sided flow. A series of different flow speeds was applied to check sensitivity to flow speed and ensure that there were no scaling effects. The graph below contains the drag coefficient areas curve fit for all size cases and flow velocities considered.
“It’s great to be finally demonstrating with hard numbers and engineering what we all knew was the case in terms of Reef cubes’ on-bottom stability performance.”
Tom Birbeck, CEO and Co-Founder