CFD Plays Well With World's Largest Boundary-Layer Wind Tunnel
The world's largest, scientific quality, boundary-layer wind tunnel was recently declared operational at the University of New Hampshire (UNH). The new wind tunnel known as the Flow Physics Facility (FPF) serves as a great example of how a wind tunnel and Computational Fluid Dynamics (CFD) can form a mutually beneficial relationship.
The specifications for the FPF are truly impressive. The open-loop wind tunnel working section is 300 feet long by 20 feet wide. The maximum wind speed is 28 mph provided by 2x400 hp fans. The FPF was designed in a modular manner with an eye to upgrade to an even more capable closed-loop wind tunnel if sufficient funding becomes available.
Before and during the construction of the US$3 million wind tunnel, CFD was used as a cost-effective method to investigate various features of the wind tunnel. A chicken-and-egg situation arises with the design of a wind tunnel in that you would really like to use a wind tunnel to design the wind tunnel. However, using the advanced modeling capabilities of current CFD packages you can break this seemingly self referral infinite loop and make headway with a wind tunnel design to avoid costly rework during the wind tunnel construction phase.
So you can see how this wind tunnel benefited from CFD, but how do we close the symbiotic circle and see how CFD might benefit from this wind tunnel? Good question. The stated aim of the FPF is to "...help engineers and scientists better understand the dynamics of boundary layers..." It just so happens that one of the areas that RANS CFD could use help with is improving turbulence models for better prediction of - you guessed it - boundary layers. The boundary layer is a critical region that forms adjacent to surfaces exposed to fluid flow. The frictional fluid forces due to viscosity in the boundary layer account for a significant portion of the drag that acts on an object (e.g., car, aircraft, ship) as that object tries to move through a fluid, such as air or water.
Expectations are high that the FPF can help provide insights and validation data for improved CFD turbulence models. And without CFD the FPF construction would have been a much more costly exercise or, worse, might not have been economically viable. Yet again we see here an example of a wind tunnel used in conjunction with CFD that results in a win-win situation rather than the zero-sum some believe exists between these two modeling techniques.
My thanks go to Joe Klewicki (professor of mechanical engineering and director of the Center for Fluid Physics at UNH) for his tour of the FPF and presentation during the joint New England SNAME/AIAA September 2011 meeting.