CFD and Wind Tunnels: Combining the Two for Aerodynamic Excellence
Industry : Aerospace
Location : Australia
Studying the effects of airflow over an object is of a paramount importance for design considerations.
Studying the effects of airflow over an object is of a paramount importance for design considerations. To understand the impact of the aerodynamic forces on engineering objects such as automobiles, aircrafts and locomotives, it is required to set up external airflow conditions that resemble actual physical scenario.
Since the design requirements for such high-speed applications is to minimize the effect of drag forces that otherwise resist the motion, it is important to have accurate information about the flow conditions to identify design constraints.
To evaluate these systems against external aerodynamics, modern engineering community rely on simulation tools such as computational fluid dynamics, which holds the ability to simulate real conditions cost-effectively. Modern CFD tools are capable to solve complex flow regimes using numerical solvers. The systematic approach to evaluate external aerodynamics using CFD involves defining the computational domain including the surface geometry of the object being studied.
This is followed by generating mesh over the domain across which the equations will be solved; and then defining the boundary conditions, which involves selecting suitable turbulence model, near wall treatments and flow conditions.
Problems with CFD:
The results obtained through CFD simulations can then be utilized to visualize flow around the geometry, and further obtain information on drag forces and vortex formation to optimize the geometrical shape of the object accordingly. However, too much reliance on simulation techniques is indeed putting the design in danger, as numerical algorithms that solve the flow equations are an approximation and not completely accurate. As such, the accuracy of CFD suffers, especially in solving turbulence.
Despite numerous models available to solve turbulence more accurately, they are tuned for specific applications and cannot be applied to every problem. Another issue lies with the mesh quality; generating the mesh that adequately resolves the boundary layers on surfaces without producing degenerate elements is often a time consuming process.
Wind Tunnel and its Problems:
Wind tunnels have been employed for aerodynamic performance evaluation ever since the first days of powered flights. These tunnels simulate the object movement through air by placing scale or real physical model within the duct and blowing or sucking air through the duct. However, modern wind tunnels are automated and highly sophisticated to capture the lift and drag forces effectively, and with much less time compared to CFD simulations.
Advanced wind tunnels employ Pressure-Sensitive Paint, which matches the colorful pressure contours as produced by CFD simulations and Particle Image Velocimetry to produce velocity fields as visualized in CFD. However, wind tunnels have a limit to accommodate geometry sizes and often utilize scale models to perform the tests. As such for the flow profile to be accurate, the Reynolds number of the scale model has to be ideally same or close enough with that of the full size model, in order to ensure that the flow measurements do not fall wrong.
Combining CFD and Wind Tunnel:
To achieve excellence in studying the aerodynamics of a particular problem, it is always profound to combine both the simulation approaches. The data obtained through wind tunnel tests can be utilized to perform a full scale aerodynamic analysis using CFD. On the contrary, the results of CFD can be validated against wind tunnel tests to identify errors and corrections required in the CFD model.
The use of CFD can also provide a ballpark idea to narrow down the design constraints and parameters, which can be used to develop a more optimized physical model for wind tunnel tests. It is clear that both wind tunnel and CFD complement each other, proving to be an ideal combination to achieve aerodynamic excellence much needed in aerospace and automotive applications.