Transonic flow, defined by the presence of subsonic and supersonic areas, poses considerable challenges and opportunities in aerodynamic design. This FREE Tutorial examines the characteristics of compressible flow over the NACA0012 airfoil, an often-analyzed symmetric profile, in the transonic regime (roughly Mach 0.8 to 1.2). This study aims to clarify the intricacies of transonic flow and its effects on aerodynamic performance by analyzing phenomena such as shock wave production, pressure distribution, and their consequent impact on lift and drag characteristics.

• These are the assumptions:
• Transonic Flow (Mach=0.8)
• Air is compressible
• NACA0012 is employed
• Angle Of Attach is 4°
• Structured Grid
• Three-dimensional (3D) Analysis

As explained in the tutorial video, the most daunting challenge through this investigation is designing a great geometry model considering suitable divisions in order to make it ready for grid production. All the blocking steps are done using Design Modeler. Then, ANSYS Meshing is adopted to divide each edge meticulously and sweep it through the depth of the model. In solver settings, given the transonic flow, it is expected to employ density-based solver. However, because of limited increase in speed of airflow, far from hypersonic or supersonic flow, we can substitute pressure-based solver, supported by Ideal-gas law which considers compressibility effects. Notably, it is more realistic to take viscosity changes into the account via Sutherland model. The angle of attack is 4° and the definition of drag and lift coefficients requires further thoughts.

Figure 1: Structured grid over NACA0012 3D model

Results

This study of transonic flow over a NACA 0012 blade at 0.8 Mach and a 4° angle of attack shows important aerodynamic effects. The flow speeds up considerably over the top of the airfoil, making a region of high speed that is probably faster than the speed of sound in that area. This is a sign of a supersonic pocket. This speeding up is caused by the airfoil’s shape and angle of attack, which creates a lower pressure area above the airfoil compared to a more even higher-pressure area below. Lift is caused by this change in pressure. However, the fast slowing down after the top surface’s peak speed points to the formation of a shock wave, which is a feature of transonic flow and a cause of wave drag.

Figure 2: velocity distribution around NACA0012 3D model CFD Simulation

Furthermore, the contours indicate potential flow separation near the trailing edge, particularly on the upper surface. This separation is suggested by the diverging pathlines and confirmed by the velocity vectors pointing away from the airfoil surface in that region. Flow separation disrupts the smooth airflow, contributing to increased drag and potentially reducing lift.  The 4° angle of attack, while contributing to lift generation, also exacerbates the adverse pressure gradient on the upper surface, promoting this flow separation.

Figure 3: velocity a) pathlines b) vectors on suction side, showing flow separation

The formation of shock waves, the presence of a supersonic pocket, and the separation of flow all show how complicated transonic flow is. These things have a big effect on aerodynamic speed, making drag higher and possibly lowering lift.