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2D Airfoil predictions using zero and one-equation models

In 2006, a joint effort between the Georgia Institute of Technology, the University of Maryland and the US Army aimed at evaluating the potential of CFD for replacing look-up tables in rotorcraft performance codes [1]. Five different CFD codes were implemented independently, all based on algebraic or one-equation models. The details of these codes are summarized in table 1↓.
Organization
CFD Code
Turbulence Model
AFDD
OVERFLOW
Spalart Allmaras
AED
FUN2D
Spalart Allmaras
GIT 1
CFL3D
Baldwin Lomax
GIT 2
Cobalt LLC
Spalart Allmaras
U of M
TURNS
Spalart Allmaras
Table 1 
Participants and solution models. AFDD stands for Army Aeroflightdynamics Directorate and AED for Aviation Engineering Directorate. Source: Smith (2006) [1]
These CFD codes were applied to predict the lift, drag and pitching moment of a SC1095 airfoil (UH-60A main rotor). The computations were run with Mach numbers ranging from 0.3 to 1 and angles of attack from -10 deg to 22 deg. The lift and drag coefficients predicted by the five codes at Mach 0.7 are compared to experimental data in Fig. 1↓.
figure images/clRANS.png figure images/cdRANS.png
Figure 1 
Lift and drag coefficients of a SC1095 airfoil at M = 0.7 predicted by five CFD codes. Source: Lynch (2011)
All models accurately predict the lift coefficient in the linear regime. However, these simple turbulence models fail at capturing separated boundary layers. This results in poor predictions above the stalling angle. The drag coefficient is also overpredicted. All these observations confirm the statements made in the theory section. Algebraic and one-equation models are good, provided the flow is simple. As long as the boundary layer remains attached and compressibility effects remain relatively weak, the CFD results are reasonable. If a flow departs from these basic conditions, the inherent assumptions in the models are violated, leading to unreliable predictions.

References

[1] M.J. Smith, M. Potsdam, T. Wong, J.D. Baeder, and S. Phanse. Evaluation of computational fluid dynamics to determine two-dimensional airfoil characteristics for rotorcraft applications. Journal of the American Helicopter Society, 51(1):70--79, 2006.