Page 26 - Computational Fluid Dynamics for Engineers

1.2 Prediction of the Maximum Lift Coefficient of Multielement Wings 11

Fig. 1.10. Flow over a typical high-lift system

slats can more than double (C/Jmax with subsequent improvement in takeoff
and landing performance. Thus, it is important to predict the performance of
(
high-lift systems that can be designed for high CL) m a x in landing configuration
and high lift-to-drag ratio in take-off configuration. The lower drag also results
in lower noise, which is necessary to comply with noise abatement regulations.
Despite the significant advances in CFD, our ability to predict the maximum
lift coefficient of multielement wings is still not satisfactory. As shown in Fig.
1.10, the flow about multielement airfoils for high lift is very complex. The main
problem is the lack of an accurate turbulence model (Chapter 3) to represent
flows with extensive separation. The problem is exacerbated by inaccuracies
of numerical solutions of the conservation equations (Chapter 2) at these flow
conditions and difficulties in modeling flow near the trailing edge of an airfoil
or wing, trailing viscous wakes that may impinge on aft elements, merging
boundary-layers, and flow separation.
In this section we describe a useful design method developed by Valarezo
and Chin [6]. This method, called "The Pressure Difference Rule", for predict-
ing the maximum lift coefficient of multielement wings is based on Hess' panel
method which is an extension of the two-dimensional panel method of Section
6.4 to three-dimensional flows. The accuracy of this method, even though the
solution is based on the reduced conservation equations and does not include the
effects of viscosity, is then demonstrated for the high-lift systems of a transport
aircraft as a function of Reynolds number. While this method is appropriate for
configuration development, it cannot predict the optimum gap/overhang loca-
tions for each of the high-lift wing components; at this time the determination
of promising range of locations is performed using two-dimensional CFD meth-
ods for multielement airfoils. The final determination of the optimal locations is
made in high-lift wind tunnel tests. The ability to predict reliably the optimal