NEWPAN2D: Tools for the Analysis and Design of Aerofoils and Wings

A principal area of focus for Flow Solutions, and one in which we provide industry leading capabilities, is the analysis and design of aerofoils. These may be purely two-dimensional. More usually however the sections form a 3D component such as an aircraft or racecar wing, the hull, keel or rudder of a boat, a propeller or ducted fan. The area of application is wide, but is concentrated on low-speed applications, i.e. subsonic Mach numbers.

Broadly speaking, available CFD codes have tended to fall into one of two camps:

• 3D analysis only, using computationally expensive methods which are impossible/impractical to use in a direct design/optimisation mode;
• 2D analysis and design (some offering inverse design capability, but with no coupling or extension to 3D). Whilst it may be attractive to the CFD code developer to concentrate on 2D-only solutions, it is rare for a purely two-dimensional result to accurately reflect the three-dimensional reality.

2D (yellow) and 3D matched (green) results on a front wing section, showing the dangers of drawing any conclusions from 2D results.

What is much more valuable of course is a 3D design solution, and it is precisely this requirement which the NEWPAN2D/NEWPAN combination is designed to provide. By coupling the two codes together, you can gain access to the inverse design and strong viscous coupling of NEWPAN2D, applied to the 3D NEWPAN results.

Key Features of NEWPAN2D

VERY fast and highly interactive; all the functionality described here executes interactively in one or two seconds at most; inviscid analysis of two-dimensional multi-element aerofoils (e.g. wing sections); application of strong viscous coupling to the basic inviscid results, giving lift, drag and detailed Cp and boundary layer characteristics up to maximum lift (i.e. with significant areas of flow separation); an inverse aerofoil redesign procedure, which allows the user to prescribe a sectional pressure distribution, and which returns the aerofoil profiles required to achieve it; redesign by direct profile modification, including rotation, translation and scaling; application of all these features to three-dimensional sectional data as generated by NEWPAN, allowing redesign and viscous coupling on sections sitting in a three-dimensional flowfield; integration of the method within the VIEWPAN framework, enabling for example easy generation of 2D sections from 3D wing elements, and comparison of results generated with existing data; integration of the method within the GEMS framework for analysis and redesign of purely two-dimensional sections.

Strong Viscous Coupling

Both NEWPAN and NEWPAN2D in their basic forms are inviscid methods. Hence their predicted pressure distributions assume full attachment, which at high incidences will not be realised; predicted lift keeps rising beyond the viscous maximum lift value.

Strong inviscid-viscous coupling provides support for accurate results even in the presence of large areas of flow separation, up to and beyond maximum lift.

NEWPAN2D provides access to a fully coupled boundary layer (viscous correction) calculation. It uses quasi-simultaneous coupling to give converged solutions with separation. The proper boundary solution is found in cases where separation is present.

It includes calculation of the boundary layer as it flows into the wake (i.e. it computes the free shear layer in the wake). This is important as, for example, the wake from a wing element can `burst' over a downstream flap (after running through the adverse pressure gradient on the off-body pressure field of the flap). This is an important stall-inducing mechanism limiting CLmax.

In contrast, other methods capable of being applied to 3D generated data offer only weak coupling - with no calculation of the wake free shear layer, and failure to converge in the presence of separation.

Inverse Design: An Aerofoil Redesign Procedure

Although it is undoubtedly useful and important to produce accurate results for the aerodynamic (or hydrodynamic) properties of your current design, this is usually only part of the battle. Of even greater interest is guidance as to how to make the design better.

The skilled aerodynamicist would like to be able to control and prescribe the aerodynamic properties - such as lift, drag, and details of the boundary layer behaviour such as transition and separation, and let the CFD software work out the geometry of the design necessary to achieve it. This is the inverse design problem, addressed by NEWPAN2D.

Unconstrained, fully 3D inverse design is feasible, though not yet possible; instead, through the NEWPAN-NEWPAN2D coupling, Flow Solutions provides for the redesign of one or more sections sitting in a NEWPAN-generated 3D flowfield (purely 2D support is also provided).

Multi-element aerofoils may be redesigned as follows. Firstly the NEWPAN2D pure 2D or 3D derived NEWPAN results for the existing geometry are computed and displayed. The inverse design method allows us to directly modify the pressures; hence we graphically edit the desired target pressures. As we do so, an integral boundary layer solution is automatically re-evaluated; this allows us to tailor the pressure gradients to achieve desirable transition and separation characteristics. When we're ready, NEWPAN2D perturbs the original aerofoil profiles to achieve a new design which generates a pressure distribution with the closest feasible match to the target pressures requested. In this manner, any one or even all of the sections of a multi-element configuration may be redesigned simultaneously.

We may perform our redesign at one of a number of possible design points, e.g. attitudes. NEWPAN2D is now able to compute and display the performance of our new design at all the other design points. We can evaluate its performance further by applying strong viscous coupling. Now we are able to refine the design further, by applying another design iteration, starting either with any of our previous designs or by returning to the original geometry. In this way a complete design history may be built up, with a number of branches down which different possible avenues are explored.

An example and discussion of inverse design applied to the front wing of a Formula One car is given here.

Redesign by Direct Geometry Modification

Often it is also useful to be able to apply direct geometric changes within the NEWPAN2D design environment. For example a flap element could be rotated by 10 degrees, or a mainplane could be scaled along its chord by 10%. Via a seamless interface to GEMS, its rich toolkit for geometry editing is also accessible. This allows NURBS editing of profiles - i.e. fitting a NURBS curve through the points, enabling smooth perturbation of the profile by moving the control points of the curve.

Uniquely, where the section is part of a 3D model, such modifications may be evaluated in seconds within the 3D flowfield. NEWPAN2D provides a means to export the redesigned sections to GEMS, which in turn provides customised functionality for reintegration of the sections into the complete 3D model. At this stage a full 3D NEWPAN rerun may be performed, which serves to validate the results predicted by NEWPAN2D. Unless huge geometric changes between configurations have been made, correlation is highly satisfactory. This technique significantly accelerates the design process.