Aerodynamic potential-flow code

In fluid dynamics, aerodynamic potential flow codes or panel codes are used to determine the fluid velocity, and subsequently the pressure distribution, on an object. This may be a simple two-dimensional object, such as a circle or wing, or it may be a three-dimensional vehicle.

A series of singularities as sources, sinks, vortex points and doublets are used to model the panels and wakes. These codes may be valid at subsonic and supersonic speeds.

History

Early panel codes were developed in the late 1960s to early 1970s. Advanced panel codes, such as Panair (developed by Boeing), were first introduced in the late 1970s, and gained popularity as computing speed increased. Over time, panel codes were replaced with higher order panel methods and subsequently CFD (Computational Fluid Dynamics). However, panel codes are still used for preliminary aerodynamic analysis as the time required for an analysis run is significantly less due to a decreased number of elements.

Assumptions

These are the various assumptions that go into developing potential flow panel methods:

However, the incompressible flow assumption may be removed from the potential flow derivation leaving:

Derivation of panel method solution to potential flow problem

(subsonic)

As Q goes from inside V to the surface of V,

For :, where the surface normal points inwards.

This equation can be broken down into both a source term and a doublet term.

The Source Strength at an arbitrary point Q is:

The Doublet Strength at an arbitrary point Q is:

The simplified potential flow equation is:

With this equation, along with applicable boundary conditions, the potential flow problem may be solved.

Required boundary conditions

The velocity potential on the internal surface and all points inside V (or on the lower surface S) is 0.

The Doublet Strength is:

The velocity potential on the outer surface is normal to the surface and is equal to the freestream velocity.

These basic equations are satisfied when the geometry is a 'watertight' geometry. If it is watertight, it is a well-posed problem. If it is not, it is an ill-posed problem.

Discretization of Potential Flow Equation

The potential flow equation with well-posed boundary conditions applied is:

The continuous surface S may now be discretized into discrete panels. These panels will approximate the shape of the actual surface. This value of the various source and doublet terms may be evaluated at a convenient point (such as the centroid of the panel). Some assumed distribution of the source and doublet strengths (typically constant or linear) are used at points other than the centroid. A single source term s of unknown strength and a single doublet term m of unknown strength are defined at a given point.

where:

These terms can be used to create a system of linear equations which can be solved for all the unknown values of .

Methods for Discretizing Panels

Some techniques are commonly used to model surfaces.[1]

Methods of determining pressure

Once the Velocity at every point is determined, the pressure can be determined by using one of the following formulas. All various Pressure coefficient methods produce results that are similar and are commonly used to identify regions where the results are invalid.

Pressure Coefficient is defined as:

The Isentropic Pressure Coefficient is:

The Incompressible Pressure Coefficient is:

The Second Order Pressure Coefficient is:

The Slender Body Theory Pressure Coefficient is:

The Linear Theory Pressure Coefficient is:

The Reduced Second Order Pressure Coefficient is:

What panel methods can't do

Potential flow software

Name License Lan Operating system Geometry import Meshing Body Representation Developer
Linux OS X Microsoft Windows Structured Unstructured Hybrid
CMARC Proprietary C Yes Yes Homepage, AeroLogic, based on PMARC-12
DesignFOIL Proprietary Wine Yes www.dreesecode.com, DreeseCode Software LLC
FlightStreamTM Proprietary Fortran / C++ Yes CAD, Discrete Yes Yes Solids Research in Flight Company
HESS Proprietary Douglas Aircraft Company
LinAir Proprietary Yes Desktop Aeronautics
MACAERO Proprietary McDonnell Aircraft
Open VOGEL GPLv3 VB.NET/C#.NET Yes Guillermo Hazebrouck, www.openvogel.com
QBlade GPLv2 C/C++ Yes TU Berlin
Quadpan Proprietary Lockheed
PanAir a502 Public domain software Fortran Yes Yes Yes Homepage, Boeing?
PANUKL freeware C++/Fortran Yes Yes Homepage, Warsaw University of Technology exports data to Calculix
PMARC Proprietary Fortran 77 UNIX NASA, descendant of VSAERO
VSAero Proprietary UNIX
Vortexje GPLv2 C++ Yes Baayen & Heinz GmbH
XFOIL GPLv2 Fortran Yes Yes Yes web.mit.edu/drela/Public/web/xfoil/
XFLR5 GPLv2 C/C++ Yes www.xflr5.com

See also

Notes

  1. Section 7.6

References

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