Free Aerodynamic / Hydrodynamic Software
Here you will find many free softwares useful for aero/hydrodynamic analysis, mainly oriented to what is called "potential flow". All Gentry theories can be analyzed and checked with them, so they are a very useful tool for a better understanding of Gentry's papers. Moreover, you will be able to shape your airfoils for best performance.
Sail7 is a full 3D multi-element Vortex Lattice Model software that allows you to design your sails and analyze them in a very interactive way. It is in fact a branch of XFLR5, from André Deperrois. As any inviscid method, it don't show separation nor boundary layer thickness, so it's appropiate mainly for small angles of attack. Final shapes can be exported to other softwares. Sail7 can also import sailcut files so you can evaluate your sail's design before building them.
JavaFoil is a multi-element 2D panel method developed by Martin Hepperle. It allows multi-element airfoil design and import/export from/to .txt files having the coordinates list. It is a very attractive tool to rapidly change the shape and visualize your results. You'll be able to plot different polars and evaluate or compare variations.
JavaFoil as Java Applet:
(JavaFoil Standalone version can be downloaded from this page also).
XFOIL Is a development of Professor Mark Drela, at the M.I.T. It is a mono-element 2D panel method software, it has a rather old interface and his use is not easy to learn, anyway it is open-source and can be embedded into other softwares. Includes some ways of viscid flow simulation, boundary layer behavior and separation (also leading and trailing edge bubbles). It is probably the most widely used foil software and clearly the preferred choice for the aerodynamicists.
XFLR5 Is an implementation of XFOIL for Windows also developed by André Deperrois that includes a 3D panel method for small aircraft and wings/foils design. It has a graphical and very easy to learn interface so is the natural introduction to XFOIL. Allows animated analysis for a range of variations applied to any design variable (i.e. angle of attack, etc.). Very popular between aeromodelists and foil designers. 2D analysis is mono-element but 3D analysis allows multiple bodies.
AVL is a 3D Vortex Lattice Model software developed also by Professor Mark Drela from the M.I.T. It's intended for small aircraft design.
FoilSim is an applet developed by NASA with educational purposes. Some simple 2D shapes can be examined varying different parameters. A complete user's manual and aerodynamic explanations are also included with the applet. It can be downloaded as a standalone application.