OVERVIEW • Introductory Remarks • Your first analysis tool – Panel Method
Introductory Remarks • Airfoil Design is an “open-ended” process. • There may not be a unique shape meeting the design specs. • Each one of you may come up with different shapes for the same design specifications. • In some cases, no airfoil shape may exist that will meet the design specifications. You may need to settle for the closest or “optimum” answer, rather than the “exact” answer. • Airfoil design is often an iterative process, where small but steady improvements are made to the airfoil shape until the design specifications are closely met.
Example: Design an airfoil that has the best L/D ratio at a specified lift coefficient. Airfoil 2 Airfoil 1 Cl Airfoil 2 Cd Target lift Airfoil 1 Target lift a Cl Two airfoils may have the same minimum “Lift to Drag ratio” at the Same target lift coefficient. In the above example, both airfoils meet the design specifications (example lowest L/D at a specified Lift) Their off-design performance may differ significantly, however.
Example 2Sometimes there may be no airfoil shape that will meet the design specifications A fish tail shape may occur Designer specifies a desired Pressure distribution Cp An open-ended trailing edge May occur. x/c If this occurs, the design specifications may need to be modified. For example, the designer may need to change the target pressure Distribution.
Design Specifications • Design specifications (or design criteria) that the airfoil should meet may vary from one application to another. Here are some examples. • Target Cl given, lowest L/D is required. • Target pressure distribution, known to have good boundary layer characteristics, is given. This requires expertise on the part of the designer. • Higher lift stall angle is required or specified. • Higher moment stall angle is required or specified. • Trailing edge (or leading edge) separation is to be eliminated. • Laminar flow is required.
Single Point vs. Multi-Point Design • In some design problems, it is enough to meet a single operation point. This is called a single point design. • For example, design an airfoil that has the lowest L/D at Cl=0.5, at a cruise Mach number of 0.75. • In other cases, the design must meet satisfactory performance at two or more conditions. This is called a multi-point design. • Example, satisfactory subsonic cruise performance (lowest L/D at M=0.78 at Cl=0.5), and acceptable supersonic dash performance ( L/D ~ 4 at M=2.0)
Steps to a Successful Design • Acquire analysis tools – potential flow analysis of flow over an airfoil, boundary layer analysis • Validate the tools before you use them. • Specify design criteria. • Start with a known airfoil shape, as close to the target as you can. • Use an iterative process to gradually improve the starting geometry/shape. • Use the new design in a 3-D analysis (or tunnel test) to make sure that the new airfoil does indeed perform better.
For this course, choose from one of the following • Laminar airfoil design • General aviation airfoil Design • Supercritical airfoil Design • Wind Turbine Airfoil Design • Tools needed: panel method or CFD method (supplied), laminar and turbulent boundary layer analyses (supplied), transition prediction method (supplied), design method (needed) • Verify your design in a 3-D lifting line or CFD method (supplied)
Assignment IDue on September 10, 2002 • Write a 5 page report summarizing • Design problem you have decided to study (one of the four choices) • Why did you choose this problem • Literature survey of previous work related to your specific design • What are some of the design criteria that have been used in past • The specific design point you would like to work on • You may change your mind later. If you decide to work on a different problem, just let me know. However, the sooner you settle on the design problem, the sooner you will get it done. • All work must be done independently. It is Ok to exchange your sources of information.
Some Sources • NASA Technical Report Server http://techreports.larc.nasa.gov/cgi-bin/NTRS/ • Papers by Eppler (laminar airfoil, general aviation airfoil) • Papers, books by: (a) Tranen, (b) Bauer-Garabedian-Korn on supercritical airfoil • Papers and website by Prof. Selig at U. Illinois • National Renewable Energy labs web site: http://www.nrel.gov/wind • I will complement this with papers by my students, colleagues, and me.
First Set of Tools • Panel method • Laminar Boundary layer Analysis • Turbulent Boundary layer Analysis • Transition prediction methods • We will start on panel method today. The source code and sample input are available at our AE 2020 web site: http://www.ae.gatech.edu/~lsankar/AE2020