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. ClCl ClCl CdCd Target lift 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. Target lift Airfoil 1 Airfoil 2 Airfoil 1
Example 2 Sometimes there may be no airfoil shape that will meet the design specifications x/c CpCp Designer specifies a desired Pressure distribution A fish tail shape may occur An open-ended trailing edge May occur. 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 C l 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 C l =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 C l =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 I Due 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/ 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 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 http://www.ae.gatech.edu/~lsankar/AE2020