DEFINING MULTIPLE FIN-SETS IN AeroCFD

2 Fin-Set Arcas Rocket
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AeroCFD
single fin-set and multiple fin-set geometry definition is easy using the efficient graphical user interface. After the airframe geometry has been defined using the second icon from the left (Generate geometry for CFD analysis) on the main screen toolbar (above) the user has the option of inserting fins on the airframe. To begin the process of inserting a single fin-set or multiple fin-sets on the airframe click the third icon from the left (Add fins to body) on  the main screen toolbar and enter the Free-Form Fin Geometry screen. Then, perform the following operations to define single fin-set and multiple fin-set geometry. A separate Fin-CFD analysis is available for determining the pressure distribution (P/Pinf), pressure coefficient distribution (Cp), Mach number distribution (Mn), density distribution (R/Rinf) and temperature distribution (T/Tinf) on the surface of thin fins. This capability is not part of the finite volume analysis output.

Free-Form Fin Geometry toolbar
1) The Plot-Region of the fins must be defined before the user can drag fin end-points into position. To define a fin-set Plot Region click the fifth icon from the left on the Free-Form Fin Geometry tool bar (above) to expose the Plot Region Dimensions
and Fin Cross-Sections Dimensions data input sections. The fin Plot Region is defined as a box located from the tip of the nose cone that will entirely enclose the fins. The "Plot-Region location from nose tip" is the first entry in the Plot-Region Dimensions section. The "Plot-Region height and width" are defined in the next data entry in the Plot-Region section. The first data entry specifies where the Plot-Region is positioned from the tip of the nose cone and the next data entry specifies the X and Y size of the Plot-Region used to define fin geometry on the airframe.

2) Next, in the Fin Cross-Section Dimensions section, insert the Total number of fins, Maximum fin thickness and if required by the cross-sectional fin shape, the location of the Maximum (fin) thickness location as a percent of fin chord length. At this point if all dimensions are properly defined a simple outline of the fin shape, not to scale, is presented in the Fin Plot-Region plot area.

3) To define a specific fin cross-sectional shape select one of the seven options listed in the pull down menu at the upper right of the Plot Regions screen. The fin cross-sectional shapes include: Double Wedge, Symmetrical Double Wedge, Double Wedge: TMAX=FN(X/C), Biconvex Section, Streamline Airfoil: X/C=50%, Round Nose Airfoil: X/C=50%, and Slender Elliptical Foil. Depending on which cross-sectional shape is selected a different leading edge factor (KLE) will be computed for supersonic flow. For subsonic flow the KLE is ignored and the drag and lift coefficients are based on subsonic derivations. The KLE Leading edge factor, Fin area, Reference area of the model, fin Sweep angle, Average chord and Semi-span are computed and displayed in the Cross-Section Dimension Results section.

4)
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lick the fifth icon from the left on the Free-Form Fin Geometry tool bar, select the number of fin end-points required and proceed to "drag" the shape end-points into position to define single and multiple fin shapes. The SHOW and HIDE plot legend contains an Up-Down control that will increase and decrease the number of fin shape points from the default of 4 shape points to a maximum of 20 shape points. To expose the Show and Hide plot legend, click the sixth icon to expose or hide the control. A color legend also appears that provides a color guide indicating the Fin Shape (Black), Body Tube Shape (Gray) and X-Y Axes (Red) of the Plot-Region. Two sets of coordinates are available to help the user rapidly position the shape points. The first set of X and Y coordinates indicates the position from the origin (0,0) of the Plot-Region to each point on the screen. The second set of coordinates, XFIN, YFIN indicates the position of the cursor and shape points from the surface of the body itself (XFIN = 0, YFIN = YBODY).

5) A summary of drag (CD), lift (CL), axial (CX) force and normal force (CY) coefficients for the fins is displayed in the Fin Drag Coefficients section. These results represent total values for all N fins defined by the user. The Fin drag and lift results are superimposed on the
AeroCFD airframe results computed in the main section of the analysis. Methods of superposition and fin interference effects techniques are employed to determine total lift and total drag effects of the fins on the body. Fin flow field effects and interference with the body are ignored because a complex 3-dimensional mesh would be required to define the endless variations required for most complex fin designs. However, a good engineering estimate of aerodynamic coefficients of a body with fins is achieved using this fin superposition methodology.

6) A
AeroCFD analysis of the two-stage ARCAS rocket example with two fin-sets instead of one fin-set operating at Mach 5 and 0.5 degrees angle of attack is presented below. This typical analysis uses a 2 to 1 flow aspect ratio to cluster the mesh near the nose and airframe of the rocket for better supersonic and hypersonic flow convergence. The high quality graphical user interface available in AeroCFD allows total set-up time for this analysis to be 10 minutes or less and computation time to be less than 5 minutes to generate the color contour plot results in Figure-3.

7) COMMON SENSE NOTES FOR MESHING, DEFINING FINS AND USING
AeroCFD
To compute non-zero fin lift force (FY) and lift coefficients (CY and CL) the user must specify non-zero angle of attack for the CFD analysis.
Maximum fin thickness location when required must be in percent chord and not fractional chord.
If an error condition occurs when generating a mesh with or without fin geometry do not save the Project file because corrupt data in the Project file will make recovering the previously saved data impossible. If an error condition occurs during meshing do not save the Project file, instead exit AeroCFD and re-enter the project data.
When generating a mesh for supersonic flow cluster the mesh around the body using an aspect ratio greater than 1.
For subsonic and supersonic flow around blunt nose cones allow a larger distance before the nose to allow the stagnation region to develop.
For supersonic flow around pointed nose cones allow only a small distance and a few mesh points in front of the nose cone.
 


Figure-1, Plot Region definitions for fin set geometry
 

Figure-2, Fin geometry after the user has dragged 8 shape-points into position using the cursor.
 

Figure-3,  AeroCFD results after completing 50 iterations for Mach 5.0 flow operating at 0.5 degrees angle of attack.