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
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
third icon from the left (Add fins to
toolbar and enter
the Free-Form Fin
Geometry screen. Then, perform the following operations to define
single fin-set and multiple fin-set geometry.
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.
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
4) Click 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
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
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
For supersonic flow around pointed nose cones allow only a small
distance and a few mesh points in front of the nose cone.