Richard E. Kreeger, Marivell Baez, Donald C. Braun, Herbert W. Schilling,

and Mary B. Vickerman

Glenn Research Center, Cleveland, Ohio

SmaggIce 2.0: Additional Capabilities for

Interactive Grid Generation of Iced Airfoils

NASA/TM—2008-214829

January 2008

AIAA–2007–0502
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Richard E. Kreeger, Marivell Baez, Donald C. Braun, Herbert W. Schilling,

and Mary B. Vickerman

Glenn Research Center, Cleveland, Ohio

SmaggIce 2.0: Additional Capabilities for

Interactive Grid Generation of Iced Airfoils

NASA/TM—2008-214829

January 2008

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

45th Aerospace Sciences Meeting and Exhibit

sponsored by the American Institute of Aeronautics and Astronautics

Reno, Nevada, January 8–11, 2007

AIAA–2007–0502
Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

Available electronically at http://gltrs.grc.nasa.gov

Level of Review: This material has been technically reviewed by technical management.
The Surface Modeling and Grid Generation for Iced Airfoils (SmaggIce) software toolkit 

has been extended to allow interactive grid generation for multi-element iced airfoils. The 

essential phases of an icing effects study include geometry preparation, block creation and 

grid generation. SmaggIce Version 2.0 now includes these main capabilities for both single 

and multi-element airfoils, plus an improved flow solver interface and a variety of additional 

tools to enhance the efficiency and accuracy of icing effects studies. An overview of these 

features is given, especially the new multi-element blocking strategy using the multiple 

wakes method. Examples are given which illustrate the capabilities of SmaggIce for 

conducting an icing effects study for both single and multi-element airfoils. 

Nomenclature 

2D two-dimensional 

3D three-dimensional 

C chord length 

CFD computational fluid dynamics 

CGNS CFD General Notation System 

LWC liquid water content, g/m3

MVD median volume diameter, μm 

SmaggIce Surface Modeling and Grid Generation for Iced Airfoils 

X x-coordinate 

Xle x-coordinate of leading edge 

I. Introduction 

wo types of problems are usually considered in icing: ice accretion and icing effects. Ice accretion geometries 

change over time in a quasi-steady manner, which requires frequent grid modifications, and is more suited to 

automated procedures. Icing effects, on the other hand, seek to determine the flow field and performance 

degradation given an ice shape, and so are more suited to interactive tools. Any computational icing effects study 

must address the issues of ice shape characterization, grid generation, and aerodynamic flow solution. 

T 

The Surface Modeling and Grid Generation for Iced Airfoils (SmaggIce) toolkit is a suite of interactive tools 

developed at the NASA Glenn Research Center1,2 which can simplify and improve an icing effects study. This suite 

of tools is used to prepare two-dimensional cross sections of iced airfoils for computational fluid dynamics (CFD) 

analysis. SmaggIce is designed to help researchers and engineers study the effects of ice accretion on airfoil 

performance, which is difficult to do with other software packages because of the complexity of ice shapes3

. CFD 

tools are used primarily for certification studies and analysis, to evaluate safety of flight with unprotected surfaces 

and to determine the need for ice protection. They are also used for the design and analysis of ice protection 

systems. Using SmaggIce to simulate flow over an iced airfoil can help to reduce the cost of performing flight and 

SmaggIce 2.0: Additional Capabilities for Interactive Grid 

Generation of Iced Airfoils 

Richard E. Kreeger, Marivell Baez, Donald C. Braun, Herbert W. Schilling, and Mary B. Vickerman 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

NASA/TM—2008-214829 1
wind-tunnel tests for certifying aircraft in natural and simulated icing conditions. The phases of a computational 

icing effects study are shown in Figure 1. 

Evaluation of the ice geometry includes identification of potential geometric problems. Ice shapes pose difficulty 

in generating the good-quality grids that are essential for predicting ice-induced complex flow. Ice geometries can 

come from experimental or predicted sources. Ice shape characterization gives a quantitative assessment and 

comparisons of ice shapes. SmaggIce provides tools to rapidly quantify ice-shape characteristics (such as horn 

height, angle and location) and calculate integrated ice area, allowing researchers to examine their effect on 

aerodynamic performance. It includes tools that examine input data for possible errors (such as tangling introduced 

during data acquisition) and allows the user to smooth ice shapes to desired levels for computational fluid dynamics 

analysis. There is a third category of use to the engineer: artificial ice. SmaggIce allows users to specify the size and 

location of simple, primitive ice shapes for parametric study. 

The SmaggIce tools are uniquely tailored for icing effects studies, including dividing the flow domain into 

blocks to set up the grid structure prior to grid generation, making changes to the grid-density distribution, and 

merging and smoothing multi-block grids. The use of a thin, tightly controlled block that wraps around the iced 

airfoil is an example of one of many unique features available for handling difficult ice geometries. The shapes of 

block edges can be changed via control points. The density and distribution of points over the iced airfoil can be 

controlled to streamline the interactive grid generation process. 

Figure 1: Phases of an Icing Effects Study 

Geometry preparation, blocking, gridding, link to flow solver

clean and/or 

iced airfoils

Modify grid

• smooth 

• divide 

• merge

Quality check

Block creation

• domain decomposition 

• block boundary 

Modify blocks discretization

• boundary shape 

• point distribution

• divide 

• merge Grid generation

Output to 

flow solver 

Geometry

• evaluation 

• preparation 

• modification

CGNS 

grid file

solution

Aero properties,

L, D

Wind-US 

flow solver

PLOT3D file

Common 

Grid file

grids, connectivity, boundary conditions

grids

GMAN or

MADCAP

Common 

solution file solution

grids, connectivity, 

boundary conditions

CGNS 

solution file

SmaggIce v2.0

Post 

Processors

link-solution

grids and 

solution

Figure 2. SmaggIce role in icing aerodynamic simulation and analysis process. 

 SmaggIce was recently expanded with additional capabilities. Version 2.0, the latest release, now accommodates 

multi-element as well as single-element airfoils for computational studies of icing effects, as shown in Figure 2. 

Blocking and grid generation for multi-element airfoils required a number of additional supporting capabilities. 

Version 2.0 automatically 

creates blocks that connect 

and fill in the space between 

elements of a multi-element 

airfoil. SmaggIce now 

allows the user to 

rediscretize boundaries by 

changing the number and/or 

distribution of points. These 

changes are then propagated 

automatically and 

recursively to both shared 

and opposite edges. In 

addition, the capability to 

divide blocks arbitrarily 

between opposite edges and 

to merge blocks, even if not 

one-to-one abutting, was 

NASA/TM—2008-214829 2
added to the block modification tools. Also, the ability to move the corner points of grid blocks was added. 

 Grid generation results in a 2D, structured, multi-block grid4,5. Grid modification tools in Version 2.0 are now 

available to stretch and refine blocks and to allow abutting mismatched grids. Evaluating the aerodynamics requires 

an interface with a flow solver. The interface with the flow solver WIND via the CFD General Notation System 

(CGNS) protocol was also improved, so that users can proceed easily from grid generation to flow simulation. The 

graphical interface of SmaggIce is shown in Figure 3, with an example grid created for a multi-element iced airfoil. 

Figure 3. SmaggIce 2.0 Graphical User Interface

II. Summary of Capabilities 

A. Geometry Preparation 

Tools to prepare an ice shape for gridding are provided in the current version of SmaggIce. Point data defining a 

clean and/or iced airfoil 2D surface must first be read in, using one of the three common types of input format 

accepted. Points on the surface of the airfoil should start at the trailing edge but may wrap either clockwise or 

counter-clockwise. Data may also define an ice shape only, which would include only the points along the ice 

surface and not the points on the airfoil surface. 

Setting a reference airfoil defines the chord length and identifies the leading edge coordinates. These values are 

needed for several SmaggIce functions, such as normalizing ice measurements or extending ice. A clean airfoil is 

usually read in and set as the reference airfoil. However, if clean airfoil geometry is not available, an iced airfoil 

may be used as the reference airfoil, provided the chord length and leading edge point are input. 

In order to prepare the 2D surface for gridding, a number of interactive ice shape control features are provided. 

These tools allow correction of obvious input errors that commonly arise in experimental ice tracings. These control 

features of SmaggIce not only prepare the ice surface for gridding and CFD flow simulation, but they also allow 

NASA/TM—2008-214829 3
users to correct any deficiencies (e.g., tangles, twists, gaps, too many or too few points) in the input data. The types 

of functions that can be applied to surfaces are smoothing, shaping, rediscretization and point redistribution. Any 

subcurve (or the entire curve) of a surface can be selected for interactive ice shape control.

Direct reshaping of the curve is done by dragging control points associated with the curve. Hyperbolic tangent 

stretching is also supported. Systematic smoothing of the iced surfaces in a controlled manner is accomplished using 

a control point formulation. With this feature, irregular ice surfaces can be smoothed to a level acceptable for grid 

generation. Smoothing is controlled by setting the number of control points in constructing curves. Rediscretization 

provides a means of increasing/decreasing the number of points, distributing the points by curvature, and controlling 

the uniformity of their distribution. 

Figure 4. Tangles 

In some input geometries, the trailing edge of the airfoil may not be closed; i.e., the first and last points on the 

upper and lower surface at the trailing edge do not coincide. In this case, one of the tools provided can extend the 

trailing edge of an “open” airfoil element with a single menu selection. For some ice shapes, only the geometry of 

the ice accretion is included, rather than the coordinates for the complete airfoil. This is also known as an “open” 

geometry, one in which the coordinates do not include the 

complete airfoil, but only the point for the ice accretion 

around the leading edge. Open geometries are frequently 

obtained from experimental tracings. In this case, the user 

may read in the clean airfoil on which the ice is based and 

SmaggIce will automatically extend the ice to include the 

clean airfoil before creating a grid. 

SmaggIce can also examine airfoil and ice geometry 

when it is read in and after modifications are made to it, to 

search for and identify twists and/or tangles in the surface. 

These twists will be visually highlighted on the screen, so 

that they can be corrected either manually or automatically. 

This is shown in Figure 4. 

B. Domain Decomposition 

The basic blocking strategy and topologies used in SmaggIce will result in high-quality multiple-block grids for 

2D iced airfoils. Methods and guidelines for generating high-quality multi-block Navier-Stokes grids, and strategies 

to provide fast convergence, have been incorporated in the development5

. 

outerblock

overlap

near field

wake airfoil

The first step in domain decomposition of a single-element airfoil is to define a wake, which extends 

downstream from the trailing edge of the iced airfoil. The wake is used to specify the cut for the C-shaped topology 

that must be used during domain decomposition. The number of points, the length, and the angle of the wake can all 

be set manually, or using typical default values. The spacing of the wake points closest to the trailing edge is based 

on the spacing of points on 

the airfoil at the trailing 

edge. The rest of the point 

spacings are set to 

accommodate the specified 

length and number of points. 

Domain decomposition 

divides the flow domain into 

blocks, and is performed 

prior to gridding. The 

domain is decomposed in 

steps, with various types of 

blocks created during each 

step. For single-element 

airfoils, the domain consists 

of three or more blocks- a 

viscous sublayer, one or 

more near field blocks, and 

an outer block that overlaps 

the near field. This is shown in Figure 5. 

Figure 5. Single-element Blocking Strategy. 

NASA/TM—2008-214829 4
The viscous sublayer block is a thin C-shaped domain that wraps around the wake and iced airfoil. It serves as a 

transition from the irregular geometry of the iced surface to a smoother outer boundary while simultaneously 

providing a very dense mesh near the no-slip boundary with firm control. The points along the inner boundary of the 

viscous sublayer block are automatically created using the points along the wake and iced airfoil. The spacing of the 

points in the radial direction is specified by three parameters: (1) the number of points in the radial direction, (2) the 

thickness of the block, and (3) the initial grid spacing adjacent to the iced airfoil and wake. Concave areas of ice can 

introduce tangles in the viscous sublayer block. These tangles are automatically indicated, and can be removed by 

reducing the thickness of the block, manually smoothing the ice surface before creating the block, or using an 

automatic process to smooth out the inner and outer edges of the viscous sublayer block. An example of this feature 

is shown in Figure 6. When the viscous sublayer domain decomposition is complete, the viscous sublayer block is 

created and a grid is generated for that block. 

The user selects the basic blocking topology 

to be used when creating the near field. Either 

single block or multiple blocks may be used. For 

clean airfoils, or airfoils with relatively simple 

rime ice shapes, a single block topology may be 

sufficient. For more complicated ice shapes, 

multiple blocks are recommended. In the case of 

airfoils with prominent glaze horns, the local 

flow field is dominated by separated flows aft of 

the suction-side horn. In such cases, the point of 

flow separation at the horn tip is critical to 

predicting the airfoil performance, but detailed 

geometry of other parts of the ice may not be as 

critical. The near field block(s) allow for easy 

accommodation of complex geometric features. 

The multiple block topology is based on userdefined radial cuts. SmaggIce allows the user to 

interactively select and move the endpoints of 

the radial cuts, then shape the cuts. Figure 6. Twists and Creases Highlighted 

The inner boundary of the near field is 

automatically defined as abutting one-to-one 

with the outer boundary of the viscous sublayer 

block. The initial grid cell spacing along that 

inner boundary is set to match the last cell 

spacing along the outer boundary of the viscous 

sublayer. The outer boundary of the near field 

consists of a semi-circle centered on the leading 

edge point and a parallelogram abutting the 

semi-circle and extending to the end of the wake. 

Parameters are specified to set the radius of the 

leading edge semi-circle, the number of points in 

the radial direction, and for radial cut topology, 

the number of radial cuts to use. When the near 

field domain decomposition is complete, the near 

field blocks are created and grids are generated 

for each of those blocks. 

in Viscous Sublayer Boundary 

Figure 7. Viscous Sublayer and Near-Field Blocks 

The user also defines parameters for 

generating the outer block. The outer boundary 

of the outer block consists of a semi-circle 

centered on the leading edge point and a 

rectangle abutting the semi-circle and extending 

to the end of the wake. The inner boundary is 

created to overlap the outer boundary of the near field. The user specifies the number of cells to overlap, the number 

of points in the radial direction and the azimuthal direction of the grid, and the outer radius of the semi-circle at the 

leading edge. After these parameters are specified, the boundaries of the outer block and its grid are generated. 

NASA/TM—2008-214829 5
Appropriate boundary conditions are assigned to block 

boundaries during domain decomposition. SmaggIce can 

accommodate either no-slip or Euler boundary conditions 

along the surface of the airfoil, as shown in Figure 8. The 

viscous sublayer block and near field blocks are created 

with one-to-one abutting connectivity to each other, while 

the near field blocks and the outer block have overlapping 

(overset) connectivity between them. These connectivities 

are stored for each block, along with the geometry. When 

modifications are made to block boundaries (rediscretizing, 

redistributing points, dividing and merging grids), the 

connectivity information is maintained or modified as 

necessary. Inverse bilinear interpolation is used to compute 

the overlapping connectivities at boundary points of the 

outermost near field blocks that overlap the outer block and 

at the boundary points of the outer block that overlap near 

field blocks. An efficient and robust algorithm was 

developed that uses fast algebraic calculations rather than an 

iterative procedure. The algorithm identifies and properly 

handles every possible degenerate special case of a 

quadrilateral cell. Figure 8. Airfoil Surface Boundary Interface

For a single-element airfoil, the required steps for initial domain decomposition and grid generation are: 

• Identify the clean airfoil (or at least specify the chord length and the leading edge point). 

• Select airfoil on which the grid will be based. 

• Create a wake. 

• Create a viscous sublayer block. This is a required, C-shape block, which wraps around the airfoil and 

extends back along the wake. 

• Create the near field block(s). This consists of at least a single block that surrounds the viscous sublayer 

block, but it may be defined as multiple blocks with radial cuts dividing them. 

• Create the outer block which overlaps the near field block(s). 

Figure 9. Radial Cuts Shaped Using Control Points 

The same tools that are 

used for geometry preparation 

also can be used to modify 

block boundaries, including 

the radial cuts that make up the 

near field in multiple-block 

topologies. This is a powerful 

feature of SmaggIce which 

allows rapid, interactive 

shaping of the overall topology 

to ensure a quality final grid. 

After a block has been created, 

the shape of the block 

boundary can be adjusted. 

Control points along the 

subcurve of the boundary can 

be dragged using the mouse, as 

shown in Figure 9. When the 

boundary is modified, the grid 

is removed, since it no longer 

matches the boundary. A 

preview of the adjusted points 

is then displayed. When the 

changes are applied, the abutting boundary from the adjacent block is automatically changed to keep the 

NASA/TM—2008-214829 6
connectivity intact. Then, grids are automatically regenerated for the modified blocks. Frequently, points need to be 

distributed along a block edge so that the cell spacings match those of the block's neighbors. SmaggIce provides a 

tool to do this. The spacing can match at either end or both ends of a block edge. 

C. Multi-element blocking 

Four methods were initially considered for multi-element blocking and grid topology. The “wake connectors” 

was the simplest conceptually, but it was found that it was hard to increase grid density in some areas, and hard to 

get grids in the small gaps between elements. Another method6

, allowed for all structured, all abutting one-to-one 

blocks. This method would have created many blocks which would quickly become tedious to modify with an 

interactive tool. A hybrid method7

 considered was very different from the abutting block approach used by 

SmaggIce, and therefore was not suitable. What was chosen was a method incorporating separate wakes off of each 

element8

. 

Domain decomposition divides the flow domain 

into blocks, as with single-element blocking, and is 

performed prior to gridding. The domain is 

likewise decomposed in steps, with various types of 

blocks created during each step. For multi-element 

airfoils, the domain consists of multiple blocks: one 

block for each element, one wrap-around block for 

each element that surrounds the viscous sublayer 

blocks and connects them together, one or more 

near field blocks, and a single outer block. 

This method required several major changes to 

SmaggIce in order to implement it. The major 

issues were as follows: (1) multiple wakes must be 

saved and associated with blocks, (2) multiple 

viscous sublayers will exist and be associated with 

blocks, (3) there will be another type of block for 

the wrap-around block- in addition to the viscous 

sublayer, near-field and outer-block types, and (4) 

the near field will connect to multiple wrap-around 

blocks, not just a single viscous sublayer. 

Figure 10. Creating Links Between Elements

As with the single-element case, the user must 

first identify the clean airfoil, or at least specify the 

chord length and the leading edge point. Then, set 

the reference airfoil as in previous versions. Following this, links must be created between the elements that make 

up the multi-element airfoil, as shown in Figure 10. These links are created one at a time, starting with the rightmost element of the airfoil. It is important to create good links between the elements in a multi-element airfoil, 

because the locations of the lines that define the links will affect the shape of the wrap-around blocks between and 

around the elements. The positions of the points of the lines that define each link can be moved. 

Next, wakes for each element in the multi-element airfoil must be generated. The user enters the parameters for 

each wake, and is given a preview wake before being asked to confirm the desired shape. The system then 

automatically propagates the number and distribution of points downstream and along the upper edge. The user then 

has the option to create critical points for each 

wake, allowing the wake to be reshaped so as not 

to interfere with other parts of the geometry. This 

is shown in Figure 11. 

Once the wakes have been defined, the viscous 

sublayer and wrap-around blocks must be created 

for each element. The block for each element of the 

airfoil contains the Navier-Stokes viscous sublayer 

for that element. As in the single-element case, the 

viscous sublayer blocks are C-shaped blocks that 

surround each element and extend back along the 

wake. Figure 11. Wake Elements Shaped Using Critical Points 

Much of this process is automated, in part to 

NASA/TM—2008-214829 7
ensure that the thickness of the viscous sublayer is not greater than 25% of the smallest distance between elements. 

The wrap-around blocks are C-shaped blocks that wrap around each viscous sublayer block and fill in the space 

between them, as shown in Figure 12. 

Figure12. Sublayer and Wrap-around Blocks for a Multi-Element Airfoil 

Next, the near field block(s) are created. This step requires at least one block that surrounds the wrap-around 

blocks, but it may be defined as multiple blocks with radial cuts dividing them. Finally, the outer block which 

overlaps the near field block(s) must be created. 

This may be easily shaped, if desired, using 

radial cuts as shown in Figure 13. The complete 

blocking of a multi-element airfoil is shown in 

Figure 14. 

For a multi-element airfoil, the required steps for 

initial domain decomposition and grid generation 

are:

• Identify the clean airfoil (or at least specify the 

chord length and the leading edge point). 

• Create links between the elements that make up 

the multi-element airfoil. 

• Create the wakes for each element in the multielement airfoil. 

• Create the viscous sublayer and wrap-around 

blocks for each element. 

Figure 13. Near-Field Blocks Shaped Using Radial Cuts

• Create the near field block(s). 

• Create the outer block which overlaps the near 

field block(s). 

SmaggIce also includes a feature for moving 

airfoil elements, with which to modify elements 

by rotating and/or translating the geometry. For 

instance, an aileron element can be translated 

and rotated about a user-specified hinge point. 

This may be useful to prepare for parametric 

studies on multi-element configurations. This 

feature is shown in Figure 15. 

D. Grid Generation 

To generate its grids, SmaggIce uses 

transfinite interpolation followed by grid 

smoothing. The transfinite interpolation is done 

using a routine from a gridding package provided 

by NASA Ames Research Center. Figure 14. Complete Blocking of Multi-Element Airfoil 

NASA/TM—2008-214829 8
SmaggIce can perform two kinds of grid smoothing. Elliptic smoothing is done using a variant of Thomas 

Middlecoff background control functions. Several parameters to the elliptic smoothing function, such as number of 

iterations and tolerance, can be modified by the user. The user also has control of elliptic smoothing parameters used 

during grid generation. Changing these parameters will affect the appearance of the grid. 

The second kind of smoothing smoothes across two 

abutting blocks. The user first selects a subcurve on the 

shared boundary between the blocks. A sub-grid consisting of 

grid cells from both blocks on either side of the subcurve is 

then smoothed using Laplace smoothing. This smoothing 

necessarily modifies the location of the selected subcurve. 

Other tools which allow the user to modify grids include 

dividing a gridded block, merging two gridded blocks which 

are abutting one-to-one, and copying the spacing of points 

along one edge of a gridded block to the interior grid lines. 

After the initial domain decomposition and grid 

generation, block boundaries may be modified to change the 

shape of the boundaries or change the point distribution. The 

number of points may not be changed at this time. This is to 

ensure that all blocks which are grid-ready and one-to-one 

abutting remain so even after modifications are made. 

Figure 15. Geometry Translation and Rotation

Grids may be modified by: 

• dividing along grid lines 

• dividing along arbitrary lines across opposite edges of the block; these lines may be re-shaped 

• merging one-to-one abutting grids or blocks that share corners 

• smoothing grids across shared edges of blocks 

• performing interior smoothing 

• setting cell spacing along an edge to match cell spacing of adjacent grids. 

 Blocking and grid generation for multi-element airfoils required a number of additional supporting 

capabilities. Version 2.0 allows the user to rediscretize boundaries by changing the number and/or distribution of 

points. These changes are then propagated automatically and recursively to both shared edges and opposite edges, as 

shown in Figure 16 via the user interface in Figure 17. In addition, the capability to divide blocks arbitrarily between 

opposite edges was added to the block modification toolkit. 

Figure 16. Re-gridding of Near-field Block Figure 17. Propagation Interface 

NASA/TM—2008-214829 9
E. Grid Quality Analysis 

Grid quality measurements can be calculated and displayed graphically on the gridded blocks as color-coded 

overlays. This allows the user to quickly identify areas of poor grid quality, which may then be repaired. Minimum 

and maximum values for these measurements are also displayed. Examples are shown in Figures 18 and 19. 

Figure 18. Grid Quality (Orthogonality) Figure 19. Invalid Cell Shapes 

These grid quality measurements include: 

• aspect ratio of grid cells 

• orthogonality of grid cells 

• skewness of grid cells 

• stretching ratio in the I direction at each grid point 

• stretching ratio in the J direction at each grid point 

• grid cell shape (twisted, concave, or degenerate cells) 

• tangled grid using value of cell Jacobian

F. Output 

When saving to a CGNS file, all boundary conditions are stored along with the geometry for each block and 

output with the geometry. The connectivity information is also output with the geometry when saving to a CGNS 

file. This saves the user the extra steps of having to use a separate program to define the boundary conditions and 

the connectivities between blocks. An alternate format for output, compatible with PLOT3D, while more generic, 

does not retain either the boundary or connectivity information. This format allows for rapid import of SmaggIce 

grids into a wide variety of grid generation and post-processing software, however. Certain geometries may also be 

saved in a point cloud format, which retains only the x, y and z coordinates of surface points. 

Output files generated by SmaggIce in the CGNS format are compatible with the WIND flow solver. This is 

another new feature that has been added for version 2.0. The use of CGNS as a file format to transfer information to 

and from other CFD applications like post-processors and flow solvers has been a long-term goal of the SmaggIce 

project. Using the CGNS format allows ice geometry, grids, boundary conditions, connectivity and solutions to all 

be saved in a single file. Additionally, SmaggIce offers a simple tool to create a generic input file for the WIND 

flow solver, in addition to the grid file. 

G. Ice Shape Characterization 

Parameters like upper horn thickness and ice mass are widely used by researchers and analysts in performing 

sensitivity studies. Sensitivity tests9

 are used to characterize the influence of small changes in environmental or 

tunnel parameters on aerodynamic effects. They provide a link between spray cloud, ice changes and aerodynamic 

effects, and allow designers to develop preliminary estimates that a minimal change in LWC or MVD would have 

on aerodynamic characteristics. 

The ice shape characterization tools incorporated in SmaggIce are used to measure and record location, length, 

angle, arc length, and ice area. This gives users the means to measure the physical characteristics of ice such as icing 

limit locations, horn height and angle, distance along the clean airfoil from the leading edge to a prominent ice 

NASA/TM—2008-214829 10
location, or the area of ice between two user-specified points. These tools provide an aid in the process of 

determining the relationship between ice characteristics and their effects on aerodynamic performance. 

Ice shapes can be measured interactively by selecting data points on the screen, recording the measurements in a 

table, and saving them to a file. In SmaggIce, an airfoil is defined as a sequence of points, i.e., (x,y) coordinates, 

starting at the upper (or lower) surface of the trailing edge and following along the surface toward the leading edge 

and then back toward the trailing edge along the lower (or upper) surface. Ice shapes are also defined by a sequence 

of points, although they do not have to include points back to the trailing edge. All modifications to the geometry 

are made to these points. 

The types of measurements that can be made include: 

• point location (e.g., ice limits) 

• distance between two points (e.g., ice horn height or width) 

• arc length 

• angle between two lines (e.g., horn angle) 

• (X-X )/C (e.g., distance from leading edge) le

• ice area 

Location and distance may be normalized by the chord length of a clean airfoil. Ice area may be normalized by 

the area of the clean airfoil. The points used in making the measurements can be selected by various methods: the 

closest point to any object, the closest point to the currently-selected object, the closest point to the reference airfoil, 

or an arbitrary point in space. 

H. Create and Place Artificial Ice 

Adding computer-generated ice shapes to the surface of a clean airfoil is used to prepare for studies of the effects 

of surface roughness as well as the effect of various ice shapes on the airfoil aerodynamic performance10. 

Figure 20. Artificial Ice Added to Clean Airfoil Geometry 

Once clean airfoil data is read in, the user may interactively add different types of geometries to the clean airfoil. 

A tool is provided to attach artificial (i.e., computer-generated) ice shapes such as forward-facing right triangle, 

backward-facing right triangle, generic triangle, rectangles, forward-facing quarter circle, backward-facing quarter 

circle, half-circles, and trapezoids to the 

surface of an airfoil. Parameters for 

location, replication, size, and number of 

points can be specified. This tool facilitates 

studies of the effects that various ice shapes 

and ice roughness have on aerodynamic 

performance. An example of artificial ice is 

shown in Figure 20. 

I. Extruded 3D Wings 

While SmaggIce is not intended as a fully three-dimensional tool, some limited 3D capabilities have been 

implemented. Airfoil geometry can be extruded to generate a surface grid of a three-dimensional wing. Sweep angle 

is an available parameter and no tip shapes are generated. When both a clean and iced airfoil section are defined, 

SmaggIce can generate a three-dimensional wing with ice that varies along the span through the application of 

sinusoidal variations in the spanwise direction. The number of cycles across the span and the amount of variation are 

available input parameters. The interpolation between the clean shape and the iced shape as a function of span is 

automatically performed. 

J. Convenience Features 

SmaggIce incorporates an array of features to improve its convenience and usability. Most notable among these 

is the undo/redo feature. SmaggIce has full undo and redo capability for all user actions that affect the data defining 

elements, wakes and blocks. The number of undo/redo levels can be set by the user. The toolkit also includes a save 

state feature, which saves all current information about elements, blocks, grids and settings to a file. A restore state 

command allows previous sessions to be loaded. 

A scripting interface is provided to allow frequently-performed commands or sequences to be repeated. Scripting 

enables the recording feature of SmaggIce, which records all of the user’s inputs to an easy-to-read text file. 

Recorded scripts are text files and can be easily edited. 

NASA/TM—2008-214829 11
Extensive help files can be displayed from within the SmaggIce interface, or can be viewed using a Web 

browser. A detailed user manual along with a small number of tutorial cases is distributed along with the code. 

III. Software Development 

Some challenges facing the software development team included the requirement for multiple platforms, 

integrating code from different contributors, different languages (mostly C, some FORTRAN), and a limited budget. 

For these reasons, the development of SmaggIce relied heavily on the use of open source tools. 

The libraries used included: 

• iniParser, a C library for maintaining preference files 

• Python, an embedded interpreter enabling scripting of SmaggIce via Python 

• CGNS, a library for storage and retrieval of CFD data 

• Libmba, a general purpose C library including a CSV file parser 

The open source tools used included: 

• Bugzilla, a bug tracking system 

• CVS, a revision control system 

• DDD, a graphical front-end for command-line debuggers such as GDB 

• Splint, for statically checking C program coding 

• gcc, a cross-platform C library 

• GNU Make, a cross-platform build tool 

• SWIG, which wraps C code so that it is callable by Python 

• Doxygen, a source code documentation generator tool 

• Cscope, a developer’s tool for browsing source code 

The SmaggIce code does some inline "self-checking". A routine called SmgSanityCheck is called after each user 

action. It checks to make sure that the state of SmaggIce is consistent and within expected bounds. If an error is 

detected, SmaggIce aborts. SmaggIce also makes use of some common software design patterns. In particular, the 

Command Pattern was used to implement undo/redo and the scriptability and recordability of SmaggIce. The 

recordability feature is used to log all user actions. This allows the user to re-run their session via a script. This is 

useful if, for example, SmaggIce aborts for any reason. 

The development approach was chosen so it could be steered by the customer. This allowed changes and 

additions to be made quickly when new requirements were discovered as development progressed. Continuous 

integration with an always-available working code and group ownership of source code helped keep development on 

schedule. SmaggIce was designed and written to run on any UNIX platform. It has been ported to run on MSWindows systems using third party X-server software available from various sources. 

IV. Future Plans 

 In the future, SmaggIce will be integrated into the NASA GlennICE computational environment, which is 

currently still being defined. Additional capabilities being considered include a quasi-3D approach for aerodynamic 

performance, enhanced block and grid modification tools, grid error estimation, and further integration between flow 

solvers and multi-disciplinary tools. Ultimately, icing effects studies will have to go to fully 3D and possibly 

unstructured grids. Plans are to continue supporting primarily Windows PC and Linux platforms. SmaggIce can also 

be used for grid generation of airfoils without ice accretion. 

V. Conclusion 

SmaggIce was recently expanded with additional capabilities. SmaggIce is one of NASA’s aircraft icing research 

codes, used for aerodynamic performance prediction of iced airfoils. Its primary purpose is to create high-quality 

structured grids for two-dimensional iced airfoils in preparation for computational fluid dynamics analysis. It 

provides tools to measure ice shape characteristics, prepare the ice surface for gridding, perform domain 

decomposition, create and modify grids, analyze grid quality and output grids for subsequent input into aerodynamic 

flow solvers. Version 2.0, the latest release, now accommodates multi-element as well as single-element airfoils for 

computational studies of icing effects. Blocking and grid generation for multi-element airfoils required a number of 

additional supporting capabilities. SmaggIce provides the tools needed to create high-quality grids for aerodynamic 

simulation of iced airfoils. 

NASA/TM—2008-214829 12
References 

1

 Vickerman, Mary B., Choo, Yung K., Schilling, Herbert W., Baez, M., Braun, Don C., Cotton, Barbara J., 

“Toward an Efficient Icing CFD Process Using an Interactive Software Toolkit – SmaggIce 2D,” AIAA Paper 2002-

380, Jan. 2002. 

2

 Vickerman, Mary B., Choo, Yung K., Schilling, Herbert W., Braun, Donald C., Baez, Marivell, Cotton, Barbara J., 

“SmaggIce: Further Progress in Software for Gridding 2D Iced Airfoils,” AIAA Paper 2005-1369, Jan. 2005. 

3

 Kreeger, Richard E., and Wright, W., “The Influence of Viscous Effects on Ice Accretion Prediction and Airfoil 

Performance Predictions,” AIAA Paper 2005-1373, Jan. 2005. 

4

 Farrell, Charles A., and Choo, Yung K., “Simulation of Two Dimensional Icing Effects on Airfoil Performance 

Using Smaggice and WIND,” AIAA Paper 2005-1370, Jan. 2005 

5

 Chi., X., Slater, J.W., Shih, T. I.-P., Zhu, B., Addy, H.E., Choo, Y.K., “Computing Aerodynamic Performance of a 

2D Iced Airfoil – Blocking Topology and Grid Generation,” AIAA Paper 2002-381, Jan. 2002. 

6

 J. Thompson, Z. Warsi and C. Mastin, Numerical Grid Generation: Foundations and Applications, Elsevier 

Science Publishing Co., Inc., 1985. 

7

 Noack, R. W., Steinbrenner, J.P., Collection of Technical Papers, AIAA 12th Computational Fluid Dynamics 

Conference, Jun. 1995. 

8

 Rumsey, C. L., Gatski, T.B., “Recent Turbulence Model Advances Applied to Multielement Airfoil 

Computations,” AIAA Paper 2000-4323, Aug. 2000. 

9

 Miller, D., Potapczuk, M., “Preliminary Investigation of Ice Shape Sensitivity to Parameter Variation,” AIAA 

Paper 2005-73, Jan. 2005. 

10 Pan, J., Loth, E., Bragg, M., “RANS Simulations of Airfoils with Ice Shapes,” AIAA Paper 2003-729, Jan. 2003. 

NASA/TM—2008-214829 13
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SmaggIce 2.0: Additional Capabilities for Interactive Grid Generation of Iced Airoils 

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14. ABSTRACT

The Surface Modeling and Grid Generation for Iced Airfoils (SmaggIce) software toolkit has been extended to allow interactive grid 

generation of multi-element iced airfoils. The essential phases of an icing effects study include geometry preparation, block creation, and 

grid generation. SmaggIce Version 2.0 now includes these main capabilities for both single and multi-element airfoils, plus an improved 

flow solver interface and a variety of additional tools to enhance the efficiency and accuracy of icing effects studies. An overview of these 

features is given, especially the new multi-element blocking strategy using the multiple wakes method. Examples are given which illustrate 

the capabilities of SmaggIce for conducting an icing effects study for both single and multi-element airfoils. 

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Aerodynamics 

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