How can I draw a model without overlapping surfaces?

Whether you import a model from a CAD package or draw it in IES software, it is important that the drawing be physically realistic. Many things that are easy to draw, or which may be drawn to facilitate manufacturing (e.g. a cylinder “force-fit” to a hole, being slightly larger than the hole it is being inserted into), are not physically possible, hence they will adversely affect attempts to use the drawing for engineering analysis. It is less work to have such problems addressed by training the person doing the drawings than it is to try to modify the drawings when they are completed.

One of the more subtle problems that comes up is “overlapping surfaces” – illustrated to the right. On top is a cylinder of copper, underneath is a block of steel. Where they meet, there is physically only a single interface. As shown in blue on the left, there is essentially a hole in the top surface of the steel block, and this hole is the interface between the two.

Consider what would happen if the drawing was made by constructing a block, then constructing a cylinder. As shown in blue on the right, the whole top surface would be one surface. Problems that result include that the boundary condition for the solution will have this as either a steel/air or a steel/copper interface – not as a combination of both. Even if the elements were to individually check what material is on each side – there would be many elements crossing the boundary of the small circle – hence the boundary would effectively become very coarse.

Since there can only be a single surface between the copper and steel, and since this surface is distinct from the rest of the top of the steel block – a model for CAE purposes needs to be drawn that way. The rest of this FAQ entry addresses how to do this in version 6.0 of IES 3D software. The principles given here can be applied to drawing in various CAD packages for those customers wishing to use our direct CAD import feature.

While recognizing holes in surfaces is very difficult in 3D, it is relatively simple in 2D. Hence, the first step is to draw the intersecting surfaces using the 2D mode. After entering the 2D mode you draw a rectangle with a circle inside it. The fact that the circle is a hole in the rectangle is automatically recognized.

Now when you switch to 3D mode the software will recognize the 2D region with a hole as a 3D surface with a hole. The hole will also be defined as a surface (though in some cases it might be convenient to undefine it).

From this point the basic problem of overlapping surfaces is solved and there are many drawing options. One simple method is to sweep (extrude) the circle upwards into the cylinder, and the top surface, plus the hole in it downwards. This produces 3 volumes as shown. In fact, for the similar problem of a copper rod sticking though a steel block, this is exactly the geometry you want. However, for the stated problem the cylindrical volume within the steel might be deemed a nuisance since it requires an extra step in operations where the steel volume is being selected. If so, it can be deleted as follows:

In IES software all you need to do is delete the 2 segments that define the circular bottom of the volume and the side length. This breaks both the volume and bottom surface of the steel (since you have removed part of the boundaries for each). Redefine the bottom surface and you can now redefine the volume as well.

You have now created the physically correct volumes and surfaces, and you can assign materials to the volumes – and any other appropriate physical parameters.

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