CST2013: Vent

Modeling: ShapesFaces and AperturesVent

Vents are used to model perforated metal screens, which are commonly used as windows or ventilation panels on metal enclosures. For frequencies at which the width of the individual apertures is significantly smaller than half a wavelength, these screens are only weakly penetrable to electromagnetic fields.

Precise modelling of individual apertures in a vent would require a cell size of just a fraction of the aperture width. This is often impractical for air vents, as the apertures may be fairly small. An alternative approach is to replace the screen with a panel of homogeneous material having the same large-scale electrical properties. Any effects due to individual apertures are then lost, but the cell size of the model may be chosen without any reference to the aperture size.

A Vent may be specified over all or part of a thin metal panel. The Solver estimates the electrical properties of the vent from the size and shape of its apertures and from the thickness and fractional open area of the screen. That part of the metal panel covered by the vent is then replaced by a panel of a special material having the appropriate properties.

Note that the original metal panel should have no thickness. The thickness of the screen is regarded as an electrical rather than a geometric property, and is not determined by the thickness of the underlying metal.

Grating Effects

At frequencies for which the aperture-spacing (pitch) of a perforated screen is greater than half a wavelength, the screen can function as a diffraction grating. That is, the screen can produce diffracted and reflected waves at angles which differ from the angle of incidence of illumination (the limit is a half rather than a full wavelength to allow for grazing incidence). Since the vent model is homogeneous it cannot produce this effect, and so should only be used when the pitch of the screen is less than half a wavelength (at the highest frequency of interest).

Even when its pitch is less than half a wavelength, a real screen will give rise to local evanescent fields in addition to the ordinary transmitted and reflected rays. For the worst case of grazing incidence, the least evanescent space-harmonic produced by a screen of pitch p, will decay by a factor of e, in going through a distance away from the screen of

At distances much larger than this, the evanescent fields will be negligible in comparison to the ordinary transmitted and reflected rays. Conversely, if an object is placed within a distance  of the screen, then coupling to the evanescent field could make a significant contribution to transmission through the screen. This imposes a further limitation on the use of the vent model; namely that there should be a clear space of (say) 2  on either side of the screen. Note that if the wavelength is much greater than twice the pitch then  ~ pitch/(2pi).

Isotropy

A round or square aperture has electrical properties which are (to a first-order approximation), isotropic in the plane of the screen. This is also true for any aperture which has rotational symmetry for any angle of less than 180° - a hexagonal aperture for example. A screen with apertures of this type will itself be isotropic. If the pattern of apertures is anisotropic - if the apertures are closely spaced in one direction and widely spaced in the perpendicular direction, then the screen may still be taken as isotropic provided that the wider spacing is less than half a wavelength.

An elongated aperture has anisotropic electrical properties. A screen with elongated apertures will itself be anisotropic if the apertures are all parallel to one another. The vent model is not capable of modelling such anisotropic screens.

If the individual apertures are anisotropic, but are at different orientations to one another, then the screen as a whole may be isotropic. The precise requirement is that the angular distribution of aperture axes has rotational symmetry for any angle of less than 180°. A screen having elongated apertures, with each aperture at 90° to its neighbors would meet this requirement. In this case, the screen should be treated as though half the apertures (the effective fraction parallel to any given polarization), were absent.

 Summary of Limitations

  In summary, the vent model should only be used if the following conditions are met:

• The aperture spacing (pitch of the screen) is less than half a wavelength at the highest frequency of interest.

• The screen is isotropic (does not consist of parallel elongated apertures).

• The aperture depth (screen thickness) is no greater than the aperture width.

• No objects are so close to the screen as to disturb the fields in the vicinity of individual apertures. If the pitch of the screen is much less than half a wavelength, then that is (say), closer than one third of the pitch.

Vent Dialog

Name: Specify a unique name for the vent.

Shape: Here you select the shape of the vent aperture. Round, square or hexagonal, or whichever of these best approximates the actual aperture shape.

Owner type: The vent can be specified over all or part of a metal panel. If you pick a face that is aligned to one of the coordinate axes before opening the Vent dialog, picked face will define the dimensions of the vent region.

If you pick two points on one face and if this face is aligned to one of the coordinate axes, then these two points will define the opposite corners of the vent region.

If there are no suitable picked points or face when the Vent dialog is opened, then the vent will be specified over the whole surface of the selected metal panel.

• Axial face: Depending on pick, the entire face or a portion of the face will be used as the vent region.

• Sheet body: The whole surface of the body will be used as the vent region.  

Vent parameters  frame

Width / Radius / Side length: Depending on shape selected, specify here the width or radius or side length of the vent apertures.

Depth: Specify the thickness of the screen.

Coverage: Specify the vent coverage in the selected region. This is a number between 0 and 1, being the fraction of the area of the screen which is occupied by apertures. It is the percentage open area expressed as a decimal fraction. The Solver does not take any account of the pattern in which the apertures are arranged - whether on rectangular grid, or in staggered rows.

Vent region frame

Xmin / Xmax / Ymin / Ymax / Zmin / Zmax: The bounding box of the vent region.

Picks: Before entering this dialog, if points are picked, this option will be enabled.

OK

            Press this button to create the vent.

Preview

 Press this button to create a preview image of the vent. This option is very useful to check the settings before you actually create the vent.

Cancel

Closes this dialog box without performing any further action.

Help

Shows this help text.