Damage by wind alone is seen to its greatest effect on leeward surfaces, wherever sharp edges project into the wind stream. This is due to the downstream
vortices induced immediately behind any sharp edge, and the vortex intensity relates to wind speed and the sharpness of the traversed edge. The vortices produced are of higher velocity than the stream itself, and are typically of the order of 1.5—2.0 times the generating wind speed.
Two other features need to be borne in mind with respect to vortex effect and Figure 2.23 helps to
explain the phenomena. Firstly, it should be noted that downstream of the corner or edge, the standing vortex generated will establish, for a given wind speed and direction, a reversal in direction of flow at the downstream face of the masonry.
This causes suctional forces to develop against the leeward face, similar to the effect over an aircraft wing, for the short section close to the corner. The suctional forces drag any loose mortar free of the joints, and hence increase the chance of arris damage as noted above. It should also be noted that wind is never constant in either speed or direction for long, as it constantly veers and backs in short time cycles, reacting to major forces in the atmosphere. Hence the constantly changing effects of the vortices aggravate the extent of scouring on the downstream face. Friction with the ground decreases, causing wind speed to increase with height. Tall ruins, such as towers or ruined abbey naves, should have carefully rounded-off tops and shoulders where dressed for water shedding, etc. These rounded edges help maintain a laminar condition to the airstream, which restricts the development of vortices.
Judicious planting around very exposed sites will help to drop the wind speed and lift the airstream to some extent. This not only creates a more visitor friendly environment, but can also reduce weathering to the fabric.
The second effect, which can be recognised both in ruins and standing buildings alike, is that of wind channelling, such as shown in Figure 2.23. The effects of other walls, roofs or objects, if at a suitable angle to the wind stream, can divert part of the stream, effectively deflecting it against a leeward corner, which might otherwise be sheltered to some extent. Hence, certain parts of a structure can receive wear over a wider range of wind directions, and localised wind degradation will be more severe.
Figure 2.24 demonstrates the basic effects of wind vortex erosion in a high wind area, although little erosion occurs on the mine chimney, because the smooth profile restricts the development of vortices; the square base structure has noticeable corner erosion just where the vortices are generated.
Figure 2.25 shows localised erosion to a round chimney, where downstream vortices have impinged on the surface, these being spun off the adjacent edge of the engine house ruin in the foreground.
In certain very severe cases, as occur frequently in maritime situations, some thought should be given to providing wind-break fences within a complex of ruins. Obviously there is a visual trade-off involved here, but occasions will arise of frequently visited sites needing them. They are best made of vertical battens about 30—35 mm wide and spaced apart a clear distance of 40 — 45 mm. They need to be 1700—2100 mm above ground level, and the tops should not be capped with a rail, but left as protruding fingers. This type of fence has been tested in high-wind areas and works efficiently to 'kill' the
Figure 2.25 Localised erosion related to downstream vortices.
stream speed by the simple process of generating opposing vortices.
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