Front Wind Research Articles

Studies on the Wind in Front and Back of the Shelter-Hedges (9)

When a windbreak is set up, the eddy occures in front and back of it. In order to know the volume of the eddy, the average wind velocity and the distance covered by the eddy before it vanishes the correlation coefficient of the turbulence is calculated as followsRT=Σ(VX′×VY′)/Σ(VX′2)Σ(VY′2)VX…the velocity of the Wind at X PointVX, VY…Mean Velocity at X and Y PointVX′=VX-VX VY′=VY-VYBelow 100cm height, the correlation of 3X/H is seen up to 8X/H, especially at 70cm to 10X/H, Above 150cm the correlation of 3X/H is large at each location. As to the correlation with the standard, it is not seen up to 8-10X/H. It is seen with 10X/H at 50cm and 8X/H at 150cm, respectively. If the time-correlation in 5 seconds is adopted for this eddy, the scale of the eddy is as followsL=RTU=5×(0.5-1.2)=2.5-6.0(m)RT…time CorrelationU…average Velocity of WindThe height of the eddy seems to be about 1.0-1.2 meters.

Studies on the wind in front and back of the shelter-hedges. (8)

Studies on the wind in front and back of the shelter-hedges. (8)

Studies on the wind in front and back of the shelter-hedges

Studies on the wind in front and back of the shelter-hedges

Studies on the wind in front and back of the shelter-hedges (6)

Studies on the wind in front and back of the shelter-hedges (6)

Studies on the wind in front and back of the Shelter-hedges (4)

Studies on the wind in front and back of the Shelter-hedges (4)

Studies on the wind in front and back of the shelter-hedges. (5)

Concerning the relation between the height of shelter-hedges and the effective area, the past researchers have put forth two different results. Namely, the one is that the higher the hedge the wider is the effective area and the other is the contrary. Theoretically speaking, the area is to be wider in proportion to the 8/7 power of H. (H is the height) The following factors may be suggested to explain how the different conclusions have been reached.1) The direction of the wind; The fence placed at right angles to the wind direction produces a remarkably different effective area from the fence placed at oblique angles.2) Turbulence; The more turbulence of the wind causes the less wide effective area; Near the seashore, owing to little turbulence in general, the effective area becomes wider, while in the inland place with woods and forests the area becomes narrower.3) The roughness on the surface of the ground; Much unevenness or undulation on the surface of the earth or much grassiness lessens the effective area.4) The structure of the shelter-hedge; The density, breadth, position of the branches, flexibility of the crown, and so on exert a great influence on the effective area, 5) The velocity of the wind; The higher velocity causes the more turbulence, and in consequence the effective area becomes less. On the contrary, the lower velocity causes the area to be wider. However, the experiments have brought about a result to the contrary.The result of the experiment:1) ConditionsTwo kinds of fences: 100cm. and 50cm. in height.The kind of fence employed: board-fence, 75% covered.The direction of the wind: right angles to the fence (variance of about 5 degrees to be included)The velocity of the wind: 6-8m/sec. Places of measurement: Three places of different turbulences.Measuring apparatus: small type of Robinson's anemometer.2) ResultThe more the turbulence the less has been the difference in ratios of the wind velocity of the two fences. The difference is to be shown in a stright line in its diagram. In the case of the 50cm high one, the difference deviates from the stright linear formula. Times the height of the hedges, in both kinds, which decrease the velocity of the wind 50%, becomes the more where the turbulence is less. In the case of the fence 100cm high, the ratio of the increase of the effective area is shown as H1.03-H1.09 (H denotes times the height of the hedge) -H1.06 as average. Now it has become clear that when the fences get higher, the effective area grow more in proportion to times of the height of the fences.

Studies on the wind in front and back of the shelter-hedges (2)

In order to investigate the contrary wind in front and back of shelter-hedges, we used board-fences with interstices of 0%, 20%, 50% and 80%, each being 90cm. high and 15m. long. In case of the hedge with 0% interstice, contrary wind was observed to 0.5 times the height of the hedge (for all the heights of 20cm, 40cm, and 60cm above ground) to the windward, and 9 times the height at the maximum to the leeward of the hedge at 20cm above the surface of the ground. Here it must be noted that the range of contrary wind to the leeward became longer the nearer to the surface. In case of the hedge with 20% interstice, the range was from 2 to 7 times the height, and in case with 50% interstice, the contrary wind was observed only at 20cm above surface ranging from 3 to 7 times the height of the hedge to its leeward, with no other remakable occurrences.The strength of the contrary wind increased according as the interstice became smaller. Wind the interstice of 0%, the velocity of the wind reached 40% of the standard wind velocity to the leeward of the hedge at a distance 3 to 4 times the height of hedge. As the interstice became wider, the spot where the wind is strong, extended to the leeward. The contrary wind lasted for comparatively short time, and in most cases it blew alternately with the normal wind. But, at a distance 3 to 4 times the height to the leeward the wind blew continuously in both cases of 0% and 20% interstices. As the wind velocity increased, the contrary wind also became stronger, but the ratio of its velocity to the normal one scarecely changes. Wide interstice being given, blowing-in caused the increase of wind velocity and at the same time the decrease of contrary wind.

Studies on the wind in front and back of the shelter-hedges

As the interstice of a wind break or hedge causes wind path, the wind velocity increases and wind erosion is often promoted.According to our investigation, the accelerated wind velocity at the interstice within a shelterbelt reached to about 120% of standard velocity. It was found that Sodegaki which was slanted off from the artificial hedge was more effective than the right-angled one.On the occation when the hedge were pierced with a road, the zigzaged Sodegaki was most effective and the blowing through the interstice was ceased.

Studies on the wind in front and back of the shelter-hedges (1)

Some characteristics of natural wind have been investigated in front and back of plain shelter-hedges, 80cm high and 15m long, of several kinds of type having interstice of 80% (type A), 50% (B), 20%(C) and 0% (D) respectively. Being inspected the horizontal distributions of wind velocity at the height of 50cm, it has been made clear that there exist the zone of acceleration just behind the hedges of tybe A and B and the zone of deceleration for C and D, which are considered to be due to the local blowing and the interruption of wind respectively.The vertical distribution of wind velocity for the hedge A has been shown to keep almost the logarithmic law except at the station extremely near the hedge, and, on the other hand, those for B, C and D have been shown to increase discrepancy from the logarithmic law at heights below 1m as the degree of interstice decreases.The degree of wind wind velocity turbulence increased just behind the hedges has tended to decrease with the distance from hedges, and it has been shown that at the distance of 20 times of hedge height the influence of hedge has either disappeared for A and B or remained for C and D.

WIND LOADS ON STRUCTURES.

Synopsis. From a review of existing knowledge of wind loads it is clear that, although many observations of wind velocities and pressures have been made, it is difficult to estimate their true significance with reference to the derivation of design loads. Velocities vary considerably over the wind front and fluctuate rapidly, and the pressures actually recorded may act on only a small area of a structure and for only a few seconds. Hitherto, practically no attention has been given to the time effect, and an attempt is made in this Paper to estimate the probable damage inflicted on a structure during a high wind. The damage is expressed in terms of the permanent plastic deformation and, hence, wind loads suitable for use in ultimate (plastic) design methods may be derived. Although some numerical results are given, it is found that available data on wind velocities and pressures are insufficient to enable full calculations to be made. The investigation shows clearly the nature of further research which should be under-taken in order to achieve a rational treatment of wind loading.