The results are presented as dimensionless variables for the mean wind speed
, the turbulence intensity
, the turbulent kinetic energy
, the vertical velocity skewness
, the aerodynamic surface roughness length
and the friction velocity
. As a reference length for measurements in the vertical direction, the height of the highest olive tree model (0.09 m) is used, designated
. The distance between measuring sections is
, and it changes between profiles in the streamwise direction (
Figure 2).
4.2. Mean Flow and Turbulence Around Olive Groves
For the three configurations tested, vertical profiles are compared along the main axis, corresponding to the central trees line, and vertical profiles on right and left sections include measurements between models. Two profiles are registered inside the olive grove from the coordinate origin, where the olive grove ends and the leeward side starts (-P1 and -P2 in
Figure 2a). The models were distributed uniformly, thus it was proven that measurements in the central section are representative of the wind velocity distribution along the crosswise section.
Nine significant points were selected within the measured area, corresponding with Profiles P1, P4 and P6, in order to make a comparison of the results for each configurations analyzed.
Figure 6 shows the position of these points in which measurements have been taken (coordinates
z and
x), according to the spatial distribution shown in
Figure 2.
Comparisons have been made between each configuration and the results obtained previously for the incoming flow (ABL) (
Figure 5). For this purpose, the values of the dimensionless wind speed, turbulence intensity and dimensionless TKE are shown. Some variation rates represented in percentages of variation were calculated, defined as:
Table 5,
Table 6 and
Table 7 show the dimensionless mean velocity difference
, the dimensionless turbulence intensity
and dimensionless turbulent kinematic energy
at the test section for the reference ABL (empty section) and the same variables for the three olive grove configurations (C1–C3). In all cases, the mean velocity (
) is lower in the presence of olive trees (for this reason, the rate shows a negative value), while the turbulence parameters (
and
) are higher.
Without an agro-forest system, the wind speed profile has a logarithmic shape (
Figure 5). With an agro-forest system, the mean wind speed decreases between
and
at Points 1 and 7 (P1 and P6 at the bottom), and around
at
(Points 3, 6 and 9), where the effect produced by the ecosystem is very weak (
Figure 7 and
Figure 8). The comparison values show that the traditional olive grove without cover (C2) is the configuration that most influences the mean flow in the area next to the models. Regarding
for the staggered configuration (C2), the percentages differ between 527% to 157% at the bottom and 1.90% to 0.72% at the top, for P1 and P6. However, the turbulent characteristics are maintained with a higher value for the case of the configuration with vegetation cover (C3), in both height and streamwise (
Table 7).
Olive agro-forest presence is noticed up to an approximate height of
, where the vertical profile is not logarithmic. The logarithmic shape is not recovered until a distance
; however, a trend is still visible. At tree height, for the traditional configuration C2, the velocity is almost zero in Profile 1 and, from Profile 2, this velocity increases in the streamwise direction (
Figure 7 and
Figure 8). Obstacle transmission is noticed from Profile 5, and it extends beyond a distance of
.
In a comparison of the traditional configurations with and without vegetation cover (C2 and C3), the profiles have a more uniform shape in the presence of vegetation cover, not only for the mean velocity
but also for the turbulence intensity
(
Figure 7 and
Figure 8). However,
is similar for a vertical distance of
, and, at a height of
, the flow is uniform, with a higher value for the case with vegetation cover.
In the following figures, the agro-forest length
L is used as a reference length in the streamwise direction. In
Figure 9 the color gradient covers the range of
, with the lowest value corresponding to blue and the highest value to red. The intermediate values correspond to orange, yellow and green shades. In the case of turbulence intensity (
Figure 10 and
Figure 11b), the red range corresponds to the lowest values and the blue range to the highest turbulence levels, ranging
. In
Figure 12, the color scale ranges from white and light yellow for lower values to dark red for higher values, with a range of values of
.
In
Figure 9 and
Figure 10, the results for intensive farming without cover (C1) are compared with those for traditional farming with vegetation cover (C3). Darker blue color indicate a greater distortion with respect to the reference values. For a height of
, the wind velocities are more uniform, up to
higher in the first case in areas close to the surface and
lower in terms of turbulence intensity. For a distance of
, the effects of intensive olive grove decrease; in contrast, these remains constant for the traditional configuration.
There are small differences between the lateral profiles and central profiles (
Figure 11), due to some differences between the areas in which measurements were taken. A higher turbulence intensity is noticed for central section measurements at a distance of
with respect to measurements for right section.
For the TKE, from a height of
, the energy dissipation decreases and stabilizes vertically (
Figure 12). The turbulence, generated at the surface, in the case of the traditional olive grove is developed earlier than in other configurations. As
Figure 12 shows, there is an increase and spatial dispersion of the TKE along the flow downwind. In a comparison of the TKE values with and without the models (
Figure 12), the flow around the olive groves has turbulence values of 300–600% higher than the free flow at a height of
. By contrast, the TKE remains constant along the streamwise distance, most noticeable effect in the case of staggered olive grove.
In
Figure 12, for intensive exploitation, a higher TKE is observed near the models up to a relative height of
. The TKE increases and remains at high level from that point and along the flow direction for the traditional olive grove, reaching a value of almost double that of the intensive olive grove.
Vertical velocity skewness was analyzed, and the results for grid and staggered with vegetation cover configurations are shown in
Figure 13. These results are in agreement with those obtained by Segalini et al. [
24].
shows significant height (
z) dependence and its behavior can be divided in three areas: (i) an area below the reference height (tree height
), in which the vertical skewness values are negative, so
is not transported vertically; (ii) a second area from the height
to about
, in which the values are positive, and a vertical transport of
is expected; and (iii) a third area that covers from
to the end of the measurements taken, in which the values become negative again, although in the highest zone (
) a similar trend to that observed in the lower part is seen, towards values close to zero.
These values are intimately related to turbulent kinetic energy, so a logical behavior can be observed if we compare them. The results obtained are consistent with those shown in the work of Hogan et al. [
34], focused on the study of air flow over different types of canopy and spatial distributions.