# Numerical Study on Tree Belt Impact on Wind Shear on Agricultural Land
## 5 Conclusions  
The study employed a realizable \(k\)-epsilon turbulence model to a large domain with porous drag. The inlet profiles of velocity, turbulent kinetic energy, and its dissipation rate were based on atmospheric research supplemented with measurement data. The terrain drag was modeled with two sets of wall functions, and it was proved that standard wall functions will not correctly predict atmospheric flows. This necessitated the use of atmospheric wall functions that resulted in very good agreement of the calculated velocity profiles with the theoretical logarithmic ones.  
To summarize clearly the practical implications of this study: when placing small-scale wind turbines downstream of tree belts, it is advisable to position the turbine rotors at heights exceedingly approximately 2.5 times the height of the tree belt. This placement avoids substantial increases in turbulence intensity and benefits from the local acceleration in wind velocity above the tree belt. Additionally, choosing tree belt porosity coefficients beyond 0.5 m\({}^{-1}\) offers diminishing returns in modifying wind velocity and turbulence characteristics, suggesting that overly dense vegetation does not significantly enhance the protective or aerodynamic performance.  
The requirements for dimensionless wall distance (y\({}^{+}\)) were met through significant mesh refinement near the ground, resulting in cell heights as small as 1 cm. The y\({}^{+}\) increased after the application of atmospheric wall functions, but remained within reasonable limits, especially considering the size of the domain.  
The belt represents an obstacle, and as such it tended to reduce the speed downstream. This decrease was compensated by a velocity increase starting at elevations 1.5 times the tree belt. The velocity deficit behind the belt and the corresponding gain at higher elevations were dependent on the belt porosity up to values of 0.5 m\({}^{-1}\). This effect is expected to diminish for further increases in porosity drag. The profiles furthest downstream of the belt never completely regained the undisturbed velocity profile, thus reducing, albeit marginally, the flow rate available for turbines.  
The belt introduced substantial turbulent intensity for higher drag coefficients. The affected zone may reach a size of 2.5 tree heights in the vertical direction and a length of 15 tree heights downstream. This should be considered carefully during installation of wind turbines.  
Future research will focus on extending the present methodology to more complex vegetation configurations, including heterogeneous tree belts with mixed porosity and height profiles. Experimental validation using wind tunnel measurements or LiDAR field data is also foreseen to strengthen the numerical findings and improve the parameterization of vegetation-induced drag in CFD models.  
Conceptualization, A.T., P.Z., F.B. and M.I.; methodology, A.T., G.P. and B.S.; software, G.P., F.B. and J.D.; validation, B.S. and M.I.; formal analysis, A.T., G.P. and P.Z.; investigation, A.T., B.S. and F.B.; resources, G.P., J.D. and M.I.; data curation, A.T., B.S. and F.B.; writing--original draft preparation, P.Z., M.I. and G.P.; writing--review and editing, A.T., P.Z., F.B. and M.I.; visualization, P.Z. and A.T.; supervision, A.T. and F.B. All authors have read and agreed to the published version of the manuscript.  
This study is financed by the European Union--NextGeneration EU, through the National Recovery and Resilience Plan of the Republic of Bulgaria, Project No. BG-RRP-2.004-0005.  
Not applicable.  
The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.  
The authors declare no conflicts of interest.