| 1. |
- Arnqvist, Johan, 1985-, et al.
(författare)
-
Flux-profile relation with roughness sublayer correction
- 2015
-
Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 141:689, s. 1191-1197
-
Tidskriftsartikel (refereegranskat)abstract
- Calculation of momentum flux using Monin–Obukhov similarity theory over forested areas is well known to underestimate the flux. Several suggestions of corrections to the standard flux-profile expression have been proposed in order to increase the magnitude of turbulent flux. The aim of this article is to find a simple, analytical representation for the characteristics of the flow within the canopy layer and the surface layer, including the roughness sublayer. A new form of the roughness sublayer correction is proposed, based on the desire to connect the shape of the roughness sublayer correction to forest characteristics. The new flux-profile relation can be used to find the flux or the wind profile whenever simple and fast estimations are needed, as for mesoscale modelling, scalar transport models, or sound propagation models.
|
|
| 2. |
- Arnqvist, Johan, 1985-, et al.
(författare)
-
Investigation of Turbulence Accuracy When Modeling Wind in Realistic Forests Using LES
- 2019
-
Ingår i: Progress In Turbulence Viii. - Cham : SPRINGER INTERNATIONAL PUBLISHING AG. - 9783030221966 - 9783030221959 ; , s. 291-296
-
Konferensbidrag (refereegranskat)abstract
- This study presents an evaluation of wind field simulations, in neutral atmospheric conditions, above a heterogeneous forest. The calculations were performed with Large-Eddy Simulation (LES) code OpenFOAM, with explicit modelling of the forest through drag coefficient and forest density. The findings indicate that a large modelling domain is needed in order to reproduce the measurements in different wind directions, since the effect of far upwind forest characteristics influence the wind and turbulence profiles. It is further shown that even though the low resolution of the LES simulations lead to slightly misrepresented single point turbulence characteristics, two point turbulence characteristics are well predicted due to spatial filtering of the small scales.
|
|
| 3. |
- Arnqvist, Johan, 1985-
(författare)
-
Mean Wind and Turbulence Conditions in the Boundary Layer above Forests
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- As wind turbines have grown, new installation areas become possible. Placing wind turbines in forested landscapes introduce uncertainties to the wind resource estimation. Even though close-to-canopy processes have been studied intensively during the last thirty years, the focus has mostly been on exchange processes and the height span of the studies has been below the rotor of a modern wind turbine.This thesis contains analysis of new measurements from a 138 m high tower in a forested landscape. The previous knowledge of near-canopy processes is extended to the region above the roughness sublayer. It is shown that above the roughness sublayer, the surface layer behaves as over low vegetation, and Monin-Obukhov similarity is shown to hold for several variables. However, in stable stratification, effects that could be linked to the boundary layer depth are shown to be present in the measurements. These include wind turning with height, the behaviour of the turbulence length scale and the curvature of the wind profile.Two new analytical models are presented in the thesis. One is a flux-profile expression in the roughness sublayer, which allows for analytical integration of the wind gradient. The model suggests that the roughness-sublayer effect depends on stratification and that the aerodynamic roughness length changes with stability. A decrease of roughness length in stable stratification is confirmed with a new method to determine the roughness length using measurements from the 138 m tower.The other model determines the spectral tensor in stable stratification using analytical solution to the rapid distortion equations for stratified shear flow, with homogeneous stratification and shear. By using a formulation for the integration time of the distortions of an isotropic spectrum, a model is derived which provides the cross spectra of velocity and temperature at any two given points in space.Finally the existence of waves in the wind over forests is investigated and it is concluded that the Kelvin-Helmholtz instability can create waves which are coherent in time and exist over the entire height span of wind turbine rotors. Linear wave theory is shown to be able to explain certain features of the waves.
|
|
| 4. |
|
|
| 5. |
- Arnqvist, Johan, 1985-, et al.
(författare)
-
Robust processing of airborne laser scans to plant area density profiles
- 2020
-
Ingår i: Biogeosciences. - : Copernicus GmbH. - 1726-4170 .- 1726-4189. ; 17:23, s. 5939-5952
-
Tidskriftsartikel (refereegranskat)abstract
- We present a new algorithm for the estimation of the plant area density (PAD) profiles and plant area index (PAI) for forested areas based on data from airborne lidar. The new element in the algorithm is to scale and average returned lidar intensities for each lidar pulse, whereas other methods do not use the intensity information at all, use only average intensity values, or do not scale the intensity information, which can cause problems for heterogeneous vegetation. We compare the performance of the new algorithm to three previously published algorithms over two contrasting types of forest: a boreal coniferous forest with a relatively open structure and a dense beech forest. For the beech forest site, both summer (full-leaf) and winter (bare-tree) scans are analyzed, thereby testing the algorithm over a wide spectrum of PAIs. Whereas all tested algorithms give qualitatively similar results, absolute differences are large (up to 400 % for the average PAI at one site). A comparison with ground-based estimates shows that the new algorithm performs well for the tested sites. Specific weak points regarding the estimation of the PAD from airborne lidar data are addressed including the influence of ground reflections and the effect of small-scale heterogeneity, and we show how the effect of these points is reduced in the new algorithm, by combining benefits of earlier algorithms. We further show that low-resolution gridding of the PAD will lead to a negative bias in the resulting estimate according to Jensen's inequality for convex functions and that the severity of this bias is method dependent. As a result, the PAI magnitude as well as heterogeneity scales should be carefully considered when setting the resolution for the PAD gridding of airborne lidar scans.
|
|
| 6. |
- Avila, M., et al.
(författare)
-
Daily cycle simulations of thermally stratified flows over forests
- 2019
-
Ingår i: Wake Conference 2019 22–24 May 2019, Visby, Sweden. - : IOP PUBLISHING LTD.
-
Konferensbidrag (refereegranskat)abstract
- The aim of the present work is to obtain a better understanding of how to model the thermally stratified wind field over a forest during full diurnal cycles. The setup of the study assumes a horizontally homogeneous forest, with the objective of finding a simple and efficient way to model the canopy flow using time-dependent input data, obtained from measurements and mesoscale simulations. With this, new insights can be gained for future microscale modelling of complex forested terrains using mesoscale input data. In terrain without forest a diurnal cycle is commonly simulated by imposing time-dependent ground temperature. However, the presence of forests partially isolates the temperature at ground level from the flow above the canopy, making this common approach ineffective. This work proposes imposing the time-dependent net radiation at the forest canopy top to drive the thermal stratification changes along the diurnal cycle. To this end, several full days of simulation are driven by prescribing the net radiative heat flux balance measured on top of the canopy, together with a geostrophic pressure gradient. The advantage of the method is its simplicity and that the input data can be easily obtained from mesoscale modelling. When compared to the observations at the Swedish site Ryningsnas, the new method dramatically improves estimations of wind speed, wind direction and turbulent kinetic energy compared to simulations that only assume neutral stratification. Out of the variables studied, temperature and turbulent heat flux profiles were the ones that qualitatively followed the measurements the best, while wind speed and turbulent kinetic energy showed a larger disagreement.
|
|
| 7. |
|
|
| 8. |
- Floors, Rogier, et al.
(författare)
-
From lidar scans to roughness maps for wind resource modelling in forested areas
- 2018
-
Ingår i: Wind Energy Science. - : COPERNICUS GESELLSCHAFT MBH. - 2366-7443 .- 2366-7451. ; 3:1, s. 353-370
-
Tidskriftsartikel (refereegranskat)abstract
- Applying erroneous roughness lengths can have a large impact on the estimated performance of wind turbines, particularly in forested areas. In this study, a new method called the objective roughness approach (ORA), which converts tree height maps created using airborne lidar scans to roughness maps suitable for wind modelling, is evaluated via cross predictions among different anemometers at a complex forested site with seven tall meteorological masts using the Wind Atlas Analysis and Application Program (WAsP). The cross predictions were made using ORA maps created at four spatial resolutions and from four freely available roughness maps based on land use classifications. The validation showed that the use of ORA maps resulted in a closer agreement with observational data for all investigated resolutions compared to the land use maps. Further, when using the ORA maps, the risk of making large errors (>25%) in predicted power density was reduced by 40-50% compared to satellite-based products with the same resolution. The results could be further improved for high-resolution ORA maps by adding the displacement height. The improvements when using the ORA maps were both due to a higher roughness length and due to the higher resolution.
|
|
| 9. |
- Hallgren, Christoffer, et al.
(författare)
-
Classification and properties of non-idealized coastal wind profiles - an observational study
- 2022
-
Ingår i: Wind Energy Science. - : Copernicus Publications. - 2366-7443 .- 2366-7451. ; 7:3, s. 1183-1207
-
Tidskriftsartikel (refereegranskat)abstract
- Non-idealized wind profiles frequently occur over the Baltic Sea and are important to take into consideration for offshore wind power, as they affect not only the power production but also the loads on the structure and the behavior of the wake behind the turbine. In this observational study, we classified non-idealized profiles as the following wind profiles having negative shear in at least one part of the lidar wind profile between 28 and 300 m: low-level jets (with a local wind maximum in the profile), profiles with a local minimum and negative profiles. Using observations spanning over 3 years, we show that these non-idealized profiles are common over the Baltic Sea in late spring and summer, with a peak of 40 % relative occurrence in May. Negative profiles (in the 28-300 m layer) mostly occurred during unstable conditions, in contrast to low-level jets that primarily occurred in stable stratification. There were indications that the strong shear zone of low-level jets could cause a relative suppression of the variance for large turbulent eddies compared to the peak of the velocity spectra, in the layer below the jet core. Swell conditions were found to be favorable for the occurrence of negative profiles and profiles with a local minimum, as the waves fed energy into the surface layer, resulting in an increase in the wind speed from below.
|
|
| 10. |
- Hallgren, Christoffer, et al.
(författare)
-
Looking for an Offshore Low-Level Jet Champion among Recent Reanalyses : A Tight Race over the Baltic Sea
- 2020
-
Ingår i: Energies. - : MDPI. - 1996-1073. ; 13:14
-
Tidskriftsartikel (refereegranskat)abstract
- With an increasing interest in offshore wind energy, focus has been directed towards large semi-enclosed basins such as the Baltic Sea as potential sites to set up wind turbines. The meteorology of this inland sea in particular is strongly affected by the surrounding land, creating mesoscale conditions that are important to take into consideration when planning for new wind farms. This paper presents a comparison between data from four state-of-the-art reanalyses (MERRA2, ERA5, UERRA, NEWA) and observations from LiDAR. The comparison is made for four sites in the Baltic Sea with wind profiles up to 300 m. The findings provide insight into the accuracy of reanalyses for wind resource assessment. In general, the reanalyses underestimate the average wind speed. The average shear is too low in NEWA, while ERA5 and UERRA predominantly overestimate the shear. MERRA2 suffers from insufficient vertical resolution, which limits its usefulness in evaluating the wind profile. It is also shown that low-level jets, a very frequent mesoscale phenomenon in the Baltic Sea during late spring, can appear in a wide range of wind speeds. The observed frequency of low-level jets is best captured by UERRA. In terms of general wind characteristics, ERA5, UERRA, and NEWA are similar, and the best choice depends on the application.
|
|
