Wind Tunnel Studies of Shear Stress Partitioning in Live Plant Canopies

The earth's surface is permanently exposed to the atmosphere and accordingly to strong wind forces in many regions. Aerodynamic entrainment, transport and redeposition of sand, soil or snow are able to considerably reshape the surface morphology and influence the environment in areas ranging from deserts to polar regions. Even in moderate climate zones, entrainment and transport of dust, particulate matter and pollen or seeds by wind may have a strong impact on the local atmosphere and vegetation. Many of these processes exert negative influences on our sensitive natural environment. Land degradation, desertification or dust storms, increased particulate matter concentrations in the atmosphere or reduced accumulation of snow in arid regions are just a few examples of the impacts of wind erosion. Vegetation on the ground can provide an efficient sheltering effect against wind erosion. Plants influence sediment erosion mainly by the following four mechanisms: by reducing the surface exposed to the wind, by trapping particles in motion, by local stress concentration and by absorbing momentum from the flow. The latter results in lower surface shear stress on the ground beneath the plant canopies. The peak of the surface shear stress is responsible for the onset of erosion and the spatial mean is commonly used to estimate particle mass fluxes. To quantify the sheltering effect of vegetation, a method called shear stress partitioning has been extensively investigated in the past. This method determines the fraction of the total fluid stress on the entire canopy acting directly on the substrate surface. However, previous studies have limitations: they were either field-based, mainly using live plants, with the limitation that wind conditions could not be controlled, or from wind tunnels using rigid and non porous plant imitations, that poorly reflect the aerodynamic behaviour of live vegetation. This study takes a new approach, performing shear stress partitioning experiments in a controlled wind tunnel environment to systematically quantify the sheltering effect of live, flexible and porous plants. Subsequently, the data was used to test and improve a theoretical model that predicts the stress partition for vegetation canopies. This dissertation is divided into four sections. The main results of each section are discussed in this thesis and have also been published as one conference (Chapter 2) and three journal articles (Chapter 3-5). In Chapter 2, the flow conditions produced in the wind tunnel over live vegetation canopies were investigated to identify the suitability of the boundary-layer flow for these new investigations of shear stress partitioning. Flow characteristics like vertical Reynolds stress and integral length scale profiles and power spectral densities were determined from two-component hot-film anemometry measurements. The results were in good agreement with established literature, suggesting that well developed boundary-layers over live vegetation canopies can be generated in this wind tunnel. In Chapter 3, the experimental setup and the building, calibration and testing of the measurement technique for measuring surface shear stress in the wind tunnel are presented. The experimental setup consisted of wooden boards in which the live plants, grown in plastic tubes, were arranged in staggered rows. The surface shear stress sensors (Irwin sensors) and the required multi-channel pressure scanner were custom designed and built. Accuracy tests verified that reliable surface shear stress measurements with an average accuracy of about ±5% can be performed when using a universal calibration function for all Irwin sensors built for this study. The surface shear stress distribution around a single wall-mounted rectangular block was measured as a test case and can serve as high-resolution validation data for CFD simulations. The surface shear stress distributions on the ground beneath the different densities of live plant canopies were measured with previously unmatched high spatial and temporal resolution, the results of which are presented in Chapter 4. Vertical velocity profiles were measured with a two-component hot-film anemometer to determine the total stress above the canopy as well as additional flow characteristics. For comparison, similar experiments were performed with rigid blocks as substitutes for the plants to systematically investigate the influence of the plants' flexibility and porosity on their sheltering effect against sediment erosion. Several distinctive differences in the sheltering effect of live plants and rigid blocks were found: (i) Flow speed-up around the blocks caused higher peak surface shear stress than in experiments with plants. (ii) The sheltered areas in the lee of the plants are significantly narrower and longer with higher surface shear stress than those found in the lee of the blocks. (iii) The streamlining behaviour of the flexible plants results in a decreasing sheltering effect at increasing wind speeds. (iv) Turbulence intensity distributions close to the ground suggest a suppression of horseshoe vortices in the plant case. Another important result is that the percentage of time when a particle entrainment threshold surface shear stress value is locally exceeded is found to be a useful parameter for determining local erosion and deposition rates. In Chapter 5, a shear stress partitioning model (Raupach 1992) was tested against the measured data. The model allows the prediction of the total shear stress on the entire canopy as well as the peak and average shear stress ratios. This study is the first, to systematically investigate the models ability to account for shape differences of various roughness elements. The model can predict the general difference between the plant and the block experiments correctly, although the model limitations were clearly revealed and are discussed in this chapter. The model constant c, relating the size of an effective shelter area and volume to flow parameters and which was poorly specified prior to this study, was found to have a value of about c = 0.27. Values for the model parameter m, which relates the peak surface shear stress to the spatial average shear stress, are difficult to determine because m was found to be a function of the roughness density, the wind velocity and the roughness element shape. A new, more physically based parameter a referred to as the peak-mean stress ratio is suggested as a substitute for m which is solely a function of the roughness element shape. According to this, values for a are much easier to determine than values for m. As a result, a method to identify values for the new a-parameter for different kinds of roughness elements is presented.

Large-Eddy Simulation of Turbulent Boundary Layer Flow through Wind Turbines and Wind Farms

Yu-Ting Wu

In this thesis, we develop a new large-eddy simulation (LES) framework to simulate atmospheric boundary layer (ABL) flows through wind turbines and wind farms. A Lagrangian scale-dependent dynamic model is used to compute the Smagorinsky model coefficient dynamically based on the local dynamics of the resolved velocity field. The turbine-generated power outputs and the turbine-induced forces (e.g., thrust, lift, drag) are parameterized using two actuator-disk models: (a) the traditional actuator-disk model without rotation (ADM-NR), which uses 1D momentum theory to relate the power output and the uniform thrust force distribution with an incoming velocity; and (b) the actuator-disk model with rotation (ADM-R), which adopts blade element theory to calculate the lift and drag forces (that produce thrust, rotor shaft torque, and power) based on the local blade and flow characteristics. The performance of the LES framework is first evaluated through comparisons with high-resolution velocity measurements collected in the wind-tunnel experiments with the single wake of a stand-alone miniature wind turbine [Chamorro and Porté-Agel, 2010a] as well as the multiple turbine wakes in a model aligned wind farm [Chamorro and Porté-Agel, 2011]. Emphasis is placed on the structure and characteristics of the simulated turbine wakes using the ADM-NR and the ADM-R. In general, the ADM-R yields improved predictions compared with the ADM-NR in the wake regions, where the ADM-NR tends to underestimate the velocity deficit and the enhancement of turbulence intensity in the wakes. Next, the LES framework is used to model multiple turbine wakes and associated power losses in a large wind farm. Here, we propose a new dynamic procedure coupled with the ADM-R to predict turbine power output based on a turbine-model-specific relationship between the shaft torque and the blade angular speed. The Horns Rev offshore wind farm is chosen as a case study since both power data from eighty Vestas V80 wind turbines and flow data from three nearby meteorological masts were available [Barthelmie et al., 2009]. The torque-speed relationship for the V80 turbine is obtained from a series of stand-alone turbine simulations. In the wind farm simulations, the ADM-R gives the best power prediction. Finally, the validated LES framework is used to study atmospheric turbulence effects on wind-turbine wakes in neutrally-stratified ABL over homogeneous flat surfaces with four different aerodynamic roughness lengths. In this study, the simulation result shows that the different turbulence intensity levels of the incoming flow lead to considerable influence on the spatial distribution of the mean velocity deficit, turbulence intensity, and turbulent shear stress in the wake region. In particular, when the turbulence intensity level of the incoming flow is higher, the turbine-induced wake (velocity deficit) recovers faster, and the locations of the maximum turbulence intensity and turbulent stress are closer to the turbine.

EPFL2013

Wind Tunnel Experiments and Modelling of Particle-Laden Boundary Layer Flows

Michael Lehning, Benjamin Andreas Walter

Particle-laden boundary flows occur in many geophysical and industrial environments yet are difficult to understand and quantitatively describe because the interactions of an often turbulent boundary layer flow with surface and particle dynamics are complex. The SLF wind tunnel allows the investigation of snow and sand particle laden boundary layer flows over their natural surfaces with and without the presence of a grass canopy.The experimental results are used to investigate the two possible approaches in describing the surface dynamics: (i) Models of particle transport, which assume a stationary flow situation and predict a mass flux for an hypothetical equilibrium; (ii) Models that take the temporal variability of the mass flux explicitly into account such as Lagrangian Stochastic particle tracking Models (LSM) on the basis of large eddy simulation (LES) or direct numerical simulation (DNS) of flow and turbulence. This presentation shows that wind tunnel data support the form of semi-empirical equilibrium models, which predict mass flux, q, as a function of the mean wind speed or the friction velocity, u, and a threshold velocity, uth: q=a(u-uth)x. For the exponent "x", a value of approximately 3, as based on theoretical considerations, is consistent with the data. This simple form of equilibrium models as well as more complicated equilibrium models are all based on the hypothesis that the surface shear stress induced by a fluid on the ground during sediment saltation is constant, i.e. independent of the magnitude of the particle mass flux (Owen's second hypothesis). Our surface shear stress measurements in a drifting-sand wind tunnel show a constant value of the fluid shear stress for saltation layers of various mass-flux magnitudes, directly validating Owen’s second hypothesis for the first time. The equilibrium models, however, only insufficiently describe the full dynamics of particle-laden flows. The second part of the presentation therefore discusses non-equilibrium features such as a high variability of the particle mass flux caused by flow turbulence and surface heterogeneity. Mass flux intermittency is primarily observed around the threshold value uth. Using a combination of LES and LSM models, we show how the simulation of individual feed-back processes leads to a more complete understanding of the mechanisms behind the flux variability.

2013

Wind tunnel measurements of sediment mass fluxes in different dense live plant canopies

Michael Lehning, Benjamin Andreas Walter

Erosion, transport and deposition of soil or snow are processes that occur when the atmospheric turbulent boundary layer interacts with the ground. Vegetation on the ground is able to significantly modify these processes, usually resulting in reduced particle mass fluxes. Previous wind tunnel studies quantifying mass fluxes as a function of the wind speed mainly investigated bare sediment surfaces without vegeta-tion. However, studies investigating the sheltering effect of vegetation against wind erosion typically focused on the influence of the canopy density on the mass flux. We present a combination of both: Wind tunnel measurements of particle mass fluxes and vertical mass flux profiles in different dense live plant canopies as a function of the wind speed. The experiments were performed in the SLF boundary layer wind tunnel with an 8 m long test section and a cross section area of one square meter. Three canopies of different densities were investigated (0; 5 and 25 plants/m2). For the experiments, natural quartz sand was spread on the ground beneath the plants (ryegrass). A sediment sampler with 60 bins was used to measure the vertical mass flux profiles up to a height of 60 cm. The results are compared to previous measurements of the surface shear stress in similar canopies. First data analyses show an exponential decrease of the total mass flux Q as a function of the canopy density as found in many other studies. For the bare sand surface, Q shows the typical (uτ - uτ,th)3 increase with increasing wind speed or skin friction velocity uτ, where uτ,th is the threshold at the onset of particle erosion. Similar relations were found for the 5 and 25 plants/m2 setups, however, with significant differ-ences compared to the bare sand surface that can be explained by the strong spatial inhomogeneity of uτ for the canopy cases.

2013

Wind Tunnel Experiments and Modelling of Particle-Laden Boundary Layer Flows

Michael Lehning, Benjamin Andreas Walter

2013

Large-Eddy Simulation of Turbulent Boundary Layer Flow through Wind Turbines and Wind Farms

Yu-Ting Wu

EPFL2013