Forest Winds in Complex Terrain Ilda Albuquerque 1 Contents Project Description Motivation Forest Complex Terrain Forested Complex Terrain 2
Project Description WAUDIT (Wind Resource Assessment Audit and Standardization), a Marie Curie Initial Training Network (ITN), funded under the EU FP7-People programme. Development state-of-the-art wind resource assessment methodologies Work Package 2.6 Forest winds in complex terrain, using computational fluid dynamics (CFD) Hosts: GL Garrad Hassan, CREST, CENER. 3 Motivation 4
Forest: Physical Description Semi-porous blockage to wind flow. Area of higher roughness. Physiological processes Mean velocity profile inflexion point Intermittent turbulent eddies Shear layer http://en.wikipedia.org 5 Forest & Canopy Models In flat terrain, above canopy region: Modified logarithmic profile using a zero displacement height (d) and surface roughness (z o ). o Determination of d and z o varies... Higher roughness Porous subdomain Porous subdomain combined with additional drag force terms in the momentum and turbulence equations Function of leaf drag area, drag coefficient and sheltering. 6
Complex Terrain: Physical Description Logarithmic profile not valid : Hills induce pressure gradients. Topography features lead to: wind speed acceleration, channelling and recirculation regions. http://www.aviationweather.ws 7 Complex Terrain: Current Model No standard models that predict wind conditions in complex terrain. Jackson and Hunt linear analysis of flow over low hills Momentum and mass conservation equations for incompressible, time-independent, neutrally stratified flow. Linearization of the momentum equations, assuming constant background flow in the horizontal but varied vertically. Vertical variation consisted in a division into separate layers where different balances are dominant: o Inner Surface (viscous forces dominate); o Upper Layer (inertial forces dominate); o Wake region. 8
Forested Complex Terrain: Description Sweep motions at the canopy top along the hill. Length scales imposed by a combination of outer layer eddy scale and hill scale pressure perturbations. Influence of drag within the canopy (where speeds are significantly reduced by canopy drag). Intermittent separation in the lee. The streamwise wind velocity upwind from the summit only correlated with the flow above the wake. The wake region the streamwise wind velocity at the canopy top is only correlated with the flow within the wake region. 9 Streamwise velocity profile in a 2D forested hill 10
Streamlines within canopy, in a 2D hill: (Finnigan) Upwind slope has a negative vertical pressure gradient : streamlines enter the canopy. Acceleration starts immediately behind the point where the pressure gradient become negative. Loss of inflexion point profile, due to rapid acceleration within canopy flow compared with the wind above canopy. Peak u(x) occurs before hill crest. At crest of the hill, inflexion point profile is shifted into the upper third of the canopy The parcels enter in the adverse pressure gradient area on the lee side of the hill, decelerating and maybe reversing direction. Wake region behind the hill: wind velocity reduced, flow capped by an elevated shear layer. 11 Forested Complex terrain: Models Challenges persist in modelling scales of turbulent motion in forested complex terrains. Diminished hill crest speed-up, canopy model versus simple roughness parameterisation.(finnigan) Models using a drag-force with a first order turbulence scheme and models using roughness-length approach: Able to simulate satisfactorily the mean flow field but the turbulence and momentum fluxes are overestimated. Main approaches in CFD currently are Reynolds Average Navier Stokes (RANS) and Large Eddy Simulations (LES). 12
Flow over trees: Large vortex structures depended on the pattern, size, and type of trees. Size, low time scale, intermittency: exclude the use of gradient diffusion models and steady state models. Flow over hills: Eddies formed in the lee of the hill have time and length scales much larger than those represented in a RANS model. LES simulations over hills with/without canopies show a coupling between intermittently separated flows in the lee of a hill with the outer-layer flow. In RANS models: there is no mechanism representing this type of connection between the inner and outer layers of the flow. 13 How to account for the unsteady phenomena using a steady model? The turbulent flows can be treat as steady if the flow statistics remain constant when calculated over a time period large enough; in this case the flow is called statistically steady. An improvement versus linear models An alternative to LES that has higher computational costs. 14
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