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Program Objectives

Flow DynamicsVarious Flow Dynamics occurring in Perdigão. Courtesy: Joe Fernando

The experiment focused on a number of science issues. For each one, a set of hypotheses was developed and approaches to test them were outlined.

Lead: University of Notre Dame
Co-Lead: University of Colorado

Scientific Questions

The list below summarizes the science questions that are being addressed by the individual science teams.

  1. Multi-scale flow interactions in complex terrain

    Lead: Fernando
    Participants: Notre Dame Group
    Learn more about our research into multi-scale flow interactions in complex terrain

  2. Influence of terrain heterogeneity

    Lead: Fernando
    Participants: Notre Dame Group
    Learn more about our research into the influence of terrain heterogeneity

  3. Gap flows

    Lead: Fernando
    Participants: Notre Dame Group
    Learn more about our research into Gap flows

  4. Transitions and diurnal cycle of the atmospheric boundary layer, and interactions between valley flows and boundary layer flow above

    Lead: Klien
    Participants: University of Oklahoma

  5. Impacts of surface inhomogeneity

    Lead: Barthelmie
    Participants: Cornell University

  6. Flow-turbine interactions and wake flows

    Lead: Barthelmie
    Participants: Cornell University

  7. Inflow, flow-turbine interaction, wake flow

    Lead: Gerz
    Participants: DLR

  8. Modeling

    Lead: Palma
    Participants: UPORTO

  9. Weather-dependent sound patterns around a wind turbine

    Lead: Gerz
    Participants: DLR

  10. Intermittent turbulence and turbulence dissipation rate measurements

    Lead: Lundquist
    Participants: Colorado University Boulder

  11. Flow-turbine interactions, especially interaction of wake with coherent structures

    Lead: Ludquist
    Participants: Colorado University Boulder

  12. Mesoscale-microscale modeling

    Lead: Chow
    Participants: University of California Berkley

  13. Wind energy resource estimation by measurements and models

    Lead: Mann

Scientific question: Multi-scale flow interactions in complex terrain

Hypotheses: Local circulation in the valley represents complex interactions between thermal circulation, regional flow and synoptic forcing. Thus, local flow is highly variable in space and time, depending on the strengths of each contributor and interactions thereof. Turbulence and mixing in the valley (and hence eddy-coefficients) are also highly variable over a range of scales.

Approach:

  1. Characterize synoptic, regional and mesoscales, including forcing such as pressure gradients and sea surface temperature (site is ~ 100 km from the coast);
  2. Measure local conditions and their evolution at selected sites (i.e. flow collision, interaction and flow distortion hotspots, identified intuitively or long-term monitoring) at high space-time resolution, including turbulence and fluxes;
  3. Identify processes and phenomena – shear instabilities, internal waves, slope flows, flow collisions – at locations in (b) via scanning Lidars, IR imaging, remote and in-situ sensing, and relate their appearance to local conditions (e.g. dimensionless parameters);
  4. Delineate physical mechanisms and interactions thereof, identify their space-time variability and parameterize relevant property fluxes;
  5. Demarcate appearance of various flow regimes as functions of (suitably scaled) property footprints.

Implications: Strong interactions of synoptic and thermally/mechanically induced flows occur at micro- or smaller scales (Sturman et al. 2003; Fernando et al. 2015), but their flux footprints are unknown. Careful multi-scale observations are invaluable for improving microscale models (Landberg et al. 2003; Liu et al. 2011).

Scientific question: Influence of terrain heterogeneity

Evolution of Wind, Temperature and Humidity Structure in the Valley is discussed below:

Hypotheses: While the assumption of an idealized two-dimensional valley within two parallel ridges, as in Perdigão, is a reasonable first step in modeling, the natural variability of topography and land use can greatly modify the flow within the valley and over the ridges. In particular, the presence of a simple “gap” in one ridge can generate secondary circulation, jetting, interacting shear layers and cross-slope flows that modify the flow and turbulence over a certain spatial extent – which is determined by the overall topography, topographic anomalies (gaps), approach flow and stability.

Approach:

  1. Characterize the approach flow, background stability and orography of the gaps;
  2. Verify Jackson & Hunt (1975) framework for low slope angles (i.e. h/L < 0.1) and no gap areas of the fore mountain at neutral stability and explain discrepancies;
  3. Map 3D velocity and turbulence fields in the vicinity of the gap and away from it at high resolution, identify and explain the differences;
  4. Measure the pressure field, local circulation, separated flow, flow structures, secondary circulation and turbulence at selected locations at high space-time resolution;
  5. Measure coherent structures at the ridge shear layers and gap-separated flows and estimate related momentum transports
  6. Identify the length scales of flow distortions, both vertical and horizontal, at the gap and away from it,
  7. Quantify internal wave radiation under stable conditions and upslope flow separation under unstable conditions at gap area and away from it as a function of flow and stratification parameter
  8. Study how the results of (a–f) depend on atmospheric stability (stable, unstable or neutral).

Implications: Topographic inhomogeneities of microscales are known to substantially modify both thermally and synoptically driven flows (Rotach & Zardi 2007), which cannot be captured by mesoscale models. The spatial extent and magnitude of this modification, as well as possible unsteady phenomena such as vortex shedding, is of importance in wind turbine siting and operations (Fesquet et al. 2009).

Scientific Question: Gap Flows

Hypotheses: The presence of a gap along the ridge leads to local flow distortions, wherein the flow converges, accelerates and jets out of the gap, which interacts with the valley as well as atmosphere aloft. Shear layers at the exit lead to high turbulence levels and enhanced vertical and horizontal mixing.

Approach: A gap in the ridge leads to a variety of phenomena governed by the additional parameters, gap width/height and gap length (streamwise)/height. Determining these phenomena are the balances of forces - inertia, pressure gradient, friction and buoyancy - in the gap (Baines 1987). Numerical simulation of flow at the Perdigão site using the VENTOS® model developed at Porto University (an EU Partner) shows that it is not possible to expect 2D flow near the gap, independent of the cross-wind direction. For winds from the SW, air appears to stall at a low-point (gap) of the downwind ridge even though it is only about 20m lower. Not surprisingly, a region of low winds, high turbulence and recirculation develops in the valley between the ridges, which persist to well above the leeward ridge top (interestingly, this is a location selected for future Vale Cobrão wind turbines!). Flow separation at the gap edges may generate highly unsteady flow structures in time dependent simulations. The simulations do not account for buoyancy effects, which are expected to change the flow character drastically (Mayr et al. 2007). No high-resolution field data are available for gaps, and we will use WindScanner and suitably located ND Triple Lidar to investigate flow structures and turbulence at the entrance/exit areas of the gap and the interactions of valley and gap flows.

Implications: The results will help parameterize subgrid process associated with gaps. High turbulence and unsteadiness are some of the effects expected, and this work will map such processes and will develop parameterizations to describe their effects.