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ValidWind™: Improved Ground-Based Remote Sensing of Low Altitude Wind Profiles

Discover the ValidWind concept, a small balloon tracer with advanced techniques and results for ground-based remote sensing of wind profiles. Explore its applications, intrinsic accuracy, aerodynamics, data processing, and validation campaigns.

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ValidWind™: Improved Ground-Based Remote Sensing of Low Altitude Wind Profiles

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  1. ValidWind™: Improved Techniques and Results for Ground-Based Remote Sensing of Low Altitude Wind Profiles Working Group on Space-Based Lidar Winds Destin, Florida February 3, 2010 Tom Apedaile Bill Bradford Alan Marchant Danny Scholes Tom Wilkerson ™ USU Research Foundation, 2009

  2. The ValidWind Concept • small balloon tracer • 11” He-filled latex • retroreflector tape • laser rangefinder • built-in inclinometer • integrated compass • bluetooth data link • 3D balloon trajectory • trajectory analysis S/W • ValidWind capabilities: • horizontal range 2km • altitude range 1km • velocity accuracy 1% at all wind speeds • direction accuracy 1º for v > 1m/s • profile resolution 20m • profiling rate 5 – 10 min

  3. Presentation Outline • The ValidWind Concept • Applications for Local Wind Profiling • Intrinsic Accuracy and Aerodynamics • Data Processing • Validation Campaigns • System Improvements

  4. Wind Turbine Micrositing • proposed wind turbine site at the mouth of Logan canyon • supplementary power for USU campus • exploit canyon drainage wind • campaign results: • nocturnal jet develops from the bottom up, then decays coherently • ideal turbine height ~ 100m (unobtrusive) • jet duration 11 hours (winter evaluation needed) Aug 19 Aug 20 time of day (MDT)

  5. Terrain-Induced Wind Field Clarkston ridge, West of Logan 9/19/09 wind vector field constructed from multiple balloon trajectories v > 7 m/s v < 7 m/s altitude w.r.t. sensor (m) leeward terrain horizontal distance (m) NE • wind field cross-section projected to plane 52 from North (the prevailing wind direction on 9/19/09) • strong updraft at the ridgeline transitions to a strong leeward downdraft • horizontal velocity steady at 7.8 ± 1.3 m/s • vertical flow shifts dramatically from +1.6 to -3.5 m/s • preferred turbine location may be slightly leeward

  6. Air Quality Campaign Danish Flats campaign. Air-quality campaign at Danish Flats treatment facility, 10/22-26/09 • ValidWind used to monitor wind profiles. • Wind data coordinated with other instruments to assess fluxes. • Wind data Balloon flights coordinated to provide profiles over the facility. • Good agreement with a 15m tower. • Comparison w/ Remtech sodar. • average speed and direction ok • sodar yields poor profile accuracy ValidWind profiling was performed over or near the processing facility.

  7. History of Balloon Tracers • PIBAL – Pilot Balloon • First described ~1872. • Developed to check upper level winds before manned balloon flights. • Passive balloon. • Direction measured by theodolite. • Range based on a typical rate of ascent. All of these balloons have limited advection accuracy. • Rawinsonde • Weather balloon with telemetry. • Trajectory traced by GPS. • Flights may last hours and extend above the troposphere. • Jimsphere • Metalized balloon, tracked by radar. • Intentionally roughened to minimize “lift instabilities.” • “Standard for upper level wind measurement.”

  8. ValidWind Intrinsic Accuracy • Balloon motion relative to the wind. • CD, Re, terminal velocity • Lift-induced perturbations. • Transient slip.

  9. Aerodynamics of a Sphere • Aerodynamic properties scale with Reynolds number: • where U = velocity (rate of rise) and D = diameter. • For ValidWind, D  13”, Vz ~ 2 m/s, Re ~ 44,000. • For weather balloons, D  2m, Vz ~ 7 m/s, Re ~ 900,000. • The drag coefficient is a function of Re defined by: transition from laminar to turbulent boundary layer subcritical flow CD = 0.4 – 0.5 for all Re of interest supercritical flow

  10. Balloon Terminal Velocity Terminal rate of rise is reached when the drag force cancels the loft: or For the nominal ValidWind parameters, Vz = 2.0 m/s. Observation: trajectory-averaged rate of rise • Loss of He reduces the loft and the expected Vz. (Latex is leaky.) • Up/down drafts contribute variability. • The results are consistent with the spherical balloon model.

  11. Lift-Induced Perturbations “Lift” is an aerodynamic force normal to motion through the fluid. The lift coefficient is analogous to the drag coefficient: Experiments and simulation show CL fluctuating with an amplitude 0.05 – 0.1 for subcritical flow. Fluctuations in CD are much smaller. The typical fluctuation frequency is with a Strouhal number S ~ 0.1 – 0.2. For ValidWind, the fluctuation frequency is 0.5 – 1 Hz. Trajectory perturbations are estimated by integrating the equation of motion: where m’ is the balloon inertial mass plus the aerodynamic “added mass.” For ValidWind, with worst-case values of CL and S, the rms velocity perturbation is sV < 0.2 m/s. This is strongly damped by trajectory averaging. For super-critical flow, CL is somewhat larger. More importantly, CL develops significant power at low frequencies.

  12. Transient Slip • How does the balloon respond to a large velocity offset? • DV with respect to the vertical terminal velocity • balloon launched, starting from rest • balloon passes through an abrupt sheer layer • Equation of motion: • ignoring perturbations due to lift and turbulence • co-moving coordinate system • Solution for transverse motion: • where q atan(V/Vz) and • Transient decay is approximately exponential with a time constant of order 1s. • For DV = 2 m/s, total slip is < 1m. It’s very hard to throw a balloon.

  13. Data Processing - Flow compass inclinometer rangefinder timetag flight reset • Raw Data collected by MatLab script pre-flight trial dust • Parse Data remove false readings • . . . e.g. rangefinder failures, background or foreground interference, telemetry errors. East North AGL Dt • Convert Data to Cartesian coordinates • accommodate asynchronous trajectory sampling • low-pass temporal filter with a uniform scale • smooth the trajectory and estimate velocity in a single step (minimize processing noise) • accommodate wind shear • Filter Data w.r.t. time & Fit Velocity Vector

  14. GQLF Trajectory Filter • Gaussian-weighted Quadratic Least squares Filter. • A type of LOESS (locally estimated scatterplot smoothing) filter. • MatLab m-file: gqlf(t, ti, X, sigma) • {X} are measurements of a cartesian coordinate corresponding to {t}. • {ti} are the evaluation times, bounded by {t}. • The weighting function is exp(-Dt2/2sigma2). s = 10s, typical • Returns NaN if data density is too low (less than 3 data points within ti±3s). • Return values: • qF.b0 profile estimate (at each value ti) balloon trajectory • qF.b1 slope estimate (dX/dt) velocity profile • qF.b2 2nd derivative estimate (d2X/dt2) ~ wind shear  vz • qF.errweighted fit error (indicates pointwise quality of b0) • Inputs need not be uniformly spaced. • Filter resolution is uniform all along the trajectory. • For ValidWind vector profiles, repeat GQLF for x, y, and z.

  15. Filtered Trajectory Danish Flats 10/26/09, 10:39 am MDT GQLF filter with s = 10s insufficient sample rate altitude (m, AGL) North East

  16. Residual Errors Flight #1, 10/26/09, Danish Flats • residual errors typically 1 - 2m, consistent with sensor precision and pointing repeatability • errors due to pointing increase with range (r x dq) • velocity uncertainty from a 2m residual is < 0.1m/s

  17. Validation Campaigns Boulder Atmospheric Observatory 300m met tower Sept 30, 2009 (Dan Wolfe, NOAA) miniMOPA Doppler Lidar Sept 29, 2009 (Alan Brewer et al., NOAA)

  18. BAO Comparisons wind speed (m/s) ValidWind vs. BAO: -0.25s ± 1.4s ValidWind results at 100m vs. BAO anemometer at 100m time (hours, MDT) similar resulst at 50, 150, & 200m wind source direction (deg) ValidWind vs. BAO: 28º ± 1.0s • wind velocities are consistent • wind direction offset  calibrate ValidWind compass

  19. miniMOPA Comparison • excellent qualitative agreement • wind directions • wind shift event at 23:00 UTC • wind profile features • arrows show wind direction; colors show wind speed

  20. miniMOPA - Agreement • quantitative agreement in the early afternoon and evening • wind velocities • profile features • lidar shows a homogeneous, area-wide wind field

  21. miniMOPA - Disagreement map of Doppler residuals over the ValidWind site • Agreement fails when: • wind shift event at 23:00 UTC • lidar shows strong wind field inhomogeneity • mMOPA verifies a local Easterly wind component

  22. ValidWind Enhancements compass tracking camera rangefinder motorized gimbal • Automatic balloon tracking • increased tracking accuracy • increased sample frequency • eliminate human limitations on balloon tracking • Real-time data processing. • manage the video tracker • automatic data parsing • field displays of trajectories and wind profiles

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