1 / 20

Javier Sanz Rodrigo Daniel Cabezón Ignacio Martí (CENER) 19-03-2009

Parameterization of the atmospheric boundary layer for offshore wind resource assessment with a limited-length-scale k-ε model. Javier Sanz Rodrigo Daniel Cabezón Ignacio Martí (CENER) 19-03-2009. Introduction. Motivation. On ABL parameterization: mixing length vs k- ε modeling .

morna
Download Presentation

Javier Sanz Rodrigo Daniel Cabezón Ignacio Martí (CENER) 19-03-2009

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Parameterization of the atmospheric boundary layer for offshore wind resource assessment with a limited-length-scale k-ε model Javier Sanz Rodrigo Daniel Cabezón Ignacio Martí (CENER) 19-03-2009

  2. Introduction • Motivation. • On ABL parameterization: mixing length vs k-ε modeling. • Mixing length parameterizations. • Verification of ABL solver on the Leipzig wind profile. • Validation with FINO-1 test case. • Conclusions. • Outlook.

  3. Motivation • OFFSHORE wind resource assessment characterized by: • Lack of measurements. • Very low roughness  important influence of thermal effects in the structure of the ABL. • Surface similarity theory (log-laws) appear to be usable above the surface layer only in neutral and unstable conditions. • STABLE atmospheric conditions are very frequent in the offshore environment, producing: • Extreme wind speed and direction shear  non-uniform wind loading. • Very low turbulence  long lasting wind turbine wakes. • It is necessary to model the entire ABL to consider stable conditions: from surface to geostrophic wind levels.

  4. On ABL parameterization • Assumptions: • Horizontally homogeneous conditions: d/dx0, d/dy0 • Hydrostatic: • 1D Momentum and Energy equations: Coriolis parameter Geostrophic Wind: Kinematic momentum and heat fluxes

  5. On ABL parameterization • Closure problem: turbulent viscosity formulation • Two unknowns: turbulent kinetic energy (k) and mixing length (lm) • Transport equation for k: • k-lm: parameterization of mixing length from ABL scales (u*,L,etc) • k-ε: mixing length indirectly obtained from turbulent dissipation rate ε, assuming ld=lm ABL flows:

  6. Mixing Length Parameterizations (k-lm) • Prognostic equation for the mixing length based on ABL scales: • u*0, u*: Friction velocity (surface or local) • z0: Roughness length • L0, L: Monin-Obukhov (surface or local) • zi: Boundary layer depth • Blackadar (1997): • Apsley and Castro (1971): • Delage (1972): • Gryning (2007): • Mahrt and Vickers (2003): (Stable) (Stable)

  7. Mixing Length Parameterizations (k-lm) • Comparison: NEUTRAL ABL • Agreement with surface layer scaling only in the first 5% • Main differences in the upper part of the ABL • Blackadar=Apsley • Mahrt and Delage models agree • Gryning model produces the largest mixing length

  8. Mixing Length Parameterizations (k-lm) • Comparison: UNSTABLE ABL • Agreement with surface layer scaling up to the first 20-30% • Blackadar=Apsley=Delage (neutral formulation) • Mahrt model between ‘neutral’ and Gryning formulation • Gryning model produces the largest mixing length

  9. Mixing Length Parameterizations (k-lm) • Comparison: STABLE ABL • Agreement with surface layer scaling only in the first 3% • Blackadar with stability function • Apsley with local L has similar performance to Mahrt and Delage models • Gryning model produces the largest mixing length with deeper influence of lmax

  10. Two equation closure (k-ε) • Standard k-ε model: • Produces a mixing length proportional to the height above the ground, i.e it is only valid in the surface boundary layer. • Too deep boundary layer due to too much mixing (turbulent viscosity) in the upper part of the ABL • Modifications to ε production term: • Detering and Etling (1985): • Apsley and Castro (1997): • Weng and Taylor (2003):

  11. Verification of ABL solver: Leipzig profile • Leipzig wind profile: Neutral ABL (z0=0.3m, u*=0.65m, Ug=17.5m) • Standard k-ε model produces a very deep ABL • Mixing-length and limited-length-scale k-ε model perform well

  12. Validation: FINO-1 (North Sea)

  13. FINO-1: MeteorologicalInstrumentation

  14. Stability distribution • Very low roughness: z0~0.2mm • Predominant non-neutral conditions. • STABLE ABL at all relevant wind speeds.

  15. Data screening • Objective: Clustering of wind profiles for stability classification according to flux M-O length at 80m. • Filtering of registers: • Open sea • Without mast distortions: 190º-250º • Velocity > 3m/s • Stationary test

  16. Preliminary Results: k-ε (Apsley and Castro) • Fitting with u*,Ug,L and lmax. • Too deep boundary layers require very low lmax (<10m) to increase wind shear and reduce turbulence intensity  Local scaling?

  17. Mixing length parameterization • Mixing length Mahrt and Vickers (2003) parameterization. • Stability function depends on local M-O length. • Low dependency with height. • FINO1: best fitting with b=7

  18. Mixing length parameterization • Mixing length Mahrt and Vickers (2003) parameterization. • Measured mixing length • FINO1: best fitting with p=3.5

  19. Conclusions and Outlook • ABL modeling requires parameterization of mixing length. • Preliminary results in offshore conditions indicate the K-εmodel produces too deep boundary layers. • k-lm model with Mahrt and Vickers parameterization looks promising. • Outlook: • Benchmarking of mixing length parameterizations. • Meso-CFD coupling: Study mesoscale databases, where geostropic winds and boundary layer depth are readily available.

More Related