Large-Eddy Simulation of Coherent Turbulence Structures Associated with Scalar Ramps Over Plant Canopies
Tóm tắt
Large-eddy simulations were performed of a neutrally-stratified turbulent flow within and above an ideal, horizontally- and vertically-homogeneous plant canopy. Three simulations were performed for shear-driven flows in small and large computational domains, and a pressure-driven flow in a small domain, to enable the nature of canopy turbulence unaffected by external conditions to be captured. The simulations reproduced quite realistic canopy turbulence characteristics, including typical ramp structures appearing in time traces of the scalar concentration near the canopy top. Then, the spatial structure of the organised turbulence that caused the scalar ramps was examined using conditional sampling of three-dimensional instantaneous fields, triggered by the occurrence of ramp structures. A wavelet transform was used for the detection of ramp structures in the time traces. The ensemble-averaged results illustrate that the scalar ramps are associated with the microfrontal structure in the scalar, the ejection-sweep structure in the streamwise and vertical velocities, a laterally divergent flow just around the ramp-detection point, and a positive, vertically-coherent pressure perturbation. These vertical structures were consistent with previous measurements made in fields or wind tunnels. However, the most striking feature is that the horizontal slice of the same structure revealed a streamwise-elongated region of high-speed streamwise velocity impacting on another elongated region of low-speed velocity. These elongated structures resemble the so-called streak structures that are commonly observed in near-wall shear layers. Since elongated structures of essentially similar spatial scales were observed in all of the runs, these streak structures appear to be inherent in near-canopy turbulence. Presumably, strong wind shear formed just above the canopy is involved in their formation. By synthesis of the ensemble-averaged and instantaneous results, the following processes were inferred for the development of scalar microfronts and their associated flow structures: (1) a distinct scalar microfront develops where a coherent downdraft associated with a high-speed streak penetrates into the region of a low-speed streak; (2) a stagnation in flow between two streaks of different velocities builds up a vertically-coherent high-pressure region there; (3) the pressure gradients around the high-pressure region work to reduce the longitudinal variations in streamwise velocity and to enhance the laterally-divergent flow and lifted updrafts downstream of the microfront; (4) as the coherent mother downdraft impinges on the canopy, canopy-scale eddies are formed near the canopy top in a similar manner as observed in conventional mixing-layer turbulence.
Tài liệu tham khảo
BergstrÖm, H. and HÖgstrÖm, U.: 1989, ‘Turbulent Exchange Above a Pine Forest. II. Organized Structures’, Boundary-Layer Meteorol. 49, 231–263.
Collineau, S. and Brunet, Y.: 1993a, ‘Detection of Turbulent Coherent Motions in a Forest Canopy. Part I: Wavelet Analysis’, Boundary-Layer Meteorol. 65, 357–379.
Collineau, S. and Brunet, Y.: 1993b, ‘Detection of Turbulent Coherent Motions in a Forest Canopy. Part I: Time-Scales and Conditional Averages’, Boundary-Layer Meteorol. 66, 49–73.
Deardorff, J. W.: 1970, ‘A Numerical Study of Three-Dimensional Turbulent Channel Flow at Large Reynolds Numbers’, J. Fluid Mech. 41, 453–480.
Finnigan, J. J.: 1979, ‘Turbulence in Waving Wheat. II. Structure of Momentum Transfer’, Boundary-Layer Meteorol. 16, 213–236.
Finnigan, J. J.: 2000, ‘Turbulence in Plant Canopies’, Annu. Rev. Fluid Mech. 32, 519–571.
Finnigan, J. J. and Shaw, R. H.: 2000, ‘A Wind-Tunnel Study of Airflow in Waving Wheat: An EOF Analysis of the Structure of the Large-Eddy Motion’, Boundary-Layer Meteorol. 96, 211–255.
Gao, W., Shaw, R. H., and Paw U, K. T.: 1989, ‘Observation of Organized Structure in Turbulent Flow Within and Above a Forest Canopy’, Boundary-Layer Meteorol. 47, 349–377.
Inoue, E.: 1955a, ‘Studies of the Phenomena of Waving Plants (“HONAMI”) Caused by Wind. Part 1: Mechanism and Characteristics of Waving Plants Phenomena’, J. Agric. Meteorol. (Japan) 11, 18–22 (in Japanese with English summary).
Inoue, E.: 1955b, ‘Studies of the Phenomena of Waving Plants (“HONAMI”) Caused by Wind. Part 2: Spectra of Waving Plants and Plants Vibration’, J. Agric. Meteorol. (Japan) 11, 87–90 (in Japanese with English summary).
Inoue, E.: 1963, ‘On the Turbulent Structure of Airflow within Crop Canopies’, J. Agric. Meteorol (Japan) 41, 317–326.
Kaimal, J. C. and Finnigan, J. J.: 1994, Atmospheric Boundary Layer Flows, Oxford University Press, Oxford, 289 pp.
Kanda, M. and Hino, M.: 1994, ‘Organized Structures in Developing Turbulent Flow Within and Above a Plant Canopy, Using a Large-Eddy Simulation’, Boundary-Layer Meteorol. 68, 237–257.
Lele, S. K.: 1992, ‘Compact Finite Difference Schemes with Spectral-Like resolution’, J. Comput. Phys. 103, 16–42.
Lin, C.-L., McWilliams, J. C., Moeng, C.-H., and Sullivan, P. P.: 1996, ‘Coherent Structures and Dynamics in a Neutrally Stratified Planetary Boundary Layer Flow’, Phys. Fluid 8, 2626–2639.
Lu, C.-H. and Fitzjarrald, D. R.: 1994, ‘Seasonal and Diurnal Variations of Coherent Structures over a Deciduous Forest’, Boundary-Layer Meteorol. 69, 43–69.
Moeng, C.-H.: 1984, ‘A Large-Eddy-Simulation Model for the Study of Planetary Boundary-Layer Turbulence’, J. Atmos. Sci. 41, 2052–2062.
Moeng, C.-H. and Sullivan, P. P.: 1994, ‘A Comparison of Shear-and Buoyancy-Driven Planetary Boundary Layer Flows’, J. Atmos. Sci. 51, 999–1022.
Moeng, C.-H. and Wyngaard, J. C.: 1988, ‘Spectral Analysis of Large-Eddy Simulations of the Convective Boundary Layer’, J. Atmos. Sci. 45, 3573–3587.
Moin, P. and Kim, J.: 1982, ‘Numerical Investigation of Turbulent Channel Flow’, J. Fluid Mech. 118, 341–377.
Orszag, S. A.: 1971, ‘On the Elimination of Aliasing in Finite-Difference Schemes by Filtering High-Wavenumber Components’, J. Atmos. Sci. 28, 1074.
Patton, E. G., Shaw, R. H., Judd, M. J., and Raupach, M. R.: 1998, ‘Large-Eddy Simulation of Windbreak Flow’, Boundary-Layer Meteorol. 87, 275–306.
Raupach, M. R. and Thom, A. S.: 1981, ‘Turbulence In and Above Plant Canopies’, Ann. Rev. Fluid Mech. 13, 97–129.
Raupach, M. R., Finnigan, J. J., and Brunet, Y.: 1996, ‘Coherent Eddies and Turbulence in Vegetation Canopies: The Mixing Layer Analogy’, Boundary-Layer Meteorol. 78, 351–382.
Schumann, U.: 1975, ‘Subgrid Scale Model for Finite Difference Simulations of Turbulent Flows in Plane Channels and Annuli’, J. Comput. Phys. 18, 376–404.
Shaw, R. H. and Patton, E. G.: 2003, ‘Canopy Element Influences on Resolved-and Subgrid-Scale Energy within a Large-Eddy Simulation’, Agric. For. Meteorol. 115, 5–17.
Shaw, R. H. and Schumann, U.: 1992, ‘Large-Eddy Simulation of Turbulent Flow Above and Within a Forest’, Boundary-Layer Meteorol. 61, 47–64.
Shaw, R. H. and Zhang, X. J.: 1992, ‘Evidence of Pressure-Forced Turbulent Flow in a Forest’, Boundary-Layer Meteorol. 58, 273–288.
Shaw, R. H., Brunet, Y., Finnigan, J. J., and Raupach, M. R.: 1995, ‘A Wind Tunnel Study of Air Flow in Waving Wheat: Two-Point Velocity Statistics’, Boundary-Layer Meteorol. 76, 349–376.
Shaw, R. H., Paw U, K. T., and, Gao W.: 1989, ‘Detection of Temperature Ramps and Flow Structures at a Deciduous Forest Site’, Boundary-Layer Meteorol. 47, 123–138.
Shaw, R. H., Paw U, K. T., Zhang, X. J., Gao, W., Den Hartog, G., and Neumann, H. H.: 1990, ‘Retrieval of Turbulent Pressure Fluctuations at the Ground Surface Beneath a Forest’, Boundary-Layer Meteorol. 50, 319–338.
Su, H.-B., Shaw, R. H., Paw U, K. T., Moeng, C.-H., and Sullivan, P. P.: 1998, ‘Turbulent Statistics of Neutrally Stratified Flow Within and Above a Sparse Forest from Large-Eddy Simulation and Field Observations’, Boundary-Layer Meteorol. 88, 363–397.
Su, H.-B., Shaw, R. H., and Paw U, K. T.: 2000, ‘Two-Point Correlation Analysis of Neutrally Stratified Flow Within and Above a Forest from Large-Eddy Simulation’, Boundary-Layer Meteorol. 49, 423–460.
Yasuda, Y. and Watanabe, T.: 2001, ‘Comparative Measurements of CO2 Flux Over a Forest Using Closed-Path and Open-Path CO2 Analysers’, Boundary-Layer Meteorol. 100, 191–208.
Williamson, J. H.: 1980, ‘Low-Storage Runge-Kutta Schemes’, J. Comput. Phys. 35, 48–56.
Zhuang, Y. and Amiro, B. D.: 1994, ‘Pressure Fluctuations during Coherent Motions and their Effects on the Budgets of Turbulent Kinetic Energy and Momentum Flux within a Forest Canopy’, J. Appl. Meteorol. 33, 704–711.