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Waving Wheat

Managed by

Data Provider: 

Ian Harman (CSIRO)

Data accesibility: 

The test case is offered to participants of the IEA Task 31 Wakebench who agree with the terms and conditions described below.

Site Description: 

The experiment is described in Brunet et al. (1994). It consists on single-point velocity statistics in a wind tunnel within and above a model of a waving wheat crop, made of nylon stalks 47 mm high and 0.25 mm wide, in a square grid of side 5 mm and frontal area index 0.47. This model was designed by Finnigan and Mulhearn (1978) with special attention to the aeroelastic properties of the crop. The canopy area density is A=10 m-1. The canopy drag coefficient Cd depends on the height. An average value of 0.68 can be assumed over the depth of the canopy, where drag = CdAU2.

A 50 mm fence at the beginning of the test section generates a deep turbulent boundary layer which develops over a 3 m long rough gravel surface, which raises progressively in order to match the estimated zero-plane displacement of the canopy (34 mm). The canopy then extends over 5.15 m in the streamwise direction occupying the full with of the tunnel. Downstream of the canopy, a raised gravel floor covers the rest of the working section.

Figure1: Sketch of the Waving Wheat experiment from Brunet et al. (1994). Dimensions in meters.

Instrumentation: 

The experiment was conducted in the Pye Laboratory wind tunnel (Wooding, 1968), CSRIO Black Mountain Laboratories, Camberra. The test section is 11 m long, 1.8 m wide and 0.65 m high. This facility enables the establishment of a zero pressure gradient boundary-layer (to within ±1 Pa/m locally and better over the full length of the tunnel).

The observations were taken using coplanar triple hot-wire anemometry, consisting of a vertically-oriented X-wire and a third vertical wire adjacent to it resulting in a lateral distance between outermost wires of 2.5 mm. A 1 KHz low-pass filter was used and the sampling period for each measuring position was 17 s. The free-stream velocity is measured with a Pitot tube.    

Measurements inside the canopy were made using a rectangular enclosure to protect the hotwire from damaging by flailing stalks.

Measurement Campaign: 

The dataset consists of the mean conditions and higher-order moments (up to third-order) of the flow over the waving wheat surface.   

Two probes were used simultaneously. Probe 1, situated at the origin of the coordinate system, was located on the tunnel centerline at 3.61 m from the leading edge of the canopy and could traverse vertically normal to the floor (along the z-axis). Probe 2 was mounted on a 3D traversing system and operated within a domain defined by -1.5 < x/h < 20 and -4 < y/h < 8. A vertical traverse consists of 12 levels at heights z/h = [1/6, 1/3, 2/3, 5/6, 1, 1.2, 1.5, 2, 3, 4, 6].

The tunnel was operated at a free-stream velocity of 10.2 m/s, resulting in a boundary layer depth of

0.5 ± 0.05 m.

The measurements in the lower canopy cannot be considered to be completely reliable because of the very high turbulence intensity levels, leading to flow reversals that are not well captured by the probes. However, the in-canopy measurements, up to 60% turbulence intensities, are expected to be a reasonable representation of reality, except at the very lowest levels.

Ensemble averaging over 71 runs of probe 2 measurements will be used to define the flow statistics. Hence, the mean vertical profiles are the result of a spatio-temporal average within a 21.5h times 6h domain. The profiles observed throughout this region do not show noticeable streamwise differences. This is also confirmed by very low mean vertical velocities of the order of ±0.02 m/s, within measurement errors. Hence, this indicates that the flow is in equilibrium and the profiles can be considered as horizontally homogeneous.

Remarks: 

The test case is suitable for the design of canopy models since it provides a fairly accurate description of the turbulent structure within and above the canopy in steady-state and horizontally homogeneous conditions.

The structure of the turbulent flow in horizontally homogeneous canopies have been the object of study of numerous experiments ranging from wind tunnel models to tall forests (Kaimal and Finnigan, 1994). All the profiles display a characteristic inflexion point near the canopy top which separates the canopy flow from the boundary layer profile above. A constant shear stress region is present in the free-stream which decreases rapidly as momentum is absorbed by the canopy. The momentum absorption is proportional to the product of the canopy drag coefficient (Cd) and the canopy-area-density (A).

Extensive literature can be found about the modeling of canopy flows. Wilson et al. (1998) mention how to simulate the vertical gradient of Reynolds stress above the canopy by introducing an effective pressure gradient defined from the measured shear stress profile. This was used by Sogachev and Panferov (2006) to simulate the Waving Wheat test case with a RANS k-w turbulent model, leading to good agreement in the mean velocity, shear stress and turbulent dissipation profiles.

References: 

Brunet Y., Finnigan J.J., Raupach M.R., 1994, A Wind Tunnel Study of Air Flow in Waving Wheat: Single-Point Velocity Statistics,Boundary-Layer Meteorol.70: 95-132

Finnigan J.J., Mulhearn P.J., 1978, Modelling Waving Crops in a Wind Tunnel,Boundary-Layer Meteorol.14: 253-277

Kaimal J.C., Finnigan J.J., 1994, Atmospheric Boundary Layer Flows: Their Structure and Measurement, Oxford University Press, pp. 304

Sogachev A., Panferov O., 2006, Modification of two-equation models to account for plant canopy,Boundary-Layer Meteorol.121: 229-266

Wilson J.D., Finnigan J.J., Raupach M.R., 1998, A first-order closure for disturbed plant-canopy flows, and its application to winds in a canopy on a ridge,Q. J. R. Meteorol. Soc.124: 705-732

Wooding, 1968, A low-speed wind tunnel for model studies in micrometeorology, II The Pye Laboratory Wind Tunnel,CSIRO Div. Plant Industry Tech25: 25-39, available from CSIRO Marine and Atmospheric Research, FC Pye Laboratory, Camberra, ACT, Australia

NDA: 

Interested participants will have to subscribe to the attached data licensing agreement agreed between CENER and CSIRO. Please send two signed copies to: Javier Sanz Rodrigo, Calle Ciudad de la Innovación 7, 31621-Sarriguren, Spain
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