7/28/2019 MeteodynWT CFD Modeling Forest Canopy Flow Input Parameters Calibration Validation Wind Engineergin Modeling

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1. Kaimal, J.C., Finnigan, J.J. (1994).Atmospheric boundary layer flows their structure and measurement, Oxford

University Press, New York

2. Meteodyn WT technical documentation

3. European standard EN 1991-1-4 (April 2005), Eurocode 1 (EC1), part 1-4: Wind Actions

atmospheric boundary layer, perturbations induced by ground surfaces with

h roughness values, such as forests, generate a high level of turbulence and

ong wind shear. Thus the effect of forest must be taken into account in wind

gineering and wind energy assessment to better estimate the mean wind profile

d turbulence intensity.

nd flow simulations have been performed with CFD software Meteodyn WT,

ich allows introducing a custom forest canopy model. The main objectives of

study are:

nvestigating the influence of changes of model parameters on the results;

alibrating the CFD model according to the mean wind speed and turbulence

ensity given by the European standard EC1, applied on flat homogenous

rains;

hecking the accuracy of the CFD results by comparison with measurements on

eal site with heterogeneous forest covering.

e study is devoted to the CFD modeling of wind flows in presence of forest. This

ster shows the methodology of the simulation, as well as the calibration process

he forest canopy model implemented in the CFD software Meteodyn WT. To

idate the CFD model, the computed mean wind profile and turbulence intensity

e compared to met mast measurements on a real site.

Abstract

CFD modeling of forest canopy flows:

Input parameters, calibration and validationJIANG Zixiao1 , Mara BULLIDO-GARCIA2, Jean-Claude HOUBART2, Cline BEZAULT2

1Meteodyn China, 2Meteodyn France

PO. ID

208

Objectives

Methods

References

EWEA 2013, Vienna, Austria: Europes Premier Wind Energy Event

teodyn WT software is used. This CFD model solves the full RANS equations

h a turbulence closure scheme obtained by the prognostic equation on the

bulent kinetic energy (TKE), including an atmospheric turbulent length scale

proach for the dissipation term.

e perturbations induced by forests are modeled by including sink terms in the

mentum conservation equations for the cells lying inside the forested volumes.

e sink terms depend on the coefficient CD, a volume drag coefficient proportional

he forest density and depending on vertical leaf area profile.

ide the canopy, both the production term and dissipation term are enhanced,

s additional terms are added into the TKE equation.

e forest canopy height hc, and the depth of the roughness sublayerhRSL which is

region where the canopy directly impinges on the flow [1], can be expressed in

ction of the roughness length z0, the parameters kcand hadd[2]:

hRSL = hc+ hadd and hc= kcz0

ere hadd is the height of the additional dissipations zone.

rthermore, modifications of the mixing length are made inside the canopy and in

additional dissipations zone.

Fig 3. Elevation (left) and roughness (right) map of the site

Fig 1. Computed wind speed compared to

Eurocode profiles in the case z0= 1.0 m

Fig 6. Measured and computed wind speed, normalized by the wind

speed at 30 m. The relative error is less than 5% for most directions.

Fig 7. Measured and computed turbulence intensity at 50 m. The

error is less than 0.005 for most directions.

Forest Model Calibration

e default values of the model parameters used in the software are kc= 30, hadd=

m, and CD = 0.005. The calibration of these parameters was done by comparing

CFD results to the vertical profiles given by Eurocode 1 (EC1), part 1-4: Wind

tions (EN 1991-1-4:2005E). In case of standard homogeneous and flat terrains,1 gives formula to estimate the mean wind speed and turbulence intensity at

ferent heights from the ground [3].

th Meteodyn WT, tests have been carried out on standard flat terrains with high

ughness lengths: 0.2 m, 0.4 m, 0.6 m, 0.8 m and 1.0m. For each roughness

gth, the simulation was performed with different settings of the following

rameters: kc, haddand CD.

e numerical tests have led kc = 20, hadd = 15 m, and CD = 0.002. Compared to

1 profiles, the RMSE errors are 0.024 for normalized velocity, and 0.009 for

bulence intensity. According to this result, the current default value of the

rameters kchas been changed to the calibrated value 20 in Meteodyn WT, which

ans that the ratio between the tree height and the roughness value should be 20

better reproduce the EC1 profile on standard terrains.

an example, the computed wind speed and turbulence intensity profiles in the

se z0 = 1.0 m are shown in Fig 1 and 2, from 10 m to 200 m above ground, and

mpared with EC1 profiles. The wind speed is normalized with the reference windeed (10 m height over an open terrain of 5 cm roughness length)

Conclusions

Validation case

In order to check the accuracy of the calibrated forest model in complex terrains, a

validation case has been carried out. The site is located in the east of France. The

canopy heights vary from 4 m to 30 m. The closest distance between the met mas

and the forested zones is approximately 200 m.

The met mast is equipped with anemometers at three heights: 10 m, 30 m and 50

m. The measurement period is 3.2 years. A strong wind filter is applied to remov

the thermal effect: only the records with speed greater than 10 m/s at 30 m heigh

are considered, leading to a total number of strong wind records equal to 5 567.

Compared to the measured data, the error on

mean wind speed, taking into account all

directions, extrapolated from 30 m height, is

1.5% at 50 m and -4.7% at 10 m (Fig 5).

The directional comparisons are shown in

Fig 6 and 7. We can observe that the

measured wind shear and turbulence intensity

are not homogenous in different directions.That is caused by the heterogeneity of

orography and forest covering. The wind characteristics are especially affected by

the distance from the met mast to upstream forest, which varies with the direction.

Fig 4. Wind frequency rose at 30 m

height at the met mast

The forest canopy model implemented in the CDF software Meteodyn WT has bee

calibrated. The calibrated model shows good coherence with the standard EC1 fo

both mean wind speed and turbulence intensity.

The wind flow over a real site has been simulated. The wind characteristics at th

met mast are obtained with a CFD approach taking into account the heterogeneit

of the orography and the forest. The results show that the CFD modeling, with the

appropriate forest model, is able to reproduce the wind speed profile and

turbulence intensity in complex terrains with an acceptable accuracy.

Fig 5. Measured and computed mean

wind speed (strong wind record)

Fig 2. C omputed turbulence intensity compared to

Eurocode profiles in the case z0= 1.0 m

Height (m) 10 50

CFD (m/s) 9.9 12.5

Measurement (m/s) 10.3 12.3

Error -4.7% 1.5%