wind energy resource evaluation in a site of central italy ... - WindSim

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As it can be seen from figures number 2.5 and 2.6, the increase of the heightdistribution factor seems to be not responsible of the variation of the verticalprofiles. Anyhow, a better accuracy of the results is obtained, as a general rule, forlow values of the HDF, which moves the vertical central cell points in a region closerto the ground, where velocity gradients are higher.Figure 2.7: Zoom of the figure 2.5.Figure 2.6 shows that the three curves don’t overlap completely, as it can be seenalso from the zoom in the further figure 2.7.The phenomenon shown in figure 2.7 is very important, and must be taken intoaccount when working with wind data measured at low height respect to the groundlevel. So it’s reasonable to run simulations with low values of height distributionfactor when a particular precision near the ground level is needed, while insituations regarding the evaluation of the wind resources at high levels thisphenomenon becomes less marked. The experience shows, moreover, that theheight distribution factor cannot be decreased indefinitely, but its value is strictlyconnected to the resolution of the horizontal grid. Then, if it is necessary to havegood precision in such low heights, also the dimension of the horizontal cells must21
e decreased, to avoid to build thin and long cells near the ground, and also avoidconnected divergence problems of the solver.Finally, another consideration must be done when varying the vertical discretizationgrading: according to Crasto G. [1], the height of the first cell near the ground shouldbe at least double the height of the roughness height.2.5 Influence of the top boundary conditionTo determine the most realistic boundary condition at the top of the domain, able todescribe the atmospheric boundary layer development over a flat terrain, two seriesof simulations have been run. In the first series the zero relative pressure wasassigned, while in the second series the wall with no friction condition was assigned.The zero relative pressure boundary condition is an outflow boundary conditionwhere the pressure is set to the atmospheric value and the velocity is evaluatedfrom the internal side of the calculation domain.The wall with no friction BC fixes a null value for the vertical component of thevelocity while the other velocity component and the pressure are calculated. This BCbehaves like a physical wall where the viscous tangential forces are equal to zero.Each series of simulations have been run for 3 different roughness values, i.e. R=0.01(corresponding to a flat area with very short grass), R=0.3 (a countryside with fewtrees and farms) and finally R=0.6 (typical value for a forest). For all the simulationsthe number of cells in the vertical direction was set to 20.The following figures show the results of the simulations, particularly the windprofile of the atmospheric boundary layer.22
Page 1 and 2: UNIVERSITÀ DEGLI STUDI DI CAGLIARI
Page 3 and 4: IndexIndex page IAcknowledgmentsVII
Page 5 and 6: CHAPTER 4: Characteristics of the s
Page 7 and 8: 6.7.1 Mean wind velocity page 1096.
Page 9 and 10: This thesis is the result of a prac
Page 11 and 12: WindSim is a software developed by
Page 13 and 14: CHAPTER 1. Atmospheric Boundary Lay
Page 15 and 16: Figure 1.1: Example of an atmospher
Page 17 and 18: 1.5 The WindSim numerical codeWindS
Page 19 and 20: (κ−ε RNG or the Low Reynolds),
Page 21 and 22: 1.5.4 RESULTSThe Results module all
Page 23 and 24: Figure 1.2: Example of TKE vertical
Page 25 and 26: CHAPTER 2. Simulations over flat te
Page 27 and 28: inlet position (0 meters), station
Page 29: 2.4 Height distribution factor sens
Page 33 and 34: Figure 2.10: Vertical profiles for
Page 35 and 36: CHAPTER 3. Simulations over 2D hill
Page 37 and 38: Table 3.1: Measurement coordinatesS
Page 39 and 40: The results shown above demonstrate
Page 41 and 42: Figure 3.6: Sketch of the 15 km lon
Page 43 and 44: Figure 3.9: Influence of the height
Page 45 and 46: Figure 3.13: Zoom of the hill top r
Page 47 and 48: 3.5.2) = ∙.The two models for att
Page 49 and 50: H [m] hill heightν[m 2 /s] air cin
Page 51 and 52: Figure 3.18: Particular of the nume
Page 53 and 54: Finally, the horizontal x velocity
Page 55 and 56: Figure 3.25: Z velocity componentFi
Page 57 and 58: 3.6 RemarksA CFD study of a boundar
Page 59 and 60: Some experimental wind data of this
Page 61 and 62: Both the pictures 4.3 and 4.4 have
Page 63 and 64: Table 4.2: Description of the color
Page 65 and 66: In this periods, data are recorded
Page 67 and 68: In the following pages, a short des
Page 69 and 70: 4.3.2 Mast 5424-15 m Period April-M
Page 71 and 72: 4.3.4 Mast 5424-15 m Period Novembe
Page 73 and 74: 4.3.6 Mast 5428-40 m Period April-M
Page 75 and 76: 4.3.8 Mast 5428-50 m Period Novembe
Page 77 and 78: 4.3.10 Mast 5428-30 m Period Novemb
Page 79 and 80: 4.4.2 Mast 542870
Page 81 and 82:
4.5.2 Mast 542872
Page 83 and 84:
CHAPTER 5. Test of the Free Stream
Page 85 and 86:
case the wind is blowing from the e
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Geometrical modelTable 5.3: Input p
Page 89 and 90:
Figure 5.6: Vertical profiles in po
Page 91 and 92:
The orthogonalization of the grid h
Page 93 and 94:
Figure 5.9: Map of speed-ups for di
Page 95 and 96:
Figure 5.30: Speed-2D at 30 m heigh
Page 97 and 98:
Figure 5.12: Speed-2D at 30 m heigh
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CHAPTER 6. Analysis of the wind fie
Page 101 and 102:
Table 6.2: Boundary conditions and
Page 103 and 104:
6.5 Vertical profiles of VelocityTo
Page 105 and 106:
characterized by high wind speed (5
Page 107 and 108:
From the error analysis, it is foun
Page 109 and 110:
Anyhow, the trend of the linear int
Page 111 and 112:
Concerning the CFD simulations, val
Page 113 and 114:
Analyzing RMS error table for tower
Page 115 and 116:
A general overview of the figure 6.
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6.7 Cross Checking of wind dataIn t
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Figure 6.8 a/bFigure 6.8 c/dFigure
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Table 6.11: RMS error for cross che
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Figure 6.9 e/fFigure 6.9 g/hFigure
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Figure 6.10 g/hFigure 6.10 i/lFigur
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to have a better agreement during t
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Figure 7.2: Weibull fitting of the
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Figure 7.5: Perspective view of the
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The calculated AEP has been reporte
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ConclusionsIn this thesis the evalu
Page 137 and 138:
Bibliography[1] CRASTO G. - Numeric
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