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

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A different behavior is obtained when the wall with no friction BC is used. Accordingto the development of the boundary layer, the effects of the roughness slow downalong the domain and the velocity deficit between two stations decreases whengoing toward the end of the flat terrain, confirming the general behavior of aturbulent neutral ABL. Due to the particular boundary conditions the same volumeflow is maintained for all the vertical stations and the streamlines remain horizontal.2.6 RemarksFlat terrains, like the model considered in this document, are not sensitive to theincrease of the vertical number of cells and then the default value (20 cells) seemsto be a good compromise between the goodness of the final results and the timetaken to run the simulation. An increase of the vertical cell number could be usefulin terrain with complex features.The height distribution factor needs to be taken into account when, in region of theflow attached to the terrain, a good precision is needed, and it should be decreasedslowly together with the increase of the horizontal resolution to avoid divergenceproblems. From this analysis a limit value of the Height distribution factor was found(HDF= 0.01).The boundary condition to be assigned to the top level of the geometrical model hasbeen analyzed, showing that mass flow rate can be conserved only assigning thewall with no friction boundary condition. This BC produces the effect of the shift ofthe free stream velocity respect to the value superimposed in the WindSim module,but the effect of velocity deficit due to the roughness value is strongly reduced,mostly for high values of the roughness height.25
CHAPTER 3. Simulations over 2D hilly terrains.3.1 IntroductionThe present chapter concerns a series of numerical simulations on the behavior ofan air flow over a single hill, characterized by a roughness value on the ground. Allsimulations have been run using a geometrical model in which a 3D “cos 2 ” hill isbuilt: this fact causes the presence of a finite Y-dimension, generating 3D effects. Toavoid these errors can affect the results, all the simulations have been runconsidering a very thin section of the 3D hill domain in Y-direction and the velocityvertical profiles, needed for further analysis, have been extracted in the central cellof the y-direction.The results obtained in this way are totally equivalent to the results that could beextracted by a “flatted” hill in y-direction, as illustrated in the next paragraph (figure3.2), and they are not affected by tridimensional flow effects.The shape of the speed vertical profiles along the hill has been also studied, and thespeed-up effect due to the variation of the terrain’s height has been highlighted.Moreover, a sensitivity study of the numerical grid for hilly terrain has been carriedout, both for horizontal and vertical discretization.Finally, a test case concerning the behavior of an air flow over a hilly terrain as beenreproduced: this case was good enough to put in evidence the separationphenomena of the flow downstream the hill, and set some important parameters ofthe WindSim software, to be used in complex terrains.3.1.1 Similitude between a 3D “cos 2 ” and “flatted” hillThe following figure 3.1 shows the shape of the geometrical model in the X-Z planeused to study the speed-up effect. The measurement stations, where speed verticalprofiles have been extracted, are also visualized.26
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 and 30: 2.4 Height distribution factor sens
Page 31 and 32: e decreased, to avoid to build thin
Page 33: Figure 2.10: Vertical profiles for
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
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case the wind is blowing from the e
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Geometrical modelTable 5.3: Input p
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Figure 5.6: Vertical profiles in po
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The orthogonalization of the grid h
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Figure 5.9: Map of speed-ups for di
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Figure 5.30: Speed-2D at 30 m heigh
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Figure 5.12: Speed-2D at 30 m heigh
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CHAPTER 6. Analysis of the wind fie
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Table 6.2: Boundary conditions and
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6.5 Vertical profiles of VelocityTo
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characterized by high wind speed (5
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From the error analysis, it is foun
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Anyhow, the trend of the linear int
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Concerning the CFD simulations, val
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Analyzing RMS error table for tower
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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
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Bibliography[1] CRASTO G. - Numeric
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