3.2.2 CFD validation: computational settings and results

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Type Frequency Angular range Accuracy Chapter 3

3-component velocity measurement 600 Hz ±45° acceptance cone wind speed: ± 0.5 m/s wind direction: ± 1° (in the pitch-yaw axes)

Fig. 3.2b shows the measured incident vertical profiles of the dimensionless mean streamwise velocity component (U/Uref) and turbulence intensity (TI/TIref). The incident profiles are those measured in the empty wind tunnel at the location where the buildings will be placed [83]. Note that the reference wind velocity, Uref, and the turbulence intensity, TIref, are taken at building height, yielding values of 13.4 m/s and 8%. The building Reynolds number is 24,745 based on the street passage width (0.028 m) and the reference wind speed of 13.4 m/s, which is well above the critical value of 11,000, for which the flow around a building can be considered as Reynolds number independent [84].

3.2.2 CFD validation: computational settings and results

The upstream and downstream lengths of the computational domain are 3H and 15H, respectively, according to the best practice guidelines for CFD simulations of wind flow in urban areas [86, 124]. Note that the upstream domain length is smaller than the value proposed by the best practice guidelines, i.e., 5H, to limit unintended changes of streamwise gradients in the vertical approach-flow profiles [49, 87, 88]. The lateral length and the height of the computational domain are chosen equal to the cross-section of wind-tunnel resulting in a blockage ratio of 2.95%, which does not exceed the maximum value recommended by the aforementioned CFD guidelines. The computational grid consists of 5,464,450 hexahedral cells with 20 cells along the passage between the buildings. The average and maximum y* values are 40 and 76, respectively. The grid resolution can ensure that the center points of wall-adjacent cells are located in the logarithmic layer of the boundary layer for the near-wall treatment employing the near-wall treatment. The boundary conditions at the domain inlet are based on the measured incident vertical profiles of mean streamwise velocity, as shown in Fig. 3.2b. The turbulent kinetic energy k is calculated from the measured incident vertical profiles of U(z) and TI (z) using Eq. (3.1). The turbulence dissipation rate ɛ is given by Eq. (3.2) as below:

��(��)=1.5(��(��)����(��))2 (3.1)

�������� ∗ 3 ��(��+��0) (3.2)

where κ, u * ABL and z0 represent the von Karman constant (= 0.42), the ABL friction velocity (= 0.55 m/s) and the aerodynamic roughness length (9×10-6 m at reduced scale), respectively. The standard wall functions [89] with roughness modification are used on the ground surface. The roughness parameters of the sand-grain roughness height ks (m) and the roughness constant Cs are determined using their consistency relationship with the aerodynamic roughness length z0 [87], (Eq. (3.3)):

9.793��0 (3.3)

Urban wind energy potential: Impacts of building corner modifications 45

In this study, ks = 0.0007 m and Cs = 0.13 are employed for the ground surface. The ground and building walls are modeled as no-slip walls. Zero static gauge pressure is employed at the outlet boundary. Symmetry conditions are imposed on the top and lateral sides of the computational domain. The commercial CFD software ANSYS/Fluent v19.0 is used to perform the simulations. The 3D Reynolds-averaged Navier–Stokes (RANS) simulations are performed using the Linear Pressure–Strain (LPS) Reynolds Stress Model (RSM) turbulence model, which is selected based on a sensitivity analysis for different turbulence models. Detailed information about this sensitivity analysis is presented in Ref. [111]. The SIMPLE algorithm is adopted to couple velocity and pressure [90]. Second-order discretization schemes are employed for both the convection terms and viscous terms of the governing equations. Convergence is obtained when the scaled residuals level off and reach a minimum of 10−5 for continuity, 10−8 for x, y, z momentum, and k, 10−6 for ɛ, and 10−7 for the six Reynolds stress tensor components. Fig. 3.3 compares the simulated and measured U/Uref and TI/TIref values along three lines in the vertical center plane (y/B = 0) at x/B = -0.97, 0 and 0.97. The CFD results are in good agreement with the experimental data where the average absolute deviations for U/Uref and TI/TIref are less than 5% and 16%, respectively.

Figure 3.3. Validation study: comparison of (a-c) dimensionless mean streamwise velocity component and (df) turbulence intensity by CFD and wind-tunnel experiments along three lines in the vertical centerplane (y/B = 0) at x/B = -0.97, 0 and 0.97.