Comparisons of wind-tunnel and field measurements are essential for checking the validity of modelling criteria and procedures. Micrometeorological data used for the subsequent comparisons are primarily from measurements in the ASL.
Stably stratified flow over a smooth surface with only a saw-tooth trip at the test-section entrance was investigated in the MWT by Arya et al (1969). A graph showing the measured values of Ri as a function of z/L (0 < z < 0.26) accompanied by a relationship for the atmosphere reported by Monin et al (1969) is given in Fig. 14.
The agreement indicates good similarity for the two scales of flow. This is further confirmed by measurements of mean velocity and temperature for both stable and unstable flows in the MWT by Chuang et al (1966). On Fig. 15 vertical profiles of these data are compared to micrometeorological data from Project Prairie Grass reported by Barad (1952) using Eqs. 2 and 3 to form the coordinates. It is evident that data from both sources follow the log-linear relationships in a similar way.
Various devices are frequently placed at the test-section entrance to increase 5. Effects of the following three configurations in the MWT are reported by Cermak (1982) for neutral flow:
(1) A 4-cm sawtooth trip for “natural” boundary-layer development,
(2) 61-cm high vortex generators designed by Counihan (1969), and
(3) 1.8-m high spires designed by Standen (1972).
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Profiles of mean and turbulent velocities were measured over three surface roughness (zo= 0.02 cm, 0.15 cm and 0.35 cm) for each entrance configuration at x = 3.3 m, 6.1 m, 12.2 m, 18.3 m, and 25.9 m downwind of the entrance. Because of space limitations, only profiles of longitudinal turbulence spectra with zo = 0.35 cm at x = 18.29 m are presented.
These are shown in Fig. 16. At x = 18.29 m all profiles approach a common form indicating that the initial flow signature introduced by the various devices is erased if the test-section length is sufficient. The spires and vortex generators increased the boundary-layer thickness 6 by 33% of 6 for the sawtooth trip. Of greater significance is the agreement with spectral model (a) in Table 2 which is based on atmospheric data.
Vertical profiles of mean velocity and turbulence intensities measured by Cochran (1992) in the thick “constant” shear layer are presented in Fig. 13. These data are compared with field data measured at the Texas Tech University field site as reported by Chok (1988) in which model heights are related to full-scale heights by a length scale of 1:50.
The mean velocity profiles presented in Fig. 17 and the longitudinal turbulence intensity given in Fig. 18 for the modelled and field ASL confirm that the simulation is satisfactory. The longitudinal turbulence spectra at a height of 4 m are presented in Fig. 19 for the TTU field site and the MWT.
Model and full-scale data are in fair agreement excepting at high frequencies for which the field instrumentation has inadequate response. However, the wind-tunnel data agree very well with model (d) of Table 2.
The foregoing comparisons give good evidence that satisfactory modelling of the ASL can be achieved. Comparisons such as those shown by Figs. 14 and 15 have provided justification for development of many BLWTs for modelling of the ABL and ASL.