## 2 Traditional ABL Wind Tunnel Simulation Technique  
According to [PERSON] and [PERSON] [5], before the mid-1950s, the wind tunnels used for research were aeronautical tunnels with short working sections. They were converted for civil engineering applications by adding passive devices, such as grids, barriers, fences, and spires, to the test section entrance in order to generate turbulence. However, the turbulence generated in these tunnels usually did not meet the basic similarity requirements necessary to obtain realistic test results for structures situated in the ABL. [PERSON]'s model law [1] changed wind tunnel construction practice fundamentally. After the model law was published, ABL wind tunnels with long working sections were constructed. According to [PERSON] and [PERSON] [6], in long wind tunnels, a boundary layer with a typical depth of 0.5 to 1 m develops naturally over a floor to the order of 20 to 30 m in length, which is covered with rough elements. Atmospheric turbulence simulations in long wind tunnels are probably the best scenarios that can be achieved at present. However, in long wind tunnels, the similarity between the turbulence in the laboratory flow and in the realistic ABL is still not generally as strong as [PERSON] had conceived. According to [PERSON] and [PERSON] [6], wind tunnel tests occur at reduced geometric scales for obvious reasons of economy and convenience. Due to scaling, the similarities of a set of dimensionless numbers (the Strouhal number, Rossby number, Reynolds number, Froude number, Prandtl number, Eckert number, and Richardson number) are jeopardized as the flow events are different for different values of dimensionless numbers. Therefore, the question of scaling opens up the whole area of physical similitude. To meet the similarity criteria for wind tunnel experiments, researchers have paid attention to the dimensionless numbers mentioned above; they believed that when these dimensionless numbers in the model test and the full scale agreed well, the similarity issue with the wind tunnel test was effectively addressed. With the characteristics of the target flow and the scale factors for similitude established, it became apparent that some of the criteria established for similarity cannot, in fact, be satisfied under typical everyday test conditions. The researchers thus launched upon a series of inevitable compromises that rendered their task complex. Wind tunnel experiments vary widely, depending on the particular objectives and available resources, and some of the commonly used tests are described below [5]. (1) Tests of local pressures using scaled static models with pressure taps: Typical scales for these tests are in the order of 1:100 to 1:500. Mean and fluctuating pressures are often measured by connecting the pressure taps on the model with pressure transducers using thin vinyl tubing. The distortion of pressure fluctuations caused by the long tubes may be corrected using Fourier transform techniques. The fluctuating pressures measured in the wind tunnel are used to calculate the characteristic pressure and suction at each point of the structure. (2) Direct measurements of overall wind loads: Typical scales for these tests are in the order of 1:100 to 1:500. The model is fixed to a base balance to measure the overall wind load acting on the model. Some balances measure all six load components (the three forces and the three moments); other balances measure only some of the load components. Specifically designed high-frequency base balances may be used to measure the fluctuating overall wind load on the model without significant distortion from natural model vibrations. The models used in these tests should have a natural frequency in excess of the most significant wind loading frequencies. (3) Section model tests: Typical scales for these tests are in the order of 1:50 to 1:100. Section model tests are used to determine the aerodynamic data, e.g., the aerodynamic derivatives and the flutter parameters of the bridge deck.