## 1 Introduction  
Due to the surface changes of the earth, such as the gorges and hills [1], the wind field within the atmospheric boundary layer (ABL) changes. This significantly affects the wind forces on structures, the dispersion of pollutants, the utilization of the wind power, and other wind-related research fields. Hilly terrain enables a significant increase of the near-surface wind speed, which results in a larger wind load on the structure. Furthermore, the hills may affect the local wind direction, which leads to a distorted directional variation of the wind [2]. In light of this, it deserves efforts made to gain insight into the wind field of the airflow over the hilly terrain. Extensive studies related to the wind field over the hilly terrain have been conducted, using the field measuring, wind-tunnel test, and computational fluid dynamics (CFD) method. Field measuring is recognized as a most direct and effective means, such as that for Kettles Mountain in Canada [3], for Sirhoowy Canyon in Wales [4], and for Askervein Mountain in Scotland [5]. The main advantage of the field measuring is that the wind field information can be intuitively and accurately observed. However, due to the various adverse external conditions, this method is hard to be performed without strong financial support. Wind tunnel testing is an important and cost-effective method, and widely adopted in current wind engineering field. [PERSON] [6] studied the wind flow of the slopes through a wind tunnel test, and presented a wind speed model for the upstream and downstream sides of the hill. [PERSON] and [PERSON] [7] proposed a wind speed profile model of a symmetrical hill with a two-dimensional (2-D) flow, and mean wind velocities were used in their work. [PERSON] [8] proposed a law of wind profile for the flow over steep slopes, and suggested that the law is merely suitable for the steep slopes less than 10 km in length and 20\({}^{\circ}\) in slope. [PERSON] and [PERSON] [9] proposed a simplified method to consider the speed-up at different heights by using the maximum speed-up, which is called the original guideline. [PERSON] [10] reviewed the results of the field measurements and wind tunnel tests, and compared the mathematical models proposed in the literature. [PERSON] found that the maximum speed-up error of the algorithm by [PERSON] [8] in the calculation of flow separation is less than 15% to 18%. [PERSON] and [PERSON] [11] conducted substantial wind tunnel tests to study the wind fields of the flow over different hill geometries, such as the shallow sinusoidal hill, the steep sinusoidal hills, the consecutive hill, and other irregularly shaped hills. They compared the wind-tunnel test results with that obtained by using CFD in association with a \(k\)-\(\varepsilon\) turbulence model, and found that the greatest mean speed-up ratio appears at the height of 5.0 m above the hill crest. For the single shallow hill the speed-up ratio is 2.13, whereas for the steep hill it is slightly less than 2.08. [PERSON] et al. [12] conducted a wind tunnel test on a circular hill with a cosine-squared cross section. They compared the results with those in the undisturbed ABL, and found that the maximum perturbations in both streamwise and vertical wind speed variances could be observed at the height of the hill and on the lee slope of the hill. The maximum speed-up ratio is 1.5, and appears at the hill crest. [PERSON] and [PERSON] [13] utilized the wind-tunnel measurements and CFD simulations to study the speed-up of the airflow over bell-shaped hills, and compared the results with that in wind-loading codes. They found that the maximum speed-up ratio obtained from the CFD simulation is 2.15, and none of the codes provides a satisfactory result for speed-up. With regard to CFD-based analysis, [PERSON] et al. [14] studied two series of two-dimensional hills, and found that the turbulence energy is dissipated due to the influence of the front hill, which results in a reduction of the wake area behind the rear hill.