# Experimental Study on the Effect of Wind Speed on Hail Impacts
## 2 Experimental Overview  
### Coupled Wind-Hail Experimental System  
This study is conducted in the smooth-flow-suction-type, single-test-section, low-speed wind tunnel at the Wind Engineering Research Center of Hunan University of Science and Technology. The test section dimensions are 21.0 m (length) \(\times\) 4.0 m (width) \(\times\) 3.0 m (height). The wind tunnel is equipped with a circular turntable adjustable over a rotation range of 0-360\({}^{\circ}\), and exhibits exceptional flow field performance, characterized by flow velocity non-uniformity below 1% and turbulence intensity below 0.5%. The maximum achievable wind speed within the tunnel is 30 m/s, approximating the wind speed of a Category 1 typhoon.  
Currently, most hail impact experiments employ pneumatic launchers [26; 27]. To simulate coupled wind-hail scenarios, this study develops a self-designed vertical hail impact testing apparatus tailored for wind tunnel experiments. The system comprises two main components: a hail launching device and a data acquisition system in Figure 1. As can be seen in Figure 1, the experiment is carried out in the turntable of the wind tunnel test section, and the hail launching system consists of a control system and a hail launcher, in which the hail launcher utilizes a customized pneumatic pumping system as the dynamic loading mechanism for the impact test, and different hail launching speeds can be achieved by adjusting the air pressure in the pneumatic chamber in the hail launcher through the control system. Velocities can be measured from the hail launcher's built-in infrared laser velocimeter and an external synchronized high-speed camera, which can also be used to detect the integrity of the hail launched by the hail launcher. This platform enables the simulation of hail impacts with different diameters and velocities under controlled wind speeds. Additionally, a hail protection net is installed at the downstream end of the wind tunnel test section to prevent fragmented hail particles from entering the fan and damaging its blades.  
The data acquisition system comprises wind speed measurement devices and impact force measurement devices. A three-dimensional pulsating anemometer (TFI) is selected to measure and monitor wind speed at a height of 50 cm above the target panel during the experiments. The impact force measurement setup consists of a custom-designed force measurement bracket and a force-measuring balance. The bracket, with a height of 50 cm, is positioned at the center of the wind tunnel turntable. A customized bracket is connected to the force-measuring balance and is securely mounted on it, as shown in Figure 2. The DELTA six-component, high-frequency, force-measuring balance (manufactured by DMS International Co., Ltd., headquartered in Hong Kong, China.) is employed in this study. This device integrates force sensors, a rigid cross frame, and a base. Data acquisition is performed at a sampling frequency of 1500 Hz, which adequately meets the requirements for capturing data under low-to-medium-velocity impacts. During the impact process, the friction between the hailstones and the panel is negligible, with only normal pressure acting on the panel. Consequently, the force-measuring balance is used to measure the normal impact force (Z-axis) under vertical impacts. The bracket is installed at a height of 50 cm, and the target panel is a flat steel plate with dimensions of 50 cm \(\times\) 50 cm \(\times\) 5 mm.