# PIV Measurements of Turbulent Airflow in a Channel with Trapezoidal Ribs on One Wall
## 2 Materials and Methods  
The experimental setup used for flow testing in a channel with one rough wall consisted of a single radial fan, a section designed to equalize the flow and a measuring channel. Due to its size, the experimental setup is presented in two figures: Figures 1 and 2. The radial fan, with inverter-controlled rotational speed, supplied air into the flow equalization section consisting of the following elements: a straightener, a diffuser with two rows of grids, and the first and second confusor connected with the measuring channel (Figure 1).  
The 1.2 m long measuring channel with a 0.20 m \(\times\) 0.02 m internal cross-section was made of mineral glass. The top wall and the side walls were smooth along their entire length, whereas the bottom wall was smooth only along its initial (0.14 m) and its final (0.1 m) sections. Along its remaining length, the bottom wall featured periodic roughness in the form of evenly spaced, isosceles trapezoid shaped ribs positioned crosswise in relation to the direction of the flow. The ribs represented a simplified laboratory model of ribbed, arched supports used in mining longwall workings. The dimensions of the ribs (in millimeters) are presented in Figure 3.  
Velocity field measurements were carried out using the PIV method. The PIVTEC GmbH Goettingen, Germany aerosol generator with Laskin atomizer nozzles supplied DiEthyl-Hexyl-Sebacat (DEHS) marker particles into the channel through the suction opening of the fan. Based on the air dynamic viscosity 18.13 Pa-s, DEHS particle diameter 1 \(\upmu\)m and density 912 kg/m\({}^{3}\), channel height 0.02 m and mean velocity 29 m/s Stokes number is about 0.004. For Stokes number smaller than one particle, fluid streamlines were followed closely. The tested area was illuminated with a light sheet generated using a neodymium laser. An scientific Complementary Metal-Oxide-Semiconductor (sCMOS) camera, with its optical axis oriented perpendicularly to the plane of the laser sheet, recorded two pictures in a short interval. Each measurement consisted of a series of double pictures captured at a frequency of 15 Hz. The pictures were then sent to a PC, where dedicated software calculated instantaneous velocity fields, which were subsequently averaged over time.  
Numerous initial tests were carried out prior to conducting the actual measurements. Optimal measuring conditions were determined on the basis of these tests and defects of the test station, e.g., a local leak of the channel, were identified and removed. It was ascertained that the length of the test station was sufficient for obtaining the required development of the velocity profile. During each of the measurements, the air velocity in the channel was controlled with a Pitot tube installed at the outlet and connected to a TSI Incorporated, Minnesota, USA Airflow meter. The Reynolds number, based on height  
Figure 1: Schematic view of the test station—flow straightening elements.  
Figure 3: Trapezoidal ribs (dimensions in mm).  
Figure 2: Schematic view of the experimental setup—the channel featuring measuring instruments.  
H = 0.02 m and the average velocity in the initial, i.e., smooth section of the channel, was Re = 32,500.