## 3 Measurement Technique and Data Reduction Methodologies
### Hot-Wire Anemometry  
Hot-wire (HW) anemometry was used to extract the turbulent structure statistically inherent in the unsteady random flow process because it offers high-resolution measurements, both temporally and spatially, with a high frequency response. A standard single hot-wire probe made of Tungsten (KANOMAX Inc., 0251R-T5) was employed. The wire had a diameter of 5 \(\upmu\)m and was 2 mm long. The prong length was 7 mm. The signal detected by the hot wire, equivalent to the flow velocity, was converted to a voltage by an A/D converter (KANOMAX Inc., constant temperature hot-wire anemometer: CTA). The voltage data were acquired by a data recorder (A&D Inc., RM-1100) at a sampling frequency (\(f\)) of 20 kHz. Data acquisition was carried out for more than 30 s at every measurement location; of the 30-s measurements, 5-s data was used to capture a boundary layer profile and 15-s data at each measurement location was used for turbulent analysis. This duration was considered to be long enough to extract the turbulent statistical feature as the characteristic frequency of the flowfield would be in the order of 1 kHz or 1 millisecond, and therefore more than 10,000 samples of characteristic turbulent feature data were expected to be acquired from each dataset. Measurement locations were \(x=600\) mm (\(x/x_{chord}=2/3\)) and \(y=-20\) mm (\(y/y_{width}=-0.031\)), as shown in Figure 5a presenting a schematic diagram of the measurement system; the probe position in the \(z\)-direction was only varied by using an electrically controlled traversing system. Total measurement locations with respect to the \(z\) position were 26, where the \(z\) position was varied from 1 to 20 mm from the wall surface. The uncertainty in positioning the \(z\) location was determined from the relationship between the read count of the stepping motor of the traversing system and the actual traversed distance. The probe height at each wind tunnel run was checked to be firmly fixed by reading the read count during each wind tunnel operation. The uncertainty for positioning was found to be negligible. The vertical position was changed, starting from \(z=2.0\) to 10 mm, which is around the upper edge of the boundary layer, with an increment of 0.5 mm. It was then increased by increments of 2 or 4 mm until it reached \(z=20\) mm. After the measurement at \(z=20\) mm, the height was changed in the other direction (in the order of decreasing height) to check for hysteresis. Figure 5b plots a representative freestream temperature change for the smooth wall configuration during a wind tunnel operation with changing of the probe position (i.e., \(z\) position). The sequential numbers on the figure indicate the order of measurement: The larger the number, the longer the wind tunnel operation time is. Obviously, the freestream temperature increases gradually as the wind tunnel operation becomes longer. Not shown here but a similar observation of large temperature variation was seen in each wind tunnel run. Because the resistance of the hot-wire probe is a function of flow properties, such as freestream velocity and temperature, an appropriate temperature correction must be made in the post-process for quantitative analysis. The details of the temperature correction will be described later. The same hot-wire probe was used for measurements for both the smooth and riblet walls in order to eliminate any uncertainty due to differences between probes. Additionally, the condition of the probe was checked by measuring its resistance and also by visual inspection. The resistance was around 4.0 \(\Omega\) before and after testing and the absence of damage to the probe was confirmed. Another fact that should be accounted for is that the tungsten probe may involve a time decay in its output under long-duration measurements where temperature changes of freestream are involved.