The definition of the coordinate system indicating sensor positions is shown in Figure 1. +V Vertical. -Y Left side along the driving direction

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1 1 Standardisation of test method for salt spreader: Air flow experiments Report 5: Three dimensional velocity and its components by Hisamitsu Takai and Jan S. Strøm, Consultants Aarhus University, Engineering Centre Bygholm, Test and Development Background The accuracy of velocity measurements using omnidirectional measurement methods like hot wire and turbine anemometers will depend on asymmetries in the airflow patterns. The smaller seize of e.g. the hot wire sensor makes it better suited for measurement points close to surfaces, but information of asymmetric flows or the presence of large scale eddies will not be revealed. Purpose The purpose was to get a better understanding on the true velocity in each point downstream of the scale model salt truck by determining the true 3D velocity at its components. Methods and instrumentation 3D flows are investigated using a HS-50 Horizontally Symmetrical Ultrasonic Research Anemometer (Doc. No PS-0032) from Gill Instruments Limited, Lymington, Hampshire. SO41 9EG UK: It measures the velocity components in 3-D. The driving speed was recorded using a hot wire air velocity meter VelociCalc model 9555 TSI Incorporated, Shoreview, MN USA. The definition of the coordinate system indicating sensor positions is shown in Figure 1. +V Vertical Driving direction +Y Right side along the driving direction +X Down wind -Y Left side along the driving direction Figure 1: Definition of the coordinate system

2 2 Figure 2 shows a photograph of the experimental setup for 3D air velocity measurements around the salt spreader model on the wind table. Figure 2: Experimental setup of air velocity instruments for the salt spreader model on the wind table The sensor head was placed in different positions behind the truuck as shown in Table 1. The Z axis position was 7 cm for all measurements. Wind speed (=Driving speed) was measured at X = -100, Y = 0, Z = 10 cm. (3 3=9 positions, 3 wind speed levels, 27 measurements in total) Table 1. Experimental design. Sensor Y axis position Sensor X axis position 5 cm 30 cm 55 cm Wind speed (= Driving speed ) -10 cm 0.4 / 2 / 10 m/s 0.4 / 2 / 10 m/s 0.4 / 2 / 10 m/s 0 cm 0.4 / 2 / 10 m/s 0.4 / 2 / 10 m/s 0.4 / 2 / 10 m/s +10 cm 0.4 / 2 / 10 m/s 0.4 / 2 / 10 m/s 0.4 / 2 / 10 m/s The dimensions of the sensor head is fairly large, however, as seen in Figure 3, which makes it impossible to determine flow fields closer than 7 cm to the ground surface. In the figure the sensor is placed at its lowest position in the centre, vertical plane. The maximum dimension of the sensor is 34 cm, so in order to place the sensor in different positions, a sensible minimum distance to a surface or object is 20 cm. The measurements at Y = ±10cm (ref. Table 1) provide average airflow velocities of the crossing from the volume behind the truck to the volume beside the truck. This have to be taken account when analysing/interpreting the data measured at Y = ±10cm.

3 Figure 3. Dimensions of the 3d ultrasonic anemometer head.

Y = 0, Z = 10 cm. TSI hot-wire anemometer is an omnidirectional sensor, which measures air velocity; but not velocity components.

The varying airflow directions seen in the figure indicate that there were turbulence at all measuring points. However, the fluctuations at the side lines, i.e. Y = ±0.

3 3 Figure 3. Dimensions of the 3d ultrasonic anemometer head. The air velocities were transferred to relative velocity by dividing them with wind velocities (driving velocities) measured by TSI hot-wire anemometer located at X = -100, Y = 0, Z = 10 cm. TSI hot-wire anemometer is an omnidirectional sensor, which measures air velocity; but not velocity components. Results Figure 4 shows the vector presentation of the velocities measured at different positions behind the truck. The wind velocity was 10 m s -1. The varying airflow directions seen in the figure indicate that there were turbulence at all measuring points. However, the fluctuations at the side lines, i.e. Y = ±0.1m, were smaller than at the centre line, i.e. Y = 0m. This may explained by the fact that the air flow measured at these positions were average velocity of the airflow behind the truck and the free airflow beside the truck. Figure 4: Vector presentations of the velocities at different measuring points behind the spreader. (Wind velocity = 10 m s -1 )

4 4 Vectors closely behind the truck, i.e. X = 0.05, Y = 0, Z = 0.07m, show upward going turbulent airflow. Blue colour indicates very low velocity, i.e. 1 to 1.5 m s -1. Vectors at 0.3m down-wind on the centre line show almost the same level of fluctuations; but the flow directions were more horizontal. The velocities increased to around 3.5 to 4 m s -1. Fluctuations, i.e. turbulence level, reduced to lower level at 0.55, down-wind on the centre line. Velocity seems to vary from 3.5 to 7.5 m s -1 (Figure 5) Figure 5: Vector presentations of the velocity components in the centre X-Y plane (top), in the horizontal X-Y plane (middle) and in the Y-Z cross-section (bottom) at different measuring points behind the spreader (Wind velocity = 10 m s -1 ) Figure 6 shows 75% percentile of X, Y and Z velocity components (in relative velocities) measured along the centre line, i.e. Y = 0, Z = 7 cm, X = 5, 30 and 55 cm. The relative velocities of X, Y and Z velocity components were obtained by dividing the velocity components by wind velocity measured by TSI hot-wire anemometer. As it can seen by Figure 6, the concept of relative velocity seems to be valid for velocity components albeit the wind velocities measured by an omni-anemometer (?) were used to normalize the velocity components, 25% of the X velocity components data were greater than 0.1 at X = 5 cm, 0.4 at X = 30 cm and 0.7 at X = 55 cm. While, the fluctuations of Y and Z components were much smaller than X component.

5 5 75% percentile of X, Y and Z velocity components (Position: Y = 0cm, Z = 7cm) 75% percentile, Relative velocity X-axis distans from the spreader cm X component Y component Z component Figure 6: 75% percentile of 1024 measurements (4 measurements pr. Sec) of 3D air velocities (relative velocity) at different distances from the spreader along the X axis. Figure 7 shows 3D resultant relative air velocities measured with 3 different wind velocity levels, i.e. 0.4, 2 and 10 m s -1. The relative velocities along the centre line, i.e. Y = 0, Z = 7, X = 5, 30 and 55 cm, increased with distance from the spreader, i.e at X = 5 cm to about 0.6 at X = 55 cm from the spreader. Relative velocity D resultant air velocity (Relative value) (Measured with 3 wind velocity levels: 0.4, 2 an 10 m/s ) X-axis distans from the spreader, cm Y=0, Z=6 cm Y=10, Z=6cm Y=-10, Z=6cm Figure 7: 3D resultant air velocity transformed to relative velocity (ref. report 4) at different distances from the spreader.

6 6 The regression model obtained from these measurements is: Y=0.0096X , R 2 =0.9615, n = 9 As relative velocities obtained with three different wind velocity levels at the same measuring points were quite similar to each other, it seems reasonable to use wind velocity (i.e. driving speed) to normalize wind velocities measured with different wind speeds (or driving speed). Conclusions Although the measuring volume of the 3d ultrasonic anemometer used in the present study was too large to observe small airflow details in the down-wind zone behind the model truck, the following conclusions maybe drawn from the study: 1) The results indicate that the air velocities relative to the driving speeds, i.e. relative velocities, can be used to normalize air velocity data obtained at different driving speeds. 2) The concept of relative velocity seems to be valid for both 3d resultant air velocity and velocity components. 3) Further study on validity of relative velocity concept for higher driving speeds with full scale conditions is needed. 4) The very low velocities i.e. relative velocity of around 0.15, and varying upward airflows were observed closely behind the spreader. This supports the earlier observations reported in report 4. Literature Jan S. Strøm and Hisamitsu Takai, Standardisation of test method for salt spreader: Air flow experiments. Report 4: Effect of driving speeds. Aarhus University, Engineering Centre Bygholm, Test and Development

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1 NordFoU: External Influences on Spray Patterns (EPAS) Report 15: Local wind profile for the test road at Bygholm Jan S. Strøm, Aarhus University, Dept. of Engineering, Engineering Center Bygholm, Horsens