Stationary 3D anemometer at 0.5 m from the ground and ca m from the driving track Driving direction. 25 m

Transcription

1 1 Standardisation of test method for salt spreader: Air flow experiments Report 6: Three dimensional velocities generated by a full scale salt truck by Hisamitsu Takai and Jan S. Strøm, Consultants Aarhus University, Engineering Centre Bygholm, Test and Development Background In the present experiments a full scale salt truck drove through stagnant air unlike the previous experiments, in which a model truck was placed stationary in moving air. It is assumed that the behaviour of the airflows around the truck in both cases have similar characteristics. However, there is a fundamental difference between airflows around a stationary truck and a moving truck. Air velocity in the driving direction behind the stationary truck placed in moving air will get closer to the free wind speed as increasing distance from the truck. Contrary, the air velocity generated by moving truck in the X-direction will decline with increasing distance from the truck. Purpose Purposes of the present study are: 1) Develop a method to explore the characteristics of airflow generated by a moving full scale salt truck. 2) Examine the validity of relative velocity concept for higher driving speeds with full scale conditions. Methods and materials The experiments were carried out in GH with a salt truck. Air in GH was not completely stagnant. South port in GH was open and thermostat controlled heating fan was running some time. The background air velocity was estimated to be less than m s -1. Figure 1 shows a sketch of the experimental site. Length of the salt truck was 8.5m. It started at point a and after about 1.5 truck length drive it reached the desired speed, i.e. ca. 10 and 20 km h -1. The actual driving speed was determined by measuring time needed to drive between points b and c, i.e. 25m. Stationary 3D anemometer at 0.5 m from the ground and ca m from the driving track Driving direction 3D anemometer mounted on the truck 25 m d: Truck stop c Driving speed measurement b a: Truck start Figure 1: Sketch of the experimental site: The truck started at a then reached the desired speed before b. The driving speed was determined by measuring the time needed to drive from b to c. The truck stopped completely just before d.

2 2 Two 3D anemometers were used to determine 3D velocities around the truck. One was placed 0.35m outside the driving track, Figure 2, and the other anemometer was mounted on the truck behind the salt spreader disc as shown in Figure 3. Figure 2: Position of the stationary 3D anemometer head outside the driving track Figure 3: Position of the 3D anemometer head behind the salt spreader disc. Results and discussions Figure 4 shows vector presentations of air velocities measured at the stationary measuring point. The 3D anemometer was placed at Car position=0. One can imagine that the truck drove with a speed of 5.63 m s -1 (20 km h -1 ) from Car position = 34m to Car position = -8.5m and the situation the truck front is at X=0 and truck end at X=8.5m. The vectors shown along the axis Car position presents the airflow measured at Car position = 0 ; but they show the airflows at different distances from the truck front. Air velocity at -8.5m (1 truck length) away from the truck was around m s -1 (blue vectors). The vectors at 0 to 8.5 show the airflow along the truck. Air velocity was about 1.5 to 2.5 m s -1. Airflow direction changed from minus-x direction to plus-x direction along the truck. The velocity peak (red vector) was measured behind the truck, i.e. about ½ truck length behind the truck. Vectors at X=8.5

3 3 to 34m show directions of large variation. This indicates that there was strong turbulence behind the truck. Figure 4: Vector presentation of air velocities measured at the stationary measuring point. Driving speed was 5.63 m s -1 (20 km h -1 ). Car position indicates the distance between anemometer and truck front. One can imagine that the truck drove from Car position = 34m to Car position = -8.5m and the situation the truck front is at X=0 and truck end at X=8.5m. Figure 5 shows vector presentations of air velocities measured closely behind the salt spreader disc. Air velocity varied from about 0.5 to 1.5 m s -1. Airflow was mostly X-minus direction. This differs from the observations with scaled model, which showed upward airflow closely behind the truck. The photograph shown in Figure 3, may suggest that the anemometer was located outside of the upward airflow zone closely behind the truck. Figure 5: Vector presentations of air velocities measured closely behind the salt spreader disc. Driving speed was 5.71 m s -1 (20.5 km h -1 ).

4 4 Rel. air velocity Horisontal, driving direction Rel. air velocity Horisontal, right angle to driving direction Rel. airvelocity Vertical direction Figure 6: Effect of driving speeds on relative velocity components. X-component = Driving direction (top), Y-component = Horizontal, right angle to driving direction (middle), Z-component = Vertical direction (bottom)

5 5 The relative velocity concept was applied to the full scale experiment, Figure 6. Two different driving speeds, i.e m s -1 (12.5 k, h -1 ) and 5.63 m s -1 (20 km h -1 ) were compared. As the relative velocity components observed with two different driving speeds were almost the same level, the relative velocity concept seems to be valid with low driving speeds. Further studies on the validity of relative velocity concept for higher driving speeds with full scale conditions are needed. The moving truck created relative velocity component in the Y direction, i.e. side wind, of about. If this relation is valid for higher driving speeds, a considerable high side wind, e.g. ca. 6.7 m s -1 when driving speed is 60 km h -1, is always, i.e. regardless of the weather, expected under practical conditions. Conclusions 3 dimensional velocity generated by moving full scale salt truck was studied. The 3D anemometer measurements of airflow around the full scale salt truck were carried out in the Spreading Laboratory at, Engineering Centre Bygholm, Test and Development. Although the driving speeds applied in the experiments were relatively low, we can conclude followings: 1) The method using stationary 3D anemometer to explore air movement generated by full scale truck showed promising results. 2) Relative velocity concept seems to be valid for full scale model with low driving speed. 3) Validity of relative velocity concept for higher driving speeds with full scale conditions has to be examined.