1 Standardisation of test method for salt spreader: Air flow experiments Report 2: Visualization of airflow patterns by Jan S. Strøm, Consultant Aarhus University, Engineering Centre Bygholm, Test and Development Objective One of the objectives of the air flow experiments was to visualize the airflow patterns and stability around a salt spreader. The main focus was an overall picture of the turbulent airflow created around the spreader models with special emphasis on airflow patterns and stability near the spreader position Methods The experiments were carried out in the wind tunnel in the Air physics laboratory at the Engineering Centre Bygholm as shown in figure 1. Figure 1. Picture inside the wind tunnel looking upstream towards the four axial supply fans. The wind tunnel is described by Strøm and Takai (2011). Average velocities up to 5 m/s could be generated in the experimental cross section, but for flow visualization low velocities are preferable. The experiments were carried out at 3 Hz controller setting equivalent to a free airflow velocity of 0.3 m/s. A scale model of a salt spraying truck was used together with a simplified box model with the same outer dimensions. In order to make laser sheet visualization more effective the box model was painted flat black as shown in figure 2.
2 Figure 2. The scale model of a salt spraying truck and the simplified box model with the same outer dimensions Smoke was created by use of Kitagawa Airflow indicator tubes No. 301 inserted in Ø6/8 mm tubes connected to an AM-TOP aquarium air pump CR-40 as shown in figure 3. According to the manufacturer the nominal flow of the pump was 2 x 2 l/min. Only one of the outlets was connected to the tube and the other outlet left open. Figure 3. Smoke was created by use of an airflow indicator tube inserted in a tube connected to an aquarium air pump The possibility of using a grid with a number of thin, short threads of wool was also evaluated for visualization of the stability and directions of the airflows, but was discarded of as inferior to laser sheet illumination of smoke. Airflow around box model At the upstream end of the box truck the airflow is divided into a flow going down and continues downstream under the truck and another flow going up and continues downstream above the truck, figure 4. Not as visible is the sidewise split in one flow on each side of the truck.
3 Figure 4. Airflow at the upstream end of the box truck model In order to study the development of the flow under the truck, smoke was supplied under the front as shown in figure 5. Figure 5. Smoke was supplied under the front of the box truck to study the development of the flow under and behind the truck The resulting flow in the centre plane behind the truck is shown in figure 6. It is seen how the flow detaches the road and curves upwards into an eddy attached to the rear end of the truck.
4 Figure 6. Airflow in the centre plane behind the truck In the horizontal plane at a height equivalent to the underside of the truck the air is seen to curve from the left hand side towards the right hand side with an eddy rotating in the centre plane close to the rear end, figure 7. Figure 7. Airflow in the horizontal plane at a height equivalent to the underside of the truck is seen to curve from the left hand side towards the right hand side In the box model three holes were drilled to provide different smoke supply; 1) low in the centre of the rear end, 2) downwards in the centre of the bottom and 3) downwards in the rear, left corner, figure 8.
5 Figure 8. In the box model three holes were drilled to provide different smoke supply In case 1) the flow in the central plane is projecting backwards for some distance due to the impulse of the smoke supply jet. It the curves upwards and enters backflow towards the truck before joining the airflow above the truck, figure 9. Figure 9. The flow created by the horizontal smoke jet in the central plane is projecting backwards for some distance due to the impulse of the smoke supply jet before curving upwards
6 In figure 10 is shown the concentrated smoke plume in the horizontal plane with backflows visible as diffuse smoke. Figure 10. Concentrated smoke plume in the horizontal plane created by the horizontal smoke jet in the central plane. In case 2) the smoke is supplied vertically downwards in the centre plane, figure 11, detaches the road in the same manner, but more clearly, as previously shown in figure 6. Figure 11. Smoke supplied vertically downwards in the centre plane shows how the airflow detaches the road in the centre plane behind the truck The airflow behind the left side of the truck is shown in figure 12 with case 3) supply of smoke. The detachment from the road is weaker than in the centre plane, and the airflow is probably turning inwards to the centre plane instead of curving upwards.
7 Figure 12. Vertical distribution of the airflow behind the left side of the truck with smoke supplied downwards in the left side aft corner This is visualized in figure 13 where the air in the horizontal plane is seen curving towards the right hand side of the wake behind an eddy rotating in the centre plane close to the rear end. Figure 13. Horizontal distribution of the airflow behind the left side of the truck with smoke supplied downwards in the left side aft corner
8 Airflow around the scale model At the upstream end of the scale mode the airflow is divided into the same four flows as shown previously for the box model. One flow is going down and continues downstream under the truck and another flow going up and continues downstream above the truck, figure 14. Figure 14. Airflow at the upstream end of the scale model truck In order to study the development of the flow behind the truck, smoke was supplied downwards in the centre plane behind the rear axles as shown in figure 15. Figure 15. Smoke was supplied downwards in the centre plane behind the rear axles to study the development of the flow behind the model truck
9 The resulting flow in the centre plane behind the truck is shown in figure 16. It is seen how the flow detaches the road in the same way as behind the box truck (figure 11) and curves upwards into an eddy attached to the rear end of the truck. Figure 16. Smoke supplied vertically downwards in the centre plane behind the rear axles shows how the airflow detaches the road behind the model truck In figure 17 the horizontal plane at a height equivalent to the underside of the truck the air is seen to curve from the left hand side towards the right hand side with an eddy rotating in the centre plane close to the rear end. Figure 17. Airflow in the horizontal plane at a height equivalent to the spreader disc is seen to curve from the left hand side towards the right hand side In figure 18 the air is seen to curve the other way from the right towards the left hand side, i.e. the opposite direction of figure 17.
10 Figure 18. Airflow in the horizontal plane at a height equivalent to the spreader disc is seen to curve from the right hand side towards the left hand side The two last figures indicates the presence of a von Kármán vortex street, i.e. a repeating pattern of swirling vortices caused by the unsteady separation of flow of a fluid. This will explain why fish tailing patterns are observed in the distribution of salt after the salt truck. According to theory a vortex street will only be observed over a given range of Reynolds numbers (Re). The range of Re values will vary with the size and shape of the body from which the eddies are being shed. Over a large Re range eddies are shed continuously from each side of the body, forming rows of vortices in its wake. The alternation leads to a vortex in one row being opposite the point midway between two vortexes in the other row, probably giving rise to the distinctive patterns reported at salt spraying. Ultimately, the energy of the vortices is consumed by viscosity as they move further downstream, and the regular pattern disappears, but at this point most of the salt may be unevenly distributed on the road. Conclusions In the centre plane behind the truck the airflow is detaching the road and curves upwards into a backflow eddy attached to the rear end of the truck before joining the airflow above the truck In the horizontal plane at a height equivalent to the underside of the truck the air was seen to curve from the one side towards the other side with an eddy rotating in the centre plane close to the rear end indicating the presence of a von Kármán vortex street, i.e. a repeating pattern of swirling vortices caused by the unsteady separation of airflow. This may explain why fish tailing patterns often are observed in the distribution of salt after a salt truck. Literature Jan S. Strøm and Hisamitsu Takai, 2011. Standardisation of test method for salt spreader, Air flow experiments. Report 1: Velocity distribution and stability around a scale model salt spreader. Aarhus University, Engineering Centre Bygholm, Test and Development