Report 08 Salt observation 19jan13

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Evan McKenzie
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1 1 Standardization of test method for salt spreader: Air flow experiments Report 8: Observation of salt particle trajectory from spreader disc to road surface by Hisamitsu Takai, Consultant Aarhus University, Engineering Centre Bygholm, Test and Development 2012 Summary Salt particles spread in air were filmed by a high speed movie camera, Cacio Exilim, with two different speeds, i.e. 210 and 1000 frames per second. Most of the salt particles flew as a lump during the first 1.5 to 2m of the pathway. Although the disintegration of the salt-lump occurred through the pathway, most of the smaller particles were drawn by the larger particles and flew together with them until they reached 1.5 to 2 m away from the spreader disc. The flying height was reduced by about 10 cm. This seems to be enough to come away from the strong turbulence created by the truck. It is reasonable to assume that the distances between the particles increases as the salt-lump enlarges volume and the smaller particles get gradually away from the influence of larger particles. For development of a simulation model of salt spreading process, further studies on this process are recommended. Mechanism behind the early development of salt-cloud, which possibly resulted in an uneven salt distribution, has to be understood. Studies on influence of salt quality including size distribution and water contents on durability of salt-lump in air are recommended. Observation of slow motion replay can provide information, which otherwise difficult to obtain. It is especially suited for visual observations. Further studies and method development are needed to be able to determine flying speeds and directions of particles by using high speed movie camera. Development of a computer simulation model is recommended. Although a simulation model can probably give only rough idea of the effects of different parameters on salt particle distribution on road, it will be useful to identify the most important parameters that have to be taken in account when standardize testing method. Objective of the study 1) To examine the applicability of high speed movie camera to analyse the motion of salt particles spread in air with high initial speeds. 2) To develop a method to study the motion of salt particles 3) To explore the motion of salt particles spread in air by a salt spreader. Methods and materials High speed movies of salt spreading process were taken by a high speed movie camera Casio Exilim. It can take high speed movies with different speeds, i.e. 30, 210, 420 and 1000 frames per second (fps). Two speeds, 210 and 1000 fps were used. Salt was spread on an asphalt test road by a salt-truck with a driving speed of 30 km h -1. Salt was also spread while the truck was not driving to take close film of the spreader and salt particles in air. For the present study rock salt was used. The slow motion replays of the processes were observed, of which results were analysed and discussed.

2 2 Results and discussions Observation 1-1 (Figure 1) Most of the salt particles flew as lumps in the first flying period. They spread out as they flew away from the spreader disc. Figure 1: Spreading of salt (3 g/m 2, 8 m) filmed by a high speed movie camera (1000 frames per sec (fps): 1 frame = 1 ms) Cacio/Still pictures/cimg4199; Hyperlink: Cacio\ out Cacio 1\CIMG4183.AVI Analyses and discussions 1-1: According to the theory of aerosol mechanics and the observations of the films, we should be able to say followings: Most of the smaller particles were drawn by the larger particles and flew together with them in the first flying period. As the salt-lump spread out the smaller particles, say <2mm, decelerate flying speed due to viscosity of the air. Larger particles, say >4mm, continue to flying through the air without losing speed significantly. Observation 1-2 (Figure 1) Salt-lumps were formed by 6 wings on the disc. Analyses and discussions a: The time between formations of one lump to the next lump was 19 frames = 19 ms = s. This indicates the rotation speed of the disc: = [s (rotation)-1 ]. = 526 [rpm] Analyses and discussions b: The diameter of the disc was 0.44 m. Then the periphery speed can be estimated to:... = [m s -1 ] We assume that the initial speed of salt-lump is equal to the periphery speed, i.e. 12 m s -1. Observation 2 (Figure 2-b) The first stripe just after the bar is stripe #4. Stripe #1, 2 and 3 are behind the bar and cannot be observed in this picture. Stripes from # 4 through #9 are clear enough to identify. The stripes #10 and above can only be identified with uncertainty by observing uneven distribution of the larger particles in the air. Analyses and discussions 2 - a: The total number of stripes, i.e. total number of salt lumps formed by the wings and still in the air is estimated to be between 15 and 16. This lead to a flying time of salt to:

$Hyperlink: Cacio\20120802 out Cacio$ AVI Analyses and discussions 2 - b: We

air on larger particles and salt-lumps is

(z-displacement) can be given by: sin 12

3 [s] 2m 2m a b Figure 2: Spreading of salt (30 km/h, 3 g/m2, 8 m) on the test road filmed by a high speed movie camera (movie speed: 210 fps; 1 frame = 4.76 ms) Cacio/Still pictures/cimg4205; Hyperlink: Cacio\ out Cacio 1\CIMG4152.AVI Analyses and discussions 2 - b: We assume that the effect of viscosity of the air on larger particles and salt-lumps is negligible. Then, the vertical distance the salt particles can reach during t-seconds (z-displacement) can be given by: sin 12 sin [m] Where: v0 = Initial velocity of the particle = 12 [m s-1] θ = Cast angle = 0 [degree ] g = 9.8 [m s-2] Distance between spreader disc and road surface (h) was 0.43 m. Thus, the estimation of the flying time seems reasonable.

4 4 The truck was moving on x direction with a driving speed of -30 kh h -1 (-8.33 m s -1 ). Salt-lumps were scattered into the air with an initial speed of 12 m s -1. Their flying speed in relation to the road is given by 12 + ( 8.33) = 3.67 [m s -1 ] The distance (X) the salt particles flew during t-seconds (x-displacement) can be given by: = cos = 3.67 cos = 1.08 [m] The distance the truck moved on x direction during the same period can be given by: = 2.46 [m] Then, the distance between the disc and the salt particles, which can be achieved during this period can be given by: 1.08 ( 2.46) = 3.45 [m] This agrees well with the observation of Figure 2-a. It showed a distance of about 3 to 3.5 m from the disc to the point the particles touch the road. Observation 3 (figure 3) Salt-lump was significantly crumbled and a salt-cloud was developed at the distance of 1.5 to 2 m from the spreader disc. Figure 3: Side view of a salt umbrella Smoke test/jss fotos/02aug12a/012 Analyses and discussions 3 a: Salt-lump of half-circle was expanded from 0.69 m (= half circle of the disc) at 0 m from the disc to 5.5 m at 1.75 m. The average distance between salt particles increased at least 8 times Real average distance should be much larger as the salt-lump loses salt under the way and expands length, width and height.

5 It is reasonable to assume that the distances between the

And, the smaller particles get gradually away from the influence of

For development of a simulation model of salt spreading process,

Analyses and discussions 3 b The time (t) needed to fly 1.

146 [s] Z-displacement can be given by: sin 12 sin 0 12 9.8 0.146 0.

during some of the test runs (figure 4-a).

5 5 It is reasonable to assume that the distances between the particles increases as the salt-lump enlarges volume. And, the smaller particles get gradually away from the influence of the larger particles. For development of a simulation model of salt spreading process, further studies on this process are recommended. Analyses and discussions 3 b The time (t) needed to fly 1.75 m from the spreader disc is given by: బ [s] Z-displacement can be given by: sin 12 sin [m] Observation 4 (Figure 4) A cloud of salt particles developed during some of the test runs (figure 4-a). It resulted in an uneven salt distribution on the road (Figure 4-b) a b Figure 4: Uneven salt distribution on road occurred occasionally. Cacio/Still pictures/cimg4217; Hyperlink:: Cacio\ out Cacio 1\CIMG4154.AVI

6 6 Analyses and discussions 4 As development of salt-cloud happened occasionally, one can consider that it should be something with variation in size distribution and water contents of salt particles. These parameters may influence on the durability of salt-lump in the air. Weaker lump may crumble quickly and develop salt-cloud in turbulent air described in Report 9: Visualization of airflow patterns behind a full scale salt truck (J.S. Strøm, 2012). This was probably happened for the test run shown in Figure 4. Saltcloud developed within 1m from the truck. Need of a simulation model Development of a computer simulation model is recommended. The observations indicate that particle size distribution and nature of salt-lump plays important role for spreading process. But, there is many other parameters influencing on the salt particle distribution on road. Some of them are not controllable, e.g. weather conditions and some other can be controlled, e.g. initial particle velocity, spreading angle, water-salt ratio, moisture content etc. For the standardisation of a test method it is important to know the effect of them on the test results and their quality, i.e. reliability and reproducibility of the measurements as well as their relevance in relation to the objectives of the test. Although a simulation model can probably give only rough idea about the effects of different parameters on salt particle distribution on road, it will be useful to identify the most important parameters that have to be taken in account when standardize testing method. Conclusions 1) Slow motion replay is suited to visual observation of particle movement. 2) Further studies and method development are needed to be able to determine flying speeds and directions of particles by using high speed movie camera. 3) Studies on disintegration process of salt-lump in air are recommended. 4) Studies on influence of salt quality including size distribution and water contents on durability of salt-lump in air (development of salt-cloud) are recommended. 5) Studies on the aerodynamics of particles of different sizes in an expanding lump in air is recommended 6) Development of a computer simulation model is recommended. Literature Strøm, Jan S, Standardization of test method for salt spreader: Air flow experiments. Report 9: Visualization of airflow patterns behind a full scale salt truck. Aarhus University, Engineering Centre Bygholm, Test and Development

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