EXPERIMENTAL STUDY ON SNOW BEHAVIOR AROUND FENCES INSTALLED ALONG ELEVATED HIGHWAY

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1 ISTP-16, 25, PRAGUE 16 TH INTERNATIONAL SYMPOSIUM ON TRANSPORT PHENOMENA EXPERIMENTAL STUDY ON SNOW BEHAVIOR AROUND FENCES INSTALLED ALONG ELEVATED HIGHWAY Akinori Nakata*, Haruo Soeda*, Junji Onishi*, Akira Kondo**, Koji Mori* and Manabu Iguchi*** *Faculty of Engineering, Osaka Electro-Communication Univ., Japan **Graduate School of Engineering, Osaka Univ. ***Graduate School of Engineering, Hokkaido Univ. Corresponding author: Keywords: scale model experiment, snow fence, highway, PIV, hydrogen bubble method Abstract Since many highways have been constructed in the regions with snowfalls in our country, it is an important issue to keep the highway safe during winter. In order to develop measures of snow protection, numerical simulation is one of the useful tools since it can be applied to investigate snow behavior efficiently under various conditions. To establish simulation models, reliable experimental data should be prepared to validate it. In the present study, a basic experiment was performed using a scale model of an elevated highway with three different snow fences, under conditions of wind blowing. The scale model was arranged in a duct filled with running water. To simulate falling snow, small bubbles were used. The bubble behavior was measured using a PIV system and their flow pattern was examined. Results seem to be quantitatively acceptable and will be used to validate the numerical simulation model. 1 Introduction In Japan, the total length of express highways exceeded 7,km in 22. Approximately a half of the total length is laid in snow-covered areas. In the rest of Japan, except for Okinawa, it snows several times a year. Therefore, some measures against snow and ice are needed for the highway safety. In urban areas, highways often use elevated constructions. In winter seasons, the temperature of the elevated road surface can be 3 6 lower than that of ground-level roads, since it is not influenced by geothermal heat. Moreover, there are less sunshine hours because safety and sound barriers create shadows. For these reasons, the snow on the road surface does not melt as much as snow on ground-level roads. The road shoulder is not always wide enough for snow blowers to operate. One more significant factor is that highway usually has high traffic flow. In some urban areas, snowfall is less frequent and therefore adequate snow protection measures are not always provided. Snowfall is usually dealt with by removal measures. Effective protection measures should be taken to increase highway safety during winter. In order to develop measures of snow protection, numerical simulation is useful as it can be used to investigate snow behavior efficiently under various conditions. In order to establish such a simulation model, reliable experimental data should be prepared to validate it. Based on the above background, a basic model experiment was carried out for snow behavior around snow fences installed on an elevated highway. For ground-level roads, both snow behavior and accumulation around snow fences have been examined by experiments using wind tunnel models [1]-[3]. However, for 1

2 elevated highways, they have not been examined. In this study, to create the condition that wind is naturally blowing across the highway, a water-filled duct system was employed. The test section has a rectangular cross section of 8mm 2 and length of 15mm. To simulate falling snow, small buoyant bubbles were used. An electrolytic process of water was utilized to generate bubbles. A two-dimensional model of an elevated highway, at a scale at 1:2 and including three different height snow fences, was placed upside down in the test section. In order to measure bubble particle behavior, the PIV (Particle Image Velocimetry) system was used. Bubble particles were made visible with a double pulse Nd:Yag laser light, and their images were captured by a CCD camera. The captured images were stored in computer memory, and the bubble behavior was analyzed. Results seem to be quantitatively acceptable and will be utilized to validate the numerical simulation model. 2 Experimental Setup Fig.1 shows a schematic of the water-filled duct system. The test section was set on the downstream side of the duct line, which has a rectangular cross section of 8mm 2 and a length of 1.35m. To make the test section visible, the front and top panels were made of clear acrylic plate. The background and bottom were painted with matte-black to minimize the reflection of the laser light. All the interior surface of duct around the test section was coated with clear hydraulic material for avoiding adhesion of bubbles that may cause optical noises in recorded images. In order to keep the velocity distribution nearly uniform across the cross section of the duct line, a flow-rectifier was inserted in the upstream side. For the measurement of the test section, the center vertical cross section was selected, since it is assumed to be twodimensional. Small bubbles were used to express falling snow. To supply the bubbles in the test section, an electrolytic process of water was applied. Small bubbles are emitted from two electrically charged electrodes in the water. Fig.2 shows a schematic of the specially made bubble generator. Two sheets of stainless wire mesh (wire diameter:.4mm, mesh pitch: 1 pitch/inch) acts as electrodes. The dimensions of each mesh sheet are 3 8mm 2 and their vertical space was fixed at 4mm. Elevated highway model (turned upside down) Test section Water-filled duct line Flow-rectifier x W:8 y Flow Water storage tank (L) Flow Pump P 135 Bubble generator Flow-meter Fig.1. Schematic of water-filled duct system 2

3 EXPERIMENTAL STUDY ON SNOW BEHAVIOR AROUND FENCES INSTALLED ALONG ELEVATED HIGHWAY 3mm 8mm Fig.2. Schematic View of Bubble Generator 4mm Fig.3 shows a cross sectional view of a twodimensional model of an elevated highway. The model is a 1:2 scale of a prototype built according to Japanese road construction standards. To avoid obscuring the laser light, the model was also made of acrylic glass. It has safety barriers constructed on both sides of the road. In the present study, a snow fence with the height of H was attached to the top of one wall, as shown by the dotted line in Fig.3. Three different snow fences of the height of 5, 1 and 15mm were selected. In the case of the practical surface road, it seems to be preferable that the height ratio of snow fence to road width, h(=h/w), is 1/1 1/7 [4]. The height ratio of the model was determined to be.69,.139 and.28. H y x W= unit:mm Fig.3. Two-Dimensional Model of an Elevated Highway The highway model was turned upside down and set in the test section. The bubble generator was placed at the bottom of the duct line upstream of the test section. Regarding the small bubbles as snow particles, snow behavior was measured using a PIV (Particle Image Velocimetry) system. The system structure and the layout were shown in Fig.4. The bubbles were visualized with a double pulse Nd:Yag laser light source maximum output of 12mJ. The visualized images were captured by a CCD camera ( pixel 2 ) synchronized with the laser pulse, and stored in a computer memory. To obtain the velocity distribution of the bubble behavior, the image data was analyzed with the FFT based cross correlation method. Nd:Yag laser light source (max. output: 12mJ) Water-filled duct line CCD camera ( pixel 2 ) Fig.4. System structure and layout Reflecting mirror 3 Preliminary Test and Discussion about Particle Size of Bubble 3.1 Measurement of Rising Velocity of Bubbles Prior to the main experiments, a preliminary test was performed in order to examine the buoyancy of bubbles emitted by the generator. The highway model was removed from the test section in Fig.1, and the bubble generator was set at the bottom of the duct, 26mm upstream of the test section. In order to maintain a consistent amount of the bubbles, the current, passing through the water between two electrodes, was kept at a constant value of 15A. Moreover, the temperature of the water was maintained at 1 3

4 .5 using a cooling pump. The water flow rate in the duct line was kept at 36L/min, using a pump with flow controller and propeller flow meter. The approaching velocity averaged in the cross section, U, is 7.5cm/s. Then the Reynolds number, R e, based upon U and the height of the duct (mm), becomes Consequently, the water flow in the duct line can be regarded as developed turbulent flow. The generated bubbles were made visible and their images were captured by the PIV system as previously noted. The intervals of the laser pulse emission (straddling time) are 1.5 3ms and the sampling frequency of a pair of images is 3.7Hz. image sets (2 images) were taken for averaging. Velocity distributions of the bubbles in the test area were analyzed and the vertical velocity components were corrected. The rising velocity around the center of test section was 13.8mm/s. 3.2 Particle Size of Bubble From the data on the rising speed of bubbles, bubble particle sizes can be estimated. Assuming that the bubble particle is a stable spherical body made of hydrogen gas, the equation of the motion of a rising bubble in water was numerically solved. Lapple s equation [5], given by a function of Reynolds Number based on the particle diameter, d, was applied to the drag coefficient of the spherical body C D = ( Re ), (1) Re where R e. Both, the terminal velocity of rising bubble, V, and the Reynolds Number, R e, were calculated and are shown in Fig.5. From the figure, the particle diameter of bubbles can be estimated to be about 2 m in this case. 4 Similarity between Snow and Bubble Particles In general, snow particles have a diameter in the range.1 5mm and a density in the range.1.5g/cm 3. In fact, the flow around a snow Terminal Velocity, V (mm/s) & Reynolds Number, Re 13.8 V Re Particle Diameter, d (µm) Fig.5 Terminal velocity of a rising bubble and the Reynolds Number particle is considered to be laminar flow. Moreover, the flow around a bubble particle is also laminar flow. The inertia force of each particle is negligible. Therefore, viscosity force, F v, and gravity force (or buoyancy), F g, are dominant for the behavior of both particles. Here we adapt the following π -number given by the ratio of F v to F [6] g. Fv µ v π vg = =, (2) 2 Fg ρ p ρ f gd where µ : viscosity coefficient of fluid, ρ p, ρ f : density of particle and fluid. Assuming that the representative velocity v=1m/s for the prototype, the π -number takes on a wide range of (for the large and heavy particle) to (for the small and light particle). On the other hand, the model s π -number becomes.25. This is within the range of the prototype. 5 Experimental Results The main experiment was performed with the highway model under the same condition as the preliminary test. Fig.6 shows an example of the images obtained for the fence height of 1mm. From all the image data, the velocity of the bubbles were analyzed and the results are shown in Fig.7(a) (c). For a 4

5 EXPERIMENTAL STUDY ON SNOW BEHAVIOR AROUND FENCES INSTALLED ALONG ELEVATED HIGHWAY reference, a case without a snow fence was shown in Fig.7(d). In the all cases, the flow separation on the top of the snow fence (or the upstream safety barrier) forms a thin elliptical separation vortex of the bubble particles. In the cases with the snow fence, the bubbles flow over the road becomes reversed and slowly falls down on the road surface. In the case of h=.69, the separated flow of bubbles reattaches near the top of the downstream safety barrier. On the other hand, in the case without the snow fence, the bubble particles fall directly on the downstream side of the road surface, since the safety barrier is too short to shield the direct flow of the bubbles. For h=.139, three different flow velocity conditions were created. The approaching velocity was varied at values of 3.8, 15 and 22.5cm/s. The corresponding Reynolds numbers, based on the height of the duct, are 291, 115, and 172 respectively. As shown in Figure 8(a), in the case of the lower Reynolds number, the separation vortex of bubbles, which was observed above, was not completely formed. Rather, the bubble particles fell down slowly on the road surface. On the other hand, in the two cases of higher Reynolds numbers, a relatively intensive vortex was observed and almost horizontal flow was formed on the road surface, as shown in Figure 8(b) and (c). Flow 1mm cm/s (a) h=.69 (b) h=.139 (c) h=.28 Fig.6 An example of the obtained images (H=1mm) (d) without fence Fig.7 Bubble behavior around the highway models with three different snow fences 5

6 5 cm/s (a) U=3.7cm/s highway, a basic experiment was performed using a water-filled duct system. Small bubbles were used to simulate falling snow, and their behavior around three different height fences was measured using a PIV system. Results seem to be quantitatively acceptable and will be utilized to validate the numerical simulation model. 5 5 Fig.8 Bubble behavior around a highway model under three different flow conditions 6 Conclusion (b) U=15cm/s (c) U=22.5cm/s In order to examine snow behavior around snow fences installed along elevated Acknowledgement The authors would like to express their sincere thanks to the Japan Highway Public Corporation for the provision of valuable information. References [1] Iversen J D. Comparison of Wind tunnel model and full scale snow fence drift, J Wind Eng. Ind. Aerodyn., Vol.8, pp , [2] Tsuchiya M, Tomabechi T, Hongo T and Ueda H. Characteristics of Snowdrift and Wind Flow Around a Fence (Property of Wind Flow Which Affects Snowdrift), Journal of Snow Engineering of Japan, Vol.17, No.4, pp.3-9, 21. [3] Sakamoto H, Moriya M, Takai K and Obata Y. Development of New Type Snow Fence with Induced Plate in the Form of Circular Arc Airfoil for Prevention pf Blowing-Snow Disasters, Transactions of the Japan Society of mechanical Engineers, Vol.68, No.673, pp.69-76, 22. [4] Hongo T, Tomabechi T. Effectiveness of wind and snow fences to protect the workmen from severe wind and snowdrift, Journal of Snow Engineering of Japan, Vol.12, No.4, pp.3-15, [5] Tanaka Z and Iinoya K. New Approximate Equation of Drag Coefficient for Spherical Particles, Journal of Chemical Engineering of Japan, Vol.3, No.2, pp , 197. [6] Sekimoto K. Scaling Laws on Sedimentation of a Turbulent Flow System, Proc. of International Symposium on Scale Modeling, pp ,