Analysis of baffle wall gap in the design of stilling basin model

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Analysis of baffle wall gap in the design of stilling basin model

  1. 1. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME 66 ANALYSIS OF BAFFLE WALL GAP IN THE DESIGN OF STILLING BASIN MODEL H. L. Tiwari M A N I T Bhopal, India ABSTRACT This paper investigates the gap of baffle wall in stilling basin design for the enhancement of energy dissipation in order to protect the downstream structures from excessive scouring. Experiments have been carried out by keeping the baffle wall at same location for Froude numbers 3.85, 2.85 and 1.85 to evolve the new stilling basin model by changing the gap beneath the wall above the basin. Performance of outlet basins having baffle wall of at same location with varying the gap were evaluated based on dimensionless number called as performance index (PI), which is the ratio of grains Froude number to scour index. Higher values of PI indicate better performance of the basin. It is concluded that by placing the appropriate baffle wall size at suitable location with proper gap, the efficiency of basin increases. Key words: Baffle wall, Performance, Scouring, Stilling - basin, Introduction Baffle wall also called as impact wall plays an important role in the design of stilling basin for dissipating the excessive energy downstream of hydraulic structure like over flow spillway, sluices, pipe outlets etc. The effect of impact wall on the flow or scour characteristics depends upon the geometry of impact wall, its location and the gap underneath the wall with basin floor. Various types of recommended stilling basins for pipe outlets using impact wall are by Bradley and Peterka [1], Vollmer and Khader [2], Garde et al. [3], Verma and Goel [4 &5], Goel [6], Tiwari et al. [7], and Tiwari [8 &9], etc. The present research paper describes about the effect of the gap beneath the wall over basin floor for a stilling basin performance. The effects of design of stilling basin affects on scour characteristics downstream of the basin. The jet of water strikes against the baffle wall, which equalizes the distribution of flow of water over the full channel width. The flow gets turned toward the upper portion of the baffle wall and comes down on the floor of stilling basin after striking the hood portion of impact wall. The dimensions and location of an impact wall is quite important for improving the dissipation of energy. The size of hood should be such that there is no possibility of splashing of water. The gap at the INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND TECHNOLOGY (IJCIET) ISSN 0976 – 6308 (Print) ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), pp. 66-71 © IAEME: www.iaeme.com/ijciet.asp Journal Impact Factor (2013): 5.3277 (Calculated by GISI) www.jifactor.com IJCIET © IAEME
  2. 2. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME 67 bottom of the baffle wall may give rise to additional horizontal shear as small portion of water would pass through this gap dissipating more energy thereby performance of the stilling basin improved. The opening should be such that no material is entrapped inside the openings. In present work, performance of stilling basin models were evaluated with varying gap from 0.5d to 1.5d of impact wall over stilling basin . All models are shown in Figs.1 and 2 MATERIALS AND METHODS Experimental Set-up The experiments were conducted in a recirculating laboratory flume of 0.95 m wide 1 m deep and 25 m long. A rectangular pipe having 10.8 cm x 6.3 cm as cross sectional area was used to represent the outlet flow. This pipe was connected with delivery pipe of diameter of 10.26 cm in centrifugal pump. The exit of pipe was kept above stilling basin by one equivalent diameter (1d). A wooden floor was provided downstream of the outlet for fixing the appurtenances in the basin. To observe the scour after the end sill of stilling basin an erodible bed was made of coarse sand. First of all, the erodible material was filled up to the height of end sill and gets leveled then normal depth was maintained over the sand bed by allowing the water from the overhead tank inside the flume by operating the tail gate. Later on a centrifugal pump of capacity 20 HP was switched on while keeping the control valve closed which was fitted into the feeding pipe. The flow into the flume was increased gradually so as to achieve required Froude number with a minimum possible disturbance to the erodible sand bed. The discharge was measured by a calibrated venturimeter installed in the feeding pipe. With the operation of tail gate, the desired steady flow condition with normal depth of flow was maintained, which was computed by Manning’s formula corresponding to the inflow Froude number Fr = V/(gd)0.5 ,where V is the average velocity in the pipe, g is the acceleration due to gravity and d is the equivalent diameter of the pipe. As soon as the the required amount of water flowing from pipe outlet, reached the erodible bed material the movement of bed materials started and the geometry of scour hole started changing with time. After one hour test run, the motor was switched off. The value of maximum depth of scour (dm) and its location from the end sill (ds) were noted. All the models were tested for constant run time of one hour and with the same erodible material for all Froude numbers. The time for each test run was kept as one hour. The value of maximum depth of scour (dm) and its location from the end sill (ds) were noted as shown in Table 4. Impact wall of size 1.5d × 3d was tested in the stilling basin model (MSM-42) for different flow conditions for all three Froude numbers at 4d location along with the end sill. Further the gap beneath the wall with basin floor was varied to 0.5d, 0.75d and 1.5d for same impact wall. Arrangements of models are shown in Table1 and Figs.1&2. Scour parameters along with PI values for all models are given in Table 2. Table 1 Testing of Models to Study the Effect of Gap of wall above Basin Floor S. No. Model Name Impact Wall with hood End Sill Gap beneath the wall above basin floor Location from outlet exit Shape Height Width Location from outlet exit 1 MSM-42 1d 4d Sloping 1d 1d 7d 2 MSM-43 0.75d 4d Sloping 1d 1d 7d 3 MSM-44 0.5d 4d Sloping 1d 1d 7d 4 MSM-45 1.5d 4d Sloping 1d 1d 7d
  3. 3. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME 68 Performance Evaluation of Models A stilling basin model that produces smaller depth of scour at a longer distance is considered to have a better performance as compared to another stilling basin which results in a larger depth of scour at a shorter distance when tested under similar flow condition [5]. The performance of a stilling basin models were evaluated for different Froude number (Fr) which is a function of channel velocity (v), the maximum depth of scour (dm) and its location from end sill (ds). A non dimensional number, called as performance index (PI) has been used for evaluating the performance of stilling basin model [7 &8]. This is given as below: ܲ‫ܫ‬ ൌ ୚୶ୢ౩ ଶୢౣට୥ ρ౩షρ౭ ρ౭ ௗఱబ (1) Where, V - the mean velocity of channel, ds - distance of maximum depth of scour from end sill, dm- depth of maximum scour, g – gravitation acceleration, ρs- density of sand, ρw density of water, d50- the particle size such that 50% of the sand particle is finer than this size, A higher value of performance index indicates a better performance of the stilling basin model. This performance index takes into consideration of flow, scour parameters (dm & ds) and properties of material used downstream of the stilling basin. Model MSM-42 with gap 1d Model MSM-43 with gap 0.5d Figure 1 Stilling Basin model with impact wall having gap 1d and 0.75d
  4. 4. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME 69 Model MSM-44 with gap 0.5d Model MSM-45 with gap 1.5d Figure 2 Stilling Basin model with impact wall having gap 0.5d and 1.5d Table 2 Performance Index for Models Tested at varying Gap (dm and ds are in cm) S. No. Model name Fr = 1.85 F r = 2.85 F r = 3.85 dm ds PI dm ds PI dm ds PI 1 MSM-42 4.2 25.2 4.88 4.2 35.4 7.82 5.2 40.2 7.86 2 MSM-43 2.8 16 4.65 4.8 32.6 6.30 7.8 41.5 5.41 3 MSM-44 2.9 15.7 4.40 4.7 28 5.52 8.1 35.5 4.45 4 MSM-45 5.1 17.9 2.85 7.4 31.5 3.95 7.6 35.8 4.79
  5. 5. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July RESULTS AND DISCUSSIONS In order to study the effect of gap between basin floor and impact wall, the impact wall of size 1.5d × 3d was tested at different gap with the basin floor along with triangular end sill secured at 7d. Models MSM-42, MSM 0.75d, 0.5d and 1.5d respectively, having the impact wall sill (1V:1H) at 7d. By keeping the impact wall at 4d mod of stilling basin model (MSM-42) was which came as 4.88, 7.82 and 7.86 for Froude numbers 1.85,2.85 and 3.85 respectively. model keeping the gap as 1d, the impact wall gap was changed to 0.75d and in similar flow condition model was tested and renamed as MSM 5.41 for Fr = 1.85, 2.85 and 3.85 respectively. conditions by varying the gap as 0.5d and and performance indices were computed as shown in Table 2. On analyzing the performances of all models it reveals that the model (MSM tested models. Table 6 and Fig. 3 also as its performance indices (4.88, 7.82 & 7.86 compared to other models, namely MSM 1d, values of performance indices increases, but when gap reached to 1.5d, these values decreases. It shows that gap 1d is appropriate, which is also established by Bradl Table 3 Variation of PI with Gap Fr MSM-44 0.5d 1.85 4.40 2.85 5.52 3.85 4.45 Fig. 3 Variation of Performance Index with Ratio of Gap to Equivalent ivil Engineering and Technology (IJCIET), ISSN 0976 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME 70 of gap between basin floor and impact wall, the impact wall of 3d was tested at different gap with the basin floor along with triangular end sill , MSM-43, MSM-44 and MSM-45 were tested at the gap of. 1d, and 1.5d respectively, having the impact wall located at 4d along with the triangular end By keeping the impact wall at 4d model was tested with gap 1d and performance 42) was evaluated by computing the values of performance index PI), which came as 4.88, 7.82 and 7.86 for Froude numbers 1.85,2.85 and 3.85 respectively. gap as 1d, the impact wall gap was changed to 0.75d and in similar flow condition model was tested and renamed as MSM-43, and its performance was evaluated as PI = 5.41 for Fr = 1.85, 2.85 and 3.85 respectively. Similarly other models were tested at similar flow 0.5d and 1.5d and renamed as MSM-44 and MSM and performance indices were computed as shown in Table 2. On analyzing the performances of all models it reveals that the model (MSM-42) having gap as 1d performs better as compared to other also explain that model MSM-42, having gap 1d performs better s its performance indices (4.88, 7.82 & 7.86) for all three Froude numbers are higher side as compared to other models, namely MSM-44, MSM-43 and MSM-45. As gap increases from 0.5 increases, but when gap reached to 1.5d, these values decreases. It which is also established by Bradley & Paterka [1]. Variation of PI with Baffle Wall Gap above Basin Floor Gap beneath Baffle wall with Basin floor MSM-43 MSM-42 MSM-45 0.75d 1d 1.5d 4.65 4.88 2.85 6.30 7.82 3.95 5.41 7.86 4.79 Variation of Performance Index with Ratio of Gap to Equivalent dia. ivil Engineering and Technology (IJCIET), ISSN 0976 – 6308 August (2013), © IAEME of gap between basin floor and impact wall, the impact wall of 3d was tested at different gap with the basin floor along with triangular end sill (1V:1H) 45 were tested at the gap of. 1d, at 4d along with the triangular end ted with gap 1d and performance evaluated by computing the values of performance index PI), which came as 4.88, 7.82 and 7.86 for Froude numbers 1.85,2.85 and 3.85 respectively. testing the gap as 1d, the impact wall gap was changed to 0.75d and in similar flow condition PI = 4.65, 6.30 and tested at similar flow 44 and MSM-45 respectively and performance indices were computed as shown in Table 2. On analyzing the performances of all 42) having gap as 1d performs better as compared to other 42, having gap 1d performs better ) for all three Froude numbers are higher side as 45. As gap increases from 0.5d to increases, but when gap reached to 1.5d, these values decreases. It [1]. 45 . of Pipe.
  6. 6. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME 71 CONCLUSIONS An experimental study was conducted in the laboratory to study the role of gap of impact wall over basin in the design of stilling basin for varying gap (0.5d, 0.75d, 1d and 1.5d) for rectangular shaped pipe outlet with 12 test runs for Froude numbers 3.85, 2.85 and 1.85. Based on the experimental results, it is concluded that gap beneath the wall from basin floor affect the performance of stilling basin significantly due to change in the flow patterns. During the study it was found that the gap of impact wall affects the flow conditions and hence scours pattern downstream of the stilling basin. REFERENCES [1] Bradley, J. N. and Peterka, A. J. Hydraulic Design of Stilling Basins, Journal of A.S.C.E., Hydraulic Engg, 83(5), 1401-1406, (1957). [2] Vollmer, E. and Khader, M. H. A. “Counter Current Energy Dissipator for Conduit Outlets”. International J. of Water Power, July, pp. 260-267,(1971). [3] Garde, R .J. Saraf, P. D. and Dahigaonkar, J. G. Evolution of Design of Energy Dissipator for Pipe Outlets. J. of Irrigation & Power, 41 (3), 145-154, (1986). [4] Verma, D. V. S. and Goel, A. Stilling Basins for Outlets Using Wedge Shaped Splitter Blocks. Journal of Irrigation and Drainage Engineering, American Society of Civil Engineering (ASCE), 126 (3), 179-184. (2000). [5] Verma, D. V. S and Goel, A. Development of Efficient Stilling Basins for Pipe Outlets. Journal of Irrigation and Drainage Engineering, American Society of Civil Engineering. 129 (3), 194-200, (2003). [6] Goel A. Design of Stilling Basin for Circular Pipe Outlet. Canadian Journal of Civil Engineering, 35(12), 1365-1374, (2008). [7] Tiwari H. L. Goel, A. and Gahlot V.K. Experimental Study of Sill controlled tilling Basin, International Journal of civil Engg. & Research, 2(2), 107-117, (2011). [8] Tiwari, H.L. Experimental Investigations of Hydraulic Energy Dissipators for Rectangular pipe outlets, Ph.D. thesis MANIT Bhopal. (2012). [9] Tiwari, H.L. (2013) Design of Stilling Basin with Impact wall and End sill, International Research Journal of Resent Sciences, 2 (3), 59-63, (2013) [10] Damodar Maity, C. Naveen Raj and Indrani Gogoi, “Dynamic Response of Elevated Liquid Storage Elastic Tanks with Baffle”, International Journal of Civil Engineering & Technology (IJCIET), Volume 1, Issue 1, 2010, pp. 27 - 45, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316. [11] Hitesh N Panchal, Dr. Manish Doshi, Anup Patel and Keyursinh Thakor, “Experimental Investigation on Coupling Evacuated Heat Pipe Collector on Single Basin Single Slope Solar Still Productivity”, International Journal of Mechanical Engineering & Technology (IJMET), Volume 2, Issue 1, 2011, pp. 1 - 9, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359. [12] Nadia Khelif, Imed Ben Slimène and M.Moncef Chalbaoui, “Intrinsic Vulnerability Analysis to Nitrate Contamination: Implications from Recharge in Fate and Transport in Shallow Groundwater (Case of Moulares-Redayef Mining Basin)”, International Journal of Civil Engineering & Technology (IJCIET), Volume 3, Issue 2, 2012, pp. 465 - 476, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.

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