Evolution of scour induced by propeller wash. School of Civil & Environmental Engineering Nanyang Technological University

Transcription

1 Evolution of scour induced by propeller wash Jian-Hao Hong Indra Susanto Yee-Meng Chiew Nian-Sheng Cheng School of Civil & Environmental Engineering Nanyang Technological University 1

2 OUTLINE Introduction Experimental setup Scouring process Equilibrium scour profiles Evolution of maximum scour depth Conclusions 2

3 3

4 Objective To investigate the development of a scour hole in non-cohesive sediments due to the jet induced by a rotating propeller. To examine the similarity of equilibrium scour profile To study the time-dependent max. scour depth and the max. equilibrium scour depth 4

5 Experimental setup = 1, 21 cm h t =6cm C y o dunes h c d 5 =.24,.34 mm Bed features Zone A small scour hole L s Zone B main scour hole Zone C Deposit mound 5

6 Experimental parameters Tests (cm) d 5 (mm) y (cm) y / Rpm F R ej investigator R ,15 R ,25 R ,61 THN ,17 THN ,716 THN ,55 THN ,23 THN ,17 THN ,716 THN ,55 THN ,23 Hamill (1987), H-1 Hamill (1987), H , , 6

7 (cm) Scouring process (1/2) 6 4 (a) Scouring processes along the central line of propeller location of propeller (b) Pictures for scouring processes Initial stage.5hr (c) Schematic diagram for scouring processes Scour hole.5hr (cm) = 1 cm, y o = 1 cm rpm = 45, d 5 =.34 mm.5 hr Flow Location of propeller ripples/dunes Deposit crest Plain view 4 2 location of propeller Developing stage 2.hr main scour hole 2.hr -2-4 small scour hole hr 2. hr 7

8 (cm) (cm) Scouring process (2/2) 4 location of propeller Stabilization stage 8.hr 8.hr location of propeller.5 hr 2. hr 8. hr Equilibrium stage 64.hr 64hr hr 2. hr 8. hr 64 hr x (cm) 8

9 Equilibrium scour profiles (1/3) y Low rpm with high clearance High rpm with low clearance Deposit crest y y C x h c,me x,me x sf x s,me yˆ 2 3 xˆ c c xˆ c xˆ c x ˆ l s,me l c,me l e,me yˆ 2 3 xˆ 1 b b xˆ b xˆ b x ˆ B.C. (1) at x =, y = ε (2) at x=x s,me ; y=,me (3) at x=x s,me ; dy/dx= (4) at x=l s,me ; y=o B.C. (1) at x = l s,me, dy(y<)/dx= dy(y>)/dx (2) at x=l s,me ; y=o (3) at x=l c,me ; y=h c,me (4) at x=l e,me ; y= 9

10 Scour depth, (mm) Scour depth, (mm) Equilibrium scour profiles (2/3) High Rpm, low offset height 3 2 (a) Equlibrium scour hole profile =.21 m, d 5 =.24 mm Low Rpm, high offset height 3 2 (b) Equlibrium scour hole profile =.21 m, d 5 =.24 mm rpm y / rpm y / Longitudinal distance, x (cm) Longitudinal distance, x (cm) 1 (a) 1 (b).5 =.21m d 5 =.25 mm y =.15m.5 =.21m d 5 =.25 mm y =.21m /,me -.5 /,me rpm y / rpm y / (x-x sf ) / (l s,me -x sf ) (x-x sf ) / (l s,me -x sf ) 1

11 Equilibrium scour profiles (3/3) 1.5 Hamill (1987) =.154m y / = 1.14 rpm = 4 /,me Data d 5 (mm) H H (x-x sf ) / (l s,me -x sf ) 11

12 Evolution of maximum scour depth,t / Data of Hamill (1987) Hamill (1987) =.154 m y / = 1.14 rpm = 4 Data d 5 (mm) H H-2.76 Modeling d (mm) 5 H H T = U o t/ / Data of present study =.21m d 5 =.24 mm y =.15m y / =.5 Simulation (a) rpm / =.21m d 5 =.24 mm y =.15m y / = 1. Simulation (b) rpm / =.21m d 5 =.34 mm y =.15m Simulation rpm y o / (c) T = U t / T = U t / T = U t / y o / =.5 y o / =1. y o / =1.5 12

13 Maximum equilibrium scour depth 1.6 Current Hamill (1987) Computed / Line of perfect agreement Measured / 13

14 Conclusions (1/2) 1. The equilibrium scour profile induced by propeller wash, including the small scour hole beneath the propeller, main scour hole and the deposition mound follows a particular geometrical similarity which can be expressed by a with suitable boundary conditions. combination of three polynomials 2. Based on the experimental data and the Buckingham π theorem, a semi-empirical formula is proposed to estimate the temporal development of the scour depth. The simulated results compare reasonably well with the experimental data. 14

15 Conclusions (2/2) 3. The maximum equilibrium scour depth increases with the increase in densimetric Froude and decrease in offset height ratio. 15

16 Thank you for your attention Questions & Comments? 16