Hydrodynamic Loads on Two-Dimensional Sheets of Netting within the Range of Small Angels of Attack

Transcription

1 Hydrodynamic Loads on Two-Dimensional Sheets of Netting within the Range of Small Angels of Attack by Mathias Paschen & Karsten Breddermann 10 th International Workshop Methods for the Development and Evaluation of maritime Technologies DEMaT 11 Split, October 26 th 29 th, 2011

2 General information Intention: Improving the understanding of fluid-structure interactions in net-like structures especially in the cod-end of trawls indication for selectivity (?) Methods used: Experimental methods: - wind tunnel tests with a stiff net grid [measurement of loads and current distribution by PIV and heated wires, several devices for current-manipulation] Theoretical methods: - Reynolds averaged Navier-Stokes (RANS) method and - Potential theory (Conformal mapping)

3 Experimental set-up bottom v Top view of the experimental set-up Parameters of the grid: l = 1450 mm b = 750 mm d = 10 mm a = 83 mm u 1 = 0.5 d/a = 0.12 distance to the bottom: 167 mm Perforated plates used for experimental simulation of porosity (clogging due to caught fish) of the aft part

4 Experimental set-up Large subsonic wind tunnel of the Chair of Ocean Engineering (cut-out) 3d positioning system nozzle net grid collector bottom 6-component balance

5 Results of experimental investigations 0,1 cd [-] 0,075 0,05 0 0,25 0,5 0,75 1 d [-] without front plate with front plate 0,125 cl [-] 0,1 0, ,25 0,5 0,75 1 d [-] without front plate with front plate Influence of clogging d on the dimensionless drag (c d ) and lift (c l ) coefficients

6 Results of experimental investigations 0,770 xpp / l [-] 0,700 0,630 0,560 x pp 0 0,25 0,5 0,75 1 l d d [-] Position of the centre of pressure of the net grid against clogging d (front plate) x pp l with ( d ) x pp l ( d 1) x pp l ( d 0) sin 2 d 2 x pp l ( d 0) x pp l ( d 0) and x pp l ( d 1)

7 v(x,z) [m/s] v(x,z) [m/s] Chair of Ocean Engineering Results of experimental investigations 20 17, , measured by heated wire v 0 = [m/s] z [mm] x = 73 [mm] x = 218 [mm] x = 363 [mm] x = 1090 [mm] 20 17,5 15 measured by PIV v [m/s] 12, z [mm] x = 73 [mm] x = 218 [mm] x = 1090 [mm] v(x,z) above net grid v(x,z): resulting velocity in x-z-plane

8 TU [%] z [mm] Results of experimental investigations Intensity of turbulence of the current T U above net grid measured by heated wire, d = 1, v 0 = [m/s] z [mm] x = 73 [mm] x = 218 [mm] x = 363 [mm] x = 1090 [mm] x [mm] Tu ~ 1.5% Tu ~ 1.5%, n.p. Tu ~ 2.7% Tu ~ 2.7%, n.p. Curve of constant intensity of turbulence T U above net grid measured by heated wire, v 0 = [m/s] (n.p.: free current without any front plate and aft plate)

9 Results of numerical simulations Prediction of the flow patterns around and through a net grid with Reynolds averaged Navier-Stokes (RANS) method here: vector plot of calculated distribution of current close to the net grid v 0 = 15 [ms- 1 ]

10 Results of numerical simulations v 0 = 15 [ms- 1 ] To limit the numerical effort for the simulations the net grid was reduced to 2 meshes in the breadth. The length of the net grid and the parameters of the meshes were the same which were used during the model tests. Number of cells and knots varied between 5.85 to 6.02 million and 1.29 to 1.39 million, respectively.

11 Results of numerical simulations front plate grid aft plate Vector plot of calculated distribution of current v 0 = 15 [ms- 1 ] dead current

12 vx, vz [m/s] Chair of Ocean Engineering Results of numerical simulations vx, vz [m/s] z = 0,133 [m] 8 4 z = 0,158 [m] ,5-1 -0,5 0 x [m] vx-conf.map. vz-conf.map. vx-cfd vz-cfd 0-1,5-1 -0,5 0 x [m] vx-conf.map. vz-conf.map. vx-cfd vz-cfd Results of compare-able numerical simulations regarding distribution of current between bottom and net grid here: conformal mapping and CFD

13 Results of numerical simulations Results of in y-z- plane, Mesh no. 2.5 v 0 = 15 [ms- 1 ]

14 Results of numerical simulations Results of in y-z- plane, Mesh no. 6.5 v 0 = 15 [ms- 1 ]

15 Results of numerical simulations Results of in y-z- plane, Mesh no. 7.5 v 0 = 15 [ms- 1 ]

16 Results of numerical simulations Results of in y-z- plane, Mesh no. 8.5 v 0 = 15 [ms- 1 ]

17 Results of numerical simulations Results of in y-z- plane, Mesh no. 9.5 v 0 = 15 [ms- 1 ]

18 Summary The results show that the experimental set-up has an enormous influence on net panels drag and lift coefficients. Marginal modifications may lead to significant changes. The results demonstrate an extensive dependency of both the installed front plate and the porosity of the aft plate on the fluid flow close to the net grid which is significant for the fluid loads. Fishing gears are often complicated and flexible systems which generally influence the fluid flow close to the netting. The influence may be significantly high. Software tools for computer aided net designs should consider these effects. Therefore, further studies on the basics of net mechanics should be focused on investigations for a trustworthy prediction of the flow through and around netting structures.

19 Thank you for your kind attention!