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Influence of the trawling gear on the drag - E-Fishing

VIEWS: 7 PAGES: 21

									Influence of the trawling
gear on the drag force

 A.Ivanović R.D.Neilson

 School of Engineering
 University of Aberdeen
 Aberdeen, UK
Outline

   Background
   Research undertaken by UoA
   Finite Element modelling
   Fluid modelling
   Otter Door Analysis
   Roller Clump Analysis
   Conclusion




                  e-fishing – 18-20 May 2010, Vigo,Spain
Background
 High percentage of fuel (60%) is used
  to tow the net itself followed by the
  otter door and roller clump (approx
  20%) and smaller percentage from the
  bridles, warps and foot ropes. [Parente
  et al., 2008]

 Single and twin trawls were compared
  in terms of their influence on fuel
  consumption and single trawl was
  deemed to be more fuel efficient.
  [Rihan D., 2005]

 Otter trawling gear has the least effect
  on the fauna but the otter doors have
  the greatest impact on the habitat
  [Kaiser et al, 2002]


                    e-fishing – 18-20 May 2010, Vigo,Spain
Background - DEGREE
 As part of DEGREE project the influence of trawling gear
  components on the seabed was investigated.

 Physical interaction between the seabed and gear
  components was examined by both physical and
  numerical modelling approaches.

      FE Model of the gear component/sediment interface (Abaqus)
      Model the dynamics of a complete trawl system (Matlab)
      Model the fluids influence on a trawl component (Cosmos
       FloWORKS)
      Laboratory testing of scaled trawl components




                    e-fishing – 18-20 May 2010, Vigo,Spain
Drag force
 For each gear component two sources of drag force are
  considered
      Due to the contact between the seabed and gear components
      Due to the fluid influence
 FE models were generated to investigate the contact
  between seabed and trawl component using Abaqus
  software.
 Cosmo FLOWorks package was used to investigate drag
  force.
 Simulations were carried out for both the roller and otter
  door.



                    e-fishing – 18-20 May 2010, Vigo,Spain
Validation
 Fluid drag results were compared with data provided by
  Morgere (CD=0.95 and CL=1.45). The values from the
  models are lower than that calculated from the
  coefficients, but the influence of the shackles etc are not
  taken in consideration in fluid analysis.

 FE models of both trawl elements were validated against
  sea-trials where profiles of the trenches we observed.
  Good comparison was found.




                  e-fishing – 18-20 May 2010, Vigo, Spain
 Soil model


 Soil is represented as a simple elastic – perfectly plastic
  material
 This model attempts to find a simple but general method
  to simulate the physical disturbance
 Soil Properties: density 1900kg/m3; Young’s modulus
  2MPa, Yield stress 18kPa and Poisson’s ratio 0.49, friction
  coefficient between the shaft and the roller 0.1.




                    e-fishing – 18-20 May 2010, Vigo,Spain
  Roller clump FE model

 3D models of the roller clump and
  the sea bed were built in the FE
  analysis
 The roller and shaft are modelled as
  discrete rigid bodies while the sea
  bed is modelled as a flexible part
 Boundary conditions – boundary
  conditions were applied at the sides
  while the bottom was constrained.
 The roller is towed on its own rather
  than being part of the whole trawl
  system
 The roller dimensions: 600mm dia,
  500mm width



                      e-fishing – 18-20 May 2010, Vigo,Spain
FE results
   Full scale roller clump model




             e-fishing – 18-20 May 2010, Vigo,Spain
Comparison between sea
trials and FE model (roller clump)
                                      (Marine Scotland – Science)




                                             roller clump on muddy sand

                            30
              Depth (mm)




                            -20


                            -70


                           -120


                           -170
                                  0    250                 500            750   1000

                                                      Distance (mm)


           e-fishing – 18-20 May 2010, Vigo,Spain
Influence of speed and soil
 Speed
   Speeds examined influence the fluid drag
     more than the drag from the contact with
     the seabed.
 Soil
   Two components: base friction and passive
     pressure from the build up of soil in front.
   Softer sediment results in higher drag force
     due to the build up of sediment in front of
     the gear element.

               e-fishing – 18-20 May 2010, Vigo,Spain
Otter Door
 The shoe of the otter
  door only is modelled
  as it is in direct
  contact with the
  seabed.
 Uneven condition of
  the seabed and vessel
  motion were not taken
  into consideration.



             e-fishing – 18-20 May 2010, Vigo,Spain
Influence of attitude
Angle of attack
 The angles of angle of
  attack varied 0-40;
  beyond 15 an almost
  linear progression is
  obtained.
 The doors used in the
  sea-trials work in the
  range 25-40 which falls
  within a near linear
  relationship.
              e-fishing – 18-20 May 2010, Vigo,Spain
Influence of attitude
Angle of attack vs spreading force
 With an increase of angle of
   attack the spreading force
   increases until it reaches 30.
 This confirms that 30 degrees is
   an optimum angle of attack
   where max spreading force can
   be obtained.




                 e-fishing – 18-20 May 2010, Vigo,Spain
Influence of attitude
 Beyond that point
  the spreading force
  decreases – due to a
  stall condition with
  separation of flow
 CFD analysis show
  similar effect where
  large eddies were
  formed behind the
  door at 40.




                e-fishing – 18-20 May 2010, Vigo,Spain
Influence of attitude
Pitch angle
 In the CFD analysis the pitch angle was varied
    from -20 to 20.
 A similar effect was obtained regardless of the
    direction of the pitch rotation of the door




                  e-fishing – 18-20 May 2010, Vigo,Spain
Influence of attitude
Pitch angle
 For FE analysis pitch
    angle varied 0-10.
 The pitch angle of 5
    generates a lower drag
    force than either 0 or
    10, which is similar to
    the findings observed
    with fluids simulations.




                   e-fishing – 18-20 May 2010, Vigo,Spain
Observations – drag forces
 Pitch angle has an influence on the drag forces
   0 degrees – the door performs more like a sledge
      (cause: large contact results in a large drag force)
   Both the penetration and the drag force at 5 pitch
      angle are less than with 0 angle (cause: reduced
      contact with the seabed)
   Drag force higher for 10 pitch angle (cause: the heel
      of the door penetrates more and builds up the sand
      in front of the door)




                  e-fishing – 18-20 May 2010, Vigo,Spain
Influence of speed

 Drag force due to soil interaction does not fluctuate
  greatly with speed
 Drag due to fluid is substantial according to the scaling
  laws
     FD = 0.5CDAv2

  1m/s – 648N ; 2m/s – 2592N
 If the two analyses were combined and the speeds are
  high the changes of drag force of saturated sediment are
  postulated to be more prominent.



                  e-fishing – 18-20 May 2010, Vigo,Spain
Influence of type of soil

 Both firm and soft soils are examined.
 The changes in soil properties results in changes in
  penetration; if the soil is stiffer the penetration is lower
  and the soil in front is pushed away by the door without
  making a significant build up of soil.




                   e-fishing – 18-20 May 2010, Vigo,Spain
Conclusions

  This study highlights the importance of both fluid and
   contact drag forces for a particular gear components
  For the door the fluid drag component is more
   important as the speed increases whereas for the
   roller clump it is the contact forces.
  The type of sediment influences the magnitude of the
   drag force which governs the fuel efficiency




                e-fishing – 18-20 May 2010, Vigo,Spain

								
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