Contra-Rotating Propellers

Overview:

  1. Introduction
  2. Computational Model
  3. Interfaces
  4. Cases
  5. Results
  6. Resources

Introduction

What are CRP and why would we use them?

  • CRP or Contra-Rotating Propellers

    • propellers with collinear axes rotating in opposite directions
    • secondary propeller harvests the energy otherwise lost in rotating flow
    • use of CRP reduces strain exerted on the main propeller
    • reduced risk of cavitation
    • positive impact on ship propulsion performance
    • improved stability
  • CRP set applications:

    • electric and hybrid propulsion systems
    • fuel cost reduction for large vessels
    • increased manoeuvrability - small vessels and torpedoes
  • Two most widely used CRP designs:

    • single shaft design
    • pod-mounted set
Single shaft CRP Pod-mounted CRP
  • This research focused on a single shaft CRP design:

    • an appropriate set of CRP propellers was chosen
    • a model set was used to match the one used in the research done by Miller [1,2]
    • Steady and unsteady hydrodynamic coefficients were analysed and
      compared to experimental data
    • the Q-Criterion was used to investigate vorticity of the flow
  • The numerical investigation of the CRP model was done using foam-extend:


Computational Model

  • The computational mesh used in the research was based on the model geometry
    used in the aforementioned research made by Miller [1,2]
  • This was done to be able to verify the results against the experimental data
    provided by the same research

Model Geometry

  • The geometry used matches the model used in the experimental study of CRP
    performed by Miller as closely as possible

  • The propeller set (type 3686-3687A):

    • two 4-bladed propellers rotating on a single shaft with collinear axes
    • front propeller (type 3686) rotates in the CW direction of x-axis
    • front propeller (type 3687A) rotates in the CCW direction of x-axis
  • Set details given in the table below:

Propeller type 3686 3687A
Position in set FORE AFT
Blades 4 4
Diameter [m] 0.2991 0.3052
Rotation CW CCW
Expanded area ratio 0.303 0.324
Section meanline NACA a = 0.8 NACA a = 0.8
Thickness Distribution NACA 66 NACA 66

Computational Meshes

  • Two computational meshes were created based on two model geometries:
    • a complete geometry of both propellers (dubbed FULL)
    • a single blade passage of each propeller (dubbed QUARTER)

  • Two computational domains were created based on the geometries shown above:

    • the Quarter domain and the Full domain were created
    • each domain was constructed from three separate regions:
      • the region around the FORE propeller
      • the region around the AFT propeller
      • the FarField region
  • The separation into subdomains (regions) was performed to:

    • give us a chance to study different interfaces used to connect
      separadted mesh regions
    • make it easier to define zones used in different approaches to resolving
      mesh motion, such as:
      • MRF zones
      • dynamicMesh zones

Full Domain

  • Details of the full geometry mesh regions:
Mesh Region Number of Cells Type NOTE
FarField 1117248 hexahedra No moving cell zones.
FORE propeller 1316400 hexahedra MRF/DynamicMesh zone: Prop1
AFT propeller 2778144 hexahedra MRF/DynamicMesh zone: Prop2
Total 5211792
  • Farfield with separate patches:

  • Internal mesh region: internal patches - used for interface setup
FarField region Propeller regions
  • To establish the communication between patches using interfaces patch pairs
    are established as follows:
    • side patches of the propeller regions are paired with matching side
      patches of the far field region
    • front patches of the two propellers are matched as a pair
    • the back patch of the FORE propeller is paired with the front patch
      of the far field region
    • the back patch of the AFT propeller is paired with the back patch
      of the far field region.

Quarter Domain

  • Details of the full geometry mesh regions:
Mesh Region Number of Cells Type NOTE
FarField 1117248 hexahedra No moving cell zones.
FORE propeller 1316400 hexahedra MRF/DynamicMesh zone: Prop1
AFT propeller 2778144 hexahedra MRF/DynamicMesh zone: Prop2
Total 5211792
  • Farfield with separate patches:

  • Internal mesh region: internal patches - used for interface setup
FarField region Propeller regions
  • Patch definition and communication was established differently for the
    quarter mesh CRP cases:
    • geometric definitions of internal patches belonging to the FarField
      region shave not changed when compared to the whole geometry mesh
    • positions of the front, back and side patches are analogous to their
      whole geometry counterparts, but they are reduced to a quarter of their
      original geometry
    • new patches defining the boundaries where the quarter regions would expand
      into the full geometry region are present (cyclic and cyclic shadow.
      • hese patches bound the cyclically periodic boundary of the propeller
        regions and are defined as cyclicGgi interfaces.

Patch Interfaces

  • Varying combinations of interfaces were used depending on on the mesh and
    geometry (quarter or whole) and whether the simulation was set up as steady state or transient

  • A generic patch type was applied to the inlet and outlet patches, whereas
    the wall type was applied to all propeller and hub parts.

  • For quarter geometry cases:

    • the interface used for internal patches is switched from ggi to overlapGgi,
      to enable communication between the full domain of the far field and the
      quarter domains of the propellers.
    • quarter cases use the cyclicGgi interface for cyclically periodic
      boundary patches.
  • A series of steady-state simulations was carried out using the mixingPlane interface, replacing the ggi interfaces.

    • for steady-state cases only !
    • the averaging performed by mixingPlane should not be applied to
      transient simulations
  • An overview of patches with matching patch types and corresponding pairs is given tabularly:

  • FarField patch info:

  • FORE propeller patch info:

  • AFT propeller patch info:

Case setup

  • An tabular overview of patches and the boundary conditions applied to them,
    for different case geometries and simulation types:

  • FarField boundary conditions (BCs):

  • FORE propeller BCs:

  • AFT propeller BCs:


Interfaces

  • A detailed overview of setup files used in setting up the CRP simulation
    described above.
  • Examples are given on how the boundary files were set up for use with:
    • overlapGgi
    • cyclicGgi
    • mixingPlane

OverlapGgi

  • An example of the code used to define an overlapGgi patch in the boundary
    file found in case_dir/constant/polymesh, with accompanying explanation:
FORE_interface_front
{
    type            overlapGgi;
    nFaces          2320;
    startFace       6358544;
    shadowPatch     FT_interface_front;
    bridgeOverlap   false;
    rotationAxis    (1 0 0);
    nCopies         4;
}
  • Overview of settings:
    • type - Defines type of boundary patch.
    • nFaces, startFace - Internally defined with mesh.
    • shadowPatch - GGI patch pair should be defined.
    • zone - Name of the faceZone analogous to the GGI patch being defined.
    • bridgeOverlap - Should be set to false for overlapGgi.
    • rotationAxis - Defines the axis of rotation.
    • nCopies - Set exact number of patch copies needed for defining a full
      geometry (360 degrees).

CyclicGgi

  • An example of the code used to define a cyclicGgi patch in the boundary
    file found in case_dir/constant/polymesh, with accompanying explanation:
  FORE_cyclic
    {
        type             cyclicGgi;
        nFaces           5064;
        startFace        6369274;
        shadowPatch      FORE_cyclic_shadow;
        zone             FORE_cyclic_zone;
        bridgeOverlap    false;
        rotationAxis     (1 0 0);
        rotationAngle    90;
        separationOffset (0 0 0);
    }
  • Overview of settings:
    • type - Defines type of boundary patch.
    • nFaces, startFace - Internally defined with mesh.
    • shadowPatch - GGI patch pair should be defined.
    • zone - Name of the faceZone analogous to the GGI patch being defined.
    • bridgeOverlap - Should be set to false for overlapGgi.
    • rotationAxis - Defines the axis of rotation.
    • nCopies - Set exact number of patch copies needed for defining a full
      geometry (360 degrees).

mixingPlane

  • An example of the code used to define a mixingPlane patch in the boundary
    file found in case_dir/constant/polymesh, with accompanying explanation:

  FORE_interface_side
    {
        type            mixingPlane;
        nFaces          6090;
        startFace       6363184;
        shadowPatch     MAIN_interface_side1;
        zone            FORE_side1_zone;
        coordinateSystem
        {
               type            cylindrical;
               name            mixingCS;
               origin          (0 0 0);
               axis            (1 0 0);
               direction       (0 1 0);
               inDegrees       false; //radians
        }
        ribbonPatch
        {
               sweepAxis       Theta;
               stackAxis       Z;
               discretisation  bothPatches;
        }
    }
  • Use of mixingPlane with theta as sweep axis and R as stack axis:

  • Use of mixingPlane with theta as sweep axis and R as stack axis:


Cases

  • Overview of case-specific settings and few selected results:

Whole case, steady-state, J=0.5


Quarter case, steady-state, J=0.5


Quarter case, mixingPlane, J=0.5


Quarter case, transient, J=1.1


Full case, transient, J=1.1


Results

Steady-state results

  • overview of hydrodynamic coefficients calculated from steady-state CRP simulations
  • comparison of mixingPlane results with overlapGgi

Hydrodynamic coefficients

  • Hydrodynamic coefficients: quarter CRP vs. whole CRP case

  • Hydrodynamic coefficients quarter CRP: simulation vs. experiment

  • Hydrodynamic coefficients: quarter CRP: ggi, mixing plane and experiment

mixingPlane vs. overlapGgi

  • pressure field at z=const. plane, quarter CRP case for J=0.5:
overlapGgi mixingPlane
  • velocity field in z = const. plane, quarter CRP case for J=0.5:
overlapGgi mixingPlane
  • velocity field at x/R = 0 and x/R = 0.334. planes, quarter CRP case for J=0.5:
overlapGgi mixingPlane
  • TKE field at z=const. plane, quarter CRP case for J=0.5:
overlapGgi mixingPlane

Transient simulation results

  • Pressure distribution on propeller surfaces for transient simulation of CRP: Fig. A
  • Velocity field in z = const. plane for transient simulation of CRP: Fig. B
  • Axial and tangential velocity at x/R = 0.334 for trans. sim. of CRP: Fig. C
  • Distribution of TKE on prop. blades for transient simulation of CRP: Fig. D
  • Total CRP (fore+aft) unsteady hydrodynamic performance for J=0.5: Fig. E
  • Unsteady hydrodynamic performance coefficients: Fig. F
Fig. A Fig. B
Fig. C Fig. D
Fig. E Fig. F

Resources

  • For a detailed overview of the problems shown above, please refer to the literature listed here.

  • Everything shown here was taken from these two sources:

    • Balatinec, Luka. An overview of rotor-stator interfaces for computational
      fluid dynamics simulations in turbomachinery
      , 2019, Master thesis,
      Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb

    • Balatinec, Luka; Jasak, Hrvoje. CFD evaluation of hydrodynamic performance
      of contra-rotating propellers (CRP) using OpenFOAM
      ,
      Propellers & Impellers - Research, Design, Construction & Applications,
      London, United Kingdom, 2019.

  • Experimental data sourced from:

    • [1] M. L. Miller. Experimental Determination of Unsteady Forces on
      Contrarotating Propellers in Uniform Flow
      .
      David W. Taylor Naval Ship R & D Center, Bethesda Md., 1976.

    • [2] M. L. Miller. _Experimental Determination of Unsteady Forces on
      Contrarotating Propellers for Application to Torpedoes.
      David W. Taylor Naval Ship R & D Center, Bethesda Md., 1981.