In the present study, the motion of a torpedo was simulated using 6 DOF (degree of freedom) solver available in the commercial CFD software ANSYS fluent. Objectives: *To identify turbulent characteristics in a typical incompressible fluid flow that interacts continuously with rigid body of the torpedo. * To study transient variations of pressure-velocity fields of the flow domain interacting with the rigid body. * To study two DOF (degree of freedom) motion of a torpedo through water at a specified rotor speed of the propeller.

1. W.A. Chanaka Sudheera
B.Sc. Eng. (Hons.)
University of Moratuwa
chanakasudheera@gmail.com
COMPUTATIONAL FLUID DYNAMICS
ME 4432

2. Motion of a torpedo through
water
Torpedo
“A cigar-shaped self-propelled underwater missile designed to be fired from a
ship or submarine or dropped into the water from an aircraft and to explode on
reaching a target”
- Oxford dictionary
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Propeller of a torpedo

3.  In the present study, the motion of a torpedo was simulated using 6 DOF (degree of
freedom) solver available in the commercial CFD software ANSYS fluent.
Introduction
3
Objectives:
*To identify turbulent characteristics in a typical incompressible fluid flow that interacts continuously with rigid body of
the torpedo.
* To study transient variations of pressure-velocity fields of the flow domain interacting with the rigid body.
* To study two DOF motion of a torpedo through water at a specified rotor speed of the propeller.

4. Mesh and boundary conditions
4
Fluid body around the torpedo
Main fluid zone that deforms
after each time step
Surface mesh of the torpedo

5. 5Inflation layers around the rigid body
Growth rate = 1.2
No. of layers = 5
Sizing function:
Proximity and curvature

6.  6 DOF solver was used for the solution and only two degrees of freedom were allowed for the
rigid body of the torpedo namely,
 Translational motion in x-direction
 Rotation around x-axis
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𝑇𝑟𝑎𝑛𝑠𝑙𝑎𝑡𝑖𝑜𝑛
𝜔
 Mass of the torpedo = 50 kg
 Angular speed of the propeller = 750 rpm
= 78.5 rads-1
 Constant density of water = 998 kgm-3

7. Solver: Pressure based solver
 Pressure based solver was used since the velocities involved in this simulation are well below the speed of sound (In fact, less
than 1/3rd of speed of sound) so that fluid can be considered to be incompressible.
Pressure-velocity coupling scheme: Coupled
 Coupled algorithm was used since it provided a faster convergence rate for dynamic mesh updates and stabilized the solution
without causing divergence at the initial time steps.
Spatial discretization:
 Gradient: Least Square Cell Based
 On irregular (skewed and distorted) unstructured tetrahedral meshes, the accuracy of the least-squares gradient method is
comparable to that of the node-based gradient (and both are much more superior compared to the cell-based gradient).
However, it is less expensive to compute the least-squares gradient than the node-based gradient. Therefore, it was selected as
the gradient method in the solver.
 Pressure: PRESTO!
Pressure actually calculates pressure on the face. This is possible using staggered grids where velocity and pressure variables are
not "co-located". PRESTO! discretization gives more accurate results since interpolation errors and pressure gradient assumptions
on boundaries are avoided. This scheme works better for problems with strong body forces (swirl).
 Second order upwind scheme was used for momentum, turbulent kinetic energy (k) and turbulent dissipation rate (∈).
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8. 8
After t= 0.266 s
After t= 0.01 s

9. 9
Flow time (s)
t = 0 to t = 0.10 s
Average velocity of torpedo (m/s) Average velocity of torpedo (m/s)
Flow time (s)
t = 0.1 to t = 0.176 s
Average velocity of torpedo (m/s)
Flow time (s)
t = 0.176 to t = 0.266 s

10. 10
After t= 0.266 s
After t= 0.01 s

11. 11
After t= 0.01 s
After t= 0.266 s
After t= 0.1 s

12. 12
After t= 0.01 s
After t= 0.1 s
After t= 0.266 s

13. 13

14. 14