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
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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
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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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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
