Computational Fluid Dynamics-Based System Identification of Marine Vehicles

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Computational Fluid

Dynamics-Based SystemIdentification of MarineVehicles

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Contents 1) Motivation2) Goal3) Conventional Practical Experiment-Based

System Identification Procedures4) The Proposed CFD-Based System

Identification Procedure5) Case Studies6) Discussion7) Conclusion

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1) Motivation 1) What is System Identification of Marine

Vehicles?

2) Importance of System Identification ofMarine Vehicles

3) Importance of Computational Fluid

Dynamics Based System Identification

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1) Motivation 1) What is System Identification of

Marine Vehicles?

′

+ = −  

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1) Motivation 2) Importance of System Identificationof Marine Vehicles

Considering the following motivating applications,Solving a dynamic model of this simple formavailable, should be of a great importance

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1) Motivation i) Deciding the suitable thrusters choices

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1) Motivation ii) Predicting the marine vehicle motion

in the fastest way possible

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1) Motivation ii) Deriving a suitable control law thatcan force the vehicle to follow acertain desired trajectory 

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1) Motivation 3) Importance of Computational FluidDynamics Based System Identification

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2) Goal Overcoming the mentioned disadvantages ofthe conventional practical approaches by:

1) Proposing an alternative procedure for the SystemIdentification based on CFD simulation rather thanpractical experiments

2) Providing a quite intuitive set-up for the CFD

simulation to help the user quickly set up a reliablesimulation

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Conventional Practical Experiment-

Based System Identification

Procedures

Current Methods :-

• Steady State Experiments

 – Constant Thrust (Zero Acceleration): DampingCoefficient s (linear and non-linear) terms.

•

Dynamic Experiments – Sinusoidal Force

 =  sin  +   

Mass and added mass term.

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The Proposed CFD-Based System

Identification Procedure

 =  ( ) ∗    +̈  ∗  

F is the sum of external force (or moment) from thrusters, gravityforce and pressure force due to fluid.

    ̈ is the acceleraon of DOF x

    ̇is the velocity of DOF x. 

spring: hydrostatic spring term.

x is the translation / rotation DOF.

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Choose a suitable thrust (or input moment for rotational DOF) that is expected to

yield a reasonable steady state velocity in a certain DOF

Discretize the problem geometry using a suitable meshing algorithm.

Solve the system of partial differential equations describing the fluid dynamics usingfinite volume method defined over the discretized geometry (mesh) subject to

suitable boundary conditions to compute the pressure force acting on the marine

vehicle due to fluid

Solve the ordinary differential equation (ODE) in the current degree of freedom

 =  ( ) ∗    +̈  ∗  

Steady

state

velocity

reached?

Use a mesh moving algorithm to adjust

the mesh so that the vehicle appears at

its new position obtained from the

solution of the ODE

Take the velocity and acceleration readings obtained from simulation results at the

different time instants as the measurements corresponding to the input thrust. Use

LSQ method to find the model parameters in the DOF under consideration that best

fit these measurements.

Repeat the whole procedure for other degrees of freedom of interest

 T  h  e c  o m p l   e t   e f   l   o w

 c  h  a r  t   o f   t   h  e p

 r  o p o s  e d 

 C  F D- b  a s  e d  m o d 

 e l   i   d  e n t   i   f   i   c  a t   i   o n

 p r  o c  e d  u r  e

Yes

No

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Case Studies

A. System Identification of an Underwater Vehicle

Simplifying Assumptions:-

• The vehicle travels at low speeds, that is, less than 2m/s.• Roll and pitch movements are passively controlled and

therefore, considered to be negligible.

• The vehicle is considered to be symmetrical about its

three planes.

• During all maneuvers the vehicle is always maintainedin a horizontal posture.

• Disturbances from the water environment on the vehicle

such as currents and waves are negligible.

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The system identification problem is reduced to the

problem of identifying the added mass and damping

parameters in four decoupled equations of theform:

    ̇+ (+ ||) = +  

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Discussion

A. Accuracy of CFD Simulation

1- under-relaxation factor

Solnew = α Solnew + (1- α) Solold where α if the under-relaxation factor

2- Mesh Quality

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B. Comparison of CFD simulation and Practical

Experiments

1- Readings of accelerations and other

measurements

2- Steady state solver

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C. Importance Analysis

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Conclusion

• Future Work

- Perform Practical identification experiments and

compare their results with the CFD based results.

- Study the reasons behind the discrepancies and

based on this study conclude a set of

recommendations based on which the accuracy of

both simulations and practical setups can beimproved.