Real-Time Modeling of Cross-Body Flow for Torpedo Tube Recovery of the Phoenix AUV

Kevin Byrne

11 March 1998

Objectives

• Accurately simulate the ocean environment for testing and development of AUV software– Wave motion

– Body-induced flow

• Extensible flow modeling methodology • Platform-independent simulation using Java

VRML• Torpedo tube docking software/hardware Design

Start of Thesis

End of Thesis

Overview of Presentation

• Wind generated waves – Theory

– Implementation

• Body induced flow– Flow field simulation

– Flat plate model

– Tube-level flow

• Phoenix AUV Execution Level– Software / Hardware

Environmental Disturbances

• Three major areas to consider:– Wind

• N/A underwater

– Ocean currents• Effects are felt by both submarine and AUV

– Wind generated waves• Have a significant effect on shallow water AUV

operations

Pierson-Moskowitz Wave Spectrum

• Given: significant wave height and frequency– This develops a wave front which simulates a fully

developed sea in the North Atlantic

– S correspond the the wave Force in Feet per Radian/Second

SH s

8 384

5

33 522 5.

.

Buoyancy Model

• Daniel Bacon’s Thesis – Integration of A Submarine into NPSNET

– Solved buoyancy problem: Split submerged body into many pieces to allow for varying buoyancy

– Previous submersibles were modeled as neutrally buoyant

Buoyancy Model

Extended Buoyancy Model Assumptions:• Wave effects on a submerged body are due to the motion

of surrounding water volume. As the water flows over that submerged body a drag force is created over the bodies exposed area.

• These forces can be accurately simulated by summing the force vectors along the submerged body.

• During shallow water operations wave movement can cause exposure of various parts of the vehicle. As this occurs vehicle buoyancy and center of buoyancy must be

adjusted.

Extended Buoyancy Model Implementation Details• Each Time Step

– The Wave height calculated for each section of AUV• P-M Spectrum Lookup based on Sea State

• Sea vs. AUV Heading Factored Into Formulation

– AUV buoyancy and center of buoyancy are adjusted as necessary.

– Water velocity due to wave motion is calculated and applied to each section of AUV as a cross-body drag force.

Fluid Mechanics:Body Induced Flow Disturbances• Forces induced by a body moving through a

medium such as water • Very important for torpedo tube docking

– Size of AUV vs Submarine

– Flow instabilities along submarine hull

• Caused by pump suction/discharge or torpedo tube door

– Docking is a high risk evolution

• Noise Hazards, Watertight Integrity, Prop/TA Fouling

Simulation of Body Induced Flow Disturbances • These forces must be applied to all vehicles upon

which they are felt• Since each body creates it’s own flow

disturbances they need to be calculated based on body characteristics

• For this simulation, the size of the AUV relative to the submarine is small enough to ignore the effects of its flow disturbances

Simulated Flow Field:General Layout• Grid attached to submarine

– 1/2 ft intervals X,Y,Z force components

– Along entire length of hull

Simulated Flow Field (cont.)Submarine • The field is attached to and surrounds the

submarine

Simulated Flow Field (cont.)

Simulated Flow FieldEffects On AUV:• The size of flow grid intervals corresponds to the

size of each AUV slice• AUV can move in field at any arbitrary angle, the

proper grid position is calculated and flow forces at that point used

• Forces are integrated along the length of the body. Since these are due to flow cross-body drag is used to incorporate the effects into the EOM.

Simulated Flow FieldEffects On AUV:

Body Induced Fluid Flow Theory

• Goals:– Must ensure valid data at each grid point

– Theory is too complicated for real-time generation

– Must have a general solution which allows for extension when more is learned about the field

• Cases Considered:– Flat Plate model

– Tube Level model

Flat Plate Theory

• Used to model uniform flow over a flat plate aligned with the flow direction– Since submarine movement causes the flow, it’s heading

is always aligned with the plate.

• Assumes over 90% of drag is pressure drag, with only a small fraction due to skin friction– This is also valid for submarine movement

• Flow caused by displacement, not friction

• Is the submarine a flat Plate from AUV’s perspective?

Flat Plate Theory (cont.)

• Yes, it is!

Flat Plate Theory (cont.)

Flat Plate Theory (cot)Implementation• Generation Program

– Originally in Fortran, Now in C++– Allows for varying flow coefficients

– Creates flow field • Bow to stern• Hull to open flow area

• Computational Advantage – Flow force has only one component direction, vice three

• Problem Simplification: One profile good for entire sub

Tube Level Flow Profile

• Needed for areas where open torpedo tube door causes flow instabilities.

• This type of flow profile is not “well Understood”• BUT,

– At 1/2 ft resolution the unknown areas can be overlooked

– Time varying behaviors - Vortex Shedding frequency

– We can bracket worst case from AUV perspective

Tube Level ProfileImplementation• Approach similar to flat plate

– If AUV is in an area where tube level profile needed, data is pulled from appropriate grid

• Five separate grids Are used to cover al tube cases – Above tube, Upper edge of door, Center of door,

Lower edge, below

• Flow force has three components– X, Y, Z

Platform Independent Simulation

• Dynamics running in Java• Virtual Environment “Viewer” in VRML• Using DIS/Java/VRML Library• Current performance:

– Java: Real-Time, without flow field integrated

– VRML: Using Java through the script node, 2-3 packets per second

Phoenix AUV Execution Level

• Hardware– Tritech DS30 Precision Doppler Sonar

• speed over ground (u and v)

• speed through water (u and v)

• vehicle altitude

• Software– Simulated sensor input– Adjusted vehicle control laws to use data

The End.

Questions?

Contact Information

Kevin Byrne, LT USN

Naval Postgraduate School

2 University Circle - SGC 2287

Monterey, CA 93943

(408) 656-4077

E-mail: kmbyrne@cs.nps.navy.mil

http://web.nps.navy.mil/~kmbyrne