ASNE International Launch & Recovery Symposium, November 19-20, 2014, Linthicum (MD).

Numerical Predictions of the Motions of Surface and Underwater Vehicles in

Waves. A True Step Forward towards Accurate L&R Simulations.

Stefano Brizzolara & Chryssostomos Chryssostomidis

Massachusetts Institute of Technology

MIT Sea Grant College Program, Department of Mechanical Engineering

Cambridge (MA), 02139. stebriz@mit.edu / chrys@mit.edu

ABSTRACT

In this paper, we use a viscous fully non-linear numerical

simulation method to predict pitch and heave responses in

waves of multiple rigid bodies, at zero speed. The two

considered vehicles are an ASV-SWATH and an AUVs. The

mutual hydrodynamic interference effects are characterized

and discussed in the paper, in the case where the underwater

vehicle is placed inside the two underwater hulls of the

SWATH vessel. We show the difference in motion

predictions obtained with the proposed fully non-linear

viscous methods versus more traditional 3D potential flow

based linear theories. Indeed these last theories are still

widely used to predict the operational capabilities of systems

L&R for underwater vehicles from surface crafts in a sea

state. We show the limitations and the level of approximation

of these low fidelity methods versus the new proposed one.

The analysis of the results of the simulations of the relative

motions in different wave lengths and amplitudes is used also

to design the best recovery system for the autonomous

surface vessel and a series of criteria for delimiting the

capabilities of the ASV L&R in rough seas.

KEY WORDS

Autonomous Surface Vehicle (ASV), Small Waterplane Area

Twin Hull (SWATH), Autonomous Underwater Vehicles

(AUV) launch and recovery, Seakeeping, Multi-body

Motions in waves, AUV and SWATH motions in waves.

1.0 INTRODUCTION

MIT Sea Grant is working towards the goal of realizing a

persistent cooperative networks of Autonomous Underwater

Vehicles (AUVs) to monitor large sea stretches. Such as

system of vehicles can open dramatically new perspectives of

ocean sensing both for scientific/civilian applications (water

quality, oceanography) as well as for other military purposes,

such for instance for security/surveillance tasks.

Besides the autonomy, communication, intelligence and

control issues, as naval architects, we are interested to solve

the technological gap which is possibly the major deterrent to

a wider diffusion at sea of such systems. This is the need of

periodic and frequent recharging of the electric power storage

devices (usually batteries). This operation currently involves

complex launch and recovery operations from manned ships.

Underwater recharging stations, as those developed by

majors AUVs producers (e.g. Battelle/Bluefin) are a viable

alternative when the operational theatre is invariable and

sufficiently close to a land-based power source.

Fig. 1. The underwater cooperative network of AUVs served and

recharged by the new ASV-SWATH

Our view of AUVs recharging is based on an autonomous or

autonomous surface vessel (ASV), specially designed for the

purpose (Brizzolara et al., 2011; US patent US8763546 B2)

which can serve the AUVs in their place of operations. This

is a desirable feature to make the underwater system truly

portable and effectively independent from land-based

electrical power sources. The SWATH-ASV, in fact, can be

transported inside a standard container unit and can be

deployed from land or form a ship.

In this context, the SWATH-ASV turns out to be the missing

chain link between the surface world and the AUVs as it

ensures the autonomous ability of launching, retrieving and

recharging in place as well as working as a means of transport

back and forth from the base. The autonomous surface craft

is the true enabler of a persistent system of AUVs for ocean

monitoring and sensing.

For the ASV to be effective, a robust launch and recovery

system has to be designed; ideally the same system would

support or include the capability to transfer energy to the

AUVs for recharging. Considering the unrestricted nature of

the AUV operational theatre, the recharging manoeuvre

should be possible also in moderate sea state conditions: so

the prediction of the motions of the surface craft and the AUV

in waves is essential. Results of this study influence the

design philosophy of the AUVs launching and recovery the

selection of the best power transfer system.

Different theoretical and numerical methods are available at

the Numerical Simulation Group of MIT Sea Grant for the

prediction of the motions of SWATH crafts and underwater

vehicles close to the free surface. The most approximate

models, but fast in computational time, are based on potential

flow assumption and a strip theory or slender theory

approach: they are neglecting viscous effects, which are

normally added as corrections in the calculation of the

response amplitude operator. These methods neglect or

approximate also three-dimensional effects, which in our

case (zero speed) are perhaps more relevant than at speed.

These approximate methods are used in a preliminary design

phase, to get a first guess on the sensitivity of the submarine

in waves, and also for preliminary sizing purposes.

A better tool recently developed for ONR is still based on

potential flow based strip theory for the calculation of

exciting and diffraction forces but it uses a fully viscous

simulation to derive the added mass and damping of a series

of 2D sections of the hull (Brizzolara et al. 2013). We have

used this approach to get a first guess of the SWATH motions

in waves, demonstrating the importance of viscous effects for

motions predictions in waves of SWATH vessels.

An increase of fidelity can be obtained with fully 3D potential

flow based (panel) methods, but they still suffer from viscous

effects, that are added as empirical corrections to the added

mass and damping (see Newman and Lee, 1991). The

ultimate and conceptually most accurate method is

represented by 3D fully viscous time domain motions

simulation in waves by unsteady (free surface) RANSE

(Reynolds-Averaged Navier-Stokes solvers). This capability

was demonstrated by Muzaferija et al. (1998) or Carrica et al.

(2007) for example in case of surface ships.

However, few published studies exist about the wave induced

motions on submerged bodies near the free surface. The state

of the art is still largely based on frequency domain potential

flow solvers with the eventual addition of (potential flow

based) second order (drift) forces. One of the first theoretical

study of this kind was published by Newman and Lee (1991)

who using asymptotic theories and empirical viscous

corrections demonstrated that second order forces are

important to predict motions of underwater bodies in

proximity of a wavy free surface. Higher fidelity results than

those of Newman’s can be obtained by modern unsteady two

phase (water and air) RANSE solvers. These solvers need to

be opportunely customized with moving mesh capabilities to

solve the rigid body motions of floating objects resulting

from the action of regular and irregular waves: among others

Huang et al. (2012) adapted a free surface capturing method

to deal with waves and adopted an overlapping mesh

technique to allow for moving bodies. In this paper, we

follow the approach first introduced by Peric and recently

validated (Hadzic et al. 2005) in case of floating bodies,

extending it to solve for the 2-DoF motions of two marine

vehicles (the AUV and the ASV-SWATH) in regular waves,

using a mesh-morphing technique to consider the motion of

the two bodies. The results presented are relative to a

medium-size autonomous underwater vehicle (e.g. REMUS-

100 or Bluefin-9) of about 2m in length and a 6m long

unconventional SWATH autonomous surface craft.

Feng et al. (1996) have recently proposed a nonlinear

numerical motion responses model for submerged slender

bodies running near the water-surface in waves. They have

been among the first to highlight that the wave drift (mean)

forces can cause large vertical motions of the submerged

body. In some cases, when the control forces of the

manoeuvring system (depth rudders) could offset the mean

drift forces, the initially submerged AUV was even emerging

to the surface. The results calculated by them were shown to

be in good agreement with those obtained from a self-

propelled model built and tested in oblique seas.

Results presented in this paper give another is based on 3D

fully viscous free surface time domain simulations of the two

vessels (the ASV and AUV) in the same wave. The final goal

is to characterize the non-linear effects which affect the

motion of AUVs when close to the free surface and the

interaction effects that may

2.0 VESSELS CHARACTERISTICS

We present here the main characteristics of the autonomous

surface and the underwater vehicles considered for the

seakeeping simulation study presented in the next section 3.

Figure 1 shows a 3D-view of the general arrangement of the

ASV-SWATH (Small Waterplane Area Twin Hull). The twin

underwater hulls have a length L=6m and a maximum

diameter of 0.65m. The whole vessel is about 4.5 meters wide

and has a full load displacement of about 4.2 metric tons. The

cruise speed is 12 knots: at this speed the shape of the

underwater hulls of the SWATH has been optimized by a

fully parametric procedure based on CFD models as better

detailed in Brizzolara & Vernengo (2011). The special

unconventional shape of the hulls and the particular canted

struts arrangement has been designed to ensure the lowest

motions in waves and the lowest wave-making resistance.

The results of a study presented two years ago at this

conference (Brizzolara & Chryssostomidis, 2012)

demonstrated that the pitch motions magnitude is reduced to

1/3 of that experienced by a conventional catamaran and

heave motion is reduced to half for wave lengths.

The propulsion system is diesel-electric with two 25kWe

gen-sets fitted in the main body of the SWATH. Two 20 kW

electric motors are fitted in the lower hulls and drive the

propellers through epicycloidal gears (red cylinders in Figure

1) to the slow turning fixed pitch propellers. Four battery

packs are fitted in the lower part of the struts (gray boxes).

Fuel and compensation ballast tanks are fitted in the central

portion of the lower hulls (red and yellow colored portions).

They are sized to ensure a sufficient reserve of energy to

cover a range of about 120 miles (at cruise speed) and

recharge about half a dozen of AUVs in a single sortie. The

center of gravity of the vessel is practically ad midship and

close to full load design waterline.

The upper structure of the SWATH (main body) is

subdivided in three sections by two watertight bulkheads. The

central section hosts the L&R and recharging system for

AUVs. Figure 1 presents the belts and winches L&R system

that was initially designed to retrieve a 2m long AUV from

the water. The whole AUV can be dismounted can fitted into

a standard 40’ container to be easily transported by a truck or

taken onboard of a ship. More details and feature about the

design of the vessel and its characteristics are given in

Brizzolara et al. (2011). The AUV considered in this study

and for the design of the ASV-SWATH is a small size

torpedo shaped vehicle, 2m in length and with a dry weight

of about 200 kg, such as REMUS-100 or Bluefin-9. The

notional vehicle shape is represented in Figure 1 in the

submerged position in between the two SWATH hull initially

imagined for the L&R maneuver. The results of the

seakeeping study demonstrate that this configuration does not

lead to safe operation in a sea state, so the idea of a belt

system for L&R close to the surface has been changed in

favor of a deep submersible cradle.

Figure 1: Layout of the ASV-SWATH for, first concept design of the AUVs Launching and Recovery system. Underwater hulls have the

unconventional shape optimized for cruise speed and the twin struts are canted to ensure optimal static and dynamic stability in waves.

3.0 U-RANSE Solver and 2-DoF Model

A state of art of unsteady RANSE finite volume solver with

mixture of fluid capturing method for free surface flows has

been adapted to the problem of the prediction of non-linear

ship motion in waves, as already tested in case of the SWATH

alone (Brizzolara & Chryssostomidis, 2012) and validated on

the Series 60 (Grasso et al., 2010). The generic CFD solver

(CD-Adapco, 2012) can use a variety of turbulent models and

solves for the dynamic equilibrium of a floating body, whose

motion is predicted integrating the 6 degree of freedom rigid

body motions ODE, coupled with the fluid flow solution

through the hydrodynamic forces acting on the body. We

setup an implicit unsteady (time domain) fully turbulent

RANSE segregated flow simulation model with a two-layers

(Rodi, 1991), realizable (Shih et al. 1994) k- turbulence

model. StarCCM+ uses a Finite-Volume RANS solver

allowing for non-structured polyhedral elements to discretize

the physical domain. SIMPLE method is used to conjugate

pressure field and velocity field, and the AMG (Algebraic

Multi-Grid) solver to accelerate the convergence of the

solution of the momentum and continuity equations at each

time step. Usually ten iterative steps are needed to minimize

the residuals and to obtain an accurate prediction of ship

motions in waves: they are necessary, each time step, to get a

converged estimation of the tangential and normal fluid

stresses on the hull which are integrated to give the body

forces.

In the presented case we use non-cubic Cartesian prismatic

cells in all domain and in a body fitted prism layer around the

hull. The transition between the prism layer and the Cartesian

cells is done with trimmed cells as captured in the mesh

section of Figure 3.

To save cells close to the hull surface, a two layer analytical

wall functions suitable for high Reynolds numbers has been

adopted to extrapolate the velocity in the prismatic layer of

cells closest to the wall. According this model, a first thin

linear (laminar) sub-layer close to the wall is considered and

then the extrapolation continues in the outer region by a usual

universal logarithmic law, valid for attached thin boundary

layers. 30<y+<100 was maintained, in order for the centroid

of the first cell near the wall to lie in the log-law region.

Figure 2: Domain used for the simulations of ASV-SWATH and

AUV seakeeping, represented in white color. Note the denser prismatic mesh around the free surface.

Steadily-progressing free surface waves are generated

imposing an initial flow in the domain and time variant inlet

and outlet velocity fields of non-linear (up to the fifth order)

Stokes waves, according to the analytical formulation of

Fenton (1985). A finite volume model with about 11.5M cells

have been created in the prismatic near field domain around

the ASV-SWATH and the AUV represented in Figure 2. The

cell size far from body is 10%L (L is the SWATH hull length),

while on the hulls surfaces is 0.25%L with a minimum of

0.02%L around the area with higher curvature (automatically

refined by the surface meshing algorithm) as it appears from

Figure 3. Two anisotropic nested cell refinements around the

free surface have been used, spanning 0.1L and 0.5L

respectively. In the inner region around the mean free surface,

the mesh is composed by prismatic cells measuring [2; 3;

0.25]%L in the longitudinal, transversal and vertical

directions, respectively. An outer refinement region spanning

about twice the size of the inner region was used to adequately

resolve the wave orbital motion in the rest of the domain.

Figure 3: Mesh refinement close to the two vehicles: a longitudinal

and a transverse plane are presented. The mesh is morphed due to a

sinkage of the AUV and a rise of the ASV-SWATH.

A small time step, lower than T/200 was used for all presented

simulation results, being T the period of the incoming regular

waves. Second order integration scheme is used for the time

integration: this ensures the best accuracy in following free

surface the wave propagation and transformation due to

diffraction and reflection on the moving bodies. Numerical

wave damping, implemented as proposed by Choi and Yoon

(2009), was applied on a surface strip at the boundaries (all

except inlet and symmetry) having a width equal to the at least

half wave length. To allow for the motion of the two bodies,

mesh morphing technique is implemented: basically, the

smaller cells closer to the moving body follow the rigid body

motion undistorted, while those more distant are subject to a

higher deformation as they move as if the volume domain was

elastically deforming around the rigid body. An example of

such a deformation is visible in Figure 3, in which the AUV

has moved down and the SWATH up with respect to the

initial position.

4.0 MULTI-BODY MOTIONS IN REGULAR WAVES

The excellent performance in terms of vertical plane motions

in bow waves was demonstrated in the paper presented two

years ago at the same ASNE L&R conference (Brizzolara &

Chryssostomidis, 2012). The comparison of the predicted

motions of the unconventional SWATH versus those of a

conventional catamaran in the same incident waves, made

clear that the ASV-SWATH is the best candidate vessel to

perform launching and recovering of AUVs at sea. In fact, the

research activities are intended to continue with the design

studies for the system to launch and retrieve onboard the AUV

from the midship hangar. Complementary studies aimed to

assess the importance of viscous and non-linear free surface

effects in the prediction of the vertical motion in waves of

SWATH crafts, concluded (Brizzolara et al., 2013) that

viscous effects are extremely important for the accurate

prediction of motions at sea of SWATH types of hull. It will

be even more they are important for the accurate prediction of

motions in waves of underwater vehicles when freely floating

close to the (wavy) free surface.

A rigorous and systematic approach has been used for

preparation of the CFD model to simulate the motions of the

two bodies in the configuration initially assumed for

recharging (or recovery). First the unsteady viscous

simulations in waves have been verified against another

results obtained with a lower fidelity (but still 3D) numerical

method. Hence the interference effects (diffraction and

refraction) between the AUV moving in proximity to the

surface craft and the SWATH have been studied.

Three systematic series of unsteady fully turbulent viscous 3D

RANSE simulations over a wide range of incoming regular

waves with different lengths on:

- AUV alone (AUV-solo)

- autonomous surface craft in isolation (SWATH-solo)

- the two above vessels simultaneously

Interesting conclusion can be drawn especially from the

comparison of the simulations of the AUV alone and those

with the AUV together with the SWATH. Each class of

simulation is presented in the following sub-sections.

4.1 SWATH-Solo

We present here the comparative results of the pitch and heave

motions predicted for the ASV-SWATH and an equivalent

catamaran design based on the MARINTEK type of hull.

Figure 4: Response amplitude operator of heave (3) motion of the ASV-SWATH and of an equivalent catamaran as a function of the

ration between the incoming wave length and the hull length L.

The catamaran design, first introduced in (Brizzolara &

Chryssostomidis, 2012) has the same waterline length, the

same displacement and the same deck area of the SWATH

vessel. In this sense it is considered “equivalent” to it. Its

shape can be well thought as representative of a catamaran

hull used in many different state of the art small autonomous

surface crafts.

Figure 5: Response amplitude operator of pitch (5) motion of the

ASV-SWATH and of an equivalent catamaran as a function of the

ration between the incoming wave length and the hull length L.

The viscous motion simulations of the SWATH in regular

waves were all repeated with respect to those presented in the

cited paper, using the new mesh and numerical algorithms

described in section 3, including the wave damping which

improved the regularity of the solution in longer waves

(>3.0), avoiding reflection of energy at the downstream

boundary. Bow regular waves having constant amplitude

equal to 3.5% L and different lengths ranging from L to 4.5L

have been considered. We deliberately avoided to assign a

constant wave slope to the incident waves, since this would

tend to enhance non-linear effects at higher wave lengths. In

case of SWATHs, a constant wave height with variable

length, tends to preserve the same type of non-linear effect

caused by the rapid shape change of the tapered struts around

the calm free surface.

Results are synthetized by the pitch and heave response

amplitude operator of Figure 4 and Figure 5. SWATH heave

response is significantly lower than that of the catamaran

which is generally twice in magnitude. This is true up to the

relative minimum at /L=3.5, where the heave motion

amplitude of the SWATH is four times smaller than the

catamaran. For longer wave lengths, the SWATH heave

response seems to increase rapidly due to the resonance peak

typical of SWATHs in longer waves. Close to resonance

viscous and non-linear free surface effects are heavily

affecting added mass and damping forces. The exact

estimation of these forces is essential to obtain reliable motion

predictions, as initially demonstrated by Brizzolara et al.

(2013) by means of a modified strip theory with numerical

viscous flow calculation of diffraction forces.

Pitch motion response of the ASV-SWATH is also

significantly lower than that predicted for the catamaran,

measuring almost one third of the latter over all the

investigated range of wave lengths. The pitch angular motion

amplitude 5 is non-dimensionalized with the wave slope ka,

where k=2 is the wave number and a is the wave

amplitude. Pitch motion response of the SWATH is subject to

a lower ramp than that observed for heave at /L=4.

Additional series of simulations have been programmed to

investigate the responses of the SWATH in even longer

waves, since the asymptotic trend of the presented RAO

curves should tend to 1 at higher wave lengths and =4 seems

still far from this asymptotic value.

4.2 AUV-Solo

In absence of useful experimental data with which to validate

the seakeeping predictions of submerged bodies near the free

surface, we performed a verification study of the results

obtained with the viscous flow solver (StarCCM+) against

those of the state of the art seakeeping code WAMIT,

developed at MIT (Lee, 1996). WAMIT permits to solve the

problem of motions of floating bodies in waves in the

frequency domain according the linear framework theory

(radiation and diffraction problems solved separately). The

unsteady RANSE solver, on the contrary, solves for the time

domain motion response of the vessels with a fully non-linear

approach which includes non-linear free surface and viscous

effects. The results of the RANSE unsteady model and

WAMIT have been obtained for an initial AUV submergence

d=L/2 measured from the free surface to the AUV axis and for

incident wave lengths ranging from L to 8L. Here and in the

rest of the paper L indicates the length of the SWATH surface

craft which is 3 times larger than that of the AUV (LAUV =L/3).

Results of the verification study are summarized in Figure 6

and Figure 7 for have and pitch RAOs as a function of the

incident wave lengths (LW=) normalized with the length of

the AUV, LAUV.

Figure 6: Heave RAO, calculated with the RANSE solver (Star) and

the panel method (WAMIT). The variability in amplitude response obtained with the non-linear viscous RANSE time domain

simulations is reported as a band around the mean value.

Figure 7: Heave RAO, calculated with the RANSE solver (Star) and the panel method (WAMIT). The variability in amplitude response

obtained with the non-linear viscous RANSE time domain

simulations is reported as a band around the mean value.

The heave RAO (empty dots) predicted in the (linear)

frequency-domain seakeeping framework with first order

diffraction forces calculated by WAMIT agrees well with the

amplitude of the fundamental oscillatory component of the

time varying motion predicted by the unsteady RANSE model

(solid squares). However at almost all wave lengths, the heave

motion predicted for the AUV-solo with the unsteady RANSE

shows large drift motions superimposed on the fundamental

oscillatory component. An example of large drift motion is

given in Figure 8 (blue curve): on top of the regular heave

oscillations, occurring at the incident wave frequency, a

longer period heave oscillation, larger in amplitude, is

observed (note that the AUV is buoyancy neutral). This is

what we call drift motion. Depending on the wave length, the

slowly varying vertical drift motion is not always as large and

irreversible as in the given example and it can be directed

upwards or downwards. Viscous effects do play a role in this

non-linear coupling, introducing dissipative damping and

consequently a shift in the phase of the forces and motion

induced by the incoming wave.

Figure 8: Heave motion vs. time predicted by the unsteady RANSE

solver for the AUV-solo (blue curve) and for the AUV together with

the ASV-SWATH (cyan curve). /LAUV=9.0 (/L=3.0).

In case of the AUV, the drift motion is a result of second order

forces, caused by non-linear coupling of pitch and heave

motions through the Froude-Krilov exciting forces. The phase

shift between the pitch periodic motion and the wave induced

flow results in a (small but) non-zero mean angle of attack

over a wave period which in turn generates a non-zero

dynamic lift component over the AUV.

These observed non-linear phenomena are quite relevant for

the assessment of the launching and recovery of AUVs in

waves at shallow depths and cannot be predicted in linear

seakeeping frameworks.

Figure 9: Pitch motion vs. time predicted by the unsteady RANSE

solver for the AUV-solo (blue curve) and for the AUV together with

the ASV-SWATH (cyan curve). /LAUV=9.0 (/L=3.0).

The same type of non-linearity is noted also on the pitch

response and the phenomenon is very dependent on incoming

wave frequency. The variability of the amplitude of the heave

and pitch periodic motion components derived from the time

histories of the simulations is included as an uncertainty band

in Figure 6 and Figure 7. For instance in a shorter incident

wave length the pitch motion

Figure 10: Heave motion vs. time predicted by the unsteady RANSE

solver for the AUV-solo (blue curve) and for the AUV together with

the ASV-SWATH (cyan curve). /LAUV=6.0 (/L=2.0).

Figure 11: Pitch motion vs. time predicted by the unsteady RANSE

solver for the AUV-solo (blue curve) and for the AUV together with

the ASV-SWATH (cyan curve). /LAUV=6.0 (/L=2.0).

Overall, the systematic series of CFD simulations evidence a

problem with the stability of the AUV heave motion and

sometimes also the pitch, due to strong second order forces

induced. The public literature contains only few papers on this

argument: Wang et al. (2003) and Dai et al. (2007), for

instance, confirm the importance of the second order forces

for the correct prediction of free floating bodies submerged at

shallow drafts. The experiments and numerical calculations of

Feng et al. (1996 and 1997) in their analogous study, confirm

that the peculiar predicted behaviour may be due to real

physical reasons and not to numerical instabilities or errors. A

numerical error can affect the simulations when the vertical

displacement of the AUV overcomes the length of the body

itself, but this happens only in two cases, among which the

one presented in Figure 8. In shorter waves this phenomenon

is still present, but its duration and amplitude (max sinkage)

is limited and once this sinking drift motion is dampened out,

it reverses and the AUVs tends to drift back up, i.e. coming

back its initial static position.

4.3 AUV and ASV-SWATH

The comparative analysis of the fully non-linear viscous time

domain motion simulations of the AUV and SWATH

together, with those presented before, permits to isolate and

discuss the interference effects of one vessel on the other. The

initial location of the SWATH and the AUV is set as the

equilibrium position in calm water corresponding to the

beginning of the recovering manoeuvre, with the AUV just at

the same depth of the SWATH hulls and in between them, as

represented in the perspective view of Figure 1.

The mutual interference effects between the two vessels are

principally due to wave diffraction and wave reflection of the

SWATH and in a minor extent due to the disturbance of the

AUV moving close to the free surface. The influence of the

SWATH disturbance on the motion of the AUV is evident

from the pitch and heave time histories presented already in

Figure 8 to Figure 11: both motions are affected and somehow

the presence of the SWATH tends to reduce (not always

though) the large heave drift motions that the submerged

vehicle experience when it is in isolation. The effect of the

SWATH on pitch varies with the incident wave length, but

generally tends to increase the pitch amplitude of the AUVs

after some time. This effect should be attributed to the

radiated waves produced by the SWATH when it reaches the

steady state oscillatory motion (usually after 5/6 periods).

The influence of the AUV on the SWATH is on the contrary

very limited. An example of time histories of heave and pitch

motions simulated with an incident wave length /L=4.0 is

given in Figure 12 and Figure 13, respectively. The difference

on both motion of the SWATH simulated in isolation (cyan

curve) versus the simulation together with the AUV (red

curve) is appreciable in this case which is one of the most

evident.

The significant relevance of viscous effects is well captured

by the plot of the vorticity field generated by the motion of

the two bodies at or in proximity of the free surface waves, as

in the example of Figure 14. The series of graphs in Figure 15

present coloured contour levels of the in-plane vorticity field

on a series of transverse planes along the length of the

SWATH, captured at the same instant in time, for wave length

/L=2.0. The complex structure of the vortex field is a result

of the viscous flow separation on the surface of the two

pitching and heaving vessels over time. It is relevant to note

that the wave amplitude considered in this study is relatively

high (with respect to the vehicle size) and corresponds to

realistic sea state conditions at which the SWATH is expected

to perform launching and recovery operations of the AUV. It

has been purposely chosen to eventually identify and

highlight dangerous non-linear phenomena avoiding to stay in

a relative small wave height which would satisfy the linear

approximations, but would be unrealistic in practice.

Figure 12: Heave motion vs. time predicted for the SWATH (red curve) and for the AUV (blue curve) simulated together and for the

SWATH-solo (cyan curve). /LAUV=12.0 (/L=4.0).

Figure 13: Pitch motion vs. time predicted for the SWATH (red

curve) and for the AUV (blue curve) together and for the SWATH-

solo (cyan curve). /LAUV=12.0 (/L=4.0).

5.0 CONCLUSIONS and FURTHER STUDIES

5.1 L&R Manoeuvre

The fully non-linear free surface unsteady viscous simulations

in waves, permit to conclude that, in spite of the excellent

seakeeping performance of the ASV-SWATH, any L&R

manoeuvre that aims to take on board the AUV from short

below the free surface has to be avoided. The large drift

motions predicted for the AUV floating in proximity of the

wavy sea surface at a depth d=L/2 render any type of

manoeuvres with the AUV close to the surface hazardous

even in mild sea states. Although this could be imagined, it

could not be demonstrated using the results of any standard

linear seakeeping method.

The first concept for launching and recovering the UAV from

the ASV-SWATH has been changed. The system initially

designed (Brizzolara & Chryssostomidis, 2013) consisted, as

per Figure 1, of two couples of electric driven self-tensioning

winches one forward and one aft, each one of them controlled

on the effective tension of the hoist and belt assembly. The

new system features a transparent (light truss construction)

cradle which is launched and retrieved from the AUV by

means of the same self-tensioning winches system. This time

the cradle though, can be lowered in water at relatively high

depth where the wave induced motion of the AUV is almost

vanished.

The lifting operation starts with the lowering of the hoists and

hoist/belt assemblies into the water, at a sufficient depth to be

less sensitive to the wave drift forces (this is relevant also for

the AUV). Only a couple of thin stainless steel cables of the

hoists are piercing the water in this lower configuration, so

the wave drifting effect is reduced to a minimum. The belts

contact surfaces are finished with a thick and dense silicon

rubber coating, in order to have the best grip on the AUV

surface.

The AUV is guided inside the cradle by a sonar beacon

positioned at the leading edge of the cradle truss of the two

hulls and when in place and correctly aligned gives the

confirmation signal to start the lifting manoeuver. During the

lift manoeuver hoists are controlled either on the basis of the

tension force acting on the hoists cable or on the acceleration

measured at the winch location. When lifted up the AUV is

secured against a soft stopper by calibrated tensioning of the

hoists.

The operational limits of this type of system will be verified

by means of multi-body seakeeping simulations, possibly

with the same types of solvers used for the presented study

coupled to a multi-body time domain method for the dynamic

of the hoists cables when they are subject to the AUV weight

and wave loads.

Other types of systems yet based on winches or on rigid

articulated structures are also under evaluation and will be

possibly topics of future research.

5.2 Motion Simulations

We have presented main technical and numerical aspects of a

RANSE based simulation model for multi-vehicle time

domain motion predictions in regular waves. In absence of

model tests results, the viscous numerical model has been

verified with the results obtained by a high order 3D panel

method which solves the linear seakeeping problem. The

viscous non-linear simulations results agree with the results

obtained by the potential flow method if only the fundamental

oscillation component is measured: they show closer

amplitudes in heave and smaller in pitch, due to expected

viscous damping. The time domain predictions, though, show

that large slowly varying motions induced by second order

drift forces for the AUV, especially when it is in isolation. The

presence of the SWATH induces a substantial disturbance on

the AUV which somehow tends to mitigate the extreme drift

motions observed at some wave lengths.

It was verified that viscous and second order (non-linear)

forces do have an important effect on the motions of AUVs in

proximity of the free surface and hence linear seakeeping

method which cannot deal with these important effects are not

appropriate to conduct launching and recovering studies of

Figure 14: Longitudinal Sections of the unsteady (viscous free surface) flow field calculated around the two bodies in a regular incoming

wave (/L=2.0, Hw/L=7%).

AUVs onto surface vessels. Studies of this kind need to be

supported by appropriate simulations with fully viscous

unsteady Reynolds Averaged Navier-Stokes solvers with

coupled 6DoF (degrees of freedom) motions of multi-bodies.

The current study has permitted to draw these conclusions and

to set-up the physical and numerical parameters needed to

obtained accurate and reliable predictions of the motions of

the two bodies in waves.

Further investigations are planned to verify, for instance, the

effect of the third degree of freedom in the vertical plane,

namely surge, in the numerical simulations. This motion was

not considered in the current simulations. Another

investigation will regard the effect of a different moving

meshing algorithm, as for instance overlapping meshes

(chimera meshes) onto the accuracy of the predicted motions.

AKNOWLEDGEMENTS

This research effort has been partly supported by ONR grant

N00014-11-1-0598, dedicated to the study of an underway

recharging system for AUVs from the special autonomous

SWATH surface vehicle and the predictions of the relative

motions in waves.

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Figure 15: Transverse Sections of the unsteady (viscous free surface) flow field calculated around the two bodies in a regular incoming

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