Issue 6 - Ifremer

H
ydrodynamics Facilities Newsletter
Issue 1
October 2006
The French public institute for marine research Ifremer contributes, through studies and expert
assessments, to knowledge about the ocean and its resources, monitoring of marine and coastal zones
and the sustainable development of maritime activities.
To these ends, it designs and operates observational, experimental and monitoring tools and facilities.
Ifremer manages the ocean research fleet for the French scientific community.
Contents
Experimental facilities
…... 1
Vortex and wake
effects on marine risers ….. 2
Flow characterization
…….... 3
around a cod-end
Fishing gear development .. 4
Reduction of turbulence
by honeycombs …..….…… 4
Towfish ……..……………. 5
Marel Smatch buoy ..……. 5
Main activities
…………... 6
As the main French research institute in marine science, Ifremer has a wide range of facilities at it's
disposal (http://www.ifremer.fr/metri/), among which are two major Hydrodynamics facilities. This
newsletter is devoted to the IFREMER free surface flume tank in Boulogne-sur-Mer. The next issue
will be devoted to the deep wave basin in Brest.
Experimental facilities
The Hydrodynamics laboratories of Boulogne-sur-mer and Brest carry out researches on
submarine devices and new offshore concepts. For that purpose, experimental and numerical
facilities are used to give hydrodynamics expertises. The Boulogne-sur-Mer unit is in charge of a
free surface hydrodynamic water tunnel while the Brest unit is in charge of an ocean engineering
basin, where tests are carried out for French or foreign partners, for development and research
projects or for assistance in confidential matter.
The activity connected to the flume tank can involve a wide variety of tests, dealing with
hydrodynamic problems as well in the field of fishing techniques as in those of new offshore concepts
(structures characterization, phenomena of fluid/structure interaction...). For that purpose, measuring
techniques specific to hydrodynamic researches are regularly developed and implemented (like LDV
and PIV laser velocimetry techniques).
The experimental principle is to provide an homogeneous
water flow around the model or the device being tested.
This last is maintained in the channel by either a towing
system or a set of force and motion sensors. A large window
placed on one side of the tunnel allows to observe the
behaviour of the models during trials and to carry out video
sequences.
The bottom of the flume is a conveyor belt which can be
synchronized with the water speed. This technique allows
the simulation of devices in contact with the bottom and
limits the boundary layer development.
The flume tank is 18 m long by 4 m wide and 2 m deep with
a side observation window of 8 m x 2 m. The flow
turbulence is less than 5 % and the flow velocity range is 0.1
to 2.2 m/s.
Instrumentation:
2D Laser Doppler Velocimeter
2D Particle Image Velocimeter
3 and 6 components loads cells
3D motions measurement system
Software:
Fluent for towed bodies, foils…
studies
In-house software for test data
acquisition, results analysis…
Presentation of IFREMER Boulogne-sur-Mer facility
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IFREMER Hydrodynamics Facilities Newsletter
Vortex and wake effects on marine risers
Area of interest
Fishing technology
Seismic devices
Offshore engineering
Renewable energy
ROV - Submarine
Other activities
Vortex and wake effects on long risers are generally less well understood than other load effects
acting on marine structures. As petroleum exploitation moves to deeper waters, avoiding
interaction and collision between risers in an array becomes more and more difficult and
expensive to obtain. Some ongoing research are performed for a better understanding of the
hydrodynamic loads acting on a riser placed on the wake of an upstream one. The main
concerns about riser interference are to be able to predict the VIV amplitude, shielding and
wake effects and also to avoid the risk of collision between closely spaced risers. This research is
conducted within a French JIP*.
An experimentation involves the study of VIV for two closely spaced vertical pivoting risers in 2 m
water depth**. Scaled model tests are performed to improve the understanding of the behaviour of
dual riser interaction in uniform and steady current. A particular attention is paid on how fluid
interaction between two cylinders of equal diameter in cross-flow can significantly modify their
structural response in term of amplitude and frequency, compared to a single cylinder (see Figure 1).
Figure 1: Rms amplitude response for
risers in tandem
Figure 2: PIV map on a circular cylinder
(Vr from 4 to 15)
Riser with strakes during trials
The risers array is tested at Reynolds
numbers from 5500 to 40000. The
Figure 3: Response of two closely
reduced mass and the natural frequency
spaced risers (upstream cylinder on the right, the downstream one on the left)
of the system are respectively 1.3 and
0.75 Hz. Experimental results presents
VIV amplitude, shielding and wake effects, drag and lift amplification, together with detailed flow
velocity measurements obtained from PIV (see Figure 2). Both inline and cross-flow responses are
presented as functions of non dimensional numbers and demonstrate that wake effects can be
relatively strong (see Figure 3) .
Due to the complexity of the problem, experimental and numerical approaches are used
simultaneously for a better understanding of the hydrodynamic loads acting on risers in tandem.
* Acknowledgement to all JIP partners: Doris Eng., EGIM, IFP, Océanide, SAIPEM
** Vortex and Wake effects on closely spaced marine risers, G. Germain & al., FIV 2006, Vancouver
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Issue 1 October 2006
http://www.ifremer.fr/boulogne/anglais/bassin.htm
Flow characterization around a cod-end
Area of interest
The selectivity of fishing gears like trawls is conditioned by the net structure and flow
characteristics. With the aim of a better understanding of interaction in presence of these
flexible structures, we carried out an experimental study to determine and to analyse the flow
over a cod-end*.
Fishing technology
Seismic devices
Offshore engineering
Renewable energy
ROV - Submarine
Other activities
Improving the knowledge of the process
involved in netting would improve their
selectivity. Notably the small fish escapement
could be facilitated by the mesh shape, the
tensions in twines, the zones of high flow or
dead water inside the fishing gear, the flow
inside the meshes… If the flow around the whole
structure is difficult to study, it is easier to focus
in a first time on parts of the whole structure
such as warps, doors or cod-end… The present
work is devoted to the cod-end and has been
done in the field of a European project
(PREMECS II – contract QSR5-2002-01328).
During the first phase of the project, tests were
carried out in order to measure the shape and the
tension in a cod-end. It was done for few netting
characteristics, several catches and type of codend. The flow has not been measured
Figure 1: Measurement near the netting
with a 2D LDV system
during this first phase due to the instabilities
of the cod-end and especially of the catch
inside the structure. So, it has been decided
to realize a second phase of trials for the
measurement of the flow inside and around a
cod-end.
To achieve this aim, a specific stiff cod-end
has been carried out to avoid instabilities.
This structure is perfectly stable in the flow
and its geometrical shape is known. It has
allowed to measure flow velocity profiles by
Laser Doppler Velocimetry (LDV, see
Figure 1) and Particle Image Velocimetry
techniques.
The pattern for all the velocity profiles
(Figure 2) shown a well developed flow, for
which the general characteristics are
preserved inside the cod-end. The profile 6,
just upstream the catch, shows very low
water speeds inside the structure. At this
point, the axial velocity component reaches
15% of the entry velocity while it reaches
50% on profile 5. In fact, near the catch
the flow escapes the cod-end, while
upstream the flow penetrates into the
structure. The water outgoes near the catch
with a pretty high velocity (the same order as
the entry velocity). The measurements
behind the catch confirmed the high
turbulent flow in this zone (see Figure 3).
Figure 2: LDV profiles of axial velocity inside and outside the
cod-end. The vertical lines indicate the netting position
(profile 2 at the entry, profile 7 at the beginning of the catch).
Figure 3: Particle Image Velocimetry map around a cod-end
* Flow characterization around a cod-end, G. Germain, J-V. Facq, D. Priour, Int. Maritime Congress, Lisbon, september 2005
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IFREMER Hydrodynamics Facilities Newsletter
Fishing gear development
Area of interest
Fishing technology
Seismic devices
Offshore engineering
Renewable energy
ROV - Submarine
Other activities
By-catch reduction devices are being used in some countries. Helps are needed to effectively
introduce and transfer TED (Turtle Excluder Device) and BRD (By-catch Reduction Device)
technology to those countries. To this end, technical developments continue on efforts to reduce
by-catch and turtle capture in shrimp trawls in Guyana and Madagascar.
The Turtle Excluder Device was created to
lessen the impact of shrimp fishing on sea
turtle populations. The TED is a metal grid of
bars attached to a shrimp trawling net. It has
an opening at either the top or the bottom,
which creates a hatch allowing larger animals,
such as sea turtles, to escape while keeping the
shrimp inside. The BRD is used to reduce the
capture of small fish by-catch.
Introduction of both TED and BRD into
shrimp fishery needs technical developments
to obtain some well fitted devices. To this
end, real scaled tests were conducted on
flume tank to increase the efficiency of trawl
nets including those devices. After tank
trials, testing onboard a commercial shrimp
trawler under normal operating conditions
were carried out.
Turtle Excluder and By-catch Reduction Devices trials
Reduction of turbulence by honeycombs
A numerical and experimental work* was carried out to evaluate the influence of honeycombs
on the reduction of turbulence. In order to assess the evolution of the turbulence through these
meshes, different characteristics of the honeycombs (length, diameter, thickness…) were tested.
So, the influence of the structural parameters on the flow was highlighted. Numerical
simulations were validated from comparisons on rate and scale of turbulence over the
structure. The calculations used a CFD code and the experimental results were obtained using
Laser Doppler Velocimetry and Particle Image Velocimetry.
Numerical and experimental axial velocity comparison
150 mm downstream the honeycomb
Comparison between
PIV and CFD velocity map at the honeycomb exit
* Reduction of turbulence by honeycombs, G. Germain, C.
Candelier, S. Blarel, 10e Journées de l’Hydrodynamique,
Nantes, France, mars 2005
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Issue 1 October 2006
http://www.ifremer.fr/boulogne/anglais/bassin.htm
Area of interest
Fishing technology
Seismic devices
Offshore engineering
Renewable energy
ROV - Submarine
Other activities
INNOVA AS and HFT AS develop new deep
towed depressor (ViperFish II) and towed
undulating instrument carrier (Turbot).
The ViperFish II is a combination of a
hydrodynamic depressor design and a
heavily ballasted clump weight, while
Turbot has been designed as a terrain
following/undulating towfish.
Turbot in action
Model tests have been carried out in the flume tank on the depressor alone and the Turbot towed
behind the depressor. Results indicated that all the design criteria had been fulfilled, with an
excellent behaviour and stability of the system. These towfish were piloted from the surface with
an optimum control of the altitude and positioning. The Turbot also showed its aptitude to track the
seabed
“Marel Smatch TS” buoy
The first specimen of the new buoy “Marel Smatch TS”, result of a collaboration between Ifremer
and NKE, was presented in preview at
Oceanology International meeting 2006. The
first trials were programmed in order to
determine the behaviour of this buoy in
current. The selected mechanical concept was
validated, and the operational limits in tidal
currents were given. The buoy weighs less
than 10 kg and its height is 1.20 m. It is easy to
handle and install from a light boat. In its first
version, the buoy analyses the temperature and
salinity. The data collected are transferred
autonomously by GSM towards a ground
station. A more complete version for the
measurement of other parameters like
turbidity, fluorescence, dissolved oxygen and
pH will also be developed.
http://www.ifremer.fr/prod/marel/mareluk.htm
Marel Smatch buoy in current
5
Boulogne-sur-Mer Hydrodynamic
Water Tunnel
Hydrodynamic tests are carried out on underwater
vehicles and new offshore structures. Studies are
mainly conducted at reduced scale in calm water or
in current at a flow speed between 0,1 and 2 m/s.
The free surface water tunnel is designed to test
devices towed on the sea bottom (fishing-gears),
submersed, propelled or anchored in deep water or
floating on the surface.
Underwater vehicle trials
(Subeo)
Study of a Remote Operated Vehicle
(Comex Pro)
Offshore structure
(Saipem)
Acknowledged experience
¾ Behavioural study and performance analysis
¾ Geometry and stress measurements
¾ Measurements of hydrodynamic coefficients
¾ Flow characterisation
¾ Shape optimiser and methods of validation
¾ Fluid structure interaction analysis
Testing facilities
Wing profil study
(Thales)
¾ P.I.V. (Particle Image Velocimetry)
¾ Laser Doppler velocimeter (2 components)
¾ Laser tomography – Flow visualisation
¾ Load cells (3 and 6 components)
Cod-end hydrodynamics
Laser velocimeter measurement
P.I.V. Flow map
IFREMER Hydrodynamics Facilities France - E-mail : ggermain@ifremer.fr - http://www.ifremer.fr/boulogne/anglais/bassin.htm