Impinging swirling jet against a wall: experimental

Impinging swirling jet against a wall: experimental
investigation by PIV and high speed visualizations
Mario Felli, Massimo Falchi, Pietro Fornari
1: Propulsion and Cavitation Laboratory, INSEAN, Italy, [email protected]
The present paper deals with the problem of an impinging swirling jet against a wall. The study concerned a
detailed experimental investigation on the jet-wall interaction using PIV and flow visualizations over a
range of operating conditions in which the distance of the ducted propeller from the wall was changed. The
influence of the impingement distance and the propeller revolution speed as well as the interaction between
the jet deformation and the perturbation induced on the wall are highlighted in the paper.
1. Introduction
In this study, the problem of the interaction between a swirling jet and a wall is addressed. Swirling
jets impinging on surfaces are widely encountered in nature (i.e. tornadoes, waterspouts, dust
devils) and in different industrial applications (i.e. heating and cooling of metals, turbine cooling,
cooling of high power electronic components). In particular, swirling jets are used in marine
engineering to carry out underwater excavation of seabed soils, for the purpose of pipeline
trenching and pre-sweeping, bed-leveling, navigation dredging . Besides its practical importance,
jets impinging on solid surfaces have also received considerable attention in literature because they
present several flow types of interest (i.e. developing wall-jets, shear layers, a potential core and a
stagnation zone) (Serrin, 1972; Shtern and Mi, 2004, Abrantes and Azevedo, 2006).
In the present work, a detailed investigation of swirl jet impingement was conducted in a large
cavitation tunnel over a range of operating conditions in which the distance of the ducted propeller
from the wall. The swirling jet was generated by a ducted propeller.
Measurements were carried out in the large cavitation tunnel of INSEAN (i.e. Italian Institute for
Naval Architecture Studies and Testing) on a large scale model of a ducted propeller jetting system.
Specifically, the study concerned: flow visualizations by standard and high speed cameras,
dynamometric measurements to assess propeller performance characteristics under impinging and
not impinging conditions and PIV measurements along a diametral plane of the jet.
The jet generation system and the experimental set up are presented in section 2. The test matrix is
described in section 3. The experimental results are documented in section 4: the results of the flow
visualizations are discussed in 4.1, the mean and fluctuating velocity field are presented in 4.2.
Then a brief summary is given in section 5.
2. Experimental set up
2.1 Jetting system model
A scale model of a ducted propeller was built at a λ=1/3 scale ratio. The model is a replica of a
ducted propeller used to carry out underwater excavation of seabed soils. Technical characteristics
of the propeller are reported in Table 1.
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λ
Z
Dp (mm)
(P/D)tip
(P/D)0.7
(P/D)hub
Scale ratio
Blade number
Overall diameter(full scale)
Pitch-diameter ratio at the tip
Pitch-diameter ratio at 0.7R
Pitch-diameter ratio at the hub
1/3
4
670
0.89
0.95
1.2
Table 1 Main features of the ducted propeller
The propeller is of relatively standard design, typical of axial-flow pumps with the geometric pitch
varying along the blade span: over-pitched in the inner-part of the blades and under-pitched in the
tip region.
A picture of the ducted propeller is shown in Fig.1. The propeller model has a gap clearance of
about 1.7 mm.
Fig. 1 Sketch of the ducted propeller
All the experiments were carried out in the INSEAN large cavitation channel. This free surface
facility has a 4 million litres capacity. The circulation water speed in the test section can varied
continuously up to 5 m/s and the free surface water pressure can be varied in the range 5 to 101 kPa.
Note that a pressure less than 101 kPa represents less than the atmospheric pressure. The test
section has a constant rectangular cross-sectional shape, and is provided with large viewing
windows on the lateral sides. The main dimensional characteristics of the test section are: length =
10 m, width = 3.6 m, max. water depth = 2.25 m, with 1.0 m of freeboard above the free water
surface. The dynamometer with the ducted propeller model was fixed in the test section with the jet
axis at right angle as to the lateral wall of the control room. This configuration meant that the
channel side wall was used to simulate the seabed. In this configuration, the ducted propeller
delivers a horizontal jet that impinges against one of the windows of the control room.
The dynamometer was set up with the shaft at 700 mm below the free surface. This choice was the
result of a trade-off between the need to minimize free-surface effects and blockage by the facility
floor, and the requirement to provide visual and instrument-lead access through a window into the
control room.
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2.2 Set up of the jet visualizations
A qualification of the jet behaviour was performed through flow visualizations at different
operating conditions. A sketch of the experimental set up of the visualizations is reported in figure
2. Flow visualizations were undertaken by injecting pressurized air into the jet. Visualizations were
performed using two cameras: a standard CCD camera placed in an underwater case to image the jet
transversely at different positions of the propeller blades ranging between 0 and 360° (configuration
1), and a high-speed-CMOS camera set up in the control room to image the jet longitudinally
through the control room window (configuration 2).
In the configuration 1, the camera was a PCO Sensicam 12 bit, with a 1280×1024 px CCD sensor
and a frame rate of 8 Hz. The camera was equipped with a 28 mm 2.8 f-number lens. The
underwater camera was synchronized with the propeller in order to acquire images at specific
angular positions. This was carried out by triggering the image acquisition with a TTL signal
generated by the synchronizer at the desired position of the propeller.
In the configuration 2, the high speed camera was a Photron-Ultima APX model, equipped with a
10 bit monochrome 1024×1024px CMOS sensor and with a 1.4 f-number 50 mm focal length lens.
Acquisitions where performed at 1kHz frame rate.
A strobe light synchronized with the propeller was used for the visualizations with the CCD
camera.
Measurements were executed at two impingement distances, corresponding to x=2D and x=3D,
rotating the propeller at 12 rps.
2.4 PIV experimental set up and image processing algorithm
A sketch of the PIV experimental set up is reported in figure 3. Image acquisition was performed
using a standard CCD camera placed inside an underwater case to image the jet transversely (Right
of Figures 3). The CCD camera was the same used for the visualizations in configuration 1 (see §
2.3). The light source was a Big Sky Quantel laser: this standard double-cavity Nd-Yag pulsed
laser, emitting at 532 nm and 200 mJ energy/pulse, is capable to shot pulses of laser light whose
durations are as low as 7ns.
The flow field evolution was reconstructed reassembling different patches by which the whole
investigation area was scanned. The PIV system was mounted on a traverse system in order to allow
the imaging of the different patches. This operation was executed with an accuracy of about 0.1
mm. The initial reference position was fixed with an accuracy of about 0.5 mm by imaging a special
target device. The number of patches ranged between 5 and 7 dependently on the investigated
impingement distances. Patches were overlapped partially in order to compensate any minimal
camera misalignment during the final reconstruction of the flow field evolution.
Fig. 2. Sketch of the experimental set up of the flow visualizations (left).
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Fig. 3 Sketch of the PIV measurement experimental set up.
Image acquisition was synchronized with the propeller at specific angular positions. More
specifically 50 angular positions with an angular step of 7.2° were acquired for each patch. The
phase sampling of the PIV images was based upon an angular triggering approach. The timing of
laser pulses and camera recording was triggered as soon as the propeller reference blade reaches the
selected angular position. This operation was repeated for each investigated angular position and a
population of 1000 images pairs per patch was collected for the statistical analysis.
The phase averaged mean and turbulent velocity corresponding to a certain angular position of the
propeller were obtained through an ensemble average of the instantaneous PIV acquisitions (one for
each image pair) collected at that angle.
Fig. 4 Sketch of the PIV measurement experimental set up (left) and arrangement of the PIV system in the test section.
Acquired images were analysed using an home-made algorithm in which the window off-set
correlation and the window deformation techniques were implemented (Westerweel 1997; Scarano,
2002). Details on the processing algorithm are documented in Di Florio et al. (2002).
Statistic evaluation was performed considering only velocity vectors that fulfill all the above
specified filters simultaneously. About 15-20 seed particles are generally present in the 32 px2
interrogation area.
Three different impingement distances with the ducted propeller at x=2D, x=3D and x=4D from the
wall were. In all the configurations the ducted propeller operated at bollard pull condition (J=0).
The propeller revolution speed was kept constant at a value of 10 rps for all the studied
configurations. This value was chosen in order to overcome that air bubbles, forming at the free
surface of the channel and passing across the laser light plane, could create dark areas in the
acquired images. In this regard, the selected speed was the maximum speed at which the
aforementioned problem was experienced to not occur.
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3. Result analysis
The transverse visualization of the jet at x=2D and x=3D are illustrated in the top side of figure 6.
The different traces of whether the hub vortex and the tip structures highlight the phase loss of the
wake just behind the duct outlet. This phenomenon, typical of the transition to the instability of a
propeller wake and already observed in literature by Di Felice et al. (2004), occurs closer and closer
to the propeller as the swirl number increases and, thus, is particularly evident at bollard pull
condition.
The trace of the hub vortex is apparent clearly in the visualizations due to its intensity, which is
strong enough to suck most of the air bubbles. The hub vortex is straight within the wallunperturbed region, after which it undergoes an abrupt change to a diverging-spirallinggeometry. In this regard, an appreciation of the hub vortex evolution in the close-to–wall region is
given by the high speed visualizations which highlight the aforementioned spiral pattern and the
hub vortex breakdown on the wall clearly (bottom of figure 3). The extension of the spiral and the
position of the straight-to-spiral transition region show a clear dependency on the impingement
distance. More specifically the amplitude and the intensity of the hub vortex spiral increase at
reducing the impingement distance.
In the near wake, the envelope of the tip structures describes a cylindrical surface whose radius is
about the same of the duct outlet. Further downstream, in the wall region, the geometry of the jet
changes and it becomes splayed out assuming the typical shape of the Type B jet (Serrin, 1972): the
jet adheres to the wall and travels outwards radially as a wall-jet flow.
In figure 7 the analysis of the PIV contour plots highlights the typical flow regions of an impinging
swirling jet with a potential core, a close-to-wall stagnation zone, shear layers and developing wall
jets (Abrantes and Azevedo, 2006). In the next paragraphs a description of the main features of the
jet at the different operating conditions is reported.
Close to the ducted propeller one finds some typical features of a propeller operating in open water
condition (propeller with no duct) (Stella et al., 2000; Di Felice et al., 2004; Felli et al., 2006).
The effect of the wall causes an abrupt deformation of the jet which tends to spread out suddenly,
moving outwards the impingement point and flowing nearly parallel to the wall in the typical
configuration of a wall-jet. Such a significant spread-out is correlated to the swirl component of the
jet. Figure 8 documents the radial distribution of kinetic energy (i.e. 0.5·(U2+V2)) just before the jet
splay-out: the energy content of the jet, about the same for the configurations at 2D and 3D
impinging distance, undergoes an evident reduction when the ducted propeller is moved at 4D.
Fig. 5 Jet evolution for the configuration with the ducted propeller at x=2D (left) and x=3D (right).
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Fig. 6 Evolution of the hub vortex in the configuration at 3D from the wall (front view)
Fig. 7 Phase averaged axial velocity (first row), vorticity and streamlines (second row) and TKE (third row) for the
impingement distances of 2D (left), 3D (center) and 4D (right)
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Fig. 8 Radial distribution of the mean (left) and turbulent (right) kinetic energy just behind the splay out region of the jet
The occurrence of a toroidal structure in the re-circulation region is highlighted by the analysis of
the vorticity field and the streamlines in figure 7.
Figure 7 shows the distribution of the in plane contribution to the turbulent kinetic energy (i.e.
0.5·(u’2+v’2)). The rate of velocity fluctuations in the jet is larger in the boundary region of the recirculating bubble and increases at reducing the impingement distance. The two shear layers
developing around the jet boundaries, one at the interface of the jet with the surrounding stagnant
fluid and the other at the interface of the jet with the slow moving fluid of the inner re-circulating
region and the instability of the spiraling hub vortex are the causes of this phenomenon.
In conclusion, the analysis of the PIV data highlights two main aspects that are likely to have an
influence on efficiency of the excavation system. On the one hand, the wall induced energy transfer
from the mean to the turbulent flow causes a reduction in excavation efficiency which increases at
the reducing impinging distance. On the other hand, the jet energy before the splay out region tends
to reduce with the impinging distance for the aforementioned wake instability, dissipation and
transition-to-turbulence effects. Therefore, there is an optimal distance for the ducted propeller from
the wall for which seabed excavation is strongest once a specific propeller rotational speed is
defined.
1. Conclusions
The dynamic of an impinging swirl jet generated by a ducted propeller was tackled experimentally
in the large cavitation tunnel of INSEAN through analyzing the jet behavior at different impinging
distances. The study consisted in phase averaged PIV measurements and high speed visualizations
carried out with the model fixed in the test section with the shaft-axis normal to the lateral wall of
the control room at the impinging distances of 2, 3 and 4D. The whole measurement area was
investigated through the composition of 30 mm2 patches. This allowed resolving the complete
evolution of the jet without jeopardizing the spatial resolution of the measurement. On the physical
side, the following conclusion can be pointed out:
- Close to the ducted propeller the typical features of a standard propeller with no duct are
highlighted by the PIV contour plots. The effect of the duct occurs in the tip region of the jet
mainly where a vorticity sheet extended all along the trailing edge of the duct is developed in
place of isolated tip vortices.
- The jet spreads out as suddenly as it starts to “feel” the effect of the wall, moving outwards and
flowing nearly parallel to the wall in the typical configuration of a wall-jet. The nature of such a
significant spread-out of the jet is the consequence of the swirl component and increasing at
increasing the swirl number. The hub-vortex rolls up around the re-circulating region describing
a spiral geometry before breaking down against the wall surface.
- The jet spread out causes the formation of a re-circulating region with a toroidal structure inside.
The latter causes a reverse flow at the prolongation of the propeller axis whose effect is
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correlated to the excavation action.
Acknowledgements
The present study was supported by the EU in the framework of the Swirl-Jet Project.
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