In the present study an experimental analysis of the velocity and pressure fields behind a marine propeller, in non cavitating regime is reported. Velocity measurements were performed in phase with the propeller angle by using 2D Particle Image Velocimetry (2D-PIV). Measurements were carried out arranging the light sheet along the mid longitudinal plane of the propeller, to investigate the evolution of the axial and the radial velocity components, from the blade trailing edge up to 2 diameters downstream. The pressure measurements were performed at four radial and eight longitudinal positions downstream the propeller model. Measurements of the pressure field were performed at different advance ratios of the propeller. Pressure data, processed by using slotting techniques, allowed to reconstruct the evolution of the pressure field in phase with the reference blade position. In addition, the correlation of the velocity and pressure signals was performed. The analysis demonstrated that, within the near wake, the tip vortices passage is the most important contribution in generating the pressure field in the propeller flow. The incoming vortex breakdown process causes a strong deformation of the hub vortex far downstream the slipstream contraction. This process contributes to the pressure generation at the shaft rate frequency

Camussi, Roberto

Di Felice, Fabio

Felli, Mario

Guj, Giulio

M. Felli

F. Di Felice

G. Guj

R. Camussi

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Analysis of the propeller wake by pressure and velocity correlation

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15th Australasian Fluid Mechanics Conference 
The University of Sydney, Sydney, Australia 
13-17 December 2004 
 
Analysis of the propeller wake by pressure and velocity correlation 
 
M. Felli1 and F. Di Felice1 
 
1INSEAN, Italian Institute for Naval Studies and Testing 
 Via di Vallerano 139, 00128 Rome, ITALY 
 
G. Guj2, R. Camussi2 
2Department of Mechanical and Industrial Engineering 
“Roma Tre” University, Via della Vasca Navale 79, 00146 Rome, ITALY 
 
 
Abstract 
In the present study an experimental analysis of the velocity and 
pressure fields behind a marine propeller, in non cavitating 
regime is reported. Velocity measurements were performed in 
phase with the propeller angle by using 2D Particle Image 
Velocimetry (2D-PIV). Measurements were carried out arranging 
the light sheet along the mid longitudinal plane of the propeller, 
to investigate the evolution of the axial and the radial velocity 
components, from the blade trailing edge up to 2 diameters 
downstream. The pressure measurements were performed at four 
radial and eight longitudinal positions downstream the propeller 
model. Measurements of the pressure field were performed at 
different advance ratios of the propeller. Pressure data, processed 
by using slotting techniques, allowed to reconstruct the evolution 
of the pressure field in phase with the reference blade position. In 
addition, the correlation of the velocity and pressure signals was 
performed. 
The analysis demonstrated that, within the near wake, the tip 
vortices passage is the most important contribution in generating 
the pressure field in the propeller flow. The incoming vortex 
breakdown process causes a strong deformation of the hub vortex 
far downstream the slipstream contraction. This process 
contributes to the pressure generation at the shaft rate frequency. 
 
Introduction  
In the last years, the accurate analysis of the fluctuating pressure 
and velocity fields, induced by a marine propeller, is required by 
the increase of both the power of the ship and the demand for a 
more comfortable vessel.  
With the present emphasis on increased ship speeds and 
consequently much higher propeller thrust, in fact, 
hydrodynamic-induced noise and vibrations has became a very 
important problem both in navy and civil naval architecture. 
Vibratory forces, which are predominantly applied at the blade 
frequency, have frequently caused local structural failure by 
producing fatigue and, in many cases, have precluded the 
occupancy of parts of passengers vessels because of the noise and 
discomfort correlated to the resonance of decks and bulkheads. 
The problem of reducing propeller induced noise and vibration 
leads to an increased complexity of blade geometry, primarily 
due to the low aspect ratio and to the skew, and have implied a 
rising interest on detailed measurements of the propeller flow 
field, to be used for both new design approach as well as for 
analysing propulsive, hydro-acoustic and structural 
performances. 
The pressure on the stern bottom of the hull, generated by a 
propeller, is strictly related to the induced velocity field, therefore 
a detailed flow field analysis, relating the flow structures and the 
pressure signal, is required to improve the knowledge on the 
noise mechanism. 
On the other hand, the experimental investigation provides 
baselines to improve and integrate theoretical predictions and to 
support the flow modelling and the validation of computational 
codes (BEM, RANS, LES).  
The flow field analysis around a propeller is complicated by 
many factors as unsteadiness, three-dimensionality, and high 
levels of turbulence. These properties has been pointed out in 
many previous Laser Doppler Velocimetry (LDV) measurements 
( [1], [2], [3], [4], [5]), but none of previous studies have been 
aimed in correlating the velocity field with the pressure signal. In 
the present study the pressure signals were correlated with the 
flow structures of the propeller slipstream, like the tip and hub 
vortex and the blade wake. The correlation between the velocity 
field and the pressure at a point is performed by using 2D-PIV 
and hydrophones.  
A Wageningen modified type model propeller (INSEAN E779A) 
was selected for the present research project for two main 
reasons. First, this model propeller has been widely studied with 
the most advanced flow measurement and visualization 
techniques, such as LDV ([3], [5]) and PIV [6]. A large amount 
of data has been collected providing a thorough documentation 
on the non-cavitating flow characteristics: the propeller 
geometry, LDV data and PIV data are now freely available for 
downloading at http://crm.insean.it/E779A. 
 
Facility and propeller model 
 Measurements were conducted in the Italian Navy Cavitation 
Tunnel (C.E.I.M.M.) This is a close jet tunnel with a 2.6 m long 
by 0.6 m span by 0.6 m deep test section. Perspex windows on 
the four walls enable optical access.  
The nozzle contraction ratio is 5.96:1 and the maximum water 
speed is 12 m/s. The highest free stream turbulence intensity in 
the test section is 2% and, in the adopted test condition, reduces 
to  0.6% in  the  propeller blade inflow at 0.7 r/R, being r the 
radial coordinate and R the propeller radius. In the test section, 
the mean velocity uniformity is within 1% for the axial 
component and 3% for the vertical component. 
The four blades propeller model E779A is skewed, with a 
uniform pitch (pitch/diameter=1.1) and with a forward rake angle 
of 4° 3”. A reference system O-XYZ, with the X axis along the 
tunnel centerline, the Y axis along the upward vertical, the Z axis 
along the horizontal towards starboard is adopted. 
Two dimensionless groups govern the propeller flow field in non-
cavitating conditions: the advance ratio   J=U∞/2nR, where U∞ is 
the free stream velocity, n the revolution frequency and 2R the 
diameter of the propeller, and the Reynolds number 
Re=C0.7V0.7/υ, where C0.7 and V0.7 are respectively the cord of 
the blade and the velocity at 0.7 r/R and υ the kinematic 
viscosity.  
 
PIV measurements experimental set up  
A sketch of the PIV experimental set up is reported in figure 1. 
The propeller model is mounted on a front dynamometer shaft. 
This arrangement of the propeller and the length of the test 
section, which is about 15 times the propeller diameter, allows 
the slipstream to develop freely in the downstream direction as in 
a real operative condition. An encoder, with a resolution of 0.1°, 
mounted on the dynamometer shaft, feeds a special signal 
processor which provides a trigger signal to a special 
synchronising device for each propeller angular position. The 
synchroniser provides a TTL trigger signal to a cross-correlation 
camera (1008 by 1018 pixel), and to a double cavity Nd-Yag 
laser (200 mJ per pulse at 12.5 Hz each), to allow image 
acquisitions for each propeller angular position. The digital cross-
correlation video camera, allows the recordings of two separate 
images (one for each laser pulse) within a few microseconds at a 
maximum camera frame rate of 15.0 Hz. By using cross-
correlation, the directional ambiguity is completely removed. The 
instantaneous velocity fields were acquired from a distance up to 
700 mm from the side window, using a 60 mm lens with 2.8 f-
number and imaging an area of about 100 mm by 100 mm. 
 
 
Figure 1.  PIV measurements experimental set up. 
 
  
The tracer particles are one of the critical aspects of the PIV 
technique, especially in case of large facilities. Since the 
technique is based on the measurement of particle displacement, 
it is fundamental that the seed accurately follows the water flow 
velocity. This requires particles having a diameter on the order of 
some µm. At the same time, it is mandatory to achieve a high 
uniform seeding density in the region of interest, at least 15 
particle pairs per interrogation window, in order to accurately 
perform auto/cross correlation analysis. To this purpose, the 
water in the tunnel was initially filtered, and then seeded with 10 
µm silver coated hollow glass spherical particles with high 
diffraction index and density of about 1.1 g/cm3. The PIV system 
was arranged to measure, in the mid longitudinal plane of the 
propeller, the axial and vertical velocity components 
simultaneously in the tunnel frame. In view of the symmetry of 
the propeller inflow and of the steady conditions, when the light 
sheet is located on the vertical radius (along the z-axis), the axial 
component of the velocity and the vertical one correspond 
respectively to the axial and the radial components in the 
propeller moving frame.  
To investigate the propeller wake up to 2 diameters downstream 
the blades trailing edge, the measurements were performed over 
3 adjacent windows by traversing the camera (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. 
   
Pressure measurements experimental set up  
Pressure measurements have been performed by using 
hydrophones (Bruel & Kjaer 8103 models), with a frequency 
response flat till 20 kHz and a diameter of 3.5 mm. These 
transducers are sensitive to the pressure fluctuations in the range 
form 0.3 Hz to 20 kHz, and hence are capable to measure the 
fluctuations of the total pressure. Hydrophones have been 
mounted on a special rake device, shown in figure 2. The 
transducers outputs were amplified by four conditioner-
amplifiers Bruel & Kjaer 2650 with band pass filter (0.3 Hz÷20 
kHz), connected with the acquisition system and a spectral 
analyzer. Simultaneously, the TTL signal was acquired to 
synchronize the angular position of the reference blade. The 
hydrophones signals were acquired for 50 s at the sample rate of 
40 kHz. 
The pressure measurements were performed at four radial and 
eight longitudinal positions downstream the propeller model. 
Pressure signals acquisition was performed at different propeller 
conditions J=0.748, 0.814, 0.880, 0.940, 1.012 that correspond to 
Re (Re x 106) 1.14, 1.15, 1.17, 1.80, 1.20. Correlation between 
pressure signal and velocity field has been performed only for 
J=0.88. 
 
 
Figure 2.  Hydrophones arrangement. 
 
 
Experimental results  
In the following we will focus on pressure measurement results 
and in the correlation of the pressure signal with the flow 
structures. 
More information about the flow field measured by PIV can be 
found in [6].  
Figure 3 shows the pressure coefficients at different radial 
positions, for the longitudinal station x/R=1.0. The signals show 
a different behavior and amplitude as a consequence of the 
different interaction with the flow structures of the propeller 
wake.  
An example of the pressure signals and the flow field measured 
by PIV for the angular position θ= 20° is reported in figure 3 
(where θ denotes the reference blade angular position). 
15th Australasian Fluid Mechanics Conference 
The University of Sydney, Sydney, Australia 
13-17 December 2004 
 
The contour plot represents the magnitude of the in-plane 
velocity components normalized with the freestream velocity. 
The main flow structures, like the viscous wake shed from the 
blade trailing edge, the tip and the hub vortex, are clearly 
apparent. 
In the same figures the pressure at the hydrophones locations is 
also highlighted. Specifically, the plots report the angular 
variation, at different radial positions, of the pressure coefficient 
fluctuations ∆kp defined as: 
                                  22 Dn
Pk p ⋅⋅
∆=∆ ρ                                      (1) 
where ∆P is the total pressure fluctuations. It is shown that the 
pressure signal at r/R=0.3 is influenced by the blade wake 
passages and by the hub vortex evolution. In the same way, at 
r/R=0.7, the signal consists of large scale fluctuations caused by 
the passage at the hydrophone location of the low pressure flow, 
coming from the face of the propeller and small scale fluctuations 
due to the passage of the blade wake.  
The maximum values of the pressure coefficient are achieved at 
r/R=0.9 simultaneously to the passage of the tip vortex core, 
pointing out that the tip vortex is the most important pressure 
fluctuations source in the propeller wake at x/R=1.0. In fact 
pressure fluctuation peaks are one order of magnitude larger at 
r/R=0.9 with respect the other locations. At r/R=1.2 the pressure 
fluctuation is very low being the transducer out the slipstream 
tube. Close to the trailing edge of the propeller and for all the 
radial positions the pressure signals are dominated by the blade 
rate.  
The described behaviour changes downstream the contraction 
due to the vortex breakdown mechanism. At this purpose, figure 
4 shows the amplitude of the blade rate and shaft harmonics, for 
the radial position r/R=0.3 (top of figure 4) and r/R=0.9 (bottom 
of figure 4). The analysis points out that near the hub (r/R=0.3), 
2-3 diameters downstream the propeller, a different mechanism is 
contributing to the generation of the pressure signal. In fact the 
transfer of the energy from the blade to the shaft rate harmonics 
is strictly related to the vortex breakdown mechanism, as shown 
in [6]. Downstream the contraction a separation of the trajectory 
of the tip vortices, due to the different blades and a strong 
spiralling and deformation process of the hub vortex, occurs. The 
slipstream starts to lose its axis-symmetry and blade periodicity 
but in the first phase of the breakdown still maintains the phase 
with the shaft revolution. This behaviour is probably due to the 
mutual tip-hub vortex interaction, as demonstrated by their 
simultaneous generation.  
The phase analysis of the correlation between the turbulence field 
with the standard deviation of the pressure signal (∆kp)std has 
been also conducted. An example of the achieved results is 
reported in Fig. (5).  
This analysis outlined the following points clarifying the wake 
characteristics: 
1. The highest values of the pressure standard deviation are 
related to the highest values of the turbulence intensity and 
mainly concentrated, for r/R=0.9, in the tip vortex core; 
 
Figure 3.  Correlation between velocity field, by PIV images, and pressure signal at θ=20°.  
The longitudinal station corresponds to  x/R=1.0. 
 
2. Turbulent blade wake, downstream of the propeller, is 
quickly diffused and dissipated by viscous effects. 
Nevertheless the standard deviation plots show two peaks 
associated respectively to blade wake and tip vortex 
passages. The maximum values are achieved in 
correspondence of the tip vortex core; 
3. The standard deviation of the pressure fluctuation in the 
downstream direction raises as a consequence of the largest 
turbulent fluctuation achieved in the tip vortex core far 
downstream the propeller. This effect is related to the vortex 
breakdown instability as shown in [6].  
 
Conclusions  
The performed experimental study allows correlating the 
propeller flow field structures, like the blade wake and the tip 
vortex, with the pressure signal at a point. The results point out 
that, within the slipstream contraction, the highest values of the 
pressure fluctuations are in correspondence of radial position 
x/R=0.9, in coincidence with the tip vortex passage. Thus this 
flow structure is the most important one in generating the 
hydrodynamic pressure field in the propeller flow.  
Far downstream the slipstream contraction the vortex breakdown 
occurs and the strong deformation of the hub vortex contributes 
in the pressure signal generation at the frequency of the shaft 
rate. 
The standard deviation of the pressure signal shows the presence 
of two peaks, with different intensity, due to blade wake and tip 
vortex passage. Furthermore, the increased values of the standard 
deviation in the downstream evolution point out the vortex 
instability phenomena as also shown by the turbulence 
distribution.  
 
Acknowledgments 
The authors are grateful to the CEIMM personnel who supported 
the PIV measurements. This work was sponsored by Italian 
Ministero delle Infrastrutture in the frame of INSEAN Research 
Plan 2000-2002. 
 
References 
 [1] Min K. S. (1978) "Numerical and experimental methods for 
prediction of field point velocities around propeller blade" 
Department of Ocean Engineering Report 78-12, MIT. 
 [2] Kobayashi S. (1982) "Propeller wake survey by laser doppler 
velocimeter" First International Symposium on the 
Application of Laser Anemometry to Fluid Mechanics, 
Lisbon. 
[3] Cenedese A., Accardo L., Milone R. (1985) "Phase sampling 
tecniques in the analysis of a propeller wake"  First 
International Conference on Laser Anemometry: Advances 
and Application, BHRA Fluid Engineering, Manchester. 
[4] Hoshino T., Oshima A. (1987) "Measurement of flow field 
around propeller by using a 3-component laser doppler 
velocimeter" Mitsubishi Tech Rev 24(1): 46-53. 
 [5] Stella A., Guj G., Di Felice F., Elefante M. (2000) 
"Experimental investigation of propeller wake evolution by 
means of LDV and flow visualizations" Journal of Ship 
Research,  44 (3):  159-173.  
[6] Di Felice F., Di Florio D., Felli M., Romano G.P. (2004) 
"Experimental Investigation of the Propeller wake at different 
Loading Condition by PIV" Journal of Ship Research, 48 (2): 
168-190. 
 
 
Figure 4. Amplitude of blade rate and shaft harmonics. 
 
 
 
 
 
Figure 5.  Correlation between turbulence and pressure signal standard deviation at θ=30°.  


Analysis of the propeller wake by pressure and velocity correlation

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