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By Shin Hyung Rhee and Shitalkumar Joshi, Fluent Inc.
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The static pressure distribution on the back (left) and face (right) sides of the propeller for an advance
ratio of 1.1, showing the expected features, including the low pressure region near the tip on the face side,
where the tip vortex originates
Compared to fans and turbines, a marine
propeller has a very complex geometry
because it has variable section profiles,
chord lengths, and pitch angles. In addition,
to meet the heavy loads required for today’s
high-speed vessels, marine propellers are usually
operated at high rotation speeds, and this
results in even higher skewness in the propeller
design. These factors make propeller flow one
of the most challenging problems in CFD and
to date, many researchers and designers have
relied on experimental techniques and simplified
numerical methods for analysis.
Recent modeling efforts have shown,
however, that CFD can provide valuable insights
into the flow field generated by a propeller,
including the forces and moments due to the
rotating blades. The inviscid potential flow methods
used previously were successfully applied
to many propellers and are still popular among
field designers. Yet this approach often
requires cumbersome procedures and a considerable
amount of preliminary knowledge.
During the past decade, CFD methods for solving
the RANS equations have been increasingly
applied to various marine propeller geometries.
While these studies have shown great advancement
in the technology, some issues still need
to be addressed for more practicable procedures.
These include mesh generation strategies
and turbulence model selection.
Regarding the mesh challenges, the strong
twisting of the blade central plane, a stagnation
point on the hub close to the blades, and
limited space for grid generation behind the
ship have combined to make structured meshes
impractical. Unstructured mesh generation
offers many benefits, but the recent introduction
of hybrid meshing has the most promise. Hybrid
meshes allow for fine resolution of the
boundary layers, and concentrated detail in
the tip vortex and wake regions.
Circumferentially averaged velocity components normalized
by tip speed, as functions of normalized radial coordinate
(r/R) on an axial plane
To properly capture the highly swirling flow
and tip vortex, the turbulence model must be
chosen appropriately. Many previous studies
have under-predicted the size and strength of
the tip vortex, and the cause is usually attributed
to an over-estimation of the isotropic turbulent
viscosity. Recent studies have shown that
both the k-w and algebraic Reynolds stress
models perform far better than previously used
models in predicting the mean flow features
of tip vortices.
In a recent study, a five-bladed, highly skewed
propeller of typical modern design, the
P5168, was simulated using the moving (rotating)
reference frame model and the k-w turbulence
model in FLUENT. The simulations
followed open water measurements1 made at
a wide range of advance ratio, J (the ratio of
freestream speed to tip speed). Thrust and torque
coefficients were found to be within 8% and
11% of measured values, respectively.
Circumferentially averaged axial, tangential, and
radial velocity components were found to reproduce
the experimental trends well. In particular,
the maximum axial velocity in the mid-span
area, where the blade has its highest pitch angle,
was captured. The tangential component was
found to increase with increasing radius, as
expected, and negative radial velocity, which
indicates flow contraction due to the propeller
action, was also in good agreement with data.
Finally, the increasing magnitude of all velocity
components with increasing load and decreasing
J was also correctly captured.
Reference:
- C. Chesnakas and S. Jessup, Proc. 22nd
Symposium on Naval Hydrodynamics,
Washington DC, 1998.
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