Geometry and mesh
Fig. 2 - CAD models
Fig. 3 - CFD external domain, with Inlet (blue) and Outlet (red)
A CAD model of the tower crane was supplied
by Terex: the geometry consisted only of the
upper part of the tower crane, specifically the
operator cabin, the concrete counterweight
and the jib. Three different crane lengths were
considered: 65, 60 and 55 meters (Fig. 2).
The crane lengths were obtained by
“cutting” the farthest part of the jib, while the
counterweights were reduced accordingly.
To accurately calculate the fluid dynamics surrounding the crane, a
suitably large external domain was considered in the CFD model. The
dimensions of the box in this case were 1.0 x 0.8 x 0.2 km (Fig. 3).
Different wind directions were evaluated by simply rotating the cylindrical
domain of the crane relative to the wind inlet. Clearly, the major challenge
of this CFD analysis concerned the over dimensions of the geometry,
together with the extremely detailed CAD of the solid parts of the crane.
Small bolts, screws, and panels were common and the geometric
anomalies within the CAD were too long and cumbersome for the user.
Fluent Meshing’s fault-tolerant mesh workflow was crucial to successfully
mesh the fluid geometry, preserving the relevant part shapes of the crane
while neglecting all other details. Thanks to this powerful tool a triangular
surface mesh of the tower crane was created, from which it was possible
to extract the fluid domain directly.
A tetrahedral and prism approach was then used to mesh the cylindrical
section of the tower crane, while the external box was meshed using a
structured strategy. The resulting meshes have very large numbers of
elements, due to the overall dimensions of the model and the level of
mesh detail (Fig. 4).
Fig. 4 - Mesh
Fig. 5 - Velocity contour on the longitudinal section
Solution and results
The Ansys CFD model was essentially configured for an external
aerodynamics problem with incompressible, turbulent, steady state
conditions.
An air temperature of 20°C was selected for the working fluid, with a
constant wind inlet velocity of 20 m/s. Three different wind directions
were considered: headwind (front), side and tailwind (rear), which were
achieved by rotating the cylindrical domain of the crane. The k-omega
shear stress transport (SST) turbulence model without ground contour
effects was used. Free flow (symmetry) conditions were used on the
sides of the bunding box.
Results were both qualitative and quantitative, in terms of the pressure
and velocity contours (Fig. 5) and overall pressure on the crane surfaces.
The shielding effect of the wider surfaces on those that follow is evident
in Fig. 5.
Terex was mainly interested in predicting the resulting forces along the
three Cartesian directions: longitudinal (X), vertical (Y) and transverse
(Z). Fig. 6 shows the cumulative plots of the X (longitudinal) force for
the case with headwind. The similarity of the results for the three cases is
noticeable, with a slight increase in overall force intensity from 55 to 65
meters, in line with expectations. It can be also noted that the operator’s
cabin and the slewing unit are responsible for most of the resulting overall
force, while the jib has a much lower effect on the force per unit length.
