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Dynamics >>25
Improving
hydraulic systems using CFD
Dr. Ulf Specht, IABG mbH, Germany
In
full-scale fatigue testing of large aircraft, detailed information
about the individual test apparatus can be used to optimize operation:
saving both time and money! Such in-depth analyses can be dealt
with easily and quickly using CFD, yielding much greater detail
and understanding than from an experimental investigation alone.
Here, STAR-Design was used to predict the pressure losses in the
hydraulic system used to exert large forces on the outer wing, causing
the wing to displace and deform (replicating a specified operating
condition). The results were compared against experiment and very
accurate agreement was achieved.
Testing environment
In order to apply the loads to the wings, a complex hydraulic system with a large number of hydraulic cylinders is required. During normal test operation, the hydraulic cylinders are controlled by servo valves, which are part of the overall control system. During switch off operation, only passive valves control the hydraulic flow and consequently the hydraulic pressure. These have to be individually adjusted during commissioning, therefore obtaining detailed information of the pressure loss between the chambers on both sides of the actuator piston. How to adjust the valves is critical.
CFD simulations
The geometry of the hydraulic channels at the actuator, the valve manifold and the valve itself were designed using STAR-Design. The volume mesh, consisting of approximately 1 million cells, was obtained by triangulating the surface in pro-STAR’s surface meshing module and generating a trimmed mesh, with three extruded layers, in pro-STAR. A section of the mesh and a pressure distribution can be seen in Figure 1. In
order
to obtain an overview, a steady-state case was analyzed first. The
flow was assumed to be steady, incompressible and turbulent. The
standard k-? model was chosen to model turbulence. The fluid under
consideration was a special hydraulic fluid.
A normal operation
temperature of 55°C and the corresponding viscosity of the fluid
were used.
A typical flow field is shown in Figure 2. It
is characterized by the strong production of swirl at the valve,
while at other positions of interest in the fluid channel, the flow
is only slightly influenced by turbulence effects. This means that
the adjustable valve is the dominant cause of the pressure drop
in the oil flow
Comparison with experiments
Experimental investigations using the same setup have been performed in order to validate the CFD simulations. The pressure losses were measured at different flow rates and orifice diameters.
The
agreement between calculation and measurement is very good (Figure
3).
Though the relation between flow rate and pressure loss
seems to be almost quadratic, it was essential to understand the
parameters that describe this relationship and to evaluate the situation
at very low flow.
As the main result, the effects of the flow of the hydraulic fluid during connection of both chambers of a loaded hydraulic actuator were understood to a much higher degree than it was possible only by experimental investigations. The effect of geometry alterations can be understood by numerical evaluation much faster than by experiment, which helps to act faster and reduce cost.
Conclusions
A
very accurate prediction of the pressure losses caused by a geometrically
complex hydraulic fluid channel including an adjustable valve can
be achieved using STAR-CD. It was demonstrated that the combination
of a numerical investigation
with experimental validation saved
time and money.
For more information contact specht@iabg.de
Figures
01: Trimmed mesh and pressure distribution
02:
Streamlines and pressure distribution
03: Comparison simulation
with experiments
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