Highly transient axi-symmetric squeeze flows
Date
2011-04-04
Authors
Krassnokutski, Alexei E. Krass de
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Abstract
The aim of this work was to use experimental, analytical and computational Computational Fluid
Dynamic - CFD methodologies to investigate so-called highly transient axi-symmetric squeeze flows.
These flows occur between two co-axial and parallel discs which are subjected to an impact, arising from
a falling mass, which induces a constant energy squeezing system, as distinct from the traditionally
investigated constant force or constant velocity squeezing systems.
Experiments were conducted using a test cell comprising two parallel discs of diameter 120 mm with a
flexible bladder used to contain fluid. This test cell was bolted onto the base of a drop-weight tester used
to induce constant energy squeeze flows. Glycerine was used as the working fluid, the temperature of
which was appropriately monitored. Disc separation, together with pressures at three radial positions,
were measured throughout the experimental stroke typically less than 10 ms duration. Two additional
pressure transducers at the same radial position as the outermost transducer were also used to monitor
and subsequently correct for minor non-axi-symmetries that arose in the system. Approximately 150
tests were conducted, embracing combinations of drop height from 0.1 to 1 m, drop mass from 10 to 55
kg and initial disc separation from 3 to 10 mm.
Three elementary features were typically observed: a distinct preliminary pressure spike 1 immediately
after impact corresponding to very large accelerations exceeding over 6 km/s2 in some experiments, a
secondary major pressure spike 2 towards the termination of the stroke corresponding to diminishing
disc separations and a bridging region 3 joining the two spikes corresponding to somewhat reduced
pressures. While pressure distributions were observed to be closely parabolic during the major pressure
spike, some uncertainty was present during the preliminary pressure spike, ascribed to sensitivities to
deviations from axi-symmetry, and the likelihood of inertially generated pressures at the edge of the disc.
The former feature appears not to have been reported on in the formal literature.
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Four analytical models were considered, invoking the parallel flow assumption in conjunction with the
Navier Stokes equations: an inviscid/inertial model, a viscous model the lubrication approximation, a
quasi-steady linear QSL model and a quasi-steady corrected linear QSCL model. The first two of these
models, on incorporation of measured disc separations, and the derived velocities and accelerations,
achieved acceptable correlations with pressure measurements largely within uncertainty bounds during
the initial impact and towards the end of the stroke, respectively. The QSL model agreed satisfactorily
with measurements throughout the entire duration of the experiment, while the QSCL model, by
incorporating non-linear effects in an approximate linear way, yielded somewhat better correlations. By
invoking the parallel flow assumption, all four models predict a parabolic radial pressure distribution.
Utilizing a hypothetical case in which variations of disc separation, velocity and acceleration were
considered employing similar magnitudes and timescales to those that were measured, outputs of the
QSL model yielded results that correlated closely with CFD predictions, while the QSCL data were
somewhat better. On the basis of the CFD data it was also inferred that, within practical uncertainty
bounds, the parallel flow assumption was valid for the range of disc separation to radius ratios embraced
in the current investigation.
Description
Keywords
Squeeze flow systems, Computational Fluid Dynamic - CFD