  Numerical simulations of the flow in an extrusion damper are performed using
a finite volume method. The damper is assumed to consist of a shaft, with or
without a spherical bulge, oscillating axially in a containing cylinder filled
with a viscoplastic material of Bingham type. The response of the damper to a
forced sinusoidal displacement is studied. In the bulgeless case the
configuration is the annular analogue of the well-known lid-driven cavity
problem, but with a sinusoidal rather than constant lid velocity. Navier slip
is applied to the shaft surface in order to bound the reaction force to finite
values. Starting from a base case, several problem parameters are varied in
turn in order to study the effects of viscoplasticity, slip, damper geometry
and oscillation frequency to the damper response. The results show that,
compared to Newtonian flow, viscoplasticity causes the damper force to be less
sensitive to the shaft velocity; this is often a desirable damper property. The
bulge increases the required force on the damper mainly by generating a
pressure difference across itself; the latter is larger the smaller the gap
between the bulge and the casing is. At high yield stresses or slip
coefficients the amount of energy dissipation that occurs due to sliding
friction at the shaft-fluid interface is seen to increase significantly. At low
frequencies the flow is in quasi steady state, dominated by viscoplastic
forces, while at higher frequencies the fluid kinetic energy storage and
release also come into the energy balance, introducing hysteresis effects.
