1,169 research outputs found
A Wiener-Laguerre model of VIV forces given recent cylinder velocities
Slender structures immersed in a cross flow can experience vibrations induced
by vortex shedding (VIV), which cause fatigue damage and other problems. VIV
models in engineering use today tend to operate in the frequency domain. A time
domain model would allow to capture the chaotic nature of VIV and to model
interactions with other loads and non-linearities. Such a model was developed
in the present work: for each cross section, recent velocity history is
compressed using Laguerre polynomials. The compressed information is used to
enter an interpolation function to predict the instantaneous force, allowing to
step the dynamic analysis. An offshore riser was modeled in this way: Some
analyses provided an unusually fine level of realism, while in other analyses,
the riser fell into an unphysical pattern of vibration. It is concluded that
the concept is promissing, yet that more work is needed to understand orbit
stability and related issues, in order to further progress towards an
engineering tool
Optimisation of confinement in a fusion reactor using a nonlinear turbulence model
The confinement of heat in the core of a magnetic fusion reactor is optimised
using a multidimensional optimisation algorithm. For the first time in such a
study, the loss of heat due to turbulence is modelled at every stage using
first-principles nonlinear simulations which accurately capture the turbulent
cascade and large-scale zonal flows. The simulations utilise a novel approach,
with gyrofluid treatment of the small-scale drift waves and gyrokinetic
treatment of the large-scale zonal flows. A simple near-circular equilibrium
with standard parameters is chosen as the initial condition. The figure of
merit, fusion power per unit volume, is calculated, and then two control
parameters, the elongation and triangularity of the outer flux surface, are
varied, with the algorithm seeking to optimise the chosen figure of merit. A
two-fold increase in the plasma power per unit volume is achieved by moving to
higher elongation and strongly negative triangularity.Comment: 32 pages, 8 figures, accepted to JP
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