58,405 research outputs found
A tale of two airfoils: resolvent-based modelling of an oscillator vs. an amplifier from an experimental mean
The flows around a NACA 0018 airfoil at a Reynolds number of 10250 and angles
of attack of alpha = 0 (A0) and alpha = 10 (A10) are modelled using resolvent
analysis and limited experimental measurements obtained from particle image
velocimetry. The experimental mean velocity profiles are data-assimilated so
that they are solutions of the incompressible Reynolds-averaged Navier-Stokes
equations forced by Reynolds stress terms which are derived from experimental
data. Spectral proper orthogonal decompositions (SPOD) of the velocity
fluctuations and nonlinear forcing find low-rank behaviour at the shedding
frequency and its higher harmonics for the A0 case. In the A10 case, low-rank
behaviour is observed for the velocity fluctuations in two bands of
frequencies. Resolvent analysis of the data-assimilated means identifies
low-rank behaviour only in the vicinity of the shedding frequency for A0 and
none of its harmonics. The resolvent operator for the A10 case, on the other
hand, identifies two linear mechanisms whose frequencies are a close match with
those identified by SPOD. It is also shown that the second linear mechanism,
corresponding to the Kelvin-Helmholtz instability in the shear layer, cannot be
identified just by considering the time-averaged experimental measurements as a
mean flow due to the fact that experimental data are missing near the leading
edge. The A0 case is classified as an oscillator where the flow is organised
around an intrinsic instability while the A10 case behaves like an amplifier
whose forcing is unstructured. For both cases, resolvent modes resemble those
from SPOD when the operator is low-rank. To model the higher harmonics where
this is not the case, we add parasitic resolvent modes, as opposed to classical
resolvent modes which are the most amplified, by approximating the nonlinear
forcing from limited triadic interactions of known resolvent modes.Comment: 32 pages, 23 figure
Validation of gyrokinetic modelling of light impurity transport including rotation in ASDEX Upgrade
Upgraded spectroscopic hardware and an improved impurity concentration
calculation allow accurate determination of boron density in the ASDEX Upgrade
tokamak. A database of boron measurements is compared to quasilinear and
nonlinear gyrokinetic simulations including Coriolis and centrifugal rotational
effects over a range of H-mode plasma regimes. The peaking of the measured
boron profiles shows a strong anti-correlation with the plasma rotation
gradient, via a relationship explained and reproduced by the theory. It is
demonstrated that the rotodiffusive impurity flux driven by the rotation
gradient is required for the modelling to reproduce the hollow boron profiles
at higher rotation gradients. The nonlinear simulations validate the
quasilinear approach, and, with the addition of perpendicular flow shear,
demonstrate that each symmetry breaking mechanism that causes momentum
transport also couples to rotodiffusion. At lower rotation gradients, the
parallel compressive convection is required to match the most peaked boron
profiles. The sensitivities of both datasets to possible errors is
investigated, and quantitative agreement is found within the estimated
uncertainties. The approach used can be considered a template for mitigating
uncertainty in quantitative comparisons between simulation and experiment.Comment: 19 pages, 11 figures, accepted in Nuclear Fusio
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