125 research outputs found
Morphing surfaces for the control of boundary layer transition
A structure configured to modify its surface morphology between a smooth state and a rough state in response to an applied stress. In demonstrated examples, a soft (PDMS) substrate is produced, and is pre-strained. A relatively stiff overlayer of a metal, such as chromium and gold, is applied to the substrate. When the pre-strained substrate is allowed to relax, the free surface of the stiff overlayer is forced to become distorted, yielding a free surface having a roughness of less than 1 millimeter. Repeated application and removal of the applied stress has been shown to yield reproducible changes in the morphology of the free surface. An application of such morphing free surface is to control a boundary layer transition of an aerodynamic fluid flowing over the surface
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
On the correspondence between Koopman mode decomposition, resolvent mode decomposition, and invariant solutions of the Navier-Stokes equations
The relationship between Koopman mode decomposition, resolvent mode
decomposition and exact invariant solutions of the Navier-Stokes equations is
clarified. The correspondence rests upon the invariance of the system operators
under symmetry operations such as spatial translation. The usual interpretation
of the Koopman operator is generalised to permit combinations of such
operations, in addition to translation in time. This invariance is related to
the spectrum of a spatio-temporal Koopman operator, which has a travelling wave
interpretation. The relationship leads to a generalisation of dynamic mode
decomposition, in which symmetry operations are applied to restrict the dynamic
modes to span a subspace subject to those symmetries. The resolvent is
interpreted as the mapping between the Koopman modes of the Reynolds stress
divergence and the velocity field. It is shown that the singular vectors of the
resolvent (the resolvent modes) are the optimal basis in which to express the
velocity field Koopman modes where the latter are not a priori known
Nonlinear interactions isolated through scale synthesis in experimental wall turbulence
An experimental investigation of nonlinear scale interactions in a forced turbulent boundary layer is presented here. A dynamic wall perturbation mechanism was used to externally force two distinct large-scale synthetic modes with well-defined spatial and temporal wave numbers in a fully turbulent flow. The focus is on characterizing the nonlinear flow response at triadically consistent wave numbers that arises from the direct interactions of the two synthetic modes. These experimental results isolate triadic scale interactions in wall turbulence in a unique fashion, and provide the ability to explore the dynamics of scale coupling in a systematic and detailed manner. The ideas advanced here are intended to contribute towards modeling efforts of high-Reynolds-number wall turbulence
Influence of a local change of depth on the behavior of bouncing oil drops
The work of Couder \textit{et al} (see also Bush \textit{et al}) inspired
consideration of the impact of a submerged obstacle, providing a local change
of depth, on the behavior of oil drops in the bouncing regime. In the linked
videos, we recreate some of their results for a drop bouncing on a uniform
depth bath of the same liquid undergoing vertical oscillations just below the
conditions for Faraday instability, and show a range of new behaviors
associated with change of depth.
This article accompanies a fluid dynamics video entered into the Gallery of
Fluid Motion of the 66th Annual Meeting of the APS Division of Fluid Dynamics.Comment: High and low resolutions videos included as ancillary file
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