218 research outputs found
Selective energy and enstrophy modification of two-dimensional decaying turbulence
In two-dimensional decaying homogeneous isotropic turbulence, kinetic energy
and enstrophy are respectively transferred to larger and smaller scales. In
such spatiotemporally complex dynamics, it is challenging to identify the
important flow structures that govern this behavior. We propose and numerically
employ two flow modification strategies that leverage the inviscid global
conservation of energy and enstrophy to design external forcing inputs which
change these quantities selectively and simultaneously, and drive the system
towards steady-state or other late-stage behavior. One strategy employs only
local flow-field information, while the other is global. We observe various
flow structures excited by these inputs and compare with recent literature.
Energy modification is characterized by excitation of smaller wavenumber
structures in the flow than enstrophy modification.Comment: 15 pages, 11 figure
Network-theoretic modeling of fluid-structure interactions
The coupling interactions between deformable structures and unsteady fluid
flows occur across a wide range of spatial and temporal scales in many
engineering applications. These fluid-structure interactions (FSI) pose
significant challenges in accurately predicting flow physics. In the present
work, two multi-layer network approaches are proposed that characterize the
interactions between the fluid and structural layers for an incompressible
laminar flow over a two-dimensional compliant flat plate at a 35-degrees angle
of attack. In the first approach, the network nodes are formed by wake vortices
and bound vortexlets, and the edges of the network are defined by the induced
velocity between these elements. In the second approach, coherent structures
(fluid modes), contributing to the kinetic energy of the flow and structural
modes, contributing to the kinetic energy of the compliant structure constitute
the network nodes. The energy transfers between the modes are extracted using a
perturbation approach. Furthermore, the network structure of the FSI system is
simplified using the community detection algorithm in the vortical approach and
by selecting dominant modes in the modal approach. Network measures are used to
reveal the temporal behavior of the individual nodes within the simplified FSI
system. Predictive models are then built using both data-driven and
physics-based methods. Overall, this work sets the foundation for
network-theoretic reduced-order modeling of fluid-structure interactions,
generalizable to other multi-physics systems.Comment: 20 pages, 10 figure
Cluster-based feedback control of turbulent post-stall separated flows
We propose a novel model-free self-learning cluster-based control strategy
for general nonlinear feedback flow control technique, benchmarked for
high-fidelity simulations of post-stall separated flows over an airfoil. The
present approach partitions the flow trajectories (force measurements) into
clusters, which correspond to characteristic coarse-grained phases in a
low-dimensional feature space. A feedback control law is then sought for each
cluster state through iterative evaluation and downhill simplex search to
minimize power consumption in flight. Unsupervised clustering of the flow
trajectories for in-situ learning and optimization of coarse-grained control
laws are implemented in an automated manner as key enablers. Re-routing the
flow trajectories, the optimized control laws shift the cluster populations to
the aerodynamically favorable states. Utilizing limited number of sensor
measurements for both clustering and optimization, these feedback laws were
determined in only iterations. The objective of the present work is not
necessarily to suppress flow separation but to minimize the desired cost
function to achieve enhanced aerodynamic performance. The present control
approach is applied to the control of two and three-dimensional separated flows
over a NACA 0012 airfoil with large-eddy simulations at an angle of attack of
, Reynolds number and free-stream Mach number . The optimized control laws effectively minimize the flight power
consumption enabling the flows to reach a low-drag state. The present work aims
to address the challenges associated with adaptive feedback control design for
turbulent separated flows at moderate Reynolds number.Comment: 32 pages, 18 figure
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