4,526 research outputs found
Feedback control of unstable steady states of flow past a flat plate using reduced-order estimators
We present an estimator-based control design procedure for flow control,
using reduced-order models of the governing equations, linearized about a
possibly unstable steady state. The reduced models are obtained using an
approximate balanced truncation method that retains the most controllable and
observable modes of the system. The original method is valid only for stable
linear systems, and we present an extension to unstable linear systems. The
dynamics on the unstable subspace are represented by projecting the original
equations onto the global unstable eigenmodes, assumed to be small in number. A
snapshot-based algorithm is developed, using approximate balanced truncation,
for obtaining a reduced-order model of the dynamics on the stable subspace. The
proposed algorithm is used to study feedback control of 2-D flow over a flat
plate at a low Reynolds number and at large angles of attack, where the natural
flow is vortex shedding, though there also exists an unstable steady state. For
control design, we derive reduced-order models valid in the neighborhood of
this unstable steady state. The actuation is modeled as a localized body force
near the leading edge of the flat plate, and the sensors are two velocity
measurements in the near-wake of the plate. A reduced-order Kalman filter is
developed based on these models and is shown to accurately reconstruct the flow
field from the sensor measurements, and the resulting estimator-based control
is shown to stabilize the unstable steady state. For small perturbations of the
steady state, the model accurately predicts the response of the full
simulation. Furthermore, the resulting controller is even able to suppress the
stable periodic vortex shedding, where the nonlinear effects are strong, thus
implying a large domain of attraction of the stabilized steady state.Comment: 36 pages, 17 figure
Variational integrators for degenerate Lagrangians, with application to point vortices
We develop discrete mechanics and variational integrators
for a class of degenerate Lagrangian systems,
and apply these integrators to a system of
point vortices. Excellent numerical behavior is observed.
A longer term goal is to use these integration
methods in the context of control of mechanical
systems, such as coordinated groups of underwater
vehicles. In fact, numerical evidence given
in related problems, such as those in [2] shows that
in the presence of external forces, these methods
give superior predictions of energy behavior
The effect of injector design on thrust- chamber erosion
Relation between injector design and erosion of ablative and pyrolytic graphite thrust chamber throa
Michigan resource inventories: Characteristics and costs of selected projects using high altitude color infrared imagery. Remote Sensing Project
The procedures and costs associated with mapping land cover/use and forest resources from high altitude color infrared (CIR) imagery are documented through an evaluation of several inventory efforts. CIR photos (1:36,000) were used to classify the forests of Mason County, Michigan into six species groups, three stocking levels, and three maturity classes at a cost of 4.28/sq. km. and 1,500 by integrating grid-geocoded land cover/use, soils, topographic, and well log data using an analytical computer program
Guide to aerial imagery of Michigan
There are no author-identified significant results in this report
Uncertainty Quantification for Airfoil Icing using Polynomial Chaos Expansions
The formation and accretion of ice on the leading edge of a wing can be
detrimental to airplane performance. Complicating this reality is the fact that
even a small amount of uncertainty in the shape of the accreted ice may result
in a large amount of uncertainty in aerodynamic performance metrics (e.g.,
stall angle of attack). The main focus of this work concerns using the
techniques of Polynomial Chaos Expansions (PCE) to quantify icing uncertainty
much more quickly than traditional methods (e.g., Monte Carlo). First, we
present a brief survey of the literature concerning the physics of wing icing,
with the intention of giving a certain amount of intuition for the physical
process. Next, we give a brief overview of the background theory of PCE.
Finally, we compare the results of Monte Carlo simulations to PCE-based
uncertainty quantification for several different airfoil icing scenarios. The
results are in good agreement and confirm that PCE methods are much more
efficient for the canonical airfoil icing uncertainty quantification problem
than Monte Carlo methods.Comment: Submitted and under review for the AIAA Journal of Aircraft and 2015
AIAA Conferenc
On self-sustained oscillations in two-dimensional compressible flow over rectangular cavities
Numerical simulations are used to investigate the resonant instabilities in two-dimensional flow past an open cavity. The compressible Navier–Stokes equations are solved directly (no turbulence model) for cavities with laminar boundary layers upstream. The computational domain is large enough to directly resolve a portion of the radiated acoustic field, which is shown to be in good visual agreement with schlieren photographs from experiments at several different Mach numbers. The results show a transition from a shear-layer mode, primarily for shorter cavities and lower Mach numbers, to a wake mode for longer cavities and higher Mach numbers. The shear-layer mode is characterized well by the acoustic feedback process described by Rossiter (1964), and disturbances in the shear layer compare well with predictions based on linear stability analysis of the Kelvin–Helmholtz mode. The wake mode is characterized instead by a large-scale vortex shedding with Strouhal number independent of Mach number. The wake mode oscillation is similar in many ways to that reported by Gharib & Roshko (1987) for incompressible flow with a laminar upstream boundary layer. Transition to wake mode occurs as the length and/or depth of the cavity becomes large compared to the upstream boundary-layer thickness, or as the Mach and/or Reynolds numbers are raised. Under these conditions, it is shown that the Kelvin–Helmholtz instability grows to sufficient strength that a strong recirculating flow is induced in the cavity. The resulting mean flow is similar to wake profiles that are absolutely unstable, and absolute instability may provide an explanation of the hydrodynamic feedback mechanism that leads to wake mode. Predictive criteria for the onset of shear-layer oscillations (from steady flow) and for the transition to wake mode are developed based on linear theory for amplification rates in the shear layer, and a simple model for the acoustic efficiency of edge scattering
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