2,390 research outputs found
Active vibration control techniques for flexible space structures
Two proposed control system design techniques for active vibration control in flexible space structures are detailed. Control issues relevant only to flexible-body dynamics are addressed, whereas no attempt was made to integrate the flexible and rigid-body spacecraft dynamics. Both of the proposed approaches revealed encouraging results; however, further investigation of the interaction of the flexible and rigid-body dynamics is warranted
On non-normality and classification of amplification mechanisms in stability and resolvent analysis
We seek to quantify non-normality of the most amplified resolvent modes and
predict their features based on the characteristics of the base or mean
velocity profile. A 2-by-2 model linear Navier-Stokes (LNS) operator
illustrates how non-normality from mean shear distributes perturbation energy
in different velocity components of the forcing and response modes. The inverse
of their inner product, which is unity for a purely normal mechanism, is
proposed as a measure to quantify non-normality. In flows where there is
downstream spatial dependence of the base/mean, mean flow advection separates
the spatial support of forcing and response modes which impacts the inner
product. Success of mean stability analysis depends on the normality of
amplification. If the amplification is normal, the resolvent operator written
in its dyadic representation reveals that the adjoint and forward stability
modes are proportional to the forcing and response resolvent modes. If the
amplification is non-normal, then resolvent analysis is required to understand
the origin of observed flow structures. Eigenspectra and pseudospectra are used
to characterize these phenomena. Two test cases are studied: low Reynolds
number cylinder flow and turbulent channel flow. The first deals mainly with
normal mechanisms and quantification of non-normality using the inverse inner
product of the leading forcing and response modes agrees well with the product
of the resolvent norm and distance between the imaginary axis and least stable
eigenvalue. In turbulent channel flow, structures result from both normal and
non-normal mechanisms. Mean shear is exploited most efficiently by stationary
disturbances while bounds on the pseudospectra illustrate how non-normality is
responsible for the most amplified disturbances at spatial wavenumbers and
temporal frequencies corresponding to well-known turbulent structures
Robust Measurement Feedback Control of an Inclined Cable
International audienceConsidering the partial differential equation model of the vibrations of an inclined cable, we are interested in applying robust control technics to stabilize the system with measurement feedback when it is submitted to external disturbances. This paper focuses indeed on the construction of a standard linear infinite dimensional state space system and an H_infinity feedback control of vibrations with partial observation of the state. The control and observation are performed using an active tendon
Stochastic Structural Stability Theory applied to roll/streak formation in boundary layer shear flow
Stochastic Structural Stability Theory (SSST) provides an autonomous,
deterministic, nonlinear dynamical system for evolving the statistical mean
state of a turbulent system. In this work SSST is applied to the problem of
understanding the formation of the roll/streak structures that arise from
free-stream turbulence (FST) and are associated with bypass transition in
boundary layers. Roll structures in the cross-stream/spanwise plane and
associated streamwise streaks are shown to arise as a linear instability of
interaction between the FST and the mean flow. In this interaction incoherent
Reynolds stresses arising from FST are organized by perturbation streamwise
streaks to coherently force perturbation rolls giving rise to an amplification
of the streamwise streak perturbation and through this feedback to an
instability of the combined roll/streak/turbulence complex. The dominant
turbulent perturbation structures involved in supporting the
roll/streak/turbulence complex instability are non-normal optimal perturbations
with the form of oblique waves. The cooperative linear instability giving rise
to the roll/streak structure arises at a bifurcation in the parameter of STM
excitation parameter. This structural instability eventually equilibrates
nonlinearly at finite amplitude and although the resulting statistical
equilibrium streamwise streaks are inflectional the associated flows are
stable. Formation and equilibration of the roll/streak structure by this
mechanism can be traced to the non-normality which underlies interaction
between perturbations and mean flows in modally stable systems.Comment: 16 pages, 24 figures, has been submitted for publication to Physics
of Fluid
Event-Based Hâ filter design for a class of nonlinear time-varying systems with fading channels and multiplicative noises
In this paper, a general event-triggered framework is developed to deal with the finite-horizon Hâ filtering problem for discrete time-varying systems with fading channels, randomly occurring nonlinearities and multiplicative noises. An event indicator variable is constructed and the corresponding event-triggered scheme is proposed. Such a scheme is based on the relative error with respect to the measurement signal in order to determine whether the measurement output should be transmitted to the filter or not. The fading channels are described by modified stochastic Rice fading models. Some uncorrelated random variables are introduced, respectively, to govern the phenomena of state-multiplicative noises, randomly occurring nonlinearities as well as fading measurements. The purpose of the addressed problem is to design a set of time-varying filter such that the influence from the exogenous disturbances onto the filtering errors is attenuated at the given level quantified by a Hâ norm in the mean-square sense. By utilizing stochastic analysis techniques, sufficient conditions are established to ensure that the dynamic system under consideration satisfies the Hâ filtering performance constraint, and then a recursive linear matrix inequality (RLMI) approach is employed to design the desired filter gains. Simulation results demonstrate the effectiveness of the developed filter design scheme
Nonlinear and adaptive control
The primary thrust of the research was to conduct fundamental research in the theories and methodologies for designing complex high-performance multivariable feedback control systems; and to conduct feasibiltiy studies in application areas of interest to NASA sponsors that point out advantages and shortcomings of available control system design methodologies
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