Mixed baroclinic convection in a cavity

We study the convective patterns that arise in a nearly semi-cylindrical cavity fed in with hot fluid at the upper boundary, bounded by a cold, porous semi-circular boundary at the bottom, and infinitely extended in the third direction. While this configuration is relevant to continuous casting processes that are significantly more complex, we focus on the flow patterns associated with the particular form of mixed convection that arises in it. Linear stability analysis and direct numerical simulations (DNS) are conducted, using the spectral element method to identify observable states. The nature of the bifurcations is determined through Stuart--Landau analysis for completeness. The base flow consists of two counter-rotating rolls driven by the baroclinic imbalance due to the curved isothermal boundary. These are however suppressed by the through-flow, which is found to have a stabilising influence as soon as the Reynolds number $Re$ based on the through-flow exceeds 25. For a sufficiently high Rayleigh number, this base flow is linearly unstable to three different modes, depending on $Re$. For $Re\leq75$, the rolls destabilise through a supercritical bifurcation into a travelling wave. For $100\leq Re \leq 110$, a subcritical bifurcation leads to a standing oscillatory mode, whereas for $Re\geq150$, the unstable mode is non-oscillatory and grows out of a supercritical bifurcation. The direct numerical simulations confirm that in all cases, the dominant mode returned by the linear stability analysis precisely matches the topology and evolution of the flow patterns that arise out of the fully nonlinear dynamics

Robust Adaptive Control of the Mold Level in the Continuous Casting Process Using Multiple Models

Abstract-In the continuous casting of steel, mold level control is fundamental for obtaining high productivity and high quality. Using conventional methods, it is difficult to achieve both stability and performance robustness because of different classes of disturbances and parameters uncertainties in the process. This paper presents a multi-model adaptive control architecture based on the so-called RMMAC methodology. With the help of precise definition of robust performance requirements, the number of models, estimators and controllers are merely derived. More importantly, the combination of robust non-adaptive mixed-µ synthesis and stochastic hypothesis testing concepts enables controller performances prediction as well as online monitoring process parameters which could be used by operators to take corrective actions. The generated signals are likewise useful for understanding the physical phenomena in the process

Nonlinear output feedback and periodic disturbance attenuation for a speed tracking of a combustion engine test bench

The quality of control actions depends strongly on the availability and the quality of signals to construct the controller. While most control design tools assume all states, hence signals, are measurable, this is often unrealistic. An observer is often necessary to use in controller implementation. This paper proposes a reduced order observer design and output feedback control for a class of nonlinear systems, namely extended Hammerstein systems. We apply the proposed design to a combustion engine testbench, to solve a set point tracking problem. As in real practice the measured signals are often affected by periodic disturbance from combustion oscillations, the controller is extended with an internal model based filter, to remove the effect of the disturbance. Some simulation results are presented, comparing the performance of the proposed output feedback with the state feedback controller

Nonlinear adaptive estimation with application to sinusoidal identification

Parameter estimation of a sinusoidal signal in real-time is encountered in applications in numerous areas of engineering. Parameters of interest are usually amplitude, frequency and phase wherein frequency tracking is the fundamental task in sinusoidal estimation. This thesis deals with the problem of identifying a signal that comprises n (n ≥ 1) harmonics from a measurement possibly affected by structured and unstructured disturbances. The structured perturbations are modeled as a time-polynomial so as to represent, for example, bias and drift phenomena typically present in applications, whereas the unstructured disturbances are characterized as bounded perturbation. Several approaches upon different theoretical tools are presented in this thesis, and classified into two main categories: asymptotic and non-asymptotic methodologies, depending on the qualitative characteristics of the convergence behavior over time. The first part of the thesis is devoted to the asymptotic estimators, which typically consist in a pre-filtering module for generating a number of auxiliary signals, independent of the structured perturbations. These auxiliary signals can be used either directly or indirectly to estimate—in an adaptive way—the frequency, the amplitude and the phase of the sinusoidal signals. More specifically, the direct approach is based on a simple gradient method, which ensures Input-to-State Stability of the estimation error with respect to the bounded-unstructured disturbances. The indirect method exploits a specific adaptive observer scheme equipped with a switching criterion allowing to properly address in a stable way the poor excitation scenarios. It is shown that the adaptive observer method can be applied for estimating multi-frequencies through an augmented but unified framework, which is a crucial advantage with respect to direct approaches. The estimators’ stability properties are also analyzed by Input-to-State-Stability (ISS) arguments. In the second part we present a non-asymptotic estimation methodology characterized by a distinctive feature that permits finite-time convergence of the estimates. Resorting to the Volterra integral operators with suitably designed kernels, the measured signal is processed, yielding a set of auxiliary signals, in which the influence of the unknown initial conditions is annihilated. A sliding mode-based adaptation law, fed by the aforementioned auxiliary signals, is proposed for deadbeat estimation of the frequency and amplitude, which are dealt with in a step-by-step manner. The worst case behavior of the proposed algorithm in the presence of bounded perturbation is studied by ISS tools. The practical characteristics of all estimation techniques are evaluated and compared with other existing techniques by extensive simulations and experimental trials.Open Acces

Aerospace Medicine and Biology: A continuing bibliography with indexes, supplement 145

This bibliography lists 301 reports, articles, and other documents introduced into the NASA scientific and technical information system in August 1975

On the oscillatory hydrodynamic instability of gravitational thermal flows of liquid metals in variable cross-section containers

Natural convective flows of liquid metals in open or closed ducts and containers play a relevant role in a variety of applications in mechanical, materials and nuclear engineering. This analysis follows and integrates the line of inquiry started in past authors’ work about the typical properties of these flows and associated hierarchy of bifurcations in rectangular geometries. The Navier Stokes and energy equations are solved in their time-dependent and non-linear formulation to investigate the onset and evolution of oscillatory disturbances and other effects breaking the initially unicellular structure of the flow. It is shown that a kaleidoscope of oscillatory patterns is made possible by the new degree of freedom represented by the opposite inclination of the walls with respect to the horizontal direction. Even minute variations in the geometry and/or initial conditions can cause significant changes. Multiple states exist which can replace each other in given sub-regions of the space of parameters. Observed regimes include: stationary convection, weakly oscillating rolls, coalescing rolls, traveling waves, and modulated (pulso-traveling) disturbances. Most interestingly, traveling waves can propagate either in the downstream or the upstream direction according to whether the walls are converging or diverging