924 research outputs found
Multigrid method for nonlinear poroelasticity equations
<p>In this study, a nonlinear multigrid method is applied for solving the system of incompressible poroelasticity equations considering nonlinear hydraulic conductivity. For the unsteady problem, an additional artificial term is utilized to stabilize the solutions when the equations are discretized on collocated grids. We employ two nonlinear multigrid methods, i.e. the “full approximation scheme” and “Newton multigrid” for solving the corresponding system of equations arising after discretization. For the steady case, both homogeneous and heterogeneous cases are solved and two different smoothers are examined to search for an efficient multigrid method. Numerical results show a good convergence performance for all the strategies.</p
Time-stepping and Krylov methods for large-scale instability problems
With the ever increasing computational power available and the development of
high-performances computing, investigating the properties of realistic very
large-scale nonlinear dynamical systems has been become reachable. It must be
noted however that the memory capabilities of computers increase at a slower
rate than their computational capabilities. Consequently, the traditional
matrix-forming approaches wherein the Jacobian matrix of the system considered
is explicitly assembled become rapidly intractable. Over the past two decades,
so-called matrix-free approaches have emerged as an efficient alternative. The
aim of this chapter is thus to provide an overview of well-grounded matrix-free
methods for fixed points computations and linear stability analyses of very
large-scale nonlinear dynamical systems.Comment: To appear in "Computational Modeling of Bifurcations and
Instabilities in Fluid Mechanics", eds. A. Gelfgat, Springe
Analysis and development of numerical methodologies for simulation of flow control with dielectric barrier discharge actuators
The aim of this thesis is to investigate and develop different numerical methodologies for
modeling the Dielectric Barrier discharge (DBD) plasma actuators for flow control purposes.
Two different modeling approaches were considered; one based on Plasma-fluid model and
the other based on a phenomenological model.
A three component Plasma fluid model based on the transport equations of charged particles
was implemented in this thesis in OpenFOAM, using several techniques to reduce the
numerical issues. The coupled plasma-fluid problem involves wide range of length and time
scales which make the numerical simulation difficult. Therefore, to obtain stable and
accurate results in a reasonable computational run time, several numerical procedures were
implemented including: semi-implicit treatment of coupling of Poisson equation and charge
density equation, super-time-stepping and operator splitting algorithm.
We examined our code for a constant positive voltage, testing for the dependency of the
behavior of the current density to the selected numerical scheme. In addition, although there
is no clear numerical or experimental benchmark case for DBD plasma actuator problem, the
developed plasma solver was compared quantitively and qualitively with several numerical
works in the literature. Afterward, the developed numerical methodology was used to explore
the possibility of influencing the flow, with higher speed, using nano-second (NS) pulsed DBD
plasma actuator. Therefore, the interaction of the transonic flow and actuation effects of
DBD plasma actuator with nano second pulsed voltage was simulated. The effect of gas
heating and body force was calculated by the plasma solver and was supplied into the gas
dynamic solver for simulating the flow field. Moreover, the results of the plasma fluid model
were used to develop an energy deposition model. It was shown that the energy deposition
model is able to capture the main features of the effect of NS DBD plasma actuators
correctly, with less computational time. It was also shown that fast energy transfer, from
plasma to fluid, leads to the formation of micro-shock waves that modify locally the features
of the transonic flow.
Although the numerical efficiency of the plasma fluid model was improved, the computational
cost of simulating the effect of DBD plasma actuator on a real scale flow situation was still
high. Therefore, a simple model for plasma discharge and its effect on the flow was
developed based on scaling of the thrust generated by DBD plasma actuators. The scaled
thrust model correctly predicts the nonlinear dependency of the thrust produced and the
applied voltage. These scales were then introduced into a simple phenomenological model to
estimate and simulate the body force distribution generated by the plasma actuator.
Although the model includes some experimental correlations, it does not need any fitting
parameter. The model was validated with experimental results and showed better accuracy
compared to previous plasma models.
Using a simple phenomenological model that was developed here, a numerical study was
conducted to investigate and compare the effect of steady and unsteady actuation for controlling the flow at relatively high Reynolds number. Firstly it was shown that the size of
the time-averaged separation bubble is greatly reduced and the flow structure is sensitive to
the frequency of burst modulation of DBD plasma actuators. The results also confirmed that
in the case of unsteady actuation, the burst frequency and burst ratio are crucial parameters
for influencing the capability of the actuators to control the flow. It was found that burst
frequencies near the natural frequencies of the system were able to excite the flow structure
in a resonance mode. This observation also confirmed that with proper frequencies of
excitation, the flow structure can be well rearranged and the flow losses can be reduced.
In the end, Plasma actuators were used for controlling the flow over the Coanda surface of
the ACHEON nozzle. When the plasma actuator was used, it was possible to postpone
separation of the flow and increase the deflection angle of the exit jet of the nozzle. To find
the optimum position of the actuators, seven DBD actuators in forward forcing mode were
placed over the Coanda surface considering the numerically obtained separation points.
Results show that when the actuator is placed slightly before the separation point, enhanced
thrust vectorizing with the use of DBD actuator is achievable. Preliminary results of the
experiments agree with planned/foreseen deflection angle obtained from numerical
computation.O objetivo deste trabalho visa a investigação e desenvolvimento de diferentes métodos
numéricos para modelação de actuadores a plasma de Descarga em Barreira Dieléctrica,
(DBD), tendo em vista o controlo do escoamento na camada limite. Esta modelação numérica
foi abordada de duas formas diferentes, uma baseada num modelo de “plasma-fluid” e outra
fundamentada num modelo fenomenológico.
Neste trabalho é usado um modelo “plasma-fluid” de três componentes que é baseado numa
equação de transporte para as partículas electricamente carregadas. Este foi implementado
no software OpenFOAM fazendo uso de diversas técnicas para minimização de problemas
numéricos que ocorriam na resolução das equações. O cálculo de um problema com
acoplamento entre plasma e fluido envolve uma gama diversa de escalas, tanto temporais
como dimensionais, trata-se então de uma simulação numérica delicada. Como tal, e por
forma a obter resultados estáveis e precisos num tempo de cálculo considerado razoável,
foram implementados diversos procedimentos numéricos, tais como o tratamento semiimplícito
do acoplamento da equação de Poisson com a equação da densidade de carga, o
super-passo-tempo e ainda um algoritmo do tipo divisão de operador.
Foi considerado o caso de uma diferença de potencial positiva, constante, e testada a
dependência da densidade de corrente com os diferentes esquemas numéricos. Apesar de não
existir atualmente uma base de dados, de tipo numérica ou experimental, com casos de teste
para actuadores a plasma tipo DBD, o modelo computacional desenvolvido para calcular o
plasma foi validado qualitativamente, bem como quantitativamente, usando os vários
trabalhos numéricos disponíveis na literatura. Após esta validação inicial, a metodologia
numérica desenvolvida foi utilizada para explorar a possibilidade de influenciar um
escoamento de maior velocidade, através de actuadores a plasma tipo DBD com impulsos de
tensão da ordem de nano-segundos (NS). Desta forma foi simulada a interacção entre um
escoamento transónico e o efeito dos actuadores a plasma tipo DBD sobre o escoamento,
usando pulsos de nano-segundos. O efeito térmico do gás, assim como a força resultante,
foram calculados usando o modelo numérico para cálculo de plasmas desenvolvido neste
trabalho. O resultado obtido é acoplado ao modelo de cálculo para a dinâmica de gases, o
que torna possível simular as condições do escoamento resultante. Adicionalmente, os
resultados do modelo de “plasma-fluid” foram reaproveitados para desenvolver um modelo de
deposição de energia. Este demonstrou ter a capacidade de capturar correctamente as
características principais do efeito de actuadores de plasma, de tipo NS-DBD, com um tempo
de computação menor. Foi demonstrada que uma rápida transferência de energia, do plasma
para o fluido, leva à formação de micro-ondas de choque que alteram localmente as
características do escoamento transónico. Apesar da eficiência numérica do modelo de
“plasma-fluid” ter sido melhorada, o seu custo computacional para a simulação de actuadores
a plasma tipo DBD à escala real continua bastante elevado. Neste sentido, a partir de uma escala de propulsão gerada pelo actuador plasma DBD, foi desenvolvido um modelo mais
simples para a descarga do plasma e para determinar os seus efeitos sobre o escoamento.
O modelo inicial previa correctamente uma dependência não-linear entre a força propulsiva
gerada e a diferença de potencial aplicada. Estas escalas foram então introduzidas num
modelo fenomenológico mais simples para estimar, e simular, a distribuição de forças geradas
pelo actuador a plasma. Apesar de o modelo incluir algumas correlações experimentais, este
não requer qualquer parâmetro de afinação. O modelo foi validado com resultados
experimentais, demonstrando melhores resultados quando comparado com outros modelos de
plasma .
Utilizando um modelo fenomenológico simplificado, que foi desenvolvido no presente
trabalho, foi feito um estudo numérico com o objetivo de investigar, e comparar, os efeitos
que uma actuação estacionária e não-estacionária exibe sobre o controlo do escoamento a
números de Reynolds relativamente elevados. Foi demostrado que a dimensão da bolha de
separação é reduzida em muito e que a estrutura do escoamento é sensível à frequência da
modulação “burst” do actuador a plasma tipo DBD. Os resultados também confirmaram que,
para o caso de actuação não-estacionária, a frequência de “burst” e o “burst ratio”, são
parâmetros cruciais para influenciar a capacidade de controlo do escoamento por parte dos
actuadores a plasma. Determinou-se que as frequências “burst”, semelhantes às frequências
naturais do sistema, são capazes de excitar as estruturas do escoamento num modo de
ressonância. Esta observação confirma igualmente que, com frequências de excitação
apropriadas, a estrutura de um escoamento de camada limite consegue ser correctamente
modificada, e que as perdas no escoamento são reduzidas. Por fim, os actuadores a plasma
foram utilizados para o controlo do escoamento sobre uma superfície Coanda de uma tubeira.
Quando nesta foi aplicado um plasma, tornou-se possível retardar a separação do escoamento
e aumentar o ângulo de deflexão do jacto gerado pelo propulsor. Por forma a encontrar a
posição óptima para os actuadores, sete actuadores de tipo DBD foram distribuídos ao longo
da superfície Coanda, tendo em consideração os pontos de separação do escoamento na
camada limite obtidos numericamente. Os resultados mostram que quando o actuador DBD é
colocado ligeiramente antes do ponto de separação do escoamento, há um aumento da
capacidade de controlo e vectorização do jacto gerado. Os resultados preliminares das
experiências efectuadas estão de acordo com o ângulo de deflexão do jacto previsto pelo
modelo computacional
Self-Consistent Modelling of Non-Thermal Atmospheric Argon Plasma During Arc Discharge and Its Interaction with Metal Electrodes
Heat transfer processes associated with arc plasmas are important for many industrial applications such as electric propulsion, plasma spray and arc welding. In these applications, an electric arc is used because it offers high energy densities and a controlled environment. However, it is sometimes not realizable or not economic to get the parameters within the high temperature region of plasma precisely by means of experimental measurements. A numerical model that offers reliable description of discharging process is a good choice. Any model of arc plasmas must contain not only the conservation of mass, momentum and energy, but also electromagnetic description that follows Maxwella s equations. Since the last 30 years, intensive researches embarking on nonequilibrium plasmas have led to fruitful achievements, among them NLTE (non-Local Thermal Equilibrium) model plays an important role in numerical modelling due to its superiority over LTE (Local Thermal Equilibrium) model in accounting for the difference of two phase temperatures (heavy species and electrons) that cannot be neglected near electrodes. However, deeper researches meet obstacles when the discharging system needs to be simulated self-consistently as a whole and with as few presumed conditions as possible. On one hand, discharging under high current operation tends to overheat its electrodes leading to melting or evaporating, particles from electrode material that enter the plasma will change its composition and the heat transfer process. On the other hand, therea s still a a mysteriousa region whose physical structure is so different from the main arc plasma region that cannot be accounted by conventional transport equations or theories without any extra treatments for it. This region, sometimes called sheath layer or space-charge layer, plays an important role in bridging the thermal and electric energy of arc column to electrodes. To develop a reasonable model in this region and make it compatible with the two other regions will extend the applicability of CFD model in discharging devices. The motivation of this doctoral thesis is based on my special interest in sheath region, or in other words, my pursuit of developing a self-consistent model that is capable of solving the whole plasma-electrode system. Concerning the complexity of sheath, no secondary physical phenomena such as melting and evaporating are considered in this study. For the main arc region, the plasma composition is calculated based on species conservation equations that consider both diffusion and production/loss activities of particles. And for the sake of high temperature of plasma core, ionization up to third level is applied. In the sheath layer, the effective sheath electrical conductivity is utilized, which is based on the assumption of Childa s collisionless sheath and Lowkea s expression. The ionization degree of plasma sheath plays an important role in this self-consistent method. To validate the model proposed here, several simple benchmark simulations are made and the numerical results concerning temperature, velocity and magnetic field yield satisfactory agreements with experimental or theoretical results. With the model being validated, a D.C. non-transferred plasma torch is studied. The total voltages of both situations are compared with experimental measurements. It shows that the sheath model developed in this scope make the numerical results closer to reality and is responsible for the strong fluctuation of arc jets, which also makes cathode surface temperature fluctuate accordingly. Finally, pros and cons of some new design patterns of plasma torches are discussed, with the multi anode/single cathode type DeltaGun simulated for the comparison of performances with the original type. It reveals that such kind of configuration helps to damp the unwanted arc fluctuation with multiple arc roots. It is also numerically confirmed that when an external coil is added around anode to produce a proper magnetic field, the temperature of anode attachment can be reduced due to enhanced circumferential movement of arc roots by Lorentz force, which lowers the possibility of erosion and promotes a longer lifetime
- …