abstract: The effect of reduced frequency on dynamic stall behavior of a pitching NACA0012 airfoil in a turbulent wake using Direct Numerical Simulations is presented in the current study. Upstream turbulence with dynamically oscillating blades and airfoils is associated with ambient flow unsteadiness and is encountered in many operating conditions. Wake turbulence, a more realistic scenario for airfoils in operation, is generated using a small solid cylinder placed upstream, the vortices shed from which interact with the pitching airfoil affecting dynamic stall behavior. A recently developed moving overlapping grid approach is used using a high-order Spectral Element Method (SEM) for spatial discretization combined with a dynamic time-stepping procedure allowing for up to third order temporal discretization. Two cases of reduced frequency (k = 0:16 and 0:25) for airfoil oscillation are investigated and the change in dynamic stall behavior with change in reduced frequency is studied and documented using flow-fields and aerodynamic coefficients (Drag, Lift and Pitching Moment) with a focus on understanding vortex system dynamics (including formation of secondary vortices) for different reduced frequencies and it’s affect on airfoil aerodynamic characteristics and fatigue life. Transition of the flow over the surface of an airfoil for both undisturbed and disturbed flow cases will also be discussed using Pressure coefficient and Skin Friction coefficient data for a given cycle combined with a wavelet analysis using Morse wavelets in MATLAB. / Dissertation/Thesis / Masters Thesis Mechanical Engineering 2017

Red blood cells (RBCs) are the most abundant cells in human blood, representing 40 to 45% of the blood volume (hematocrit). These cells have the particular ability to deform and bridge together to form aggregates under very low shear rates. The theory and mechanics behind aggregation are, however, not yet completely understood. The purpose of this work is to provide a novel method to analyze, understand and mimic blood behaviour in microcirculation. The main objective is to develop a methodology to quantify and characterize RBC aggregates and hence enhance the current understanding of the non-Newtonian behaviour of blood at the microscale. For this purpose, suspensions of porcine blood and human blood are tested in vitro in a Poly-di-methylsiloxane (PDMS) microchannel to characterize RBC aggregates within these two types of blood. These microchannels are fabricated using standard photolithography methods. Experiments are performed using a micro Particle Image Velocimetry ( PIV) system for shear rate measurements coupled with a high speed camera for the flow visualization. Corresponding numerical simulations are conducted using a research Computational Fluid Dynamic (CFD) solver, Nek5000, based on the spectral element method solution to the incompressible non-Newtonian Navier-Stokes equations. RBC aggregate sizes are quantified in controlled and measurable shear rate environments for 5, 10 and 15% hematocrit. Aggregate sizes are determined using image processing techniques. Velocity fields of the blood flow are measured experimentally and compared to numerical simulations using simple non-Newtonian models (Power law and Carreau models). This work establishes for the first time a relationship between RBC aggregate sizes and corresponding shear rates in a microfluidic environment as well as one between RBC aggregate sizes and apparent blood viscosity at body temperature in a microfluidic controlled environment. The results of the investigation can be used to help develop new numerical models for non-Newtonian blood flow, provide a better understanding of the mechanics of RBC aggregation and help determine aggregate behaviour in clinical settings such as for degenerative diseases like diabetes and heart disease.

In the thesis we study incompressible turbulent flow using spectral element method (SEM). We present fundamentals of SEM which can be seen as a combination of spectral method and finite element method. Turbulent flow is described with the help of two-equation k-ω turbulence models. We give reasons for the choice of Kolmogorov's and Wilcox model and implement them in 2D into an existing SEM C++ framework, Nektar++. Analytical solutions of Navier- Stokes equation were found and used as test cases for the implementation of the models. We simulated turbulent flow in channel and compared the results with direct numerical simulation. 1

QC 20101105

The development of adaptive mesh refinement capabilities in the field of computational fluid dynamics is an essential tool for enabling the simulation of larger and more complex physical problems. While such techniques have been known for a long time, most simulations do not make use of them because of the lack of a robust implementation. In this work, we present recent progresses that have been made to develop adaptive mesh refinement features in Nek5000, a code based on the spectral element method. These developments are driven by the algorithmic challenges posed by future exascale supercomputers. First, we perform the study of the strong scaling of Nek5000 on three petascale machines in order to assess the scalability of the code and identify the current bottlenecks. It is found that strong scaling limit ranges between 5, 000 and 220, 000 degrees of freedom per core depending on the machine and the case. The need for synchronized and low latency communication for efficient computational fluid dynamics simulation is also confirmed. Additionally, we present how Hypre, a library for linear algebra, is used to develop a new and efficient code for performing the setup step required prior to the use of an algebraic multigrid solver for preconditioning the pressure equation in Nek5000. Finally, the main objective of this work is to develop new methods for estimating the error on a numerical solution of the Navier–Stokes equations via the resolution of an adjoint problem. These new estimators are compared to existing ones, which are based on the decay of the spectral coefficients. Then, the estimators are combined with newly implemented capabilities in Nek5000 for automatic grid refinement and adaptive mesh adaptation is carried out. The applications considered so far are steady and two-dimensional, namely the lid-driven cavity at Re = 7, 500 and the flow past a cylinder at Re = 40. The use of adaptive mesh refinement techniques makes mesh generation easier and it is shown that a similar accuracy as with a static mesh can be reached with a significant reduction in the number of degrees of freedom. / <p>QC 20171114</p>

No description available.

In this thesis, we mainly investigate the application of a nodal discontinuous Galerkin spectral element method (DGSEM) for simulating processes in column liquid chromatography. Additionally, we investigate the effectiveness of a total variation diminishing in the mean (TVDM) limiter in controlling spurious oscillations related to the Gibbs phenomenon. With an order-of-accuracy test, we demonstrated that our nodal DGSEM achieved and, in multiple instances, even exceeded theoretical convergence rates, especially with an increased number of elements, validating the use of high-order basis functions for achieving high-order accuracy. We also demonstrated how setup parameters could affect process outcomes, which suggests that numerical simulations can help guide the development of experimental methods since they can explore the solution space of an optimization problem much faster than experimental procedures by leveraging computational speed. Finally, we showed that the TVDM limiter successfully eliminated severe oscillations and negative concentrations near shock regions but introduced significant smearing of the shocks. These findings validate the nodal DGSEM as a highly accurate and reliable tool for detailed modeling of column liquid chromatography, which is essential for improving efficiency, yield, and product quality in biopharmaceutical manufacturing.

This thesis deals with Large Eddy Simulations (LES) of the Atmospheric Boundary Layer (ABL), focusing on studying the resolution dependence of turbulent passive scalar transport within the layer. The ABL is the lowest part of the atmosphere, where humans live and conduct most of their daily activities. Here, a scalar was injected at four different heights in a mixed shear- and convective-driven ABL, which was simulated using the Spectral Element Method (SEM) code Nek5000. The statistics of the four scalars were analysed and their resolution dependence was studied and compared to that of non-scalar quantities. No significant resolution dependence was found with regards to non-scalar quantities, while scalar quantities show a rather strong dependence on resolution especially in the first quarter of the simulation. Negative concentration values are found within the layer and some approaches to solve the problem are proposed. Statistics alone provide an accurate description of the general ABL behaviour, but are found to be insufficient to capture the dynamics of the scalar injection, which ought to be analysed with more advanced methods (e.g. modal decomposition). The structures arising within the layer are also analysed, and further work regarding the study of scalar fronts is suggested.

The vulnerability of a ship to the impact of underwater explosions (UNDEX) and how to incorporate this factor into early-stage ship design is an important aspect in the ship survivability study. In this dissertation, attention is focused on the cost-efficient simulation of the ship response to a far-field UNDEX which involves fluid shock waves, cavitation, and fluid-structural interaction. Traditional fluid numerical simulation approaches using the Finite Element Method to track wave propagation and cavitation requires a high-level of mesh refinement to prevent numerical dispersion from discontinuities. Computation also becomes quite expensive for full ship-related problems due to the large fluid domain necessary to envelop the ship. The burden is aggravated by the need to generate a fluid mesh around the irregular ship hull geometry, which typically requires significant manual intervention. To accelerate the design process and enable the consideration of far-field UNDEX vulnerability, several contributions are made in this dissertation to make the simulation more efficient. First, a Cavitating Acoustic Spectral Element approach which has shown computational advantages in UNDEX problems, but not systematically assessed in total ship application, is used to model the fluid. The use of spectral elements shows greater structural response accuracy and lower computational cost than the traditional FEM. Second, a novel fully automatic all-hexahedral mesh generation scheme is applied to generate the fluid mesh. Along with the spectral element, the all-hex mesh shows greater accuracy than the all-tetrahedral finite element mesh which is typically used. This new meshing approach significantly saves time for mesh generation and allows the spectral element, which is confined to the hexahedral element, to be applied in practical ship problems. A further contribution of this dissertation is the development of a surrogate non-numerical approach to predict structural peak responses based on the shock factor concept. The regression analysis reveals a reasonably strong linear relationship between the structural peak response and the shock factor. The shock factor can be conveniently employed in the design aspects where the peak response is sufficient, using much less computational resources than numerical solvers. / Doctor of Philosophy / The vulnerability of a ship to the impact of underwater explosions (UNDEX) and how to incorporate this factor into early-stage ship design is an important aspect in the ship survivability study. In this dissertation, attention is focused on the cost-efficient simulation of the ship response to a far-field UNDEX which involves fluid shock waves, cavitation, and fluid-structural interaction. Traditional fluid numerical simulation approaches using the Finite Element Method to track wave propagation and cavitation requires a highly refined mesh to deal with large numerical errors. Computation also becomes quite expensive for full ship-related problems due to the large fluid domain necessary to envelop the ship. The burden is aggravated by the need to generate a fluid mesh around the irregular ship hull geometry, which typically requires significant manual intervention. To accelerate the design process and enable the consideration of far-field UNDEX vulnerability, several contributions are made in this dissertation to make the simulation more efficient. First, a Cavitating Acoustic Spectral Element approach, which has shown computational advantages in UNDEX problems but not systematically assessed in total ship application, is used to model the fluid. The use of spectral elements shows greater structural response accuracy and lower computational cost than the traditional FEM. Second, a novel fully automatic all-hexahedral mesh generation scheme is applied to generate the fluid mesh. Along with the spectral element, the all-hex mesh shows greater accuracy than the all-tetrahedral finite element mesh which is typically used. A further contribution of this dissertation is the development of a non-numerical approach which can approximate peak structural responses comparable to the numerical solution with far less computational effort.

To obtain predictive modeling of a spacecraft, the author investigates the effects of adding cables to a simple structure with the goal of developing an understanding of how cables interacting with a structure. In this research, the author presents predictive and accurate modeling of a cable-harnessed structure by means of the Spectral Element Method (SEM). A double beam model is used to emulate a cable-harnessed structure. SEM modeling can define the location and the number of connections between the two beams in a convenient fashion. The proposed modeling is applied and compared with the conventional FEM. The modeling approach was compared to and validated by measurement data. The validated modeling was implemented to investigate the effect of the number of connections, of the spring stiffness of interconnections, and of mass portion of an attached cable. Damping has an important role in structural design because it reduces the dynamic response, thereby avoiding excessive deflection or stress, fatigue loads, and settling times. Experimental results with some specimens indicate a clear change of damping on the main structure with the inclusion of cable dynamics. The author investigated the modification of the damping of the host structure induced by various attached cables. The identification of a damping matrix is performed using measured data. The effect of the flexibility of a cable harness on damping is observed through experiments with various types of cables. The effect of the number of connections on damping is also investigated by changing the number of connections. Moreover, to overcome the sensitivity to noise in measured data of damping matrix identification approach, various methods are compared with a simulated lumped model and real test results. An improved damping matrix identification approach is proposed and can generate the unique damping matrix over the full frequency range of interest. / Ph. D.