The effect of initial conditions in combined shear- and buoyancy- driven mixing was investigated through the use of an implicit large eddy simulation code under active development at Los Alamos National Laboratory and Texas A&M University. Alterations were done over several months both at Los Alamos National Laboratory and at the Texas A&M University campus, and include a transition from tilted rig to convective channel arrangement, introduction of an inertial reference frame, alteration of boundary conditions, etc. This work resulted in the development of a numerical framework with the capability to model various shear and Atwood number arrangements such as those seen in an inertial confinement fusion environment. In order to validate the code, it was compared to three published experiments, one with Atwood number 0.46 (White et al. 2010), one with high Atwood number 0.6 (Banerjee et al. 2010), and one with very low Atwood number 0.032 (Akula et al. 2012). Upon validating the code, pure Rayleigh-Taylor and pure Kelvin-Helmholtz instabilities were modeled along with five intermediate cases of increasing shear and constant density gradient. Plots of mixing width, Richardson number, growth parameter, and molecular mixing were compared in order to determine at what level of shear the minimum amount of mixing occurs. The results of height gradient and Reynolds number were to previous experiments and theory. The least amount of molecular mixing at the centerline was found to be when the system had a low Atwood number (0.032) and a multimode initial interface perturbation. While the increase in modes of the interface perturbation did not result in a significant change in the growth parameter, the level of molecular mixing at the centerline substantially decreased. As shear was increased in the system, the mixing width and molecular mixing subsequently increased. For this reason, the shear in the system should be eliminated, or at least minimized, if at possible so as to prevent any additional amalgamation in the system. Analysis of the Reynolds number revealed that with an increase in velocity difference between the fluid layers, the value consequently increased. This trend matches with theoretical results as the value is a function of the mixing width and velocity, thus further validating the code. Analysis of the transitional Richardson number revealed that it had a smaller value in the computational case over the experiment, but this fact can be attributed the difference in mixing width between the two methods. The development of the numerical framework with the capability to model various shear and Atwood number arrangements offers the platform for future study of hydrodynamic instabilities.

Thesis (Ph. D.)--University of Washington, 1999. / Vita. Includes bibliographical references (leaves 98-114).

Konstanz, Univ., Diplomarb., 2002.

Thesis (M. S.)--Civil and Environmental Engineering, Georgia Institute of Technology, 2006. / Kim, Jin-Yeon, Committee Member ; Qu, Jianmin, Committee Member ; Jacobs, Laurence, Committee Chair. Includes bibliographical references.

Thesis (P.D.)--University of Wisconsin--Madison, 1984. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 248-258).

Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.

Στην παρούσα εργασία γίνεται μελέτη της χρήσης του διαφορισμού στις ασύρματες επικοινωνίες. Αρχικά, γίνεται η εισαγωγή και ορίζεται το πεδίο μελέτης. Στη συνέχεια, μοντελοποιείται το ασύρματο κανάλι και παρουσιάζονται οι δυσκολίες που προκύπτουν στη μετάδοση. Συγκρίνεται το κανάλι διαλείψεων (fading channel) με το κανάλι λευκού προσθετικού Γκαουσιανού Θορύβου (AWGN). Στη συνέχεια, γίνεται εισαγωγή στην έννοια του διαφορισμού και περιγράφεται ο διαφορισμός στο χώρο με τη χρήση πολλαπλών κεραιών. Ακολουθεί η ανάλυση ενός συστήματος SIMO οπου γίνεται διαφορισμός μόνο στο δέκτη. Ύστερα, εξετάζεται ένα σύστημα MISO οπου γίνεται διαφορισμός μόνο στον πομπό. Εξετάζονται διάφοροι τρόποι μετάδοσης όπως το σχήμα Alamouti. Το επόμενο κεφάλαιο αποτελεί την εισαγωγή στα συστήματα ΜΙΜΟ και περιγράφεται η ορθογωνοποίηση ενός καναλιού ΜΙΜΟ με τη χρήση SVD. Μελετάται η βέλτιστη κατανομή ενέργειας από τον πομπό. Στη συνέχεια, αναλύονται μέθοδοι ισοστάθμισης για την ανάκτηση των δεδομένων που μεταδίδονται. Εξετάζεται ο αποσυσχετιστής (decorrelator) και ο ισοσταθμιστής ελάχιστου μέσου τετραγωνικού σφάλματος (ΜΜSE). Ακολουθεί η εξέταση μη γραμμικών ισοσταθμιστών ανάδρασης απόφασης και γίνεται αναφορά στην αρχιτεκτονική V-BLAST. Στη συνέχεια γίνεται αναφορά στο βέλτιστο δέκτη και γίνεται σύγκριση του δέκτη αυτού με τους ισοσταθμιστές. / Study of space diversity in wireless communications. The first part is the introduction and the field of study is defined. The next chapter introduces the fading channel. The performance of the Rayleigh fading channel and the AWGN channel are compared. Later, follows the introduction to diversity. We examine a SIMO and a MISO channel. In the MISO channel, emphasis is given on the Alamouti scheme. Later, MIMO channels are discussed. We talk about transmission using Singular Value Decomposition with or without waterfilling. Later, we discuss the equalizers used in MIMO communications. Zero Forcing and Minimum Mean Square Error (MMSE) equalizers are discussed. The next topic of study is decision feedback equalizers. Finally we discuss the optimal receiver architecture and compare its performance against the aforementioned equalizers.

Approved for public release; distribution is unlimited. / Wireless communication is employed to connect mobile computers in a networked environment for information exchange. In a tactical space, sensors and computers typically need to operate on-the-move while transmitting data over both short and long distances in different terrain and conditions. The wireless communication is thus susceptible to effects of Doppler shift and channel fading. In addition, when security and anti jamming features are required, such as frequency-hopping techniques, then coherent signal detection is difficult and noncoherent modulation is used instead. Our study will focus on the bit error rate (BER) performance analysis of noncoherent orthogonal modulation, specifically M-ary frequency-shift keying (MFSK) and code-shift keying (CSK) modulation, in both additive white Gaussian noise (AWGN) and for a Rayleigh fading channel with Doppler shift. The potential applications include communications between mobile computer-sensor devices, such as a mobile ground control station maintaining a datalink with UAV. / Civilian

This thesis includes studies of developing distributed optical fiber vibration sensor based on Rayleigh backscattering with broad frequency response range and high spatial resolution. Distributed vibration sensor based on all-polarization-maintaining configurations of the phase-sensitive optical time domain reflectometry (OTDR) is developed to achieve high frequency response and spatial resolution. Signal fading and noise induced by polarization change can be mitigated via polarization-maintaining components. Pencil-break event is tested as a vibration source and the layout of the sensing fiber part is designed for real applications. The spatial resolution is 1m and the maximum distance between sensing fiber and vibration event is 18cm. Wavelet denoising method is introduced to improve the performance of the distributed vibration sensor based on phase-sensitive OTDR in standard single-mode fiber. Noise can be reduced more effectively by thresholding the wavelet coefficient. Sub-meter spatial resolution is obtained with a detectable frequency up to 8 kHz. A new distributed vibration sensor based on time-division multiplexing (TDM) scheme is also studied. A special probe waveform including a narrow pules and a quasi-continuous wave can combine the conventional phase-sensitive OTDR system and polarization diversity scheme together in one single-mode fiber without crosstalk. Position and frequency of the vibration can be determined by these two detection systems consecutively in different time slots. Vibration event up to 0.6 MHz is detected with 1m spatial resolution along a 680m single-mode sensing fiber. Continuous wavelet transform (CWT) is investigated to study the non-stationary vibration events measured by our phase OTDR system. The CWT approach can access both frequency and time information of the vibration event simultaneously. Distributed vibration measurements of 500Hz and 500Hz to 1 kHz sweep events over 20 cm fiber length are demonstrated using a single-mode fiber. Optical frequency-domain reflectometry (OFDR) for vibration sensing is proposed for the first time. The local Rayleigh backscatter spectrum shift in time sequence could be used to determine dynamic strain information at a specific position of the vibrated state with respect to that of the non-vibrated state. Measurable frequency range of 0-32 Hz with the spatial resolution of 10 cm is demonstrated along a 17 m fiber.

La Multi-Channel Acquisition of Surface Waves (MASW) est devenu populaire depuis quelques années pour l'auscultation non destructive de milieux tabulaires naturels ou artificiels (Béton). Cette méthode, basée sur le comportement dispersif des ondes de surface, comporte deux étapes principales. Une courbe de dispersion des vitesses de phase des ondes de surface est, dans un premier temps, extraite d'un sismogramme par une transformée du champ d'onde (transformée p-?). L'inversion de cette dernière conduit enfin à une interprétation du milieu ausculté sous la forme d'un profil 1D des vitesses de propagation des ondes transverses en fonction de la profondeur. Cette dissertation propose une évaluation de la faisabilité de l’utilisation de la MASW pour le contrôle non destructif de structures souterraines. Cette étude s’est focalisée à évaluer puis à procéder aux différentes modifications nécessaires pour son utilisation en milieu présentant une surface concave (surface très répandue dans les structures de type galeries, tunnel ou puits) / Multi-Channel Acquisition of Surface Waves (MASW) has become very popular in recent years for non destructive testing of both layered natural and artificial (concrete) media. This method, based on the dispersive behaviour of surface waves, consists of two major steps. A phase velocity dispersion curve is first extract from the seismogram using a wave field transform (p-? transform). Then the inversion of this latter produces a 1D interpretation of the medium in terms of transverse wave velocities versus depth. While these two major steps of the method are well-documented for plane stratified media, it’s not the case when the investigated structure has a complex geometry. This dissertation deals with the evaluation of the feasibility to use MASW for non destructive evaluation of underground structures. After a brief survey of the problems that could be encountered with tunnel non destructive evaluation, this study focuses to determine and then proceed to the modifications needed to adapt the method for a concave geometry structure use