Добірка наукової літератури з теми "Boudary layers"

In-situ measurements of the structural bending moments and torque about the shank-fluke interface of the dolos armor unit have been made for 42-ton (36-metric tonne) dolosse at Crescent City, California jetty. The measurements include the static loads on the dolosse as well as wave induced forces. The data were obtained from internal strain gages cast into the dolos during construction along with a special data acquisition system. The measurement system was also capable of capturing impact forces caused by dolos rocking or movement. Measurements were made during the winter storm seasons from January 1987 through May 1988. Coincident with the structural measurements, wave height and period were measured at several water depths approaching the breakwater, including a site directly in front of the dolos test section. The Crescent City jetty is a shallow water breakwater with depth limited waves in about 10 meters of water depth. The structural measurements were made from 14 dolos units arranged in a rectangular section on the top layer of the trunk portion of the jetty. Four of these dolosse are also instrumented with an accelerometer platform to measure motion with six degrees of freedom. In addition, there are three instrumented dolosse on the bottom layer of the breakwater. These dolosse measure the static stress due to the units placed on top of them, as well as pulsating forces. The structural and wave measurements, reported here, are supplemented with hydrostatic pore pressure measurements in the core material of the breakwater, and by aerial photogrammetric motion analysis (Kendall, 1988), land based surveys, boudary condition surveys, hydrographic surveys, and side scan sonar surveys.

We present a statistical model for the distribution of increments of wind velocity and temperature of turbulence data measured in the atmospheric surface layer above the Amazonian forest, on a micrometeorological tower in Rebio-Jaru Reserve (10°04' S; 61°56' W) during LBA (Large Scale Biosphere Atmosphere Experiment in Amazonia) wet season campaign. The data were measured using 3D Campbell sonic anemometer and a Campbell fast response thermometer located at a height of 66 m (the forest canopy has a mean height of 35 m; but some of the higher tree branches may reach the height of 45 m), at a sampling rate of 60 Hz. The goal of this investigation is to detect noextensivity properties of the turbulent field in such a peculiar environment. To do this we calculate the probability density function (PDF) for wind velocity increments Vr(x) = V(x) - V(x+r) (and temperature increments) at different length scales r. The preliminary results show some evidence that the nonextensive thermostatistics modelling proposed by Tsallis (1988) provides a new and simple framework for explaining the statistical behavior of fully developed mechanical turbulence. We also investigate the relationship between intermittency and nonextensivity using a single parameter q, from Tsallis thermostatistics. The results of the wind velocity turbulent signals measured at the level of 66m show a very good agreement with the Tsallis model but the temperature signals show some discrepancy with respect to the proposed model. Physical discussions are proposed to explain our results.

To develop higher efficiency and lower emission gasoline engines, ultra-lean burning under high Reynolds number conditions is desired. It is believed that enhancement of tumble flow in the engine cylinder is effective for increase of turbulent intensity, resulting in improvement of characteristics of flame propagation and ignition under a strong discharge, while the enhancement of tumble flow might cause heat loss from the wall. Investigations of characteristics of turbulence and distributions of wall and gas temperature in engine cylinders are still challenging due to transient and high pressure, and due to cycle-to-cycle variations. In the previous study (Jainski et al., 2013), a micro particle image velocimetry (micro PIV) measurements were conducted in an engine cylinder at up to 1100 rpm and the characteristics of velocity boundary layer around a cylinder head were investigated. The study has shown that the log-law does not properly present the measured velocity distributions near the wall. In our previous study (Shimura et al., 2018), a micro PIV was conducted in a motored engine cylinder to investigate velocity boundary layer characteristics near piston top before the top dead center (TDC) at a constant engine speed of 2000 rpm to deepen understanding characteristics of velocity boundary layer in engine cylinder with tumble flow. The velocity boundary layer was well fitted to the Blasius theory at 30 CAD before TDC and deviated from the theory at 15 CAD before TDC. However, the obtained data was two components of velocity in the measurement plane, which means that effects of magnitude of velocity were not clear in the previous measurement. In this study, stereoscopic micro PIV was conducted to elucidate the effects of magnitude and direction of velocity on the characteristics of velocity boundary layer near the piston top in the tumble enhanced SI IC optical engine. The tumble enhanced SI IC optical engine used in the previous study (Shimura et al., 2018; Matsuda et al., 2019) was used also in this study. The bore is 75 mm and the stroke is 112.5 mm. Length of the connecting rod is 250 mm. The engine has two intake valves of the diameter 29 mm and two exhaust valves of the diameter 25 mm. The compression ratio is 13.0. The optical access is achieved through the quartz glass cylinder. A tumble enhancing intake port is used for the sake of improvement of ignition and flame propagation. The engine speed can be set up to 2000 rpm at the maximum. The overall flow fields taken by a preliminary PIV experiment can be seen in the literatures (Shimura et al., 2018; Matsuda et al., 2019). The laser beams for PIV are from two Nd:YAG lasers (LOTIS, LS-2131, 150 mJ/pulse, 532 nm) are led to the same optical axis by a mirror and a polarizing beam splitter. The laser beam is formed into laser sheet of 180 µm thickness by laser sheet forming optics and led into the engine cylinder. The scattering light was collected by long distance microscope lenses (Quester, SZM100) and imaged onto CCD cameras (Princeton Technology, ES4020) in the stereoscopic alignment with 18 degrees. To compensate for the difference in the focal length caused by the quartz engine liner, a cylindrical lens of 1000 mm focal length was placed in front of each long distance microscope lens. SiO2 of 1 µm mean diameter was used for tracer particles. The micro PIV was operated at about 6.6 Hz to be synchronized with engine speed. The time separation of the successive particle images was 1.5 µs. The field of view of the micro PIV was 3.5 mm × 3.5 mm on the piston top including central axis of the cylinder. Here, x and y coordinates are set to the direction from the exhaust to the intake valves and the direction from the piston to the pent roof, respectively. z coordinates is perpendicular to x and y axes, and the orizin of the coordinates is set at the center of the piston top. The spatial resolution of PIV, which is defined by the size of interrogation region, is 108.8 µm × 54.4 µm. Vector spacing is 54.4 µm × 27.2 µm. The first vector position is about 27.2 µm away from the wall. The measurements were conducted at 345 CAD. The engine was motored at 2000 rpm and operated for three intake valve open timings of -30 CAD. The operation condition of the engine tested contain strong cycle-to-cycle variations, which results in the large root-mean-square values of velocity fluctuation near the center of the piston top (Shimura et al., 2018). To evaluate flow characteristics in the cycle-to-cycle variations, conditional averaging based on magnitude of fluid velocity is used in this study. Figure 1(a) shows a histogram of the magnitude of combined velocity of u and w. The magnitude of velocity can be considered as momentum of fluid because few fluctuation of density is considered and temperature boundary layer is enough thin compared to the velocity boundary layer. The large variations in the momentum can be observed in Fig. 1(a). The large variations are considered to be caused by the variations of tumble core locations. The fraction of the large momentum Here, the momentum are classified into C1 to C4 based on fractions (C1: 54.4%, C2: 19.5%, C3: 19.5%, C4: 6.6%). Figure 1(b) and (c) shows mean velocity distribution classified into C1 and C4 in Fig. 1(a). The distribution is fitted to the log-law velocity profile of developed wall turbulence. The mean velocity profile for C1 shows large discrepancy from that of general turbulent boundary layers, while that for C4 show relatively close to that of general turbulent boundary layer. C2 and C3, which are not shown here, have trend between the C1 and C4 profiles. These results show that the velocity profiles which can be assumed to be the developed turbulent boudary layer in the targeted condition is less than half of cycles, which means partial applicability of conventional CFD models for prediction of boundary layer of the engine condition.

The laminar-turbulent transition in viscous fluid flows is one of the most intriguing problems in fluid dynamics today. In view of the enormous applications it has in a variety of fields such as aircraft design, turbomachinery, etc., scientists have now realized the importance of tackling this problem effectively. Three-dimensional flows are usually associated with pressure gradient, streamline curvature, streamline convergence / divergence etc., all acting simultaneously. Towards a better understanding of the transition process and modeling the transition zone, it is important to study the effect of each of these parameters on the transitional flow. The present work aims at studying experimentally the effect of lateral streamline convergence alone on the laminar-turbulent transition zone under constant stream-wise pressure. The experimental setup consists of a low turbulence wind tunnel with its test section modified to cause lateral streamline convergence under constant pressure. This is achieved by converging the side-walls and appropriately diverging the roof, thus maintaining a constant stream-wise pressure. The half angle of convergence is chosen as 100 , which is approximately the same as the half of the turbulent spot envelope in constant pressure two-dimensional flows. Experiments are carried out to analyze the development of the laminar and transitional boundary layers, intermittency distribution in the transition zone and the overall characteristics of an artificially induced turbulent spot. The laminar velocity profiles are found to be of the Blasius type for two-dimensional constant pressure flows. However, the converging streamlines are found to contribute to an increased thickness of the boundary layer as compared to the corresponding two-dimensional flow. The intermittency distribution in the transition zone is found to follow the universal intermittency distribution for two-dimensional constant pressure flow. A simple linear-combination model for two-dimensional flows is found to perform very well in predicting the measured velocity profiles in the transition zone. An artificially introduced turbulent spot is found to propagate along a conical envelope with an apex cone angle of 220 which is very nearly the value for a corresponding constant pressure two-dimensional flow. The spot shapes and celerities are also comparable to those in two-dimensional flow. In summary, the present study brings out many similarities between a constant pressure laterally converging flow and a constant pressure two-dimensional flow.

Dans cette thèse, nous contribuons à la modélisation, la simulation et l'optimisation d'une nouvelle génération de matrices de micro-miroirs conçue par le Laboratoire d'Astrophysique de Marseille (LAM). Une contribution est également apportée au développement du logiciel de calcul symbolique MEMSALab qui assistera la construction de modèles multi-échelles pour des matrices de microsystèmes. Le couplage entre le comportement élastique quasi-statique non linéaire d'une cellule de la matrice de micro-miroirs et le champ électrostatique utilisé pour son actionnement a été simulé.Une fois validée, cette simulation a été utilisée pour étudier le phénomène de pull-in ainsi que son optimisation. Ensuite, un modèle homogénéisé du champ électrostatique dans le vide entourant la matrice de micro-miroirs est construit à l'aide d'une méthode asymptotique. Les contributions au développement de MEMSALab constituent en l'introduction d'une théorie d'extension et de combinaison de preuves qui sera utilisée pour construire des modèles asymptotiques par un procédé de complexifications successives. Enfin, un langage spécifique complet pour l'utilisation de MEMSALab est présenté et illustré par des exemples significatifs. En particulier, il a été utilisé pour coder la construction d'un modèle homogénéisé qui sert d'art initial à la méthode extension-combinaison
In this thesis, we contribute to the modeling, simulation and optimization of a new generation of micro mirror arrays designed by the Astrophysics Laboratory of Marseille (LAM). A contribution is also made to the development of MEMSALab a symbolic computation software package designed to asist multiscale model derivation for microsystem arrays. The coupling between the quasi-static nonlinear behavior of a cell of the micro-mirror array and the electrostatic field used for its actuation is simulated. The simulation is then used to study the phenomenon of pull-in and its optimization. Then, a homogenized model for the electrostatic field in the vacuum space surrounding in the micro-mirror array has been built using an asymptotic method. The contributions to the development of MEMSALab consist in the introduction of an extension and combination theory that will be used to construct multiscale models based on various asymptotic approaches by a process of successive complexifications. Finally, a complete specification language for using MEMSALab is presented and illustrated by significant examples. In particular, it was used to encode the derivation of a homogenized model that serves as an initial state to the extension-combination method

L'indagine ha riguardato il profilo del vento nei primi 30 metri dello strato limite atmosferico stabile nell'ambito della teoria di similarità locale. Ad oggi, diversi esperimenti hanno confermato la validità della teoria per strati-limite su terreni livellati e superfici omogenee. Tali condizioni ideali sono però infrequenti nella realtà ed è perciò importante capire quali siano i limiti della similarità locale per strati-limite su terreni complessi e superfici disomogenee. Entrambe le condizioni sono presenti a Ny-Alesund (Svalbard, Norvegia) dove il Consiglio Nazionale delle Ricerche (CNR), nel 2009, ha installato una torre di 30 m, la Amudsen-Nobile Climate Change Tower (CCT), per lo studio dello strato-limite artico. Il lavoro di tesi ha riguardato misure di vento e turbolenza acquisite sulla CCT da maggio 2012 a maggio 2014. Il confronto tra le velocità del vento misurate dagli anemometri installati sulla CCT, ha rivelato criticità nel dato sonico manifestatesi con sovrastime sistematiche e maggiore erraticità rispetto alle misure provenienti dagli anemometri a elica. Un test condotto fra diversi metodi per il calcolo dei gradienti verticali della velocità del vento ha rivelato scarsa sensibilità dei risultati ottenuti al particolare metodo utilizzato. Lo studio ha riguardato i gradienti verticali adimensionali della velocità del vento nei primi 30-m dello strato limite stabile. Deviazioni significative tra i tra le osservazioni e i valori predetti dalla similarità locale sono state osservate in particolare per i livelli più distanti dal suolo e per valori crescenti del parametro di stabilità z/L (L, lunghezza di Obukhov locale). In particolare, si sono osservati gradienti adimensionali inferiori a quelli predetti dalle più usate relazioni di flusso-gradiente. Tali deviazioni, presenti perlopiù per z/L>0.1, sono state associate ad un effetto di accentuazione della turbolenza da parte delle irregolarità del terreno. Per condizioni meno stabili, z/L<0.1, scarti positivi tra i gradienti osservati e quelli attesi sono stati attribuiti alla formazione di strati limite interni in condizioni di vento dal mare verso la costa. Sono stati proposti diversi metodi per la stima dell'effetto della self-correlazione nella derivazione delle relazioni di flusso-gradiente, dovuta alla condivisione della variabile u*. La formula per il coefficiente lineare di self correlazione e le sue distribuzioni di probabilità empiriche sono state derivate e hanno permesso di stimare il livello di self-correlazione presente nel dataset considerato.

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This work presents a comparative study between four different sliding mode variable structure control strategies (SMVSC) applied to autonomous underwater vehicles (AUV) positioning in 6 DOF, under the influence of wind, waves and marine currents. The addressed strategies are the conventional CEV-MD control based on Lyapunov stability, the CEV-MD control based on the equivalent control, the CEV-MD control based on the input-output stability and the CEVMD adaptive control. The accomplished comparisons seek a satisfactory tradeoff between the tracking performance and the closed-loop system stability in light of eliminating the chattering phenomenon. In that sense, the analysis and synthesis of the respective SMVSC control laws is carried out fromthe Lyapunov Stability Theory and the Barbalat s Lemma. As well as numerical simulations are implemented to obtaining the respective performances of each SMVSC control strategy presented.
Este trabalho apresenta um estudo comparativo entre quatro diferentes estratégias de controle de estrutura variável por modos deslizantes (CEV-MD) aplicadas ao posicionamento de veículos subaquáticos autônomos (VSA) em 6 GDL, sob a influência de ventos, ondas e correntes marinhas. As estratégias abordadas são o controle CEV-MD convencional baseado na estabilidade de Lyapunov, o controle CEV-MD baseado no controle equivalente, o controle CEV-MD baseado na estabilidade entrada-saída e o controle CEV-MD adaptativo. As comparações realizadas visam a eliminação do do fenômeno do chattering buscando um compromisso satisfatório entre o desempenho de rastreamento e a estabilidade do sistema em laço fechado. Nesse sentido, a análise e síntese das respectivas leis de controle CEV-MD é realizada a partir da Teoria de Estabilidade de Lyapunov e do Lema de Barbalat. Assim como simulações numéricas são implementadas para a obtenção dos respectivos desempenhos de cada estratégia de controle CEV-MD apresentada.