<p>In sheet metal forming processes, friction has decisive effects on the strain distribution in the deformed sheets and the quality of the final product due to the large surface/thickness ratio of the blank sheets. It is well known that friction in sheet forming operations is dependent on local contact conditions such as surface roughness, contact pressure and sliding velocity. Adding complexity to this frictional behavior, some rolled sheets have oriented surface roughness and show considerable frictional anisotropy. A constant friction model without consideration of these relevant phenomena is regarded as the reason why sheet metal forming simulations often fail to produce satisfying results despite the well developed material models. </p> <p>To develop a friction model which considers both of the varying conditions of local contact and the frictional anisotropy was the aim of this thesis. For this purpose, the analysis method of the friction test (draw-bend test) had to be examined for the capability to evaluate these parameters independently. Through careful study using finite element simulations, it was found that the conventional method has shortcomings in addressing pressure dependent friction due to the pressure non-uniformity existing in the test. Therefore, a new analysis method, which can evaluate pressure dependency of a friction coefficient, was developed. In the new method, contact pressure maps obtained from simulations were included in the analysis of test data.</p> <p>The new analysis method was applied to friction measurement of aluminum sheets with known anisotropic mill finish, and friction coefficients were obtained as functions of contact pressure, sliding velocity and sliding direction. In the obtained friction model, a friction coefficient is a continuous surface over the domain of contact pressure and sliding velocity. Lastly, the new friction model was implemented into a finite element code and the model was validated through circular cup drawing experiments and simulations. The comparisons showed good agreements in the aspects of punch force, cup size and failure location. Thus, the newly developed model can accurately predict the effects of anisotropic friction in sheet metal forming processes. </p> / Thesis / Doctor of Philosophy (PhD)

The threshold friction velocities of potential dust sources in the US Southwest were measured in the field using a Portable Wind Tunnel, which is based on the Desert Research Institute's Portable In-Situ Wind Erosion Laboratory (PI-SWERL). A mix of both disturbed and undisturbed surfaces were included in this study. It was found that disturbed surfaces, such as those at the Iron King Mine tailings site, which is part of the EPA's Superfund program and contains surface concentrations of arsenic and lead reaching as high as 0.5% (w/w), had lower threshold friction velocities (0.32 m s⁻¹ to 0.40 m s⁻¹) in comparison to those of undisturbed surfaces (0.48 to 0.61 m s⁻¹). Surface characteristics, such as particle size distribution, had effects on the threshold friction velocity (smaller grain sized distributions resulted in lower threshold friction velocities). Overall, the threshold friction velocities of disturbed surfaces were within the range of natural wind conditions, indicating that surfaces disturbed by human activity are more prone to causing windblown dust.

abstract: Understanding and predicting climate changes at the urban scale have been an important yet challenging problem in environmental engineering. The lack of reliable long-term observations at the urban scale makes it difficult to even assess past climate changes. Numerical modeling plays an important role in filling the gap of observation and predicting future changes. Numerical studies on the climatic effect of desert urbanization have focused on basic meteorological fields such as temperature and wind. For desert cities, urban expansion can lead to substantial changes in the local production of wind-blown dust, which have implications for air quality and public health. This study expands the existing framework of numerical simulation for desert urbanization to include the computation of dust generation related to urban land-use changes. This is accomplished by connecting a suite of numerical models, including a meso-scale meteorological model, a land-surface model, an urban canopy model, and a turbulence model, to produce the key parameters that control the surface fluxes of wind-blown dust. Those models generate the near-surface turbulence intensity, soil moisture, and land-surface properties, which are used to determine the dust fluxes from a set of laboratory-based empirical formulas. This framework is applied to a series of simulations for the desert city of Erbil across a period of rapid urbanization. The changes in surface dust fluxes associated with urbanization are quantified. An analysis of the model output further reveals the dependence of surface dust fluxes on local meteorological conditions. Future applications of the models to environmental prediction are discussed. / Dissertation/Thesis / Doctoral Dissertation Mechanical Engineering 2019

This study aims to better understand the characteristics of the tidal- current profiles and the near-bed boundary layer structures off the southwestern coast of Taiwan. The velocity profile is measured by a bottom-mounted ADCP. Six experiments were conducted, each lasted 10~20 days and the water depth ranging 12~18 m. Twenty-minute averaged velocity profiles have been fitted to a logarithmic form with 4% accuracy. The friction velocity (u*) and roughness length (z0) are then derived from the slope and intercept of the best-fitted straight lines. Our results show that the profile shape and friction velocity vary tidally, the latter reaches O(0.06)ms-1 during peak current flow. The magnitude of z0 is large and scattered, but it shows a general trend of decrease with increasing flow speed. The observed log-layer height increases, and the bottom drag coefficient (CD) decrease, respectively with increasing flow speed. Measurements also show that water turbidity increases with rainfall, as a result the z0 and CD also increase. Finally, harmonic analysis of the tidal currents indicate significant changes between winter (homogeneous) and summer (stratified) conditions. In winter the vertical variation of orientation and phase is small, whereas in summer there was a 150 orientation and 250 phase difference (the bottom currents lead the surface currents) between the near surface and near bed regions.

abstract: It is estimated that wind induced soil transports more than 500 x 106 metric tons of fugitive dust annually. Soil erosion has negative effects on human health, the productivity of farms, and the quality of surface waters. A variety of different polymer stabilizers are available on the market for fugitive dust control. Most of these polymer stabilizers are expensive synthetic polymer products. Their adverse effects and expense usually limits their use. Biopolymers provide a potential alternative to synthetic polymers. They can provide dust abatement by encapsulating soil particles and creating a binding network throughout the treated area. This research into the effectiveness of biopolymers for fugitive dust control involved three phases. Phase I included proof of concept tests. Phase II included carrying out the tests in a wind tunnel. Phase III consisted of conducting the experiments in the field. Proof of concept tests showed that biopolymers have the potential to reduce soil erosion and fugitive dust transport. Wind tunnel tests on two candidate biopolymers, xanthan and chitosan, showed that there is a proportional relationship between biopolymer application rates and threshold wind velocities. The wind tunnel tests also showed that xanthan gum is more successful in the field than chitosan. The field tests showed that xanthan gum was effective at controlling soil erosion. However, the chitosan field data was inconsistent with the xanthan data and field data on bare soil. / Dissertation/Thesis / Ph.D. Civil and Environmental Engineering 2011

Wave heights and periods of surface waves in the Baltic Sea have been modelled for a two-year period (1999-2000) with the wave model Hypas on an 11x11-km grid scale. There is a clear seasonal variation with higher waves during winter and lower during summer. This is mainly a reflection of the wind climate in the area where the winters are windier than the summers. The largest waves are found in the Skagerrak and over the deeper, eastern areas in the Baltic Proper. In the Baltic Sea, the surface waves influence the bottom sediment by initiating resuspension down to 80 m depths. This process is dependent not only on the waves but also on the varying grain size diameters. Fine and medium sand resuspend more often than other sediment types, and these sediments cover together about 25% of the Baltic Proper area. On average sediment is here resuspended 4-5 times per month with a duration for each event of 22 hours. The highest resuspension frequencies are found on the eastern and southern side of the Baltic Proper. During resuspension sediment grains are lifted up into the water mass and matters earlier bound in the sediment can be released. This may stimulate both production and degradation of organic matter.

A fundamental stage in rocky-shore seaweed life history is the recruitment process involving external fertilisation and then settlement of the propagules on a suitable substrate. The ultimate step in this settlement stage is the crossing of the viscous sub-layer and attachment to the substrate. Given the extreme conditions met in the intertidal zone, propagules can be dislodged at any time before they secure a strong enough anchoring. Flow conditions and propagule properties are key to this process. The settlement process under turbulent conditions was recreated within a stirred benthic chamber for five different species. Whereas propagule properties (size, density) vary with species, and propagules are adapted to the different conditions in the intertidal, they exhibit the same settlement behaviour. They nevertheless exhibit different settling velocities and settlement thresholds. Several methods of characterisation of the tank flow from particle tracking velocimetry and acoustic Doppler velocimetry data are reviewed, as well as an analytical model. Turbulent settling was found to be independent of the well-mixed tank bulk flow and to depend only on the boundary-layer mechanics. A model of settlement threshold is presented from which propagule mucilage adhesiveness estimates are derived, leading to good correlations between adult plant exposure and the stickiness of its propagules and to the conclusion that settlement can only occur in calm conditions. To extend the work, computational fluid dynamic techniques are developed by extending the Gerris Flow Solver. A 2-D approach to tank modelling and a pilot study of expansion to 3-D is described. This extends the perspective given by the experiments, notably through output of the hydrodynamical forces experienced by the propagules. Finally, in the view of realizing direct numerical simulations of propagule behaviour in the viscous sub-layer, a new and unique 2-D/3-D fully conservative solid/fluid interaction model is developed and tested with success.

Cette thèse a pour but d'étudier une nouvelle décharge nanoseconde pulsée à barrière diélectrique basée sur 3 électrodes (SL-DBD pour « SLiding DBD » en anglais), de la comparer à la décharge nanoseconde conventionnelle utilisant 2 électrodes (NS-DBD), et d'évaluer sa capacité à être utilisée soit comme capteur de vitesse pariétale, soit comme actionneur électromécanique pour le contrôle d'écoulement. Dans la première partie, les propriétés électriques des deux décharges sont caractérisées, permettant ainsi d'identifier le paramètre électrique clé qui permet de passer d'un régime de décharge à un autre. Des visualisations par caméra intensifiée ont confirmé cette transition lorsque le champ électrique moyen devient supérieur à 6.5 kV/cm. Des diagnostiques mécaniques (Schlieren et mesures de pression) ont permis de caractériser précisément l'onde de pression générée par les deux décharges.Ensuite, l'influence d'un écoulement sur le comportement électrique de la SL-DBD a été étudiée, mettant en évidence que le courant « collecté » par l'électrode (3) était à peu près proportionnel à la vitesse de l'écoulement en proche paroi. Même s'il reste encore de nombreux points à vérifier, ce résultat encourageant permet d'envisager l'utilisation de la SL-DBD comme capteur de vitesse et/ou de frottement pariétal.Enfin, la troisième partie est consacrée à l'effet de la SL-DBD sur des écoulements aérodynamiques, dans le but de les manipuler. Plusieurs configurations ont été étudiées (profil d'aile, marche descendante, plaque plane) et les résultats ont permis de montrer la complexité des phénomènes physiques à l'origine du contrôle, sans pour autant pouvoir totalement les expliquer. / This thesis aims to study new design of nanosecond pulsed dielectric barrier discharge in 3 electrodes configuration: the sliding discharge (SL-DBD), to compare it to the conventional nanosecond DBD in 2 electrodes geometry (NS-DBD) and to show also its capacity to be used as a friction velocity or wall shear stress sensor and to be used as electromechanical actuator for flow control.In its first part, the electrical properties of these two discharges are characterized and point out the key parameter governing the transition of one regime to another. The visualizations with an intensified camera confirm this transition when the mean electric field increases over 6.5 kV/cm. Therefore they extend further and cover the inter-electrode gap. Mechanical diagnostics (Schlieren and pressure measurements) characterize the pressure wave generated by these discharges. In the second part, the electrical characterization of the SL-DBD under flow conditions shows that the courant « collected » by the third electrode is almost proportional to the wall flow velocity. However, even if other studies needed to be performed, these encouraging results reveal the ability of the SL-DBD to be used as a friction velocity or a wall shear stress sensor. Finally, the third part is addressed to the effect of the SL-DBD on aerodynamics flows in order to manipulate them. Several configurations are studied (airfoil, backward facing step, flat plate) and the results have shown the complexity of the physicals phenomena governing the control authority, without being able to fully explain them.