Dynamics of shallow flows with and without background rotation

Large-scale oceanic and atmospheric flows tend to behave in a two-dimensional way. To further understand such flows, a large scientific effort has been devoted to the study of perfect two-dimensional flows. For the last 30 years, there has been a large interest in experimentally validating the results from numerical and theoretical studies concerning two-dimensional flows, particularly twodimensional turbulence and spatially periodic two-dimensional flows. Inspired by geophysical flows, experimentalists have used stratification, shallow fluid layer configurations, and background rotation to enforce the two-dimensionality of flows in the laboratory. However, as all these methods have shortcomings, it is difficult to achieve a perfectly two-dimensional flow in the laboratory. The work presented in this thesis focuses on two of the common methods used to enforce the two-dimensionality of flows: the shallow layer configuration and background rotation. To further understand the effect of these methods on the two-dimensionality of flows, we studied the dynamics of generic elementary vortical structures in a shallow fluid layer with and without background rotation. Through the analytical and numerical study of a decaying axisymmetric monopolar vortex, we revised the usual argument for considering shallow flowsas two-dimensional. This argument is based on the continuity equation, and it states that the vertical velocity can be neglected if the ratio of vertical to horizontal length scales of the flow is small. By performing numerical simulations and a perturbation analysis for shallow flows, it was shown that this argument is not valid in general, and that the two-dimensionality of the flow does not depend exclusively on the aspect ratio. Instead, it also depends on the dynamics of the flow; particularly, a shallow flow behaves in a two-dimensional way if the flow evolution is dominated by bottom friction over the whole fluid depth. These results were supported by the numerical and experimental study of a more complex flow structure, namely a dipolar vortex, in a shallow fluid layer. For the study of decaying dipolar vortices, numerical simulations were performed using a finite element code. The flow was initialized with a Lamb–Chaplygin dipolar vortex with a Poiseuille-like vertical profile, after which it was left to evolve freely. The 3D structure of the vortex was obtained using the 2 vortex detection criterion. Using this tool, it was observed how the vortex is gradually distorted due to the secondary 3D motions. An experimental investigation of an electromagnetically forced dipolar vortex, where Particle Image Velocimetry (PIV) was used to calculate the velocity field in a horizontal cross-section of the flow, supports the numerically obtained results. It is assumed that flows subjected to strong background rotation behave like two-dimensional flows due to the reduction of gradients in the direction parallel to the rotation axis, as stated by the Taylor–Proudman theorem. This phenomenon results in the formation of columnar structures. In the current work, it was found that the flow can behave in a two-dimensional way as long as the rotation rate is fast enough, irrespective of the aspect ratio. In other words, this is true even if the fluid depth is of the same order as the thickness of the Ekman boundary layer, for which case no columnar structures are formed. This is attributed to the linear coupling between primary and secondary motions. From the study of decaying vortical structures, it was concluded that neither adding background rotation to a shallow flow nor decreasing the depth of a rotating flow necessarily increases the degree of two-dimensionality of the flow. The last two chapters of this thesis are dedicated to the study of a shallow dipolar structure that is continuously driven by time-independent electromagnetic forcing. For a shallow structure without background rotation, it was observed that for weak forcing the flow can be considered indeed as twodimensional. However, every shallow flow, even for very small fluid depths, becomes three-dimensional for a sufficiently high forcing magnitude. An equivalent result was obtained for a similar flow subjected to background rotation. The change in behavior is associated with a change in the vertical profile of the horizontal velocity, which is clearly absent in perfectly two-dimensional flow. The results presented in this thesis confirm that under certain conditions shallow flows and flows subjected to background rotation can behave as a twodimensional flow. However, more importantly, it is shown that there are clear limits to this behavior. This work presents a better understanding of the basic dynamics of shallow flows with and without background rotation and of the extent to which these flows can be considered as quasi-two-dimensional

Biopolymer photonics: from nature to nanotechnology

Biopolymers offer vast potential for renewable and sustainable devices. While nature mastered the use of biopolymers to create highly complex 3D structures and optimized their photonic response, artificially created structures still lack nature's diversity. To bridge this gap between natural and engineered biophotonic structures, fundamental questions such as the natural formation process and the interplay of structural order and disorder must be answered. Herein, biological photonic structures and their characterization techniques are reviewed, focusing on those structures not yet artificially manufactured. Then, employed and potential nanofabrication strategies for biomimetic, bio-templated, and artificially created biopolymeric photonic structures are discussed. The discussion is extended to responsive biopolymer photonic structures and hybrid structures. Last, future fundamental physics, chemistry, and nanotechnology challenges related to biopolymer photonics are foreseen.Peer ReviewedPostprint (published version

Parametrization of relative humidity- and wavelength-dependent optical properties of mixed Saharan dust and marine aerosol

Aerosol particles interact with sunlight through scattering and absorption and have therefore a direct radiative effect. Hygroscopic aerosol particles take up water and are able to grow in size below 100% relative humidity, which involves the change of optical properties and the direct radiative effect. The change of aerosol optical properties for aerosol mixtures under humidification is presently not well understood, especially for the largest particle sources worldwide. The present PhD-thesis quantifies wavelength- and humidity-dependent aerosol optical properties for a mixture of Saharan mineral dust and marine aerosol. For quantification, an aerosol model was developed, which based on in-situ measurements of microphysical and optical properties at Cape Verde. With this model, aerosol optical properties were calculated from the dry state up to 90% relative humidity. To validate the model, a measure of the total extenuated light from particles under ambient conditions was used. Finally, the humidity dependence of aerosol optical properties for marine aerosol, Saharan dust aerosol, and a mixture of both species was described by two empirical equations. With the wavelength of the incident visible solar radiation, relative humidity, and dry dust volume fraction, the humidity dependence of optical properties can be calculated from tabulated values. To calculate radiative effects, aerosol optical properties were used as input parameters for global circulation models including radiative transfer. Due to the complexity of aerosol related processes, they have been treated implicitly, meaning in parameterized form. For modelling purposes, the present PhD-thesis provides a solution to include humidity effects of aerosol optical properties.Aerosolpartikel wechselwirken durch Streu- und Absorptionsprozesse mit der einfallenden Sonnenstrahlung und haben somit einen direkten Strahlungseffekt. Bei relativen Feuchten bis 100% können Aerosolpartikel aufquellen und somit ihre Größe ändern. Im Zuge des Aufquellens, ändern sich die optischen Eigenschaften und somit auch der direkte Strahlungseffekt der Aerosolpartikel. Speziell für Mischungen von verschiedenen Aerosolspezies ist die Änderung der optischen Eigenschaften des Aerosols durch Feuchte Einfuss noch nicht ausreichend verstanden. Gegenstand der vorliegenden Arbeit ist daher die Quantifizierung der wellenlängen- und feuchteabhängigen optischen Eigenschaften einer Mischung von Saharastaub- und marinen Aerosol. Die zur Quantifizierung notwendigen Daten wurden im Rahmen einer Feldmessung von mikrophysikalischen- und optischen Aerosol-Eigenschaften auf den Kapverdischen Inseln gesammelt. Auf Grundlage dieser Messungen wurde ein Aerosol-Modell entwickelt. Dieses Modell wurde daraufhin verwendet, um Berechnungen von optischen Aerosol-Eigenschaften bei relativen Feuchten bis 90% durchzuführen. Eine Messung der Lichtschwächung durch Aerosolpartikel unter Umgebungsbedingungen wurde verwandt, um das Modell bei Umgebungsfeuchten zu validieren. Die Wellenlängen- und Feuchteabhängigkeit der optischen Eigenschaften des Aerosols wurde parametrisiert und konnte anhand von zwei Parametergleichungen bestimmt werden. Unter Benutzung von tabellierten Werten und der Wellenlänge des einfallenden sichtbaren Sonnenlichtes, der relativen Feuchte, sowie der Staubvolumenfraktion, kann die Feuchteabhängigkeit von wichtigen Aerosol-optischen Eigenschaften für Saharastaub, marinen Aerosol und einer Mischung aus beiden Komponenten bestimmt werden. Globale Zirkulationsmodelle, die auch eine Berechnung von Strahlungseffekten durch Aerosolpartikel beinhalten, nutzen Aerosol-optische Eigenschaften als Eingabeparameter. Durch zunehmende Komplexitiät zur Beschreibung von Wechselwirkungen in der Atmosphäre, sind einfache Parametrisierungen unabdingbar. Die vorliegende Arbeit liefert daher einen wichtigen Beitrag für die Modellierung von Strahlungseffekten durch Aerosolpartikel und somit zum Verständnis des Strahlungshaushaltes der Erde

Culinary fluid mechanics and other currents in food science

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