The present thesis is an experimental study of some aspects of unsteady two dimensional boundary layers subject to adverse pressure gradient. An adverse pressure gradient usually leads to boundary layer separation or an instability which may result in transition to turbulence. Unsteady boundary layer separation is not yet fully understood and there is no specific criterion proposed in literature for its occurrence. The details of separation depend on the Reynolds number, the geometry of the body (streamlined or bluff) and the type of imposed unsteady motion (impulsive, oscillatory etc.). Similarly there are many unknowns with respect to instability and transition in unsteady boundary layers, especially those having a streamwise variation. For unsteady flows it is useful to break up the pressure gradient term in the unsteady boundary layer equation into two components:(Formula) is the velocity at the edge of the boundary layer. The first term of the right hand side of this equation may be called the temporal component (Πt) which signifies acceleration or deceleration in time of the free stream and the second term is the spatial component (Πx) which represents the spatial or convective acceleration of the free stream. Many of the studies on instability in unsteady flows found in literature are carried out in straight tubes or channels, where the Πx term is absent. However, in many cases, especially in biological systems both terms are present. An example is the unsteady flow over the moving body of a fish. To study the effects of Πt and Πx on unsteady separation and instability we have built an unsteady water tunnel where the two components can be systematically varied. The flow is created by a controlled motion of a piston. By a suitable combination of the geometry of the model and the piston motion, different types of separation bubbles may be generated. In our studies the piston motion follows a trapezoidal variation: constant acceleration from rest, followed by constant velocity and then deceleration to zero velocity. We have chosen two geometries. One is a bluff body and thus has a high value of Πx and other is a small angle diffuser with a divergence angle 6.2° and thus having a small value of Πx. Upstream and downstream of the diffuser are long lengths of constant cross section. We have performed experiments with the above mentioned geometries placed in the tunnel test section. Flow is visualized using the laser induced fluorescence technique by injecting a thin layer of fluorescein dye on the test wall. Numerical simulations have been done using the software FLUENT. Boundary layer parameters like boundary layer, displacement and momentum thicknesses are calculated from the simulations and used to analyze the experimental results. For the flow in the diffuser, quasi-steady stability analysis of the instantaneous velocity profiles gives a general idea of stability behavior of the flow. Two types of experiments have been done with the bluff body. One is the unsteady boundary layer separation and the formation of the initial vortex for a flow that is uniformly accelerated from rest. We have found some scalings for the formation time (tv) of the separation vortex. The second type of experiment was to study the vortex shedding from the separating shear layer after the boundary layer has fully separated. At high enough Reynolds number shear layer vortices are seen to shed from the separation bubble. The Strouhal number based on the momentum thickness and the velocity at the edge of the boundary layer just upstream of the separation point is found to vary between 0.004 and 0.008. This value is close to the Strouhal number value of 0.0068 found in laminar separation bubbles on a flat plate. The second part of the study concerns with the evolution of the flow in the small angle diffuser with a mild variation of the spatial component of the pressure gradient. From the experimental visualizations we have found that the ratio of Πx and Πt at the start of the deceleration phase of the piston motion is an important parameter that determines the type of instability. This value of Πx/Πt is controlled by controlling the piston deceleration: a large deceleration gives a low Πx/Πt value and a low deceleration gives a large Πx/Πt value. Three types of instabilities have been observed in our experiments. In Type I, the first vortex forms at the maximum pressure gradient point (MPGP) and which grows disproportionately with time. However, instability vortices are seen later at other locations around the MPGP. In type II an array of vortices over a certain length are observed; the vortices grow with time. In Type III, which we observe for low decelerations, we observe initial vortices only in the diffuser section in the deceleration phase of the piston motion. Type III instability is similar to the one observed in dynamic stall experiments. In all cases the instability is very localized - it occurs only over some length of the boundary layer. Transition to turbulence, which is also localized, is observed at higher Reynolds numbers. The non-dimensional time for vortex formation is not very different from that found in straight channel experiments. Quasi-steady linear stability analyses for the boundary layer at the MPGP both for the top and the bottom walls show that the flow is absolutely unstable for some cases. In summary, the thesis looks at in a unified way the separation and instability of unsteady boundary layers with reverse flow. It is hoped that the results will be useful in predicting and understanding onset of separation and instability in practically occurring unsteady flows.

Particle-based models play a central role in many simulation techniques used for example in thermodynamics, molecular biology, material sciences, or astrophysics. Such simulations are carried out by directly calculating interactions on a set of individual particles over many time steps. Clusters of particles form higher-order structures like drops or waves. The interactive visual inspection of particle datasets allows gaining in-depth insight, especially for initial exploration tasks. However, their visualization is challenging in many ways. Visualizations are required to convey structures and dynamics on multiple levels, such as per-particle or per-structure. Structures are typically dense and highly dynamic over time and are thus likely subject to heavy occlusion. Furthermore, since simulation systems become increasingly powerful, the number of particles per time step increases steadily, reaching data set sizes of trillions of particles. This enormous amount of data is challenging not only from a computational perspective but also concerning comprehensibility. In this work, the idea of Focus+Context is applied to particle visualizations. Focus+Context is based on presenting a selection of the data – the focus – in high detail, while the remaining data – the context – is shown in reduced detail within the same image. This enables efficient and scalable visualizations that retain as much relevant information as possible while still being comprehensible for a human researcher. Based on the formulation of the most critical challenges, various novel methods for the visualization of static and dynamic 3D and nD particle data are introduced. A new approach that builds on global illumination and extended transparency allows to visualize otherwise occluded structures and steer visual saliency towards selected elements. To address the time-dependent nature of particle data, Focus+Context is then extended to time. By using an illustration-inspired visualization, the researcher is supported in assessing the dynamics of higher-order particle structures. To understand correlations and high dimensional structures in higher dimensional data, a new method is presented, based on the idea of depth of field.