Glass fiber based polymer composites based injection molded parts provide a light-weight high-strength alternative for use in automobile applications. These composites have enhanced mechanical properties compared to those of pure polymers, if the fibers are oriented in the right direction. One of the major challenges in processing of these composites is to control the fiber orientation in the final product. The evolution of short glass fiber orientation in a center-gated disk was experimentally determined along the radial direction at three different heights representative of the shell, transition and core layers, respectively. Orientation data along the shell and transition layers in the lubrication region show shear flow effects, which tends to align the fibers along the flow direction. In the core layer, where the extension in the "-direction dominates, fibers tend to get aligned along the "-direction. In the frontal flow region orientation in the flow direction drops in all three layers due to fountain flow effects. Fiber orientation predictions in coupled and decoupled transient simulations using the Folgar-Tucker model, and the two slow versions of the Folgar-Tucker model, namely the slip Folgar-Tucker model and the reduced strain closure (RSC) model were compared with the experimental data. Measured inlet orientation was used in all simulations and model parameters were determined by fitting model predictions to rheological data under startup of shear. Pseudo-concentration method was implemented for the modeling of the advancing front and fountain flow effects in the region near the front. Discontinuous Galerkin finite element method and a third order Runge-Kutta total variance diminishing time integration scheme were implemented for the solution of the orientation and transport equations. In the lubrication region of the shell layer, all three orientation models provided a good match with the experimental data. In the frontal region, fountain flow simulations showed characteristic features seen in r- and z-profiles of orientation, although the experimental data showed these features at a relatively larger distance behind the front while the simulations predicted these effects only up to a small distance behind the front. On the other hand, orientation predictions with the Hele-Shaw flow approximation showed significant over-predictions in the frontal region. With model parameters determined from fitting to rheological data, coupling did not show any significant improvements. However, with the use of a smaller value of the fiber interaction parameter, coupling showed significant improvement in orientation predictions in all three layers in the frontal region. The simulation scheme was extended to long fiber systems by comparing available long fiber orientation data in a center-gated disk with model predictions using the Bead-Rod model which considers fiber bending, a property exhibited by long semi-flexible fibers. The Bead-Rod model showed improvements over rigid fiber models in the lubrication region of the shell layer. However, close to the front, both models showed similar predictions. In fountain flow simulations, the flow features seen in the r- and z-profiles were much better predicted with both the models while Hele-Shaw flow approximation showed over-prediction of orientation in the flow direction, especially in the shell layer. / Ph. D.

The flow field of a twin jet impinging on ground plane with and without free-stream and at low jet-height-to-diameter ratios was investigated using the Particle Image Velocimetry (PIV) technique. Detailed, time-averaged flow field data are obtained via the high-resolution and the high-sampling rate instantaneous velocity field that is made available via the PIV technique. A model of twin jet issuing from 0.245m circular plate, with 0.019m jet exit diameter, and with jet span to diameter ratio of 3.0 is placed in a water tunnel with the jets in tandem arrangement with respect to the free-stream. The recently upgraded PIV system, in the ESM department fluid mechanics laboratory at VA-Tech, allowed us to capture instantaneous velocity field images of about 0.076m x 0.076m, at 512(H)x512(V) frame resolution. Sampling rates of 1000 and 1200 fps were employed. Understanding the flow field at lower heights is of crucial significance to the VSTOL aircraft application. Huge jet thrust is required to initiate the take-off operation due to the high lift loss encountered while the airframe is in proximity to the ground. Therefore, jet-height-to-diameter ratios of 2 and 4 were employed in this study. Jet-to-free-stream velocity ratios of 0.12, 0.18 and 0.22 were employed in addition to the no-free-stream case. In the current study, only time-averaged flow field properties were considered. These properties were extracted from the available instantaneous velocity field data. In order to provide some details in the time-averaged velocity field, the data were obtained along several planes of interrogation underneath the test model in the vicinity of the twin jet impinging flow. Images were captured in a single plane normal to the free-stream and five planes parallel to the free-stream. A vortex-like flow appears between the main jet and the fountain upwash. This flow is found to experience spiral motion. The direction of such flow spirals is found to be dependent on the jet exit height above the ground, and on the jet-to-free stream, velocity ratios. The flow spirals out towards the vortex flow periphery and upon increasing the free-stream it reverses its direction to be inward spiraling towards the core of the vortex. The flow reversal at certain height of the jet above the ground depends on the free-stream velocity. In our discussion, more emphasis is given to the case of jet-height-to-diameter ratio of two. We also found that the largest turbulent kinetic energy production rate is found to be at the fountain upwash formation zone. / Master of Science