Introduced in 1991, solid ink color printing technology is widely used in the office printing, prepress proofing, and wide format color printing markets. Ink droplets are first deposited on a rotating drum and then transferred to paper to reproduce images with high print quality. The objective of this thesis is to develop scientific knowledge of ink droplet deposition, which is needed for precise image buildup on the drum surface. The first problem studied in the thesis is droplet formation from the printhead with varied working voltages and jetting frequencies. Attention is paid to the formation of satellite droplets, the contraction of ligaments and the startup of high frequency jetting. The jetting conditions for obtaining consistent droplet generation with satellites are determined. A theoretical model is developed to predict the lifetime of ligaments. The second problem we studied is the deposition of single droplets on solid surfaces. The surface texture and final shape of deposited droplets are correlated with impact conditions, which include printhead temperature, substrate temperature, distance from printhead to substrate, and the type of substrate surface. An analytical model is developed to evaluate the interaction of oscillation and viscous damping in the droplet during impact. The third problem covered in the thesis is the deposition of multiple ink droplets on the drum surface. Interaction between droplets causes drawback effect, which degrades print quality. We define a parameter to quantify the drawback effect with varied deposition conditions. A simple model is provided to predict conditions for making continuous lines based on the results of two ink droplets deposition. To understand the hydrodynamics in causing drawback effect, a series of experiments using large liquid droplets are carried out. Focus is put on the evolution of spread length and dynamics of contact line. Correlations for maximum and minimum spread lengths are developed, which are used to reveal the cause of drawback effect in the deposition of ink droplets.

Introduced in 1991, solid ink color printing technology is widely used in the office printing, prepress proofing, and wide format color printing markets. Ink droplets are first deposited on a rotating drum and then transferred to paper to reproduce images with high print quality. The objective of this thesis is to develop scientific knowledge of ink droplet deposition, which is needed for precise image buildup on the drum surface. The first problem studied in the thesis is droplet formation from the printhead with varied working voltages and jetting frequencies. Attention is paid to the formation of satellite droplets, the contraction of ligaments and the startup of high frequency jetting. The jetting conditions for obtaining consistent droplet generation with satellites are determined. A theoretical model is developed to predict the lifetime of ligaments. The second problem we studied is the deposition of single droplets on solid surfaces. The surface texture and final shape of deposited droplets are correlated with impact conditions, which include printhead temperature, substrate temperature, distance from printhead to substrate, and the type of substrate surface. An analytical model is developed to evaluate the interaction of oscillation and viscous damping in the droplet during impact. The third problem covered in the thesis is the deposition of multiple ink droplets on the drum surface. Interaction between droplets causes drawback effect, which degrades print quality. We define a parameter to quantify the drawback effect with varied deposition conditions. A simple model is provided to predict conditions for making continuous lines based on the results of two ink droplets deposition. To understand the hydrodynamics in causing drawback effect, a series of experiments using large liquid droplets are carried out. Focus is put on the evolution of spread length and dynamics of contact line. Correlations for maximum and minimum spread lengths are developed, which are used to reveal the cause of drawback effect in the deposition of ink droplets.

Two active flow control methods are investigated numerically to understand the mechanism by which they control aerodynamics in the presence of severe flow separation on an airfoil. In particular, synthetic jets are applied to separated flows generated by additional surface feature (the actuators) near the trailing edge to obtain Coanda-like effects, and an impulse jet is used to control a stalled flow over an airfoil. A moving-grid scheme is developed, verified and validated to support simulations of external flow over moving bodies. Turbulent flow is modeled using detached eddy simulation (DES) turbulence models in the CFD code CDP (34) developed by Lopez (54). Synthetic jet actuation enhances turbulent mixing in flow separation regions, reduces the size of the separation, deflects stream lines closer to the surface and changes pressure distributions on the surface, all of which lead to bi-directional changes in the aerodynamic lift and moment. The external flow responds to actuation within about one convective time, which is significantly faster than for conventional control surfaces. Simulation of pitching airfoils shows that high-frequency synthetic jet affects the flow independently of the baseline frequencies associated with vortex shedding and airfoil dynamics. These unique features of synthetic jets are studied on a dynamically maneuvering airfoil with a closed-loop control system, which represents the response of the airfoil in wind-tunnel experiments and examines the controller for a rapidly maneuvering free-flight airfoil. An impulse jet, which is applied upstream of a nominal flow separation point, generates vortices that convect downstream, interact with the separating shear layer, dismantle the layer and allow following vortices to propagate along the surface in the separation region. These following vortices delay the separation point reattaching the boundary layer, which returns slowly to its initial stall condition, as observed in wind-tunnel experiments. A simple model of the impulse jet actuator used herein is found to be sufficient to represent the global effects of the jet on the stalled flow because it correctly represents the momentum injected into the flow. / text

Boundary layer separation is an unwanted phenomenon in most technical applications, as for instance on airplane wings, ground vehicles and in internal flow systems. If separation occurs, it causes loss of lift, higher drag and energy losses. It is thus essential to develop methods to eliminate or delay separation.In the present experimental work streamwise vortices are introduced in turbulent boundary layers to transport higher momentum fluid towards the wall. This enables the boundary layer to stay attached at  larger pressure gradients. First the adverse pressure gradient (APG) separation bubbles that are to be eliminated are studied. It is shown that, independent of pressure gradient, the mean velocity defect profiles are self-similar when the scaling proposed by Zagarola and Smits is applied to the data. Then vortex pairs and arrays of vortices of different initial strength are studied in zero pressure gradient (ZPG). Vane-type vortex generators (VGs) are used to generate counter-rotating vortex pairs, and it is shown that the vortex core trajectories scale with the VG height h and the spanwise spacing of the blades. Also the streamwise evolution of the turbulent quantities scale with h. As the vortices are convected downstream they seem to move towards a equidistant state, where the distance from the vortex centres to the wall is half the spanwise distance between two vortices. Yawing the VGs up to 20° do not change the generated circulation of a VG pair. After the ZPG measurements, the VGs where applied in the APG mentioned above. It is shown that that the circulation needed to eliminate separation is nearly independent of the pressure gradient and that the streamwise position of the VG array relative to the separated region is not critical to the control effect. In a similar APG jet vortex generators (VGJs) are shown to as effective as the passive VGs. The ratio VR of jet velocity and test section inlet velocity is varied and a control effectiveness optimum is found for VR=5. At 40° yaw the VGJs have only lost approximately 20% of the control effect. For pulsed VGJs the pulsing frequency, the duty cycle and VR were varied. It was shown that to achieve maximum control effect the injected mass flow rate should be as large as possible, within an optimal range of jet VRs. For a given injected mass flow rate, the important parameter was shown to be the injection time t1. A non-dimensional injection time is defined as t1+ = t1Ujet/d, where d is the jet orifice diameter. Here, the optimal  t1+ was 100-200. / QC 20100825