One of the last remaining challenges preventing the laminarization of sweptwings is the control of unstable crossflow vortices. In low-disturbance environments the transition from laminar to turbulent flow on the swept-wing initially takes the path of receptivity, where surface roughness or disturbances in the environment introduce shortwavelength disturbances into the boundary layer. This is followed by development and linear growth of stationary crossflow vortices that modify the mean flow, changing the stability characteristics of the boundary layer. Finally, breakdown to turbulence occurs over a short length scale due to the high-frequency secondary instability. The receptivity mechanism is the least understood, yet holds the most promise for providing a laminar flow control strategy. Results of a 3-year flight test program focused on receptivity measurements and laminar flow control on a 30-degree swept-wing are presented. A swept-wing test article was mounted on the port wing of a Cessna O-2A aircraft and operated at a chord Reynolds number of 6.5 to 7.5 million. Spanwise-periodic, micronsized discrete roughness elements were applied at the leading edge of the swept-wing in order to excite the most unstable crossflow wavelength and promote early boundary layer transition. An infrared camera was used to detect boundary-layer transition due to changes in leading-edge roughness. Combined with the IR camera, a new technique of calibrating surface-mounted hotfilms was developed for making disturbance-amplitude measurements downstream of modulated roughness heights. This technique proved to be effective at measuring disturbance amplitudes and can be applied in future tests where instrumentation is limited. Furthermore, laminar flow control was performed with subcritically-spaced roughness. A 100% increase in the region of laminar flow was achieved for some of the conditions tested here.

Laminar flow control has been studied for several decades in an effort to achieve higher efficiencies for aircraft. Successful implementation of laminar flow control technology on transport aircraft could significantly reduce drag and increase operating efficiency and range. However, the crossflow instability present on swept-wing boundary layers has been a chief hurdle in the design of laminar wings. The use of spanwise-periodic discrete roughness elements (DREs) applied near the leading edge of a swept-wing typical of a transport aircraft represents a promising technique able to control crossflow and delay transition to accomplish the goal of increased laminar flow. Recently, the Flight Research Laboratory at Texas A&M University conducted an extensive flight test study using DREs on a swept-wing model at chord Reynolds numbers in the range of eight million. The results of this study indicated DREs were able to double the laminar flow on the model, pushing transition back to 60 percent chord. With the successful demonstration of DRE technology at these lower chord Reynolds numbers, the next logical step is to extend the technology to higher Reynolds numbers in the range of 15 to 20 million typical of smaller transport aircraft. To conduct the flight tests at the higher Reynolds numbers, DREs will be placed on a wing glove attached to the aircraft wing. However, a feasibility study was necessary before initiating the flight-testing. First, a suitable aircraft able to achieve the Reynolds numbers and accommodate a wing glove was identified. Next, a full CFD analysis of the aircraft was performed to determine any adverse effects on the wing flow-field from the aircraft engines. This required an accurate CAD model of the selected aircraft. Proper modeling techniques were needed to represent the effects of the aircraft engine. Once sufficient CFD results were obtained, they were used as guidance for the placement of the glove. The attainable chord Reynolds numbers based on the recommendations for the wing glove placement then determined if the selected aircraft was suitable for the flight-testing.

A series of flight tests was performed using a swept-wing model mounted on a Cessna O-2 aircraft. The crossflow waves on the airfoil were excited by pneumatic spanwise-periodic distributed roughness elements (DREs). The objective of the experiment was to determine the roughness receptivity i.e. the relationship between roughness height and the amplitude of the unstable crossflow wave. The local skin-friction variation was measured using an array of calibrated and temperature-compensated hotfilm sensors. The amplitudes of the disturbance shear stress were compared to the amplitudes of the DREs. It was found that there is a relationship between the shear stress and DRE amplitude that needs to be studied more before any definitely conclusions can be made. It was also found that the sensitivity of the crossflow to DREs is highly dependent on the freestream turbulence levels.

In this study laminar forced film condensation of vapor flowing in cross-flow direction through the annular space between two concentric cylinders was investigated numerically. To achieve this, governing equations of the vapor and the condensate flow in cross-flow direction between two concentric cylinders were developed. After obtaining the equations in integral forms by using the finite difference technique the vapor boundary layer thicknesses on the inner and outer cylinders and the condensate layer thickness was obtained as a function of the angular position on the cylinders. It was assumed that the condensation took place on the outer surface of the inner cylinder only and the outer cylinder was assumed to be insulated. The computer program developed is capable to calculate the condensate film thickness, vapor boundary layer thickness, the heat flux and the heat transfer coefficient and the interface velocity between the condensate and the vapor layer as a function of the angular position on the cylinders. Effects of changing the free stream velocity flowing in the channel, the radius of the inner cylinder, the temperature difference between the saturated vapor and the wall and the annular space between the concentric cylinders were investigated numerically by using the computer program and the results were presented graphically. Results showed that by increasing the free stream velocity of the vapor in the core, the film thickness decreased and by increasing the radius of the inner cylinder, the temperature difference between the saturated vapor and the wall and the annular space, the film thickness increased.

The problem of the forced film condensation heat transfer of pure steam and steam-air mixture flowing downward a tier of horizontal cylinders is investigated numerically and experimentally. Liquid and vapor-air mixture boundary layers were solved by an implicit finite difference scheme. The effects of the free stream non-condensable gas (air) concentration, free stream velocity (Reynolds number), cylinder diameter, temperature difference and angle of inclination on the condensation heat transfer are analyzed. Inline and staggered tubes arrangements are considered. The mathematical model takes into account the effect of staggering of the cylinders and how condensation is affected at the lower cylinders when condensate does not fall on to the center line of the cylinders. An experimental setup was also manufactured and mounted at METU workshop. A set of experiments were conducted to observe the condensation heat transfer phenomenon and to verify the theoretical results. Condensation heat transfer results are available in ranges from (U&amp / #61605 / = 1 - 30 m/s) for free stream velocity, (m1,&amp / #61605 / = 0.01 -0.8) for free stream air mass fraction, (d = 12.7 -50.8 mm) for cylinder diameter and (T&amp / #61605 / -Tw =10-40 K) for temperature difference. Results show that / a remarked reduction in the vapor side heat transfer coefficient is noticed when very small amounts of air mass fractions present in the vapor. In addition, it decreases by increasing in the cylinder diameter and the temperature difference. On the other hand, it increases by increasing the free stream velocity (Reynolds number). Average heat transfer coefficient at the middle and the bottom cylinders increases by increasing the angle of inclination, whereas, no significant change is observed for that of the upper cylinder. Although some discrepancies are noticed, the present study results are inline and in a reasonable agreement with the theory and experiment in the literature. Down the bank, a rapid decrease in the vapor side heat transfer coefficient is noticed. It may be resulted from the combined effects of inundation, decrease in the vapor velocity and increase in the non-condensable gas (air) at the bottom cylinders in the bank. Differences between the present study results and the theoretical and the experimental data may be resulted from the errors in the numerical schemes used. These errors include truncation and round off errors, approximations in the numerical differentiation for interfacial fluxes at the vapor-liquid interface, constant properties assumption and approximations in the initial profiles. Mixing and re-circulation in the steam-air mixture at the lower tubes may be the other reasons for these deviations.

Thesis (Ph. D.)--State University of New York at Binghamton, Thomas J. Watson School of Engineeering and Applied Science, Department of Mechanical Engineering, 2009. / Includes bibliographical references.

In this project we studied the effect of different parameters on heat transfer in two-dimensional microchannels, microtubes and also tube-in-block heat exchangers for various engineering applications. These included the use of flow restrictions to enhance local heat or mass transfer rate, enhancement of conjugate heat transfer with discrete heating and magnetic coolers (or heaters) associated with magnetic refrigeration system. The results of this research will help in designing the heating or cooling systems and selection of their appropriate geometrical dimensions and materials for specific applications. Types of problems studied in this project are: steady state analysis of forced convection around a restricted flow passage in a micro channel, effect of protrusions in a microtube or microchannel for conjugate heat transfer with uniform or discrete heating and transient analysis of heat transfer in trapezoidal microchannels under time varying heat source. For each of these problems a numerical simulation model was developed. The mass, momentum, and energy conservation equations were solved in the fluid region and energy conservation in the sold region to arrive at the velocity and the temperature distributions. Detailed parametric study was carried out for each problem. The parameters were shape and size of the restriction/protrusion, number of restrictions/protrusions, wall thickness, Reynolds number, solid materials and working fluids. The results showed that in microtubes the local value of Nusselt number increases at the restriction/protrusion and the global value for the whole tube is somewhat lower. For a two-dimensional microchannel, both the local Nusselt number at the restriction/protrusion as well as the global Nusselt number for the entire channel is higher. In the trapezoidal channels the results showed that with the increase in Reynolds number, the outlet temperature decreased and the average heat transfer coefficient increased. With an increase in magnetic field there was an increase in the solid fluid interface temperature and in turn the average heat transfer coefficient increased. With a decrease in the channel height and width there was an increase in the average Nusselt number in the channel.

Interactions of cells with their chemical microenvironments are critical to many polarized processes, including differentiation, migration, and pathfinding. To investigate such cellular events, tools are required that can rapidly reshape the microscopic chemical landscapes presented to cultured cells. Existing chemical dosing technologies rely on use of pre-fabricated chemical gradients, thus offering static cell-reagent interactions. Such interactions are particularly limiting for studying migration and chemotaxis, during which cells undergo rapid changes in position, morphology, and intracellular signaling. This dissertation describes the use of laminar streams, containing cellular effector molecules, for precise delivery of effectors to selected subcellular regions. In this approach, cells are grown on an ultra-thin polymer membrane that serves as a barrier to an underlying reagent reservoir. By using a tightly-focused pulsed laser beam, micron-diameter pores can be ablated in the membrane upstream of desired subcellular dosing sites. Emerging through these pores are well-defined reagent streams, which dose the targeted regions. Multiple pores can be ablated to allow parallel delivery of effector molecules to an arbitrary number of targets. Importantly, both the directionality and the composition of the reagent streams can be changed on-the-fly under a second to present dynamically changing chemical signals to cells undergoing migration. These methods are applied to study the chemotactic responses of neutrophil precursor cells. The subcellular localization of the chemical signals emerging through pores is found to influence the morphological evolution of these motile cells as they polarize and migrate in response to rapidly altered effector gradients. / text

Most embryonic development and tissue self-assembly requires the integration of cell movements within multiple cell layers composed of different cell types, which are integrated with the signaling networks in these 3D environments. Although the role of cell mechanics in tissue self-assembly has been demonstrated, little is known about the mechanical responses of 3D multi-layer tissues to chemical cues. To investigate the collective movements within multilayered tissues, I developed a novel microfluidic technique capable of removing desired height or width of tissue from a composite tissue. I call this technique "3D tissue-etching" because it is analogous to techniques used in the microelectromechanics (MEMS) field where complex 3D structures are built by successively removing material from a monolithic solid through subtractive manufacturing. I used a custom-designed microfluidic control system to deliver a range of tissue etching reagents (detergents, chelators, proteases, etc.) to specific regions of multilayered tissues microsurgically isolated from embryos of the African Clawtoed frog, Xenopus laevis. Xenopus embryos and explanted tissues have long been used to elucidate signaling and other cellular processes during development and here provide an ideal model 3D tissue etching. Long exposure to a narrow etchant stream cuts completely through cell-cell layers to expose the substrate. By reducing the exposure time a single layer may be removed. By controlling the width of the etchant and the exposure time a broader swath of the surface layer may be removed. For more refined etching, after removal of a broad swath the resistance circuits can be switched and a second narrow stream can remove only a single narrow band within the swath exposed cells. I developed tissue-etching techniques that allow me to shape complex multi-layered embryonic tissues. The ability to control 3D stimulation and the form of multicellular tissues will provide extend the tools of tissue engineering to synthesize highly complex 3D integrated multicellular biosystems. Integration of tissue etching in my custom microfluidic system provides a "test-bed" where a range of hypotheses concerning the control and regulation of development and cell differentiation can be implemented and tested.

No description available.