Daniel R. Sabatino

Boundary Layer Influence on the Unsteady Horseshoe Vortex Flow and Surface Heat Transfer

The spatial-temporal flow field and associated surface heat transfer within the leading edge, end-wall region of a bluff body were examined using both particle image velocimetry and thermochromic liquid crystal temperature measurements. The horseshoe vortex system in the end-wall region is mechanistically linked to the upstream boundary layer unsteadiness. Hairpin vortex packets, associated with turbulent boundary layer bursting behavior, amalgamate with the horseshoe vortex resulting in unsteady strengthening and streamwise motion. The horseshoe vortex unsteadiness exhibits two different natural frequencies: one associated with the transient motion of the horseshoe vortex and the other with the transient surface heat transfer. Comparable unsteadiness occurs in the end-wall region of the more complex airfoil geometry of a linear turbine cascade. To directly compare the horseshoe vortex behavior around a turning airfoil to that of a simple bluff body, a length scale based on the maximum airfoil thickness is proposed.

Pyrolytic Graphite Heat Sinks: A Study of Circuit Board Applications

Pyrolytic graphite (PG) is of interest as a heat sink and heat exchanger material because it has a thermal conductivity greater than copper with a density less than aluminum. However, its anisotropic molecular structure creates a significant challenge to the realization of practical thermal performance benefits. The current computational study compares the performance of a high-thermal conductivity PG, Pyroid® HT, to aluminum over a design space that focuses on circuit board component cooling. The applications considered in this paper include flat-plate thermal spreaders and finned heat sinks. The results are presented in a form that can be used to provide material and geometry selection during preliminary design. The competition between the size of the heat source, the thickness of the spreader, its size, and the strength of the forced convection is quantitatively presented. In addition, the thicknesses that provide the optimal performance are determined and the performance is compared with a thickness that would be approximately cost neutral. A single isotropic analytical fin model is used to quantitatively establish the fin sizes and aspect ratios that would demonstrate the largest performance benefit if made from PG. Finally, the impact of Pyroid® on the size of finned heat sinks is examined.

ITAPS: A Process and Toolset to Support Aircraft Level System Integration Studies

Integrated Total Aircraft Power Systems (ITAPS) is a process and toolset developed within the United Technologies Corporation to support capability based technology planning. This process identifies integrated system solutions and the associated enabling component technologies that deliver improved aircraft capabilities. ITAPS TM has been developed and refined through the execution of more than 25 different system architecture studies for both military and commercial vehicle platforms. ITAPS TM enables a diverse collection of experts to leverage a documented body of technical knowledge, add their own new ideas, assemble, evaluate, and select the best architectures as well as create the required technology-development plans. A set of collaboration tools has been developed that decompose the integrated power system design problem into topical elements that can be readily addressed by individual discipline experts. These discipline experts make local design decisions and are provided with rapid feedback regarding their impact on key vehicle metrics, thus enabling the optimization of power and propulsion system architectures. The decomposition of system design problem is maintained throughout the evaluation process and leveraged to create an interactive study result. The ability to generate results in this manner allows the study team to instantly assess new alternative system configurations that may be posed during the technical review of results. This paper provides an overview of the ITAPS TM process and the associated collaboration tools.

Characterizing the formation and regeneration of hairpin vortices in a laminar boundary layer

A free surface water channel is used to study hairpin vortex formation created by fluid injection through a narrow slot into a laminar boundary layer. Particle image velocimetry flow-field measurements of injections into quiescent cross-flow conditions confirm that elongated ring vortices are produced with a nondimensionalized circulation strength that is approximately linear with formation time. Unlike circular ring vortices, a limiting strength is not observed at a nondimensional formation time of 4 due to the proximity of the counter-rotating vortex pair. Identical injections are made into a laminar boundary layer at different free-stream velocities and streamwise slot positions (485 ≤ Reδ∗ ≤ 584) with average injection velocity ratios between 0.08 and 0.16. Visualizations indicate that the shear layer between the low x-momentum injected fluid and the boundary layer creates a Kelvin-Helmholtz instability that forms the hairpin vortex head which then monotonically decreases in circulation strength with ...

The Control of Sand Curtains Used in Abrasion Testing

Abrasion testing that follows the ASTM G65-04(2010) test method involves the delivery of a test sand to an abrasion wheel that removes material from a test specimen. The sand flow is controlled by a supply valve and delivered to a nozzle in order to form a uniform “curtain.” It has been found that as the nozzle exit area is increased, the sand flow becomes widely dispersed and unstable. An experimental study was performed to determine the cause of this effect and to identify a means to mitigate it. By visualizing the inlet and outlet flow of different nozzles it was found that the dispersed flow occurs when the feed tube does not completely fill with sand. This sparsely packed flow in the nozzle is able to exit with a wide range of exit angles and yields the dispersed flow pattern as well as a higher flow rate. It was found that when the flow is controlled at the nozzle outlet, allowing the system to completely fill with sand before the flow is initiated, a desirable narrow sand curtain forms because the particles are more closely packed and thus their motion is more constrained. The narrow curtain is associated with lower flow rates than the dispersed pattern for the same geometry and once established will remain stable until the flow is terminated.