Showing papers by "Terrence W. Simon published in 1994"

Fluid Mechanics and Heat Transfer Measurements in Transitional Boundary Layers Conditionally Sampled on Intermittency

Abstract: An experimental investigation of transition on a flat-plate boundary layer was performed. Mean and turbulence quantities, including turbulent heat flux, were sampled according to the intermittency function. Such sampling allows segregation of the signal into two types of behavior-laminarlike and turbulentlike. Results show that during transition these two types of behavior cannot be thought of as separate Basius and fully turbulent profiles, respectively. Thus, simple transition models in which the desired quantity is assumed to be an average, weighted on intermittency, of the laminar and fully turbulent values may not be entirely successful

An Application of Octant Analysis to Turbulent and Transitional Flow Data

Abstract: A technique called “octant analysis” was used to examine the eddy structure of turbulent and transitional heated boundary layers on flat and curved surfaces. The intent was to identify important physical processes that play a role in boundary layer transition on flat and concave surfaces. Octant processsing involves the partitioning of flow signals into octants based on the instantaneous signs of the fluctuating temperature, t′, streamwise velocity, u′, and cross-stream velocity, v′. Each octant is associated with a particular eddy motion. For example, u′ 0, t′>0 is associated with an ejection or “burst” of warm fluid away from a heated wall. Within each octant, the contribution to various quantities of interest (such as the turbulent shear stress, −u′v′ , or the turbulent heat flux, v′t′ ) can be computed. By comparing and contrasting the relative contributions from each octant, the importance of particular types of motion can be determined. If the data within each octant are further segregated based on the magnitudes of the fluctuating components so that minor events are eliminated, the relative importance of particular types of motion to the events that are important can also be discussed. In fully developed, turbulent boundary layers along flat plates, trends previously reported in the literature were confirmed. A fundamental difference was observed in the octant distribution between the transitional and fully turbulent boundary layers, however, showing incomplete mixing and a lesser importance of small scales in the transitional boundary layer. Such observations were true on both flat and concave walls. The differences are attributed to incomplete development of the turbulent kinetic energy cascade in transitional flows. The findings have potential application to modeling, suggesting the utility of incorporating multiple length scales in transition models.

Velocity and temperature profiles in turbulent boundary layer flows experiencing streamwise pressure gradients

Abstract: The standard turbulent law of the wall, devised for zero pressure gradient flows, has been previously shown to be inadequate for accelerating and decelerating turbulent boundary layers. In this paper, formulations for mean velocity profiles from the literature are applied and formulations for the temperature profiles are developed using a mixing length model. These formulations capture the effects of pressure gradients by including the convective and pressure gradient terms in the momentum and energy equations. The profiles which include these terms deviate considerably from the standard law of the wall; the temperature profiles more so than the velocity profiles. The new profiles agree well with experimental data. By looking at the various terms separately, it is shown why the velocity law of the wall is more robust to streamwise pressure gradients than is the thermal law of the wall. The modification to the velocity profile is useful for evaluation of more accurate skin friction coefficients from experimental data by the near-wall fitting technique. The temperature profile modification improves the accuracy with which one may extract turbulent Prandtl numbers from near-wall mean temperature data when they cannot be determined directly.

Turbulence measurements in a heated, concave boundary layer under high free-stream turbulence conditions

Abstract: Turbulence measurements for both momentum and heat transfer are taken in a low-velocity, turbulent boundary layer growing naturally over a concave wall. The experiments are conducted with negligible streamwise acceleration and a nominal free-stream turbulence intensity of {approximately} 8%. Comparisons are made with data taken in an earlier study in the same test facility but with a 0.6% free-stream turbulence intensity. Results show that elevated free-stream turbulence intensity enhances turbulence transport quantities like {ovr uv} and {ovr vt} in most of the boundary layer. In contrast to the low-turbulence cases, high levels of transport of momentum are measured outside the boundary layer. Stable, Goertlerlike vortices, present in the flow under low-turbulence conditions, do not form when the free-stream turbulence intensity is elevated. Turbulent Prandtl numbers, Pr{sub t}, within the log region of the boundary layer over the concave wall increase with streamwise distance to values as high as 1.2. Profiles of Pr{sub t} suggest that the increase in momentum transport with increased free-stream turbulence intensity precedes the increase in heat transport. Distributions of near-wall mixing length for momentum remain unchanged on the concave wall when free-stream turbulence intensity is elevated. Both for this level of free-stream turbulence and for themore » lower level, mixing length distributions increase linearly with distance from the wall, following the standard slope. However, when free-stream turbulence intensity is elevated, this linear region extends farther into the boundary layer, indicating the emerging importance of larger eddies in the wake of the boundary layer with the high-turbulence free stream. Because these eddies are damped by the wall, the influence of the wall grows with eddy size.« less

Effects of dissolved gases on subcooled flow boiling from small heated regions with and without streamwise concave curvature

Abstract: Coolants such as fluorocarbon liquids usually contain high levels of dissolved gases. When heated, these gases are liberated from the liquid; if the liquid is boiling, these gases may influence the supply of liquid to the boiling surface. In this study, the effects of dissolved air in perfluorinated hydrocarbon, FC-72, on flow boiling heat transfer characteristics were experimentally investigated over a range of heat flux from the onset of nucleate boiling (ONB) to the critical heat flux (CHF). The experiments were conducted in both straight and curved channels. The boiling surface was a 3 mm/spl times/3 mm patch located on the channel wall. In the curved flow, it was located at the 90/spl deg/ position of a U-bend on the concave wall. Gas content was varied from 1.16/spl times/10/sup -5/ to 2.55/spl times/10/sup -3/ mole/mole, velocity ranged from 0.8 to 6.9 m/s, and pressure was controlled to either 1.41 or 1.92 atm. Subcooling, based on total pressure, was maintained at 27.5/spl deg/C. The data show that reduction of incipience superheat at ONB due to dissolved gases under these forced-convection conditions is much less than with pool boiling or low-velocity flow boiling. Boiling curves for gassy and degassed cases differ at low heat flux levels but merge at higher heat fluxes. The merging may indicate that the near-wall liquid layer is being degassed at higher heat fluxes. Though the high-heat-flux portions of the boiling curves were apparently unaffected by the content of dissolved gas in the approaching liquid stream, the critical heat flux was decreased by as much as 10%. Explanations of this behavior are presented in terms of the two mechanisms for the liquid supply to the macrolayer. >

Effects of dissolved gases on subcooled flow boiling from small heated regions with and without streamwise concave curvature

Abstract: Coolants such as fluorocarbon liquids usually contain high levels of dissolved gases. When heated, these gases are liberated from the liquid; if the liquid is boiling, these gases may influence the supply of liquid to the boiling surface. In this study, the effects of dissolved air in perfluorinated hydrocarbon, FC-72, on flow boiling heat transfer characteristics were experimentally investigated over a range of heat flux from the onset of nucleate boiling (ONB) to the critical heat flux (CHF). The experiments were conducted in both straight and curved channels. The boiling surface was a 3 mm/spl times/3 mm patch located on the channel wall. In the curved flow, it was located at the 90/spl deg/ position of a U-bend on the concave wall. Gas content was varied from 1.16/spl times/10/sup -5/ to 2.55/spl times/10/sup -3/ mole/mole, velocity ranged from 0.8 to 6.9 m/s, and pressure was controlled to either 1.41 or 1.92 atm. Subcooling, based on total pressure, was maintained at 27.5/spl deg/C. The data show that reduction of incipience superheat at ONB due to dissolved gases under these forced-convection conditions is much less than with pool boiling or low-velocity flow boiling. Boiling curves for gassy and degassed cases differ at low heat flux levels but merge at higher heat fluxes. The merging may indicate that the near-wall liquid layer is being degassed at higher heat fluxes. Though the high-heat-flux portions of the boiling curves were apparently unaffected by the content of dissolved gas in the approaching liquid stream, the critical heat flux was decreased by as much as 10%. Explanations of this behavior are presented in terms of the two mechanisms for the liquid supply to the macrolayer. >