A numerical study of three-dimensional vortex breakdown

A numerical simulation of bubble-type vortex breakdown using a unique discrete form of the full 3-D, unsteady incompressible Navier-Stokes equations was performed. The Navier-Stokes equations were written in a vorticity-velocity form and the physical problem was not restricted to axisymmetric flow. The problem was parametized on a Rossby- Reynolds-number basis. Utilization of this parameter duo was shown to dictate the form of the free-field boundary condition specification and allowed control of axial breakdown location within the computational domain. The structure of the breakdown bubble was studied through time evolution plots of planar projected velocity vectors as well as through plots of particle traces and vortex lines. These results compared favorably with previous experimental studies. In addition, profiles of all three velocity components are presented at various axial stations and a Fourier analysis was performed to identify the dominant circumferential modes. The dynamics of the breakdown process were studied through plots of axial variation of rate of change of integrated total energy and rate of change of integrated enstrophy, as well as through contour plots of velocity, vorticity and pressure

A Study of School Desegregation with a Specific Look at the Rochester – West Irondequoit Inter – Cultural Program

This paper looks at the racial disparities within the educational system and negative consequences imposed on African American/minority children by that system. The author proposes full racial integration, with school populations reflecting the demographics of the wider community. He examines a volunteer transfer program, focusing on 25 high-achieving first graders who transferred from Rochester city school #19 to six neighborhood schools in the West Irondequoit Central School district in 1965. A control group of 25 high-achieving first graders remained at school #19. The author describes the cultural and communication difficulties encountered by the children in their new, predominantly white environment. He also recounts the various reactions of teachers, administrators, and parents to the program. This paper provides contemporary insight into the school desegregation methods and attitudes of the 1960s

Numerical analysis of a vortex controlled diffuser

A numerical study of a prototypical vortex controlled diffuser is performed. The basic diffuser geometry consists of a step expansion in a pipe of area ratio 2.25:1. The incompressible Reynolds averaged Navier-Stokes equations, employing the K-epsilon turbulence model, are solved. Results are presented for bleed rates ranging from 1 to 7 percent. Diffuser efficiencies in excess of 80 percent are obtained. Reattachment lengths are reduced by a factor of up to 3. These results are in qualitative agreement with previous experimental work. However, differences in some basic details of experimentally observed and the present numerically generated flowfields exist. The effect of swirl is also investigated

Linear stability theory and three-dimensional boundary layer transition

The viewgraphs and discussion of linear stability theory and three dimensional boundary layer transition are provided. The ability to predict, using analytical tools, the location of boundary layer transition over aircraft-type configurations is of great importance to designers interested in laminar flow control (LFC). The e(sup N) method has proven to be fairly effective in predicting, in a consistent manner, the location of the onset of transition for simple geometries in low disturbance environments. This method provides a correlation between the most amplified single normal mode and the experimental location of the onset of transition. Studies indicate that values of N between 8 and 10 correlate well with the onset of transition. For most previous calculations, the mean flows were restricted to two-dimensional or axisymmetric cases, or have employed simple three-dimensional mean flows (e.g., rotating disk, infinite swept wing, or tapered swept wing with straight isobars). Unfortunately, for flows over general wing configurations, and for nearly all flows over fuselage-type bodies at incidence, the analysis of fully three-dimensional flow fields is required. Results obtained for the linear stability of fully three-dimensional boundary layers formed over both wing and fuselage-type geometries, and for both high and low speed flows are discussed. When possible, transition estimates form the e(sup N) method are compared to experimentally determined locations. The stability calculations are made using a modified version of the linear stability code COSAL. Mean flows were computed using both Navier Stokes and boundary-layer codes

Effect of turbulence models on criticality conditions in swirling flows

The critical state of vortex cores downstream of vortex breakdown has been studied. Base vortical flows were computed using the Reynolds-averaged, axisymmetric Navier-Stokes equations. Standard K - epsilon, RNG and second-order Reynolds stress models were employed. Results indicate that the return to supercriticality is highly dependent on the turbulence model. The K - epsilon model predicted a rapid return of the vortex to supercritical conditions, the location of which showed little sensitivity to changes in the swirl ratio. The Reynolds stress model predicted that the vortex remains subcritical to the end of the domain for each of the swirl ratios employed, and provided results in qualitative agreement with experimental work. The RNG model produced intermediate results, with a downstream movement in the critical location with increasing swirl. Calculations for which area reductions were introduced at the exit in a subcritical flow were also performed using the Reynolds stress model. The structure of the resulting recirculation zone was altered significantly. However, when area reductions were employed within supercritical flows as predicted using the two-equation models, no significant influence on the recirculation zone was noted

Smooth-Wall Boundary Conditions for Dissipation-Based Turbulence Models

It is shown that the smooth-wall boundary conditions specified for commonly used dissipation-based turbulence models are mathematically incorrect. It is demonstrated that when these traditional wall boundary conditions are used, the resulting formulations allow an infinite number of solutions. Furthermore, these solutions do not enforce energy conservation and they do not properly enforce the no-slip condition at a smooth surface. This is true for all dissipation-based turbulence models, including the k-ε, k-ω, and k-ζ models. Physically correct wall boundary conditions must force both k and its gradient to zero at a smooth wall. Enforcing these two boundary conditions on k is sufficient to determine a unique solution to the coupled system of differential transport equations. There is no need to impose any wall boundary condition on ε, ω, or ζ at a smooth surface and it is incorrect to do so. The behavior of ε, ω, or ζ approaching a smooth surface is that required to force both k and its gradient to zero at the wall

An Assessment of Five Turbulence Models in Predicting Turbulent Separation

ABSTRACT Four different turbulence models were employed to predict the flow over a wall-mounted Glauert-Goldschmied body. The models evaluated include: 1) two-layer , 2) shear stress transport, 3) low-Reynolds number , 4) Spalart-Allmaras, and 5) . Calculations were performed for both an uncontrolled case, and a controlled-flow case which used steady suction through a slot located at the 65% chord station. The flow conditions include a freestream Mach number of approximately 0.1, and a chord Reynolds number of just under 1 million. For each model, the numerical results over predicted the experimentally determined re-attachment length. An examination of streamwise velocity profiles at several stations downstream of the trailing edge revealed considerable variation in the predictions of the five turbulence models

validation and Enhancement of Computational Fluid Dynamics and Heat Transfer Predictive Capabilities for Generation IV Reactor Systems

Nationwide, the demand for electricity due to population and industrial growth is on the rise. However, climate change and air quality issues raise serious questions about the wisdom of addressing these shortages through the construction of additional fossil fueled power plants. In 1997, the President's Committee of Advisors on Science and Technology Energy Research and Development Panel determined that restoring a viable nuclear energy option was essential and that the DOE should implement a R&D effort to address principal obstacles to achieving this option. This work has addressed the need for improved thermal/fluid analysis capabilities, through the use of computational fluid dynamics, which are necessary to support the design of generation IV gas-cooled and supercritical water reactors

Seasonal evolution of Aleutian low pressure systems: Implications for the North Pacific subpolar circulation

The seasonal change in the development of Aleutian low pressure systems from early fall to early winter is analyzed using a combination of meteorological reanalysis fields, satellite sea surface temperature (SST) data, and satellite wind data. The time period of the study is September–December 2002, although results are shown to be representative of the long-term climatology. Characteristics of the storms were documented as they progressed across the North Pacific, including their path, central pressure, deepening rate, and speed of translation. Clear patterns emerged. Storms tended to deepen in two distinct geographical locations—the Gulf of Alaska in early fall and the western North Pacific in late fall. In the Gulf of Alaska, a quasi-permanent “notch” in the SST distribution is argued to be of significance. The signature of the notch is imprinted in the atmosphere, resulting in a region of enhanced cyclonic potential vorticity in the lower troposphere that is conducive for storm development. Later in the season, as winter approaches and the Sea of Okhotsk becomes partially ice covered and cold, the air emanating from the Asian continent leads to enhanced baroclinicity in the region south of Kamchatka. This corresponds to enhanced storm cyclogenesis in that region. Consequently, there is a seasonal westward migration of the dominant lobe of the Aleutian low. The impact of the wind stress curl pattern resulting from these two regions of storm development on the oceanic circulation is investigated using historical hydrography. It is argued that the seasonal bimodal input of cyclonic vorticity from the wind may be partly responsible for the two distinct North Pacific subarctic gyres

The western Arctic boundary current at 152°W : structure, variability, and transport

Author Posting. © The Author(s), 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 56 (2009): 1164-1181, doi:10.1016/j.dsr2.2008.10.014.From August 2002 to September 2004 a high-resolution mooring array was maintained across the western Arctic boundary current in the Beaufort Sea north of Alaska. The array consisted of profiling instrumentation, providing a timeseries of vertical sections of the current. Here we present the first-year velocity measurements, with emphasis on the Pacific water component of the current. The mean flow is characterized as a bottom-intensified jet of O(15 cm s-1) directed to the east, trapped to the shelfbreak near 100 m depth. Its width scale is only 10-15 km. Seasonally the flow has distinct configurations. During summer it becomes surface-intensified as it advects buoyant Alaskan Coastal Water. In fall and winter the current often reverses (flows westward) under upwelling-favorable winds. Between the storms, as the eastward flow re-establishes, the current develops a deep extension to depths exceeding 700 m. In spring the bottom-trapped flow advects winter-transformed Pacific water emanating from the Chukchi Sea. The year-long mean volume transport of Pacific Water is 0.13±0.08 Sv to the east, which is less than 20% of the long-term mean Bering Strait inflow. This implies that most of the Pacific water entering the Arctic goes elsewhere, contrary to expected dynamics and previous modeling results. Possible reasons for this are discussed. The mean Atlantic water transport (to 800 m depth) is 0.047±0.026 Sv, also smaller than anticipated.AN was funded by the Swedish Research Council; RP, PF, and DT were funded by grant N00014-02-1-0317 of the Office of Naval Research