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Flow separation

About: Flow separation is a research topic. Over the lifetime, 16708 publications have been published within this topic receiving 386926 citations.


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TL;DR: In this paper, the circulation in an ocean basin containing an island is studied under nearly geostrophic, beta plane dynamics, and the estimate of the transport between the island and the oceanic basin's boundary is studied.
Abstract: The circulation in an ocean basin containing an island is studied under nearly geostrophic, beta plane dynamics. The model is a fluid of uniform density driven by wind forcing or sources and sinks of mass at the upper boundary of the flow. The circulation is studied analytically, numerically, as well as in the laboratory through the device of the “sliced cylinder” model for the ocean circulation. Of particular interest is the estimate of the transport between the island and the oceanic basin’s boundary. The model is conceived as relevant to both the wind-driven circulation as well as the circulation of abyssal waters around deep topographic features such as mid-ocean ridge segments. Godfrey’s Island Rule for the transport is rederived in general form and the validity of the original approximation of Godfrey (1989) is examined in a variety of circumstances. In particular, the role of dissipative boundary layers and inertial effects such as vortex shedding are scrutinized to determine their role in determining the net transport around the island. Linear theory in many cases predicts a recirculation on the eastern side of the island, provided the meridional extent of the island is large enough. The existence of the recirculation, containing trapped fluid, is confirmed in both laboratory and numerical experiments and the evolution of the structure of the recirculation is examined as a function of the boundary layer Reynolds number. In both the laboratory and numerical studies, the recirculation predicted by linear theory is joined and then superseded by an inertial recirculation springing from boundary layer separation as the Reynolds number increases past a critical value. Even in the linear limit it is shown that the recirculation region, which is closed in quasigeostrophic theory, is subject to a small leak due to planetary geostrophic effects, which prediction is confirmed in the laboratory. The original island rule of Godfrey yields an estimate of the transport which is surprisingly robust and generally within 75% of the values measured in our numerical experiments. Agreement is moderately good when island western boundary layer transport is used as a basis for comparison. Several cases are discussed, however, in which the assumptions made by Godfrey are violated. One occurs when the frictional boundary layers of the island and the basin boundary overlap. We derive a threshold width for the gap for the case where the island is close to a northern or southern boundary of the basin and show how the transport is increasingly blocked as the gap is reduced. A second case occurs when the island is thin and zonally elongated so that the dissipative effects on the northern and southern boundaries of the island become important. Here the vorticity balance assumed in the simple Island Rule is fundamentally altered, and we extend the Island Rule to account for the new dissipation.

107 citations

Journal ArticleDOI
TL;DR: In this paper, a generalized Rayleigh discriminant is computed from a two-dimensional numerical simulation of the basic flow in the same geometry, and it is shown that three regions of the 2D flow are potentially unstable through the centrifugal instability.
Abstract: Three-dimensional stationary structure of the flow over a backward-facing step is studied experimentally. Visualizations and Particle Image Velocimetry (PIV) measurements are investigated. It is shown that the recirculation length is periodically modulated in the spanwise direction with a well-defined wavelength. Visualizations also reveal the presence of longitudinal vortices. In order to understand the origin of this instability, a generalized Rayleigh discriminant is computed from a two-dimensional numerical simulation of the basic flow in the same geometry. This study reveals that actually three regions of the two-dimensional flow are potentially unstable through the centrifugal instability. However both the experiment and the computation of a local Gortler number suggest that only one of these regions is unstable. It is localized in the vicinity of the reattached flow and outside the recirculation bubble.

107 citations

Journal ArticleDOI
TL;DR: In this article, the authors describe an experimental study of the flow field and wall pressure fluctuations induced by quasi-two-dimensional incompressible turbulent boundary layers overflowing a forward-facing step (FFS).
Abstract: This work describes an experimental study of the flow field and wall pressure fluctuations induced by quasi-two-dimensional incompressible turbulent boundary layers overflowing a forward-facing step (FFS). Pressure fluctuations are measured upstream and downstream of an instrumented FFS step model installed inside a large scale recirculation water tunnel, while two-dimensional (2D) velocity fields are measured close to the step via 2D particle image velocimetry (PIV). The overall flow physics is studied in terms of averaged velocity and vorticity fields for different Reynolds numbers based on the step’s height. The wall pressure statistics are analyzed in terms of several indicators, including the root mean squares and probability density functions of the pressure fluctuations, demonstrating that the most relevant flow structure is the unsteady recirculation bubble formed at the reattachment region downstream of the step. Pressure spectra and cross correlations are computed as well, and the convection vel...

106 citations

Journal ArticleDOI
TL;DR: In this article, von Karman et al. investigated the boundary layer interaction at Mach 5.8 and found that turbulent boundary layer heat transfer coefficients for supersonic and hypersonic flow were positively associated with skin friction.
Abstract: 21 Feldman, S., "Hypersonic gasdynamic charts for equilibrium air," Avco Rept. (1957). 22 Nagamatsu, H. T., Workman, J. B., and Sheer, R. E., Jr., "Hypersonic nozzle expansion of air with atom recombination present," J. Aerospace Sci. 28, 833-837 (1961). 23 von Karman, T. and Tsien, H. S., "Boundary layer in compressible fluids," J. Aeronaut. Sci. 6, 227-232 (1938). 24 Kendall, J. M., Jr., "An experimental investigation of leading edge shock wave-boundary layer interaction at Mach 5.8," J. Aeronaut. Sci. 24,47-56 (1957). 25 Howarth, L., Modern Developments in Fluid Dynamics High Speed Flow (Oxford University Press, London, 1953), Vol. 1, p. 402. 26 von Karman, T., "Turbulence and skin friction," J. Aeronaut. Sci. 1,1-20 (1934). 27 Van Driest, E. R., "Turbulent boundary layer in compressible fluids," J. Aeronaut. Sci. 18,145-160 (1951). 28 Li, T. Y. and Nagamatsu, H. T., "Effects of density fluctuations on the turbulent skin friction of an insulated flat plate at high supersonic speeds," J. Aeronaut. Sci. 18, 696-697 (1951). 29 Persh, J., "An analytical investigation of turbulent boundary layer heat transfer coefficients for supersonic and hypersonic flow," Naval Ordnance Lab. Rept. 4099 (1955). 30 Lin, C. C., Turbulent Flows and Heat Transfer: High Speed Aerodynamics and Jet Propulsion (Princeton University Press, Princeton, N. J., 1959), Vol. V.

106 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023177
2022333
2021361
2020394
2019403
2018371