The Experts below are selected from a list of 279 Experts worldwide ranked by ideXlab platform
Yuansen Ting - One of the best experts on this subject based on the ideXlab platform.
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on the red giant branch ambiguity in the Surface Boundary condition leads to 100 k uncertainty in model effective temperatures
The Astrophysical Journal, 2018Co-Authors: Jieun Choi, Aaron Dotter, Charlie Conroy, Yuansen TingAbstract:The effective temperature (Teff) distribution of stellar evolution models along the red giant branch (RGB) is sensitive to a number of parameters including the overall metallicity, elemental abundance patterns, the efficiency of convection, and the treatment of the Surface Boundary condition. Recently there has been interest in using observational estimates of the RGB Teff to place constraints on the mixing length parameter, a_MLT, and possible variation with metallicity. Here we use 1D MESA stellar evolution models to explore the sensitivity of the RGB Teff to the treatment of the Surface Boundary condition. We find that different Surface Boundary conditions can lead to +/- 100 K metallicity-dependent offsets on the RGB relative to one another in spite of the fact that all models can reproduce the properties of the Sun. Moreover, for a given atmosphere T-tau relation, we find that the RGB Teff is also sensitive to the optical depth at which the Surface Boundary condition is applied in the stellar model. Nearly all models adopt the photosphere as the location of the Surface Boundary condition but this choice is somewhat arbitrary. We compare our models to stellar parameters derived from the APOGEE-Kepler sample of first ascent red giants and find that systematic uncertainties in the models due to treatment of the Surface Boundary condition place a limit of ~100 K below which it is not possible to make firm conclusions regarding the fidelity of the current generation of stellar models.
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On the Red Giant Branch: Ambiguity in the Surface Boundary Condition Leads to ≈100 K Uncertainty in Model Effective Temperatures
The Astrophysical Journal, 2018Co-Authors: Jieun Choi, Aaron Dotter, Charlie Conroy, Yuansen TingAbstract:The effective temperature (Teff) distribution of stellar evolution models along the red giant branch (RGB) is sensitive to a number of parameters including the overall metallicity, elemental abundance patterns, the efficiency of convection, and the treatment of the Surface Boundary condition. Recently there has been interest in using observational estimates of the RGB Teff to place constraints on the mixing length parameter, a_MLT, and possible variation with metallicity. Here we use 1D MESA stellar evolution models to explore the sensitivity of the RGB Teff to the treatment of the Surface Boundary condition. We find that different Surface Boundary conditions can lead to +/- 100 K metallicity-dependent offsets on the RGB relative to one another in spite of the fact that all models can reproduce the properties of the Sun. Moreover, for a given atmosphere T-tau relation, we find that the RGB Teff is also sensitive to the optical depth at which the Surface Boundary condition is applied in the stellar model. Nearly all models adopt the photosphere as the location of the Surface Boundary condition but this choice is somewhat arbitrary. We compare our models to stellar parameters derived from the APOGEE-Kepler sample of first ascent red giants and find that systematic uncertainties in the models due to treatment of the Surface Boundary condition place a limit of ~100 K below which it is not possible to make firm conclusions regarding the fidelity of the current generation of stellar models.
Tobias Kukulka - One of the best experts on this subject based on the ideXlab platform.
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Lagrangian Investigation of Wave-Driven Turbulence in the Ocean Surface Boundary Layer
Journal of Physical Oceanography, 2019Co-Authors: Tobias Kukulka, Fabrice VeronAbstract:AbstractTurbulent processes in the ocean Surface Boundary layer (OSBL) play a key role in weather and climate systems. This study explores a Lagrangian analysis of wave-driven OSBL turbulence, base...
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Interaction of Langmuir Turbulence and Inertial Currents in the Ocean Surface Boundary Layer under Tropical Cyclones
Journal of Physical Oceanography, 2018Co-Authors: Dong Wang, Tobias Kukulka, Brandon G. Reichl, Tetsu Hara, Isaac Ginis, Peter P. SullivanAbstract:AbstractBased on a large-eddy simulation approach, this study investigates the response of the ocean Surface Boundary layer (OSBL) and Langmuir turbulence (LT) to extreme wind and complex wave forc...
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observations of turbulence in the ocean Surface Boundary layer energetics and transport
Journal of Physical Oceanography, 2009Co-Authors: Gregory P Gerbi, John H Trowbridge, Eugene A Terray, Albert J Plueddemann, Tobias KukulkaAbstract:Observations of turbulent kinetic energy (TKE) dynamics in the ocean Surface Boundary layer are presented here and compared with results from previous observational, numerical, and analytic studies. As in previous studies, the dissipation rate of TKE is found to be higher in the wavy ocean Surface Boundary layer than it would be in a flow past a rigid Boundary with similar stress and buoyancy forcing. Estimates of the terms in the turbulent kinetic energy equation indicate that, unlike in a flow past a rigid Boundary, the dissipation rates cannot be balanced by local production terms, suggesting that the transport of TKE is important in the ocean Surface Boundary layer. A simple analytic model containing parameterizations of production, dissipation, and transport reproduces key features of the vertical profile of TKE, including enhancement near the Surface. The effective turbulent diffusion coefficient for heat is larger than would be expected in a rigid-Boundary Boundary layer. This diffusion coefficient is predicted reasonably well by a model that contains the effects of shear production, buoyancy forcing, and transport of TKE (thought to be related to wave breaking). Neglect of buoyancy forcing or wave breaking in the parameterization results in poor predictions of turbulent diffusivity. Langmuir turbulence was detected concurrently with a fraction of the turbulence quantities reported here, but these times did not stand out as having significant differences from observations when Langmuir turbulence was not detected.
Gregory P Gerbi - One of the best experts on this subject based on the ideXlab platform.
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The Role of Whitecapping in Thickening the Ocean Surface Boundary Layer
Journal of Physical Oceanography, 2015Co-Authors: Gregory P Gerbi, S. E. Kastner, Genevieve BrettAbstract:AbstractThe effects of wind-driven whitecapping on the evolution of the ocean Surface Boundary layer are examined using an idealized one-dimensional Reynolds-averaged Navier–Stokes numerical model. Whitecapping is parameterized as a flux of turbulent kinetic energy through the sea Surface and through an adjustment of the turbulent length scale. Simulations begin with a two-layer configuration and use a wind that ramps to a steady stress. This study finds that the Boundary layer begins to thicken sooner in simulations with whitecapping than without because whitecapping introduces energy to the base of the Boundary layer sooner than shear production does. Even in the presence of whitecapping, shear production becomes important for several hours, but then inertial oscillations cause shear production and whitecapping to alternate as the dominant energy sources for mixing. Details of these results are sensitive to initial and forcing conditions, particularly to the turbulent length scale imposed by breaking wa...
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observations of turbulence in the ocean Surface Boundary layer energetics and transport
Journal of Physical Oceanography, 2009Co-Authors: Gregory P Gerbi, John H Trowbridge, Eugene A Terray, Albert J Plueddemann, Tobias KukulkaAbstract:Observations of turbulent kinetic energy (TKE) dynamics in the ocean Surface Boundary layer are presented here and compared with results from previous observational, numerical, and analytic studies. As in previous studies, the dissipation rate of TKE is found to be higher in the wavy ocean Surface Boundary layer than it would be in a flow past a rigid Boundary with similar stress and buoyancy forcing. Estimates of the terms in the turbulent kinetic energy equation indicate that, unlike in a flow past a rigid Boundary, the dissipation rates cannot be balanced by local production terms, suggesting that the transport of TKE is important in the ocean Surface Boundary layer. A simple analytic model containing parameterizations of production, dissipation, and transport reproduces key features of the vertical profile of TKE, including enhancement near the Surface. The effective turbulent diffusion coefficient for heat is larger than would be expected in a rigid-Boundary Boundary layer. This diffusion coefficient is predicted reasonably well by a model that contains the effects of shear production, buoyancy forcing, and transport of TKE (thought to be related to wave breaking). Neglect of buoyancy forcing or wave breaking in the parameterization results in poor predictions of turbulent diffusivity. Langmuir turbulence was detected concurrently with a fraction of the turbulence quantities reported here, but these times did not stand out as having significant differences from observations when Langmuir turbulence was not detected.
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Observations of turbulent fluxes and turbulence dynamics in the ocean Surface Boundary layer - Observations of Turbulent Fluxes and Turbulence Dynamics in the Ocean Surface Boundary Layer
2008Co-Authors: Gregory P GerbiAbstract:Abstract : This study presents observations of turbulence dynamics made during the low winds portion of the Coupled Boundary Layers and Air-Sea Transfer experiment (CBLAST-low) in the ocean Surface Boundary layer. Observations include turbulent fluxes, turbulent kinetic energy, and the length scales of flux-carrying and energy-containing eddies. The observations of turbulent fluxes allowed the closing of heat and momentum budgets across the air-sea interface. The flux-carrying eddies are similar in size to those expected in rigid-Boundary turbulence, but energy-containing eddies are smaller than those in rigid Boundary turbulence. The observations confirm previous speculation that Surface wave breaking provides a Surface source of turbulent kinetic energy that dissipates as it is transported to depth. A model that includes the effects of shear production, transport, and dissipation is able to reproduce the enhancement of turbulent kinetic energy near the ocean Surface. The ocean Surface Boundary layer is observed to have small but finite temperature gradients that are related to the Boundary fluxes of heat and momentum, as assumed by closure models. However, the turbulent diffusivity of heat in the Surface Boundary layer is larger than predicted by rigid-Boundary closure models. This discrepancy can be explained by the addition of wave breaking to the rigid-Boundary model.
T. Yokomizo - One of the best experts on this subject based on the ideXlab platform.
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moving Surface Boundary layer control as applied to slender and bluff bodies
1999Co-Authors: V. J. Modi, S R Munshi, T. YokomizoAbstract:Fluid dynamics and dynamics of two-dimensional airfoils as well as bluff bodies are studied in presence of the Moving Surface Boundary-layer Control (MSBC) using wind tunnel tests, numerical finite element analysis and flow visualization. Results suggest significant increase in the maximum lift coefficient and delay in the stall for airfoils, and decrease in the drag for bluff bodies. The concept proved effective in suppressing vortex resonance and galloping type of instabilities encountered by tall buildings, bridges, smokestacks and similar bluff structures.
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Pressure Distribution on a Roof in Presence of the Moving Surface Boundary-layer Control
Journal of Visualization, 1998Co-Authors: V. J. Modi, T. YokomizoAbstract:The paper studies effect of momentum injection, accomplished through circular cylindrical rotating elements, on the pressure distribution at the roof of a model house. Extensive wind tunnel tests, complemented by flow visualization, suggest that such Moving Surface Boundary-layer Control (MSBC) effectively delays separation and significantly increases the pressure, both on leeward and windward Surfaces. This would assist in protection of the roof against wind storms and snow accumulation.
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Moving Surface Boundary-layer control: Studies with bluff bodies and application
AIAA Journal, 1991Co-Authors: V. J. Modi, M. S. U. K. Fernando, T. YokomizoAbstract:The concept of moving Surface Boundary-layer control has proved quite successful in increasing lift and delaying stall of slender bodies like airfoil sections. We assess effectiveness of the concept in reducing drag of bluff bodies, such as a two-dimensional flat plate at large angles of attack, rectangular prisms, and three-dimensional models of trucks, through an extensive wind-tunnel test program.
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Moving Surface Boundary-layer control as applied to two-dimensional and three-dimensional bluff bodies
Journal of Wind Engineering and Industrial Aerodynamics, 1991Co-Authors: V. J. Modi, M. S. U. K. Fernando, T. YokomizoAbstract:Abstract The concept of moving Surface Boundary-layer control has proved quite successful in increasing lift and delaying stall of slender bodies such as airfoil sections. This paper assesses the effectiveness of the concept in reducing drag of bluff bodies such as a two-dimensional flat plate at large angles of attack, rectangular prisms and three-dimensional models of trucks through an extensive wind tunnel test program. Results suggest that injection of momentum through moving Surfaces, achieved here by the introduction of bearing-mounted, motor-driven, hollow cylinders, can significantly delay separation of the Boundary layer and reduce the pressure drag. A flow visualization study, conducted in a closed-circuit water tunnel using slit lighting and polyvinylchloride tracer particles, complements the wind tunnel tests. It shows, rather dramatically, the effectiveness of the moving Surface Boundary-layer control.
Fernando Porté-agel - One of the best experts on this subject based on the ideXlab platform.
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Effect of Roughness on Surface Boundary Conditions for Large-Eddy Simulation
Boundary-Layer Meteorology, 2006Co-Authors: Rob Stoll, Fernando Porté-agelAbstract:An important parameterization in large-eddy simulations (LESs) of high-Reynolds-number Boundary layers, such as the atmospheric Boundary layer, is the specification of the Surface Boundary condition. Typical Boundary conditions compute the fluctuating Surface shear stress as a function of the resolved (filtered) velocity at the lowest grid points based on similarity theory. However, these approaches are questionable because they use instantaneous (filtered) variables, while similarity theory is only valid for mean quantities. Three of these formulations are implemented in simulations of a neutral atmospheric Boundary layer with different aerodynamic Surface roughness. Our results show unrealistic influence of Surface roughness on the mean profile, variance and spectra of the resolved velocity near the ground, in contradiction of similarity theory. In addition to similarity-based Surface Boundary conditions, a recent model developed from an a priori experimental study is tested and it is shown to yield more realistic independence of the results to changes in Surface roughness. The optimum value of the model parameter found in our simulations matches well the value reported in the a priori wind-tunnel study.
