Kolmogorov Flow and the Role of Surface Friction

2013 ◽  
pp. 189-194
Author(s):  
Felix V. Dolzhansky
Keyword(s):  
Surface Friction ◽  
Kolmogorov Flow

The Role of Reynolds Number Effect and Tip Leakage in Compressor Geometry Scaling at Low Turbulent Reynolds Numbers

2020 ◽  
Vol 142 (3) ◽  
Author(s):  
Markus Diehl ◽  
Christoph Schreiber ◽  
Jürg Schiffmann
Keyword(s):  
Reynolds Number ◽  
Surface Friction ◽  
Reynolds Numbers ◽  
Small Scale ◽  
Tip Leakage ◽  
Reynolds Number Effect ◽  
Geometric Scaling ◽  
Compressor Performance ◽  
The Impact

Abstract In compressor design, a convenient way to save time is to scale an existing geometry to required specifications, rather than developing a new design. The approach works well when scaling compressors of similar size at high Reynolds numbers but becomes more complex when applied to small-scale machines. Besides the well-understood increase in surface friction due to increased relative surface roughness, two other main problems specific to small-scale turbomachinery can be specified: (1) the Reynolds number effect, describing the non-linear dependency of surface friction on Reynolds number and (2) increased relative tip clearance resulting from manufacturing limitations. This paper investigates the role of both effects in a geometric scaling process, as used by a designer. The work is based on numerical models derived from an experimentally validated geometry. First, the effects of geometric scaling on compressor performance are assessed analytically. Second, prediction capabilities of reduced-order models from the public domain are assessed. In addition to design point assessment, often found in other publications, the models are tested at off-design. Third, the impact of tip leakage on compressor performance and its Reynolds number dependency is assessed. Here, geometries of different scale and with different tip clearances are investigated numerically. Fourth, a detailed investigation regarding tip leakage driving mechanisms is carried out and design recommendations to improve small-scale compressor performance are provided.

scholarly journals Role of Surface Friction on Shallow Nonprecipitating Convection

2018 ◽  
Vol 75 (1) ◽  
pp. 163-178 ◽  
Author(s):  
Seung-Bu Park ◽  
Steven Böing ◽  
Pierre Gentine
Keyword(s):  
Mass Flux ◽  
Cloud Cover ◽  
Layer Structure ◽  
Topological Analysis ◽  
Surface Friction ◽  
Cloud Layer ◽  
Cloud Base ◽  
Identification Algorithm ◽  
Entrainment Rate

The role of surface friction on shallow nonprecipitating convection is investigated using a series of large-eddy simulations with varying surface friction velocity and with a cloud identification algorithm. As surface friction intensifies, convective rolls dominate over convective cells and secondary overturning circulation becomes stronger in the subcloud layer, thus transporting more moisture upward and more heat downward between the subcloud and cloud layers. Identifying individual clouds, using the identification algorithm based on a three-dimensional topological analysis, reveals that intensified surface friction increases the number of clouds and the degree of tilting in the downstream direction. Highly intensified surface friction increases wind shear across the cloud base and induces cloud tilting, which leads to a vertically parabolic profile of liquid water mixing ratio instead of the classical two-layer structure (conditionally unstable and trade inversion layers). Furthermore, cloud tilting induces more cloud cover and more cloud mass flux much above the cloud base (e.g., 0.8 < z < 1.2 km), but less cloud cover and less cloud mass flux in the upper cloud layer (e.g., z > 1.2 km) because of increased lateral entrainment rate. Similarly, profiles of directly measured entrainment and detrainment rates show that detrainment in the lower cloud layer becomes smaller with stronger surface friction.

scholarly journals Intensification of tropical cyclones in the GFS model

2009 ◽  
Vol 9 (4) ◽  
pp. 1407-1417 ◽  
Author(s):  
J. C. Marín ◽  
D. J. Raymond ◽  
G. B. Raga
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclones ◽  
Surface Friction ◽  
Sea Surface ◽  
Global Forecast System ◽  
Forecast System ◽  
Gfs Model ◽  
Environmental Air ◽  
The Relationship

Abstract. Special forecasts from the Global Forecast System (GFS) model were used in this study to evaluate how the intensification process in a tropical cyclone is represented in this model. Several tropical cyclones that developed in 2005 were analyzed in terms of the storm-scale circulation rather than more traditional measures such as maximum wind or minimum central pressure. The primary balance governing the circulation in the planetary boundary layer is between the convergence of environmental vorticity, which tends to spin up the storm, and surface friction, which tends to spin it down. In addition, we employ recently developed ideas about the relationship between precipitation and the saturation fraction of the environment to understand the factors controlling mass, and hence vorticity convergence. The budget of moist entropy is central to this analysis. Two well-known governing factors for cyclone intensification emerge from this study; surface moist entropy fluxes, dependent in the model on sea surface temperature and cyclone-generated surface winds, and ventilation of the system by dry environmental air. Quantitative expressions for the role of these factors in cyclone intensification are presented in this paper.

scholarly journals Intensification of tropical cyclones in the GFS model

2008 ◽  
Vol 8 (5) ◽  
pp. 17803-17839
Author(s):  
J. C. Marín ◽  
D. J. Raymond ◽  
G. B. Raga
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclones ◽  
Surface Friction ◽  
Sea Surface ◽  
Global Forecast System ◽  
Forecast System ◽  
Gfs Model ◽  
Environmental Air ◽  
The Relationship

Abstract. Special forecasts from the Global Forecast System (GFS) model were used in this study to evaluate how the intensification process in a tropical cyclone is represented in this model. Several tropical cyclones that developed in 2005 were analyzed in terms of the storm-scale circulation rather than more traditional measures such as maximum wind or minimum central pressure. The primary balance governing the circulation in the planetary boundary layer is between the convergence of environmental vorticity, which tends to spin up the storm, and surface friction, which tends to spin it down. In addition, we employ recently developed ideas about the relationship between precipitation and the saturation fraction of the environment to understand the factors controlling mass, and hence vorticity convergence. The budget of moist entropy is central to this analysis. Two well-known governing factors for cyclone intensification emerge from this study; surface moist entropy fluxes, dependent in the model on sea surface temperature and cyclone-generated surface winds, and ventilation of the system by dry environmental air. Quantitative expressions for the role of these factors in cyclone intensification are presented in this paper.

Examples of the role of surface friction in downslope windstorms

1990 ◽  
Vol 43 (1-4) ◽  
pp. 163-172 ◽  
Author(s):  
E. Richard ◽  
P. Mascart ◽  
E. C. Nickerson
Keyword(s):  
Surface Friction ◽  
Downslope Windstorms

scholarly journals The Role of Surface Friction in Downslope Windstorms

1989 ◽  
Vol 28 (4) ◽  
pp. 241-251 ◽  
Author(s):  
Evelyne Richard ◽  
Patrick Mascart ◽  
Everett C. Nickerson
Keyword(s):  
Surface Friction ◽  
Downslope Windstorms

scholarly journals The Role of Surface Drag in Mesocyclone Intensification Leading to Tornadogenesis within an Idealized Supercell Simulation

2017 ◽  
Vol 74 (9) ◽  
pp. 3055-3077 ◽  
Author(s):  
Brett Roberts ◽  
Ming Xue
Keyword(s):  
Pressure Gradient ◽  
Surface Friction ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Surface Drag ◽  
Vertical Pressure ◽  
Low Level ◽  
Physical Mechanisms ◽  
Key Factor

Abstract The idealized supercell simulations in a previous study by Roberts et al. are further analyzed to clarify the physical mechanisms leading to differences in mesocyclone intensification between an experiment with surface friction applied to the full wind (FWFRIC) and an experiment with friction applied to the environmental wind only (EnvFRIC). The low-level mesocyclone intensifies rapidly during the 3 min preceding tornadogenesis in FWFRIC, while the intensification during the same period is much weaker in EnvFRIC, which fails to produce a tornado. To quantify the mechanisms responsible for this discrepancy in mesocyclone evolution, material circuits enclosing the low-level mesocyclone are initialized and traced back in time, and circulation budgets for these circuits are analyzed. The results show that in FWFRIC, surface drag directly generates a substantial proportion of the final circulation around the mesocyclone, especially below 1 km AGL; in EnvFRIC, circulation budgets indicate the mesocyclone circulation is overwhelmingly barotropic. It is proposed that the import of near-ground, frictionally generated vorticity into the low-level mesocyclone in FWFRIC is a key factor causing the intensification and lowering of the mesocyclone toward the ground, creating a large upward vertical pressure gradient force that leads to tornadogenesis. Similar circulation analyses are also performed for circuits enclosing the tornado at its genesis stage. The frictionally generated circulation component is found to contribute more than half of the final circulation for circuits enclosing the tornado vortex below 400 m AGL, and the frictional contribution decreases monotonically with the height of the final circuit.

scholarly journals The Role of Boundary Layer Processes in Limiting PV Homogenization

2009 ◽  
Vol 66 (6) ◽  
pp. 1612-1632 ◽  
Author(s):  
Yang Zhang ◽  
Peter H. Stone ◽  
Amy Solomon
Keyword(s):  
Boundary Layer ◽  
Heat Flux ◽  
Temperature Gradient ◽  
Surface Friction ◽  
Mean Flow ◽  
Eddy Heat Flux ◽  
Boundary Layer Processes ◽  
The Mean ◽  
Pv Gradient

Abstract A β-plane multilevel quasigeostrophic channel model with interactive static stability and a simplified parameterization of atmospheric boundary layer physics is used to study the role of different boundary layer processes in eddy equilibration and their relative effect in maintaining the strong boundary layer potential vorticity (PV) gradient. The model results show that vertical thermal diffusion, along with the surface heat exchange, is primarily responsible for limiting PV homogenization by baroclinic eddies in the boundary layer. Under fixed SST boundary conditions, these two processes act as the source of the mean flow baroclinicity in the lower levels and result in stronger eddy heat fluxes. Reducing surface friction alone does not result in efficient elimination of the boundary layer PV gradient, but the equilibrium state temperature gradient is still largely influenced by surface friction and its response to changes in surface friction is not monotonic. In the regime of strong surface friction, with reduced poleward eddy heat flux, a strong temperature gradient is still retained. When the surface friction is sufficiently weak along with the stronger zonal wind, the critical level at the center of the jet drops below the surface. As a result, in the lower levels, the eddy heat flux forcing on the mean flow moves away from the center of the jet and the equilibrium state varies only slightly with the strength of the vertical momentum diffusion in the boundary layer.

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