Effects of the Large-Scale Flow on Characteristic Features of the Sea Breeze

Emission and fuel consumption in swirl-supported diesel engines strongly depend on the in-cylinder turbulent flows. But the physical effects of squish flow on the tangential flow and turbulence production are still far from well understood. To identify the effects of squish flow, Particle image velocimetry (PIV) experiments are performed in a motored optical diesel engine equipped with different bowls. By comparing and associating the large-scale flow and turbulent kinetic energy (k), the main effects of the squish flow are clarified. The effect of squish flow on the turbulence production in the r−θ plane lies in the axial-asymmetry of the annular distribution of radial flow and the deviation between the ensemble-averaged swirl field and rigid body swirl field. Larger squish flow could promote the swirl center to move to the cylinder axis and reduce the deformation of swirl center, which could decrease the axial-asymmetry of annular distribution of radial flow, further, that results in a lower turbulence production of the shear stress. Moreover, larger squish flow increases the radial fluctuation velocity which makes a similar contribution to k with the tangential component. The understanding of the squish flow and its correlations with tangential flow and turbulence obtained in this study is beneficial to design and optimize the in-cylinder turbulent flow.

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Turbulent drag in a rotating frame

Journal of Fluid Mechanics ◽

10.1017/jfm.2016.214 ◽

2016 ◽

Vol 794 ◽

Cited By ~ 12

Author(s):

Antoine Campagne ◽

Nathanaël Machicoane ◽

Basile Gallet ◽

Pierre-Philippe Cortet ◽

Frédéric Moisy

Keyword(s):

Large Scale ◽

Scaling Law ◽

Stereoscopic Particle Image Velocimetry ◽

Turbulence Theory ◽

Water Tank ◽

Wave Interactions ◽

Turbulent Drag ◽

Image Velocimetry ◽

Weak Turbulence Theory ◽

Large Scale Flow

What is the turbulent drag force experienced by an object moving in a rotating fluid? This open and fundamental question can be addressed by measuring the torque needed to drive an impeller at a constant angular velocity ${\it\omega}$ in a water tank mounted on a platform rotating at a rate ${\it\Omega}$. We report a dramatic reduction in drag as ${\it\Omega}$ increases, down to values as low as 12 % of the non-rotating drag. At small Rossby number $Ro={\it\omega}/{\it\Omega}$, the decrease in the drag coefficient $K$ follows the approximate scaling law $K\sim Ro$, which is predicted in the framework of nonlinear inertial-wave interactions and weak-turbulence theory. However, stereoscopic particle image velocimetry measurements indicate that this drag reduction instead originates from a weakening of the turbulence intensity in line with the two-dimensionalization of the large-scale flow.

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Investigation of the Large Scale Flow Structures in Dual Planar Attaching Jets

44th AIAA Aerospace Sciences Meeting and Exhibit ◽

10.2514/6.2006-313 ◽

2006 ◽

Author(s):

Nan Gao ◽

Dan Ewing

Keyword(s):

Large Scale ◽

Flow Structures ◽

Large Scale Flow

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Barotropic Regeneration of Upper-Level Synoptic Disturbances in Different Configurations of the Zonal Weather Regime

Journal of the Atmospheric Sciences ◽

10.1175/2008jas2603.1 ◽

2008 ◽

Vol 65 (10) ◽

pp. 3159-3178 ◽

Cited By ~ 10

Author(s):

Gwendal Rivière

Keyword(s):

Large Scale ◽

Deformation Field ◽

Regeneration Process ◽

Barotropic Model ◽

Weather Regime ◽

The North ◽

The Difference ◽

Upper Level ◽

Composite Maps ◽

Large Scale Flow

Barotropic dynamics of upper-tropospheric midlatitude disturbances evolving in different configurations of the zonal weather regime (i.e., in different zonal-like large-scale flows) were studied using observational analyses and barotropic model experiments. The contraction stage of upper-level disturbances that follows their elongation stage leads to an increase of eddy kinetic energy that is called the barotropic regeneration process in this text. This barotropic mechanism is studied through notions of barotropic critical regions (BtCRs) and effective deformation that have been introduced in a previous paper. The effective deformation field is equal to the difference between the square of the large-scale deformation magnitude and the square of the large-scale vorticity. Regions where the effective deformation is positive correspond to regions where the large-scale flow tends to strongly stretch synoptic disturbances. A BtCR is an area separating two large-scale regions of positive effective deformation, one located upstream and on the south side of the jet and the other downstream and on the north side. Such a region presents a discontinuity in the orientation of the dilatation axes and is a potential area where the barotropic regeneration process may occur. Winter days presenting a zonal weather regime in the 40-yr ECMWF Re-Analysis dataset are decomposed, via a partitioning algorithm, into different configurations of the effective deformation field at 300 hPa. A six-cluster partition is obtained. Composite maps of the barotropic generation rate for each cluster exhibit a succession of negative and positive values on both sides of the BtCRs. It confirms statistically that the barotropic regeneration mechanism occurs preferentially about BtCRs. Numerical experiments using a forced barotropic model on the sphere are performed. Each experiment consists of adding a synoptic-scale perturbation to one of the zonal-like jet configurations found in observations, which is kept fixed with time. The combined effects of the effective deformation and nonlinearities are shown to be crucial to reproduce the barotropic regeneration process about BtCRs.

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Mechanism of large-scale flow reversals in turbulent thermal convection

Science Advances ◽

10.1126/sciadv.aat7480 ◽

2018 ◽

Vol 4 (11) ◽

pp. eaat7480 ◽

Cited By ~ 6

Author(s):

Yin Wang ◽

Pik-Yin Lai ◽

Hao Song ◽

Penger Tong

Keyword(s):

Thermal Convection ◽

Large Scale ◽

Careful Analysis ◽

Extreme Value Statistics ◽

Convection Cell ◽

Heat Accumulation ◽

Thermal Plumes ◽

Minute Amount ◽

Large Scale Flow ◽

Experimental Findings

It is commonly believed that heat flux passing through a closed thermal convection system is balanced so that the convection system can remain at a steady state. Here, we report a new kind of convective instability for turbulent thermal convection, in which the convective flow stays over a long steady “quiet period” having a minute amount of heat accumulation in the convection cell, followed by a short and intermittent “active period” with a massive eruption of thermal plumes to release the accumulated heat. The rare massive eruption of thermal plumes disrupts the existing large-scale circulation across the cell and resets its rotational direction. A careful analysis reveals that the distribution of the plume eruption amplitude follows the generalized extreme value statistics with an upper bound, which changes with the fluid properties of the convecting medium. The experimental findings have important implications to many closed convection systems of geophysical scale, in which massive eruptions and sudden changes in large-scale flow pattern are often observed.

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Climate Drivers Linked to Changing Seasonality of Alaska Coastal Tundra Vegetation Productivity

Earth Interactions ◽

10.1175/ei-d-15-0013.1 ◽

2015 ◽

Vol 19 (19) ◽

pp. 1-29 ◽

Cited By ~ 25

Author(s):

Peter A. Bieniek ◽

Uma S. Bhatt ◽

Donald A. Walker ◽

Martha K. Raynolds ◽

Josefino C. Comiso ◽

...

Keyword(s):

Land Surface ◽

Large Scale ◽

Vegetation Index ◽

Normalized Difference Vegetation Index ◽

Sea Breeze ◽

Tundra Vegetation ◽

Land Surface Temperatures ◽

Alaskan Tundra ◽

Northern Alaska

Abstract The mechanisms driving trends and variability of the normalized difference vegetation index (NDVI) for tundra in Alaska along the Beaufort, east Chukchi, and east Bering Seas for 1982–2013 are evaluated in the context of remote sensing, reanalysis, and meteorological station data as well as regional modeling. Over the entire season the tundra vegetation continues to green; however, biweekly NDVI has declined during the early part of the growing season in all of the Alaskan tundra domains. These springtime declines coincide with increased snow depth in spring documented in northern Alaska. The tundra region generally has warmed over the summer but intraseasonal analysis shows a decline in midsummer land surface temperatures. The midsummer cooling is consistent with recent large-scale circulation changes characterized by lower sea level pressures, which favor increased cloud cover. In northern Alaska, the sea-breeze circulation is strengthened with an increase in atmospheric moisture/cloudiness inland when the land surface is warmed in a regional model, suggesting the potential for increased vegetation to feedback onto the atmospheric circulation that could reduce midsummer temperatures. This study shows that both large- and local-scale climate drivers likely play a role in the observed seasonality of NDVI trends.

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