Convective Momentum Transfer Research Articles

Corrigendum to The effect of the convective momentum transfer on the acoustic boundary condition of perforated liners with grazing mean flow

Corrigendum to The effect of the convective momentum transfer on the acoustic boundary condition of perforated liners with grazing mean flow

Corrigendum to The effect of the convective momentum transfer on the acoustic boundary condition of perforated liners with grazing mean flow

Corrigendum to The effect of the convective momentum transfer on the acoustic boundary condition of perforated liners with grazing mean flow

The effect of the convective momentum transfer on the acoustic boundary condition of perforated liners with grazing mean flow

The interaction of sound with sound-permeable hard walls subjected to grazing mean flow is investigated with a focus on the sound-induced exchange of streamwise momentum between the mean flow and the wall. Two generic wall types have to be distinguished, the homogeneously permeable wall and the wall with clearly separated openings, which is a more realistic model of technically feasible walls. To begin with, the focus is on the shear stress that drives the dynamics of the shearing mean flow over the homogeneous wall. This is analyzed by means of two simple mathematical models of shear stress diffusion, which come as two equivalent pairs of differential equations either for the acoustic shear stress and the wall-normal displacement, or for the streamwise and the wall-normal components of the acoustic velocity. The physical analysis is concentrated on the relation between shear stress and the wall-normal displacement of the fluid elements, which determines the effective admittance of the wall. The shear stress is represented by the momentum transfer impedance which is defined as the ratio between the acoustic wall shear stress and the in-wall velocity evaluated at the wall. It turns out that the strong increase of the acoustic wall shear stress due to transfer of mean flow momentum to the wall is the dominating mechanism which affects the effective admittance of the wall. Nevertheless, the suitability of the momentum transfer impedance as part of a complete boundary condition of the wall is questioned. The disagreement between the predicted momentum transfer impedance and some rare experimental data obtained with real inhomogeneous walls is considered as a strong indication that some further mechanisms are invoked by the inhomogeneity of real walls which are briefly discussed with regard to future studies.

A Multiscale Model for the Intraseasonal Impact of the Diurnal Cycle over the Maritime Continent on the Madden–Julian Oscillation

Abstract The eastward-propagating Madden–Julian oscillation (MJO) typically exhibits complex behavior during its passage over the Maritime Continent, sometimes slowly propagating eastward and other times stalling and even terminating there with large amounts of rainfall. This is a huge challenge for present-day numerical models to simulate. One possible reason is the inadequate treatment of the diurnal cycle and its scale interaction with the MJO. Here these two components are incorporated into a simple self-consistent multiscale model that includes one model for the intraseasonal impact of the diurnal cycle and another one for the planetary/intraseasonal circulation. The latter model is forced self-consistently by eddy flux divergences of momentum and temperature from a model for the diurnal cycle with two baroclinic modes, which capture the intraseasonal impact of the diurnal cycle. The MJO is modeled as the planetary-scale circulation response to a moving heat source on the synoptic and planetary scales. The results show that the intraseasonal impact of the diurnal cycle during boreal winter tends to strengthen the westerlies (easterlies) in the lower (upper) troposphere in agreement with the observations. In addition, the temperature anomaly induced by the intraseasonal impact of the diurnal cycle can cancel that from the symmetric–asymmetric MJO with convective momentum transfer, yielding stalled or suppressed propagation of the MJO across the Maritime Continent. The simple multiscale model should be useful for the MJO in observations or more complex numerical models.

Zonal Momentum Balance in the Tropical Atmospheric Circulation during the Global Monsoon Mature Months

AbstractIn this paper, zonal momentum balances of the tropical atmospheric circulation during the global monsoon mature months (January and July) are analyzed in three dimensions based on the ECMWF Interim Re-Analysis (ERA-Interim). It is found that the dominant terms in the balance of the atmospheric boundary layer (ABL) in both months are the pressure gradient force, the Coriolis force, and friction. The nonlinear advection term plays a significant role only in the Asian summer monsoon regions within the ABL. In the upper troposphere, the pressure gradient force, the Coriolis force, and the nonlinear advection are the dominant terms. The transient eddy force and the residual force (which can be explained as convective momentum transfer over open oceans) are secondary, yet cannot be neglected near the equator. Zonal-mean equatorial upper-troposphere easterlies are maintained by the absolute angular momentum advection associated with the cross-equatorial Hadley circulation. Equatorial upper-troposphere easterlies over the Asian monsoon regions are also controlled by the absolute angular momentum advection but are mainly maintained by the pressure gradient force in January. The equivalent linear Rayleigh friction, which is widely applied in simple tropical models, is calculated and the corresponding spatial distribution of the local coefficient and damping time scale are estimated from the linear regression. It is found that the linear momentum model is in general capable of crudely describing the tropical atmospheric circulation dynamics, yet the caveat should be kept in mind that the friction coefficient is not uniformly distributed and is even negative in some regions.

Proper orthogonal decomposition investigation of turbulent Rayleigh-Bénard convection in a rectangular cavity

We consider the large-eddy simulation (LES) of turbulent Rayleigh-Bénard convection for air in a parallepipedic cavity of ratio (1:5:1) over the range Ra = 6 × 108 up to Ra = 1010 previously studied in Sergent and Le Quéré (Proceedings of the 13th European Turbulence Conference, 2011). Using proper orthogonal decomposition (POD) analysis, we confirm the existence of a large-scale circulation (LSC) consisting of quasi-stationary cross-stream rolls (y-rolls) which are aligned with the small direction of the box. Strong changes in the LSC are observed to take place over a few hundred convective time units, defined as \documentclass[12pt]{minimal}\begin{document}$\kappa /(L_{x}^{2} Ra^{1/2})$\end{document}κ/(Lx2Ra1/2), where κ is the fluid diffusivity, Lx is the height of the box and Ra is the Rayleigh number. We also show the existence of a secondary flow, which consists of horizontal rolls (z-rolls) surrounding the core of the cavity and orthogonal to the cross-stream rolls. The amplitude of these longitudinal rolls oscillates on a time scale of 50 convective units. The longitudinal rolls are associated with strong variations in the vertical momentum transfer, while cross-stream rolls are primarily responsible for more than half of the convective heat transfer and make little contribution to the convective momentum transfer. Integration of a simplified dynamical model with stationary y-rolls leads to an oscillation in the z-rolls with a characteristic period of 70 units, which supports the idea that the oscillation of the secondary rolls is determined by the primary flow structure. Using 2D linear stability analysis based on the contributions from POD modes, we show that the presence of longitudinal shear leads to the stabilization of the high wavenumber range, and we predict a roll size which is reasonably close to that observed in the LES. We conjecture that changes in the large-scale circulation of the flow are related to the fluctuating shear created by the secondary rolls and higher-order modes outside the boundary layer.

Critical roles of convective momentum transfer in sustaining the multi-scale Madden–Julian oscillation

To understand the nature and role of multi-scale interaction involved in the Madden–Julian oscillation (MJO), a dynamical model is built based on two essential processes: the convective complex of the MJO modulates the strength and location of synoptic-scale motions, which in turn feed back to the MJO through the convective momentum transfer (CMT). Our results exhibit that: (1) The lower tropospheric easterly CMT coming from the 2-day waves slows down the MJO dramatically; (2) although the lower tropospheric westerly CMT coming from the superclusters can produce the horizontal quadrupole vortex and vertical westerly wind-burst structures of the MJO, it drives the large-scale motions to propagate eastward too fast; (3) the planetary boundary layer provides an instability source for the MJO and pulls the MJO to propagate eastward at a speed of 0∼10 ms−1; and (4) the optimal structure of the multi-scale MJO should be: the stronger superclusters/2-day waves prevail in the rear/front part of the MJO and produce lower tropospheric westerly/easterly CMT there. These theoretical results emphasize the role of CMT and encourage further observations in the multi-scale MJO.

複断面蛇行河道における洪水流の増水期と減水期の流れ構造

Flood flows in meandering compound channels show highly three-dimensional structure due to complex shape, roughness distribution and flow unsteadiness. In this study detailed measurement of velocity and water depth was carried out for unsteady flow in a compound meandering channel to clarify the differences in flow structure and the momentum transfer mechanism between main channel and flood plains during rising and falling stages. It was demonstrated that the convective momentum transfer between main channel and flood plains is more active during rising stage than falling stage.

Percolation disorder in viscous and nonviscous flow through porous media.

The complete set of Navier-Stokes equations has been numerically solved for two-dimensional random pore networks subjected to site percolation disorder. Both viscous and nonviscous flow regimes have been investigated to emphasize the effect of structure and phenomenology on deviation from the classical Darcy law of permeability. It is shown that near the percolation threshold, different scaling laws apply for distinct flow conditions. At this transition region, discrepancies in the critical exponents reflect the influence of the convective momentum transfer mechanism on the overall behavior of the disordered physical system.

A Wave-CISK Model of Squall Lines

Abstract The wave-CISK model of Raymond incorporating lag effects in the updraft and downdraft is implemented as an initial value problem in physical space. Examples of both midlatitude and tropical squall lines are successfully simulated. Diagnosis of the model shows that condensational heating in the updraft is the primary driving mechanism of a mature squall line, with evaporative cooling and convective momentum transfer playing subsidiary roles. However, an instability involving the displacement of boundary layer air by downdrafts apparently plays an important role in squall line initiation. A weakness of the model is its inability to predict the direction of squall line propagation relative to the low level wind shear. This is traced to the insensitivity of the convective parameterization to midlevel entrainment. However, unlike strict two-dimensional squall lint models, the parameterization allows cross-stream mass transfer to occur in the context of overall slab symmetry. Such transfer is dynamical...

梅雨前線帯に観測された中規模擾乱と関連する降雨細胞

梅雨前線帯に観測された中規模擾乱と関連する降雨細胞