scholarly journals Mountain waves produced by a stratified shear flow with a boundary layer. Part II: Form drag, wave drag, and transition from downstream sheltering to upstream blocking

2020 ◽  
Author(s):  
François Lott ◽  
Bruno Deremble ◽  
Clément Soufflet
Keyword(s):  
Reynolds Stress ◽  
Large Scale ◽  
Wave Drag ◽  
Wave Crest ◽  
Form Drag ◽  
Mountain Waves ◽  
Reflected Waves ◽  
Stable Case ◽  
Large Scale Flow ◽  
Neutral Case

AbstractThe non-hydrostatic version of the mountain flow theory presented in Part I is detailed. In the near neutral case, the surface pressure decreases when the flow crosses the mountain to balance an increase in surface friction along the ground. This produces a form drag which can be predicted qualitatively. When stratification increases, internal waves start to control the dynamics and the drag is due to upward propagating mountain waves as in part I. The reflected waves nevertheless add complexity to the transition. First, when stability increases, upward propagating waves and reflected waves interact destructively and low drag states occur. When stability increases further, the interaction becomes constructive and high drag state are reached. In very stable cases the reflected waves do not affect the drag much. Although the drag gives a reasonable estimate of the Reynolds stress, its sign and vertical profile are profoundly affected by stability. In the near neutral case the Reynolds stress in the flow is positive, with maximum around the top of the inner layer, decelerating the large-scale flow in the inner layer and accelerating it above. In the more stable cases, on the contrary, the large-scale flow above the inner layer is decelerated as expected for dissipated mountain waves. The structure of the flow around the mountain is also strongly affected by stability: it is characterized by non separated sheltering in the near neutral cases, by upstream blocking in the very stable case, and at intermediate stability by the presence of a strong but isolated wave crest immediately downstream of the ridge.

Mountain waves produced by a stratified shear flow with a boundary layer: transition from downstream sheltering to upstream blocking

2020 ◽  
Author(s):  
Francois Lott ◽  
Bruno Deremble ◽  
Clément Soufflet
Keyword(s):  
Boundary Layer ◽  
Reynolds Stress ◽  
Internal Waves ◽  
Large Scale ◽  
Wave Drag ◽  
Mountain Waves ◽  
Reflected Waves ◽  
Stable Case ◽  
Large Scale Flow ◽  
Neutral Case

<p>A non-hydrostatic theory for mountain flow with a boundary layer of constant eddy viscosity is presented. The theory predicts that dissipation impacts the dynamics over a an inner layer which depth δ is predicted by viscous critical level theory. In the near neutral case, the surface pressure decreases when the flow crosses the mountain to balance an increase in surface friction along the ground. This produces a form drag which can be predicted quantitatively. With stratification, internal waves start to control the dynamics and produce a wave drag that can also be predicted. For weak stratification, upward propagating mountain waves and reflected waves interact destructively and low drag states occur, whereas for moderate stability they interact constructively and high drag states are reached. In very stable cases the reflected waves do not affect the drag much.</p><p>The sign and vertical profiles of the Reynolds stress are profoundly affected by stability. In the neutral case and up to the point where internal waves interact constructively, the Reynolds stress in the flow is positive, with maximum around the top of the inner layer, decelerating the large scale flow in the inner layer and accelerating it above. In the stable case, the opposite occurs, and the large scale flow above the inner layer is decelerated as expected for dissipated mountain waves. These opposed behaviors challenge how mountain form drag and mountain wave drag should be parameterized in large-scale models.</p><p>The structure of the flow around the mountain is also strongly affected by stability: it is characterized by non separated sheltering in the neutral case, by upstream blocking in the very stable case, and at intermediate stability by the presence of a strong but isolated wave crest immediately downstream of the ridge.</p>

Transient Mountain Waves and Their Interaction with Large Scales

2007 ◽  
Vol 64 (7) ◽  
pp. 2378-2400 ◽  
Author(s):  
Chih-Chieh Chen ◽  
Gregory J. Hakim ◽  
Dale R. Durran
Keyword(s):  
Potential Vorticity ◽  
Large Scale ◽  
Wave Drag ◽  
Mountain Waves ◽  
Flow Acceleration ◽  
Spatial Response ◽  
Flow Deceleration ◽  
Large Scale Flow ◽  
The Impact ◽  
Zonal Momentum

Abstract The impact of transient mountain waves on a large-scale flow is examined through idealized numerical simulations of the passage of a time-evolving synoptic-scale jet over an isolated 3D mountain. Both the global momentum budget and the spatial flow response are examined to illustrate the impact of transient mountain waves on the large-scale flow. Additionally, aspects of the spatial response are quantified by potential vorticity inversion. Nearly linear cases exhibit a weak loss of domain-averaged absolute momentum despite the absence of wave breaking. This transient effect occurs because, over the time period of the large-scale flow, the momentum flux through the top boundary does not balance the surface pressure drag. Moreover, an adiabatic spatial redistribution of momentum is observed in these cases, which results in an increase (decrease) of zonally averaged zonal momentum south (north) of the mountain. For highly nonlinear cases, the zonally averaged momentum field shows a region of flow deceleration downstream of the mountain, flanked by broader regions of weak flow acceleration. Cancellation between the accelerating and decelerating regions results in weak fluctuations in the volume-averaged zonal momentum, suggesting that the mountain-induced circulations are primarily redistributing momentum. Potential vorticity anomalies develop in a region of wave breaking near the mountain, and induce local regions of flow acceleration and deceleration that alter the large-scale flow. A “perfect” conventional gravity wave–drag parameterization is implemented on a coarser domain not having a mountain, forced by the momentum flux distribution from the fully nonlinear simulation. This parameterization scheme produces a much weaker spatial response in the momentum field and it fails to produce enough flow deceleration near the 20 m s−1 jet. These results suggest that the potential vorticity sources attributable to the gravity wave–drag parameterization have a controlling effect on the longtime downstream influence of the mountain.

Mountain Waves, Downslope Winds, and Low-Level Blocking Forced by a Midlatitude Cyclone Encountering an Isolated Ridge

2017 ◽  
Vol 74 (2) ◽  
pp. 617-639 ◽  
Author(s):  
Maximo Q. Menchaca ◽  
Dale R. Durran
Keyword(s):  
Large Scale ◽  
Cold Front ◽  
Wave Structure ◽  
High Mountain ◽  
Mountain Waves ◽  
The North ◽  
Downslope Winds ◽  
Northern Half ◽  
Lee Waves ◽  
Large Scale Flow

Abstract The interaction of a midlatitude cyclone with an isolated north–south mountain barrier is examined using numerical simulation. A prototypical cyclone develops from an isolated disturbance in a baroclinically unstable shear flow upstream of the ridge, producing a cold front that interacts strongly with the topography. The structure and evolution of the lee waves launched by the topography are analyzed, including their temporal and their north–south variation along the ridge. Typical mountain wave patterns are generated by a 500-m-high mountain, but these waves often exhibit significant differences from the waves produced in 2D or 3D simulations with steady large-scale-flow structures corresponding to the instantaneous conditions over the mountain in the evolving flow. When the mountain height is 2 km, substantial wave breaking occurs, both at low levels in the lee and in the lower stratosphere. Despite the north–south uniformity of the terrain profile, large north–south variations are apparent in wave structure and downslope winds. In particular, for a 24-h period beginning after the cold front passes the upstream side of the ridge toward the south, strong downslope winds occur only in the northern half of the lee of the ridge. Just prior to this period, the movement of the cold front across the northern lee slopes is complex and accompanied by a burst of strong downslope winds and intense vertical velocities.

scholarly journals The Impact of Mountain Waves on an Idealized Baroclinically Unstable Large-Scale Flow

2018 ◽  
Vol 75 (9) ◽  
pp. 3285-3302 ◽  
Author(s):  
Maximo Q. Menchaca ◽  
Dale R. Durran
Keyword(s):  
Wave Breaking ◽  
Large Scale ◽  
Lower Stratosphere ◽  
Unstable Wave ◽  
Mean Flow ◽  
Mountain Waves ◽  
Upper Troposphere ◽  
Low Level ◽  
Flow Deceleration ◽  
Large Scale Flow

Abstract The feedback of mountain waves and low-level blocking on an idealized baroclinically unstable wave passing over an isolated ridge is examined through numerical simulation. Theoretical analysis implies that the volume-integrated perturbation momentum budget is dominated by mean-flow deceleration, the divergence of vertical fluxes of horizontal momentum, and the Coriolis force acting on the perturbation ageostrophic wind. These do indeed appear as the dominant balances in numerically computed budgets averaged over layers containing 1) wave breaking in the lower stratosphere, 2) flow blocking with wave breaking near the surface, and 3) a region of pronounced horizontally averaged mean-flow deceleration in the upper troposphere where there is no wave breaking. The local impact of wave breaking on the jet in the lower stratosphere is dramatic, with winds in the jet core reduced by almost 50% relative to the no-mountain case. Although it is the layer with the strongest average deceleration, the local patches of decelerated flow are weakest in the upper troposphere. The cross-mountain pressure drag over a 2-km-high ridge greatly exceeds the vertical momentum flux at mountain-top level because of low-level wave breaking, blocking, and lateral flow diversion. These pressure drags and the low-level momentum fluxes are significantly different from corresponding values computed for simulations with steady forcing matching the instantaneous conditions over the mountain in the evolving large-scale flow.

A proposed wavy shield for suppression of supersonic jet noise utilizing reflections

2021 ◽  
Vol 20 (1-2) ◽  
pp. 4-34
Author(s):  
Reda R Mankbadi ◽  
Saman Salehian
Keyword(s):  
Large Scale ◽  
Flat Surface ◽  
Near Field ◽  
Supersonic Jet ◽  
Jet Noise ◽  
Dominant Frequency ◽  
Wavy Surface ◽  
Initial Region ◽  
Reflected Waves ◽  
Free Jet

In this work we propose replacing the conventional flat-surface airframe that shields the engine by a wavy surface. The basic principle is to design a wavy pattern to reflect the incoming near-field flow and acoustic perturbations into waves of a particular dominant frequency. The reflected waves will then excite the corresponding frequency of the large-scale structure in the initial region of the jet’s shear layer. By designing the frequency of the reflected waves to be the harmonic of the fundamental frequency that corresponds to the radiated peak noise, the two frequency-modes interact nonlinearly. With the appropriate phase difference, the harmonic dampens the fundamental as it extracts energy from it to amplify. The outcome is a reduction in the peak noise. To evaluate this concept, we conducted Detached Eddy Simulations for a rectangular supersonic jet with and without the wavy shield and verified our numerical results with experimental data for a free jet, as well as, for a jet with an adjacent flat surface. Results show that the proposed wavy surface reduces the jet noise as compared to that of the corresponding flat surface by as much as 4 dB.

Large-scale flow field visualization for aneurysm treatment

2001 ◽  
Vol 9 (1) ◽  
pp. 3-7
Author(s):  
Damon Liu ◽  
Mark Burgin ◽  
Walter Karplus ◽  
Daniel Valentino
Keyword(s):  
Flow Field ◽  
Large Scale ◽  
Aneurysm Treatment ◽  
Large Scale Flow

scholarly journals Assessment of Numerical Methods for Plunging Breaking Wave Predictions

2021 ◽  
Vol 9 (3) ◽  
pp. 264
Author(s):  
Shanti Bhushan ◽  
Oumnia El Fajri ◽  
Graham Hubbard ◽  
Bradley Chambers ◽  
Christopher Kees
Keyword(s):  
Solitary Wave ◽  
Wave Breaking ◽  
Large Scale ◽  
Turbulence Models ◽  
Navier Stokes ◽  
Wave Crest ◽  
Breaking Wave ◽  
Rans Turbulence Models ◽  
Run Up ◽  
Better Than

This study evaluates the capability of Navier–Stokes solvers in predicting forward and backward plunging breaking, including assessment of the effect of grid resolution, turbulence model, and VoF, CLSVoF interface models on predictions. For this purpose, 2D simulations are performed for four test cases: dam break, solitary wave run up on a slope, flow over a submerged bump, and solitary wave over a submerged rectangular obstacle. Plunging wave breaking involves high wave crest, plunger formation, and splash up, followed by second plunger, and chaotic water motions. Coarser grids reasonably predict the wave breaking features, but finer grids are required for accurate prediction of the splash up events. However, instabilities are triggered at the air–water interface (primarily for the air flow) on very fine grids, which induces surface peel-off or kinks and roll-up of the plunger tips. Reynolds averaged Navier–Stokes (RANS) turbulence models result in high eddy-viscosity in the air–water region which decays the fluid momentum and adversely affects the predictions. Both VoF and CLSVoF methods predict the large-scale plunging breaking characteristics well; however, they vary in the prediction of the finer details. The CLSVoF solver predicts the splash-up event and secondary plunger better than the VoF solver; however, the latter predicts the plunger shape better than the former for the solitary wave run-up on a slope case.

Effects of Squish Flow on Tangential Flow and Turbulence in a Diesel Engine

2019 ◽  
Vol 141 (5) ◽  
Author(s):  
Yanzhe Sun ◽  
Kai Sun ◽  
Tianyou Wang ◽  
Yufeng Li ◽  
Zhen Lu
Keyword(s):  
Diesel Engine ◽  
Turbulent Flows ◽  
Large Scale ◽  
Radial Flow ◽  
Tangential Component ◽  
Tangential Flow ◽  
Image Velocimetry ◽  
Turbulence Production ◽  
Large Scale Flow ◽  
Main Effects

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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