Trapped Lee Wave Interference in the Presence of Surface Friction

Abstract Trapped lee wave interference over double bell-shaped obstacles in the presence of surface friction is examined. Idealized high-resolution numerical experiments with the nonhydrostatic Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) are performed to examine the influence of a frictional boundary layer and nonlinearity on wave interference and the impact of interference on wave-induced boundary layer separation and the formation of rotors. The appearance of constructive and destructive interference, controlled by the ratio of the ridge separation distance to the intrinsic horizontal wavelength of lee waves, is found to be predicted well by linear interference theory with orographic adjustment. The friction-induced shortening of intrinsic wavelength displays a strong indirect effect on wave interference. For twin peak orography, the interference-induced variation of wave amplitude is smaller than that predicted by linear theory. The interference is found to affect the formation and strength of rotors most significantly in the lee of the downstream peak; destructive interference suppresses the formation and strength of rotors there, whereas results for constructive interference closely parallel those for a single mountain. Over the valley, under both constructive and destructive interference, rotors are weaker compared to those in the lee of a single ridge while their strength saturates in the finite-amplitude flow regime. Destructive interference is found to be more susceptible to nonlinear effects, with both the orographic adjustment and surface friction displaying a stronger effect on the flow in this state. “Complete” destructive interference, in which waves almost completely cancel out in the lee of the downstream ridge, develops for certain ridge separation distances but only for a downstream ridge smaller than the upstream one.

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Compressible large eddy simulation of the unsteady evolution process in a LPT Cascade with incoming wakes

Proceedings of the Institution of Mechanical Engineers Part G Journal of Aerospace Engineering ◽

10.1177/0954410020967535 ◽

2020 ◽

pp. 095441002096753

Author(s):

Yunfei Wang ◽

Huanlong Chen ◽

Huaping Liu ◽

Yanping Song ◽

Fu Chen

Keyword(s):

Boundary Layer ◽

Large Eddy Simulation ◽

Boundary Layer Separation ◽

Main Flow ◽

Reynolds Stresses ◽

Flow Area ◽

Eddy Simulation ◽

Reduced Frequency ◽

Large Eddy ◽

The Impact

An in-house large eddy simulation (LES) code based on three-dimensional compressible N-S equations is used to research the impact of incoming wakes on unsteady evolution characteristic in a low-pressure turbine (LPT) cascade. The Mach number is 0.4 and Reynolds number is 0.6 × 105 (based on the axial chord and outlet velocity). The reduced frequency of incoming wakes is Fred = 0 (without wakes), 0.37 and 0.74. A detailed analysis of Reynolds stresses and turbulent kinetic energy inside the boundary layer has been carried out. Particular consideration is devoted to the transport process of incoming wakes and the intermittent property of the unsteady boundary layer. With the increase of reduced frequency, the inhibiting effect of wakes on boundary layer separation gradually enhances. The separation at the rear part of the suction side is weakened and the separation point moves downstream. However, incoming wakes lead to an increase in dissipation and aerodynamic losses in the main flow area. Excessive reduced frequency ( Fred = 0.74) causes the main flow area to become one of the main source areas of loss. An optimal reduced frequency exists to minimize the aerodynamic loss of the linear cascade.

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Blowing Snow as a Natural Glaciogenic Cloud Seeding Mechanism

Monthly Weather Review ◽

10.1175/mwr-d-15-0241.1 ◽

2015 ◽

Vol 143 (12) ◽

pp. 5017-5033 ◽

Cited By ~ 16

Author(s):

Bart Geerts ◽

Binod Pokharel ◽

David A. R. Kristovich

Keyword(s):

Boundary Layer ◽

Weather Conditions ◽

Boundary Layer Separation ◽

Ice Crystals ◽

Size Distributions ◽

Blowing Snow ◽

Free Atmosphere ◽

Layer Separation ◽

Orographic Clouds ◽

The Impact

Abstract Winter storms are often accompanied by strong winds, especially over complex terrain. Under such conditions freshly fallen snow can be readily suspended. Most of that snow will be redistributed across the landscape (e.g., behind obstacles), but some may be lofted into the turbulent boundary layer, and even into the free atmosphere in areas of boundary layer separation near terrain crests, or in hydraulic jumps. Blowing snow ice crystals, mostly small fractured particles, thus may enhance snow growth in clouds. This may explain why shallow orographic clouds, with cloud-top temperatures too high for significant ice initiation, may produce (usually light) snowfall with remarkable persistence. While drifting snow has been studied extensively, the impact of blowing snow on precipitation on snowfall itself has not. Airborne radar and lidar data are presented to demonstrate the presence of blowing snow, boundary layer separation, and the glaciation of shallow supercooled orographic clouds. Further evidence for the presence of blowing snow comes from a comparison between snow size distributions measured at Storm Peak Laboratory (SPL) on Mount Werner (Colorado) versus those measured aboard an aircraft while passing overhead, and from an examination of snow size distributions at SPL under diverse weather conditions. Ice splintering following the collision of supercooled droplets on rimed surfaces such as trees does not appear to explain the large concentrations of small ice crystals sometimes observed at SPL.

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A nonlinear, asymptotic investigation of the stationary modes of instability of the three-dimensional boundary layer on a rotating disc

Proceedings of the Royal Society of London Series A - Mathematical and Physical Sciences ◽

10.1098/rspa.1987.0128 ◽

1987 ◽

Vol 413 (1845) ◽

pp. 497-513 ◽

Cited By ~ 12

Keyword(s):

Boundary Layer ◽

Three Dimensional ◽

Nonlinear Effects ◽

Finite Amplitude ◽

Neutral Stability ◽

Lower Branch ◽

Rotating Disc ◽

Dimensional Boundary ◽

High Reynolds ◽

Set Up

There exist two types of stationary instability of the flow over a rotating disc corresponding to the upper, inviscid mode and the lower-branch mode, which has a triple-deck structure, of the neutral stability curve. The linear problem has been investigated by P. Hall ( Proc. R. Soc. Lond. A 406, 93-106 (1986)) and the asymptotic structure of the wavenumber and orientation of these modes has been obtained. Here, a nonlinear investigation of high Reynolds number, stationary instabilities in the three-dimensional boundary layer on a rotating disc is given for the lower branch mode. By considering nonlinear effects and following the framework set up by Hall, asymptotic solutions are obtained that enable the finite amplitude growth of a disturbance close to the neutral location to be described.

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Reynolds number influence on the formation of vortical structures on a pitching flat plate

Interface Focus ◽

10.1098/rsfs.2016.0079 ◽

2017 ◽

Vol 7 (1) ◽

pp. 20160079 ◽

Cited By ~ 7

Author(s):

Alexander Widmann ◽

Cameron Tropea

Keyword(s):

Boundary Layer ◽

Reynolds Number ◽

Leading Edge ◽

Boundary Layer Separation ◽

Flow Structures ◽

Flat Plates ◽

Time Resolved ◽

Flow Phenomena ◽

Wall Interaction ◽

The Impact

The impact of chord-based Reynolds number on the formation of leading-edge vortices (LEVs) on unsteady pitching flat plates is investigated. The influence of secondary flow structures on the shear layer feeding the LEV and the subsequent topological change at the leading edge as the result of viscous processes are demonstrated. Time-resolved velocity fields are measured using particle image velocimetry simultaneously in two fields of view to correlate local and global flow phenomena in order to identify unsteady boundary-layer separation and the subsequent flow structures. Finally, the Reynolds number is identified as a parameter that is responsible for the transition in mechanisms leading to LEV detachment from an aerofoil, as it determines the viscous response of the boundary layer in the vortex–wall interaction.

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Wave-Induced Boundary Layer Separation in the Lee of the Medicine Bow Mountains. Part II: Numerical Modeling

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-14-0381.1 ◽

2015 ◽

Vol 72 (12) ◽

pp. 4865-4884 ◽

Cited By ~ 9

Author(s):

Vanda Grubišić ◽

Stefano Serafin ◽

Lukas Strauss ◽

Samuel J. Haimov ◽

Jeffrey R. French ◽

...

Keyword(s):

Boundary Layer ◽

Hydraulic Jump ◽

Rapid Change ◽

Dynamical Evolution ◽

Boundary Layer Separation ◽

Layer Separation ◽

Medicine Bow Mountains ◽

Short Period ◽

Lee Waves ◽

Lee Wave

Abstract Mountain waves and rotors in the lee of the Medicine Bow Mountains in southeastern Wyoming are investigated in a two-part paper. Part I by French et al. delivers a detailed observational account of two rotor events: one displays characteristics of a hydraulic jump and the other displays characteristics of a classic lee-wave rotor. In Part II, presented here, results of high-resolution numerical simulations are conveyed and physical processes involved in the formation and dynamical evolution of these two rotor events are examined. The simulation results reveal that the origin of the observed rotors lies in boundary layer separation, induced by wave perturbations whose amplitudes reach maxima at or near the mountain top. An undular hydraulic jump that gave rise to a rotor in one of these events was found to be triggered by midtropospheric wave breaking and an ensuing strong downslope windstorm. Lee waves spawning rotors developed under conditions favoring wave energy trapping at low levels in different phases of these two events. The upstream shift of the boundary layer separation zone, documented to occur over a relatively short period of time in both events, is shown to be the manifestation of a transition in flow regimes, from downslope windstorms to trapped lee waves, in response to a rapid change in the upstream environment, related to the passage of a short-wave synoptic disturbance aloft. The model results also suggest that the secondary obstacles surrounding the Medicine Bow Mountains play a role in the dynamics of wave and rotor events by promoting lee-wave resonance in the complex terrain of southeastern Wyoming.

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Rotor and Subrotor Dynamics in the Lee of Three-Dimensional Terrain

Journal of the Atmospheric Sciences ◽

10.1175/2007jas2352.1 ◽

2007 ◽

Vol 64 (12) ◽

pp. 4202-4221 ◽

Cited By ~ 66

Author(s):

James D. Doyle ◽

Dale R. Durran

Keyword(s):

Internal Structure ◽

Large Scale ◽

Three Dimensional ◽

Leading Edge ◽

Surface Friction ◽

Vortex Sheet ◽

Boundary Layer Separation ◽

Shear Instability ◽

Main Rotor ◽

Lee Wave

Abstract The internal structure and dynamics of rotors that form in the lee of topographic ridges are explored using a series of high-resolution eddy-resolving numerical simulations. Surface friction generates a sheet of horizontal vorticity along the lee slope that is lifted aloft by the mountain lee wave at the boundary layer separation point. Parallel-shear instability breaks this vortex sheet into small intense vortices or subrotors. The strength and evolution of the subrotors and the internal structure of the main large-scale rotor are substantially different in 2D and 3D simulations. In 2D, the subrotors are less intense and are ultimately entrained into the larger-scale rotor circulation, where they dissipate and contribute their vorticity toward the maintenance of the main rotor. In 3D, even for flow over a uniform infinitely long barrier, the subrotors are more intense, and primarily are simply swept downstream past the main rotor along the interface between that rotor and the surrounding lee wave. The average vorticity within the interior of the main rotor is much weaker and the flow is more chaotic. When an isolated peak is added to a 3D ridge, systematic along-ridge velocity perturbations create regions of preferential vortex stretching at the leading edge of the rotor. Subrotors passing through such regions are intensified by stretching and may develop values of the ridge-parallel vorticity component well in excess of those in the parent, shear-generated vortex sheet. Because of their intensity, such subrotor circulations likely pose the greatest hazard to aviation.

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Upstream boundary-layer separation in stratified flow

Journal of Fluid Mechanics ◽

10.1017/s002211207100185x ◽

1971 ◽

Vol 48 (4) ◽

pp. 791-800 ◽

Cited By ~ 4

Author(s):

John W. Miles

Keyword(s):

Reverse Flow ◽

Separation Bubble ◽

Dynamic Pressure ◽

Boundary Layer Separation ◽

Wave Mode ◽

Finite Amplitude ◽

Channel Height ◽

Lee Wave ◽

Limiting Case ◽

Thin Barrier

Stratified, inviscid channel flow over a thin barrier or into an abrupt contraction is considered on the hypotheses that the upstream dynamic pressure and density gradient are constant (Long's model) for those parametric régimes in which the hypotheses are tenable for finite-amplitude disturbances, namely k < 2 for the barrier and k < 1 for the contraction, where k = NH/πU is an inverse Froude number based on the Vaisälä frequency N, the channel height H, and the upstream velocity U. Reverse flow in the neighbourhood of the forward stagnation point, which implies the formation of an upstream separation bubble, is found for certain critical ranges of k. The maximum barrier height for which the dominant lee-wave mode can exist without reversed flow either upstream or downstream of the barrier is 0·34H. The limiting case of a half space is considered briefly, and forward separation is found for κ = Nh/U > κs, where κs = 2·05 for a thin barrier and 1·8 for a semi-circular barrier. The corresponding values for reverse flow in the lee-wave field are κc = 1·73 and 1·3, respectively.

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Investigation of Low Solidity LP Turbine Cascade With Flow Control: Part 1—Active Flow Control Using Jet-Flap

Volume 7: Turbomachinery, Parts A, B, and C ◽

10.1115/gt2010-22328 ◽

2010 ◽

Cited By ~ 1

Author(s):

Ping-Ping Chen ◽

Wei-Yang Qiao ◽

Hua-Ling Luo ◽

Farhan Ali Hashmi

Keyword(s):

Boundary Layer ◽

Flow Control ◽

Boundary Layer Separation ◽

Turbine Cascade ◽

Suction Surface ◽

Layer Separation ◽

Turbine Cascades ◽

Lp Turbine ◽

The Impact ◽

Highly Loaded

Increasing the airfoil lift and decreasing the solidity of turbine cascade are the effective ways to decrease blade count which lead to the reduction of weight and hardware cost of gas turbine in aircraft engine. The challenge with this effort is to prevent the flow separation on blade suction surface and to keep the efficiency at high levels. Recent investigations on the blade-flap have demonstrated dramatic reduction in the separation losses of turbine. It would be very attractive to integrate the blade-flap in the design of enhanced loaded turbine. The critical science that will enable this design innovation is a comprehensive understanding of the effect of flow control device on the boundary layer separation. The purpose of the present work was to investigate the impact of turbine cascade solidity on loss mechanisms (airfoil lift level) and to study the feasibility to develop low solidity and highly loaded LP turbine cascade blade using blade flap. This paper is the Part I of the study concerned with performance improvement of low solidity and highly loaded LP turbine cascade blade with jet-flap. The Part II is concerned with the Gurney-flap. Investigation on three turbine cascades with same type of airfoil but different solidity is presented in this paper. These turbine cascades are all constructed with the P&W LPTs highly loaded airfoil Pack B. Two dimensional steady Reynolds-averaged Navier-Stokes equations are solved for the flow of these cascades. It is shown that appropriate jet flap could decrease turbine cascade solidity about 12.5% without the considerable increase in loss, the flow deflection of the turbine cascade mainstream can be increased by jet-flap, and then contribute to increased blade loading. Because of the augmented deflection of the cascade mainstream, the flow velocity at suction side of the adjacent blade increases. This results in extension of the flow accelerating region and reduction of flow diffusion on the blade suction surface, consequently there is a delay in the boundary layer separation and/or makes the reattachment point advanced. In fact, the neighboring blade boundary layer flow is affected by the deflection of the mainstream, not on the flow of local boundary directly.

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