A Novel Vibrating Ribbon Technique

2005 ◽  
Author(s):  
Jonathan H. Watmuff
Keyword(s):  
Test Section ◽  
Vibration Amplitude ◽  
Wave Amplitude ◽  
Separation Bubble ◽  
Wind Tunnel Test ◽  
Mean Flow ◽  
Test Plate ◽  
Numerical Predictions ◽  
The Mean

A novel vibrating ribbon apparatus is described that is active over the full span of a wind tunnel test section. The spanwise uniformity of the vibration amplitude and other ribbon characteristics are considered in detail. The height of each end of the ribbon above the test plate can be adjusted in situ, while the ribbon is vibrating and with flow in the test section, thereby allowing the response of the layer to be easily tuned. The growth of the wave amplitude downstream of the ribbon is shown to agree with numerical predictions. However, two or three wavelengths of development are required before the wave amplitude follows the predicted growth. The flow around an inactive ribbon is examined using a commercial CFD solver and features such as a miniature separation bubble just downstream of the ribbon are revealed. The distance required for the mean flow to recover from the disturbance introduced by the ribbon is greater when the ribbon is located further from the wall. The mean flow recovers to form a boundary layer that is slightly thicker than the undisturbed flow. Experimental measurements indicate that the distance required for the wave motions to follow predicted behavior is about 4 or 5 times larger than distance for recovery of the mean flow.

scholarly journals Investigation of dominant traveling 10-day wave components using long-term MERRA-2 database

2021 ◽  
Vol 73 (1) ◽  
Author(s):  
Chunming Huang ◽  
Wei Li ◽  
Shaodong Zhang ◽  
Gang Chen ◽  
Kaiming Huang ◽  
...  
Keyword(s):  
Flow Instability ◽  
Transmission Condition ◽  
Wave Number ◽  
Mean Flow ◽  
Excited Wave ◽  
Propagating Waves ◽  
Zonal Wave Number ◽  
The Mean

AbstractThe eastward- and westward-traveling 10-day waves with zonal wavenumbers up to 6 from surface to the middle mesosphere during the recent 12 years from 2007 to 2018 are deduced from MERRA-2 data. On the basis of climatology study, the westward-propagating wave with zonal wave number 1 (W1) and eastward-propagating waves with zonal wave numbers 1 (E1) and 2 (E2) are identified as the dominant traveling ones. They are all active at mid- and high-latitudes above the troposphere and display notable month-to-month variations. The W1 and E2 waves are strong in the NH from December to March and in the SH from June to October, respectively, while the E1 wave is active in the SH from August to October and also in the NH from December to February. Further case study on E1 and E2 waves shows that their latitude–altitude structures are dependent on the transmission condition of the background atmosphere. The presence of these two waves in the stratosphere and mesosphere might have originated from the downward-propagating wave excited in the mesosphere by the mean flow instability, the upward-propagating wave from the troposphere, and/or in situ excited wave in the stratosphere. The two eastward waves can exert strong zonal forcing on the mean flow in the stratosphere and mesosphere in specific periods. Compared with E2 wave, the dramatic forcing from the E1 waves is located in the poleward regions.

Turbulent Flow Around a Bluff Rectangular Plate. Part II: Numerical Predictions

1991 ◽  
Vol 113 (1) ◽  
pp. 60-67 ◽  
Author(s):  
N. Djilali ◽  
I. S. Gartshore ◽  
M. Salcudean
Keyword(s):  
Rectangular Plate ◽  
Marked Improvement ◽  
Separation Bubble ◽  
Reattachment Length ◽  
Mean Flow ◽  
Discretization Methods ◽  
Streamline Curvature ◽  
Pressure Distributions ◽  
Numerical Predictions ◽  
Good Agreement

This paper presents calculations of the time-averaged separated-reattaching flow around a bluff rectangular plate, using a finite difference procedure and the k-ε turbulence model. Two discretization methods are used: the hybrid differencing scheme, and the bounded skew hybrid differencing scheme. The latter, although superior to the former for all grid distributions, results in a reattachment length about 30 percent shorter than the measured value. When a modification which takes into account streamline curvature is incorporated into the k-ε model, a marked improvement in the predictions is obtained. A reattachment length of 4.3 plate thicknesses (D), compared to an experimental value of 4.7D, is obtained, and the predicted mean flow field, turbulent kinetic energy and pressure distributions within the separation bubble are found to be in good agreement with experiments.

scholarly journals Strong Intraseasonal Variability of Meridional Currents near 5°N in the Eastern Indian Ocean: Characteristics and Causes

2017 ◽  
Vol 47 (5) ◽  
pp. 979-998 ◽  
Author(s):  
Gengxin Chen ◽  
Weiqing Han ◽  
Yuanlong Li ◽  
Michael J. McPhaden ◽  
Ju Chen ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Rossby Waves ◽  
Intraseasonal Variability ◽  
Upper Ocean ◽  
Mean Flow ◽  
Eastern Boundary ◽  
Typical Period ◽  
The Indian Ocean ◽  
The Mean

AbstractThis paper reports on strong, intraseasonal, upper-ocean meridional currents observed in the Indian Ocean between the Bay of Bengal (BOB) and the equator and elucidates the underlying physical processes responsible for them. In situ measurements from a subsurface mooring at 5°N, 90.5°E reveal strong intraseasonal variability of the meridional current with an amplitude of ~0.4 m s−1 and a typical period of 30–50 days in the upper 150 m, which by far exceeds the magnitudes of the mean flow and seasonal cycle. Such prominent intraseasonal variability is, however, not seen in zonal current at the same location. Further analysis suggests that the observed intraseasonal flows are closely associated with westward-propagating eddylike sea surface height anomalies (SSHAs) along 5°N. The eddylike SSHAs are largely manifestations of symmetric Rossby waves, which result primarily from intraseasonal wind stress forcing in the equatorial waveguide and reflection of the equatorial Kelvin waves at the eastern boundary. Since the wave signals are generally symmetric about the equator, similar variability is also seen at 5°S but with weaker intensity because of the inclined coastline at the eastern boundary. The Rossby waves propagate westward, causing pronounced intraseasonal SSHA and meridional current in the upper ocean across the entire southern BOB between 84° and 94°E. They greatly weaken in the western Indian Basin, but zonal currents near the equator remain relatively strong.

 Generation of planetary waves by gravity-wave drag in the middle atmosphere

2021 ◽  
Author(s):  
Hye-Yeong Chun ◽  
Byeong-Gwon Song ◽  
In-Sun Song
Keyword(s):  
Large Scale ◽  
Middle Atmosphere ◽  
Weather Forecasting ◽  
Wave Theory ◽  
Wave Drag ◽  
Planetary Waves ◽  
Mean Flow ◽  
Long Time ◽  
The Mean

<p>Large-scale atmospheric circulation has been represented mostly by interaction between the mean flow and planetary waves (PWs). Although the importance of gravity waves (GWs) has been recognized for long time, contribution of GWs to the large-scale circulation is receiving more attention recently, with conjunction to GW drag (GWD) parameterizations for climate and global weather forecasting models that extend to the middle atmosphere. As magnitude of GWD increases with height significantly, circulations in the middle atmosphere are determined largely by interactions among the mean flow, PWs and GWs. Classical wave theory in the middle atmosphere has been represented mostly by the Transformed Eulerian Mean (TEM) equation, which include PW and GW forcing separately to the mean flow. Recently, increasing number of studies revealed that forcing by combined PWs and GWs is the same, regardless of different PW and GW forcings, implying a compensation between PWs and GWs forcing. There are two ways for GWs to influence on PWs: (i) changing the mean flow that either influences on waveguide of PWs or induces baroclinic/brotropic instabilities to generate in situ PWs, and (ii) generating PWs as a source of potential vorticity (PV) equation when asymmetric components of GWD exist. The fist mechanism has been studies extensively recently associated with stratospheric sudden warmings (SSWs) that are involved large amplitude PWs and GWD. The second mechanism represents more directly the relationship between PWs and GWs, which is essential to understand the dynamics in the middle atmosphere completely (among the mean flow, PWs and GWs). In this talk, a recently reported result of the generation of PWs by GWs associated with the strongest vortex split-type SSW event occurred in January 2009 (Song et al. 2020, JAS) is presented focusing on the second mechanism.  </p>

Explicit Algebraic Reynolds-stress Modeling of Pressure-induced Separating Flows in the Presence of Sidewalls

2021 ◽  
Author(s):  
Abdelouahab Mohammed Taifour ◽  
Julien Weiss ◽  
Louis Dufresne
Keyword(s):  
Reynolds Stress ◽  
Test Section ◽  
Three Dimensional ◽  
Wind Tunnel Test ◽  
Shear Stresses ◽  
Mean Flow ◽  
Flow Topology ◽  
Three Dimensional Flow ◽  
Dimensional Flow ◽  
Good Agreement

Abstract RANS approach is used to simulate the steady state of a family of pressure-induced turbulent separation bubbles in the presence of sidewalls. Different turbulence models are employed with a specific emphasis on the BaSeLine Explicit Algebraic Reynolds Stress Model (BSL-EARSM) and the simulations are compared with experimental data. The separation and reattachment of a flat-plate turbulent boundary layer is generated through a combination of adverse and favorable pressure gradients (APG-FPG) by numerically reproducing the geometry of the wind-tunnel test section used for the experiments. Three cases are considered, a large (LB) and a medium (MB) bubble presenting mean backflow, and a small bubble (SB) without mean-flow reversal. This is achieved by varying the streamwise position of the APG/FPG transition. Good agreement between the BSL-EARSM-computed solutions and the experimental results are obtained for wall-pressure and skin-friction distributions on the centerline plane of the test section as well as for the overall three-dimensional flow topology. However, both detachment and reattachment are predicted too early and the bubble length is slightly overestimated for Cases LB and MB. For Case LB, the streamwise Reynolds stress is estimated fairly well but its production is somewhat delayed. Normal and shear stresses are in good agreement with the experiments in the upstream part of the bubble but are significantly over-estimated in the reattachment region. The k ?? ! Shear-Stress Transport (SST) model with the so-called reattachment modification performs better than the other tested linear-eddy-viscosity models but it is still unable to reproduce accurately the three-dimensional flow topology even for the 'simplest' case SB. Overall, the results suggest that BSL-EARSM is the most suitable turbulence model for this flow configuration.

scholarly journals Quantifying the Airflow Distortion over Merchant Ships. Part I: Validation of a CFD Model

2006 ◽  
Vol 23 (3) ◽  
pp. 341-350 ◽  
Author(s):  
Bengamin I. Moat ◽  
Margaret J. Yelland ◽  
Robin W. Pascal ◽  
Anthony F. Molland
Keyword(s):  
Wind Speed ◽  
Charles Darwin ◽  
Reverse Direction ◽  
Cfd Model ◽  
Mean Flow ◽  
Flow Distortion ◽  
Flux Measurements ◽  
Computational Fluid Dynamics Cfd ◽  
The Mean

Abstract The effects of flow distortion created by the ship’s hull and superstructure bias wind speed measurements made from anemometers located on ships. Flow distortion must be taken into account if accurate air–sea flux measurements are to be achieved. Little work has been undertaken to examine the wind speed bias due to flow distortion in wind speed reports from voluntary observing ships (VOS). In this first part of a two-part paper the accuracy of the computational fluid dynamics (CFD) code VECTIS in simulating the airflow over VOS is investigated. Simulations of the airflow over a representation of the bridge of a VOS are compared to in situ wind speed measurements made from six anemometers located above the bridge of the RRS Charles Darwin. The ship’s structure was ideal for reproducing the flow over VOS when the wind is blowing onto either beam. The comparisons showed VECTIS was accurate to within 4% in predicting the wind speed over ships, except in extreme cases such as wake regions or the region close to the bridge top where the flow may be stagnant or reverse direction. The study showed that there was little change in the numerically predicted flow pattern above the bridge with change in Reynolds number between 2 × 105 and 1 × 107. The findings showed that the CFD code VECTIS can reliably be used to determine the mean flow above typical VOS.

Numerical Predictions for the Flow Induced by an Enclosed Rotating Disc

1988 ◽  
Author(s):  
John W. Chew ◽  
Craig M. Vaughan
Keyword(s):  
Experimental Data ◽  
Reasonable Agreement ◽  
Mixing Length ◽  
Mean Flow ◽  
Rotating Disc ◽  
Numerical Predictions ◽  
Stationary Disc ◽  
The Mean ◽  
Radial Outflow ◽  
Good Agreement

Finite difference solutions are presented for turbulent flow in the cavity formed between a rotating and a stationary disc, with and without a net radial outflow of fluid. The mean flow is assumed steady and axisymmetric and a mixing length model of turbulence is used. Grid dependency of the solutions is shown to be acceptably small and results are compared with other workers’ experimental data. Theoretical and measured disc moment coefficients are in good agreement, while theoretical and measured velocities are in reasonable agreement. It is concluded that the mixing-length model is sufficiently accurate for many engineering calculations of boundary layer dominated flows in rotating disc systems.

An Experimental Study of the Mean Flow and Turbulence Structure Of Cross-Flow Over Tube Bundles

1996 ◽  
Vol 210 (4) ◽  
pp. 317-331 ◽  
Author(s):  
S Balabani ◽  
M Yianneskis
Keyword(s):  
Pressure Drop ◽  
Laser Doppler Anemometry ◽  
Cross Flow ◽  
Turbulence Structure ◽  
Mean Flow ◽  
Time Resolved ◽  
Numerical Predictions ◽  
Dissipation Rates ◽  
Tube Bundles ◽  
The Mean

The velocity characteristics of cross-flow over tube bundles were investigated in a water tunnel. Three tube arrays with a transverse pitch ratio of 3.6 were studied: an in-line and two staggered arrays with longitudinal pitch ratios of 2.1, 1.6 and 2.1 respectively. The mean velocities, turbulence levels, spectra, time and length scales and dissipation rates were determined from ensemble-averaged and time-resolved laser Doppler anemometry (LDA) measurements. The pressure drop across the bundles was also measured. The staggered arrays were found to generate higher levels of turbulence and a higher pressure drop. Turbulence kinetic energy reaches a maximum downstream of the second row in staggered arrays. The wake regions in both geometries are anisotropic with transverse r.m.s. velocities being higher than axial ones. Increasing the longitudinal spacing in the staggered configuration results in lower r.m.s. levels in the wakes and alteration of the recirculation characteristics. A discrete periodicity with a Strouhal number of 0.26 was identified in the 3.6 times 1.6 staggered array which is associated with vortex shedding. Turbulence scales and dissipation rates were estimated and compared with numerical predictions.

Coherent Structures In Open-Channel Flows Over a Fixed Dune

2005 ◽  
Vol 127 (5) ◽  
pp. 858-864 ◽  
Author(s):  
Wusi Yue ◽  
Ching-Long Lin ◽  
Virendra C. Patel
Keyword(s):  
Free Surface ◽  
Coherent Structures ◽  
Open Channel ◽  
Surface Deformation ◽  
Channel Flows ◽  
Mean Flow ◽  
Open Channel Flows ◽  
Near Surface ◽  
Numerical Predictions ◽  
The Mean

Turbulent open-channel flow over a two-dimensional laboratory-scale dune is studied using large eddy simulation. Free surface motion is simulated using level set method. Two subgrid scale models, namely, dynamic Smagorinsky model and dynamic two-parameter model, are employed for assessing model effects on the free surface flow. The present numerical predictions of mean flow field and turbulence statistics are in good agreement with experimental data. The mean flow can be divided into two zones, an inner zone where turbulence strongly depends on the dune bed geometry and an outer layer free from the direct influence of the bed geometry. Streaky structures are observed in the wall layer after flow reattachment. Quadrant two events are found to prevail in near-wall and near-surface motions, indicating the predominance of turbulence ejections in open-channel flows. Large-scale coherent structures are produced behind the dune crest by a strong shear layer riding over the recirculation zone. These quasistreamwise tubelike vortical structures are transported downstream with the mean flow and most are destructed before arriving at the next crest. Free surface deformation is visualized, demonstrating complex patterns of upwelling and downdraft.

Effects of Streamwise Aspect Ratio on Turbulent Flows Over Forward-Backward Facing Steps

2020 ◽  
Author(s):  
Heath Chalmers ◽  
Xingjun Fang ◽  
Mark F. Tachie
Keyword(s):  
Boundary Layer ◽  
Aspect Ratio ◽  
Turbulent Boundary Layer ◽  
Turbulent Flows ◽  
Separation Bubble ◽  
Step Height ◽  
Stream Velocity ◽  
Mean Flow ◽  
Aspect Ratios ◽  
The Mean

Abstract Separated and reattached turbulent flows induced by two-dimensional forward-backward-facing steps with different streamwise lengths submerged in a thick turbulent boundary layer are investigated using a time-resolved particle image velocimetry. The examined aspect ratios of the step range from 1 to 8, and the Reynolds number based on the free-stream velocity and step height is 13 200. The thickness of the oncoming turbulent boundary layer is 6.5 times the step height. The effects of varying aspect ratio of the steps on the mean flow, Reynolds stresses, triple correlations and unsteadiness of turbulent separation bubbles are studied. It was found that the mean flow reattaches over the step for forward-backward facing steps with aspect ratios of 2 and higher. The temporal variation of the first proper orthogonal decomposition (POD) mode and reverse flow area, which is used to examine the flapping motion of separation bubble, shows remarkable synchronization.

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