Separating and Reattaching Flow Structure in a Suddenly Expanding Rectangular Duct

1995 ◽  
Vol 117 (1) ◽  
pp. 17-23 ◽  
Author(s):  
G. Papadopoulos ◽  
M. V. O¨tu¨gen
Keyword(s):  
Aspect Ratio ◽  
Flow Structure ◽  
Three Dimensional ◽  
Side Wall ◽  
Stream Velocity ◽  
Rectangular Duct ◽  
Mean Flow ◽  
Wall Effects ◽  
Velocity Measurements ◽  
Aspect Ratios

The incompressible turbulent flow over a backward-facing step in a rectangular duct was investigated experimentally. The side wall effects on the core flow were determined by varying the aspect ratio (defined as the step span-to-height ratio) from 1 to 28. The Reynolds number, based on the step height and the oncoming free-stream velocity, was 26,500. Detailed velocity measurements were made, including the turbulent stresses, in a region which extended past the flow reattachment zone. Wall static pressure was also measured on both the step and flat walls. In addition, surface visualizations were obtained on all four walls surrounding the separated flow to supplement near-wall velocity measurements. The results show that the aspect ratio has an influence on both the velocity and wall pressure even for relatively large aspect ratios. For example, in the redevelopment region downstream of reattachment, the recovery pressure decreases with smaller aspect ratios. The three-dimensional side wall effects tend to slow down the relaxation downstream of reattachment for smaller aspect ratios as evidenced by the evolution of the velocity field. For the two smallest aspect ratios investigated, higher centerplane streamwise and transverse velocities were obtained which indicate a three-dimensional mean flow structure along the full span of the duct.

scholarly journals The wave-induced flow of internal gravity wavepackets with arbitrary aspect ratio

2017 ◽  
Vol 834 ◽  
pp. 385-408 ◽  
Author(s):  
T. S. van den Bremer ◽  
B. R. Sutherland
Keyword(s):  
Aspect Ratio ◽  
Three Dimensional ◽  
Mean Flow ◽  
One Dimensional ◽  
Unidirectional Flow ◽  
Aspect Ratios ◽  
Non Local ◽  
Flow Through ◽  
Induced Flow ◽  
Wave Induced

We examine the wave-induced flow of small-amplitude, quasi-monochromatic, three-dimensional, Boussinesq internal gravity wavepackets in a uniformly stratified ambient. It has been known since Bretherton (J. Fluid Mech., vol. 36 (4), 1969, pp. 785–803) that one-, two- and three-dimensional wavepackets induce qualitatively different flows. Whereas the wave-induced mean flow for compact three-dimensional wavepackets consists of a purely horizontal localized circulation that translates with and around the wavepacket, known as the Bretherton flow, such a flow is prohibited for a two-dimensional wavepacket of infinite spanwise extent, which instead induces a non-local internal wave response that is long compared with the streamwise extent of the wavepacket. One-dimensional (horizontally periodic) wavepackets induce a horizontal, non-divergent unidirectional flow. Through perturbation theory for quasi-monochromatic wavepackets of arbitrary aspect ratio, we predict for which aspect ratios which type of induced mean flow dominates. We compose a regime diagram that delineates whether the induced flow is comparable to that of one-, two- or compact three-dimensional wavepackets. The predictions agree well with the results of fully nonlinear three-dimensional numerical simulations.

Mean Wind Flow Field around Idealized Block Arrays with Various Aspect Ratios

2013 ◽  
Vol 393 ◽  
pp. 767-773 ◽  
Author(s):  
Azli Abd Razak ◽  
Aya Hagishima ◽  
Naoki Ikegaya ◽  
Mohd Faizal Mohamad ◽  
Sheikh Ahmad Zaki
Keyword(s):  
Aspect Ratio ◽  
Flow Structure ◽  
Urban Canopy ◽  
Streamwise Velocity ◽  
Area Ratio ◽  
Mean Flow ◽  
Canopy Layer ◽  
Mean Velocity ◽  
Aspect Ratios ◽  
Mean Wind Speed

This study investigates the characteristic of spatially averaged mean velocity profile and the flow pattern within urban canopy layer especially in pedestrian level using CFD technique. Large eddies simulation (LES) was used to perform a series of simulation of the flow around block arrays with staggered arrangement under various conditions of aspect ratio, αp (roof-to-frontal area ratio) from 0.33 to 3.0. The spatially-average profiles of both mean wind speed and streamwise velocity over various block arrays were compared with each other. The analysis clarified the following two facts. 1) The vertical mean flow structure inside the canyon change due to change a plan area ratio and block aspect ratio. 2) The horizontal mean flow structure around the block change if pedestrian level change form z = 0.05h to z = 0.25h. This can be translated to the effect of high-rise building to the flow around the building.

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.

Unravelling the large-scale circulation modes in turbulent Rayleigh–Bénard convection

2021 ◽  
Author(s):  
Susanne Horn ◽  
Peter J. Schmid ◽  
Jonathan M. Aurnou
Keyword(s):  
Large Scale ◽  
Three Dimensional ◽  
Dynamic Mode Decomposition ◽  
Dynamic Mode ◽  
Mean Flow ◽  
Large Scale Circulation ◽  
Convective Systems ◽  
Aspect Ratios ◽  
Mode Decomposition ◽  
Coherent Flow Structure

Abstract The large-scale circulation (LSC) is the most fundamental turbulent coherent flow structure in Rayleigh-B\'enard convection. Further, LSCs provide the foundation upon which superstructures, the largest observable features in convective systems, are formed. In confined cylindrical geometries with diameter-to-height aspect ratios of Γ ≅ 1, LSC dynamics are known to be governed by a quasi-two-dimensional, coupled horizontal sloshing and torsional (ST) oscillatory mode. In contrast, in Γ ≥ √2 cylinders, a three-dimensional jump rope vortex (JRV) motion dominates the LSC dynamics. Here, we use dynamic mode decomposition (DMD) on direct numerical simulation data of liquid metal to show that both types of modes co-exist in Γ = 1 and Γ = 2 cylinders but with opposite dynamical importance. Furthermore, with this analysis, we demonstrate that ST oscillations originate from a tilted elliptical mean flow superposed with a symmetric higher order mode, which is connected to the four rolls in the plane perpendicular to the LSC in Γ = 1 tanks.

Power Augmentation of a HAWT by Mie-type Tip Vanes, considering Wind Tunnel Flow Visualisation, Blade-Aspect Ratios and Reynolds Number

2003 ◽  
Vol 27 (3) ◽  
pp. 183-194 ◽  
Author(s):  
Yukimaru Shimizu ◽  
Edmond Ismaili ◽  
Yasunari Kamada ◽  
Takao Maeda
Keyword(s):  
Reynolds Number ◽  
Wind Tunnel ◽  
Aspect Ratio ◽  
Reynolds Numbers ◽  
Flow Visualisation ◽  
Velocity Measurements ◽  
Horizontal Axis Wind Turbine ◽  
Aspect Ratios ◽  
Detailed Surface ◽  
Blade Tips

Wind tunnel results are reported concerning the effects of blade aspect ratio and Reynolds number on the performance of a horizontal axis wind turbine (HAWT) with Mie-type1 tip attachments. The flow behaviour around the blade tips and the Mie-type tip vanes is presented. Detailed surface oil film visualization and velocity measurements around the blade tips, with and without Mie vanes, were obtained with the two-dimensional, Laser-Doppler Velocimetry method. Experiments were performed with rotors having blades with different aspect ratio and operating at different Reynolds numbers. The properties of the vortices generated by the Mie vanes and the blade tips were carefully studied. It was found that increased power augmentation by Mie vanes is achieved with blades having smaller aspect ratio and smaller Reynolds number.

Three-Dimensional Thermocapillary Convection in Open Cylindrical Annuli

2000 ◽  
Author(s):  
Bok-Cheol Sim ◽  
Abdelfattah Zebib
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Critical Frequency ◽  
Thermocapillary Convection ◽  
Three Dimensional ◽  
Time Dependent ◽  
Infinite Layer ◽  
Aspect Ratios ◽  
Temperature Oscillations ◽  
Good Agreement

Abstract Three-dimensional, time-dependent thermocapillary convection in open cylindrical containers is investigated numerically. Results for aspect ratios (Ar) of 1, 2.5, 8, and 16 and a Prandtl number of 6.84 are obtained to compare the results of numerical simulations with ongoing experiments. Convection is steady and axisymmetric at sufficiently low values of the Reynolds number (Re). Transition to oscillatory states occurs at critical values of Re which depend on Ar. With Ar = 1.0 and 2.5, we observe, respectively, 5 and 9 azimuthal wavetrains travelling clockwise at the free surface near the critical Re. With Ar = 8.0 and 16.0, there are substantially more, but pulsating waves near the critical Re. In the case of Ar = 16.0, which approaches the conditions in an infinite layer, our results are in good agreement with linear theory. While the critical Reynolds number decreases with increasing aspect ratio in the case of azimuthal rotating waves, it increases with increasing aspect ratio in the case of azimuthal pulsating waves. The critical frequency of temperature oscillations is found to decrease linearly with increasing Ar. We have also computed supercritical time-dependent states and find that while the frequency increases with increasing Re near the critical region, the frequency of supercritical convection decreases with Re.

The Effect of Aspect-Ratio on the Development of the Large-Scale Structures in the Three-Dimensional Wall Jet

2005 ◽  
Author(s):  
Joseph W. Hall ◽  
Daniel Ewing
Keyword(s):  
Aspect Ratio ◽  
Large Scale ◽  
Pressure Fluctuations ◽  
Three Dimensional ◽  
Decomposition Analysis ◽  
Wall Pressure ◽  
Symmetric Mode ◽  
Large Scale Structures ◽  
Aspect Ratios ◽  
Scale Structures

The development of the large-scale structures in three-dimensional wall jets exiting rectangular nozzles with aspect-ratios of 1 and 4 was investigated using simultaneous measurements of the fluctuating wall pressure across the jet. The pressure fluctuations in the jets were asymmetric and caused the fluctuating wall pressure to be poorly correlated across the jet centerline. A Proper Orthogonal Decomposition analysis indicated that both the first and second modes make similar contributions to the variance of the fluctuating pressure, and were symmetric and antisymmetric, respectively, and the interplay between these modes caused the asymmetry in the instantaneous pressure fluctuations across the jet centreline. A wavelet analysis of the instantaneously reconstructed pressure fields indicated that the fluctuations were predominantly in two frequency bands near the jet centerline, but were only contained in one band on the outer lateral edges of the jet, indicating there were two different large-scale motions present. The development of large-scale structures in the two jets initially differed in the intermediate field with the antisymmetric mode being more prominent in the square jet and the symmetric mode being more prominent in the larger aspect-ratio jet. Further downstream, the symmetric mode was more prominent in both jets.

scholarly journals Stability analysis of the elliptic cylinder wake

2014 ◽  
Vol 763 ◽  
pp. 302-321 ◽  
Author(s):  
Justin S. Leontini ◽  
David Lo Jacono ◽  
Mark C. Thompson
Keyword(s):  
Stability Analysis ◽  
Aspect Ratio ◽  
Three Dimensional ◽  
Elliptic Cylinder ◽  
Cylinder Wake ◽  
Long Wavelength ◽  
Aspect Ratios ◽  
Numerical Stability Analysis ◽  
Spatiotemporal Symmetries ◽  
Three Dimensional Simulations

AbstractThis paper presents the results of numerical stability analysis of the wake of an elliptical cylinder. Aspect ratios where the ellipse is longer in the streamwise direction than in the transverse direction are considered. The focus is on the dependence on the aspect ratio of the ellipse of the various bifurcations to three-dimensional flow from the two-dimensional Kármán vortex street. It is shown that the three modes present in the wake of a circular cylinder (modes A, B and QP) are present in the ellipse wake, and that in general they are all stabilized by increasing the aspect ratio of the ellipse. Two new pertinent modes are found: one long-wavelength mode with similarities to mode A, and a second that is only unstable for aspect ratios greater than approximately 1.75, which has similar spatiotemporal symmetries to mode B but has a distinct spatial structure. Results from fully three-dimensional simulations are also presented confirming the existence and growth of these two new modes in the saturated wakes.

scholarly journals Three-dimensional instabilities in compressible flow over open cavities

2008 ◽  
Vol 599 ◽  
pp. 309-339 ◽  
Author(s):  
GUILLAUME A. BRÈS ◽  
TIM COLONIUS
Keyword(s):  
Compressible Flow ◽  
Stokes Equations ◽  
Three Dimensional ◽  
Low Frequency ◽  
Vortical Flow ◽  
Cavity Flows ◽  
Two Dimensional ◽  
Mean Flow ◽  
Aspect Ratios ◽  
Open Cavities

Direct numerical simulations are performed to investigate the three-dimensional stability of compressible flow over open cavities. A linear stability analysis is conducted to search for three-dimensional global instabilities of the two-dimensional mean flow for cavities that are homogeneous in the spanwise direction. The presence of such instabilities is reported for a range of flow conditions and cavity aspect ratios. For cavities of aspect ratio (length to depth) of 2 and 4, the three-dimensional mode has a spanwise wavelength of approximately one cavity depth and oscillates with a frequency about one order of magnitude lower than two-dimensional Rossiter (flow/acoustics) instabilities. A steady mode of smaller spanwise wavelength is also identified for square cavities. The linear results indicate that the instability is hydrodynamic (rather than acoustic) in nature and arises from a generic centrifugal instability mechanism associated with the mean recirculating vortical flow in the downstream part of the cavity. These three-dimensional instabilities are related to centrifugal instabilities previously reported in flows over backward-facing steps, lid-driven cavity flows and Couette flows. Results from three-dimensional simulations of the nonlinear compressible Navier–Stokes equations are also reported. The formation of oscillating (and, in some cases, steady) spanwise structures is observed inside the cavity. The spanwise wavelength and oscillation frequency of these structures agree with the linear analysis predictions. When present, the shear-layer (Rossiter) oscillations experience a low-frequency modulation that arises from nonlinear interactions with the three-dimensional mode. The results are consistent with observations of low-frequency modulations and spanwise structures in previous experimental and numerical studies on open cavity flows.

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