Flow characteristics and shear-layer vortex shedding of double concentric jets

AIAA Journal ◽  
1997 ◽  
Vol 35 ◽  
pp. 887-892
Author(s):  
R. F. Huang ◽  
C. L. Lin
Keyword(s):  
Shear Layer ◽  
Vortex Shedding ◽  
Flow Characteristics

Flow Characteristics and Shear-Layer Vortex Shedding of Double Concentric Jets

AIAA Journal ◽  
1997 ◽  
Vol 35 (5) ◽  
pp. 887-892 ◽  
Author(s):  
R. F. Huang ◽  
C. L. Lin
Keyword(s):  
Shear Layer ◽  
Vortex Shedding ◽  
Flow Characteristics

Channel-Confined Wake Structure Interactions Between Two Permeable Side-By-Side Bars of a Square Cross-Section

2021 ◽  
Author(s):  
Saqib Jamshed ◽  
Amit Dhiman
Keyword(s):  
Cross Section ◽  
Vortex Shedding ◽  
Pressure Coefficient ◽  
Flow Characteristics ◽  
Darcy Number ◽  
Drag Forces ◽  
Coefficient Distribution ◽  
Wake Patterns ◽  
Lower Permeability ◽  
The Impact

Abstract The current research focuses on the laminar flow through permeable side-by-side bars of a square cross-section in a channel-confined domain. Vorticity generation on the leeward sides of the permeable bodies further necessitates the study for a better understanding of underlying physics. Reynolds number Re and Darcy number Da are varied from 5 to 150 and 10-6 to 10-2, respectively, at transverse gap ratios s/d=2.5-10. In the perspective of periodic unsteady flow, critical Re for the onset of vortex shedding is analyzed. Streamlines, vorticity, pressure coefficient distribution, and velocity profiles are discussed to identify the wake patterns. In lower permeability level, vortex-shedding from the permeable square cylinders is observed either in synchronized anti-phase mode or a single large vortex street with a synchronized in-phase pattern in the near wake. A steady-state wake pattern symmetric and flocked towards the centerline is observed for all s/d at a higher permeability level regardless of Re. Wake patterns are not altered for Da=10-6-10-3; instead, prompt extermination of the two vortex streets downstream is observed at Da=10-3 as compared to Da=10-6. The impact of s/d, Re, and permeability on the drag is examined. A jump in the flow characteristics and drag forces is noticed at higher Re for the mid-range Da remarkably at lower s/d. For the extent of high permeability, the drag coefficient asymptotically gets closer to zero.

scholarly journals A large-eddy simulation on a deep-stalled aerofoil with a wavy leading edge

2017 ◽  
Vol 813 ◽  
pp. 23-52 ◽  
Author(s):  
Rafael Pérez-Torró ◽  
Jae Wook Kim
Keyword(s):  
Vortex Shedding ◽  
Flow Characteristics ◽  
Leading Edge ◽  
Domain Size ◽  
Aerodynamic Characteristics ◽  
Streamwise Vortices ◽  
Aerodynamic Forces ◽  
Laminar Separation ◽  
Eddy Simulation ◽  
Large Eddy

A numerical investigation on the stalled flow characteristics of a NACA0021 aerofoil with a sinusoidal wavy leading edge (WLE) at chord-based Reynolds number $Re_{\infty }=1.2\times 10^{5}$ and angle of attack $\unicode[STIX]{x1D6FC}=20^{\circ }$ is presented in this paper. It is observed that laminar separation bubbles (LSBs) form at the trough areas of the WLE in a collocated fashion rather than uniformly/periodically distributed over the span. It is found that the distribution of LSBs and their influence on the aerodynamic forces is strongly dependent on the spanwise domain size of the simulation, i.e. the wavenumber of the WLE used. The creation of a pair of counter-rotating streamwise vortices from the WLE and their evolution as an interface/buffer between the LSBs and the adjacent fully separated shear layers are discussed in detail. The current simulation results confirm that an increased lift and a decreased drag are achieved by using the WLEs compared to the straight leading edge (SLE) case, as observed in previous experiments. Additionally, the WLE cases exhibit a significantly reduced level of unsteady fluctuations in aerodynamic forces at the frequency of periodic vortex shedding. The beneficial aerodynamic characteristics of the WLE cases are attributed to the following three major events observed in the current simulations: (i) the appearance of a large low-pressure zone near the leading edge created by the LSBs; (ii) the reattachment of flow behind the LSBs resulting in a decreased volume of the rear wake; and, (iii) the deterioration of von-Kármán (periodic) vortex shedding due to the breakdown of spanwise coherent structures.

scholarly journals Numerical Study of Flow Characteristics Around Confined Cylinder using OpenFOAM

2018 ◽  
Vol 7 (4.35) ◽  
pp. 617
Author(s):  
P. Mathupriya ◽  
L. Chan ◽  
H. Hasini ◽  
A. Ooi
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Numerical Study ◽  
Vortex Formation ◽  
Plane Channel ◽  
Flow Characteristics ◽  
Strong Interactions ◽  
Two Dimensional ◽  
Computational Fluid Dynamics Cfd ◽  
Shedding Frequency

The numerical study of the flow over a two-dimensional cylinder which is symmetrically confined in a plane channel is presented to study the characteristics of vortex shedding. The numerical model has been established using direct numerical simulation (DNS) based on the open source computational fluid dynamics (CFD) code named OpenFOAM. In the present study, the flow fields have been computed at blockage ratio, β of 0.5 and at Reynolds number, Re of 200 and 300. Two-dimensional simulations investigated on the effects of Reynolds number based on the vortex formation and shedding frequency. It was observed that the presence of two distinct shedding frequencies appear at higher Reynolds number due to the confinement effects where there is strong interactions between boundary layer, shear layer and the wake of the cylinder. The range of simulations conducted here has shown to produce results consistent with that available in the open literature. Therefore, OpenFOAM is found to be able to accurately capture the complex physics of the flow.

On vortex formation from a cylinder. Part 1. The initial instability

1988 ◽  
Vol 190 ◽  
pp. 491-512 ◽  
Author(s):  
M. F. Unal ◽  
D. Rockwell
Keyword(s):  
Reynolds Number ◽  
Kinetic Energy ◽  
Circular Cylinder ◽  
Shear Layer ◽  
Vortex Shedding ◽  
Vortex Formation ◽  
Absolute Instability ◽  
Near Wake ◽  
Lower Range ◽  
Fluctuation Amplitude

Vortex shedding from a circular cylinder is examined over a tenfold range of Reynolds number, 440 ≤ Re ≤ 5040. The shear layer separating from the cylinder shows, to varying degrees, an exponential variation of fluctuating kinetic energy with distance downstream of the cylinder. The characteristics of this unsteady shear layer are interpreted within the context of an absolute instability of the near wake. At the trailing-end of the cylinder, the fluctuation amplitude of the instability correlates well with previously measured values of mean base pressure. Moreover, this amplitude follows the visualized vortex formation length as Reynolds number varies. There is a drastic decrease in this near-wake fluctuation amplitude in the lower range of Reynolds number and a rapid increase at higher Reynolds number. These trends are addressed relative to the present, as well as previous, observations.

Reduced-order analysis of buffet flow of space launchers

2017 ◽  
Vol 815 ◽  
pp. 1-25 ◽  
Author(s):  
Vladimir Statnikov ◽  
Matthias Meinke ◽  
Wolfgang Schröder
Keyword(s):  
Shear Layer ◽  
Vortex Shedding ◽  
Separation Bubble ◽  
Dynamic Mode Decomposition ◽  
Recirculation Region ◽  
Dynamic Mode ◽  
Main Body ◽  
Reduced Order ◽  
Order Analysis ◽  
Wall Pressure Fluctuations

A reduced-order analysis based on optimized dynamic mode decomposition (DMD) is performed on the turbulent wake of a generic axisymmetric space launcher configuration computed via a zonal large-eddy simulation at the free stream Mach number $Ma_{\infty }=0.8$ and the Reynolds number based on the main body diameter $Re_{D}=6\times 10^{5}$ to investigate the buffet phenomenon. The transonic wake is characterized by an unsteady recirculation region occurring around the nozzle due to the separation of the turbulent boundary layer at the main body shoulder and subsequent dynamic interaction of the unstable free-shear layer with the nozzle surface. This results in strongly periodic and antisymmetric wall pressure fluctuations, for which three distinct frequency ranges are identified using conventional spectral analysis, i.e. $Sr_{D}\approx 0.1$, $Sr_{D}\approx 0.2$ and $Sr_{D}\approx 0.35$. For the spatially integrated side (buffet) loads on the nozzle, the second range is found to be energetically most dominant. To clarify the origin of the detected wake dynamics, the underlying spatio-temporal coherent modes are extracted using DMD. Subsequent analysis of the reduced-order modelled flow field based on the identified DMD modes reveals that at $Sr_{D}\approx 0.1$ a longitudinal cross-pumping motion of the separation bubble takes place, caused by a harmonic antisymmetric oscillation of the main recirculation vortex in the streamwise direction. At $Sr_{D}\approx 0.2$, a cross-flapping motion of the shear layer is determined, triggered by antisymmetric vortex shedding which is in phase with the cross-pumping motion such that it occurs at twice the frequency value. The last range of $Sr_{D}\approx 0.35$ is attributed to a swinging motion of the shear layer caused by a higher harmonic of the vortex shedding mode. Conclusively, the controversial aspect of the true three-dimensional shape of the antisymmetric mode at $Sr_{D}\approx 0.2$ that dominates the buffet phenomenon is scrutinized. Inclined elongated closed-loop vortices are identified that are shed in alternating sequence from azimuthally opposite positions in a longitudinal plane of symmetry that changes its momentary orientation irregularly, maintaining an axisymmetric time-averaged field and spatially isotropic buffet loads.

scholarly journals Experimental and Numerical Research on Flow-Induced Vibration in Valves

2017 ◽  
Vol 13 (2) ◽  
Author(s):  
Asmaa Ali Hussein
Keyword(s):  
Finite Element ◽  
Flow Rate ◽  
Vortex Shedding ◽  
Flow Characteristics ◽  
Operating Mode ◽  
Structural Vibration ◽  
Piping System ◽  
Flow Induced Vibration ◽  
Finite Element Software ◽  
Operation Conditions

Abstract   All central air conditioning systems contain piping system with various components, sizes, material, and layouts. If such systems in operating mode, the flow in piping system and its component such as valves can produce severe vibration due to some flow phenomenon’s. In this research, experimental measurements and numerical simulation are used to study the flow-induced vibration in valves. Computational fluid dynamics (CFD) concepts are included with one-way and two-way fluid-structure interaction concepts by using finite element software Package (ANSYS 14.57). Detection analysis is performed on flow characteristics under operation conditions and relations with structural vibration. Most of real geometrical, operational, and boundary conditions are simulated to obtain best similarity with real operation conditions. Comparisons performed between experimental data and numerical results (one-way and two-way simulation) to verify the results. The main conclusion was drawn from the study that the dominant source of vibration for valve is the water pulsation in addition to amount of water hammering. In addition, the main source of water pulsation in globe valve is the vortex shedding and pressure difference between upstream and downstream of valve. The vibration amplitude was increased with increasing flow rate until to be maximum when the flow rate around 30% and then decreased until flow rate reaches to around 85% and then trends to be constant. Keywords: Flow-induced vibration, vortex shedding, pressure pulsation, valve, finite element, ANSYS, fast Fourier transform (FFT).

scholarly journals Impact of Different Vegetation Zones on the Velocity and Discharge of Open-Channel Flow

2021 ◽  
Author(s):  
Xiaonan Tang ◽  
Yutong Guan ◽  
Yuxiang Hu
Keyword(s):  
Shear Layer ◽  
Management Practices ◽  
Open Channel ◽  
Flow Characteristics ◽  
Flow Region ◽  
Free Flow ◽  
Flow Process ◽  
Riparian Management ◽  
Free Zone ◽  
Rigid Vegetation

Different types of vegetation widely exist in rivers and wetlands. The vegetation will affect the ecological environment and flow process, thus becoming increasingly significant in river engineering and aquatic environmental management. Previous research on vegetated flow is mainly to understand the flow structure of open channels with fully covered one-layer vegetation. However, vegetation often grows along a river bank and co-exists in different heights. The present paper presents experimental results about the flow characteristics of an open-channel with two sides covered by differently layered vegetation, focusing on the effect of vegetation on the velocity distribution and discharge. Two heights of dowels in 10 cm and 20 cm were used to simulate rigid vegetation and arranged in a linear form on both sides of a channel bed under emergent and fully submerged flow conditions. The velocity at different positions was obtained using ADV (Acoustic Doppler Velocimetry). Measured results demonstrate that there exists a shear layer between free-flow and vegetated zones, indicating that the flow transition occurs between fast-moving flow in the free zone and slowly obstructed flow in the vegetated zone and induces a high shear layer and transverse coherent vortices near the interface. Furthermore, compared with the emergent condition, the discharge through the free-flow region slightly decreases under full submerged conditions while the discharge in the vegetated region increases, indicating that the vegetation does not significantly change the discharge percentage in the free region. These findings on differently-layered vegetation would help riparian management practices to maintain healthy ecological and habitat zones.

CFD Simulations of Helical Strakes Reducing Vortex Induced Motion of a Semi-Submersible

2018 ◽  
Author(s):  
Jiawei He ◽  
Decheng Wan ◽  
Zhiqiang Hu
Keyword(s):  
Vortex Shedding ◽  
Flow Characteristics ◽  
Boundary Region ◽  
Detached Eddy Simulation ◽  
Cfd Simulations ◽  
Mooring Lines ◽  
Boundary Area ◽  
Eddy Simulation ◽  
Helical Strakes ◽  
Induced Motion

This paper describes a set of VIM CFD simulations for a semi-submersible with and without helical strakes. The numerical investigations are conducted under low Reynolds number (Re) using naoe-FOAM-SJTU, a solver developed based on the open source framework OpenFOAM. The self-developed six degree-of-freedom (6DoF) motion module and mooring system module are applied to model motions of semi-submersible and the constraint of mooring lines, respectively. To carry out the calculations, turbulence closure has been chosen the Shear Stress Transport (SST) based Delay Detached eddy simulation (DDES), which uses the RANS model inside the boundary region and LES model outside the boundary area. This allows a realistic simulation within the boundary region where the vortex shedding is taking place, while not using unnecessary amounts of computational power. The Vortex Induced Motion (VIM) of semi-submersible with and without helical strakes was compared against each other for different reduced velocities (Ur). The flow characteristics of the semi-submersible platform is studied based on the characteristics of vortex shedding. For different current incident angles, time histories, trajectories and vorticity of the semi-submersible at different reduced velocities are reported. The result shows our CFD solver naoe-FOAM-SJTU is applicable and reliable to study VIM of semi-submersibles.

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