scholarly journals Near-Field Flow Structure and Entrainment of a Round Jet at Low Exit Velocities: Implications on Microclimate Ventilation

Computation ◽  
2020 ◽  
Vol 8 (4) ◽  
pp. 100
Author(s):  
Alan Kabanshi
Keyword(s):  
Flow Structure ◽  
Nozzle Exit ◽  
Near Field ◽  
Jet Flow ◽  
Field Vector ◽  
Statistical Characteristics ◽  
Round Jets ◽  
Nozzle Exit Velocity ◽  
Development And Validation ◽  
Ventilation Systems

This paper explores the flow structure, mean/turbulent statistical characteristics of the vector field and entrainment of round jets issued from a smooth contracting nozzle at low nozzle exit velocities (1.39–6.44 m/s). The motivation of the study was to increase understand of the near field and get insights on how to control and reduce entrainment, particularly in applications that use jets with low-medium momentum flow like microclimate ventilation systems. Additionally, the near field of free jets with low momentum flow is not extensively covered in literature. Particle image velocimetry (PIV), a whole field vector measurement method, was used for data acquisition of the flow from a 0.025 m smooth contracting nozzle. The results show that at low nozzle exit velocities the jet flow was unstable with oscillations and this increased entrainment, however, increasing the nozzle exit velocity stabilized the jet flow and reduced entrainment. This is linked to the momentum flow of the jet, the structure characteristics of the flow and the type or disintegration distance of vortices created on the shear layer. The study discusses practical implications on microclimate ventilation systems and at the same time contributes data to the development and validation of a planned computational turbulence model for microclimate ventilation.

Instability Modal Behavior of the Acoustically Excited Impinging Plane Jet With a Small Cylinder

2004 ◽  
Vol 20 (2) ◽  
pp. 145-157 ◽  
Author(s):  
Fei-Bin Hsiao ◽  
I-Che Hsu ◽  
Cheng-Chiang Hsu
Keyword(s):  
Flow Field ◽  
Coherent Structures ◽  
Nozzle Exit ◽  
Near Field ◽  
Jet Flow ◽  
Core Region ◽  
Far Field ◽  
Potential Core ◽  
Modal Behavior ◽  
Small Cylinder

AbstractThe Instability modal behavior of coherent structures in a jet-small cylinder impinging flow field is extensively studied by hot-wire anemometry measurements. The free jet is employed with a small cylinder of 3 mm in diameter located in the potential core region at the impinging length of L/H = 1.5 for the near field impingement and L/H = 4 for the far field impingement. The jet exit velocity is operated at 10 m/sec with the Reynolds number of 1.03 × 104 based on the nozzle exit width H = 15mm. The impinging jet is locally excited at the nozzle exit with varicose mode (m =0) and sinuous mode (m = 1) disturbances at the fundamental frequency of the natural jet flow. Data indicate that the jet flow is greatly altered and significantly enhanced by strengthening the coherent structures of the flow due to resonance according to the feedback mechanism. Although the original natural jet preferably exhibits the varicose mode, the strong sinuous mode is dominant in the flow field owing to the presence of the small cylinder in the potential core region. In the near field impingement, the wake region behind the cylinder preserves the pure sinuous mode to where the jet vortices merge and then mildly fades out. Whereas in the jet shear layer, the sinuous mode exists in the initial portion and gradually transforms to the varicose mode. In the far field impingement, the alternate mode dominates in each frequency stage in pure impinging case and the modal behavior follows the selected mode with the introducing acoustic waves in the acoustic excitation cases.

Effect of momentum flux distribution on multiple round jets

2018 ◽  
Vol 90 (2) ◽  
pp. 452-460 ◽  
Author(s):  
Kannan B.T. ◽  
Panchapakesan N.R.
Keyword(s):  
Flow Field ◽  
Nozzle Exit ◽  
Momentum Flux ◽  
Flux Distribution ◽  
Near Field ◽  
Secondary Flows ◽  
Positive Pressure ◽  
Content Type ◽  
Round Jets ◽  
Air Jet

Purpose This study aims to investigate the effects of nozzle momentum flux distribution on the flow field characteristics. Design/methodology/approach The nozzle configuration consists of a central nozzle surrounded by four nozzles. All nozzles have the same diameter and constant separation between nozzles. OpenFOAM® is used for simulating the jet flow. Reynolds-averaged Navier-Stokes (RANS) equations are solved iteratively with a first-order closure for turbulence. Pitot-static tube with differential pressure transducer is used for mean velocity measurements. The comparison of computed results with experimental data shows similar trend and acceptable validation. Findings According to the results, the momentum flux distribution significantly alters the near field of multiple turbulent round jets. Highly non-linear decay region in the near field is found for the cases having higher momentum in the outer jets. As a result of merging, increased positive pressure is found in the mixing region. Higher secondary flows and wider mixing region are reported as a result of momentum transfer from axial to lateral directions by Reynolds stresses. Research limitations/implications The present study is limited to isothermal flow of air jet in air medium. Social implications Optimum momentum flux distribution in multijet injector of a combustor can reap better mixing leading to better efficiency and lesser environmental pollution. Originality/value As summary, the contributions of this paper in the field of turbulent jets are following: simulations for various momentum distribution cases have been performed. In all the cases, the flow at the nozzle exit is subsonic along with constant velocity profile. To simulate proper flow field, a large cylinder-type domain with structured grid is used with refinements toward the nozzle exit and jet axis. The results show that the non-linearity increases with increase in momentum of outer jets. Longer merging zones are reported for cases with higher momentum in outer nozzles using area-averaged turbulent kinetic energy. Similarly, wider mixing regions are reported using secondary flow parameter and visualizations.

PULSE RADIOGRAPHY INVESTIGATION OF LIQUID JET FLOW STRUCTURE IN A NONTRANSPARENT MEDIUM

1993 ◽  
Vol 1 (1) ◽  
pp. 69-73
Author(s):  
V. L. Ovsiannikov ◽  
A. R. Berngardt ◽  
E. I. Pal'chikov
Keyword(s):  
Flow Structure ◽  
Jet Flow ◽  
Liquid Jet

scholarly journals Flow structure and near-field dispersion in arrays of building-like obstacles

2014 ◽  
Vol 125 ◽  
pp. 52-68 ◽  
Author(s):  
Omduth Coceal ◽  
Elisa V. Goulart ◽  
Simon Branford ◽  
T. Glyn Thomas ◽  
Stephen E. Belcher
Keyword(s):  
Flow Structure ◽  
Near Field
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