Flow structures in initial region of two interacting parallel plane jets

doi:10.1016/0894-1777(89)90006-X

Experimental Thermal and Fluid Science

Volume 2, Issue 4, October 1989, Pages 431-449

https://doi.org/10.1016/0894-1777(89)90006-X Get rights and content

Abstract

The initial region of two interacting parallel plane jets with a truncated bluff body at the plane of symmetry is studied with velocity measurements in the frequency and time domains. The mean flows of the two plane jets are nearly parallel to the plane of symmetry, and their inner and outer mixing regions are found. Within these mixing regions their respective trains of coherent structures, which are of vortical form, are established. The inner vortices are inwardly rotational, while the outer vortices rotate outwardly. The successive initial vortices in the inner mixing region seem to undergo two processes of either pairing or amalgamation. The former results in the formation of a nearly circular coherent structure, while the latter results in the elongated structure. Both the inner and other structures experience fairly rapid decay. The process of decay also seems to be either the usual decay or the division of the coalesced structure back into the individual vortical structures followed by their decay.

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There are more references available in the full text version of this article.

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The present computational study aims to investigate and improve the turbulent dual jet cooling performance on an undulated surface with varying profiles. This study also aims to highlight the flow and thermal features arising owing to an undulated surface having amplitude (UA) between 0.0 and 0.7. The transient RANS equations are considered to simulate the complex behaviour of turbulent dual jet. Four Reynolds-Averaged Navier-Stokes (RANS) turbulence models, namely, standard k-ω, shear-stress transport (SST) k-ω, renormalization group (RNG) k-ϵ, and realizable k-ϵ models, are used to predict the thermal performance and flow field of a turbulent dual jet for a fixed Reynolds number (Re) of 10,000 and offset ratio of 2. A constant wall temperature and constant wall heat flux boundary conditions are used. For the amplitude between 0.0 and 0.1, a periodic vortex shedding phenomenon near the jet exit is observed. However, two stable counter-rotating vortices are observed with no periodic vortex shedding for higher values of UA ≥ 0.3. The axial position of the merge point reduces with a rise in the amplitude. It is also observed that the velocity profile becomes self-similar and the inflection point in the velocity profile rises near the wall region with amplitude. The time series of U and V velocity signals unveil sinusoidal oscillations and display almost the same peak to peak amplitude for UA ≤ 0.1 at various times. The FFT of U velocity signals displays a solo peak dominant frequency which is the same as shown by V velocity in the FFT plot corresponding to the Strouhal number of the vortex shedding phenomenon. The average Nusselt number is increased up to 63.89% and 65.03% for higher amplitude for the constant heat flux and constant temperature boundary conditions. Correlations have been also proposed for the average Nusselt number, the average heat flux and the merge point. The outcomes of the present research can be implemented for better design in automobile industries and utilization of cooling or heating jets in electronics, metal, and material processing.
Effect of excitation Strouhal number on velocity field and mixing capability of acoustically excited dual jets
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Citation Excerpt :
The entrainment and spreading rates in the combined region were larger than those in the single free jet, which is attributable to significant increases in turbulence intensity and Reynolds shear stress relative to the single jet. The lateral distributions of mean velocities and cross-spectra of velocity fluctuations in the initial region of two parallel plane jets were examined by Ko and Lau (1989). Two trains of coherent structures were observed within the inner and outer mixing regions.
The present study experimentally investigated the effect of the excitation Strouhal number on the velocity field and mixing capability of excited dual jets. The dual jets were acoustically excited by a loudspeaker. The velocity pulsation at the jet exit was examined using a hotwire anemometer. The evolution process of the excited dual jets was captured using the smoke flow visualization technique. The jet spread width was measured from a long-exposure image by using the edge-detection method. The time-averaged velocity fields were measured using the particle image velocimetry (PIV) technique. The mixing capability was examined by tracer-gas concentration detection. Two characteristic flow modes, namely the bifurcated and parallel vortices, were classified in the domain of pulsation intensity and excitation Strouhal number. The excitation Strouhal number dominated the change in the characteristic flow modes. The leading vortices induced jet flow bifurcation in the bifurcated vortices mode, while the vortices broke up early in the parallel vortices mode. The recirculation region, merging length, combined length, jet spread, and later distribution of the dual jets excited in the bifurcated mode were larger than those in the parallel vortices mode. The tracer-gas value in the bifurcated vortices mode was smaller than that in the parallel vortices mode; therefore, the mixing capability in the bifurcated vortices mode was better than that in the parallel vortices mode. The unexcited and excited dual jets were compared to evaluate the effect of acoustic excitation on the flow and mixing characteristics.
Effects of pulsation intensity on the flow and dispersion of pulsed dual plane jets
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Citation Excerpt :
The peak carbon dioxide concentration values decreased to about 0.6% at Ip = 1.6. At y/d = 12 (Fig. 14(b)), the carbon-dioxide concentrations of the non-pulsed jets exhibited a large central peak of about 4.5% because the axial level was in the merging region between the merging and combined points [9,10,17,36,52–55]. The carbon dioxide concentrations of the pulsed jets were smaller than those at y/d = 6.
The effects of jet pulsation intensity on the flow and dispersion characteristics of dual parallel plane jets were experimentally studied. The jet pulsations were generated by acoustic excitations. The laser-light-sheet-assisted smoke flow visualization method was used to study the flow evolution processes. The lateral spreading characteristics of the jets were measured using the binary edge detection method. A hotwire anemometer was used to measure the velocity pulsations, mean velocities, turbulence intensities, and Lagrangian turbulent length and time scales. The dispersion characteristics of the jet fluids were measured by the tracer-gas concentration detection method. The jet Reynolds number and excitation Strouhal number were fixed at 600 and 0.106, respectively. The jet pulsation intensities were varied from 0 to 1.6. As the jets were pulsed, the turbulence intensity, lateral spread width, and jet fluid dispersion capability markedly increased compared with those of the non-pulsed jets. The larger the pulsation intensity was, the higher the turbulence intensity, lateral spread width, and jet fluid dispersion. The Lagrangian time and length scales of the turbulent eddies decreased as the jet pulsation intensity increased because the mushroom-shaped structures generated by the jet pulsations broke up into small turbulent eddies caused by the strong vortex-stretching effect.
Cold flow characteristics of a novel high-hydrogen Micromix model burner based on multiple confluent turbulent round jets
2021, International Journal of Hydrogen Energy
As a promising commercial hydrogen-rich gas turbine combustion technology, micro-mixing combustion has been characterized for its excellent performance with low NOx emissions. New flame stabilization mechanism of micro-mixing flames may produce new design criteria. In order to explore that, cold flow characteristics of a novel Micromix model burner based on multiple confluent round jets has been studied experimentally and numerically, which is considered to be the basis for the exploration. A three-dimensional laser Doppler velocimetry system (3D-LDV) was used to measure the flow field of the model burner. It was found that the cold flow characteristics of the burner were different from the twin plane jets, the twin round jets, and the low Reynolds number confluent round jets. Compared to which, the interior of the micro-mixing nozzle is at a very high turbulence intensity level, and the jets merging point of the burner moved upstream; however, the position of the combined point of the burner was close to the confluent round jets. There is no recirculation region between jets near the burner outlet when the nozzle spacing was equal to 3 times the nozzle diameter and the Reynolds number was less than 16,702. The steady computational Reynolds averaged equations (RANS) model results were used to compare with the experimental results. It was found that the RANS results can match the experimental results well, and the three RANS models predict the spatial mixing deficiency less than 1% at the outlet, indicating that the fuel and air were almost completely premixed uniformly.
Flow and mixing characteristics of dual parallel plane jets subject to acoustic excitation
2021, European Journal of Mechanics, B/Fluids
Citation Excerpt :
The half-width of the combined flow grew linearly with the downstream distance, but the spread angle was slightly lower than that of the single jet. Ko and Lau [5] used the hot-wire anemometer to measure the mean flow velocities, turbulence intensities, and Reynolds stresses of dual parallel plane jets in the initial region. They found the internal and external mixing regions of the jet and their respective coherent vortex structures in the flow.
The effects of jet pulsation intensity and excitation Strouhal number on the flow and dispersion characteristics of dual parallel plane jets at a low Reynolds number of 200 were experimentally studied so that the transition from laminar to turbulence was focused. A hot-wire anemometer was used to record the jet velocities. The flow patterns were obtained using the laser-light sheet assisted smoke flow visualization technique. Jet spread widths were derived from the long-exposure images using the binary edge detection method. The jet fluid dispersion characteristics were examined using the tracer-gas concentration detection technique. Four characteristic flow modes (discrete coherent vortices, contiguous coherent vortices, early vortex breakup, and lateral dispersion) were identified in the domain of the jet pulsation intensity and excitation Strouhal number. At small jet pulsation intensities, discrete and contiguous coherent vortices appeared at low and high excitation Strouhal numbers, respectively. The vortices appearing in the jet shear layers broke up slowly and caused small jet expansions and turbulent intensities, thus inducing a low efficiency of jet fluid dispersion. The early vortex breakup mode was induced by the low-frequency back-and-forth flow motions in the near region at large jet pulsation intensities and low excitation Strouhal numbers. Large turbulence fluctuations caused by the early vortex breakup increased the jet dispersion significantly. At large jet pulsation intensities and excitation Strouhal numbers, high-frequency back-and-forth flow motions in the region near the jet exits significantly reduced the time-averaged axial momentum and induced large lateral jet expansion and jet fluid dispersions.
Investigation of large-scale coherent structures and momentum interference of twin rectangular jets
2020, Experimental Thermal and Fluid Science
Citation Excerpt :
In spite of the need to understand the characteristics of the LSS in twin jets, especially within the inner shear layers, and their impact on the flow physics as the nozzle spacing is varied, the literature on this topic is quite limited. Ko and Lau [4] were the first to investigate the LSS in twin jets and examined the converging and merging regions of twin parallel plane jets. It was observed that the peak in power spectral density diminishes as the inner shear layer develops in the merging region.
An experimental study was performed to investigate the effects of nozzle spacing between twin rectangular jets on the non-linear momentum interference and spatial evolution of large-scale flow structures. Five test cases of different spacing ratios (S/d_e = 1.8, 2.8, 3.7, 5.5 and 7.3) were examined. Two-point correlation, joint probability density function and swirling strength were used to characterize the large-scale structures. Turbulent/non-turbulent interface along the inner and outer shear layers were investigated. The results showed that due to the interactions in the merging region, the spatial correlation of streamwise velocity fluctuations was considerably lower along the inner shear layer than along the outer shear layer. As a result of the mixing process, the swirling strength of retrograde vortices generated along the inner shear layer was also lower than that of the prograde vortices. Along the symmetry line, longitudinal and transverse integral length scales increased linearly as the downstream distance from the jets increases. For the longitudinal length scale, the slope of the linear region was independent of the spacing ratio, while for the transverse length scale, the slope increases as the spacing ratio increases. For cases of S/d_e < 3.7, due to the suppression effects of the adjacent jet, the jump in the conditionally-averaged mean streamwise velocity profile around the turbulent/non-turbulent interface was lower for the inner shear layer compared to the outer shear layer.

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View full text

Flow structures in initial region of two interacting parallel plane jets

Abstract

J. Sound Vib.

J. Sound Vib.

J. Sound Vib.

Coherent Structures in Outer Mixing Region of Annular Jets

J. Fluid Mech.

The Inner Regions of Annular Jets

J. Fluid Mech.

Wake and Wake-induced Shear-layer Excitation in an Annular Jet

Phys. Fluids

Annular Jets of Different Diameter Ratios

Aero Quart.