Heat transfer in production and decay regions of grid-generated turbulence

Melina, Gianfrancesco; Bruce, Paul J.; Hewitt, Geoff; Vassilicos, J. C.

doi:10.1016/j.ijheatmasstransfer.2017.02.024

Cited by 20 publications

(28 citation statements)

References 51 publications

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“…However, it has been later realized to vary dramatically depending on grid geometry, inflow conditions and experimental facility, etc. [16][17][18]24,25]. The flow structure and turbulence statistics, in Sections 3.1 and 3.2, evidenced the importance of the smaller scales in addition to the largest scale.…”

Section: Revisit the Wake Interaction Length Scale Modelmentioning

confidence: 86%

“…Previously, the magnitude of the peak turbulence intensity was proposed to be proportional to c d t 0 /L 0 for fractal square grids, where c d is the drag coefficient of the largest bars of the grid [17]. Subsequent research focused on the largest scale being the dominant factor to determine the peak turbulence intensity magnitude, i.e., Tu peak ∝ c d t 0 /L 0 [18,25]. This relation has shown to be true for both multi-scale and single-scale grids having different ratios of t 0 /L 0 .…”

Section: Revisit the Wake Interaction Length Scale Modelmentioning

confidence: 99%

“…The relationship Tu peak ∝ c d t 0 /L 0 suggests that the additional fractal scales would have little to no impact on the magnitude of the peak turbulence intensity, as long as the dimensions of the largest bar remain constant (t 0 /L 0 ). Wind-tunnel experiments on turbulent flows generated by a fractal grid, a single square grid, and a conventional grid, all having different t 0 /L 0 were reported by Melina et al (2016Melina et al ( , 2017 [18] . This study confirmed that a single square grid with larger t 0 /L 0 and lower blockage ratio can increase turbulence intensity and heat transfer compared to the other grids [19].…”

Section: Introductionmentioning

confidence: 99%

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Turbulence Enhancement by Fractal Square Grids: Effects of Multiple Fractal Scales

Omilion

Turk

Zhang

2018

Fluids

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Multi-scale fractal grids can be considered to mimic the fractal characteristic of objects of complex appearance in nature, such as branching pulmonary network and corals in biology, river network, trees, and cumulus clouds in geophysics, and the large-scale structure of the universe in astronomy. Understanding the role that multiple length scales have in momentum and energy transport is essential for effective utilization of fractal grids in a wide variety of engineering applications. Fractal square grids, consisted of the basic square pattern, have been used for enhancing fluid mixing as a passive flow control strategy. While previous studies have solidified the dominant effect of the largest scale, effects of the smaller scales and the interaction of the range of scales on the generated turbulent flow remain unclear. This research is to determine the relationship between the fractal scales (varying with the fractal iteration N), the turbulence statistics of the flow and the pressure drop across the fractal square grids using well-controlled water-tunnel experiments. Instantaneous and ensemble-averaged velocity fields are obtained by a planar Particle Image Velocimetry (PIV) method for a set of fractal square grids (N = 1, 2 and 4) at Reynolds number of 3400. The static pressure drop across the fractal square grid is measured by a differential pressure transducer. Flow fields indicate that the multiple jets, wakes and the shear layers produced by the multiple scales of bars are the fundamental flow physics that promote momentum transport in the fractal grid generated turbulence. The wake interaction length scale model is modified to incorporate the effects of smaller scales and thereof interaction, by the effective mesh size M e f f and an empirical coefficient β. Effectiveness of a fractal square grid is assessed using the gained turbulence intensity and Reynolds shear stress level at the cost of pressure loss, which varies with the distance downstream. In light of the promising capability of the fractal grids to enhance momentum and energy transport, this work can potentially benefit a wide variety of applications where energy efficient mixing or convective heat transfer is a key process.

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Section: Revisit the Wake Interaction Length Scale Modelmentioning

confidence: 86%

Section: Revisit the Wake Interaction Length Scale Modelmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Turbulence Enhancement by Fractal Square Grids: Effects of Multiple Fractal Scales

Omilion

Turk

Zhang

2018

Fluids

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“…However, the effect of the grid parameters on the turbulent field was ignored. Melina [23], Ahmadi-Baloutaki [24], and Wang [25] applied the gridgenerated turbulence directly, without studying the turbulence characteristics.…”

Section: State Of the Artmentioning

confidence: 99%

Numerical and Experimental Studies on Grid-Generated Turbulence in Wind Tunnel

Liu¹,

Zhang²,

Wu³

et al. 2017

JESTR

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The grid-generated turbulence in wind tunnel is commonly applied in aerodynamic performance experiments of various industrial structures. However, the characteristics and application conditions of the grid-generated turbulence in the wind tunnel are often affected by the grid shape and the test position, respectively. Therefore, in order to clarify the influence of the grid shape and the test position on the grid-generated turbulence in wind tunnel, a computational fluid dynamics (CFD) numerical study on the turbulence generated by two different grids was presented herein, with the wind tunnel experimental validation. Firstly, time histories of the wind speed at 19 monitoring points behind the grid in wind tunnel were collected. Secondly, the entire turbulent field generated by two different grids was simulated using CFD. Then, the characteristics of the turbulence generated by the grids in wind tunnel were investigated. Finally, the accuracy of the CFD simulations compared with the wind tunnel test was studied. Results show that the grid blockage and the interference effect lead to a significant increase of 200% in the turbulence intensity near the grid. Moreover, the grid-generated turbulence conforms to the isotropic assumption only after a specific distance of 2.0-4.0 m from the grid. In addition, CFD errors are generally within 15%-20% of the corresponding measurements in the wind tunnel. This study provides guidance for selecting the installation location of the structure model in a wind tunnel.

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“…Since Rouse (1939) first used OGT to study sediment suspension [20], the OGT was employed to explore the mechanics of the sediment transport [21,22], the interfacial mixing in stratified flows [2,23], the mass transfer across a shear-free water-air interface [24,25], the desorption of contaminants from sediment [26,27], the rate of frazil ice growth [28] and the turbulent thermal diffusion effect [6]. With so many application experiences, despite the emergence of the alternative loudspeaker and jet approaches which are relatively expensive, the OGT was still widely used in recent years [5,[29][30][31][32][33][34].…”

Section: Introductionmentioning

confidence: 99%

An Experimental Method for Generating Shear-Free Turbulence Using Horizontal Oscillating Grids

Zhang

Yang

et al. 2020

Water

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An experimental apparatus driven by horizontal oscillating grids in a water tank is proposed for generating shear-free turbulence, which is measured using Particle Image Velocimetry (PIV). The performances of the proposed apparatus are investigated through the instantaneous and root-mean-square (RMS) velocity, Reynolds stress, length and time scale, frequency spectra and dissipation rate. Results indicate that the turbulence at the core region of the water tank, probably 8 cm in length, is identified to be shear-free. The main advantage of the turbulence driven by horizontal oscillating mode is that the ratios of the longitudinal turbulent intensities to the vertical values are between 1.5 and 2.0, consistent with those ratios in open-channel flows. Additionally, the range of the length scale can span the typical sizes of suspended particles in natural environments, and the dissipation rate also agrees with those found in natural environments. For convenience of experimental use, a formula is suggested to calculate the RMS flow velocity, which is linearly proportional to the product of oscillating stroke and frequency. The proposed experimental method in this study appears to be more appropriate than the traditional vertical oscillating mode for studying the fundamental mechanisms of vertical migratory behavior of suspended particles and contaminants in turbulent flows.

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Heat transfer in production and decay regions of grid-generated turbulence

Cited by 20 publications

References 51 publications

Turbulence Enhancement by Fractal Square Grids: Effects of Multiple Fractal Scales

Turbulence Enhancement by Fractal Square Grids: Effects of Multiple Fractal Scales

Numerical and Experimental Studies on Grid-Generated Turbulence in Wind Tunnel

An Experimental Method for Generating Shear-Free Turbulence Using Horizontal Oscillating Grids

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