Direct numerical simulation of heat transfer from a cylinder immersed in the production and decay regions of grid-element turbulence

Abstract

The present direct numerical simulation (DNS) study, the first of its kind, explores the effect that the location of a cylinder, immersed in the turbulent wake of a grid-element, has on heat transfer. An insulated single square grid-element is used to generate the turbulent wake upstream of the heated circular cylinder. Due to fine-scale resolution requirements, the simulations are carried out for a low Reynolds number. Three locations downstream of the grid-element, inside the production, peak and decay regions, respectively, are considered. The turbulent flow in the production and peak regions is highly intermittent, non-Gaussian and inhomogeneous, while it is Gaussian, homogeneous and fully turbulent in the decay region. The turbulence intensities at the location of the cylinder in the production and decay regions are almost equal at 11 %, while the peak location has the highest turbulence intensity of 15 %. A baseline simulation of heat transfer from the cylinder without oncoming turbulence was also performed. Although the oncoming turbulent intensities are similar, the production region increases the stagnation point heat transfer by 63 %, while in the decay region it is enhanced by only 28 %. This difference cannot be explained only by the increased approaching velocity in the production region. The existing correlations for the stagnation point heat transfer coefficient are found invalid for the production and peak locations, while they are satisfied in the decay region. It is established that the flow in the production and peak regions is dominated by shedding events, in which the predominant vorticity component is in the azimuthal direction. This leads to increased heat transfer from the cylinder, even before vorticity is stretched by the accelerating boundary layer. The frequency of oncoming turbulence in production and peak cases also lies close to the range of frequencies that can penetrate the boundary layer developing on the cylinder, and therefore the latter is very responsive to the impinging disturbances. The highest Nusselt number along the circumference of the cylinder is shifted 45 degrees from the front stagnation point. This shift is due to the turbulence-generating grid-element bars that result in the prevalence of intense events at the point of maximum Nusselt number compared to the stagnation point.

SPTV sheds light on flow dynamics of fractal-induced turbulence over a plate-fin array forced convection

“…Besides, the fractal induced velocity fluctuations at several locations within ( X / X * = 0.12) and after ( X / X * = 0.25) the fin array are found to be strongly non-Gaussian. These two observations are consonant with the earlier studies, where velocity fluctuations are Gaussian at X / X * < 0.1 but highly non-Gaussian between 0.1 < X / X * < 0.26 24 , 38 , 42 , 78 , 84 . Remarkably, the gaussianity behaviour leeward of SFG (0.12 < X / X * < 0.25) is highly local position-dependent.…”

“…Previous researchers 24 , 38 , 42 , 78 , 84 have extensively observed that immediately leeward of fractal insert ( X / X * < 0.1), the channel centreline velocity fluctuations are Gaussian with skewness of zero and flatness of 3. As the flow travels further downstream (0.1 < X / X * < 0.26), the velocity fluctuations appear to be strongly non-Gaussian, with the largest negative values of skewness and the largest positive values of flatness occurring at X / X *≈0.2.…”

“…To date, the prevailing notion among researchers is that forced convective heat transfer coefficient increases with turbulence intensity via a substantial restructuring of flow-boundary layer fundamental characteristics 12 , 35 , 37 , 41 , 74 . Melina et al 37 and Paul et al 38 reported that the production region of SFG-induced turbulence has a stronger dominance over heat transfer than the decay region, despite the fact that the turbulence intensity in both regimes was kept similar. Figure 8 a demonstrates the profile of SFG-induced centreline I T,E and U’ rms,E / U ∞ (without the presence of fins) along the streamline direction, X/X * where X is normalised using the wake interaction length scale as defined by Mazellier and Vassilicos 42 , viz.…”

“…Cafiero et al 35 , 36 experimentally reported that fractal grid is able to significantly augment circular impinging jets forced convection as compared to a regular grid of equivalent blockage ratio and pumping power due to the high turbulence intensity and high magnitude and persistence streamwise vorticity immediate leeward of the fractal grid. Furthermore, several investigations have revealed the fact that vortex shedding, as well as turbulence integral length scale, play critical roles in thermal dissipation 12 , 37 , 38 . Although many studies have unravelled the undertaking of turbulence integral length scale upon forced convective heat transfer, the basic correlation of both remains contentious 12 , 37 – 41 .…”

Evolution of passive scalar statistics in a spatially developing turbulence

“…The code is parallelized using the PETSc libraries [47]. More details about the solver can be found in Paul et al [37], Paul [48], Paul et al [49,50].…”