Effect of shear on coherent structures in turbulent convection

Shevkar, Prafulla P.; Gunasegarane, G. S.; Mohanan, Sanal K.; Puthenveettil, Baburaj A.

doi:10.1103/physrevfluids.4.043502

Cited by 9 publications

(21 citation statements)

References 25 publications

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“…Such a flow and deformation is expected in a rising plume when the fluid elements get stretched vertically, resulting in dominant compression perpendicular to these plumes in a horizontal plane. These negative λ D regions tend to align along the dominant shear direction at higher Ra; plumes are known to align along the direction of large-scale flow Shevkar et al 2019). At higher Ra, the mean spacing between these regions is smaller, resulting in the length of these regions being larger, similar to the trend observed for the length of plume regions.…”

Section: Deformation Mechanism and Strain Rate Tensorsupporting

confidence: 58%

“…We attribute this non-orthogonality to the presence of shear, which in some extreme scenarios can even introduce non-hyperbolic FTLE ridges (Haller 2002). Regions inside the plumes, however, are known to have relatively low shear (Shevkar et al 2019;Blass et al 2020), owing to which ridges in the −λ 2 field are likely to represent attracting LCS even at high Ra. In summary, attracting LCS in regions where repelling LCS are either absent or sufficiently weak are interpreted as the centrelines of plume regions.…”

Section: Lcs and Plumesmentioning

confidence: 99%

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On separating plumes from boundary layers in turbulent convection

et al. 2022

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We present a simple, novel kinematic criterion – that uses only the horizontal velocity fields and is free of arbitrary thresholds – to separate line plumes from local boundary layers in a plane close to the hot plate in turbulent convection. We first show that the horizontal divergence of the horizontal velocity field ( $\boldsymbol {\nabla _H} \boldsymbol {\cdot } \boldsymbol {u}$ ) has negative and positive values in two-dimensional (2D), laminar similarity solutions of plumes and boundary layers, respectively. Following this observation, based on the understanding that fluid elements predominantly undergo horizontal shear in the boundary layers and vertical shear in the plumes, we propose that the dominant eigenvalue ( $\lambda _D$ ) of the 2D strain rate tensor is negative inside the plumes and positive inside the boundary layers. Using velocity fields from our experiments, we then show that plumes can indeed be extracted as regions of negative $\lambda _D$ , which are identical to the regions with negative $\boldsymbol {\nabla _H} \boldsymbol {\cdot } \boldsymbol {u}$ . Exploring the connection of these plume structures to Lagrangian coherent structures (LCS) in the instantaneous limit, we show that the centrelines of such plume regions are captured by attracting LCS that do not have dominant repelling LCS in their vicinity. Classifying the flow near the hot plate based on the distribution of eigenvalues of the 2D strain rate tensor, we then show that the effect of shear due to the large-scale flow is felt more in regions close to where the local boundary layers turn into plumes. The lengths and areas of the plume regions, detected by the $\boldsymbol {\nabla _H}\boldsymbol {\cdot }\boldsymbol {u}$ criterion applied to our experimental and computational velocity fields, are then shown to agree with our theoretical estimates from scaling arguments. Using velocity fields from numerical simulations, we then show that the $\boldsymbol {\nabla _H}\boldsymbol {\cdot }\boldsymbol {u}$ criterion detects all the upwellings, while the available criteria based on temperature and flux thresholds miss some of these upwellings. The plumes detected by the $\boldsymbol {\nabla _H}\boldsymbol {\cdot }\boldsymbol {u}$ criterion are also shown to be thicker at Prandtl numbers ( $Pr$ ) greater than one, expectedly so, due to the thicker velocity boundary layers of the plumes at $Pr>1$ .

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Section: Deformation Mechanism and Strain Rate Tensorsupporting

confidence: 58%

Section: Lcs and Plumesmentioning

confidence: 99%

On separating plumes from boundary layers in turbulent convection

et al. 2022

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“…Recently, Shevkar et al. (2019) used an external blower to directly shear the thermal BL, and found that shear can align the line plumes and increase their mean spacing.…”

Section: Introductionmentioning

confidence: 99%

“…Solomon & Gollub (1990) used a magnetically stirred thin layer of mercury to shear the bottom of a water layer, and they observed a slight increase in Nusselt number. Recently, Shevkar et al (2019) used an external blower to directly shear the thermal BL, and found that shear can align the line plumes and increase their mean spacing.…”

Section: Introductionmentioning

confidence: 99%

Exploring the plume and shear effects in turbulent Rayleigh–Bénard convection with effective horizontal buoyancy under streamwise and spanwise geometrical confinements

2022

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We present an experimental and numerical study of turbulent thermal convection in the presence of an effective horizontal buoyancy that generates extra shear at the boundary. Geometrical confinements are also applied by varying the streamwise and spanwise aspect ratios of the convection cell to condense the plumes. With these, we systematically explore the effects of plume and shear on heat transfer. It is found that a streamwise confinement results in increased plume coverage but decreased shear compared with spanwise confinement. The fact that streamwise confinement leads to a higher vertical heat transfer efficiency than the spanwise confined case suggests that the increase of plume coverage is the dominant effect responsible for the enhanced heat transfer. Our results highlight the potential applications of coherent structure manipulation in efficient passive heat transfer control and thermal engineering. We also analyse the energetics of the present system and derive the expression of mixing efficiency accordingly. The mixing efficiency is found to increase with both the buoyancy ratio and streamwise dimension.

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“…Furthermore, recent experiments by Ref. [152] investigated the plume spacing in sheared convection and found a scaling law that connects the mean spacing of the plumes with Re w , Ra, and P r.…”

Section: Introductionmentioning

confidence: 99%

Sheared Rayleigh-Bénard turbulence

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Effect of shear on coherent structures in turbulent convection

Cited by 9 publications

References 25 publications

On separating plumes from boundary layers in turbulent convection

On separating plumes from boundary layers in turbulent convection

Exploring the plume and shear effects in turbulent Rayleigh–Bénard convection with effective horizontal buoyancy under streamwise and spanwise geometrical confinements

Sheared Rayleigh-Bénard turbulence

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