PIV measurements of the K-type transition in natural convection boundary layers

Zhao, Yongling; Lei, Chengwang; Patterson, John C.

doi:10.1016/j.expthermflusci.2018.09.007

Cited by 27 publications

(8 citation statements)

References 38 publications

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“…This is not the case for the buoyancy layer, and regions of negative correlation coefficients flank a region of positive correlation coefficient in the spanwise direction. It should be noted that the LSMs discussed in the present study differ from the Λ-shaped structures observed in transitional NCBLs (Zhao et al 2017(Zhao et al , 2019.…”

Section: Two-point Correlationscontrasting

confidence: 79%

“…The differences between the wall-normal coherence of the LSMs in the turbulent buoyancy layer and canonical wall-bounded turbulence can be attributed to the presence of different vortex structures due to the change in the sign of the mean shear in the outer layer (Nakao et al 2017). At this stage, the role of hairpin vortices or similar in NCBL transition and turbulence is not fully understood (Pallares et al 2010;Abramov et al 2014;Nakao et al 2017;Zhao et al 2017Zhao et al , 2019 and no definitive conclusions can be drawn that relate hairpin-like vortex structures observed in prior studies to the two-point correlations discussed in this study.…”

Section: Wall-normal Coherencementioning

confidence: 99%

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Large-scale motions in a turbulent natural convection boundary layer immersed in a stably stratified environment

Maryada¹,

Armfield²,

Dhopade³

et al. 2023

J. Fluid Mech.

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This study investigates the coherence of turbulent fluctuations in a turbulent vertical natural convection boundary layer immersed in a stably stratified medium (turbulent buoyancy layer). A turbulent buoyancy layer of a fluid having a Prandtl number of $0.71$ at a Reynolds number of $800$ is numerically simulated using direct numerical simulation. The two-point correlations reveal that the streamwise velocity fluctuations are coherent over large streamwise distances, with the length scale of the streamwise coherence being greater than the boundary layer thickness. This is due to large-scale motions (LSMs), similar to the LSMs observed in canonical wall-bounded turbulence despite the stark differences in flow dynamics. Both high-speed (positive) and low-speed (negative) streamwise velocity fluctuations form LSMs, with their streamwise length scales increasing with increasing wall-normal distance. High-speed LSMs are composed of upwash flow with high temperatures, while low-speed LSMs are composed of downwash flow with low temperatures. Both high-speed and low-speed LSMs meander appreciably in the streamwise direction, with the degree of meandering being correlated with the sign of the spanwise velocity fluctuations. The LSMs exhibit coherence across significant wall-normal distances and contribute significantly to the turbulence production in the outer layer. Examining the one-dimensional energy spectra of the turbulent buoyancy layer shows that the LSMs are the dominant energy-containing motions, implying that the length scale of the energy-containing range is of the order of boundary layer thickness. Notably, wall-normal velocity, spanwise velocity and buoyancy fluctuations do not form LSMs with streamwise length scales comparable to streamwise velocity fluctuations.

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Section: Two-point Correlationscontrasting

confidence: 79%

Section: Wall-normal Coherencementioning

confidence: 99%

Large-scale motions in a turbulent natural convection boundary layer immersed in a stably stratified environment

Maryada¹,

Armfield²,

Dhopade³

et al. 2023

J. Fluid Mech.

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“…Dring and Gebhart, 17 Knowles and Gebhart, 21 and Dring and Gebhart 22 later studied spatial evolution of frequency bands in natural convection boundary layers, and the frequencyfiltering effect was discussed more explicitly. As the advancement of measurement technique particle image velocimetry, most recent experimental studies by Zhao et al 23,24 examined flow structures of natural convection boundary layers in periodic flows undergoing natural or controlled transitions to turbulence. Wavelengths of flow structures have been obtained for natural convection boundary layers at Pr > 1.…”

Section: Introductionmentioning

confidence: 99%

On the selection of perturbations for thermal boundary layer control

Zhao

Liu

et al. 2019

Physics of Fluids

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The convective instability of the natural convection boundary layers of air (Pr = 0.7) in the laminar-to-turbulent transition regime (Ra = 8.7 × 10 7 -1.1 × 10 9 ) is investigated by stability analysis in the framework of direct numerical simulations. To understand the spatial and temporal evolution of the convective instability of the thermal boundary layers, small-amplitude random-mode numerical perturbations are first introduced into the boundary condition of the boundary layer flow. The prescribed full spectral perturbations (i.e., white noise) are mostly damped out immediately by a limited upstream boundary layer. A low-frequency band is initially distinct in the upstream near the leading edge but decays spatially as the instability propagates downstream. In contrast, a high-frequency band emerges to finally become the most dominant frequency band in the thermal boundary layer transition regime. To obtain further insights into the nature of the established high-frequency band, single-mode perturbations of various frequencies are then introduced into the boundary layer near the leading edge. It is found that a single-mode perturbation at the peak frequency within the high-frequency band excites the maximum response of the thermal boundary layer, suggesting that the peak frequency is in fact the characteristic frequency or resonance frequency of the thermal boundary layer. The dimensionless form of the dependence of the characteristic frequency on Ra is then found to be fc = 0.07Ra 2/3 . The single-mode perturbation numerical experiments also revealed the propagation speed of convective instability waves, which was significantly greater than the convection speed of the thermal boundary layer. The smaller the Ra, the larger the difference between the two propagation speeds. A semi-analytical scaling of the wave propagation speed in the form csc ∼ Ra 1/2 y 1/2 Pr was derived (y denoting the streamwise location of the boundary layer), providing a predictive correlation that can be used for thermal boundary layer control.

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“…[Patterson et al 2002] und [Zhao et al 2016]) und Geschwindigkeiten (vgl. [Zhao et al 2019]) beobachtet, die deutlich von einem quasi-stationärem Verlauf abweichen. Im Bereich der (teil-)turbulenten instationären freien Konvektionsströmungen befasst sich ein Großteil der bisherigen Betrachtungen mit der phänomenologischen und analytischen Beschreibung von Störfrequenzen, die einer statistisch stationären Strömung (teilweise zeitvariabel) aufgeprägt werden.…”

Section: Beschreibung Von Instationären Freien Konvektionsströmungenunclassified

Experimentelle Betrachtung der Wärmeübertragung durch instationäre freie Konvektion an der vertikalen Platte

Schaub¹

2019

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Kurzinhalt In der vorliegenden Dissertation werden die Auswirkungen von instationären Prozessen in freier Konvektion an der vertikalen Platte auf die Wärmeübertragung an Luft experimentell untersucht. Die dabei erhobenen Messwerte werden auf der Grundlage einer phänomenologischen Betrachtung in ein analytisches Berechnungsverfahren überführt. So legt eine physikalische Interpretation nahe, dass nach einer plötzlichen Veränderung der Wärmestromdichte ein Überschuss an potentieller Energie in der Strömungsgrenzschicht entsteht, der in einem anschließenden Ausgleichsvorgang durch zusätzliche Konvektionsstrukturen (insb. Kelvin-Helmholtz-Wirbel) in kinetische Energie umgewandelt wird. Betrachtet werden zyklische und sprungartige Variationen der Randbedingungen, deren praktische Anwendung erst durch die Etablierung der elektronischen Leistungsregelung relevant wurde. Derartige Betriebsweisen erlauben beispielsweise für die Wärmeübergabe von Raumheizsystemen eine Intensivieru...

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PIV measurements of the K-type transition in natural convection boundary layers

Cited by 27 publications

References 38 publications

Large-scale motions in a turbulent natural convection boundary layer immersed in a stably stratified environment

Large-scale motions in a turbulent natural convection boundary layer immersed in a stably stratified environment

On the selection of perturbations for thermal boundary layer control

Experimentelle Betrachtung der Wärmeübertragung durch instationäre freie Konvektion an der vertikalen Platte

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