A Wind Tunnel Study of Thermally Stratified Boundary Layers over Rough Surfaces (original) (raw)
Related papers
International Journal of Environmental Research and Public Health
The aim of this paper is to analyse the thermal effects in a wind tunnel experiment to simulate the planetary boundary layer (PBL). Experiments were performed in the wind tunnel of the Laboratory of Constructions Aerodynamics at the Federal University of Rio Grande do Sul, Porto Alegre, Rio Grande do Sul State, Brazil. This wind tunnel is a closed return low-speed wind tunnel specifically designed for dynamic and static studies on civil construction models. As a novelty, one of the experimental sections of the wind tunnel was equipped with a metal sheet with Peltier elements coupled to it. In other words, thermal effects generating new flow patterns become feasible and open pathways to compare wind tunnel simulations to those in the PBL. Furthermore, measurements obtained with the smooth floor of the wind tunnel were repeated under the same conditions with the addition of the roughness in the floor, and the mechanical turbulence generated by the surface roughness significantly ampli...
Some Further Aspects of Stable Boundary-Layer Simulation in a Stratified-Flow Wind Tunnel
Boundary-Layer Meteorology
It is demonstrated that the vertical profile of gradient Richardson number, Ri, can be shaped by control of the working-section inlet temperature profile. In previous work (Hancock and Hayden in Boundary-Layer Meteorol 168:20–57, 2018; 175:93–112, 2020; 180:5–26, 2021) the inlet temperature profile had been specified but without control of the profile of RiRiinthedeveloped−flowregionoftheworkingsection.Controloftheinlettemperatureprofileisprovidedby15inletheaters(spreaduniformlyacrosstheheightoftheworkingsection),allowingcontrolofthetemperaturegradientoverthebulkoftheboundarylayer,andtheoveralltemperaturelevelabovethatofthesurface.ThebulkRichardsonnumberforthe11casescoverstherange0.01–0.17(thereisnooverlyinginversion).IntheupperRi in the developed-flow region of the working section. Control of the inlet temperature profile is provided by 15 inlet heaters (spread uniformly across the height of the working section), allowing control of the temperature gradient over the bulk of the boundary layer, and the overall temperature level above that of the surface. The bulk Richardson number for the 11 cases covers the range 0.01–0.17 (there is no overlying inversion). In the upperRiinthedeveloped−flowregionoftheworkingsection.Controloftheinlettemperatureprofileisprovidedby15inletheaters(spreaduniformlyacrosstheheightoftheworkingsection),allowingcontrolofthetemperaturegradientoverthebulkoftheboundarylayer,andtheoveralltemperaturelevelabovethatofthesurface.ThebulkRichardsonnumberforthe11casescoverstherange0.01–0.17(thereisnooverlyinginversion).Intheupper\approx≈2/3oftheboundarylayertheReynoldsstressesandturbulentheatfluxarecontrolledbythegradientinmeantemperature,whileinthelower≈ 2/3 of the boundary layer the Reynolds stresses and turbulent heat flux are controlled by the gradient in mean temperature, while in the lower≈2/3oftheboundarylayertheReynoldsstressesandturbulentheatfluxarecontrolledbythegradientinmeantemperature,whileinthelower\approx$$ ≈ 1/3 they a...
The effect of stable thermal stratification on turbulent boundary layer statistics
The effects of stable thermal stratification on turbulent boundary layers are experimentally investigated for smooth and rough walls. For weak to moderate stability, the turbulent stresses are seen to scale with the wall shear stress, compensating for changes in fluid density in the same manner as done for compressible flows. This suggests little change in turbulent structure within this regime. At higher levels of stratification turbulence no longer scales with the wall shear stress and turbulent production by mean shear collapses, but without the preferential damping of near-wall motions observed in previous studies. We suggest that the weakly stable and strongly stable (collapsed) regimes are delineated by the point where the turbulence no longer scales with the local wall shear stress, a significant departure from previous definitions. The critical stratification separating these two regimes closely follows the linear stability analysis of Schlichting (Z. Angew. Math. Mech., vol. 15 (6), 1935, pp. 313–338) for both smooth and rough surfaces, indicating that a good predictor of critical stratification is the gradient Richardson number evaluated at the wall. Wall-normal and shear stresses follow atmospheric trends in the local gradient Richardson number scaling of Sorbjan (Q.), suggesting that much can be learned about stratified atmospheric flows from the study of laboratory scale boundary layers at relatively low Reynolds numbers.
Wind Tunnel Experiments on Turbulent Boundary Layer Flows
Boundary Layer Flows - Modelling, Computation, and Applications of Laminar, Turbulent Incompressible and Compressible Flows [Working Title]
The knowledge and experimental development of boundary layer turbulent flows is extremely important in applications related to the building aerodynamics, wind comfort, atmospheric dispersion, and even aeronautics. The Aerodynamic Laboratories of the UFRGS and UNNE have been making joint activities related to wind engineering such as those mentioned earlier for more than 25 years. In this work, a compilation of different experiments on turbulent boundary layer flows realized in these both laboratories is carried out. The characteristics of flows that develop on a smooth surface of the wind tunnel are experimentally evaluated. Then, reduced-scale models of atmospheric boundary layer flows are analyzed including the effects of turbulence generators and surface roughness. Special attention on the behavior of the turbulent parameters in the case of experimental studies using low mean velocity is paid. Finally, some comments referring to recent studies on thermal effects in turbulent boun...
Journal of the Serbian Society for Computational Mechanics, 2022
In this article we present an experimental and numerical study of the behavior of the boundary layer type viscous flow in the presence of the thermal effect. The flow was held in a threedimensional field with a uniform infinite velocity in the case of an adiabatic wall with heat input. The presented experimental work was performed in the Thermal Laboratory (LET) of the Prime Institute of Poitiers (France). It describes the analysis of a turbulent boundary layer created in a wind tunnel on the surface of a flat plate covered with epoxy resin. An HP 6012A power supply system was used to provide circulating heat flux to heat the flat plate to 80°C by the Joule effect. The numerical result shows a clear difference in the evolution of the thermal boundary layer between the three temperatures of the wall.
Turbulent boundary layer response to the introduction of stable stratification
The response of an initially neutral rough-wall turbulent boundary layer to a change in wall temperature is investigated experimentally. The change causes a localized peak in stable stratification that diffuses and moves away from the wall with downstream distance. The streamwise and wall-normal components of turbulent velocity fluctuations are damped at similar rates, even though the stratification only directly impacts the wall-normal component. The Reynolds shear profiles reveal the growth of an internal layer that scales approximately with the bulk Brunt–Väisälä frequency.
NUMERICAL STUDY OF TURBULENT BOUNDARY LAYER FLOWS OVER ROUGH SURFACES–PART II: TEMPERATURE PROFILES
Turbulent transfer of momentum and heat over rough surfaces are numerically simulated. The effects of sudden changes are predicted in the cases of turbulent flow around surface-mounted two-dimensional ribs when subjected to a sudden change in surface roughness and temperature. A particular interest of this study is to investigate the sudden changes in the surface roughness for developing thermal boundary layer flow. A two-equation k-eps turbulence model is employed to simulate the turbulent transport. Equations of boundary-layer type were used and a forward marching method was employed for sweeping the computational domain. Wall functions that take into account surface roughness are used to specify the boundary conditions at the surface. The effects of the sudden changes of roughness accounted for the wall functions in the k-eps turbulence model are compared with available experimental data. Four configurations are simulated here, namely one extensive uniformly smooth surface, one extensively uniform rough case, and two cases where the surface roughness varies suddenly from a smooth to a rough and from a rough to a smooth surface. Results are presented for velocity and skin friction coefficient, in addition to comparisons with experimental data. The use of a parabolic solver showed good agreement with experimental values for the mean and local quantities.
Wind-Tunnel Simulation of Weakly and Moderately Stable Atmospheric Boundary Layers
Boundary-Layer Meteorology, 2018
The simulation of horizontally homogeneous boundary layers that have characteristics of weakly and moderately stable atmospheric flow is investigated, where the wellestablished wind engineering practice of using 'flow generators' to provide a deep boundary layer is employed. Primary attention is given to the flow above the surface layer, in the absence of an overlying inversion, as assessed from first-and second-order moments of velocity and temperature. A uniform inlet temperature profile ahead of a deep layer, allowing initially neutral flow, results in the upper part of the boundary layer remaining neutral. A non-uniform inlet temperature profile is required but needs careful specification if odd characteristics are to be avoided, attributed to long-lasting effects inherent of stability, and to a reduced level of turbulent mixing. The first part of the wind-tunnel floor must not be cooled if turbulence quantities are to vary smoothly with height. Closely horizontally homogeneous flow is demonstrated, where profiles are comparable or closely comparable with atmospheric data in terms of local similarity and functions of normalized height. The ratio of boundary-layer height to surface Obukhov length, and the surface heat flux, are functions of the bulk Richardson number, independent of horizontal homogeneity. Surface heat flux rises to a maximum and then decreases.