Turbulent Transport Characteristics in a Low-Speed Boundary Layer Subjected to Adverse Pressure (original) (raw)
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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 heat flux measurements in a transitional boundary layer
During an experimental investigation of the transitional boundary layer over a heated flat plate, an unexpected result was encountered for the turbulent heat flux (bar-v't'). This quantity, representing the correlation between the fluctuating normal velocity and the temperature, was measured to be negative near the wall under certain conditions. The result was unexpected as it implied a counter-gradient heat transfer by the turbulent fluctuations. Possible reasons for this anomalous result were further investigated. The possible causes considered for this negative bar-v't' were: (1) plausible measurement error and peculiarity of the flow facility, (2) large probe size effect, (3) 'streaky structure' in the near wall boundary layer, and (4) contributions from other terms usually assumed negligible in the energy equation including the Reynolds heat flux in the streamwise direction (bar-u't'). Even though the energy balance has remained inconclusive, non...
Effect of wall heating on turbulent boundary layers with temperature-dependent viscosity
Journal of Fluid Mechanics, 2013
Direct numerical simulations (DNS) of turbulent boundary layers over isothermally heated walls were performed, and the effect of viscosity stratification on the turbulence statistics and skin friction were investigated. An empirical relation for temperaturedependent viscosity for water was adopted. Based on the free-stream temperature (30 • C), two wall temperatures (70 • C and 99 • C) were selected. In the heated flows, the turbulence energy diminishes in the buffer layer, but increases near the wall. The reduction in turbulence kinetic energy in the buffer layer is accompanied by smaller levels of Reynolds shear stresses and, hence, weaker turbulence production. The enhanced turbulence energy near the wall is attributed to enhanced transfer of energy via additional diffusion-like terms due to the viscosity stratification. Despite the lower fluid viscosity near the wall, dissipation is also increased owing to the augmented nearwall fine-scale motion. Wall heating results in reduction in the skin-friction coefficient by up to 26 %. An evaluation of the different contributions to the skin friction demonstrates that drag reduction is primarily due to the changes in the Reynolds shear stresses across the boundary layer. Quadrant and octant analyses showed that ejections (Q2) and sweeps (Q4) are significantly reduced, a result further supported by an examination of outer vortical structures from linear stochastic estimation of the ejection events and spanwise vortices.
International Journal of Heat and Mass Transfer, 2009
A simple one-point closure for the inner region of turbulent boundary layers subjected to adverse pressure gradient is introduced. The use of local wall variables leads to self-similarity for the temperature distribution but not the velocity. A turbulent velocity scale directly related to the pressure parameter that maintains constant the total shear stress in the inner layer is used to define an eddy viscosity and diffusivity. The predicted velocity and temperature profiles agree reasonably well with the experiments. The essence of the formulation explains why the turbulent heat flux scaled by the local inner variables is merely unaffected contrarily to the Reynolds shear stress distribution in wall units that is significantly sensitive to the imposed pressure gradient.
WALL PROPERTIES AND HEAT TRANSFER IN NEAR-WALL TURBULENT FLOW
Numerical Heat Transfer, Part A: Applications, 2004
Direct numerical simulation of a passive scalar in fully developed turbulent channel flow is used to show that Nusselt number is not only a function of Reynolds and Prandtl number, but also depends on properties of a heating wall. Variable thickness of the heating wall and variable heater properties, combined in a fluid-solid thermal activity ratio
The thermal entrance region in fully developed turbulent flow
AIChE Journal, 1960
The temperature profile and the local rate of heat transfer from the wall were measured a t 0.453, 1.13, 4.12, and 9.97 tube diameters downstream from a step increase in wall temperature for air in fully developed turbulent flow a t Reynolds numbers of 15,000 and 65,000 in a 1.52-in. tube. The velocity profile and the pressure were also measured a t these lengths.
Heat Transfer and Fluid Dynamics Measurements in Accelerated Rough-Wall Boundary Layer
Volume 2: Combustion and Fuels; Oil and Gas Applications; Cycle Innovations; Heat Transfer; Electric Power; Industrial and Cogeneration; Ceramics; Structures and Dynamics; Controls, Diagnostics and Instrumentation; IGTI Scholar Award, 1993
The combined effects of freestream acceleration and surface roughness on heat transfer and fluid dynamics in the turbulent boundary layer were investigated experimentally. The experiments included a variety of flow conditions ranging from aerodynamically-smooth through transitionally-rough to fullyrough boundary layers with accelerations ranging from moderate to modestly strong. Two well-defined rough surfaces composed of 1.27 mm diameter hemispheres spaced 2 and 4 diameters apart, respectively, in staggered arrays on otherwise smooth surfaces were used as the test surfaces. The first 1.5 m of the test section had zero-pressure gradient followed by a 0.4 m accelerated region with the remaining 0.4 m adjusted to zeropressure gradient. The Stanton number for the rough-wall experiments decreased or increased for accelerated rough-wall cases compared to zero-pressure gradient cases depending on flow conditions. For fully-rough boundary layers, Stanton numbers increased with acceleration compared to zero-pressure gradient at the same x-position. For aerodynamically-smooth and transitionally-rough boundary-layer flows, the effect of acceleration was not similar to that of fully-rough flows and was highly dependent upon the flow conditions. The acceleration caused a decrease in the relative turbulence level over the rough surface. The profiles of un for the accelerated runs were lower than those of zero-pressure gradient cases, and a substantial decrease in the Reynolds shear stress (..17C,i) component was observed when acceleration was applied.
International Journal of Heat and Mass Transfer, 1995
In the present work, asymptotic methods are used to derive new expressions for the law of the wall, for tiae law of the wake and for the skin-friction and the Stanton number equations, for compressible turbulent boundary layers with heat and mass transfer. The results are compared with previous theories and experiments showing good agreement. The two parameters in the law of the wall, the angular coefficient and the linear coefficient of the straight part of the velocity and the temperature profiles plotted in appropriate logarithmic coordinates, are shown to vary with Math number. Only the second of these parameters, the linear coefficient of the straight line, is shown to vary with the injection rate. No dependence of these parameters on Eckert number, E could be assessed. Also, it emerges from the present analysis that the dissil:ation effects become important only when E = 0(u~-~), u~ = non-dimensional friction velocity.