Flow and thermal structures in a transitional boundary layer (original) (raw)
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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...
Heat transfer measurements in transitional boundary layers
International Journal of Heat and Mass Transfer, 2001
High velocity boundary layer transition experiments are performed in a Ludwieg tube setup. At a Mach number of 0.36, the transition is studied by using several turbulence generating grids. These grids cause turbulence levels varying from 0.25% to 3.5%. It is found that, depending on the turbulence level, dierent intermittency distributions should be used to describe the transition zone well. For low turbulence levels, the Narasimha and the Johnson models, which are based on turbulent spots, show good agreement with the measurements. For intermediate levels, the front part of the transition zone follows a distribution which is described by turbulent spots which decrease in size. In these cases, the latter part of transition also shows agreement with the Narasimha and Johnson models. A major dierence with thè classical' intermittency distributions is obtained for high turbulence levels. Assuming that for these levels non-growing turbulent spots are initiated in the whole transition zone, an exponential intermittency is derived. In the measurements these distributions indeed are found.
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.
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.
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.
Experimental Study of Boundary Layer Transition on a Heated Flat Plate
1991
A detailed investigation to document momentum and thermal development of boundary layers undergoing natural transition on a heated flat plate was performed. Experimental results of both overall and conditionally sampled characteristics of laminar, transitional and low Reynolds number turbulent[boundary layers are presented. Measurements were done in a low-speed, closed-loop wind tunne1_with a freestream velocity of I00 ft/s and zero pressure gradient over a range of freestream turb_lence intensities from 0.4 to 6 percent. The distributions of skin friction, heat transfer rate and Reynolds shear stress were a11 consistent with previously published data. Reynolds analogy factors for momentum thickness Reynolds number, Re8 < 2300 were found to be well predicted by laminar and turbulent correlations which accounted for an unheated starting length and uniform heat flux. A small dependence of turbulent results on the freestream turbulence intensity was observed. 19. Security Classif. (of the report) 20, Security Classif. (of this page) 21. No. of pages Unclassified Unclassified 16 NASA FORM 1626 OCT 88
The boundary layer transition over a flat tilted pl ate has been studied by means of heat transfer meas urements. A heat flux sensor has been developed, in order to me asure the efficiency of convective heat transfer fo r various types of surfaces or flows. Its operation at constant tem perature allows direct and fast measurements of hea t flux. The present paper reports on the development of the sen sor and presents its application to the study of tr ansition in a boundary layer depending on the angle of incidence of the external flow. An exponential relationship b etween critical Reynolds Number and pressure gradient parameter has been found.
1965
An experimental investigation was carried out to determine the effects Sof surface roughness on the turbulent boundary lay,r skin friction and heat transfer rates in air at a Mach number of h.93. Four flat plate model configurations were tested, one with a smooth surface and three with rough ourfaceG consisting of 900 V-grooves oriented perpendicular to the flow direction. Simultaneous measurements of the local skin friction and heat transfer were made uoing a floating-elcment skin friction bulance and an insulated-mass calorimeter for Reynolds numbers near 10 million and wallto-free stream temperature ratios from 2.9 to 5.2. The results of these measurements and of boundary layer pressure surveys are presented in both graphical and tabular form and are compared with theoretical predictions.
Boundary layer turbulence in transitional and developed states
Physics of Fluids, 2012
Using the recent DNS by Wu and Moin (2010) of a flat-plate boundary layer with a passively heated wall, statistics of the turbulence in transition at Re θ = 500 where spots merge (distributions of the mean velocity, rms velocity and vorticity component fluctuations, kinetic energy production and dissipation rates and enstrophy) have been compared to these statistics for the developed boundary layer turbulence at Re θ = 1850. When the distributions in the transitional region, determined in narrow planes 0.03Re θ wide, exclude regions and times when the flow is not turbulent, they closely resemble the distributions in the developed turbulent state at the higher Reynolds number, especially in the buffer and sublayers. The skin friction coefficient, determined in this conditional manner in the transitional flow is, of course, much larger than that obtained by including both turbulent and non-turbulent information there, and is consistent with a value obtained by extrapolating from the developed turbulent region. Individual hairpin vortices have been identified using vorticity lines in the transitional and developed turbulence, and they have quite similar characteristics in both cases. Some of these vortices appear to emerge out of sheets of unorganized vorticity in the viscous sublayer. Hairpin vortices are closely associated with the processes that transport momentum and heat within the boundary layer. An octant analysis based on the combinations of signs of the velocity and temperature fluctuations, u, v and θ, shows that this transport is predominantly of the mean gradient type over much of both the transitional and developed flows. The results add strong evidence to the view that there is little difference between the structure and transport processes of a developed turbulent boundary layer and of turbulent spots that appear in transition.
Turbulent Transport Characteristics in a Low-Speed Boundary Layer Subjected to Adverse Pressure
Proc. 36th Heat Transfer …, 1999
Thermal anemometry measurements were performed to evaluate the heat and momentum transport characteristics of wall turbulence over a slightly heated, smooth flat plate with a stepchange in wall temperature. Single-wire, X-probe and triple-wire sensors were employed to measure mean and fluctuating velocity and temperature as well as Reynolds stress and heat flux productions. "Equilibrium" boundary layers were considered for mild (β ≈ 0.8, Re θ ≈ 3500) and moderate (β ≈ 1.8, Re θ ≈ 3800) adverse-pressure-gradient (APG) conditions for a wall-to-freestream temperature difference ∆T of approximately 12 o C. The base case for zero-pressuregradient (ZPG) conditions (β ≈ 0, Re θ ≈ 2700) was also investigated. The origins of the momentum and thermal boundary layers did not coincide, resulting in a layer development ∆/∆-T of approximately 0.8, 1.2 and 1.5 for ZPG, mild and moderate APG, respectively. Findings suggest that the mean flow field and the fluctuating streamwise and normal flow fields responded proportionally to the magnitude of the adverse-pressure gradient present. The failure of the lawof-the-wall for velocity for the APG conditions considered was not severe. And the equilibrium condition of the flow was maintained through a balance of adverse pressure and turbulent stress production. The Reynolds analogy was confirmed for ZPG conditions while, in adverse pressure, the turbulent stress production scaled with the streamwise heat flux. The heat flux production was found to be self-similar for the pressure gradient cases investigated.