Turbulent Free Convection Boundary Layer Research Articles

Disturbing action of a cubical obstacle on the turbulent vertical-plate free convection boundary layer: RANS-based simulation

Steady-state RANS simulations were performed to establish disturbing action of a cubical obstacle on the turbulent free convection boundary layer in air along a vertical isothermally heated plate. In the case considered, the size of the cube mounted on the plate is about one-third the full thickness of the incoming boundary layer. Using the k-ω SST turbulence model, refined-grid numerical solutions have been obtained assuming the obstacle surfaces either adiabatic or isothermal. Characteristic three-dimensional features of the free convection flow past the cubical obstacle, such as the horseshoe-like vortex structures and the near-wake recirculation, are analyzed in comparison with the previously studied case dealing with the disturbing action of a circular cylinder of unit aspect ratio. Data on heat transfer augmentation near the obstacle are presented and discussed.

Effect of the horseshoe-shaped vortex on heat transfer in vicinity of the leading edge of a cylinder immersed in the turbulent free convection boundary layer on a vertical plate

Measurement data obtained with a thermo-resistance (wolfram-wire) probe for fluctuating temperature field in the front of a finite-height adiabatic circular cylinder disturbing the turbulent free convection boundary layer on a heated vertical flat plate are presented. The measurements were focused on detection of the effects attributed to action of the horseshoe-shaped vortex structures developing in the flow separation zone upstream of the obstacle. It is shown that near-wall air temperature gradients at some measurement positions are dramatically higher, if compared with profiles measured far away from the obstacle. It points in particular to highly intensified heat transfer in the flow region under study. The experimental mean temperature profiles are compared with results of previous RANS- and LES-based numerical studies performed by the authors under conditions that were close to those adopted in the experiments. It is shown that the LES-predicted temperature distributions are in a good accordance with the experimental results. The steady-state solution obtained with the RANS approach partially overpredicts the deformation of the temperature field attributed to action of the horseshoe-shaped vortices, which are substantially unsteady in the reality.

Experience in DNS of turbulent free convection in air along an isothermal vertical plate

The paper presents results of direct numerical simulation of the turbulent free convection boundary layer developing along a vertical heated plate up to the local Grashof numbers, Grx, of 8.1010 (Pr=0.7). A span-distributed macro-roughness of a selected form is used for enhancing transition to turbulence, so that the Grx–value corresponding to the end point of transition would be close to that reported for the case of natural transition. The results obtained for the first-and the second-order turbulent flow and heat transfer statistics in the developed turbulent regime are in a good accordance with the experimental data reported by Tsuji and Nagano (1988).

Computational and experimental study of 3D flow near the cube immersed in the turbulent free convection boundary layer on a vertical heated plate

Results of a coordinated computational and experimental study of time-averaged velocity and temperature fields in the vicinity of an adiabatic cube that is inserted into the turbulent free convection vertical-plate boundary layer are presented. The numerical simulation was based on the steady-state RANS approach using the k-ω SST turbulence model and a version of the differential Reynolds stress model (DRSM). The hot-wire technique was used for velocity measurements, simultaneously with temperature measurements by an accompanied “cold” wire. Two turbulence models produced similar vortex flow structures near the obstacle; however some important quantitative distinctions were observed. The velocity value profiles predicted for the middle vertical plane showed generally a good accordance with the hot-wire measurements carried out in this plane, especially in the DRSM case.

Experience in using a synthetic turbulence generator for eddy-resolving simulation of the free convection boundary layer on a vertical plate

The paper presents results of eddy-resolving numerical simulation of the turbulent free convection boundary layer that develops along a vertical isothermally heated plate. The problem of definition of boundary conditions at the computational domain inlet, where a time-varying flow field has to be specified, is considered. Mean characteristics of inflow are defined from results of RANS simulation of a 2D boundary layer, and a synthetic turbulence generator is used to introduce unsteady fluctuations at the inlet. Calculations are performed with an in-house unstructured finite-volume code; results of comparison with known experimental data are presented. The influence of the choice of the turbulence model used for RANS-based calculations of the mean inlet flow profiles is considered.

Vertical-plate free convection boundary layer disturbed by a row of circular cylinders: RANS-based numerical simulation

This paper presents the results of RANS-based numerical simulation of 3D flow dynamics and heat transfer in the turbulent free-convection boundary layer disturbed by a transverse periodic row of finite-height cylindrical obstacles. The disturbing row elements are mounted on a vertical isothermally heated plate along which the free-convection flow develops starting from a 2D state prescribed at the computational domain inlet. The height-to-diameter ratio of the cylindrical obstacles is varied from 1 to 3, and their surface is treated as adiabatic or isothermal. Vortex flow structures and local heat transfer patterns in the front and in the rear of the introduced macroroughness are analyzed.

Disturbing effects of a cylinder-form macro-roughness on the turbulent free-convection boundary layer: Large Eddy Simulation

The paper presents results of eddy-resolving numerical simulation of non-isothermal turbulent flow in the vicinity of an adiabatic circular cylinder that disturbs the turbulent free-convection boundary layer developing along a vertical heated plate. The computational domain used includes (i) a relatively long section, where the upstream free-convection boundary layer goes over to its natural state from the model state prescribed at the inlet boundary with a synthetic turbulence generator, (ii) a mid-section covering the disturbing cylinder, mounted normally on the plate, (iii) an outlet section that is much longer than the cylinder diameter. The mathematical model is based on the Navier-Stokes equation written with the Boussinesq approximation. In the case simulated, the height of the cylinder (equal to its diameter) is about two times less than the approaching boundary layer half-width, if estimated from the velocity profile. Characteristic features of unsteady phenomena in the flow disturbed by the macro-roughness are analyzed with a focus on behavior of arising horseshoe-like vortex structures. Quantitative data on heat transfer augmentation near the cylinder is presented.

Turbulent Free-Convection Boundary Layer on a Vertical Heated Plate: Regularities of the Temperature Layer

Analysis is made of the results of experimental investigations of temperature and velocity profiles in a turbulent free-convection boundary layer on a vertical heated plate. An inference is made of the absence of adequately clear concepts of the velocity and temperature laws of the wall, as well as of the urgency of determining the main (universal) temperature, velocity, and linear scales which provide for the similarity of velocity and temperature profiles in the boundary layer cross sections. Main attention is given to the investigation of regularities of the temperature layer. In contrast to the existing one-parameter concepts of the temperature law of the wall, which are based on the assumption that the temperature profiles are independent of the dynamic characteristics, a two-parameter temperature law of the wall is suggested which takes into account the effect of the Grashof number, i.e., of the dynamic characteristics on the thermal ones. Based on the results of analysis of available experimental data, the suggested temperature law of the wall is realized in the form of a two-parameter family of logarithmic temperature profiles and a model of effective thermal diffusivity. A complete description is given of the structure of the inner region, namely, of the thermal sublayer and of the transition (buffer) and logarithmic regions. A full-scale algebraic model of turbulent thermal diffusivity for the entire layer as a whole is suggested. The similarities and differences of characteristics are observed between the flow being treated and a forced-convection turbulent boundary layer on a flat plate.

An Experimental Study of Turbulent Free Convection Boundary Layer in Air along a Vertical Plate using LDV

Local heat transfer data were obtained for turbulent free convection on a vertical surface with uniform heat generation in air and are correlated well by the relation : Nux = 0.175 (Grx*Pr)0.256 for 8×1012 <Grx*Pr <2×1014 An LDV and a thermocouple were used to measure the longitudinal and normal components of velocity, u and v, and the temperature, t, in a fully developed turbulent boundary layer. Appreciable differences of turbulence quantity distrigutions, (u'2)1/2, u'v' and v't', between the present and the previously obtained results using hot wire sensors, have been found. Present LDV results of (u'2)1/2 and u'v' have a tendency to be more similar to the previously calculated results by the mean turbulent energy method.

Interferometric measurement of fully turbulent free convective heat transfer coefficients

A Mach-Zehnder interferometer provides information about the local temperature surrounding a heated body. When using this type of interferometer, the measurement of turbulent heat transfer coefficients is often difficult to achieve, and as a result most interferometric studies of heat transfer rates have been limited to laminar flow. A differential interferometer, on the other hand, provides a measure of the temperature gradient and this type of interferometer produces fringe patterns that are an excellent means of observing the flow structure within a turbulent free convection boundary layer. Parallel fringe patterns can easily be converted to local heat transfer coefficients. Heat transfer coefficients measured in air above an isothermal inclined flat plate are presented. These are in excellent agreement with previously published results.

Turbulent free convection flow

The results of an experimental investigation of a turbulent free convection boundary layer are presented for a fluid with a low Prandtl number (mercury). The boundary layer was formed along the isothermally heated vertical wall of a cell. The measured mean temperature profiles and turbulent temperature distributions, indicated the existence of a fully developed boundary layer. In the recorded temperature spectra the high frequencies region, associated with dissipation, was found to exhibit an ƒ −3 dependence (ƒ is the frequency of the temperature fluctuations). The presence and development of a “convection subrange” was also identified, dependent upon the Rayleigh number and the location of the point of measurement with respect to the wall.

Turbulent natural convection on a vertical plate and in a vertical layer

An experimental investigation was carried out on the hydrodynamics of a turbulent freeconvection boundary layer at a vertical plate and in a vertical film of ethyl alcohol closed at both ends. Herein are presented longitudinal component profiles of the mean velocity vector and fluctuation rate of the same velocity vector component. On the basis of the present data and experimental results of other authors, the existence is shown of a quasi-stationary wall layer in the turbulent boundary layer at a vertical plate, the thickness and maximum velocity of which satisfy the condition Re 1Pr 1 2 = U max δ1 (va) 1 1 = const. It is found experimentally that along the middle of the vertical cross-section in the central part of the film the temperature gradient is dT/ dx = const. Within the region of constant temperature gradient in the turbulent boundary layer there is a quasi-stationary wall layer for which Re 1= 1 2 = U max δ1 v = const. The number Re 1, is a function of the relative temperature gradient 1 ΔT dT dx in the external side of the boundary layer.

Observations on the structure of a turbulent free convection boundary layer

The paper presents the results of experiments on the turbulent boundary layer formed adjacent to a heated vertical plate immersed in water. A brief presentation of gross features is followed by a detailed discussion of fluctuations and their attendant structural implications. Conclusions are drawn regarding the scale, intensity and intermittency of turbulence and the concept of a laminar sublayer.

An Analysis of Free-Convection about a Vertical Plate

With the use of the integrated momentum and energy equations, an approximate calculation was carried out for the flow and heat transfer in the laminar and turbulent free-convection boundary layer on a vertical flat plate. The velocity profiles employed for laminar layer are of a polinomial of the fifth order and an exponential function of y, which satisfy the following boundary condition in addition to the usual ones : [numerical formula] The calculated results agree practically well, over the entire range of Pr, with the Ostrach's. The method of attack for turbulent flow is similar in principle to that of Eckert and Jackson, though some different profiles are used for temperature and velocity fields. When the turbulent region coexists with the laminar region near the leading edge, the average Nusselt number was proved to be [numerical formula] where Grc is the critical Grashof number, at which the transition from laminar to turbulent occurs.