Flow over riblet curved surfaces

The present work studies the mechanics of turbulent drag reduction over curved surfaces by riblets. The effects of surface modification on flow separation over steep and smooth curved surfaces are investigated. Four types of two-dimensional surfaces are studied based on the morphometric parameters that describe the body of a blue whale. Local measurements of mean velocity and turbulence profiles are obtained through laser Doppler anemometry (LDA) and particle image velocimetry (PIV).Indisponível

Asymptotic analysis of turbulent boundary-layer flow of purely viscous non-Newtonian fluids

The asymptotic structure of turbulent boundary layers of purely viscous non-Newtonian systems is investigated through the intermediate variable technique. The cases of power-law and Carreau fluids are discussed in detail. Results show that a classical two-layered structure persists, with a viscous layer thickness that is dependent on the power-law index, n, and a logarithmic solution in the fully turbulent region. For Carreau fluids, in general, a three-layered structure emerges, with two nested viscous sub-layers. Experimental and numerical data from other authors are used to determine the functional behaviour of the linear coefficient of the log-law with n.Indisponível

Wall shear stress measurements and parametric analysis of impinging wall jets

This paper investigates the influence of some governing parameters on the near wall characteristics of a circular impinging jet onto a smooth flat plate. Laser Doppler anemometry (LDA) is used to characterize the mean and turbulent fields including the wall shear stress. The experiments were conducted at one nozzle-to-plate space (H/D = 2) and Reynolds number of 47,100. The work makes a parametric analysis of impinging jets based on (i) conventional parameters that include the nozzle diameter, the nozzle-to-plate distance and the bulk velocity of the jet and (ii) gross parameters like the jet momentum flux. Parametrization schemes based on conventional quantities are shown to be very sensitive to the particular choice of reference quantity, resulting in functional behaviours that can be represented through either power law or linear expressions. On the other hand, it is shown that the jet momentum flux and the kinematic viscosity suffice to determine the mean and fluctuating flow parameters, even in the initial region of wall jet development (1 < r/D < 5). With the latter choice, the streamwise variation of the maximum mean velocity and maximum Reynolds longitudinal stress are shown to decay according to power law expressions. A particular near wall parametrization scheme for the mean velocity profile that resorts to a scaling procedure based on the stream-wise evolution of the flow characterized by its maximum velocity is also presented. Higher-order moments of the velocity fluctuations are discussed.Indisponível

Velocity and temperature profiles, wall shear stress and heat transfer coefficient of turbulent impinging jets

The purpose of this work is to present a set of empirical equations that can be used to predict the flow dynamics and the heat transfer properties of impinging jets over a wide range of Reynolds number, nozzle-to-plate spacing and radial position along the impingement plate. The parametrization scheme proposed by Loureiro and Silva Freire (Int. J. Heat Mass Transfer, 55 (2012), 6400–6409) is used here for the prediction of the mean flow field properties. In particular, the scaling for maximum velocity distribution along the impingement plate is extended to account for nozzle-to-plate distance and Reynolds number dependence. A new methodology for the calculation of the wall shear stress is also presented. The experimental data set of Guerra et al. (Int. J. Heat Mass Transfer, 48 (2005), 2829–2840) is used to propose a description of the full mean temperature profile for the wall jet region that follows a Weibull distribution. In all, eleven different experimental data sets are considered to propose working expressions that include a piecewise Nusselt number expression that furnishes a solution valid over the whole domain of the impingement plate, including the stagnation point and the wall jet region. New values are proposed for the power indexes and multiplicative parameters. The parametric analysis considers that the flow properties can be determined in terms of gross parameters like the free-jet momentum flux.Indisponível

Slug flow in horizontal pipes with transpiration at the wall

The present work investigates the behaviour of slug flows in horizontal pipes with a permeable wall. Measurements of pressure drop and of local velocity are given for nine different flow conditions. The liquid phase velocity was measured with laser Doppler anemometry. Single-phase data are compared with the results of other authors. The influence of flow transpiration and of roughness on the features of slug flows is shown to be pronounced. A Shadow Sizer system coupled with Particle Image Velocimetry is used to account for the properties of the slug cell.Indisponível

Transient thermal boundary layers over rough surfaces

The transient heat transfer convection in a turbulent boundary layer is theoretically and experimentally studied when a time-periodical step-wise heat flux is imposed upon a rough surface. In particular, the behavior of the displacement height for the thermal boundary layer and the validity of the Reynolds analogy are analyzed. The local boundary layer characteristics, friction velocity, friction temperature and the displacement heights for the velocity and the temperature profiles, are evaluated through the graphical method of Perry and Joubert (1963) [3]. The results indicate that the displacement heights assume very different values and that the Reynolds analogy hypothesis is satisfied.Indisponível

Experimental Investigation of Turbulent Boundary Layers over Steep Two-dimensional Elevations

This work presents a laboratory study on the behavior of turbulent boundary layers over steep topographic elevations. Two main topics of interest are addressed here: (i) to investigate and characterize the separated flow region that is formed on the leeside of a steep elevation, and (ii) to evaluate the effects of flow stability conditions on the properties of boundary layers also subject to surface changes in elevation. To carry out this task, water channel and wind-tunnel investigations were conducted. For the former research topic, a neutrally stratified boundary layer was simulated in the water-channel of the Hydraulics Laboratory of FEUP. Mean and turbulent velocities were measured through laser-Doppler anemometry. Results provided a thorough description of the inner layers along the hill and inside the recirculation region. The refined near-wall data has contributed to the calculation of the friction velocity along the hill through different methods. For the latter topic, neutral, stable and unstable boundary layers were simulated in a wind-tunnel in the Laboratory of Turbulence Mechanics of COPPE/UFRJ. Simultaneous velocity and temperature fields were measured with the aid of thermal anemometry. These results allowed the characterization of the effects of the stratification on the speed-up factor, i.e. the maximum acceleration of the flow on hilltop. The present paper has introduced the concept of the heat up/down factor, in order to study the behaviour of the temperature field on the crest of the elevation.Indisponível

Note on a Parametric Relation for Separating Flow over a Rough Hill

A parametrization method used to account for the effects of flow separation and wall roughness on the lower boundary condition for turbulent boundary layers is investigated against direct numerical simulation and laser Doppler anemometry data. The numerical simulation represents flow over a smooth, flat surface with a prescribed external adverse pressure gradient. The water-channel experiments cover flow over smooth and rough hills for two specified Reynolds numbers. Global optimization algorithms based on four different direct search methods are used to assess the parametrization function, C, in terms of local mean velocity profiles and the parametrization parameters u * (friction velocity), ∂ x p (local pressure gradient), z 0 (effective roughness) and d (zero-plane displacement). The study investigates regions of attached and reversed flows, and forty-two velocity profiles are compared with the proposed expression for the function C, including two profiles that satisfy the solution of Stratford.Indisponível

Length–area–volume of long bubbles in horizontal slug flow

The morphology of long bubbles in horizontal slug flows is experimentally investigated through images obtained with a high-speed camera. The flow conditions resulted in very long and interface-perturbed bubbles, so that a special procedure needed to be developed to correctly identify all of their geometric features. The procedure furnishes a complete characterization of the three-dimensional aspects of the nose, main body, hydraulic jump and tail of bubbles, including their fractal dimension and gas–liquid interface area. Image treatment and numerical computations were carried out with Wolfram’s Mathematica 8.0 software system. The results are compared with a previously advanced theory. Characteristic volumes and areas of long bubbles in slug flows are correlated through simple power-law expressions.Indisponível

The numerical computation of near-wall turbulent flow over a steep hill

The present work performs a detailed comparison between numerical computations for the flow over a two dimensional steep hill and some newly obtained laboratory data. Six turbulence models were tested: four eddy viscosity models (k– , RNG- , k–o, SST) and two second-moment models (SSG–RSM- , BSL–RSM-o). The experiments were conducted in a water channel and were specially planned such that the large separated flow region that is formed on the lee side of the hill could be well scrutinized. The experimental results include complete profiles of the mean velocity components and of the twodimensional Reynolds stress tensor and were obtained through the laser Doppler anemometry. A particular concern of this work has been to achieve a detailed experimental and numerical characterization of the near-wall flow region. As such, for most of the measuring stations, at least eight points were located in the viscous sublayer. The work also shows the distribution of wall-shear stress in detail. The o-equation-based models were observed to perform much better than the -equationbased models. The length of separated flow region, mean velocity profiles and wall-shear stress were all reasonably well predicted. The flow properties on the hill top were particularly difficult to describe. The turbulence properties in the reversed flow region were best simulated by the BSL–RSM model. r 2008 Elsevier Ltd. All rights reserved.Indisponível