Riblet drag reduction in mild adverse pressure gradients: A numerical investigation

Riblet films are a passive method of turbulent boundary layer control that can reduce viscous drag. They have been studied with great detail for over 30 years. Although common riblet applications include flows with Adverse Pressure Gradients (APG), nearly all research thus far has been performed in channel flows. Recent research has provided motivation to study riblets in more complicated turbulent flows with claims that riblet drag reduction can double in mild APG common to airfoils at moderate angles of attack. Therefore, in this study, we compare drag reduction by scalloped riblet films between riblets in a zero pressure gradient and those in a mild APG using high-resolution large eddy simulations. In order to gain a fundamental understanding of the relationship between drag reduction and pressure gradient, we simulated several different riblet sizes that encompassed a broad range of s⁺ (riblet width in wall units), similarly to many previously published experimental studies. We found that there was only a slight improvement in drag reduction for riblets in the mild APG. We also observed that peak values of streamwise turbulence intensity, turbulent kinetic energy, and streamwise vorticity scale with riblet width. Primary Reynolds shear stresses and turbulence kinetic energy production however scale with the ability of the riblet to reduce skin-friction.     

An experimental investigation of the static pressure fluctuation mechanism for porous transonic wind tunnel wall configurations

An experimental investigation of the static pressure fluctuation generation mechanism was performed for different transonic wind tunnel test section perforated wall configurations. Different hole diameters and geometrical configurations were investigated. Most tests were carried out with isolated perforations, while some were done with a three hole, 16° perforation pattern. To suppress the oscillation amplitudes generated by perforations, splitter-plates as flow conditioning devices along the perforations were implemented on a large number of perforated transonic test section wall samples. It was found that all the hole configurations tested, regardless of diameter or shape, resonate at discrete frequencies which order themselves along several modes.     

Experimental investigation of the recovery of the total pressure in a hypersonic wind tunnel with conical and profiled nozzles

Measurements have been made of the coefficient of recovery of the total pressure of a gas flow exhausting from axisymmetric and conical profiled hypersonic nozzles into a cylindrical channel of diameter equal to or greater than the nozzle exit and also in the presence of an Eiffel chamber. The experiments were made at Mach numbers M = 4.83–12.4 in the isentropic core. It is shown that the values of differ slightly (by 5%) from the corresponding value for a normal shock wave at the number M determined for a onedimensional flow by the ratio of the area of the cylindrical channel to the area of the critical section of the nozzle.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 170–173, January–February, 1984.     

基于风洞试验的不同坡度多跨锯齿屋面风压特性研究

多跨锯齿屋面因其具有跨度大、结构轻盈的特点成为典型的风灾易损结构.目前国内规范对锯齿屋盖的风荷载规定较为粗略,简单给定了统一的风压系数参考值,在实际工程应用中具有一定的局限性.本文以某4跨锯齿房屋为研究对象,通过风洞试验测试了15°、21°、26°、30°和40°等5种不同屋面坡度对锯齿屋盖风压特性的影响.试验结果表明...     

A comparison of PIV measurements of canopy turbulence performed in the field and in a wind tunnel model

Particle image velocimetry (PIV) has been used to compare between turbulence characteristics just within and above a mature corn canopy and those of a model canopy setup in a wind tunnel (WT). The laboratory normalized mean velocity profile is adjusted using variable mesh screens to match the normalized mean shear of the corn field (CF) data. The smallest resolved scale in the field is about 15 times the Kolmogorov length scale (η_CF ≈ 0.4 mm), whereas in the WT it is 5 times η_WT (η_WT ≈ 0.15 mm). In both cases, the mean velocity and turbulence statistics are consistent with those measured using single point sensors. However, the profiles of normalized Reynolds shear stress in the field and the laboratory differ. Turbulent spectral densities calculated from PIV spatial and time series in the field display an inertial range spanning three decades. In the laboratory due to lower Reynolds numbers, the inertial range shrinks to two decades. Quadrant-Hole analysis is applied to Reynolds shear stress, vorticity magnitude and dissipation rates. In quadrants 1–3, the WT and field conditionally sampled stresses show similar trends. However, a conflicting trend is found in the sweep quadrant. The analysis confirms that sweep and ejections dominate the momentum flux and dissipation rate.The content of this paper, entitled “Applying PIV for Measuring Turbulence just within and above a Corn Canopy,” was presented at the 6th International Symposium on Particle Image Velocimetry at Pasadena, CA, USA, September 21–23, 2005.     

CFD wind tunnel test: Field velocity patterns of wind on a building with a refuge floor

This paper reports a CFD wind tunnel study of wind patterns on a square-plan building with a refuge floor at its mid-height level. In this study, a technique of using calibrated power law equations of velocity and turbulent intensity applied as the boundary conditions in CFD wind tunnel test is being evaluated by the physical wind tunnel data obtained by the Principal Author with wind blowing perpendicularly on the building without a refuge floor. From the evaluated results, an optimised domain of flow required to produce qualitative agreement between the wind tunnel data and simulated results is proposed in this paper. Simulated results with the evaluated technique are validated by the wind tunnel data obtained by the Principal Author. The results contribute to an understanding of the fundamental behaviour of wind flow in a refuge floor when wind is blowing perpendicularly on the building. Moreover, the results reveal that the designed natural ventilation of a refuge floor may not perform desirably when the wind speed on the level is low. Under this situation, the refuge floor may become unsafe if smoke was dispersed in the leeward side of the building at a level immediately below the refuge floor.     

Optimal design accounting for uncertainty in loading amplitudes: A numerical investigation

A numerical investigation is performed addressing the optimal design of stiff structures accounting for uncertainty in loading amplitudes. A minimum volume problem is endowed with a stochastic compliance constraint handling normal distributions and solved adopting mathematical programming. The formulation, originally conceived for a single load case, is extended to handle multiple load cases. Numerical simulations are performed to test the proposed algorithms, pointing out features of the numerical procedures and peculiarities of the stochastic-based optimal solutions achieved for different values of the second-order moments. Comparisons with respect to conventional deterministic layouts are provided as well.     

Characterization of plunging liquid jets: A combined experimental and numerical investigation

This paper presents a combined experimental and numerical study of the flow characteristics of round vertical liquid jets plunging into a cylindrical liquid bath. The main objective of the experimental work consists in determining the plunging jet flow patterns, entrained air bubble sizes and the influence of the jet velocity and variations of jet falling lengths on the jet penetration depth. The instability of the jet influenced by the jet velocity and falling length is also probed. On the numerical side, two different approaches were used, namely the mixture model approach and interface-tracking approach using the level-set technique with the standard two-equation turbulence model. The numerical results are contrasted with the experimental data. Good agreements were found between experiments and the two modelling approaches on the jet penetration depth and entraining flow characteristics, with interface tracking rendering better predictions. However, visible differences are observed as to the jet instability, free surface deformation and subsequent air bubble entrainment, where interface tracking is seen to be more accurate. The CFD results support the notion that the jet with the higher flow rate thus more susceptible to surface instabilities, entrains more bubbles, reflecting in turn a smaller penetration depth as a result of momentum diffusion due to bubble concentration and generated fluctuations. The liquid average velocity field and air concentration under tank water surface were compared to existing semi-analytical correlations. Noticeable differences were revealed as to the maximum velocity at the jet centreline and associated bubble concentration. The mixture model predicts a higher velocity than the level-set and the theory at the early stage of jet penetration, due to a higher concentration of air that cannot rise to the surface and remain trapped around the jet head. The location of the maximum air content and the peak value of air holdup are also predicted differently.     

A numerical investigation into the behavior of ground-supported concrete silos filled with saturated solids

This paper introduces a finite-element solution for simulating the filling process of ground-supported concrete silos filled with saturated granular material. An elasto-plastic axisymmetric finite-element model is used to represent both the granular material and the concrete silo. The interaction between the two materials is modeled using interface elements to allow for relative movement. The filling process is idealized via a multi-stage numerical technique capable of representing both undrained and drained conditions for the granular material. The effects of the relative stiffness between the foundation and wall are examined, as are the boundary conditions at the top of the structure (the roof details).Depending on the drainage properties of the stored material, the effect of the filling process may be time-dependent. The excess pore water pressure resulting from the filling process may cause a substantial increase in the hoop stresses in the wall. The predicted internal forces may be influenced by the foundation rigidity, but not by the boundary condition at the top of the wall.The results of these analyses may be used to design experiments to evaluate existing silos, or to develop filling strategies to minimize loads on existing structures.     

A numerical approach for pressure transient analysis of a vertical well with complex fractures

A new well test model for a vertical fractured well is developed based on a discrete-fracture model in which the fractures are discretized as one dimensional (1-D) entities. The model overcomes the weakness of complex meshing, a large number of grids, and instability in conventional stripe-fracture models. Then, the discrete-fracture model is implemented using a hybrid element finite-element method. Triangular elements are used for matrix and line elements for the fractures. The finite element formulation is validated by comparing with the semi-analytical solution of a single ver-tical fractured well. The accuracy of the approach is shown through several examples with different fracture apertures, fracture conductivity, and fracture amount. Results from the discrete-fracture model agree reasonably well with the stripe-fracture model and the analytic solutions. The advantages of the discrete-fracture model are presented in mesh gen-eration, computational improvement, and abilities to handle complex fractures like wedge-shaped fractures and fractures with branches. Analytical results show that the number of grids in the discrete-fracture model is 10%less than stripe-fracture model, and computational efficiency increases by about 50%. The more fractures there are, the more the com-putational efficiency increases.     

Decay of confined,two-dimensional,spatially periodic arrays of vortices: A numerical investigation

The disarrangement of a perturbed lattice of vortices was studied numerically. The basic state is an exponentially decaying, exact solution of the Navier-Stokes equations. Square arrays of vortices with even numbers of vortex cells along each side were perturbed and their evolution was investigated. Whether the energy in the perturbation grows somewhat before it decays or decays monotonically depends on the initial strength of the vortices of the basic state, the extent of lateral confinement and the structure of the perturbation. The critical condition for temporally local instability, i.e. the critical amplitude of the basic state that must be exceeded to allow energy transfer from the basic state to the perturbation, is discussed. In the strongly confined case of a square lattice of four vortices the appearance of enchancement of global rotation is the result of energy transfer from the basic state to a temporally local unstable mode. Energy is transferred from the basic state to larger-scaled structures (inverse cascade) only if the scales of the larger structures are inherently contained in the initial structure of the perturbation. The initial structure of the double array of vortices is not maintained except for a very special form of perturbation. The facts that large scales decay more slowly than small scales and that, when non-linearities are sufficiently strong, energy is transferred from one scale to another explain the differences in the disarrangement process for different initial strengths of the vortices of the basic state. The stronger vortices, i.e. the vortices perturbed in a manner that increases their strength, tend to dominate the weaker vortices. The pairing and subsequent merging (or capture) of vortices of like sense into larger-scale vortices are described in terms of peaks in the evolution of the square root of the palinstrophy divided by the enstrophy.     

The influence of the statistical properties of self-affine surfaces in elastic contacts: A numerical investigation

In the last years, an increasing number of papers has been published in the field of contact mechanics between rough fractal surfaces. The increase in research is motivated by the wide variety of natural and industrial processes that involve formation of rough surfaces and interfaces, characterized by self-similarity or self-affine properties on multiple scales. In this paper, the contact between a linear elastic half-space and a rough self-affine fractal rigid surface is studied by employing a numerical method recently developed by the authors (Putignano et al., 2012). The paper aims at investigating the influence of surface parameters as fractal dimensions, mean square slope and mean square roughness on the relation between the contact area, the load and the average separation. The results show that, for relatively small loads, the real contact area–load relationship coefficient of proportionality $\kappa$ takes the universal value $\kappa =2$ independent of the statistical properties and fractal dimension D_f of the rough surface. This universal constant is just in between the two values predicted respectively by Bush et al. (1975) and Persson (2001). We also find that the average separation vs. load relation is affected by the fractal dimension D_f of the rough surface, as higher D_f lead to an increase of the average separation. Finally, in this work, we also study the behavior of the power spectral densities of the elastically deformed surface and of the distribution of local separations. We find that the trend of this quantities is in agreement with recent theoretical predictions.     

A numerical investigation of new film cooling hole configuration at the leading edge of asymmetrical turbine blade: part A

The focus of the first part of this numerical study is to investigate the effects of two new configurations: (1) slot with cylindrical end and (2) slot with median cylindrical hole, generated by the combination between two film cooling configurations: cylindrical hole and uniform slot. Computational results are presented for a row of coolant injection holes on each side of an asymmetrical turbine blade model near the leading edge. For each configuration, three values of the radius are taken: R = 0.4, R = 0.8 and R = 1.2. The six cases simulations, thus obtained, are conducted for the same density ratio of 1.0 and the same inlet plenum pressure. A new parameter, Rc, is defined to measure the rate of blade coverage by the film cooling. Results show that, at the pressure side; for the two new configurations, the six studied cases exceed the case baseline in cooling effectiveness term with the best result obtained for R = 0.8 (case 2). For the suction side, only configurations with R = 0.4 (cases 1 and 4) provide an increase of film effectiveness compared to the case baseline. The following configuration: Cases 1 or 4 at the suction side and case 2 at the pressure side, gets the best thermal protection because of their higher coverage and strong cooling effectiveness.     

Combining magnetic resonance measurements with numerical simulations – Extracting blood flow physiology information relevant to the investigation of intracranial aneurysms in the circle of Willis

Cerebral aneurysms in the region of the circle of Willis have an incidence of 3–6% in western populations and involve the risk of rupture with subsequent subarachnoidal bleeding. The patient specific blood flow patterns are of substantial importance for understanding the pathogenesis of the lesions and may eventually contribute to deciding on the most efficient treatment procedure for a specific patient.A non-invasive method for performing in vivo measurements on blood velocity is 4D phase-contrast magnetic resonance angiography (PC-MRA), on the basis of which a flow field with all its parameters can be simulated. We are using this approach to investigate the hemodynamic parameters in the circle of Willis and, by analyzing the values at common locations of aneurysms, trying to find potential parameters to predict the development of aneurysms. Methodologically, we are acquiring the artery geometry with 3D-time-of-flight magnetic resonance (TOF) measurements and the blood velocity in the feeding arteries with 4D PC-MRA measurements in a healthy volunteer. These measurements are combined with computational fluid dynamics (CFD) to describe detailed hemodynamic patterns within the circle of Willis.