Three dimensional magnetohydrodynamic flow between two porous disks

This is a study of conducting flow in the gap between two parallel co-axial nonconducting disks of which one is rotating and the other stationary in the presence of a uniform axial magnetic field. The effect of uniform suction or injection on the velocity distribution is investigated and asymptotic solutions are obtained for R≪M ². Expressions for the average normal force and the torque on the disks are obtained. We find that all components of velocity are affected by uniform suction or injection and in particular we note that the effect of suction or injection on the radial component of velocity predominates over the effect of rotation for a given Hartmann number.     

Transient growth mechanisms of low Reynolds number flow over a low-pressure turbine blade

A direct transient growth analysis for three-dimensional perturbations to flow past a periodic array of T-106/300 low-pressure turbine fan blades is presented. The methodology is based on a singular value decomposition of the flow evolution operator, linearised about a steady or periodic base flow. This analysis yields the optimal growth modes. Previous work on global mode stability analysis of this flow geometry showed the flow is asymptotically stable, indicating a non-modal explanation of transition may be more appropriate. The present work extends previous investigations into the transient growth around a steady base flow, to higher Reynolds numbers and periodic base flows. It is found that the notable transient growth of the optimal modes suggests a plausible route to transition in comparison to modal growth for this configuration. The spatial extent and localisation of the optimal modes is examined and possible physical triggering mechanisms are discussed. It is found that for longer times and longer spanwise wavelengths, a separation in the shear layer excites the wake mode. For shorter times and spanwise wavelengths, smaller growth associated with excitation of the near wake are observed.     

Three dimensional flow motions in the viscous sublayer

We employ novel digital Fresnel reflection holography to capture the 3 D flows within the viscous sublayer of a smooth-wall turbulent channel flow at Re τ= 400. The measurements reveal unsteady and diverse flow patterns in the sublayer including nearly uniform high and low speed flows and strong small-scale(on the order of viscous wall units) spanwise meandering motions. The probability density functions(PDFs) of wall shear stresses show a clear discrepancy in high stress range with those from direct numerical simulation(DNS), which is attributed to the unresolved streamwise and spanwise motions by DNS. Moreover,the PDF of Lagrangian particle accelerations yields a stretched exponential shape like that in homogenous isotropic turbulence, indicating strong intermittency in the sublayer. We find a significant fraction of high accelerations is associated with the small-scale meandering motions. Our study helps explain the effect of sublayer-scale roughness on reducing drag and flow separation reported in the literature.     

Supercritical and subcritical dynamic flow-induced instabilities of a small-scale wind turbine blade placed in uniform flow

There is a growing interest in extracting more power per turbine by increasing the rotor size in offshore wind turbines. As a result, the turbine blades will become longer and therefore more flexible, and a flexible blade is susceptible to flow-induced instabilities. In order to design and build stable large wind turbine blades, the onset of possible flow-induced instabilities should be considered in the design process. Currently, there is a lack of experimental work on flow-induced instabilities of wind turbine blades. In the present study, a series of experiments were conducted and flow-induced instabilities were observed in wind turbine blades. A small-scale flexible blade based on the NREL 5 MW reference wind turbine blade was built using three-dimensional printing technique. The blade was placed in the test section of a wind tunnel and was subjected to uniform oncoming flow, representing the case of a parked wind turbine blade. The blade׳s tip displacement was measured using a non-contacting displacement measurement device as the oncoming wind speed was increased. At a critical wind speed, the blade became unstable and experienced limit cycle oscillations. The amplitude of these oscillations increased with increasing wind speed. Both supercritical and subcritical dynamic instabilities were observed. The instabilities were observed at different angles of attack and for blades both with and without a geometric twist. It was found that the blade twist had a significant influence on the observed instability: a blade without a twist experienced a strong subcritical instability.     

On the flow instabilities in the end-wall region of a surface mounted obstacle

Visualization of flow in the around a surface mounted obstacle revealed that the pressure gradients imposed on the boundary layer amplified the formation of turbulent spots in the end-wall region. It was observed that the juncture vortices exhibited Λ waveform instability even during the stretching phase that was followed by a rapid amplification and formation of a turbulent spot. The turbulent spot spread laterally as it convected past the attachment and corner region of the end-wall flow. At the Reynolds number used in this investigation the instabilities were always initiated in the region where the primary singular point is located.     

Performance assessment of a non-linear eddy-viscosity turbulence model applied to the anisotropic wake flow of a low-pressure turbine blade

The wake flow produced by a low-pressure turbine blade is modeled using a non-linear eddy-viscosity turbulence model. The theoretical benefit of using a non-linear eddy-viscosity model is strongly related to the capability of resolving highly anisotropic flows in contrast to the linear turbulence models, which are unable to correctly predict anisotropy. The main aim of the present work is to practically assess the performance of the model, by examining its ability to capture the anisotropic behavior of the wake-flow, mainly focusing on the measured velocity and Reynolds-stress distributions and to provide accurate results for the turbulent kinetic energy balance terms. Additionally, the contribution of each term of its non-linear constitutive expression for the Reynolds stresses is also investigated, in order to examine their direct effect on the modeling of the wake flow. The assessment is based on the experimental measurements that have been carried-out by the same group in Thessaloniki, Sideridis et al. (2011). The computational results show that the non-linear eddy viscosity model is capable to predict, with a good accuracy, all the flow and turbulence parameters while it is easy to program it in a computer code thus meeting the expectations of its originators.     

Three dimensional free convection flow and heat transfer along a porous vertical plate

The free convection flow along a vertical porous plate with transverse sinusoidal suction velocity distribution is investigated. Due to this type of suction velocity at the plate the flow becomes three dimensional one. For the asymptotic flow condition, the wall shear stress in the direction of main flow for different values of buoyancy parameter G is obtained. For G=0, the skin friction in the direction of free stream and the rate of heat transfer from the plate to the fluid are given. It is found that these results differ from those obtained by Gersten and Gross.     

Differential interferometry with adjustable spatial carrier fringes for turbine blade cascade flow investigations

 Digital evaluated differential interferometry using adjustable spatial carrier fringes was applied to flow measurements in a transonic turbine blade cascade. The interferograms were evaluated using a two-dimensional Fourier Analysis. This evaluation provided density gradient maps of the flow field in a digital form. A specially designed interferometer was used allowing adjustment of sensitivity and superposition of carrier-fringe system separately. This type of interferometer is also highly insensitive to vibration noise. For a turbine blade cooling film a comparison of this type of differential interferometry with holographic interferometry and a Schlieren visualisation is also given. As a result differential interferometry using adjustable spatial carrier fringes and digital fringe analysis gave density-gradient maps similar to Schlieren recordings but of quantitative nature, thus enabling compensation of wave front distortions. Integration of these density gradient maps resulted in density maps which were then compared to recordings done with pulsed holographic interferometry. Received: 27 December 1996/Accepted: 30 April 1997     

One‐dimensional simulation of supercritical flow at a confluence by means of a nonlinear junction model applied with the RKDG2 method

We investigate the one‐dimensional computation of supercritical open‐channel flows at a combining junction. In such situations, the network system is composed of channel segments arranged in a branching configuration, with individual channel segments connected at a junction. Therefore, two important issues have to be addressed: (a) the numerical solution in branches, and (b) the internal boundary conditions treatment at the junction. Going from the advantageous literature supports of RKDG methods to a particular investigation for a supercritical benchmark, the second‐order Runge–Kutta discontinuous Galerkin (RKDG2) scheme is selected to compute the water flow in branches. For the internal boundary handling, we propose a new approach by incorporating the nonlinear model derived from the conservation of the momentum through the junction. The nonlinear junction model was evaluated against available experiments and then applied to compute the junction internal boundary treatment for steady and unsteady flow applications. Finally, a combining flow problem is defined and simulated by the proposed framework and results are illustrated for many choices of junction angles. Copyright © 2007 John Wiley & Sons, Ltd.     

方形钝体受限绕流的三维数值模拟

采用一种具有二阶精度的分裂步有限元方法作为大涡模拟的空间离散格式,经过标准算例的验证后,对Re=1.0×104条件下的方形钝体三维受限湍流绕流流场进行了数值模拟.计算中,为消除初始效应,略去初始段的计算结果.数值分析表明在均匀来流条件下,该湍流场沿槽道轴面对称,并呈现出一定的拟周期特性.在流场特性分析的基础上,进行了湍流能耗场的分析,结果表明,方形钝体受限绕流的能耗主要集中在大涡丰富的流动区段内.计算过程反映出,采用该空间离散格式的大涡模拟方法,能够捕捉到非常丰富的涡系及涡动的时变过程,适用于方形钝体受限绕流的数值模拟.     

Three dimensional finite element analysis of the flow of polymer melts

The finite element simulation of a selection of two- and three-dimensional flow problems is presented, based upon the use of four different constitutive models for polymer melts (Oldroyd-B, Rolie-Poly, Pom-Pom and XPP). The mathematical and computational models are first introduced, before their application to a range of visco-elastic flows is described. Results demonstrate that the finite element models used here are able to re-produce predictions made by other published numerical simulations and, significantly, by carefully conducted physical experiments using a commercial-grade polystyrene melt in a three-dimensional contraction geometry. The paper also presents a systematic comparison and evaluation of the differences between two- and three-dimensional simulations of two different flow regimes: flow of an Oldroyd-B fluid around a cylinder and flow of a Rolie-Poly fluid into the contraction geometry. This comparison allows new observations to be made concerning the relatively poor quality of two-dimensional simulations for flows in even quite deep channels.     

Laminar boundary layer separation at a fin-body junction in a hypersonic flow

Planar laser-induced fluorescence (PLIF) imaging was performed to visualize the fin bow shock, separation shock, viscous shear layer and recirculation region of the flowfield at the junction of a blunt fin and a flat plate. Making use of the temperature dependence of the PLIF technique, images were made sensitive to temperature to provide qualitative information on the flowfield. The PLIF technique was also used as the basis for a flow-tagging technique, making it possible to measure a velocity component and to demonstrate the reverse flow of the separated region. Flow visualisation of the plane of symmetry allowed determination of the point of boundary layer separation, the angle of the separation shock and the bow shock standoff distance. These parameters were compared with predictions made by computational fluid dynamic simulations of the flowfield. Good agreement between theory and experiment was achieved. Comparisons between theoretical and experimental velocity measurements showed good agreement. Received 17 October 2000 / Accepted 13 November 2000     

涡轮叶片的持久寿命预测模型

镍基高温合金用于制造发动机的高压涡轮叶片.为了提高涡轮叶片持久寿命设计参数选取和设计方法的可靠性,从涡轮叶片代表性部位取材并设计、加工试验试件,进行持久寿命试验.试验过程中记录试件的变形量,进而推算出其蟠变应变,然后利用修正θ-Project Concept法来建立其持久寿命预测方程,并对其进行验证.     

Wake impacts on aerodynamic and aeroelastic behaviors of a horizontal axis wind turbine blade for sheared and turbulent flow conditions

The magnitude and temporal variations of wind speed considerably influence aerodynamic and structural responses of MW-sized horizontal axis wind turbines. Thus, this paper investigates the variations in airloads and blade behavior of a wind turbine blade resulting from operations in sheared and turbulent flow conditions. First, in order to validate the present methods, comparisons of aerodynamic results were made among the blade element momentum method, free-wake method, and numerical results from the previous studies. Then, the validated methods were applied to a national renewable energy laboratory 5 MW reference wind turbine model for fluid–structure interaction analyses. From the numerical simulations, it can be clearly seen that unfavorable airloads and blade deformations occur due to the sheared and turbulent flow conditions. In addition, it is clear that wake impacts are not as substantial at those of high wind speeds; however, the effects obviously affect the aerodynamic and structural behaviors of the blade at lower wind speeds. Therefore, it is concluded that the numerical results markedly indicate the demand for accurate assessment of wake dynamics for accurate estimations of the aerodynamic and structural responses for sheared and turbulent flow environments.     

Three dimensional unsteady convection and mass transfer flow through porous medium

The problem of three dimensional unsteady convection flow through a porous medium, with effect of mass transfer bounded by an infinite vertical porous plate is discussed, when the suction at the plate is transverse sinusoidal and the plate temperature oscillates in time about a constant mean. Assuming the free stream velocity to be uniform, approximate solutions are obtained for the flow field, the temperature field, the skin-friction and the rate of heat transfer. The dependence of solution on Pr (Prandtl number), Gr (Grashof number based on temperature), Gc (modified Grashof number based on concentration difference), Sc (Schimdt number), the frequency and the permeability parameter is also investigated.     

Dynamic study of a wind turbine blade with horizontal axis

The study of the dynamic behavior of a wind turbine with horizontal axis can be undertaken by various methods of analysis. The effects of the change of the aerodynamic flow (in the steady and unsteady cases), the variation of parameters of the cinematic movement (angle of attack, pitch angle and yaw angle) and the definition of subsystems characteristics that makes the wind turbine (blade, nacelle and pylon) allow one to characterize the structural dynamic behavior of the wind turbine. It is therefore necessary to develop these items. Once this is done, the structural dynamic behavior of the system can be improved. The term `improve' means the increase of the life duration by mastering the fatigue effects and the reduction of cost without sacrificing the aerodynamic output. The present study aims to examine the behavior of the blade, which is the main part of the wind turbine in that it that transmits forces to all other parts of the structure. The model is based on the theory of three-dimensional beams, under the assumption of variable sections of the type NACA 4415 airfoil, and takes into account membrane, transversal shear, flexion and free torsion effects. With regards to the aerodynamic loads (the lift, the drag and the pitching moment), a validation has been undertaken by considering experimental data and numerical results obtained by a CFD code (Fluent). The forces are obtained by means of a parametric CAD method interpolation of the aerodynamic poles by Bézier patch under geometrical constraints solved by a Simplex type algorithm. The emphasis is put on dynamic aspects by a complete processing of the dynamic equilibrium equation, applied to the wind turbine blade with horizontal axis.     

Turbulent kinetic energy balance measurements in the wake of a low-pressure turbine blade

The turbulent kinetic energy budget in the wake generated by a high lift, low-pressure two-dimensional blade cascade of the T106 profile was investigated experimentally using hot-wire anemometry. The purpose of this study is to examine the transport mechanism of the turbulent kinetic energy and provide validation data for turbulence modeling. Point measurements were conducted on a high spatial resolution, two-dimensional grid that allowed precise derivative calculations. Positioning of the probe was achieved using a high accuracy traversing mechanism. The turbulent kinetic energy (TKE) convection, production, viscous diffusion and turbulent diffusion were all obtained directly from experimental measurements. Dissipation and pressure diffusion were calculated indirectly using techniques presented and validated by previous investigators. Results for all terms of the turbulent kinetic energy budget are presented and discussed in detail in the present work.     

The split of vertical annular flow at a large diameter T junction

Measurements and observations have been made of the split of gas—liquid flow at a T junction where the main pipe was vertical and the side arm horizontal. All three pipes connecting to the junction were of 0.125 m dia. The gas and liquid flow rates were chosen to ensure annular flow in the inlet pipe. The resulting data have been compared with existing models.     

Multi-objective optimization of discrete film hole arrangement on a high pressure turbine end-wall with conjugate heat transfer simulations

This paper discussed a method of combining a full automatic multi-objective optimization and conjugate heat transfer calculation to obtain optimal cooling layouts on a transonic high pressure guide vane under a realistic turbine working condition. The improvement in cooling design from the optimized models was analyzed in detail, along with a discussion of sensitivities of two objective functions to five design variables. The full automatic method comprises the process of geometry creation, mesh generation, numerical solution and post data analysis. The vane is solid and the end-wall is arranged in a linear cascade. On the end-wall, film holes are all cylindrical and classified in five regions, with region A near the leading edge of the vane, region B near the suction side, regions C and D near the pressure side, and region E for the rest. Five design variables are three pitch-to-hole ratios in regions B, D, E and two compound angles of film holes in regions A and D. Two selected objective functions are area averaged overall cooling effectiveness of the end-wall and aerodynamic losses in a cross-plane at x/C_ax = 1.06 just downstream of the outlet of the cascade. For the optimization process, the multi-objective genetic algorithm based on the Non-dominated Sorted Genetic Algorithm-II was applied. The Latin hypercube sampling method was used to choose 21 experimental design points in the design space, which are also the sources for constructing the surrogate models with the Kriging model. The results demonstrate that the method using full automatic optimization and conjugate heat transfer calculation has achieved an increase of 8.7%–9.5% in area-averaged overall cooling effectiveness and a reduction of about 4.8%–6.1% in aerodynamic losses. The highest increase in cooling effectiveness exists in the region near the pressure side with a mild increase in the middle of the passage. The largest heat flux reduction exists in the regions near the pressure side and the crown of the suction side. The change of compound angle in region A near the leading edge has a negligible influence on overall cooling effectiveness but a high impact on aerodynamic losses. It's advisable to maintain the compound angle and pitch-to-diameter ratio at low values in region D near the pressure side to obtain high cooling performance.