Modern CFD Research Articles

CFD를 이용한 모형선과 실선 스케일의 반류 비교

Assessment of hydrodynamic performance of a ship hull has been focused on a model ship rather than a full-scale ship. In order to design the propeller of a ship, model-scale wake is often extended to full-scale based upon an empirical method or designer's experience, since wake measurement data for a full-scale ship is very rare. Recently modern CFD tools made some success in reproducing wake field of a model ship, which implicates that there are some possibilities of the accurate prediction of full-scale wakes. In this paper firstly the evaluation of model-scale wake obtained by Fluent package was performed. It was found that CFD calculation with the Reynolds-stress model (RSM) provided much better agreement with wake measurement in the towing tank than with the realizable k-<TEX>$\varepsilon$</TEX> model (RKE). In the next full-scale wake was calculated using the same package to find out the difference between model and full-scale wakes. Three hull forms of KLNG, KCS, KVLCC2 having measurement data open for the public, were chosen for the comparison of resistance, form factor, and propeller plane wake between model ships and full-scale ships.

Dynamic Analysis of Francis Runners - Experiment and Numerical Simulation

The present paper shows the results of numerical and experimental modal analyses of Francis runners, which were executed in air and in still water. In its first part this paper is focused on the numerical prediction of the model parameters by means of FEM and the validation of the FEM method. Influences of different geometries on modal parameters and frequency reduction ratio (FRR), which is the ratio of the natural frequencies in water and the corresponding natural frequencies in air, are investigated for two different runners, one prototype and one model runner. The results of the analyses indicate very good agreement between experiment and simulation. Particularly the frequency reduction ratios derived from simulation are found to agree very well with the values derived from experiment. In order to identify sensitivity of the structural properties several parameters such as material properties, different model scale and different hub geometries are numerically investigated. In its second part, a harmonic response analysis is shown for a Francis runner by applying the time dependent pressure distribution resulting from an unsteady CFD simulation to the mechanical structure. Thus, the data gained by modern CFD simulation are being fully utilized for the structural design based on life time analysis. With this new approach a more precise prediction of turbine loading and its effect on turbine life cycle is possible allowing better turbine designs to be developed.

AD-based perturbation methods for uncertainties and errors

The progress of automatic differentiation (AD) and its impact on perturbation methods is the object of this paper. AD studies show an important activity for developing methods addressing the management of modern CFD kernels, taking into account the language evolution and intensive parallel computing. The evaluation of a posteriori error analysis and of resulting correctors will be addressed. Recent works in the AD-based construction of second-derivatives for building reduced-order models based on a Taylor formula will be presented on the test case of a steady compressible flow around an aircraft.

Assessment of intermittency transport equations for modeling transition in boundary layers subjected to freestream turbulence

The γ– Re θ transition model of Menter et al. [Menter, F.R., Langtry, R.B., Volker, S., Huang, P.G., 2005. Transition modelling for general purpose CFD codes. ERCOFTAC International Symposium Engineering Turbulence Modelling and Measurements] is a highly generalized transport equation model in which it has been developed based on the concept of local variables compatible with modern CFD methods where the unstructured grid and the parallel computing technique are usually integrated in. To perform the prediction with this model, two essential parameters, F length which is used to control the length of the transition region and Re θc which is used to control the onset of the transition location, must be specified to close the model. At present, both parameters are proprietary and their formulations are unpublished. For the first time here, the relations for both parameters are formulated by means of numerical experiments and analysis under the assumption of Re θc = Re θt corresponding with the bypass transition behavior. Based on this analysis, the optimized values of the parameters are found and their relations can be constructed as follows: Re θc = 803.73( Tu ∞, le + 0.6067) −1.027 and F length = 163 ln( Tu ∞, le) + 3.625. The performance of this transition model is assessed by testing with the experimental cases of T3AM, T3A, and T3B. Detailed comparisons with the predicted results by the transition models of Suzen and Huang [Suzen, Y.B., Huang, P.G., 2000. Modeling of flow transition using an intermittency transport equation. J. Fluids Eng. 122, 273–284] and Lodefier et al. [Lodefier, K., Merci, B., De Langhe, C., Dick, E., 2003. Transition modelling with the SST turbulence model and intermittency transport equation. ASME Turbo Expo, Atlanta, GA, USA, June 16–19], and also with the predicted results by the k– ε model of Launder and Sharma [Launder, B.E., Sharma, B., 1974. Application of the energy dissipation model of turbulence to the calculation of flow near a spinning disk. Letters in Heat and Mass Transfer 1, 131–138] and the SST model of Menter [Menter, F.R., 1994. Two-equation eddy-viscosity turbulence models for engineering applications. AIAA 32, 1598–1605] are presented here. Results show that the proposed relations for F length and Re θc can work well with the model to give good agreement in predicting the transition.

Foreword to the Special Issue on Computational Heat Transfer

Foreword to the Special Issue on Computational Heat Transfer

Transition Modelling for General Purpose CFD Codes

The paper addresses modelling concepts based on the RANS equations for laminar-turbulent transition prediction in general-purpose CFD codes. Available models are reviewed, with emphasis on their compatibility with modern CFD methods. Requirements for engineering transition models suitable for industrial CFD codes are specified. A new concept for transition modeling is introduced. It is based on the combination of experimental correlations with locally formulated transport equations. The concept is termed LCTM – Local Correlation-based Transition Model. An LCTM model, which satisfies most of the specified requirements is described, including results for a variety of different complex applications. An incremental approach was used to validate the model, first on 2D flat plates and airfoils and then on to progressively more complicated test cases such as a three-element flap, a 3D transonic wing and a full helicopter configuration. In all cases good agreement with the available experimental data was observed. The authors believe that the current formulation is a significant step forward in engineering transition modeling, as it allows the combination of transition correlations with general purpose CFD codes. There is a strong potential that the model will allow the 1st order effects of transition to be included in everyday industrial CFD simulations.

SU‐FF‐I‐91: Computational Representation of In‐Vivo Acquired Stenotic Renal Artery Geometries Using Turbulence Modeling

Purpose: To report on the in‐vivo simulation of arterial blood flow hemodynamics using anatomically realistic CFD models constructed through non‐invasive CT imaging, in order to facilitate therapeutic decision‐making. The methodology is pivotally applied in a case of severe RAS. Method and Materials: The highly accurate SST turbulence model is introduced in the simulation of blood flow in the severely constricted region at the ostium of a human renal artery, obtained from CT scans. This work also proposes a systematic creation and examination of simulated cases of the particular arterial segment after endovascular stent deployment. Results: The significance of specific factors relevant to the arterial geometry and their impact on blood flow across the arterial segment are investigated. The increased resistance to flow due to the presence of the atheromatic plaque is quantified. The presence of a stent at the blood inlet of the artery leads to predictable higher velocity profiles at the outlet. The internal differential pressure profiles, the WSS, and the net forces acting on the plaque at peak pulse in the RAS case are found to be considerably higher than the non‐obstructed one. Each of these hemodynamic factors is shown to retain sufficient high contrast between stenosed and unimpeded cases in view of their consideration as potential indicators of clinically significant RAS. Conclusion: The proposed methodology provides insight on a RAS development, with estimates of a healthy situation and after a potential stent implementation, using modern CT and CFD methods. It allows interventionalists to quantitatively evaluate the hemodynamic significance of RAS both non‐invasively and within reasonable time. It provides diagnostic indicators for the selection of angioplasty and/or stenting, ideally even before the patient becomes symptomatic.

On the combination of CFD and rate-based modelling in the simulation of reactive separation processes

Reactive separations (RS) combining mass transfer with simultaneous chemical reactions inside one column unit provide an important synergistic effect and bring about several advantages. However, the design and modelling of the RS columns are more sophisticated than traditional operations and thus rigorous and reliable description methods are necessary. Besides, in RS, the influence of column internals increases significantly. These internals have to enhance both separation and reaction and maintain a sound balance between them. This is the reason why, frequently, the application of process-specific column internals, rather than internals currently available on the market, is desirable. In this paper, a new modelling methodology for RS is suggested which exploits a combination of modern CFD facilities and the rate-based process simulation approach. The task of CFD is to obtain hydrodynamic and mass transfer correlations necessary for the process description. These correlations are then used in a rate-based simulator to determine required column profiles of the process variables. Both CFD and process simulation are described and illustrated with industrially relevant examples. The sensitivity studies provide information on the influence of important hardware parameters and show optimisation tendencies. The CFD simulations can be regarded as virtual experiments carried out without really existing column units which help to predict the performance of the internals by varying their geometrical and structural properties. Supplemented by the rate-based simulations, the suggested method can be viewed as a principal way towards the virtual prototyping of internals.

On the development of new column internals for reactive separations via integration of CFD and process simulation

Reactive separations combining mass transfer with simultaneous chemical reactions impose additional requirements on the applied column internals. Traditionally, reactive separation processes have been optimised via operational parameter adjustment. By this way, a direct influence of the internals geometry and structure cannot be taken into consideration. Therefore, the application of process-specific column internals, rather than of the internals currently available on the market, is desirable. Development of new internals is a major focus of interest in an European project entitled Intelligent Column Internals for Reactive Separations. In particular, such development is achieved by the application of modern CFD facilities combined with the rigorous, rate-based process simulation. An important challenge is to provide an opportunity of obtaining hydrodynamic and mass-transfer correlations, necessary for the process description, not only from the experimental measurements, but also directly from the CFD simulations. In this paper, a way to generate virtual correlations is demonstrated for a single-phase flow through structured packings. The rate-based simulation is illustrated with the heterogeneously catalysed synthesis of n-hexyl acetate via reactive distillation.

Development of a New Flap for a Light Utility Transport Aircraft

The Bush Hawk XP, which is produced by Found Aircraft of Canada, is a small, piston-engined utility transport aircraft suitable for bushplane operations. The original version was made in the 1960s, and it uses a sealed, plain hinged flap. In the latest production version the gross weight is increased substantially, necessitating an improved flap design to achieve better airfield distances and climb performance.This paper describes the aerodynamic design and development of a new single-slotted flap for the Bush Hawk. The flap shape and locations when deflected were optimized using modern CFD methods, and wind tunnel tests were bypassed. The features of the flap aerodynamic design, the aircraft structure, and the flap drive systems are described. Flight test results are presented for the Bush Hawk with the new flap; they show outstanding performance in its category.

An investigation of the grid fins as control surface devices for missiles

AbstractRecently, grid fins have been receiving increasing attention as a practical and efficient means of controlling missile trajectory. Preliminary studies at IAR have demonstrated that modern CFD methods can be used for computing flows past complex grid fin type configurations, and that these methods are more soundly physically based than the earlier vortex lattice and/or shock expansion methods.The current paper addresses the issue of the grid fin size, in terms of both the panel thickness and the frontal shape. The study covers three thicknesses for the grid fin panel, with front shapes having a simple blunt square face, as well as a sharp knife-edge shape. In addition, an important aspect of the present investigation is to quantify the aerodynamic effect of the ramp fairing installed immediately upstream of the blunt base upon which the grid fin assembly resides. A comparison of flow field characteristics and aerodynamic coefficients of the grid fin assembly, with and without the fairing ramp, would provide a direct means of evaluating the effect of the ramp. At this stage, the investigations are based on Euler calculations. The present study focuses on a standard grid fin configuration mounted on a generic cylindrical body.

回流式風洞における煙を使用した可視化実験装置

An apparatus for flow visualization around oscillating wing was designed and assembled. To validate the modern CFD. the experimental data not only about the pressure distributions on the wing surfaces but also the flow patterns around the wing are needed. The present system aims to offer the experimental information on the flow patterns around oscillating 2-D wings. In this report, the outlines of the driving mechanism for the 2-D wing and the smoke generator are presented together with the related other devices. It is shown that the system works well in the low-turbulence wind tunnel at NAL and expected that it can be used for the research of low speed unsteady aerodynamics and aeroelasticity.

Turbulent channel flow simulations using a coarse-grained extension of the lattice Boltzmann method

A coarse-grained version of the lattice Boltzmann (LB) method is developed with the intent of enhancing its geometrical flexibility so as to be able to tackle a wider class of flows of engineering interest. To this purpose, the original uniform LB technique is combined with standard finite-volume techniques based upon a blend of piecewise constant and piecewise linear interpolation schemes. A series of validation tests for the three-dimensional channel flow with one-dimensional (cross-channel) statistical behaviour are presented. The main conclusion is that, although the method does indeed mark a significant stride forward with respect to the original uniform LB scheme, better interpolation schemes should be developed before the coarse-grain LB can become fully competitive with modern CFD schemes.