Unsteady Flow Phenomena Research Articles

1628 軸流圧縮機動翼列の失速点近傍における非定常流動現象の数値解析(J12-1 流体機械に関連した流体解析と数値最適化技術(1),J12 流体機械に関連した流体解析と数値最適化技術)

The tip clearance of rotor blades is one of the important factors to be more investigated, because the rotating stall inception originates at a tip leakage flow region in most of axial flow compressors. However, the effect of the tip clearance on unsteady flow field at near-stall condition has not been investigated in detail. In the present work, the unsteady flow phenomena in a low-speed axial flow compressor stage with the large and small tip clearances at near-stall condition have been investigated numerically by URANS (Unsteady Reynolds-averaged Navier-Stokes) simulations. The results show the clear difference caused by the effect of the tip clearance on the flow field at near-stall condition. Namely, the occurrence of separation at the blade suction surface induced instability for small tip clearance, while the occurrence of tip leakage vortex breakdown induced instability for large tip clearance.

Field Velocity Approach and Geometric Conservation Law for Unsteady Flow Simulations

A technique to model unsteady flow phenomenon which involve simultaneous elastic motions of the boundaries and interaction with gust fields is discussed. The field velocity approach is used to include the effect of the gust fields caused by a vortex wake in to the flow computations. Elastic deformations of the boundaries causes changes in grid cell volumes which requires the rigorous enforcement of the Geometric Conservation Law. Mathematical modeling required to enforce conservation when performing such unsteady flow computations is detailed. The application of the technique developed to a variety of problems from simple 2-D model problems to more complex realistic problems are detailed. The predictions obtained are validated with exact analytical results/ experimental data as appropriate. Overall, the field velocity approach together with a technique to enforce the Geometric Conservation Law is found to provide a powerful tool for unsteady flow simulation.

The use of turbulent inflow conditions for the modelling of a high aspect ratio jet

The aim of this work is to investigate the capabilities of a turbulent inflow method. The application selected for this study is the high aspect ratio jet. The complexities associated to the numerical modelling of a high aspect ratio jet are embedded in its physical complexity. Consequently, the numerical modelling does not only require a high mesh resolution, but furthermore it requires a careful mesh construction, inflow conditions and subgrid-scale modelling to make an accurate computation of the unsteady flow phenomena. The results indicate that increased grid resolution and enhanced turbulence modelling reduce the effect of the imposed flow fluctuations. It is concluded that for a high aspect ratio free-jet turbulent inflow conditions are effective if the mesh resolution is insufficient to trigger shear-layer instabilities. Applied with sufficient mesh resolution the onset of vortex motions will occur in the shear layer, hence there is limited inflow sensitivity.

Modelling fluid flow: the state of the art

Application of CFD-Techniqus in Fluid Machinery.- Modelling Separation from Curved Surfaces with Anisotropy-Resolving Turbulance Closures and Related Supplementary Observations on LES.- Development of Fluid Mechanics Methods in the 20th Century and the Application to Laminar and Turbulent Flow Investigations.- Theodor Von Karman: A Global Life.- Workshop on Complex Flows with Chemical Reactions.- Numerical Modelling of Turbomachinery fluid dynamics.- Challenges in Optimisation of Axial Flow Turbomachinery Blades for 3D Flow, Including Sweep and Dihedral Effects.- Flame Stabilization of Highly Diffusive Gas Mixtures in Porous Inert Media.- A Hybrid Approach to Model Combustion Instabilities.- Modelling of Reaction and Transport in Chemical Vapor Deposition: Predictions of Growth and Uniformity of Tin Oxide Films.- Electromagnetically Forced Convection during Solidification of a Binary Metal Alloy.- Sound Generated by an Unsteady Flow Field, Using a Hybrid Method.- Influence of Cross-Sectional Configuration on Karman Vortex Excitation.- A New Wall-Function Strategy Applied to Mixed Convection in Vertical Annular Passages.- RANS simulations for film-cooling analysis and design.- Very Large Eddy Simulation for the Prediction of Unsteady Vortex Motion.- A New Stabilized Finite Element Method for Advection-Diffusion-Reaction Equations Using Quadratic Elements.- Non-Uniform Flow in a High Speed Compressor due to Asymmetric Tip Clearance.- Deterministic Unsteady Vorticity Field in a Driven Axisymmetric Cavity Flow.- An Application of the Buffer Layer Technique to Computations of Flow in Turbomachinery.- CFD-Analysis of Secondary Flows and Pressure Losses in a NASA Transonic Turbine Cascade.- Numerical Optimisation of High Pressure Steam Turbine Stage.- An Experimental Investigation of the Unsteady Flow in a Two-Stage Low-Pressure Research Turbine.- About the Onset of Partload Instability of Swept and Unswept Axial Pump Blades.- Mixing Interface Algorithm for 3D Turbulent Flow Analysis of the GAMM Francis Turbine.- Study of Blade to Blade Flows and Circumferential Stall Propogation in Radial Diffusers and Radial Fans by Vortex Cloud Analysis.- New Approach to Radial Compressor Return Channel Design.- The Flow and Head Distribution within the Volute of a Centrifugal Pump in Comparison with the Characteristics of the Impeller without Casing.- Investigation of Unsteady Flow Phenomena in a Side Channel Vacuum Pump by Particle- image Velocimetry.

Detached Eddy Simulation of Film Cooling Performance on the Trailing Edge Cutback of Gas Turbine Airfoils

The present study deals with the unsteady flow simulation of trailing edge film cooling on the pressure side cut back of gas turbine airfoils. Before being ejected tangentially on the inclined cut-back surface, the coolant air passes a partly converging passage that is equipped with turbulators such as pin fins and ribs. The film mixing process on the cut back is complicated. In the near slot region, due to the turbulators and the blunt pressure side lip, turbulence is expected to be anisotropic. Furthermore, unsteady flow phenomena like vortex shedding from the pressure side lip might influence the mixing process (i.e., the film cooling effectiveness on the cut-back surface). In the current study, three different internal cooling designs are numerically investigated starting from the steady RaNS solution, and ending with unsteady detached eddy simulations (DES). Blowing ratios M=0.5; 0.8; 1.1 are considered. To obtain both, film cooling effectiveness as well as heat transfer coefficients on the cut-back surface, the simulations are performed using adiabatic and diabatic wall boundary conditions. The DES simulations give a detailed insight into the unsteady film mixing process on the trailing edge cut back, which is indeed influenced by vortex shedding from the pressure side lip. Furthermore, the time averaged DES results show very good agreement with the experimental data in terms of film cooling effectiveness and heat transfer coefficients.

Numerical simulations of the unsteady flowfield of helicopter rotors on moving embedded grids

An Euler solver was developed for the prediction of 3-D unsteady flow phenomena that occur on helicopter rotors in forward flight, based upon a cell-centered finite volume approximation and dual-time method. In order to reduce false diffusion of vorticity and account for the rigid blade motions in rotation, flapping and pitching, a moving embedded grids methodology was adopted. Aiming at the key identification issue of donor elements in embedded grids, the present analysis implements a new searching scheme, the Pseudo-Searching Scheme of Donor Elements (PSSDE), for improvement purposes. The efficient dual-time method and moving embedded grids methodology make it feasible to compute the complicate flowfield of helicopter rotors on typical PC computers. Preliminary simulation results on surface pressure distributions were presented for a two-bladed Caradonna model rotor in forward flight at different collective pitch angles and flapping angles. Good agreements are found with measurements or other calculated results. Then unsteady lifting results, simulated for three different blade tip planforms, demonstrate the benefit of swept tips in suppressing supercritical flow. Additionally, the vorticity magnitude contours give some qualitative pictures on the evolvement of the distorted rotor wake.

Unsteady Near‐Critical Flows in Microgravity

This paper presents analysis of the different time scales associated with unsteady fluid flow phenomena near the thermodynamical critical point and that are typical for experiments carried out in microgravity. A focus of the paper is modeling the initial stage of convection under low and zero gravity on the basis of the two-dimensional Navier-Stokes equations for a compressible gas with the Van der Waals state equation. We also consider a thermoacoustic problem on the basis of three-dimensional linearized equations for an isentropic inviscid gas near the critical point in zero gravity. We compare the heat transfer due to unsteady convection and the piston effect in an enclosure with side heating in zero and low gravity with pure conductivity.

Recent developments in delta wing aerodynamics

AbstractRecent developments in delta wing aerodynamics are reviewed. For slender delta wings, recent investigations shed more light on the unsteady aspects of shear-layer structure, vortex core, breakdown and its instabilities. For nonslender delta wings, substantial differences in the structure of vortical flow and breakdown may exist. Vortex interactions are generic to both slender and nonslender wings. Various unsteady flow phenomena may cause buffeting of wings and fins, however, vortex breakdown, vortex shedding, and shear layer reattachment are the most dominant sources. Dynamic response of vortex breakdown over delta wings in unsteady flows can be characterised by large time lags and hysteresis, whose physical mechanisms need further studies. Unusual flow–structure interactions for nonslender wings in the form of self-excited roll oscillations have been observed. Recent experiments showed that substantial lift enhancement is possible on a flexible delta wing.

ケーシング部からの噴き出し流れを有するタービン段の空力性能及び非定常流れ挙動 : CFD及びEFDによる解明(OS13-3 流体機械に関連した内部複雑流動の解析と計測(流体機械 回転流れ))

ケーシング部からの噴き出し流れを有するタービン段の空力性能及び非定常流れ挙動 : CFD及びEFDによる解明(OS13-3 流体機械に関連した内部複雑流動の解析と計測(流体機械 回転流れ))

UNSTEADY FLOW PHENOMENON IN A HEATED MICROCHANNEL AT HIGH HEAT FLUXES

An experimental study was performed on the unsteady flow phenomena of the two-phase flow in a single microchannel with the inside diameter of 500 μm. Depending on the inlet liquid temperature, two types of unsteady flow were identified. The first type is the long period/large amplitude liquid/two-phase alternative flow, occurring at the liquid temperature of the water tank of 100°C. It behaves the large amplitude oscillations of fluid/wall temperatures and mass flux which have clear switch points from liquid to two-phase flow with a relative long section of the two-phase flow between them. It is conformed such that slow transition flow can be self-sustained with the feedback control of the mass flux and the entrance liquid temperature of the microchannel, incorporating the system geometry, dimensions, and the microchannel itself. The second type of unsteady flow appears when the liquid temperature of the water tank varying from room to 80°C, is named as the long period/small amplitude periodic subcooled flow boiling. The outlet temperature displays the positive signal pulse periodically, followed by negative pressure drop signal pulse. The flow is indicated to exhibit the liquid/two-phase alternative flow with a short section of boiling and bubble emission between them. The oscillation time period can be up to 50 sec at heat flux just beyond the boiling incipience, and is in the range of 5 to 10 sec at high heat flux approaching the critical heat flux. For a given pressure drop and inlet liquid temperature, the mean mass flux only has a slight decrease with increasing the heat flux after the boiling incipience, providing the evidence that the periodic flow possesses a short section of two-phase flow for a full cycle.

Comparison of computed water hammer pressures with test results for the Çatalan power plant in Turkey

The closure of a flow control mechanism at the end of a penstock may cause significant transient pressures. The governing equations of this unsteady flow phenomenon, the conservation of mass and conservation of momentum, are solved by the method of characteristics using various computational schemes. A study was carried out to compare the computational results of the transient pressures with measured prototype data for the Çatalan Power Plant in Turkey. Prototype data were collected during the closure of the wicket gates of a Francis turbine from the test procedures of "load rejection", "emergency shut down", and "quick stop". The numerical results for the transient pressures just upstream of the turbine are compared with the measured data. The general agreement between the theoretical and experimental results is found to be quite reasonable.Key words: transient pressure, water hammer, power plant, Francis turbine, penstock, method of characteristics.

Standardized pseudospectral formulation of the inviscid supsersonic blunt body problem

A highly accurate pseudospectral numerical approximation to the generalized coordinate, nonconservative form of the Euler equations is implemented for supersonic flow over an axisymmetric blunt body geometry; shock fitting is employed to maintain global accuracy and minimize the corrupting influence of numerical viscosity. The variables in the Euler equations as well as the physical grid coordinates are collocated via Lagrange interpolating polynomials and the problem is then cast in the standard form of a large system of ordinary differential equations, d x / dτ= q ( x ) , which can be solved using standard solution techniques that do not require an explicit criteria for the minimum time step. Code verification is performed by demonstrating through a series of grid refinement tests that the error in the approximation to a Taylor–Maccoll solution converges to 10 −12. Grid refinement tests for flow over a blunt body show convergence of the numerical error also to 10 −12. The code is validated for supersonic flow over a blunt body by comparison with the modified Newtonian approximation for the surface pressure distribution and empirical predictions for the shock shape. The ability of the method to capture unsteady flow phenomena is demonstrated on the problem of a planar acoustic wave interacting with an attached shock.

Cavitation phenomena in mechanical heart valves: the role of squeeze flow velocity and contact area on cavitation initiation between two impinging rods

In this study, the closing dynamics of two impinging rods were experimentally analyzed to simulate the cavitation phenomena associated with mechanical heart valve closure. The purpose of this study was to investigate the cavitation phenomena with respect to squeeze flow between two impinging surfaces and the parameter that influences cavitation inception. High-speed flow imaging was employed to visualize and identify regions of cavitation. The images obtained favored squeeze flow as an important mechanism in cavitation inception. A correlation study of the effects of impact velocities, contact areas and squeeze flow velocity on cavitation inception showed that increasing impact velocities results in an increase in the risk of cavitation. It was also shown that for similar impact velocities, regions near the point of impact were found to cavitate later for those with smaller contact areas. It was found that the decrease in contact areas and squeeze flow velocities would delay the onset and reduce the intensity of cavitation. It is also interesting to note that the squeeze flow velocity alone does not provide an indication if cavitation inception will occur. This is corroborated by the wide range of published critical squeeze flow velocity required for cavitation inception. It should be noted that the temporal acceleration of fluid, often neglected in the literature, can also play an important role on cavitation inception for unsteady flow phenomenon. This is especially true in mechanical heart valves, where for the same leaflet closing velocity, valves with a seat stop were observed to cavitate earlier. Based on these results, important inferences may be made to the design of mechanical heart valves with regards to cavitation inception.

Investigations of Unsteady and Asymmetric Flow Phenomena in Continuous Casting Moulds by Advanced Simulation Techniques

For further improvement of the continuous casting process a more detailed insight into the unsteady flow conditions in the mould is of great importance. Due to the difficulties to obtain this information directly in an operational plant, simulation techniques can be successfully used to extend the relevant knowledge. Therefore, simultaneously measurements of both, unsteady flow velocity using particle‐image‐velocimetry and mould level fluctuations using ultrasonic sensors, were carried out in physical models providing correlation between casting parameters and asymmetric flow behaviour. The numerical simulation of unsteady multiphase flow conditions shows time‐dependent varying local mould level fluctuations indicating an asymmetrical distribution across the mould. Both simulation approaches provide important information for increase in process stability, productivity and product quality.

Experimental investigation of unsteady flow phenomena in a three-stage axial compressor

In the past years, a three-stage axial compressor equipped with a modern controlled diffusion airfoil (CDA) blading has been investigated in much detail, applying state-of-the-art steady and unsteady measurement techniques, at RWTH Aachen University. The compressor under investigation exhibits design features of real industrial compressors. By performing high-resolution measurements both in space and time, a thorough insight into various flow phenomena in the compressor has been achieved, leading to a better understanding of various flow phenomena such as rotor—stator interaction, tip clearance flow and viscous flow effects in a multistage compressor environment. After a short summary of some performance characteristics at design and off-design, this paper focuses on the analysis of interaction phenomena present in the three-stage axial compressor. The interaction phenomena are described on a more global scale. In order to quantify the upstream and downstream influence of the three rotor blades, a suitable parameter is presented.

Numerical Simulation of Unsteady Turbulent Flow Around Maneuvering Prolate Spheroid

A three-dimensional Reynolds averaged Navier-Stokes method for unsteady turbulent flow around a maneuvering vehicle was developed and applied to a model problem concerning an extreme case of submarine maneuvers. A body force term is added in the momentum equations to take into account the inertial motion in the body-fixed coordinate system. The Spalart and Allmaras turbulence model is employed for turbulence closure. An artificial compressibility is introduced into the continuity equation for velocity-pressure coupling. The governing equations are discretized by second-order accurate finite volume method in space and second-order accurate backward scheme in time. The computational results are analyzed with global and local quantities and validated by comparison with experimental data. Overall, the present method performs quite well in predicting the unsteady flow phenomena associated with a maneuvering prolate spheroid, and the results compare well with available experimental data

비정상 압력측정을 통한 축류휀 유동특성에 관한 연구

This paper presents an experimental study on the unsteady flow phenomena such as leakage flow and rotating stall which have influences on the performance and stability of an axial flow fan. For this study, unsteady pressure were measured using high frequency pressure transducers mounted on the casing wall of rotor passage and analyzed by Double Phase-Locked Averaging Technique. As the flow rate was reduced to near stall point, the pressure difference between the pressure and the suction side of the blade was increased especially near the leading edge and the lowest pressure zone of suction side was gradually developed. From the result of unsteady pressure field on the casing wall, one period of rotating stall was divided into three zones and the flow characteristics on each zone were described in detail.

Characteristics of Unsteady-Flow Phenomena in a Channel Radial Diffuser

Detailed measurements of the unsteady periodic-flow characteristics were taken in a radial vaneless diffuser. The measurements were made using hot-wire miniature probes and several static- and dyna...

Unsteady flow phenomena: implications on the design of experimental facilities

Unsteady flow phenomena: implications on the design of experimental facilities

Comparison of numerical methods applied to the flow over wall-mounted cubes

Latest developments in the simulation of turbulence by detached eddy simulation (DES) have suggested that this technique might be able to replace large eddy simulation (LES) within the next decade. The results of the flow past a square cylinder show that this approach is quite inexpensive compared to LES while capturing the most important features of the flow. This study extends the range of applications of DES towards a fully unsteady three-dimensional case with strong streamline curvature, which is known to be a major problem for Reynolds-averaged Navier–Stokes equation (RANS) methods. The case considered is the turbulent flow over wall-mounted cubes at a Reynolds number of Re=1.3×10 4. The results demonstrate that DES is able to capture the most dominant flow patterns like LES, while RANS only gives a only a poor representation of the unsteady flow phenomena.