scholarly journals Analysis of Extensive Cross-Flow Separation Using Higher-Order RANS Closure Models

2003 ◽  
Author(s):  
Joseph Morrison ◽  
Argyris Panaras ◽  
Thomas Gatski ◽  
G Georgantopoulos
Keyword(s):  
Flow Separation ◽  
Cross Flow ◽  
Higher Order ◽  
Closure Models

CFD Investigation of the Flow of Trailing Edge Cooling Slots

2018 ◽  
Author(s):  
Y. Jiang ◽  
N. Gurram ◽  
E. Romero ◽  
P. T. Ireland ◽  
L. di Mare
Keyword(s):  
Mach Number ◽  
Kinetic Energy ◽  
Flow Separation ◽  
Film Cooling ◽  
High Speed ◽  
Turbine Blades ◽  
Cross Flow ◽  
Turbulent Energy ◽  
Trailing Edge ◽  
Flow Interactions

Slot film cooling is a popular choice for trailing edge cooling in high pressure (HP) turbine blades because it can provide more uniform film coverage compared to discrete film cooling holes. The slot geometry consists of a cut back in the blade pressure side connected through rectangular openings to the internal coolant feed passage. The numerical simulation of this kind of film cooling flows is challenging due to the presence of flow interactions like step flow separation, coolant-mainstream mixing and heat transfer. The geometry under consideration is a cutback surface at the trailing edge of a constant cross-section aerofoil. The cutback surface is divided into three sections separated by narrow lands. The experiments are conducted in a high speed cascade in Oxford Osney Thermo-Fluids Laboratory at Reynolds and Mach number distributions representative of engine conditions. The capability of CFD methods to capture these flow phenomena is investigated in this paper. The isentropic Mach number and film effectiveness are compared between CFD and pressure sensitive paint (PSP) data. Compared to steady k–ω SST method, Scale Adaptive Simulation (SAS) can agree better with the measurement. Furthermore, the profiles of kinetic energy, production and shear stress obtained by the steady and SAS methods are compared to identify the main source of inaccuracy in RANS simulations. The SAS method is better to capture the unsteady coolant-hot gas mixing and vortex shedding at the slot lip. The cross flow is found to affect the film significantly as it triggers flow separation near the lands and reduces the effectiveness. The film is non-symmetric with respect to the half-span plane and different flow features are present in each slot. The effect of mass flow ratio (MFR) on flow pattern and coolant distribution is also studied. The profiles of velocity, kinetic energy and production of turbulent energy are compared among the slots in detail. The MFR not only affects the magnitude but also changes the sign of production.

The Application of Eddy-Viscosity Stress Limiters for Modeling Cross-Flow Separation

2010 ◽  
Vol 132 (9) ◽  
Author(s):  
P. A. Gregory ◽  
P. N. Joubert ◽  
M. S. Chong ◽  
A. Ooi
Keyword(s):  
Kinetic Energy ◽  
Skin Friction ◽  
Turbulent Kinetic Energy ◽  
Flow Separation ◽  
Eddy Viscosity ◽  
Cross Flow ◽  
The Body ◽  
Mean Flow ◽  
Viscosity Models ◽  
Experimental Apparatus

The ability of eddy-viscosity models to simulate the turbulent wake produced by cross-flow separation over a curved body of revolution is assessed. The results obtained using the standard k−ω model show excessive levels of turbulent kinetic energy k in the vicinity of the stagnation point at the nose of the body. Additionally, high levels of k are observed throughout the wake. Enforcing laminar flow upstream of the nose (which replicates the experimental apparatus more accurately) gives more accurate estimates of k throughout the flowfield. A stress limiter in the form of Durbin’s T-limit modification for eddy-viscosity models is implemented for the k−ω model, and its effect on the computed surface pressures, skin friction, and surface flow features is assessed. Additionally, the effect of the T-limit modification on both the mean flow and the turbulent flow quantities within the wake is also examined. The use of the T-limit modification gives significant improvements in predicted levels of turbulent kinetic energy and Reynolds stresses within the wake. However, predicted values of skin friction in regions of attached flow become up to 50% greater than the experimental values when the T-limit is used. This is due to higher values of near-wall turbulence being created with the T-limit.

Frequency Domain Model for Prediction of Stochastic Vortex Induced Vibrations for Deep Water Risers

2008 ◽  
Author(s):  
Halvor Lie ◽  
Carl M. Larsen ◽  
Karl Erik Kaasen
Keyword(s):  
Fatigue Damage ◽  
Travelling Waves ◽  
Cross Flow ◽  
Higher Order ◽  
Domain Model ◽  
Limited Information ◽  
Time Sharing ◽  
New Model ◽  
Discrete Frequency ◽  
Uniform Current

This paper describes a new model for prediction of fatigue damage from VIV in risers. The method will overcome some of the shortcomings of previous methods. A fully 3D model is proposed, “cross-flow” and “in-line” response are predicted, response at higher order harmonic components will be added, and the stochastic nature of the response is accounted for by introducing a time varying envelope function combined with “time sharing” between dominating response frequencies. A model that reflects this behaviour is considered to be more realistic and is more likely to predict lower fatigue damage than the traditional discrete-frequency models. The model will predict a response that will appear as a combination of standing and travelling waves depending on boundary conditions, damping and load distribution. Fatigue damage will therefore become more evenly distributed along the riser, and less concentrated at anti-nodes for (dominating modes) than seen from traditional discrete frequency models. The proposed model needs empirical coefficients for simultaneous IL and CF response. In principle this requires a data base of added mass, excitation and damping coefficients for varying flow conditions and response frequencies, combinations of response amplitudes in both directions, varying phase between the two response components and even the presence of higher order motion components. Such data do not exist. We have therefore proposed to use the limited information we have on this matter at present. Future improvement of the model might therefore be possible if more data becomes available. The new model will be implemented in the VIVANA program and the enhancement of the code is in progress. The paper will present the background of the model, the basic assumption of the new model and a comparison between preliminary results obtained from a preliminary code and model test results. The cases include both 2D uniform current conditions and 3D (non-uniform) current conditions.

Control of Heat Transfer in Separated Flows With the Help of Miniturbulators

2010 ◽  
Author(s):  
T. V. Bogatko ◽  
A. Yu. D’yachenko ◽  
V. I. Terekhov ◽  
N. I. Yarygina
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Flow Separation ◽  
Separation Zone ◽  
Cross Flow ◽  
Separated Flow ◽  
Separation Point ◽  
Upstream Region ◽  
Maximum Heat

In the present paper, the influence of vorticity layer on the turbulent separated flow and heat transfer in a cross-flow cavity was experimentally examined. The vorticity layer was generated by a miniturbulator installed in the upstream region of the flow separation point. As the miniturbulator, a small cross-flow rib was used whose height was one order of magnitude smaller than the cavity depth. The variable parameters were the angle of wall inclination in the cavity, the rib height, and the rib-to-cavity separation. The additional vortical disturbances introduced into the recirculation zone were found to exert an appreciable influence on the vortex formation pattern and on the distribution of pressure and heat-transfer coefficients. The experimental data were compared to computation data obtained with the Fluent 6 software. Numerical data on the dynamic and thermal characteristics of flows past a system comprising a sudden pipe expansion and a low-height diaphragm installed in the upstream region of the flow separation point are also presented. It is found that such a diaphragm, used to modify the characteristics of the separated flow, results in a change of the length and intensity of the eddying flow in the separation zone. The vortex sheet produced by the diaphragm interacts with the primary eddy, makes the separation zone more extended, and shifts, even to a greater extent, the point at which the heat-transfer coefficient attains its maximum in the downstream direction. The maximum heat-transfer coefficient turns out to be increased in comparison with undisturbed flow. Both the location of the diaphragm and the diaphragm height strongly affect the heat-transfer characteristics.

Investigation of Flow Separation Inside a Conical Rocket Nozzle With the Aid of an Annular Cross Flow

2007 ◽  
Author(s):  
Alexander Weiss ◽  
Sylvester Abanteriba ◽  
Thomas Esch
Keyword(s):  
Flow Separation ◽  
Supersonic Nozzle ◽  
Cross Flow ◽  
Boundary Layer Separation ◽  
Working Fluid ◽  
Rocket Engines ◽  
Exit Plane ◽  
Conical Nozzle ◽  
Large Area ◽  
Combustion Gases

Flow separation is a phenomenon that occurs in all kinds of supersonic nozzles sometimes during run-up and shut-down operations. Especially in expansion nozzles of rocket engines with large area ratio, flow separation can trigger strong side loads that can damage the structure of the nozzle. The investigation presented in this paper seeks to establish measures that may be applied to alter the point of flow separation. In order to achieve this, a supersonic nozzle was placed at the exit plane of the conical nozzle. This resulted in the generation of cross flow surrounding the core jet flow from the conical nozzle. Due to the entrainment of the gas stream from the conical nozzle the pressure in its exit plane was found to be lower than that of the ambient. A Cold gas instead of hot combustion gases was used as the working fluid. A mathematical simulation of the concept was validated by experiment. Measurements confirmed the simulation results that due to the introduction of a second nozzle the pressure in the separated region of the conical nozzle was significantly reduced. It was also established that the boundary layer separation inside the conical nozzle was delayed thus allowing an increased degree of overexpansion. The condition established by the pressure measurements was also demonstrated qualitatively using transparent nozzle configurations.

scholarly journals On a solution of the closure problem for dry convective boundary layer turbulence and beyond

2021 ◽  
Keyword(s):  
Boundary Layer ◽  
Convective Boundary Layer ◽  
Higher Order ◽  
Turbulence Closure ◽  
Analytical Models ◽  
Central Feature ◽  
Closure Problem ◽  
Convective Boundary ◽  
Higher Order Moments ◽  
Closure Models

Abstract We consider the closure problem of representing the higher order moments (HOMs) in terms of lower-order moments, a central feature in turbulence modelling based on the Reynolds-Averaged Navier-Stokes (RANS) approach. Our focus is on models suited for the description of asymmetric, non-local and semi-organized turbulence in the dry atmospheric convective boundary layer (CBL). We establish a multivariate probability density function (PDF) describing populations of plumes which are embedded in a sea of weaker randomly spaced eddies, and apply an assumed Delta-PDF approximation. The main content of this approach consists of capturing the bulk properties of the PDF. We solve the closure problem analytically for all relevant higher order moments (HOMs) involving velocity components and temperature and establish a hierarchy of new non-Gaussian turbulence closure models of different content and complexity ranging from analytical to semi-analytical. All HOMs in the hierarchy have a universal and simple functional form. They refine the widely used Millionshchikov closure hypothesis and generalize the famous quadratic skewness-kurtosis relationship to higher-order. We examine the performance of the new closures by comparison with measurement, LES and DNS data and derive empirical constants for semi-analytical models, which are best for practical applications. We show that the new models have a good skill in predicting the HOMs for atmospheric CBL. Our closures can be implemented in second-, third- and fourth-order RANS turbulence closure models of bi-, tri-and four-variate levels of complexity. Finally, several possible generalizations of our approach are discussed.

Measurements of the Turbulence Structure in the Vicinity of a 3-D Separation

1996 ◽  
Vol 118 (2) ◽  
pp. 268-275 ◽  
Author(s):  
C. J. Chesnakas ◽  
R. L. Simpson
Keyword(s):  
Flow Separation ◽  
Three Dimensional ◽  
Cross Flow ◽  
Prolate Spheroid ◽  
Laser Doppler Velocimeter ◽  
Turbulence Structure ◽  
Separation Region ◽  
The Cross ◽  
And Diffusion ◽  
Boundary Layer Edge

The flow in the cross-flow separation region of a 6:1 prolate spheroid at 10 deg angle of attack, ReL = 4.20 × 106, was investigated using a novel, miniature, 3-D, fiber-optic Laser Doppler Velocimeter (LDV). The probe was used to measure three simultaneous, orthogonal velocity components from within the model, from approximately y+ = 7 out to the boundary layer edge. Velocity, Reynolds stress, and velocity triple product measurements are presented. These measurements are used to calculate the skin friction and to examine the convection, production, and diffusion of turbulent kinetic energy (TKE) about the three-dimensional separation. Comparisons of the measured production and diffusion of TKE in the cross-flow separation region—as well as in nonseparated regions of the flow—to the production and diffusion predicted by several models for these terms are shown.

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