Decrease of the Effective Reynolds Number with Eddy-Viscosity Subgrid Modeling

AIAA Journal ◽  
2005 ◽  
Vol 43 (2) ◽  
pp. 437-439 ◽  
Author(s):  
Christophe Bogey ◽  
Christophe Bailly
Keyword(s):  
Reynolds Number ◽  
Eddy Viscosity ◽  
Subgrid Modeling ◽  
Effective Reynolds Number

An Investigation Into the Potential of Low-Reynolds Number Eddy Viscosity Turbulent Flow Models to Predict Electronic Component Operational Temperature

2005 ◽  
Vol 127 (1) ◽  
pp. 67-75 ◽  
Author(s):  
Peter Rodgers ◽  
Vale´rie Eveloy ◽  
M. S. J. Hashmi
Keyword(s):  
Thermal Analysis ◽  
Reynolds Number ◽  
Eddy Viscosity ◽  
Predictive Accuracy ◽  
Low Reynolds Number ◽  
Turbulence Models ◽  
Thermal Design ◽  
Junction Temperature ◽  
Temperature Prediction ◽  
Low Reynolds

The flow modeling approaches employed in computational fluid dynamics (CFD) codes dedicated to the thermal analysis of electronic equipment are generally not specific for the analysis of forced airflows over populated electronic boards. This limitation has been previously highlighted (Eveloy, V. et al., 2004, IEEE Trans. Compon., Packag., Technol. 27, pp. 268–282), with component junction temperature prediction errors of up to 35% reported. This study evaluates the potential of three candidate low-Reynolds number eddy viscosity turbulence models to improve predictive accuracy. An array of fifteen board-mounted PQFPs is analyzed in a 4 m/s airflow. Using the shear stress transport k-ω model, significant improvements in component junction temperature prediction accuracy are obtained relative to the standard high-Reynolds number k-ε model, which are attributed to better prediction of both board leading edge heat transfer and component thermal interaction. Such improvements would enable parametric analysis of product thermal performance to be undertaken with greater confidence in the thermal design process, and the generation of more accurate temperature boundary conditions for use in Physics-of-Failure based reliability prediction methods. The case is made for vendors of CFD codes dedicated to the thermal analysis of electronics to consider the adoption of eddy viscosity turbulence models more suited to board-level analysis.

scholarly journals On the need for a nonlinear subscale turbulence term in POD models as exemplified for a high-Reynolds-number flow over an Ahmed body

2014 ◽  
Vol 747 ◽  
pp. 518-544 ◽  
Author(s):  
Jan Östh ◽  
Bernd R. Noack ◽  
Siniša Krajnović ◽  
Diogo Barros ◽  
Jacques Borée
Keyword(s):  
Reynolds Number ◽  
High Reynolds Number ◽  
Eddy Viscosity ◽  
Initial Conditions ◽  
Base Flow ◽  
Reduced Order ◽  
State Dependent ◽  
Galerkin Models ◽  
High Reynolds ◽  
Ahmed Body

AbstractWe investigate a hierarchy of eddy-viscosity terms in proper orthogonal decomposition (POD) Galerkin models to account for a large fraction of unresolved fluctuation energy. These Galerkin methods are applied to large eddy simulation (LES) data for a flow around a vehicle-like bluff body called an Ahmed body. This flow has three challenges for any reduced-order model: a high Reynolds number, coherent structures with broadband frequency dynamics, and meta-stable asymmetric base flow states. The Galerkin models are found to be most accurate with modal eddy viscosities as proposed by Rempfer & Fasel (J. Fluid Mech., vol. 260, 1994a, pp. 351–375; J. Fluid Mech. vol. 275, 1994b, pp. 257–283). Robustness of the model solution with respect to initial conditions, eddy-viscosity values and model order is achieved only for state-dependent eddy viscosities as proposed by Noack, Morzyński & Tadmor (Reduced-Order Modelling for Flow Control, CISM Courses and Lectures, vol. 528, 2011). Only the POD system with state-dependent modal eddy viscosities can address all challenges of the flow characteristics. All parameters are analytically derived from the Navier–Stokes-based balance equations with the available data. We arrive at simple general guidelines for robust and accurate POD models which can be expected to hold for a large class of turbulent flows.

An Investigation of Reynolds Lapse Rate for Highly Loaded Low Pressure Turbine Airfoils With Forward and Aft Loading

2011 ◽  
Author(s):  
M. Eric Lyall ◽  
Paul I. King ◽  
Rolf Sondergaard ◽  
John P. Clark ◽  
Mark W. McQuilling
Keyword(s):  
Reynolds Number ◽  
Eddy Viscosity ◽  
Low Pressure ◽  
Lapse Rate ◽  
Freestream Turbulence ◽  
Number Range ◽  
Low Pressure Turbine ◽  
Turbine Airfoils ◽  
Hot Film ◽  
Highly Loaded

This paper presents an experimental and computational study of the midspan low Reynolds number loss behavior for two highly loaded low pressure turbine airfoils, designated L2F and L2A, which are forward and aft loaded, respectively. Both airfoils were designed with incompressible Zweifel loading coefficients of 1.59. Computational predictions are provided using two codes, Fluent (with k-k1-ω model) and AFRL’s Turbine Design and Analysis System (TDAAS), each with a different eddy-viscosity RANS based turbulence model with transition capability. Experiments were conducted in a low speed wind tunnel to provide transition models for computational comparisons. The Reynolds number range based on axial chord and inlet velocity was 20,000 < Re < 100,000 with an inlet turbulence intensity of 3.1%. Predictions using TDAAS agreed well with the measured Reynolds lapse rate. Computations using Fluent however, predicted stall to occur at significantly higher Reynolds numbers as compared to experiment. Based on triple sensor hot-film measurements, Fluent’s premature stall behavior is likely the result of the eddy-viscosity hypothesis inadequately capturing anisotropic freestream turbulence effects. Furthermore, rapid distortion theory is considered as a possible analytical tool for studying freestream turbulence that influences transition near the suction surface of LPT airfoils. Comparisons with triple sensor hot-film measurements indicate that the technique is promising but more research is required to confirm its utility.

A Velocity and Length Scale Approach to k–ε Modeling

1996 ◽  
Vol 118 (4) ◽  
pp. 857-863 ◽  
Author(s):  
O. Kwon ◽  
F. E. Ames
Keyword(s):  
Reynolds Number ◽  
Kinetic Energy ◽  
Turbulent Flows ◽  
Dissipation Rate ◽  
Eddy Viscosity ◽  
Normal Component ◽  
Transport Equations ◽  
Length Scale ◽  
Wall Region ◽  
Wide Range

This paper describes a velocity and length scale approach to low-Reynolds-number k–ε modeling, which formulates the eddy viscosity on the normal component of turbulence and a length scale. The normal component of turbulence is modeled based on the dissipation and distance from the wall and is bounded by the isotropic condition. The model accounts for the anisotropy of the dissipation and the reduced length of mixing in the near wall region. The kinetic energy and dissipation rate were computed from the k and ε transport equations of Durbin (1993). The model was tested for a wide range of turbulent flows and proved to be superior to other k–ε based models.

Linear dynamics of wind waves in coupled turbulent air–water flow. Part 2. Numerical model

1996 ◽  
Vol 308 ◽  
pp. 219-254 ◽  
Author(s):  
J. A. Harris ◽  
S. E. Belcher ◽  
R. L. Street
Keyword(s):  
Reynolds Number ◽  
Numerical Model ◽  
Water Waves ◽  
Eddy Viscosity ◽  
Base Flow ◽  
Viscosity Model ◽  
Subsequent Paper ◽  
Form Drag ◽  
New Model ◽  
Eddy Viscosity Model

We develop a numerical model of the interaction between wind and a small-amplitude water wave. The model first calculates the turbulent flows in both the air and water that would be obtained with a flat interface, and then calculates linear perturbations to this base flow caused by a travelling surface wave. Turbulent stresses in the base flow are parameterized using an eddy viscosity derived from a low-turbulent-Reynolds-number κ – ε model. Turbulent stresses in the perturbed flow are parameterized using a new damped eddy viscosity model, in which the eddy viscosity model is used only in inner regions, and is damped exponentially to zero outside these inner regions. This approach is consistent with previously developed physical scaling arguments. Even on the ocean the interface can be aerodynamically smooth, transitional or rough, so the new model parameterizes the interface with a roughness Reynolds number and retains effects of molecular stresses (on both mean and turbulent parts of the flow).The damped eddy viscosity model has a free constant that is calibrated by comparing with results from a second-order closure model. The new model is then used to calculate the variation of form drag on a stationary rigid wave with Reynolds number, R. The form drag increases by a factor of almost two as R drops from 2 × 104 to 2 × 103 and shows remarkably good agreement with the value measured by Zilker & Hanratty (1979). These calculations show that the damped eddy viscosity model captures the physical processes that produce the asymmetric pressure that leads to form drag and also wave growth.Results from the numerical model show reasonable agreement with profiles measured over travelling water waves by Hsu & Hsu (1983), particularly for slower moving waves. The model suggests that the wave-induced flow in the water is irrotational except in an extremely thin interface layer, where viscous stresses are as likely to be important as turbulent stresses. Thus our study reinforces previous suggestions that the region very close to the interface is crucial to wind-wave interaction and shows that scales down to the viscous length may have an order-one effect on the development of the wave.The energy budget and growth rate of the wave motions, including effects of the sheared current and Reynolds number, will be examined in a subsequent paper.

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