Non-equilibrium Turbulence Research Articles

Nonequilibrium turbulence modeling study on light dynamic stall of aNACA0012 airfoil

A computational study on the nonequilibrium turbulence modeling effects for the prediction of the light stall phenomenon has been done for the NACA0012 airfoil. For this, an unsteady thin-layer Navier-Stokes solver was developed that is capable of solving the flowfield around an airfoil undergoing unsteady harmonic motion. In the program, the Baldwin-Lomax and Cebeci-Smith turbulence models were used as baseline models, and the Johnson-King turbulence model was used to study the nonequilibrium effects. It was found that the nonequilibrium effects are important for the prediction of the light stall, and only the Johnson-King model yields light stall hysteresis loop that is similar to the experiment. It was also found that the wind-tunnel wall effects are important, and a mean angle-of-attack increase in the computation was necessary to yield a better agreement with the experiment.

Calculation of a circular jet in crossflow with a multiple-time-scale turbulence model

Calculation of a three-dimensional turbulent flow of a jet in a crossflow using a multiple-timescale turbulence model is presented. The turbulence in the forward region of the jet is in a stronger non-equilibrium state than that in the wake region of the jet, while the turbulence level in the wake region is higher than that in the front region. The calculated flow and the concentration fields are in very good agreement with the measured data, and it indicates that the turbulent transport of mass, concentration and momentum is strongly governed by the non-equilibrium turbulence. The capability of the multiple-time-scale turbulence model to resolve the non-equilibrium turbulence field is also discussed.

New nonequilibrium turbulence model for calculating flows over airfoils

ANONEQUILIBRIUM turbulence closure model, patterned after the Johnson-King model, has been developed for computing two-dimensional turbulent flows about complex airfoil shapes. The influence of history effects in the wake region of the boundary layer is described by an ordinary differential equation developed from the turbulent kinetic energy equation. The performance of the present model has been evaluated by computing the flow around three airfoils using the Reynolds time-averaged Navier-Stokes equations/These equations were solved using an implicit, upwind, finite- volume scheme. Excellent agreement between numerical and experimental results was obtained for both attached and separated turbulent flows about the NACA 0012 airfoil, the RAE 2822 airfoil, and the Integrated Technology A 153W airfoil. Contents The majority of turbulent flow calculations are based on the Reynolds time-averaged equations. These equations can be obtained by using Favre's mass- weighted averaging. Closure of the Reynolds time-averaged equations requires the modeling of the turbulent stresses and heat-flux terms. In most turbulence models, the turbulent stresses are determined from the Boussinesq approximation: 2 2 (1)

Turbulence phenomena in a multiple normal shock wave/turbulent boundary-layer interaction

An experimental investigation of a Mach 1.61 multiple normal shock wave/turbulent boundary-layer interaction in a rectangular, nearly constant area duct is discussed with an emphasis on the turbulence phenomena. The two-component laser Doppler velocimeter measurements reveal a large amplification of the turbulence kinetic energy and Reynolds stress through the interaction. The leading shock in the multiple shock pattern causes a significant distortion of the turbulent stress tensor. Partial recovery occurs immediately downstream of the first shock. The trailing shocks in the system are much weaker than the first shock and tend to maintain the nonequilibrium turbulence structure, with complete recovery occurring well downstream of the interaction.

Experience with the Johnson-King turbulence model in a transonic turbine cascade flow solver

Experience is described of the application of the Dawes Navier-Stokes solver to a turbine test case involving shock-boundary-layer interaction. Modifying the implementation of the algebraic eddy-viscosity turbulence model gave improved agreement between the predictions and the experimental results. In order to improve the predictions still further, the nonequilibrium turbulence model of Johnson and King was incorporated into the code. This gave limited further improvement in the overall loss prediction, but the base pressure prediction was not improved. Only a minor shock-induced separation was predicted, compared with the increasing major separation observed in cascade tests with increasing outlet Mach number, although it is necessary to be aware of the possible differences between the real test flow and the two-dimensional steady flow that was modeled. Better prediction of the test flow may require the incorporation of higher-order turbulence models, in which the physics of the flow is more fully represented. Additionally, three-dimensional and unsteady capability may be needed.

ALGEBRAIC STRESS MODEL PREDICTION OF A PLANE VERTICAL PLUME

A finite-volume prediction is obtained for a plane vertical plume using nonequilibrium algebraic turbulence models for the Reynolds stress and turbulent heat flux. Vie turbulence models include approximations for convection and diffusion. Unlike previous studies, the full two-dimensional form of the model equations is retained, and the transport equations are solved elliptically. A major conclusion of this work is that the nonequilibrium algebraic stress model (ASM), when solved in an elliptic framework, yields a realistic prediction for a vertical plume. This result resolves the difficulty encountered by previous workers who solved the parabolic equations.

Improvements to a nonequilibrium algebraic turbulence model

It has been noted that while the nonequilibrium turbulence model of Johnson and King (1985, 1987) performed significantly better than alternative methods, differences between predicted and observed shock locations for certain weak interactions are produced due to a defficiency in the model's inner eddy viscosity formulation. A novel formulation for the model is presented which removes this deficiency, while satisfying the law of the wall for adverse pressure-gradient conditions better than either the original formulation or mixing-length theory.

Laminar diffusion flamelet models in non-premixed turbulent combustion

The laminar flamelet concept views a turbulent diffusion flame as an ensemble of laminar diffusion flamelets. Work relevant to the flamelet concept is spread over various fields in the literature: laminar flame studies, asymptotic analysis, theory of turbulence and percolation theory. This review tries to gather and integrate this material in order to derive a self-consistent formulation. Under the assumption of equal diffusivities a coordinate-free formulation of the flamelet structure is given. This assumption is relaxed and flow dependent effects are considered. It is shown that the steady laminar counterflow diffusion flame exhibits a very similar scalar structure as unsteady distorted mixing layers in a turbulent flow field. Therefore the counterflow geometry is proposed to be the most representative steady flow field to study chemistry models and molecular transport effects in laminar flamelets. The conserved scalar model is interpreted as the most basic flamelet structure. Non-equilibrium calculations are reviewed. The coupling between non-equilibrium chemistry and turbulence is achieved by the statistical description of two parameters: the mixture fraction and the instantaneous scalar dissipation rate. The hypothesis of statistical independence of these two parameters is discussed. Calculation methods for the marginal distributions are reviewed. It is shown how local quenching of diffusion flamelets leads to a reduction of burnable flamelets. However, there are burnable flamelets in a turbulent flame which are not reached by an ignition source. This phenomenon is described by percolation theory. Complementary approaches related to local quenching effects and connectedness are combined to derive criteria for the stabilization of lifted flames and to blow out. Further applications of the flamelet concept are reviewed and work to be done is discussed.

Survey

The College of Science and Technology at Nihon University and the Institute for Fusion Theory at Hiroshima University discuss the history of the role of theory and simulation in fusion-oriented research. Recent activities include a one-dimensional tokamak transport code at Nagoya University and three-dimensional resistive MHD simulation studies of spheromaks. Other recent activities discussed include the tokamak computer code system TRITON, transport flux in currentless ECH-produced plasma in Heliotron-E, and thermal electron transport in the presence of a steep temperature gradient. The Japan-U.S. Joint Institute for Fusion Theory's present activities are discussed, including subject areas in three-dimensional simulation studies, nonequilibrium statistical physics, anaomalous transport and drift wave turbulence and hot-electron physics.

Turbulence Modeling of Flowfields with Massive Surface Ablation

A turbulence model accounting for the various physical phenomena in the viscous shock layer of a massively blown spherically blunted probe has been developed. In particular, an allowance has been made in the turbulence model for the longitudinal and normal pressure gradient effects, peaked massive ablation, and the nonequilibrium turbulence effects. The need for finding a boundary-layer edge is avoided. A computational mesh and convergence criterion more appropriate for the blown surface is employed in obtaining the results which include the effect of surface roughness.