Timo Siikonen

AN ARTIFICIAL COMPRESSIBILITY METHOD FOR INCOMPRESSIBLE FLOWS

An artificial compressibility method characterized by the pressure-based algorithm is developed on a nonorthogonal collocated grid for incompressible fluid flow problems, using a cell-centered finite-volume approximation. Unlike the traditional pseudo-compressibility concept, the continuity constraint is perturbed by the material derivative of pressure, the physical relevance of which is to invoke matrix preconditionings. The approach provokes density perturbations, assisting the transformation between primitive and conservative variables. To account for the flow directionality in the upwinding, a rotational matrix is introduced to evaluate the convective flux. A rational means of reducing excessive numerical dissipation inherent in the pressure–velocity coupling is contrived which has the expedience of greater flexibility and increased accuracy in a way similar to the MUSCL approach. Numerical experiments in reference to a few laminar flows demonstrate that the overall artifacts expedite enhanced robustness and anticipated oscillation damping properties adhering to the factored pseudo-time integration procedure.

A pressure-correction method for solving fluid flow problems on a collocated grid

Abstract A pressure-correction method for the SIMPLE-like algorithm is proposed on a curvilinear collocated grid for the solution of two-dimensional incompressible fluid flow problems, using a vertex-based finite-volume approximation. In the pressure-correction equation, a weighting factor (a fictitious time step) is used as a substitute for the nodal contributions of the momentum equations. It serves as a limiter for the mass imbalance and provides an opportunity to avoid the pressure underrelaxation even when the source term effect in the momentum equations is dominant. The primitive formulation utilizes either the original Rhie-Chow (ORC) or the modified Rhie-Chow (MRC) flux correction at the cell face in discretizing the continuity equation to prevent the pressure oscillations. A comparative evaluation of the ORC and MRC schemes based on the computed results for a buoyancy-driven laminar flow in a half-concentric annulus shows that, on average, the MRC approach produces satisfactory stabilization for ...

An improved simple method on a collocated grid

A pressure-based method characterized by the SIMPLE algorithm is developed on a nonorthogonal collocated grid for solving two-dimensional incompressible fluid flow problems, using a cell-centered finite-volume approximation. The concept of artificial density is combined with the pressure Poisson equation that provokes density perturbations, assisting the transformation between primitive and conservative variables. A nonlinear explicit flux correction is utilized at the cell face in discretizing the continuity equation, which functions effectively in suppressing pressure oscillations. The pressure-correction equation principally consolidates a triplicate-time approach when the Courant number CFL > 1. A rotational matrix, accounting for the flow directionality in the upwinding, is introduced to evaluate the convective flux. The numerical experiments in reference to a few familiar laminar flows demonstrate that the entire contrivance executes a residual smoothing enhancement, facilitating an avoidance of the pressure underrelaxation. Consequently, included benefits are the use of larger Courant numbers, enhanced robustness, and improved overall damping properties of the unfactored pseudo-time integration procedure.A pressure-based method characterized by the SIMPLE algorithm is developed on a nonorthogonal collocated grid for solving two-dimensional incompressible fluid flow problems, using a cell-centered finite-volume approximation. The concept of artificial density is combined with the pressure Poisson equation that provokes density perturbations, assisting the transformation between primitive and conservative variables. A nonlinear explicit flux correction is utilized at the cell face in discretizing the continuity equation, which functions effectively in suppressing pressure oscillations. The pressure-correction equation principally consolidates a triplicate-time approach when the Courant number CFL > 1. A rotational matrix, accounting for the flow directionality in the upwinding, is introduced to evaluate the convective flux. The numerical experiments in reference to a few familiar laminar flows demonstrate that the entire contrivance executes a residual smoothing enhancement, facilitating an avoidance of ...

Improved Low-Reynolds-Number One-Equation Turbulence Model

DOI: 10.2514/1.J050651 A low-Reynolds-number extension of the Baldwin-Barth one-equation turbulence model is proposed and evaluated. Theeddy viscosity damping functionfinvokes anelliptic relaxation approach to account forthe distinct effects of low Reynolds number and wall proximity. The k and � , which render the undamped eddy viscosity, are evaluated using the Rk 2 =� transport equation together with the Bradshaw and other empirical relations. The modelcoefficients/functionspreservetheanisotropiccharacteristicsofturbulenceinthesensethattheyaresensitized to nonequilibrium flows. The model is validated against a few well-documented flow cases, yielding predictions in good agreement with the direct numerical simulation and experimental data. The R-transport equation having two different source terms replicates almost analogous results. Comparisons indicate that the present model offers considerable improvement over the modified Spalart-Allmaras one-equation model and competitiveness with the shear stress transport k-! model.

A DUAL-DISSIPATION SCHEME FOR PRESSURE-VELOCITY COUPLING

Within the framework of the SIMPLE algorithm, a dual-dissipation scheme is proposed on nonorthogonal collocated grids for incompressible fluid flow problems, using a cell-centered finite-volume approximation. The dissipative mechanism employs dynamic limiters to control the amount of dissipation, preserving expediences of greater flexibility and increased accuracy in a way similar to the MUSCL approach. The artificial density concept is combined with the pressure Poisson equation, facilitating an avoidance of pressure underrelaxation. To account for the flow directionality in the upwinding, a rotational matrix is evoked to evaluate the convective flux.

Wall-Distance-Free Version of Spalart-Allmaras Turbulence Model

A wall-distance-free modification to the Spalart–Allmaras one-equation turbulence model is proposed and evaluated. The k and ϵ that render the hybrid timescale are determined using the ν˜ transport equation together with the Bradshaw and other empirical relations. The model coefficients and other empirical functions are constructed to preserve some anisotropic characteristics of turbulence for application to nonequilibrium turbulent flows. The modified Spalart–Allmaras model is applied to calculate a few well-documented flows, yielding predictions in good agreement with the direct numerical simulation and experimental data. Comparisons demonstrate that the modified SA model offers some improvement over the original Spalart–Allmaras model and is competitive with the shear-stress transport k-ω model.

Simulation of HVAC flow noise sources with an exit vent as an example

ABSTRACT The control of flow noise is an important design aspect nowadays in the HVAC industry. However, acoustic design has largely been based on trial and error, which leaves the physical mechanisms responsible for the noise problems unexplained. Thus, a method is needed to analyse these mechanisms. The standard flow-modelling software used in industry misses the link between the flow and the far-field sound pressure. In the automotive and aviation industry, acoustic solver software products have been introduced to cover this gap. In this paper, a trade-off solution is proposed based on using standard flow simulation software to simulate the transient acoustic source field. This approach can be used to tackle some basic questions like where the noise is generated, which of the alternative geometrical variations is likely to produce least noise, or whether there could be distinct tones present at some frequencies. Source field analysis is also a useful intermediate phase prior to implementing an acoustic solver.

RAS one-equation turbulence model with non-singular eddy-viscosity coefficient

ABSTRACT A simplified consistency formulation for Pk/ε (production to dissipation ratio) is devised to obtain a non-singular Cμ (coefficient of eddy-viscosity) in the explicit algebraic Reynolds stress model of Gatski and Speziale. The coefficient Cμ depends non-linearly on both rotational/irrotational strains and is used in the framework of an improved RAS (Rahman–Agarwal–Siikonen) one-equation turbulence model to calculate a few well-documented turbulent flows, yielding predictions in good agreement with the direct numerical simulation and experimental data. The strain-dependent Cμ assists the RAS model in constructing the coefficients and functions such as to benefit complex flows with non-equilibrium turbulence. Comparisons with the Spalart–Allmaras one-equation model and the shear stress transport k-ω model demonstrate that Cμ improves the response of RAS model to non-equilibrium effects.

Application of a New One-Equation Turbulence Model to Computation of Separated Flows

Accurate turbulence modeling remains a critical problem in the prediction capability of computational fluid dynamics. One particular flow regime lacking accurate simulation is instances of separated flow. In this paper the Rahman-Agarwal-Siikonen (RAS) model is used to simulate the flow of several canonical separated flow cases. The commercially available software ANSYS FLUENT and the open source software OpenFOAM are used for the flow calculations. It is shown that the RAS model significantly increases the accuracy of flow simulations compared to the commonly used Spalart-Allmaras (SA) and Shear-Stress-Transport (SST) k-ω models.

An improved version of one-equation RAS turbulence model

An improved version of a recently developed one–equation turbulence model called RAS (Rahman–Agarwal–Siikonen) is proposed to account for the distinct effects of low–Reynolds number (LRN) and wall proximity. The turbulent kinetic energy k and the dissipation rate ǫ are evaluated using the R = (k/ǫ)–transport equation together with the Bradshaw and other empirical relations. The associated coefficients are constructed such as to preserve the anisotropic characteristics of turbulence encountered in non–equilibrium flows. In the current version, several improvements to the original RAS model are made which include the introduction of a near– wall eddy–viscosity damping function. An anisotropic destruction coefficient is used to obtain a faster decaying behavior of turbulence destruction in the outer region of the boundary/shear layer, thereby precluding the free–stream dependency. The source term in the transport equation is independent of the Reynolds stress tensor. A comparative assessment of the improved RAS model with the Spalart–Allmaras (SA) one–equation model and the shear stress transport (SST) k–ω model is provided for well– documented non–equilibrium turbulent flows.