Aleksandar Jemcov

A Primitive Variable Central Flux Scheme for All Mach Number Flows

A new finite volume solver for all Mach number flows suitable for the solution on unstructured meshes with the collocated arrangement of variables is presented. The solver is based on the primitive variable equations. The newly proposed method utilizes the fractional step method that resembles the projection methods used in incompressible flow simulations. The proposed solver is suitable for the computations ranging from incompressible to supersonic flows. A notable characteristic of the newly proposed method is that the new formulation of the compressible pressure equation contains the pressure Laplacian term pre-multiplied by the time step size. Unlike the incompressible projection methods, the new pressure equation is derived from the discrete form of the continuity equation, resulting in the equation that has the potential (elliptic) and the hyperbolic (convective) parts of the pressure field separated. Central flux formulation is used for the approximation of numeric fluxes, resulting in small numerical dissipation thus making it suitable for the high fidelity computations including direct and large eddy simulations. A set of validation problems is presented in this work demonstrating the main characteristics of the newly proposed solver.

MODELING AN ENCLOSED, TURBULENT REACTING METHANE JET WITH THE PREMIXED CONDITIONAL MOMENT CLOSURE METHOD

Here the premixed Conditional Moment Closure (CMC) method is used to model the recent PIV and Raman turbulent, enclosed reacting methane jet data from DLR Stuttgart [1]. The experimental data has a rectangular test section at atmospheric pressure and temperature with a single inlet jet. A jet velocity of 90 m/s is used with an adiabatic flame temperature of 2,064 K. Contours of major species, temperature and velocities along with velocity rms values are provided.The conditional moment closure model has been shown to provide the capability to model turbulent, premixed methane flames with detailed chemistry and reasonable runtimes [2]. The simplified CMC model used here falls into the class of table lookup turbulent combustion models where the chemical kinetics are solved offline over a range of conditions and stored in a table that is accessed by the CFD code. Most table lookup models are based on the laminar 1-D flamelet equations, which assume the small scale turbulence does not affect the reaction rates, only the large scale turbulence has an effect on the reaction rates. The CMC model is derived from first principles to account for the effects of small scale turbulence on the reaction rates, as well as the effects of the large scale mixing, making it more versatile than other models. This is accomplished by conditioning the scalars with the reaction progress variable. By conditioning the scalars and accounting for the small scale mixing, the effects of turbulent fluctuations of the temperature on the reaction rates are more accurately modeled.The scalar dissipation is used to account for the effects of the small scale mixing on the reaction rates. The original premixed CMC model used a constant value of scalar dissipation, here the scalar dissipation is conditioned by the reaction progress variable. The steady RANS 3-D version of the open source CFD code OpenFOAM is used. Velocity, temperature and species are compared to the experimental data. Once validated, this CFD turbulent combustion model will have great utility for designing lean premixed gas turbine combustors.Copyright

Acceleration and Stabilization of Algebraic Multigrid Solver Applied to Incompressible Flow Problems

Acceleration and stabilization of the Algebraic Multigrid solver (AMG) through n-th order Recursive Projection Method (RPM(n)) is described. It is shown that significant acceleration can be obtained if RPM(n) is applied to AMG during the inner iteration loop in a typical implicit incompressible CFD codes. In addition to accelerating the solution, RPM(n) provides increased stability to the AMG Solver extending it beyond its normal range of applicability in terms of matrix conditioning and M-matrix properties. RPM(n) algorithm allows the use of agglomerative AMG solver with simple smoothers to be effectively applied to matrices that are not an M-matrix. Theoretical foundations of RPM(n)-AMG algorithm are presented with some practical aspects of the algorithm implementation. Numerical experiments that involve pressure correction matrices of various sizes that appear in segregated pressure based algorithms together with coupled momentum and pressure matrices stemming from coupled pressure based algorithms are used to illustrate the effectiveness of the method.

Exploratory investigation of the HIPPO gas-jet target fluid dynamic properties

Abstract In order to optimize the performance of gas-jet targets for future nuclear reaction measurements, a detailed understanding of the dependence of the gas-jet properties on experiment design parameters is required. Common methods of gas-jet characterization rely on measuring the effective thickness using nuclear elastic scattering and energy loss techniques; however, these tests are time intensive and limit the range of design modifications which can be explored to improve the properties of the jet as a nuclear reaction target. Thus, a more rapid jet-characterization method is desired. We performed the first steps towards characterizing the gas-jet density distribution of the HIPPO gas-jet target at the University of Notre Dames Nuclear Science Laboratory by reproducing results from 20 Ne( α , α ) 20 Ne elastic scattering measurements with computational fluid dynamics (CFD) simulations performed with the state-of-the-art CFD software ANSYS Fluent . We find a strong sensitivity to experimental design parameters of the gas-jet target, such as the jet nozzle geometry and ambient pressure of the target chamber. We argue that improved predictive power will require moving to three-dimensional simulations and additional benchmarking with experimental data.

The Aero-Optical Environment Around a Helicopter Computed using the Compressible Vorticity Confinement Method

The methodology and results of a computational investigation into the nearfield aerooptic effects on a helicopter-borne optical system are presented. The approach investigated in the study was to incorporate a vorticity-confinement method into a commerciallyavailable CFD code, thereby allowing the rotor wake to be modeled in a less computationally-expensive manner with a higher degree of accuracy. This method is expected to produce more-accurate vortex dynamics, including vortex interactions and other secondary effects, thus leading to improved information regarding the aero-optic environment encountered by an optical system mounted on a helicopter.

Computation of the Aero-Optical Effect of a Helicopter Rotor Wake Using Unsteady RANS and LES

The aero-optical characteristics of the wake beneath a hovering helicopter was computed using Unsteady RANS (URANS) and Large-Eddy Simulations (LES). The computational methodology consisted of combining a URANS simulation with a vorticity-confinement method to produce a model of the large-scale vortex wake of the helicopter. An LES simulation of the local flow around a vortex segment was then performed and the solutions were superimposed onto the large-scale vortex structures computed using the URANS CFD, and time-resolved optical wavefronts were computed on a beam of light passing through the flow. The computed optical aberrations of the wake can be used to develop adaptive-optic systems or other beam-control approaches.

Numerical Investigations of Slot-type Film Cooling of HPT Rotor for Small Engines

Two dimensional (2D) and three dimension (3D) numerical simulations and 3D structural analysis were used to understand aero-dynamical, thermal mechanical and structural behaviors of the slot-type film cooling of a relatively small turbine rotor blade. Different blowing ratios were examined. The mechanisms of the performance of slot-type film cooling in both 2D and 3D conditions were analyzed in detail, based on the numerical results. Overall, the slot-type film cooling had much better cooling effects over the discrete-hole film cooling by avoiding the 3D flow near the coolant jet due to the non-uniformity of the coolant jet in the third dimension and the discontinuity of the jetting area, which brought the hot gas of the mainstream flow to the blade surface and worsened the cooling effect. The rotating effects were considered. Regions on the rotor blade where the Coriolis effect should be taken account were provided according to the operating conditions and the real flow field. The structural reliability based on the 3D finite element modeling (FEM) was examined, and was proved to be feasible. Take the advantage of the model design and manufacturing technique, reconsidering the slot-type film cooling could help further improve the turbine cooling effectiveness.

High Resolution Central Scheme with Dominant Wave Dissipation for Cell Centered Unstructured Meshes

A cell centered high resolution scheme with the dissipation term based on dominant wave approach is examined here. A simplied dominant eigenvalue expression based on the momentum equations together with low Mach number precondtioning is introduced. The dominant wave scheme was implemented in the unstructured cell centered framework capable of supporting polyhedral meshes. Second order of accuracy is achieved by using slope limiters for the primitive data reconstructed at the nite volume cell faces. The main advantage of the scheme is the resulting simplied dissipation term that does not require computation of eigenvectors while retaining the high resolution capabilities. It is shown that the dominant wave scheme has similar resolution to the ux dierence upwind scheme based on the Roe Riemann solver. In addition, the dominant wave scheme has similar resolution characteristics compared to other central schemes such as the Kurganov-Tadmor scheme, and comparison to the HLLC scheme showed similar resolution capabilities. A number of computational tests in 1D, 2D and 3D are performed in order to assess the resolution capabilities of the scheme.

Entropy stable multi-dimensional dissipation function for the roe scheme on unstructured meshes

Flux difference schemes based on the Roe approximate Riemann solver provide for sharp resolution of shock waves and contact discontinuities. Despite its good properties, the Roe scheme does not possess entropy stability that is required for the scheme to provide physically correct solutions. In addition, since the approximate Riemann solver was developed using one-dimensional considerations, the carbuncle instability appears around stagnation points for mesh-aligned flows. A solution to both entropy stability and the carbuncle problem is considered here. An alternative form of the dissipation function is developed that addresses both problems. The new dissipation function is based on the multidimensional modification to dissipation function of Roe scheme. The new scheme is shown to maintain sharp resolution comparable to the Roe flux difference scheme while satisfying entropy stability and removing the carbuncle type of instability.

Nonlinear Flow Solver Acceleration by Reduced Rank Extrapolation

Convergence acceleration of nonlinear flow solvers through use of vector sequence extrapolation techniques is presented. In particular, suitability of the Reduced Rank Extrapolation (RRE) algorithm for the use of convergence acceleration of nonlinear flow solvers is examined. In the RRE algorithm, the solution is obtained through a linear combination of Krylov vectors with weighting coefficients obtained by minimizing L2 norm of error in this space with properly chosen constraint conditions. This process effectively defines vector sequence extrapolation process in Krylov subspace that corresponds to the GMRES method applied to nonlinear problems. Moreover, when the RRE algorithm is used to solve nonlinear problems, the flow solver plays the role of the preconditioner for the nonlinear GMRES method. Benefits of the application of the RRE algorithm include better convergence rates, removal of residual stalling and improved coupling between equations in numerical models. Proposed algorithm is independent of the type of flow solver and it is equally applicable to explicit, implicit, pressure and density based algorithms.