Permuted proper orthogonal decomposition for analysis of advecting structures

Flow data are often decomposed using proper orthogonal decomposition (POD) of the space–time separated form, $\boldsymbol {q}'\left (\boldsymbol {x},t\right )=\sum _j a_j\left (t\right )\boldsymbol {\phi }_j\left (\boldsymbol {x}\right )$ , which targets spatially correlated flow structures in an optimal manner. This paper analyses permuted POD (PPOD), which decomposes data as $\boldsymbol {q}'\left (\boldsymbol {x},t\right )=\sum _j a_j\left (\boldsymbol {n}\right )\boldsymbol {\phi }_j\left (s,t\right )$ , where $\boldsymbol {x}=(s,\boldsymbol {n})$ is a general spatial coordinate system, $s$ is the coordinate along the bulk advection direction and $\boldsymbol {n}=(n_1,n_2)$ are along mutually orthogonal directions normal to the advection characteristic. This separation of variables is associated with a fundamentally different inner product space for which PPOD is optimal and targets correlations in $s,t$ space. This paper presents mathematical features of PPOD, followed by analysis of three experimental datasets from high-Reynolds-number, turbulent shear flows: a wake, a swirling annular jet and a jet in cross-flow. In the wake and swirling jet cases, the leading PPOD and space-only POD modes focus on similar features but differ in convergence rates and fidelity in capturing spatial and temporal information. In contrast, the leading PPOD and space-only POD modes for the jet in cross-flow capture completely different features – advecting shear layer structures and flapping of the jet column, respectively. This example demonstrates how the different inner product spaces, which order the PPOD and space-only POD modes according to different measures of variance, provide unique ‘lenses’ into features of advection-dominated flows, allowing complementary insights.

Effects of ‘Oxy’ jet in cross flow on the combustion instability and NOx emissions in lean premixed flame

Combustion instability and nitrogen oxides emission are crucial factors for modern gas turbine combustors, which seriously hampers the research and development of advanced combustors. To eliminate combustion instability and NOx emissions simultaneously, effects of the ?Oxy? (CO2/O2, N2/O2, Ar/O2and He/O2) jet in cross flow(JICF)on combustion instability and NOx emissions are experimentally studied. In this research, the flow rate and oxygen ratio of the combustor are varied to evaluate the control effectiveness. Results denotes that all the four oxy fuel gas: CO2/O2, N2/O2, Ar/O2and He/O2, could suppress combustion instability and NOx emissions. The CO2/O2dilution can achieve a better damping results than the other three cases. There are peak values or lowest points of sound pressure amplitude as the parameter of ?Oxy? JICF changes. Mode transition appears in both acoustic signal and CH* chemiluminescence of the flame. But the turning point of mode transition is different. Under the CO2/O2cases, the NOx emission decreases from 22.3ppm to 15.2ppm, the damping ratio of NOxis 40.39%. The flame shape and length were changed under different JICF dilutions. This research could promote the application of jet in cross flow methods on combustion instability or pollutant emissions in gas turbines.

LES/FMDF of turbulent reacting jet in cross-flow

Purpose This study aims to numerically investigate the flow features and mixing/combustion efficiencies in a turbulent reacting jet in cross-flow by a hybrid Eulerian-Lagrangian methodology. Design/methodology/approach A high-order hybrid solver is employed where, the velocity field is obtained by solving the Eulerian filtered compressible transport equations while the species are simulated by using the filtered mass density function (FMDF) method. Findings The main features of a reacting JICF flame are reproduced by the large-eddy simulation (LES)/FMDF method. The computed mean and root-mean-square values of velocity and mean temperature field are in good agreement with experimental data. Reacting JICF’s with different momentum ratios are considered. The jet penetrates deeper for higher momentum ratios. Mixing and combustion efficiency are improved by increasing the momentum ratio. Originality/value The authors investigate the flow and combustion characteristics in subsonic reacting JICFs for which very limited studies are reported in the literature.

Flow Characteristics of a Model Can-Type Combustor

Numerical simulations are performed on a can-type combustor in order to indentify various flow features. Elementary flow features like jet-in-crossflow, opposed jets, swirl and recirculation zones are present in a combined form. Momentum flux ratio between axial air flow and radial primary air jets from the circumference of the combustor is an important parameter. While increasing mass flow rate of the radial jet, the jet-in-cross flow structure changes to opposed jet. Swirl of axial jet is favourable for opposed jet configuration. A step provided at the close vicinity of the primary jet allows the primary jet expansion and its flow structure is affected at the entrance of the combustor to favour for jet-in-cross flow configuration. The opposed jet configuration provides large recirculation at the upstream due to merging of vortices arising from swirl of axial jet and upstream movement of primary air jet. This helps in holding the flame and also for effective combustion. However, there is a lower temperature region at the downstream core of the combustor due to penetration of primary air jets. On the other hand, jet-in-crossflow configuration has shown hotter zone at the downstream core of the combustor. Hence, effective utilization of flow structure can be considered in future design.

Low Speed Wind Tunnel Flow Diagnostics and Benchmark Cases for Thermal Fluids CFD Validation Efforts

A low speed, closed loop wind tunnel at Texas A&M University is presented for the study of turbulent mixing produced by a variety of flows types. Anticipated experiments range from canonical “unit flows” to more complex combinations of flows and geometries. Originally located at the University of Pittsburgh, the facility has since been re-located to the Thermal Hydraulics Verification and Validation (THVV) laboratory at Texas A&M University. The tunnel has undergone considerable modification and updated diagnostics prompting renewed interest in flow quality assessment. This includes a thorough mapping of the tunnel inlet velocity profile provided by Particle Image Velocimetry (PIV) measurements. Additional temperature and gage pressure measurements complete the assessment of system capabilities. These preliminary diagnostics yield empirically determined boundary conditions and fluid property correlations necessary for Computational Fluid Dynamics (CFD) model validation. The article concludes with the presentation of two unit flow types, including flow past a cylinder, with three distinct cross sections, and a single round jet in cross flow at three velocity ratios. The unit flows serve as initial benchmarks for THVV simulation efforts. Key validation metrics are presented for each benchmark including ensemble averaged velocities, Reynolds stresses, and proper orthogonal decomposition (POD) eigenvectors.