Jordan A. Denev

Large eddy simulation of a swirling transverse jet into a crossflow with investigation of scalar transport

The flow field of a turbulent jet emerging from a straight round pipe into a laminar crossflow is investigated by means of large eddy simulations. The concentration of a passive scalar, introduced with the jet, is calculated in order to quantify the mixing of the jet and the crossflow. In the jet, swirl is introduced by means of body forces and a range of jet swirl numbers from S = 0 up to S = 0.6 is studied. The impact of the jet swirl on the flow field, on the coherent structures, and on the mixing efficiency is investigated and quantified by means of various analyses. It is found that for all swirl numbers larger than zero a clear asymmetry appears in all quantities studied. Additional to the two hanging vortices at both sides of the jet a third vortex is introduced by the swirling pipe flow which interacts with the former. This feature is described in detail as it is not mentioned in the literature. For the strongest swirl investigated a recirculation zone near the jet exit is observed. Despite the asymmetry and even with a recirculation zone at the outlet, the counter-rotating vortex pair still exists in all cases in the downstream flow, where it entrains a large amount of crossflow fluid into the jet. The near field, however, is altered by the jet swirl in several respects. The jet more and more approaches the bottom wall with increasing swirl. As a result, the entrainment is gradually attenuated due to the larger blocking of the secondary flow by the wall. Increased swirl increases both the turbulent kinetic energy in the pipe and the vorticity of the average flow field near the jet exit, and thus stimulates the mixing in these regions. However, this stimulating effect is overwhelmed by the closer position of the jet trajectory to the wall of the channel with increasing swirl, which in turn reduces entrainment of fresh crossflow fluid into the jet. As a final result of these two competing effects, the overall mixing efficiency of a jet into a crossflow is merely unchanged with the addition of swirl. Various mixing indices, both spatial and temporal, are used for this analysis. Their respective advantages and disadvantages are discussed and detailed illustrations provide a sound understanding of their behavior.

On the impact of tangential grid refinement on subgrid-scale modelling in large eddy simulation

The paper presents Large Eddy Simulations of plane channel flow at a friction Reynolds number of 180 and 395 with a block-structured Finite Volume method. Local grid refinement near the solid wall is employed in order to reduce the computational cost of such simulations or other simulations of wall-bounded flows. Different subgrid-scale models are employed and different expressions for the length scale in these models are investigated. It turns out that the numerical discretization has an non-negligible impact on the computed fluctuations.

Assessment of eddy resolving techniques for the flow over periodically arranged hills up to Re=37,000

The turbulent flow over periodically arranged geometrically two-dimensional hills in a channel at a Reynolds number of Re=37,000 has been considered as benchmark case for various eddy-resolving methods. The aim of this study is to assess various LES models and numerical approaches in a turbulent flow detaching from a curved surface. We compare results of a Cartesian grid solver using the immersed boundary method with various curvilinear approaches ranging from standard eddy-viscosity subgrid-scale models to hybrid LES-RANS models. The results are validated by a recent experiment conducted in a water channel by particle image velocimetry and laser-Doppler anemometry.

Direct Numerical Simulation of a Round Jet into a Crossflow – Analysis and Required Resources

Results from two Direct Numerical Simulations of a round jet in crossflow with velocity ratio of 3.3 are presented. The Reynolds number was 650 and 325. A passive scalar with Schmidt number of unity is introduced with the jet. The boundary conditions for both, jet and crossflow are laminar. This provides an unambiguous definition of the setup and favours its use as a test case. Transition of the jet was identified by an abrupt expansion of the average scalar field. The higher Reynolds number leads to a transition at 3.49 diameters downstream of the jet exit, the lower one – at 4.41 diameters. The higher Reynolds number flow exhibits smaller turbulent structures, but despite this and the different location of the transition, the trajectories of the two flows are close to each other. The computational technique employed is a block-structured Finite-Volume method with local grid refinement at block boundaries implemented in the code LESOCC2. This allowed efficient distribution of cells so that 89% of them could be clustered in the vicinity of the jet exit and in the transition region. Issues of parallelization and efficiency are addressed in the text.

Wavelet-Adapted Sub-grid Scale Models for LES

The paper presents two novel turbulent models for LES based on a wavelet decomposition. This approach, denoted WALES, is simple and easy to implement. Tests on a number of flows using grids with different resolution near walls show that the models exhibit the same quality as the Smagorinsky model without the need of wall functions or near-wall damping. In the paper the basic wavelet framework and two such models are described in detail. Physical benefits of the models due to the use of wavelets are discussed. Results obtained with the models are compared to those using the Smagorinsky model, to experimental results and to results from Direct Numerical Simulations. The agreement achieved is generally good.

Direct Numerical Simulation, Analysis and Modelling of Mixing Processes in a Round Jet in Crossflow

Direct Numerical Simulations (DNS) for the flow with transport of passive scalars at different Schmidt numbers and chemical reactions at different Damkohler numbers has been carried out. As a result, a comprehensive database for studying mixing phenomena and chemical reactions in a jet in crossflow has been generated. Results obtained concerning instantaneous mixing structures and laminar to turbulent flow transition has been compared to companion experimental data showing a good agreement. The database obtained was used to perform various analyses of the mixing including a priori testing of mixing models (hypotheses) used in the Reynolds-averaged approach. By means of Lagrangian methods flow regions of intensive mixing have been identified. For the present configuration they have been found to be attributed to helical movement of fluid particles. Also regions, where the streamlines of the averaged flow diverge from each other downstream exhibit good local mixing. In the last part of the work bi-orthogonal wavelets are employed to construct novel, multiscale models for Large Eddy Simulations (LES). These are validated and tested with DNS of turbulent channel flow as well as with the present DNS database for the jet in crossflow. Unlike the Smagorinsky model, the new wavelet models have been found to produce no eddy-viscosity in the laminar inflow region of the round pipe from which the jet originates.

DNS and LES of Scalar Transport in a Turbulent Plane Channel Flow at Low Reynolds Number

The paper reports on DNS and LES of plane channel flow at Rei¾?= 180 and compares these to a DNS with a higher order convection scheme. For LES different subgrid-scale models like the Smagorinsky, the Dynamic Smagorinsky and the Dynamic Mixed Model were used with the grid being locally refined in the near-wall region. The mixing of a passive scalar has been simulated with two convection schemes, central differencing and HLPA. The latter exhibits numerical diffusion and the results with the central scheme are clearly superior. LES with this scheme reproduced the budget of the scalar variance equation reasonably well.

Direct Numerical Simulation of Mixing and Chemical Reactions in a Round Jet into a Crossflow — a Benchmark

A benchmark simulation of the jet in crossflow (JICF) configuration is presented in detail. A Direct Numerical Simulation (DNS) was carried out with a low Reynolds number equal to 275 and a jet-to-crossflow velocity ratio equal to 2.4. The benchmark is carefully selected to provide reference data concerning the following phenomena: the flowfield, the mixing process of passive scalars and three chemical reactions. The data presented concern both instantaneous and time-averaged values as well as the corresponding fluctuations. To facilitate the quantitative comparison with the data from the present work various one-dimensional plots are presented. To allow easy repetition of the present numerical benchmark, both the jet and the crossflow are supplied at laminar flow conditions. As a result of this a transition zone occurs which in turn constitutes a severe test for any simulation methodology.

Two-Point Correlations of a Round Jet into a Crossflow – Results from a Direct Numerical Simulation

The paper presents results for the two-point correlation coefficient of two velocity components and a passive scalar for jets in crossflow. The data were obtained from two Direct Numerical Simulations carried out at two different Reynolds numbers (650 and 325) with a jet-to-crossflow velocity ratio of 3.3 in both cases. Results along the trajectory of the jet show the larger size of the turbulent eddies for the smaller Reynolds number. The integral scale of the turbulent eddies increases downstream. In the studied region, this scale appears to be larger along the crossflow than along the direction of the jet. The required resources for the investigation are described.

Numerical Simulation of Turbulent Flows in Complex Geometries Using the Coherent Vortex Simulation Approach Based on Orthonormal Wavelet Decomposition

Applications of the wavelet based coherent vortex extraction method are presented for homogeneous isotropic turbulence for different Reynolds numbers. We also summarize the developed adaptive multiresolution method for evolutionary PDEs. Then we show first fully adaptive computations of 3d mixing layers using Coherent Vortex Simulation. Features like local scale dependent time stepping are also illustrated and examples for one dimensional problems are given. Test cases on complex geometries like the periodic hill flow (with Reynolds numbers up to 37000) and an annular burner with a swirl number of S = 0.6 have been calculated based on the developed wavelet decomposition models. The extensive results presented show the robustness and good accuracy of the adopted wavelet approach for the various flows simulated.