Direct numerical simulation of single-species and binary-species boundary layers at high pressure

Direct numerical simulations of single-species and binary-species temporal boundary layers at high pressure are performed. The main objective is to investigate flow physics in a binary-species boundary layer at high pressure, with special attention to mass diffusion. The working fluids are nitrogen in the single-species cases, and a mixture of nitrogen and methane in the binary-species cases. An investigation of mean profiles shows that velocity and temperature profiles have steep gradients near the wall, whereas the mass fraction profiles do not have a steep gradient. This result indicates that the similarity law between velocity, temperature and mass fraction does not hold true in the binary-species boundary layer. The comparison of turbulent fluctuations shows that the qualitative characteristics of velocity and temperature are similar to each other. In contrast, profiles of mass fraction fluctuations are largely different from those of velocity and temperature. This result indicates that mass diffusion is not similar to momentum and thermal diffusions in binary-species boundary layers

Large Eddy Simulations of high pressure jets: Effect of subgrid scale modeling

The focus of this study is on developing high-fidelity models and simulations for fully turbulent high-pressure flows. To this end, two Large Eddy Simulation (LES) models are considered and simulations are conducted with these models for initial conditions for which experimental data is available. In the first LES, denoted as Standard LES (SLES), the only subgrid-scale (SGS) model used is that stemming from the convective terms of the conservation equations. In the second LES, a complementary SGS model for the difference between the gradient of the filtered pressure and the gradient of the pressure computed as a function of the filtered flow field is used in the momentum equation additional to the SGS model employed in SLES; this second model is labeled PLES. The comparison of the SLES results with experimental data is favorable, however, PLES visibly enhances the accuracy of the results compared to the same data. A detailed analysis reveals that in PLES the dense-fluid core persists further downstream than in SLES, unsteadiness is increased, and mixing is enhanced further downstream. The differences between PLES and SLES occur in a narrow radial ring of radius three jet diameters around the jet, although they persist at large distances downstream from the inflow. It is recommended that details experimental data should be obtained in this elongated ring region in order to more completely evaluate the potential of PLES compared to SLES

Numerical aspects for physically accurate Direct Numerical Simulations of turbulent jets

Numerical simulations of turbulent isothermal round jets are performed at Reynolds number (based on jet diameter and jet orifice velocity) of 5000 and Mach number of 0.6 to assess influences of initial perturbation, numerical characterization, and boundary treatments on turbulence statistics. The aper documents preliminary results from an investigation eventually aimed at examining multicomponent species injection and mixing at high-pressure conditions relevant to combustion in diesel, gas-turbine and liquid-rocket engines. Lack of reliable experimental or computational results at high pressures of interest make code validation at those conditions infeasible. Therefore, single-species simulations at perfect-gas conditions are first performed to assess turbulence statistics sensitivity to numerical setup. Two cases with different inflow velocity perturbation amplitude are considered to examine influences on jet flow transition and self-similarity

Turbulent high-pressure reaction-rate modeling using the Double-conditioned Conditional Source-term Estimation method

An evaluation of the submodels constituting the Double-Conditioned Source-term Estimation (DCSE) method for utilization in Large Eddy Simulation (LES) is presented which makes use of a single realization of a turbulent high-pressure reactive-flow database obtained from Direct Numerical Simulation (DNS). A filtered and coarsened DNS (FCDNS) field is first created to mimic a real LES field, and the FCDNS values for thermodynamic variables (temperature, density and species mass fractions) are used as inputs to the DCSE model to predict the FCDNS reaction rates. It is found that there are significant errors in the predictions of the filtered reaction rate compared to the template. These errors are attributed to important aspects of the model, some of which are discussed here, namely, the integral inversion necessary to find the conditional-filtered values of the thermodynamic variables and the modelling of the joint probability density function of the conditioning variables, the latter being by far the largest source of error. Assuming a β-PDF for the marginal PDF of the reaction progress variable is shown to be a particularly poor choice, as is the assumption that the conditioning variables are statistically independent