A Simple Approach for Specifying Velocity Inflow Boundary Conditions in Simulations of Turbulent Opposed-Jet Flows

Abstract

A new methodology is developed to specify inflow boundary conditions for the velocity field at the nozzle exit planes in turbulent counterflow simulations. The turbulent counterflow configuration consists of two coaxial opposed nozzles which emit highlyturbulent streams of varying species compositions depending on the mode considered. The specification of velocity inflow boundary conditions at the nozzle exits in the counterflow configuration is non-trivial because of the unique turbulence field generated by the turbulence generating plates (TGPs) upstream of the nozzle exits. In the method presented here, a single large-eddy simulation (LES) is performed in a large domain that spans the region between the TGPs of the nozzles, and the time series of the velocity fields at the nozzle exit planes are recorded. To provide inflow boundary conditions at the nozzle exit planes for simulations under other conditions (e.g., different stream compositions, bulk velocity, TGP location), transformations are performed on the recorded time series: the mean and r.m.s. (root-mean-square) quantities of velocity, as well as the longitudinal integral length scale on the centerline, at the nozzle exits in simulations are matched to those observed in experiments, thereby matching the turbulent Reynolds number Re t . The method is assessed by implementing it in coupled large-eddy simulation/probability density function (LES/PDF) simulations on a small cylindrical domain between the nozzle exit planes for three different modes of the counterflow configuration: N 2 vs. N 2 ; N 2 vs. hot combustion products; Ranjith R. Tirunagari Flow Turbulence Combust and CH 4 /N 2 vs. O 2 . The inflow method is found to be successful as the first and second moments of velocity from the LES/PDF simulations agree well with the experimental data on the centerline for all three modes. This simple yet effective inflow strategy can be applied to eliminate the computational cost required to simulate the flow field upstream of the nozzle exits. It is also emphasized that, in addition to the predicted time series data, the availability of experimental data close to the nozzle exit planes plays a key role in the success of this method.

An investigation of turbulent premixed counterflow flames using large-eddy simulations and probability density function methods

“…The specification of the velocity inflow boundary conditions at the nozzle exit planes for the chosen computational domain is non-trivial. The inflow boundary condition method described in [37] is employed and found to be successful. 3.…”

“…quantities of the axial and radial velocities, and the longitudinal integral length scale on the centerline, at the top nozzle exit plane in the simulations to those measured at a distance of 0.5 mm downstream of the top nozzle exit in the experiments. The transformation procedure is described in detail in [37]. The hot products stream is represented by a steady velocity field with only radial profiles of the mean axial and radial velocities (i.e., no fluctuations).…”

“…The scaling for the mean axial velocity is performed so that the volume flow rate is matched to that of the experiments and the scaling for the mean radial velocity is performed so that the mean stagnation plane is near the mid-plane. The velocity boundary conditions treatment of the hot products stream is also discussed in detail in [37].…”

Characterization of extinction/reignition events in turbulent premixed counterflow flames using strain-rate analysis

“…The specification of velocity boundary conditions at the inflows of the computational domain (i.e., at the nozzle exit planes) is non-trivial. The inflow methodology described in [24] is employed to match the turbulent Reynolds number Re t in the simulations to that of the experiments. A total of approximately 0.3M LES grid cells and 6M particles (where 1M = 10 6 ) are used in the LES/PDF simulations.…”