Jet With No Cross Flow

1. RANS Simulations of Unstart Due to Mass Injection J. Fike, K. Duraisamy, J. Alonso Introduction and Motivation Jet into Confined Supersonic Cross Flow Effects of Uncertainty Possible Sources of Error and Uncertainty The overarching goal of the PSAAP program at Stanford is to develop a predictive capability to determine the operability limits of a scramjet engine and to quantify the uncertainties involved in the simulation. At the core of this effort is the development of computational tools capable of simulating the flow in an operating scramjet and predicting the conditions under which unstart will occur. Several campaigns of experiments and simulations are underway to aid in the verification and validation (V&V) of our simulations. Fuel injection is inherent to the operation of a scramjet. As such, the phenomenon of jet injection is the focus of several of these experimental/computational efforts. Various assumptions and approximations are necessary in order to perform a simulation. For instance, a mathematical model of the physics needs to be chosen and this will inevitably contain errors. Also, the exact flow conditions and geometry of an experiment are never perfectly known. A major effort of the PSAAP program is to quantify the errors and uncertainties that these assumptions introduce, and assess their effect on the predictions of the quantity of interest that the simulations provide. Initial flow field with jet off Under the nominal conditions the simulations do not match the experimental results. However, there are many possible sources of error and uncertainty. • Differences in geometry (Plenum diameter, tip radius, etc.) • Boundary layer thickness (Measurements underway) • Inlet flow conditions (Pressure, Temperature, Velocity) • Choice of turbulence model and initial conditions • Uncertainty in experimental measurements (Plenum pressure, etc.) • CO2/Air mixture for visualization, simulations use only air Unstart Margin Characterization Inlet Flow Conditions Simulations and experiments are carried out at the nominal flow conditions for a range of plenum pressures, and the shock position is determined. The figures to the right show sample results for a simulation under nominal conditions with a plenum pressure of 400 kPa. The shock locations are then plotted versus plenum pressure to create an unstart margin curve, as shown below. • Perturbing the inlet conditions can have a significant effect • The magnitude of the perturbations was chosen to have same impact on the square root of the jet momentum ratio, R Unstart Due to Mass Injection Boundary Layer Thickness Fuel injection into a scramjet is modelled as a non-reacting jet injected into a confined supersonic crossflow. The jet creates a blockage in the flow, causing a complex shock system. If the blockage is too great, the system unstarts due to an upstream propagating shock/flow separation system. The strength of the jet is controlled by changing the plenum pressure. These experiments are performed by Hyungrok Do, Seong-kyun Im, and Prof. Mark Cappelli. • Little impact on unstart margin curves for most thicknesses • For the thickest case, the splitter plates are not able to isolate the core flow from the top and bottom wall boundary layers The plot to the left shows both experimental and computational results. The plot shows the effect of changing both the spacing between the splitter plates and the amount of CO2 used in the experiment. Effect of CO2 Effect of various uncertainties • The original experiments used Air with 25% of CO2 (by volume) as a working fluid for visualization purposes • Original simulations did not consider this effect and thus misrepresented mean flow characteristics • New set of simulations suggest large sensitivity to CO2 levels and new experiments were conducted to confirm this sensitivity Comparison of Unsteady Flow Features Ensemble simulations • Simulations were conducted at 5% CO2 including inflow (temperature) variations • Computations envelope measurements Snapshots from a time accurate simulation are compared to Planar-Laser Rayleigh Scattering images of the experiment. The simulations and experiments produce qualitatively similar flow features. There is a bow shock immediately in front of the jet accompanied by thick separated boundary layers. Further upstream, there is a pseudo-shock/separated flow system that propagates upstream and out past the isolator tips. Jet With No Cross Flow As a first step, experiments and simulations are performed for a jet of air exhausting into the tunnel geometry with no cross flow. The tunnel is at 200 torr, and the plenum is highly pressurized producing an under-expanded jet. Simulations are performed for a range of plenum pressures in order to match the Mach disk height from the experiment. MM Measurements (5% CO2) Density Gradient Pressurized jet plenum Splitter Plates M=4.6 Inflow Outflow PLRS images by H. Do, S. Im and M. Cappelli Schlieren image by H. Do, S. Im and M. Cappelli Acknowledgments This work was supported by the United States Department of Energy under the Predictive Science Academic Alliance Program (PSAAP) at Stanford University