Jet With No Cross Flow

RANS Simulations of Unstart Due to Mass InjectionJ. Fike, K. Duraisamy, J. Alonso

AcknowledgmentsThis work was supported by the United States Department of Energy under the Predictive Science Academic Alliance Program (PSAAP) at Stanford University

Introduction and Motivation

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.

Jet into Confined Supersonic Cross Flow

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

Possible Sources of Error and UncertaintyUnder the nominal conditions the simulations do not match the experimental results. However, there are many possible sources of error and uncertainty.

Unstart Due to Mass Injection

Effects of Uncertainty

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.

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.

DensityGradient

Unstart Margin Characterization

Comparison of Unsteady Flow Features

M=4.6 Inflow Splitter Plates Outflow

Pressurized jet plenum

Initial flow fieldwith jet off

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.

The plot to the left shows both experimental and computational results.

Inlet Flow Conditions

Boundary Layer Thickness

Effect of CO2

Ensemble simulations

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

Simulations were conducted at 5% CO2 including inflow (temperature) variations Computations envelope measurements

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

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

The plot shows the effect of changing both the spacing between the splitter plates and the amount of CO2 used in the experiment.

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.

Schlieren image by H. Do, S. Im and M. Cappelli PLRS images by H. Do, S. Im and M. Cappelli

€

R =ρu2( )

jet

ρu2( )∞

=γ jetPjetM jet

2

γ∞P∞M∞2

MMMeasurements (5% CO2)

Effect of various uncertainties