No abstract

No abstract

The scalar dissipation rate and the scalar variance are important modeling parameters in large eddy simulations (LES) of reacting flows. Approaches ranging from constant coefficient algebraic equations to transport equations with complex closure models have been proposed to describe these parameters. While currently available approaches capture many of the physics behind the dissipation and variance, their modeling remains challenging. Here, two direct numerical simulations (DNS) are used to analyze LES dissipation rate and variance models, and to propose a new model for the dissipation rate that is based on a transport equation. The first DNS that is considered is a non-premixed auto-igniting C 2 H 4 jet flame simulation performed by Yoo et al. (Proc. Comb. Inst., 2010). An LES of this case is run using algebraic models for the dissipation rate and variance. It is shown that the algebraic models do not accurately reproduce the subfilter DNS fields. This motivates the introduction of a transport equation model for the LES dissipation rate. Closure of the equation is addressed by formulating a new adapted dynamic approach. This approach borrows dynamically computed information from LES quantities that, unlike the dissipation rate, do not reside on the smallest flow length scales. The adapted dynamic closure is analyzed by considering a second DNS of scalar mixing in homogeneous isotropic turbulence. Data from this second DNS is used to confirm that the adapted dynamic procedure successfully closes the dissipation rate equation over a wide range of LES filter widths. The first reacting jet case is then returned to and used to test the LES transport equation models. These models are shown to predict the dissipation rate and variance fields more accurately than the the algebraic LES models.

No abstract

To further develop and demonstrate the applicability of liquefying-fuel hybrid rocket technology to low-cost launch applications, a small team of engineers is developing a medium-scale liquefying-fuel hybrid sounding rocket using storable propellants (paraffin wax and N 2 O) that will carry a 5 kg payload to the edge of space. This rocket, known as Peregrine, is being developed by engineers from NASA Ames, Stanford University, Space Propulsion Group Inc. (SPG, Sunnyvale, CA) and NASA Wallops, with a launch from Wallops anticipated at some point in the future. This paper focuses on the propulsion ground test results obtained to date.Results from the first round of ground testing showed that the design peak oxidizer mass flux of 1300 kg/m 2 -sec in the fuel port was too high to achieve satisfactory flame holding with any of the injector configurations tested. Furthermore, a strong first logitudinal acoustic mode response was observed in a majority of the tests and the average c* combustion efficiency design target of greater than 95 percent was not achieved. To address these issues, the motor was redesigned to accommodate a larger fuel grain and a second phase of ground testing is currently underway. Recent ground test results show significant improvements in combustion efficiency (average c* efficiency of 93% achieved in the most recent test) and combustion stability, but further development is required to bring the propulsion system to operational status.