Jason Rabinovitch

Effect of a splitter plate on the dynamics of a vortex pair

An experimental and numerical study was performed to investigate and compare the behavior of a counter-rotating vortex pair and a single vortex in the vicinity of a wall. This analysis is motivated by the theoretical equivalence, in the inviscid limit, between these two configurations. A wind tunnel with two NACA0012 profiles mounted vertically with an optional splitter plate in the center and a stereoscopic particle image velocimetry system was used to experimentally study these interactions. Many significant differences were found between the two configurations, including the growth of the Crow instability in the two vortex configuration, but not in the one vortex/wall configuration. The numerical results re-enforced the experimental results, and emphasized the fundamental physical differences between the two configurations. While modeling a vortex wall system with an image vortex may give correct integral results for loads experienced by blades, this model does not accurately describe the downstream dynamics of the vortex system.

Pyrolysis Gas Composition for a Phenolic Impregnated Carbon Ablator Heatshield

Published physical properties of phenolic impregnated carbon ablator (PICA) are compiled, and the composition of the pyrolysis gases that form at high temperatures internal to a heatshield is investigated. A link between the composition of the solid resin, and the composition of the pyrolysis gases created is provided. This link, combined with a detailed investigation into a reacting pyrolysis gas mixture, allows a consistent, and thorough description of many of the physical phenomena occurring in a PICA heatshield, and their implications, to be presented.

An Adaptive Mesh Refinement Concept for Viscous Fluid-Structure Computations Using Eulerian Vertex-Based Finite Volume Methods

Embedded Boundary Methods (EBMs) [1] for the solution of Computational Fluid Dynamics (CFD) and Fluid-Structure Interaction (FSI) problems are typically formulated in the Eulerian setting, which makes them more attractive than Chimera and Arbitrary Lagrangian-Eulerian methods when the structure undergoes large structural motions and/or deformations. In the presence of viscous flows however, they necessitate Adaptive Mesh Refinement (AMR) because unlike Chimera and ALE methods, they do not track boundary layers [2]. In general, AMR gives rise to non-conforming mesh configurations that can complicate the semi-discretization process. This is the case when this process is carried out using the popular vertex-based finite volume method and dual cells. Perhaps for this reason, most of the literature on AMR in the context of EBMs and the FV method has focused so far on cell-centered schemes, where the treatment of non-conforming mesh configurations is straightforward [1]. Specifically, most if not all local refinement strategies developed in this context generate “hanging” nodes in the refined mesh that can be easily dealt with using cell-centered but not vertex-based methods. In the latter case, flux assembly after a mesh refinement step becomes a problematic issue, due to presence of dual cells. This talk proposes a simple approach for resolving this issue by appropriately managing the construction of dual cells past each refinement step. The talk will recall the motivations for EBMs and vertex-based FV methods, explain the aforementioned AMR issue that arises in their context, present a method for resolving it, and illustrate this method with the application of the EBM known as FIVER (Finite Volume method with Exact two-phase Riemann solvers) [3, 4] to various examples including the prediction of the aerodynamic performance of a Formula 1 car.

Rate-Controlled Constrained Equilibrium for Nozzle and Shock Flows

The performance of different constraints for the rate-controlled constrained-equilibrium (RCCE) method is investigated in the context of modeling reacting flows characteristic of hypervelocity ground testing facilities and reentry conditions. Although the RCCE approach has been used widely in the past, its application in non-combustion-based reacting flows is rarely done; the flows being investigated in this work do not contain species that can easily be classified as reactants and/or products. The effectiveness of different constraints is investigated before running a full computational simulation, and new constraints not reported in the existing literature are introduced. A constraint based on the enthalpy of formation is shown to work well for the two gas models used for flows that involve both shocks and steady expansions.

A Computationally Efficient Approach to Hypersonic Reacting Flows

Reproducing hypersonic flight conditions in ground based facilities is a very challenging problem due to the high enthalpies required in the test gas, while simultaneously trying to match the post shock gas composition seen in flight. Experimental facilities have made significant progress in reproducing accurately hypersonic flows, but these facilities generally face issues associated with high operating costs and relatively short and noisy test times. This necessitates the need for high fidelity numerical simulations to aid with the investigation of these flows.

Proposed Vertical Expansion Tunnel

It is proposed that the adverse effects from secondary diaphragm rupture in an expansion tunnel may be reduced or eliminated by orienting the tunnel vertically, matching the test gas pressure and the accelerator gas pressure, and initially separating the test gas from the accelerator gas by density stratification. This proposed configuration is termed the vertical expansion tunnel. Two benefits are 1) the removal of the diaphragm particulates in the test gas after its rupture, and 2) the elimination of the wave system that is a result of a real secondary diaphragm having a finite mass and thickness. An inviscid perfect-gas analysis and quasi-one-dimensional Euler computations are performed to find the available effective reservoir conditions (pressure and mass specific enthalpy) and useful test time in a vertical expansion tunnel for comparison to a conventional expansion tunnel and a reflected-shock tunnel. The maximum effective reservoir conditions of the vertical expansion tunnel are higher than the reflected-shock tunnel but lower than the expansion tunnel. The useful test time in the vertical expansion tunnel is slightly longer than the expansion tunnel but shorter than the reflected-shock tunnel. If some sacrifice of the effective reservoir conditions can be made, the vertical expansion tunnel could be used in hypervelocity ground testing without the problems associated with secondary diaphragm rupture.

Development of Venus drill

Venera 13 was the first Venus surface mission with sampling capabilities. Its rotary drill successfully penetrated the surface and pneumatically transferred material to the science instrument within the insulated interior of the spacecraft. Follow-on missions in the Venera and Vega program repeated that feat. These missions demonstrated that it is possible for an electric motor to function at Venus conditions and that it is possible to drill Venus surface material and pneumatically transfer captured sample under Venus conditions. Unfortunately, design details for the sampling system do not exist or cannot be located. Hence for any future Venus surface missions, the sampling technology has to be designed without any prior knowledge of materials or methods. To help advance Venus sampling technology, Honeybee Robotics in partnership with NASA JPL has been developing critical components that would make Venus sampling possible. To date, three types of motors (Switched Reluctance, Brushless DC, and Stepper), a Pulsed Injection Position Sensor (PIPS) for commutation and position control, and planetary gearboxes have been fabricated and tested at Venus Temperature and/or Venus Temperature and Pressure. This paper summarizes past work and presents the current state-of-art technology related to Venus sampling drill for the New Frontiers Venus lander proposal called In-situ Surface and Atmospheric Geochemical Explorer (VISAGE).

Numerical modeling and analysis of early shock wave interactions with a dense particle cloud

Dense compressible multiphase flows exist in variable phase turbines, explosions, and fluidized beds, where the particle volume fraction is in the range 0.001 <α d < 0.5 .A simple model problem that can be used to study modeling issues related to these types of flows is a shock wave impacting a particle cloud. In order to characterize the initial shock-particle interactions when there is little particle movement, a two-dimensional (2-D) model problem is created where the particles are frozen in place. Qualitative comparison with experimental data indicates that the 2-D model captures the essential flow physics. Volume-averaging of the 2-D data is used to reduce the data to one dimension, and x-t diagrams are used to characterize the flow behavior. An equivalent one-dimensional (1-D) model problem is developed for direct comparison with the 2-D model. While the 1-D model characterizes the overall steady-state flow behavior well, it fails to capture aspects of the unsteady behavior. As might be expected, it is found that neglecting the unclosed fluctuation terms inherent in the volume-averaged equations is not appropriate for dense gas-particle flows.