Vineet Ahuja

Frequency-domain prediction of turbofan noise radiation

This paper describes a frequency-domain numerical method for predicting noise radiation from ducted fans, including acoustic treatment and non-uniform background flow effects. The method solves the Euler equations linearized about a mean flow in the frequency domain. A pseudo-time derivative term is added to the frequency-domain equations so that a time marching technique can be employed to drive the acoustic field to steady state explicitly. This approach makes distributed parallel computing more viable for equations of this type and will allow for future use of well-known convergence acceleration techniques, such as multigrid, to obtain the solutions efficiently. Simulations of the JT15D static test inlet are performed including the effects of liners, and the results are compared with experimental data. A generic engine geometry is used for demonstrating further the prediction capability of the code, calculating the attenuation effects of different liner impedances and liner installation locations on the radiated sound fields.

Modeling three-dimensional solidification with magnetic fields and reduced gravity

Abstract Two interacting systems of partial differential equations governing three-dimensional laminar flow of an incompressible viscous fluid undergoing solidification or melting while under the influence of an externally applied magnetic field have been formulated and integrated numerically. The model includes effects of Joule heating, latent heat, and arbitrary magnitude and orientation of gravity and the magnetic field. It allows for arbitrary temperature-dependent physical properties within the melt and the solid phase. The mushy region is captured by varying viscosity orders of magnitude in the mushy region and by allowing latent heat of phase change to be an arbitrary function of temperature. The uniqueness of this approach is in the fact that both liquid and solid phases are treated as incompressible liquids with the solid phase having an extremely high viscosity. It was found numerically that the magnetic field strength and orientation can significantly influence flow field velocity and vorticity, amount of accrued solid, and the solid/liquid interface shape.

COMPUTATIONAL SIMULATIONS OF FORE AND AFT RADIATION FROM DUCTED FANS

In this paper, we extend the work of Ozyoruk and Long, for predicting farfield noise from ducted fans using a higher order coupled Euler/Navier-Stokes solver along with a Kirchhoff formulation, to full engine configurations on distributed parallel computers. Incorporating the exhaust along with the inlet raises many issues relating to appropriate grid topologies, multigrid acceleration and the unsteady numerics. These issues are addressed in this paper and simulations pertaining to a realistic engine geometry are presented. Qualitative comparisons are in good agreement with results from a frequency domain code, pertaining to radiation from a dipole in a cylinder. Simulations relating to radiation from an engine geometry, and with and without the centerbody, with and without mass flow are also shown.

Impeller Design of a Centrifugal Fan with Blade Optimization

A method is presented for redesigning a centrifugal impeller and its inlet duct. The double-discharge volute casing is a structural constraint and is maintained for its shape. The redesign effort was geared towards meeting the design volute exit pressure while reducing the power required to operate the fan. Given the high performance of the baseline impeller, the redesign adopted a high-fidelity CFD-based computational approach capable of accounting for all aerodynamic losses. The present effort utilized a numerical optimization with experiential steering techniques to redesign the fan blades, inlet duct, and shroud of the impeller. The resulting flow path modifications not only met the pressure requirement, but also reduced the fan power by 8.8% over the baseline. A refined CFD assessment of the impeller/volute coupling and the gap between the stationary duct and the rotating shroud revealed a reduction in efficiency due to the volute and the gap. The calculations verified that the new impeller matches better with the original volute. Model-fan measured data was used to validate CFD predictions and impeller design goals. The CFD results further demonstrate a Reynolds-number effect between the model- and full-scale fans.

Transient Simulations of Valve Motion in Cryogenic Systems

Valve systems in rocket propulsion systems and testing facilities are constantly subject to dynamic events resulting from the timing of valve motion leading to unsteady fluctuations in pressure and mass flow. Such events can also be accompanied by cavitation, resonance, system vibration leading to catastrophic failure. High-fidelity dynamic computational simulations of valve operation can yield important information of valve response to varying flow conditions. Prediction of transient behavior related to valve motion can serve as guidelines for valve scheduling, which is of crucial importance in engine operation and testing. In this paper, we present simulations of valve motion utilizing a multi-element unstructured computational approach that permits the use of variable grid topologies, thereby permitting solution accuracy and resolving important flow physics in the seat region of the valve. The approach is based on a mesh library strategy in which generalized mesh movement is applied between a sequence of meshes and the solution is transferred between meshes at specific valve positions pre-defined in the library. We discuss important valve flow characteristics such as valve response to variable plug control speeds as part of our simulations. Furthermore, we present an alternative methodology that utilizes a single grid that deforms and adapts during valve motion.

Computational Analyses of Pressurization in Cryogenic Tanks

A comprehensive numerical framework utilizing multi-element unstructured CFD and rigorous real fluid property routines has been developed to carry out analyses of propellant tank and delivery systems at NASA SSC. Traditionally CFD modeling of pressurization and mixing in cryogenic tanks has been difficult primarily because the fluids in the tank co-exist in different sub-critical and supercritical states with largely varying properties that have to be accurately accounted for in order to predict the correct mixing and phase change between the ullage and the propellant. For example, during tank pressurization under some circumstances, rapid mixing of relatively warm pressurant gas with cryogenic propellant can lead to rapid densification of the gas and loss of pressure in the tank. This phenomenon can cause serious problems during testing because of the resulting decrease in propellant flow rate. With proper physical models implemented, CFD can model the coupling between the propellant and pressurant including heat transfer and phase change effects and accurately capture the complex physics in the evolving flowfields. This holds the promise of allowing the specification of operational conditions and procedures that could minimize the undesirable mixing and heat transfer inherent in propellant tank operation. In our modeling framework, we incorporated two different approaches to real fluids modeling: (a) the first approach is based on the HBMS model developed by Hirschfelder, Beuler, McGee and Sutton and (b) the second approach is based on a cubic equation of state developed by Soave, Redlich and Kwong (SRK). Both approaches cover fluid properties and property variation spanning sub-critical gas and liquid states as well as the supercritical states. Both models were rigorously tested and properties for common fluids such as oxygen, nitrogen, hydrogen etc were compared against NIST data in both the sub-critical as well as supercritical regimes.

A GENERALIZED MULTI-PHASE FRAMEWORK FOR MODELING CAVITATION IN CRYOGENIC FLUIDS

Abstract A generalized multi-phase formulation for cavitation in fluids operating at temperatures elevated relative to their critical temperatures is presented. The thermal effects and the accompanying property variations due to phase change are modeled rigorously. Thermal equilibrium is assumed and fluid thermodynamic properties are specified along the saturation line using the NIST-12 databank. Fundamental changes in the physical characteristics of the cavity when thermal effects become pronounced are identified; the cavity becomes more porous, the interface less distinct, and has increased entrainment when temperature variations are present. Quantitative estimates of temperature and pressure depressions in both liquid nitrogen and liquid hydrogen were computed and compared with experimental data of Hord [l] for hydrofoils. Excellent estimates of the leading edge temperature and pressure depression were obtained while the comparisons in the cavity closure region were reasonable. Liquid nitrogen cavities were consistently found to be in thermal equilibrium while liquid hydrogen cavities exhibited small, but distinct, non-equilibrium effects.

Progress in Modeling Pressurization in Propellant Tanks

Summary The pressure of propellant and oxidizer tanks has to be maintained within a narrow margin and is critical to the proper functioning of the liquid propulsion system. Several control mechanisms such as venting and spray bars are specifically deployed in the tankage to ensure that the pressure in the tank is restricted to the design margins. In this paper a suite of predictive tools have been developed that can aid in the design and management of propellant tanks. The analysis tools comprise of a multi-node lumped parameter code, a multi-phase CFD code, and a hybrid procedure that utilizes CFD in conjunction with a lumped parameter based internal boundary between the ullage and the liquid. Each tool is specifically tailored towards a certain class of tank pressurization problems: for example the multi-node lumped parameter approach is particularly suited for long duration space tank applications, while the CFD approach is applicable to short duration injection based pressurization problems where mixing is the dominant physical mechanism. In long duration cases, where injection or venting based control systems can significantly alter the flow in the tank the hybrid approach is more appealing. A series of test cases from different regimes of tank pressurization were considered in this paper with the three approaches. The first simulation was based on the Saturn AS-203 self-pressurization fuel tank experiment in a low gravity environment. The complete duration of the test was analyzed with the multi-node lumped parameter code with average heat loads through the tank walls and the

Analysis of Flame Deflector Spray Nozzles in Rocket Engine Test Stands

The development of a unified tightly coupled multi-phase computational framework is described for the analysis and design of cooling spray nozzle configurations on the flame deflector in rocket engine test stands. An Eulerian formulation is used to model the disperse phase and is coupled to the gas-phase equations through momentum and heat transfer as well as phase change. The phase change formulation is modeled according to a modified form of the Hertz-Knudsen equation. Various simple test cases are presented to verify the validity of the numerical framework. The ability of the methodology to accurately predict the temperature load on the flame deflector is demonstrated though application to an actual sub-scale test facility. The CFD simulation was able to reproduce the result of the test-firing, showing that the spray nozzle configuration provided insufficient amount of cooling.

Analyses Of Transient Events In Complex Valve and Feed Systems

Valve systems in rocket propulsion systems and testing facilities are constantly subject to dynamic events resulting from the timing of valve motion leading to unsteady fluctuations in pressure and mass flow. Such events can also be accompanied by cavitation, resonance, system vibration leading to catastrophic failure. High-fidelity dynamic computational simulations of valve operation can yield important information of valve response to varying flow conditions. Prediction of transient behavior related to valve motion can serve as guidelines for valve scheduling, which is of crucial importance in engine operation and testing. In this paper, we present simulations of the diverse unsteady phenomena related to valve and feed systems that include valve stall, valve timing studies as well as cavitation instabilities in components utilized in the test loop.