Engineering Papers⌕ Search

Engineering topics

Cantwell, Brian J.

Publications and source records attributed to Cantwell, Brian J..

13 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2013

Mass Flow Rate and Isolation Characteristics of Injectors for Use with Self-Pressurizing Oxidizers in Hybrid Rockets

Self-pressurizing rocket propellants are currently gaining popularity in the propulsion community, particularly in hybrid rocket applications. Due to their high vapor pressure, these propellants can be driven out of a storage tank without the need for complicated pressurization systems or turbopumps, greatly minimizing the overall system complexity and mass. Nitrous oxide (N2O) is the most commonly used self pressurizing oxidizer in hybrid rockets because it has a vapor pressure of approximately 730 pounds per square inch (5.03 megapascals) at room temperature and is highly storable. However, it can be difficult to model the feed system with these propellants due to the presence of two-phase flow, especially in the injector. An experimental test apparatus was developed in order to study the performance of nitrous oxide injectors over a wide range of operating conditions. Mass flow rate characterization has been performed to determine the effects of injector geometry and propellant sub-cooling (pressurization). It has been shown that rounded and chamfered inlets provide nearly identical mass flow rate improvement in comparison to square edged orifices. A particular emphasis has been placed on identifying the critical flow regime, where the flow rate is independent of backpressure (similar to choking). For a simple orifice style injector, it has been demonstrated that critical flow occurs when the downstream pressure falls sufficiently below the vapor pressure, ensuring bulk vapor formation within the injector element. It has been proposed to leverage the insensitivity of critical mass flow rate to downstream pressure as a means of preventing the occurrence of feed system coupled combustion instabilities in hybrid rockets utilizing nitrous oxide. Additionally, observations indicate that the existence of two-phase flow can attenuate pressure fluctuations traveling upstream through the injector, providing a degree of isolation between the feed line and downstream pressure disturbances. In consideration of safety, carbon dioxide (CO2) has been used as an analog to nitrous oxide in many of these studies. It has been observed experimentally that carbon dioxide serves as a good analog to nitrous oxide in both the single-phase and two-phase flow regimes.

Oxidizer↗
NASA NTRS2004

Study of Rapid-Regression Liquefying Hybrid Rocket Fuels

A report describes experiments directed toward the development of paraffin-based hybrid rocket fuels that burn at regression rates greater than those of conventional hybrid rocket fuels like hydroxyl-terminated butadiene. The basic approach followed in this development is to use materials such that a hydrodynamically unstable liquid layer forms on the melting surface of a burning fuel body. Entrainment of droplets from the liquid/gas interface can substantially increase the rate of fuel mass transfer, leading to surface regression faster than can be achieved using conventional fuels. The higher regression rate eliminates the need for the complex multi-port grain structures of conventional solid rocket fuels, making it possible to obtain acceptable performance from single-port structures. The high-regression-rate fuels contain no toxic or otherwise hazardous components and can be shipped commercially as non-hazardous commodities. Among the experiments performed on these fuels were scale-up tests using gaseous oxygen. The data from these tests were found to agree with data from small-scale, low-pressure and low-mass-flux laboratory tests and to confirm the expectation that these fuels would burn at high regression rates, chamber pressures, and mass fluxes representative of full-scale rocket motors.

Zilliac, Greg↗
NASA NTRS1998

A New View of the Dynamics of Reynolds Stress Generation in Turbulent Boundary Layers

The structure of a numerically simulated turbulent boundary layer over a flat plate at Re(theta) = 670 was studied using the invariants of the velocity gradient tensor (Q and R) and a related scalar quantity, the cubic discriminant (D = 27R(exp 2)/4 + Q(exp 3)). These invariants have previously been used to study the properties of the small-scale motions responsible for the dissipation of turbulent kinetic energy. In addition, these scalar quantities allow the local flow patterns to be unambiguously classified according to the terminology proposed by Chong et al. The use of the discriminant as a marker of coherent motions reveals complex, large-scale flow structures that are shown to be associated with the generation of Reynolds shear stress -u'v'(bar). These motions are characterized by high spatial gradients of the discriminant and are believed to be an important part of the mechanism that sustains turbulence in the near-wall region.

Cantwell, Brian J.↗
NASA NTRS1997

Direct Numerical Simulation of Incompressible Pipe Flow Using a B-Spline Spectral Method

A numerical method based on b-spline polynomials was developed to study incompressible flows in cylindrical geometries. A b-spline method has the advantages of possessing spectral accuracy and the flexibility of standard finite element methods. Using this method it was possible to ensure regularity of the solution near the origin, i.e. smoothness and boundedness. Because b-splines have compact support, it is also possible to remove b-splines near the center to alleviate the constraint placed on the time step by an overly fine grid. Using the natural periodicity in the azimuthal direction and approximating the streamwise direction as periodic, so-called time evolving flow, greatly reduced the cost and complexity of the computations. A direct numerical simulation of pipe flow was carried out using the method described above at a Reynolds number of 5600 based on diameter and bulk velocity. General knowledge of pipe flow and the availability of experimental measurements make pipe flow the ideal test case with which to validate the numerical method. Results indicated that high flatness levels of the radial component of velocity in the near wall region are physical; regions of high radial velocity were detected and appear to be related to high speed streaks in the boundary layer. Budgets of Reynolds stress transport equations showed close similarity with those of channel flow. However contrary to channel flow, the log layer of pipe flow is not homogeneous for the present Reynolds number. A topological method based on a classification of the invariants of the velocity gradient tensor was used. Plotting iso-surfaces of the discriminant of the invariants proved to be a good method for identifying vortical eddies in the flow field.

Loulou, Patrick↗
NASA NTRS1994

The effect of initial conditions on the development of temporally evolving planar three dimensional incompressible wakes

A set of direct numerical simulations of temporally evolving incompressible plane wakes started from a variety of initial conditions have been performed. The intent is to explore the effect of initial conditions on the development of three dimensionality. It was found that the existence of an oblique disturbance at a streamwise wavelength equal to the two-dimensional subharmonic wavelength plays a very important role in the development of the wake. The Reynolds number and the presence or absence of longer wavelength disturbances were also found to be important at late times in the development of the flow. Disturbance phase was found to have an effect on the details of the structure of the wake, but has relatively little effect on the wake growth.

Sondergaard, Rolf↗
NASA NTRS1989

Investigation of an excited jet diffusion flame at elevated pressure

Experiments are carried out for studying the relationship between flow structure, flow excitation, and the reaction process in the near field of a low-speed coflowing jet diffusion flame. The effect of axial forcing and increasing pressure on the structure and controllability of the flame is studied to elucidate some of the underlying control mechanisms. The experiments are conducted in a variable-pressure flow facility permitting the study of reacting flows in the 10-1000 kPa (0.1-10 atm) pressure range. The flame is excited by a small-amplitude periodic fluctuation added to the central fuel jet exit velocity. Among the varied excitation frequencies employed, a narrow band is found in which several of the flow instabilities seem to be in coincidence, causing periodic breakup of the flame into a series of distinct eddies. This is called the strongly-coupled state. Effects of excitation frequencies at and above the strongly-coupled range are presented.

Strawa, Anthony W.↗
NASA NTRS1989

The effect of Mach number on the stability of a plane supersonic wave

The influence of compressibility on the mechanisms governing the various stages of transition in a supersonic wake is investigated. Results from linear stability theory are used to provide physical insights into the observed reduction in growth rate at high Mach numbers. A newly developed hybrid algorithm is used to solve the compressible inviscid linear disturbance equations. Growth rates for both antisymmetric and symmetric modes of two-dimensional and oblique waves are computed for a wide range of Mach numbers. Results from two and three-dimensional direct numerical simulations of a forced compressible time-developing wake are presented in order to understand the nonlinear stages of transition at high Mach numbers. Observed nonlinear growth rate comparisons are made for wakes at two different Mach numbers. The reduction in growth rate at high Mach numbers is explained by examining contour plots of baroclinic torques and the product of dilatation and vorticity.

Chen, Jacqueline H.↗
NASA NTRS1989

Direct numerical simulations of transition in a compressible wake

Direct numerical simulations of a compressible wake are performed to determine the sensitivity of large-scale motions to initial phase lags between the most unstable two-dimensional wave and a pair of oblique waves, and to determine the effect of Mach number on the development of three-dimensional motions. The vorticity dynamics of the flow are studied, and it is determined that, depending upon the initial phase lag, the interaction of the streamwise vorticity with the spanwise vortices may result in the formation of closed vortex loops, or in the removal of streamwise vorticity from the saddle region between the spanwise vortices. Due to lower growth rates experienced at high Mach numbers, the roll-up process is delayed, and the resultant nonlinear motions are much more diffuse.

Chen, Jacqueline H.↗
NASA NTRS1989

The topology and vorticity dynamics of a three-dimensional plane compressible wake

The three-dimensional aspects of transition in a low Mach number plane compressible wake are studied numerically. Comparisons are made between the topology of the velocity field and the vorticity dynamics of the flow based on results from direct numerical simulations of the full compressible Navier-Stokes equations. The velocity field is integrated to obtain instantaneous streamlines at different stages in the evolution. A generalized three-dimensional critical point theory is applied to classify the critical points of the velocity field.

Chen, Jacqueline H.↗
NASA NTRS1986

Viscous starting jets

The transient motion which is produced when a viscous incompressible fluid is forced from an initial state of rest is studied. The equations for unsteady particle paths, written in terms of similarity variables, are analyzed as a quasi-autonomous system with the Reynolds number treated as a parameter. By finding and classifying critical points in the system's phase portrait, the flow structure is examined. It is shown that: (1) bifurcations in the phase portrait occur at specific values of the Reynolds number of the flow in question, and (2) the exact solutions of the Stokes equations for the low-Reynolds-number limit contain two critical Reynolds numbers and three distinct states of motion which culminate in the onset of a vortex roll-up.

Cantwell, Brian J.↗
NASA NTRS1986

Computation of transonic separated wing flows using an Euler/Navier-Stokes zonal approach

A computer program called Transonic Navier Stokes (TNS) has been developed which solves the Euler/Navier-Stokes equations around wings using a zonal grid approach. In the present zonal scheme, the physical domain of interest is divided into several subdomains called zones and the governing equations are solved interactively. The advantages of the Zonal Grid approach are as follows: (1) the grid for any subdomain can be generated easily; (2) grids can be, in a sense, adapted to the solution; (3) different equation sets can be used in different zones; and, (4) this approach allows for a convenient data base organization scheme. Using this code, separated flows on a NACA 0012 section wing and on the NASA Ames WING C have been computed. First, the effects of turbulence and artificial dissipation models incorporated into the code are assessed by comparing the TNS results with other CFD codes and experiments. Then a series of flow cases is described where data are available. The computed results, including cases with shock-induced separation, are in good agreement with experimental data. Finally, some futuristic cases are presented to demonstrate the abilities of the code for massively separated cases which do not have experimental data.

Kaynak, Uenver↗