The stability of a steady, vortical flow in a cylindrical container with one rotating endwall has... more The stability of a steady, vortical flow in a cylindrical container with one rotating endwall has been experimentally examined to gain insight into the process of vortex breakdown. The dynamics of the flow are governed by the Reynolds number (Re) and the aspect ratio of the cylinder. Re is given by _R2/v, where _2 is the speed of rotation of the endwall, Wei-Ping Cheng, John Pye, Karl yon Ellenrider, and the rest of our research group for providing crucial help with the experiments and analysis. • The machinists Aldo, Tom, Matt, and Bob for their expertise and help in consU_cting the experimental apparatus.
The report summarizes the design, fabrication, safety features, environmental impact, and operati... more The report summarizes the design, fabrication, safety features, environmental impact, and operation of the Ames Hybrid-Fuel Combustion Facility (HCF). The facility is used in conducting research into the scalability and combustion processes of advanced paraffin-based hybrid fuels for the purpose of assessing their applicability to practical rocket systems. The facility was designed to deliver gaseous oxygen at rates between 0.5 and 16.0 kg/sec to a combustion chamber operating at pressures ranging from 300 to 900. The required run times were of the order of 10 to 20 sec. The facility proved to be robust and reliable and has been used to generate a database of regression-rate measurements of paraffin at oxygen mass flux levels comparable to those of moderate-sized hybrid rocket motors.
This project extends the study of the structure of wall-bounded flows using the topological prope... more This project extends the study of the structure of wall-bounded flows using the topological properties of eddying motions as developed by Chong et al. (1990), Soria et al. (1992, 1994), and as recently extended by Blackburn et al. (1996) and Chacin et al. (1996). In these works, regions of flow which are focal in nature are identified by being enclosed by an isosurface of a positive small value of the discriminant of the velocity gradient tensor. These regions resemble the attached vortex loops suggested first by Theodorsen (1955). Such loops are incorporated in the attached eddy model versions of Perry & Chong (1982), Perry et al. (1986), and Perry & Marusic (1995), which are extensions of a model first formulated by Townsend (1976). The DNS data of wall bounded flows studied here are from the zero pressure gradient flow of Spalart (1988) and the boundary layer with separation and reattachment of Na & Moin (1996). The flow structures are examined from the viewpoint of the attached ...
The paper is intended to present both experimental and numerical approaches for estimating the he... more The paper is intended to present both experimental and numerical approaches for estimating the heat exchange per thermal radiation towards the walls of lab-scale paraffin-based thrust chambers. Two firing tests of a lab-scale gaseous-oxygen/paraffin-wax hybrid rocket engine have been performed to apply such methods. In particular, the radiative wall heat flux has been evaluated by both spectroscopic measurements and discrete transfer method computations. Details of such approaches are given together with results achieved and suggestions for future improvements.
46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, 2010
Research is ongoing at Stanford University on the catalytic decomposition of nitrous oxide as an ... more Research is ongoing at Stanford University on the catalytic decomposition of nitrous oxide as an environmentally friendly monopropellant for propulsion and power applications. Two meso-scale planar monopropellant thrusters have been successfully tested: one made of monolithic silicon carbide utilizing a commercially supplied catalyst, and the other made of Hastelloy-X ® utilizing a rhodium based catalyst prepared in-house. The unique planar geometry shows a pathway towards scaling down to micro-scale through MEMS-batch fabrication techniques which enable low-cost, mass produced devices with improved thrust to weight ratios and faster start-up transients. The in-house catalyst showed higher reactivity as compared to the commercially supplied catalyst by initiating the decomposition reaction at a temperature of 200°C corresponding to 50W input power. Bed loadings up to 8.67 kg/m 2 /sec were achieved at a c star efficiency of 71%. Both devices are compared to a previously reported, scaled-up, cylindrical thruster. Nomenclature A o = flow control orifice throat area c* = characteristic exhaust velocity C d = discharge coefficient γ = heat capacity ratio m = mass flow rate P 1 = upstream pressure P crit = static pressure at orifice throat P c = test article chamber pressure R c = universal gas constant T c = test article chamber temperature Y = expansion factor ρ = density of the gas D
A recently developed mixing length model of the turbulent shearing stress in wall bounded flows h... more A recently developed mixing length model of the turbulent shearing stress in wall bounded flows has been used to formulate a universal velocity profile (UVP) that provides an effective replacement for the widely used Coles wall-wake formulation. Comparisons with both direct numerical simulation and experimental data demonstrate the ability of the profile to approximate a wide variety of wall-bounded flows. The UVP is uniformly valid from the wall to the boundary layer edge and for all Reynolds numbers from zero to infinity. There is no presumption of logarithmic dependence of the velocity profile outside the viscous wall layer so the profile can accurately approximate low Reynolds number turbulent boundary layers. The effect of a pressure gradient is included in the UVP through the introduction of a modified Clauser parameter that correlates well with the parameters that determine the wake portion of the velocity profile. The inherent dependence of the UVP on Reynolds number, extend...
The idea of the hybrid rocket is to store the oxidizer as a liquid and the fuel as a solid, produ... more The idea of the hybrid rocket is to store the oxidizer as a liquid and the fuel as a solid, producing a design that minimizes the chance of a chemical explosion. While the hybrid enjoys many safety and environmental advantages over conventional systems, large hybrids have not been commercially viable. The reason is that traditional systems use polymeric fuels that evaporate too slowly, making it difficult to produce the high thrust needed for most applications. Research at Stanford University and Space Propulsion Group (SPG) has led to the development of paraffin-based fuels that burn at regression rates 3–4 times that of polymeric fuels. Under the action of the oxidizer flow, the new fuels form a thin, hydrodynamically unstable liquid layer on the melting surface of the fuel. Entrainment of droplets from the liquid–gas interface can substantially increase the rate of fuel mass transfer, leading to a much higher surface regression rate than can be achieved with a conventional fuel. ...
The geometry of the velocity field in a numerically simulated incompressible turbulent boundary l... more The geometry of the velocity field in a numerically simulated incompressible turbulent boundary layer over a flat plate at Re theta=670 has been studied using the invariants of the velocity gradient tensor. These invariants are computed at every grid point in the flow and used to form the discriminant. Of primary interest are those regions in the flow where the discriminant is positive; regions where, according to the characteristic equation, the eigenvalues of the velocity gradient tensor are complex. An observer moving with a frame of reference which is attached to a fluid particle lying within such a region would see a local flow pattern of the type stable-focus-stretching or unstable-focus-compressing. When the flow is visualized this way, continuous, connected, large-scale structures are revealed that extend from the point just below the buffer layer out to the beginning of the wake region. These structures are aligned with the mean shear close to the wall and arch in the cross...
Paper published as part of the special topic on Tribute to Frank M. White on his 88th Anniversary... more Paper published as part of the special topic on Tribute to Frank M. White on his 88th Anniversary This paper was selected as Featured ARTICLES YOU MAY BE INTERESTED IN Complete and incomplete similarity for the mean velocity profile of turbulent pipe and channel flows at extreme Reynolds number
A new solid fuel formulation called SP7 was developed for application in a hybrid rocket propulsi... more A new solid fuel formulation called SP7 was developed for application in a hybrid rocket propulsion system for a potential Mars Ascent Vehicle (MAV). The new fuel offers good propulsive performance (Isp) while meeting the storage and operation requirements placed upon the proposed mission. Mixed Oxides of Nitrogen (MON) are selected for the oxidizer due to the low freezing points possible with these materials. The low temperature capabilities of the fuel and oxidizer reduce the required energy associated with thermal management systems. Evaluation of the propulsive performance of SP7 was completed with two oxidizers, N2O and MON, in a 2.7-in hybrid rocket motor. In addition to the baseline fuel, metallized formulations with 20% by weight aluminum particles were also tested. Ignition and stable combustion were demonstrated with both oxidizers over a wide range of operating conditions. Static test firing of SP7 demonstrated the ability for this fuel to meet the propulsion requirements of the as designed potential MAV mission.
Recent interest in supersonic combustion and problems of transatmospheric flight has prompted ren... more Recent interest in supersonic combustion and problems of transatmospheric flight has prompted renewed research efforts in laminar-turbulent free shear flow transition. In the present work, linear stability theory and direct numerical simulations are used to study the effect of Mach ...
The most important unanswered questions in turbulence regard the nature of turbulent flow in the ... more The most important unanswered questions in turbulence regard the nature of turbulent flow in the limit of infinite Reynolds number. The Princeton superpipe (PSP) data comprise 26 velocity profiles that cover three orders of magnitude in the Reynolds number from $Re=19\,639$ , to $Re=20\,088\,000$ based on pipe radius and pipe centreline velocity. In this paper classical mixing length theory is combined with a new mixing length model of the turbulent shear stress to solve the streamwise momentum equation and the solution is used to approximate the PSP velocity profiles. The model velocity profile is uniformly valid from the wall to the pipe centreline and comprises five free parameters that are selected through a minimization process to provide an accurate approximation to each of the 26 profiles. The model profile is grounded in the momentum equation and allows the velocity derivative, Reynolds shear stress and turbulent kinetic energy production to be studied. The results support t...
Abstract : A three year study of the effect of initial conditions on the topology and fine scale ... more Abstract : A three year study of the effect of initial conditions on the topology and fine scale structure of temporally evolving wakes has been completed. Early in the project new topological analysis tools were developed and used to analyze available simulations of the plane mixing layer. During the course of these preliminary studies it was found that certain geometrical features of the fine scale motions in the mixing layer were very similar to those seen in simulations of forced isotropic turbulence. As a consequence, we adopted a two-track approach to the work; one track was to gather together simulation data on as wide a variety of shear flows as possible with the objective of identifying universal feature of the fine scale structure. The second track was to carry out a new direct simulation of the plane wake in which systematic variations of the initial conditions were used to identify structural features of the developed flow which depend on initial conditions. The fundamental questions we tried to address were: What features of the fine scale motions in turbulent flow are universal, what features are not, and, in the case of the plane wake, what information about the near wake survives far downstream?
A computer program called Transonic Navier Stokes (TNS) has been developed which solves the Euler... more 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.
The stability of a steady, vortical flow in a cylindrical container with one rotating endwall has... more The stability of a steady, vortical flow in a cylindrical container with one rotating endwall has been experimentally examined to gain insight into the process of vortex breakdown. The dynamics of the flow are governed by the Reynolds number (Re) and the aspect ratio of the cylinder. Re is given by _R2/v, where _2 is the speed of rotation of the endwall, Wei-Ping Cheng, John Pye, Karl yon Ellenrider, and the rest of our research group for providing crucial help with the experiments and analysis. • The machinists Aldo, Tom, Matt, and Bob for their expertise and help in consU_cting the experimental apparatus.
The report summarizes the design, fabrication, safety features, environmental impact, and operati... more The report summarizes the design, fabrication, safety features, environmental impact, and operation of the Ames Hybrid-Fuel Combustion Facility (HCF). The facility is used in conducting research into the scalability and combustion processes of advanced paraffin-based hybrid fuels for the purpose of assessing their applicability to practical rocket systems. The facility was designed to deliver gaseous oxygen at rates between 0.5 and 16.0 kg/sec to a combustion chamber operating at pressures ranging from 300 to 900. The required run times were of the order of 10 to 20 sec. The facility proved to be robust and reliable and has been used to generate a database of regression-rate measurements of paraffin at oxygen mass flux levels comparable to those of moderate-sized hybrid rocket motors.
This project extends the study of the structure of wall-bounded flows using the topological prope... more This project extends the study of the structure of wall-bounded flows using the topological properties of eddying motions as developed by Chong et al. (1990), Soria et al. (1992, 1994), and as recently extended by Blackburn et al. (1996) and Chacin et al. (1996). In these works, regions of flow which are focal in nature are identified by being enclosed by an isosurface of a positive small value of the discriminant of the velocity gradient tensor. These regions resemble the attached vortex loops suggested first by Theodorsen (1955). Such loops are incorporated in the attached eddy model versions of Perry & Chong (1982), Perry et al. (1986), and Perry & Marusic (1995), which are extensions of a model first formulated by Townsend (1976). The DNS data of wall bounded flows studied here are from the zero pressure gradient flow of Spalart (1988) and the boundary layer with separation and reattachment of Na & Moin (1996). The flow structures are examined from the viewpoint of the attached ...
The paper is intended to present both experimental and numerical approaches for estimating the he... more The paper is intended to present both experimental and numerical approaches for estimating the heat exchange per thermal radiation towards the walls of lab-scale paraffin-based thrust chambers. Two firing tests of a lab-scale gaseous-oxygen/paraffin-wax hybrid rocket engine have been performed to apply such methods. In particular, the radiative wall heat flux has been evaluated by both spectroscopic measurements and discrete transfer method computations. Details of such approaches are given together with results achieved and suggestions for future improvements.
46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, 2010
Research is ongoing at Stanford University on the catalytic decomposition of nitrous oxide as an ... more Research is ongoing at Stanford University on the catalytic decomposition of nitrous oxide as an environmentally friendly monopropellant for propulsion and power applications. Two meso-scale planar monopropellant thrusters have been successfully tested: one made of monolithic silicon carbide utilizing a commercially supplied catalyst, and the other made of Hastelloy-X ® utilizing a rhodium based catalyst prepared in-house. The unique planar geometry shows a pathway towards scaling down to micro-scale through MEMS-batch fabrication techniques which enable low-cost, mass produced devices with improved thrust to weight ratios and faster start-up transients. The in-house catalyst showed higher reactivity as compared to the commercially supplied catalyst by initiating the decomposition reaction at a temperature of 200°C corresponding to 50W input power. Bed loadings up to 8.67 kg/m 2 /sec were achieved at a c star efficiency of 71%. Both devices are compared to a previously reported, scaled-up, cylindrical thruster. Nomenclature A o = flow control orifice throat area c* = characteristic exhaust velocity C d = discharge coefficient γ = heat capacity ratio m = mass flow rate P 1 = upstream pressure P crit = static pressure at orifice throat P c = test article chamber pressure R c = universal gas constant T c = test article chamber temperature Y = expansion factor ρ = density of the gas D
A recently developed mixing length model of the turbulent shearing stress in wall bounded flows h... more A recently developed mixing length model of the turbulent shearing stress in wall bounded flows has been used to formulate a universal velocity profile (UVP) that provides an effective replacement for the widely used Coles wall-wake formulation. Comparisons with both direct numerical simulation and experimental data demonstrate the ability of the profile to approximate a wide variety of wall-bounded flows. The UVP is uniformly valid from the wall to the boundary layer edge and for all Reynolds numbers from zero to infinity. There is no presumption of logarithmic dependence of the velocity profile outside the viscous wall layer so the profile can accurately approximate low Reynolds number turbulent boundary layers. The effect of a pressure gradient is included in the UVP through the introduction of a modified Clauser parameter that correlates well with the parameters that determine the wake portion of the velocity profile. The inherent dependence of the UVP on Reynolds number, extend...
The idea of the hybrid rocket is to store the oxidizer as a liquid and the fuel as a solid, produ... more The idea of the hybrid rocket is to store the oxidizer as a liquid and the fuel as a solid, producing a design that minimizes the chance of a chemical explosion. While the hybrid enjoys many safety and environmental advantages over conventional systems, large hybrids have not been commercially viable. The reason is that traditional systems use polymeric fuels that evaporate too slowly, making it difficult to produce the high thrust needed for most applications. Research at Stanford University and Space Propulsion Group (SPG) has led to the development of paraffin-based fuels that burn at regression rates 3–4 times that of polymeric fuels. Under the action of the oxidizer flow, the new fuels form a thin, hydrodynamically unstable liquid layer on the melting surface of the fuel. Entrainment of droplets from the liquid–gas interface can substantially increase the rate of fuel mass transfer, leading to a much higher surface regression rate than can be achieved with a conventional fuel. ...
The geometry of the velocity field in a numerically simulated incompressible turbulent boundary l... more The geometry of the velocity field in a numerically simulated incompressible turbulent boundary layer over a flat plate at Re theta=670 has been studied using the invariants of the velocity gradient tensor. These invariants are computed at every grid point in the flow and used to form the discriminant. Of primary interest are those regions in the flow where the discriminant is positive; regions where, according to the characteristic equation, the eigenvalues of the velocity gradient tensor are complex. An observer moving with a frame of reference which is attached to a fluid particle lying within such a region would see a local flow pattern of the type stable-focus-stretching or unstable-focus-compressing. When the flow is visualized this way, continuous, connected, large-scale structures are revealed that extend from the point just below the buffer layer out to the beginning of the wake region. These structures are aligned with the mean shear close to the wall and arch in the cross...
Paper published as part of the special topic on Tribute to Frank M. White on his 88th Anniversary... more Paper published as part of the special topic on Tribute to Frank M. White on his 88th Anniversary This paper was selected as Featured ARTICLES YOU MAY BE INTERESTED IN Complete and incomplete similarity for the mean velocity profile of turbulent pipe and channel flows at extreme Reynolds number
A new solid fuel formulation called SP7 was developed for application in a hybrid rocket propulsi... more A new solid fuel formulation called SP7 was developed for application in a hybrid rocket propulsion system for a potential Mars Ascent Vehicle (MAV). The new fuel offers good propulsive performance (Isp) while meeting the storage and operation requirements placed upon the proposed mission. Mixed Oxides of Nitrogen (MON) are selected for the oxidizer due to the low freezing points possible with these materials. The low temperature capabilities of the fuel and oxidizer reduce the required energy associated with thermal management systems. Evaluation of the propulsive performance of SP7 was completed with two oxidizers, N2O and MON, in a 2.7-in hybrid rocket motor. In addition to the baseline fuel, metallized formulations with 20% by weight aluminum particles were also tested. Ignition and stable combustion were demonstrated with both oxidizers over a wide range of operating conditions. Static test firing of SP7 demonstrated the ability for this fuel to meet the propulsion requirements of the as designed potential MAV mission.
Recent interest in supersonic combustion and problems of transatmospheric flight has prompted ren... more Recent interest in supersonic combustion and problems of transatmospheric flight has prompted renewed research efforts in laminar-turbulent free shear flow transition. In the present work, linear stability theory and direct numerical simulations are used to study the effect of Mach ...
The most important unanswered questions in turbulence regard the nature of turbulent flow in the ... more The most important unanswered questions in turbulence regard the nature of turbulent flow in the limit of infinite Reynolds number. The Princeton superpipe (PSP) data comprise 26 velocity profiles that cover three orders of magnitude in the Reynolds number from $Re=19\,639$ , to $Re=20\,088\,000$ based on pipe radius and pipe centreline velocity. In this paper classical mixing length theory is combined with a new mixing length model of the turbulent shear stress to solve the streamwise momentum equation and the solution is used to approximate the PSP velocity profiles. The model velocity profile is uniformly valid from the wall to the pipe centreline and comprises five free parameters that are selected through a minimization process to provide an accurate approximation to each of the 26 profiles. The model profile is grounded in the momentum equation and allows the velocity derivative, Reynolds shear stress and turbulent kinetic energy production to be studied. The results support t...
Abstract : A three year study of the effect of initial conditions on the topology and fine scale ... more Abstract : A three year study of the effect of initial conditions on the topology and fine scale structure of temporally evolving wakes has been completed. Early in the project new topological analysis tools were developed and used to analyze available simulations of the plane mixing layer. During the course of these preliminary studies it was found that certain geometrical features of the fine scale motions in the mixing layer were very similar to those seen in simulations of forced isotropic turbulence. As a consequence, we adopted a two-track approach to the work; one track was to gather together simulation data on as wide a variety of shear flows as possible with the objective of identifying universal feature of the fine scale structure. The second track was to carry out a new direct simulation of the plane wake in which systematic variations of the initial conditions were used to identify structural features of the developed flow which depend on initial conditions. The fundamental questions we tried to address were: What features of the fine scale motions in turbulent flow are universal, what features are not, and, in the case of the plane wake, what information about the near wake survives far downstream?
A computer program called Transonic Navier Stokes (TNS) has been developed which solves the Euler... more 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.
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