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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2010

Labyrinth Seal Flutter Analysis and Test Validation in Support of Robust Rocket Engine Design

High energy-density turbomachines, like the SSME turbopumps, utilize labyrinth seals, also referred to as knife-edge seals, to control leakage flow. The pressure drop for such seals is order of magnitude higher than comparable jet engine seals. This is aggravated by the requirement of tight clearances resulting in possible unfavorable fluid-structure interaction of the seal system (seal flutter). To demonstrate these characteristics, a benchmark case of a High Pressure Oxygen Turbopump (HPOTP) outlet Labyrinth seal was studied in detail. First, an analytical assessment of the seal stability was conducted using a Pratt & Whitney legacy seal flutter code. Sensitivity parameters including pressure drop, rotor-to-stator running clearances and cavity volumes were examined and modeling strategies established. Second, a concurrent experimental investigation was undertaken to validate the stability of the seal at the equivalent operating conditions of the pump. Actual pump hardware was used to construct the test rig, also referred to as the (Flutter Rig). The flutter rig did not include rotational effects or temperature. However, the use of Hydrogen gas at high inlet pressure provided good representation of the critical parameters affecting flutter especially the speed of sound. The flutter code predictions showed consistent trends in good agreement with the experimental data. The rig test program produced a stability threshold empirical parameter that separated operation with and without flutter. This empirical parameter was used to establish the seal build clearances to avoid flutter while providing the required cooling flow metering. The calibrated flutter code along with the empirical flutter parameter was used to redesign the baseline seal resulting in a flutter-free robust configuration. Provisions for incorporation of mechanical damping devices were introduced in the redesigned seal to ensure added robustness

El-Aini, Yehia↗
NASA NTRS1979

On the equivalence between semiempirical fracture analyses and R-curves

The relationship between several semiempirical fracture analyses (SEFA) and the R-curve concept of fracture mechanics is examined. The conditions for equivalence between a SEFA and an R-curve are derived. A hypothetical material is employed to study the relationship analytically. Equivalent R-curves are developed for real materials using data from the literature. For each SEFA there is an equivalent R-curve whose magnitude and shape are determined by the SEFA formulation and its empirical parameters. If the R-curve is indeed unique then the various empirical parameters cannot be constant, and vice versa. However, for one SEFA the differences are small enough that they may be within the range of normal data scatter for real materials.

Orange, T. W.↗
NASA NTRS1980

A relation between semiempirical fracture analyses and R-curves

The relations between several semiempirical fracture analyses (SEFA) and the R-curve concept of fracture mechanics are examined and the conditions for equivalence between a SEFA and an R-curve are derived. A hypothetical material is employed to study the relation analytically. Equivalent R-curves are developed for several real materials using data from the literature. For each SEFA there is an equivalent R-curve whose magnitude and shape are determined by the SEFA formulation and its empirical parameters. If the R-curve is indeed unique, then the various empirical parameters cannot be constant, and vice versa. However, for one SEFA the differences are small enough that they may be within the range of normal data scatter for real materials.

Orange, T. W.↗
NASA NTRS2015

Impact of AMS-02 Measurements on Reducing GCR Model Uncertainties

For vehicle design, shield optimization, mission planning, and astronaut risk assessment, the exposure from galactic cosmic rays (GCR) poses a significant and complex problem both in low Earth orbit and in deep space. To address this problem, various computational tools have been developed to quantify the exposure and risk in a wide range of scenarios. Generally, the tool used to describe the ambient GCR environment provides the input into subsequent computational tools and is therefore a critical component of end-to-end procedures. Over the past few years, several researchers have independently and very carefully compared some of the widely used GCR models to more rigorously characterize model differences and quantify uncertainties. All of the GCR models studied rely heavily on calibrating to available near-Earth measurements of GCR particle energy spectra, typically over restricted energy regions and short time periods. In this work, we first review recent sensitivity studies quantifying the ions and energies in the ambient GCR environment of greatest importance to exposure quantities behind shielding. Currently available measurements used to calibrate and validate GCR models are also summarized within this context. It is shown that the AMS-II measurements will fill a critically important gap in the measurement database. The emergence of AMS-II measurements also provides a unique opportunity to validate existing models against measurements that were not used to calibrate free parameters in the empirical descriptions. Discussion is given regarding rigorous approaches to implement the independent validation efforts, followed by recalibration of empirical parameters.

Slaba, T. C.↗
NASA NTRSDate not supplied

Progress in Simulated Water Well Performance on Mars

Recent studies of human Mars missions considered the impacts of an abundant supply of in-situ, accessible water on these mission scenarios. Discovery of exposed water ice scarps in Martian mid-latitudes has bolstered the evidence for massive amounts of almost pure water in buried deposits in regions considered candidates for these future human missions. This paper describes progress towards adapting a long-standing terrestrial technique for accessing and extracting water from these mid-latitude sources of ice. This approach relies on mechanical drills to access the ice through overlying debris. Once the ice layer has been reached, a technique known as a Rodriguez Well, or Rodwell, is used to melt the ice, store the resulting water in a subsurface cavity until needed, and then pump the water to the surface for use. Previous work by the authors utilized a computer simulation to predict the performance of a Martian Rodwell. This simulation was originally developed to predict performance in terrestrial Polar Regions. Whereas the basic approach is appropriate for a similar well on Mars, several parameters had been empirically derived and required experiments simulating the Martian environment to determine the values appropriate for a Martian Rodwell simulation. These experiments have now been completed and preliminary values have been determined for the empirical parameters. Test results are consistent with a dimensionless Sh(Ra) correlation developed by Bower and Saylor (2009) and based on lab tests of water evaporating from a pool into a large air chamber. They are also consistent with correlations derived by Ingersoll (1970) and by Hecht (2002), based on natural-convection heat transfer. Each of these earlier correlations found Sh~Ra1/3, with the 1/3rd power characteristic of large Ra where the boundary layer over the pool surface is turbulent. A somewhat surprising result was that this power-law was found to extend into a range of Ra where laminar flow would be expected. Another surprising result was found in that instead of following established correlations for natural-convection heat transfer over flat plates, the dimensionless heat-transfer rates (Nu) were much larger and approximately independent of Ra. This suggests that mass transfer from the pool enhances convective heat transfer, possibly by enhanced mixing in the boundary layer. Although this possibility has been suggested in the literature, if it is proven to be true under these conditions it could play an important role in the use of a Rodwell on Mars, where evaporative mass transfer is enhanced relative to terrestrial wells.

Human Mars Mission↗
NASA NTRS2006

An Initial Non-Equilibrium Porous-Media Model for CFD Simulation of Stirling Regenerators

The objective of this paper is to define empirical parameters for an initial thermal non-equilibrium porous-media model for use in Computational Fluid Dynamics (CFD) codes for simulation of Stirling regenerators. The two codes currently used at Glenn Research Center for Stirling modeling are Fluent and CFD-ACE. The codes porous-media models are equilibrium models, which assume solid matrix and fluid are in thermal equilibrium. This is believed to be a poor assumption for Stirling regenerators; Stirling 1-D regenerator models, used in Stirling design, use non-equilibrium regenerator models and suggest regenerator matrix and gas average temperatures can differ by several degrees at a given axial location and time during the cycle. Experimentally based information was used to define: hydrodynamic dispersion, permeability, inertial coefficient, fluid effective thermal conductivity, and fluid-solid heat transfer coefficient. Solid effective thermal conductivity was also estimated. Determination of model parameters was based on planned use in a CFD model of Infinia's Stirling Technology Demonstration Converter (TDC), which uses a random-fiber regenerator matrix. Emphasis is on use of available data to define empirical parameters needed in a thermal non-equilibrium porous media model for Stirling regenerator simulation. Such a model has not yet been implemented by the authors or their associates.

Tew, Roy C.↗
NASA NTRS1980

On the equivalence between semiempirical fracture analyses and R-curves

The relationship between the R-curves and semiempirical fracture analyses (SEFA) is investigated theoretically using a hypothetical material. Equivalent R-curves (ERC) are developed for real materials using data from the literature. It is shown that for each SEFA there is an ERC whose magnitude and shape are determined by the SEFA formulation and its empirical parameters. The ERC is equivalent in that it predicts exactly the same relationship between the fracture stress and the initial crack length (residual strength) as the SEFA. If the effective R-curve is unique, then the various empirical parameters cannot be constant, and vice versa. However, for one of the SEFA examined, Newman's SEFA, parameter variations are small enough to be within the range of normal data scatter for real materials.

Orange, T. W.↗
NASA NTRS2021

A New Method for Predicting Turbulent Flow Reattachment Location: Application of Stratford Criterion for Reattaching Flows

A new method for predicting turbulent flow reattachment location has been developed. The method predicts the reattachment location using the surface static pressure distribution and requires only a single empirical parameter. The method is based on the well-known Stratford’s flow separation criterion, which results from an approximate solution to the equations of motion within the boundary layer. Because of the large pressure rise, the original Stratford separation criterion is not valid for reattaching flows. In order to apply the Stratford criterion for reattaching flows, the reference location was moved to the pressure drop conditions instead of pressure rise conditions. In addition, the pressure distribution was scaled using a technique that was originally introduced for supersonic base flows. A new empirical parameter was also obtained for the modified Stratford criterion. The method has been tested for 30 different separated flow configurations from the literature. The variety of test cases include subsonic flows, transonic flows, 2D flows, and axisymmetric flows, with experimental as well as high resolution numerical simulation results. The flow reattachment predictions with the new method agree very well with the reported data. The error is less than 5% and typically about 2%. The new method is an extremely simple and a quick way of finding the reattachment location. Given the complexities of the existing experimental techniques for finding the reattachment location, the method easily and quickly provides the flow reattachment location, which could be used to estimate the physical extent of separation bubbles, as a performance index for flow control methods, or to assess the predictive capability of turbulence models.

Flow reattachment location↗
NASA NTRS1977

A reexamination of solar upper photosphere models, the calcium abundance, and empirical damping parameters

A general iterative method is described for constructing thermal models of the solar photosphere consistent with observed strong and weak lines of Ca I and Ca II. Calibrations of the solar calcium abundance and the van der Waals parameter for important Ca I and Ca II lines are obtained using plane-parallel largely LTE model atmospheres, and these thermal models are modified for better fits to the measured Ca II H and K inner wing shapes. Possible sources of error in this semiempirical approach are evaluated. The derived hydrogen van der Waals broadening is compared with theoretical estimates of that broadening as well as with experimental measurements of the helium broadening

Ayres, T. R.↗
NASA NTRS1996

Empirical Line Parameters of NH(sub 3) from 4791 to 5294 cm(sup -1)

To support remote sensing of planetary atmospheres at 2??laboratory spectra of NH(sub 3) andf enriched (sup 14)NH(sub 3) and (sup 15)NH(sub 3) were recorded at 0.011 cm(sup -1) resolution with the McMath Fourier transform spectrometer (FTS) located at...

NH3 remote sensing planetary atmospheric study↗
NASA NTRS2006

An Initial Non-Equilibrium Porous-Media Model for CFD Simulation of Stirling Regenerators

The objective of this paper is to define empirical parameters (or closwre models) for an initial thermai non-equilibrium porous-media model for use in Computational Fluid Dynamics (CFD) codes for simulation of Stirling regenerators. The two CFD codes currently being used at Glenn Research Center (GRC) for Stirling engine modeling are Fluent and CFD-ACE. The porous-media models available in each of these codes are equilibrium models, which assmne that the solid matrix and the fluid are in thermal equilibrium at each spatial location within the porous medium. This is believed to be a poor assumption for the oscillating-flow environment within Stirling regenerators; Stirling 1-D regenerator models, used in Stirling design, we non-equilibrium regenerator models and suggest regenerator matrix and gas average temperatures can differ by several degrees at a given axial location end time during the cycle. A NASA regenerator research grant has been providing experimental and computational results to support definition of various empirical coefficients needed in defining a noa-equilibrium, macroscopic, porous-media model (i.e., to define "closure" relations). The grant effort is being led by Cleveland State University, with subcontractor assistance from the University of Minnesota, Gedeon Associates, and Sunpower, Inc. Friction-factor and heat-transfer correlations based on data taken with the NASAlSunpower oscillating-flow test rig also provide experimentally based correlations that are useful in defining parameters for the porous-media model; these correlations are documented in Gedeon Associates' Sage Stirling-Code Manuals. These sources of experimentally based information were used to define the following terms and parameters needed in the non-equilibrium porous-media model: hydrodynamic dispersion, permeability, inertial coefficient, fluid effective thermal conductivity (including themal dispersion and estimate of tortuosity effects}, and fluid-solid heat transfer coefficient. Solid effective thermal conductivity (including the effect of tortuosity) was also estimated. Determination of the porous-media model parameters was based on planned use in a CFD model of Infinia's Stirling Technology Demonstration Convertor (TDC), which uses a random-fiber regenerator matrix. The non-equilibrium porous-media model presented is considered to be an initial, or "draft," model for possible incorporation in commercial CFD codes, with the expectation that the empirical parameters will likely need to be updated once resulting Stirling CFD model regenerator and engine results have been analyzed. The emphasis of the paper is on use of available data to define empirical parameters (and closure models) needed in a thermal non-equilibrium porous-media model for Stirling regenerator simulation. Such a model has not yet been implemented by the authors or their associates. However, it is anticipated that a thermal non-equilibrium model such as that presented here, when iacorporated in the CFD codes, will improve our ability to accurately model Stirling regenerators with CFD relative to current thermal-equilibrium porous-media models.

Tew, Roy↗
NASA NTRS2006

Cascaded Perforates as One-Dimensional, Bulk Absorbers

Porous cell honeycomb liners for aircraft engine nacelles offer the possibility of exploiting extended reaction effects to improve liner attenuation bandwidth as generally attributed to the performance of bulk absorbers. This paper describes an analytical procedure, starting with an impedance prediction model for a single perforated plate, to estimate the bulk-absorber parameters for a cascade of such perforates - a first step to modeling a porous wall honeycomb structure. The objective is to build confidence in a lumped element impedance model, when applied to a uniformly-spaced set of porous plates to predict its .bulk. absorber properties. The model is based upon a modified version of the two-parameter flow resistance model of the form A + BV(sub inc), where A and B are physics-based, semi-empirical parameters that are adjusted to provide an optimum fit to a composite dataset from three plate porosities of 2.5, 5 and 10%. The composite dataset is achieved by reformulating the two-parameter flow resistance model into a .reduced pressure drop coefficient. dependency on perforate hole Reynolds number. The resulting impedance model is employed to calculate surface impedance spectra for N and 2N-layer perforate cascades. The well-known two-thickness method for experimental determination of bulk-absorber parameters is then applied to these .synthesized. data sets to predict the characteristic impedance and propagation constant for the perforate cascades. These results are then compared with experimental results reported in a companion paper.

Parrott, T. L.↗
NASA NTRS1993

Relationship of attenuation in a vegetation canopy to physical parameters of the canopy

A discrete scatter model is employed to compute the radiometric response (i.e. emissivity) of a layer of vegetation over a homogeneous ground. This was done to gain insight into empirical formulas for the emissivity which have recently appeared in the literature and which indicate that the attenuation through the canopy is proportional to the water content of the vegetation and inversely proportional to wavelength raised to a power around unity. The analytical result assumes that the vegetation can be modeled by a sparse layer of discrete, randomly oriented particles (leaves, stalks, etc.). The attenuation is given by the effective wave number of the layer obtained from the solution for the mean wave using the effective field approximation. By using the Ulaby-El Rayes formula to relate the dielectric constant of the vegetation to its water content, it can be shown that the attenuation is proportional to water content. The analytical form offers insight into the dependence of the empirical parameters on other variables of the canopy, including plant geometry (i.e. shape and orientation of the leaves and stalks of which the vegetation is comprised), frequency of the measurement and even the physical temperature of the vegetation. Solutions are presented for some special cases including layers consisting of cylinders (stalks) and disks (leaves).

Karam, M. A.↗
NASA NTRS1992

Macrosegregation During Steady-State Arrayed Growth of Dendrites in Directionally Solidified Pb-Sn Alloys

The paper examines the growth parameter dependence of the longitudinal macrosegregation due to interdendritic thermosolutal convection during directional solidification of Pb-Sn alloys with a steady-state dendritic arrayed morphology. The experimentally observed longitudinal macrosegregation can be described by an empirical parameter, effective partition coefficient (kE), obtained from Cs = kEC0(1 - fs) super KI-1, where the original tin content is C0 and that corresponding to the fraction distance solidified (Fs) is Cs. Channel formation is strongly associated with the onset of thermosolutal convection in the interdendritic melt. The critical effective Rayleigh number for the onset of channel formation is about 0.07.

Tewari, S. N.↗
NASA NTRS2017

Creep, Fatigue and Environmental Interactions and Their Effect on Crack Growth in Superalloys

Complex interactions of creep/fatigue/environment control dwell fatigue crack growth (DFCG) in superalloys. Crack tip stress relaxation during dwells significantly changes the crack driving force and influence DFCG. Linear Elastic Fracture Mechanics, Kmax, parameter unsuitable for correlating DFCG behavior due to extensive visco-plastic deformation. Magnitude of remaining crack tip axial stresses controls DFCG resistance due to the brittle-intergranular nature of the crack growth process. Proposed a new empirical parameter, Ksrf, which incorporates visco-plastic evolution of the magnitude of remaining crack tip stresses. Previous work performed at 704C, extend the work to 760C.

environmental interactions↗
NASA NTRS1995

Dynamic Instability of Solidification Fronts and Formation of Transverse Layered Structures

A plane front of crystallization in a unidirectional process of solidification of a binary melt or a solution is shown to be unstable with respect to virtual perturbations of the front velocity within some region of physical and operating parameters. This dynamic instability has nothing in common with morphological instability of the front also possible under certain conditions. It gives rise to the origination of a self-oscillating regime of solidification. The oscillations of the front velocity are accompanied by oscillations of other relevant variables, such as the melt concentration and temperature at the front. What is most important is that this provides for a natural explanation of phenomena of the formation of ordered striations commonly observed when producing alloys and withdrawing crystals. A theory of nonlinear instability of the solidification front is developed which also allows for properties of both self-oscillating regimes of solidification and transverse layered structures it allows to be described in an explicit form, without introducing any additional hypotheses and empirical parameters. Apart from other things, the properties are strongly dependent on convective transport originated in consequence of a difference between the densities of the liquid melt and solid alloy. The theory results in a remarkably good agreement with experimental evidence on crystal withdrawal obtained in normal gravity and under space flight conditions. The problem of interplay between modes of the dynamic and morphological types of instability is also discussed. There are parametric regions in which either only one of these types occurs or both of them arise simultaneously.

Buyevich, Yu A.↗
NASA NTRS1973

A provisional analysis of two-dimensional turbulent mixing with variable density

A predictive method for the titled flows based on the Prandtl energy method was developed and assessed by comparing predicted results with experimental results. For constant-density flows, both gross properties such as spreading rate and maximum turbulent kinetic energy and detailed properties such as mean shear stress distributions are shown to be well predicted. For variable-density flows, considerable attention is devoted to the inclusion in the analysis of the added effect of pressure fluctuations and of the variation in the several extant empirical parameters on the turbulent kinetic energy. It is found that a variation with Mach number of the characteristic Reynolds number for turbulent transport is needed to account for the observed decrease in spreading rate. The predictions which result from these considerations are compared with the limited experimental data presently available for the two crucial cases: compressible adiabatic mixing and low-speed isothermal mixing of two dissimilar gases.

Libby, P. A.↗