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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS2023

Experimental Investigation of Distributed Sand-Grain Roughness Effects on Transition Onset and Turbulent Heating at Mach 6

An experimental investigation of distributed sand-grain surface roughness effects on hypersonic boundary-layer transition and turbulent heating has been performed. Two representative entry vehicle geometries, a sphere-cone aeroshell and a spherical-cap aeroshell, were considered. Cast ceramic models of each geometry were fabricated with distributed sand-grain roughness patterns of different heights that simulated an ablated thermal protection system. Wind tunnel testing was performed at Mach 6 over a range of Reynolds numbers sufficient to produce laminar, transitional, and turbulent flow. Aeroheating and boundary-layer transition onset data were obtained using global phosphor thermography. The experimental heating data are presented herein, as are comparisons to laminar and turbulent smooth-wall heat transfer distributions from computational flow field simulations. The boundary-layer transition data were found to correlate with a functional representation developed in prior roughness studies, although the data scatter was greater owing to the height variability of distributed sand-grain roughness.

Brian R Hollis↗
NASA NTRSDate not supplied

Roughness Parameter Optimization of the McClain Model in GlennICE

Aircraft icing remains a significant threat to aviation safety. Software that predicts the impingement and ice accretion on full aircraft geometries and aircraft components are in demand and NASA Glenn is committed to produce software that meets this need. One of the key parameters affecting an accurate prediction of iced geometry is the effect of ice roughness on the heat transfer coefficient. While many efforts have been made to implement the roughness in the flow solver, this report takes a correlation for roughness height distribution that is based on experimental measurements and demonstrates how to relate those measurements to an augmentation to the heat transfer coefficient provided by the flow solution. The outcome of this effort was the calibration of defaults for user supplied parameters to this correlation through comparison with 95 large glaze conditions from experiment by adjusting user-supplied parameters in the roughness augmentation equation.

Aircraft Icing↗
NASA NTRS2022

Analysis of the Smap Roughness Parameter and the Smap Vegetation Optical Depth

The SMAP product provides the soil moisture (SM) computed using three different retrieval algorithms: the single channel H and V algorithms (SCA-H and SCA-V), and the dual-channel algorithm (DCA) which in addition provides the vegetation optical depth (VOD). The roughness model and the roughness parameters play an important role in the determination of the soil moisture and the VOD. In this regard, the SMAP SCA and DCA utilize different approaches to incorporate the effect of roughness. In this work we will summarize those approaches and we will evaluate the effect of the DCA approach on the retrieval of VOD. We will compare the SMAP DCA roughness parameter h with topographic parameters such as DEM height, DEM slope, DEM height standard deviation and DEM slope standard deviation.

Mousavi, M.↗
NASA NTRS2018

Physics-Based Retrieval of Surface Roughness Parameters for Bare Soils from Combined Active-Passive Microwave Signatures

In the past the effect of soil roughness was often considered secondary within the determination of soil moisture from remote sensing data. Several studies showed that accurate determination of soil roughness leads to an improved estimation of soil moisture. Two default parameters to describe the surface roughness are the standard deviation of the surface height variation 𝑠 and the surface correlation length 𝑙 with its corresponding autocorrelation function. Both parameters (𝑠,𝑙) affect the emissivity measured by radiometers as well as the backscattering observed by radars. In this study, we develop a physics-based approach to retrieve 𝑠 and 𝑙 by combining both microwave signals based on active-passive microwave covariation. To test the approach, containing a forward model and a retrieval algorithm, we used active/passive microwave data measured with the ComRAD truck-based SMAP simulator at L-band. Results and validations with corresponding field measurements on ground show that 𝑠 and 𝑙 can be estimated simultaneously when using this approach. The physics-based retrieval algorithm works robustly for two investigated test fields having an RMS-Error of 0.68 cm and 0.69 cm between the microwave-based and field-measured 𝑠-values, and of 3.13 cm and 3.04 cm for 𝑙-values. The first validation of the results reveals that the influence of the autocorrelation function, needed within the retrieval, is distinct.

correlation length↗
NASA NTRSDate not supplied

Analysis and Comparison of Surface Roughness Effects on Pressure Data from SLS Wind Tunnel Test

Unsteady flows are becoming a more characterized field of study as advancements in technology is allowing for more high-speed, time-resolved data acquisition. It is important to understand these flows as the shock-wave boundary-layer interactions and separation they are involved with can generate extreme loads caused by high pressure and temperature on the body of a flight vehicle. It is vital to characterize these loads, as they can impede upon the structural integrity of a high-speed system. A common instrument utilized today to characterize pressure distributions caused by unsteady flows are unsteady pressure transducers. Set flush with the surface of a flight vehicle or wind tunnel model, these transducers can provide a pressure distribution, capturing the frequency of unsteady loads that could impinge upon the test article. A couple negative aspects of this instrumentation is that they are expensive, have long lead times, and a pressure distribution can only be determined in one discrete location where the transducer is installed. An optical diagnostic known as unsteady pressure-sensitive paint (uPSP) can help to satisfy these deficiencies. This is a spray paint that can be applied to a surface of a model to provide a global pressure distribution across the whole painted region.2 A complete picture of the fluid dynamic pressures occurring on the surface of a test article can be analyzed, especially in locations where it may have been impossible to install a transducer. Pressure-sensitive paint is also less expensive than traditional pressure transducers, which are, on average, about $2,000 each including the cost of installation.3 Unsteady PSP will not replace unsteady pressure transducers, but the transducers can be used to validate the PSP response to the flow and measure frequencies higher than the frequency response of the paint. One issue that can arise using these tools simultaneously, is that the surface roughness of the uPSP can affect the pressure readings of the transducers. Applying the uPSP to the surface of the model is going to affect the flow in some way as the topography of the model is now different than if it was not painted. One study with steady PSP conducted by Amer, Obara, and Liu at NASA Langley concluded that the PSP surface roughness was minimally intrusive at different test conditions with various lift, drag, etc., however, it was noted that data from some pressure transducers had decreased, likely due to the uneven application of the PSP in the localized region of the transducers.9 Sugioka et. al. tested various formulations of uPSP to study surface roughness effects on a Common Research Model (CRM) in transonic flow. The influence of uPSP on the surface of the model, depending on how it was formulated, was noted to have the possibility of relocating the shock wave present in the flow. This study was conducted to examine and compare transducer data acquired in December 2017 of a 4% scale model of the Space Launch System (SLS) performed at NASA Ames Research Center (ARC) Unitary Plan Wind Tunnel (UPWT). One goal of the test was to demonstrate to the customer the potential of the uPSP system developed at NASA ARC. The effect of uPSP on transducer signals also became an interest in this experiment and is the reason for this data analysis. The results of this study were discussed with the SLS team to help them make decisions regarding the next test on an SLS model with the PSP and Unitary Plan Wind Tunnel teams.

pressure-sensitive paint↗
NASA NTRS1994

(abstract) Measurement of Aerodynamic Roughness Using Radar Backscatter over Vegetated Surfaces

Gathering of local information from remote platforms is often a more cost-effective way of obtaining the required data type than are local measurements. A good example is the collection of wind regime data in order to infer the amount and type of aeolian transport at a given site. This paper describes a potential mechanism for remotely inferring the wind regime from synthetic aperture radar data and describes a project to estimate the practicality of using such a mechanism. Both radar backscatter and wind flow are functions of the roughness of the site under study. It is therefore reasonable to suspect that a fairly well-behaved relationship might exist between normalized radar backscatter coefficient and aerodynamic roughness length. Aeolian transport of small particles depends in turn on wind flow and is an important quantity to measure for several economic-related reasons, but the direct measurement of wind flow regime generally involves construction of wind towers and many days of data collection, making such data extremely expensive and prohibitive in areas that cannot be easily accessed. For both desert and vegetated sites where extensive wind profile data already exist, a NASA airborne multifrequency, multipolarization radar was used to acquire radar data at several incidence angles. At the vegetated sites, vegetation height varied from tens of centimeters to several meters, and surface roughness was a few centimeters.

radar↗
NASA NTRSDate not supplied

Experimental Characterization of Surface Roughness and Geometric Evaluations of Thin-Wall Laser Powder Directed Energy Deposition

Additive Manufacturing (AM) has offered many new design and manufacturing opportunities for components across various industries. As AM evolves there is a need to better understand outputs of the process including geometric limitations and surface roughness based on specific application requirements. One possible application area of AM are components using thin-wall (<2 mm) microchannel heat exchangers for subsystems across aerospace, and industrial applications. AM offers the opportunity to significantly reduce the cost and lead time associated with fabricating these complex parts with internal features such as an array of cooling channels. Laser Powder Bed Fusion (L-PBF) has been continuously demonstrated as a technique to fabricate heat exchangers with internal geometric complexities. However, L-PBF is limited in the scale at which parts can be fabricated. Laser Powder Directed Energy Deposition (LP-DED) has been demonstrated as an AM technology to significantly increase the scale of which thin-wall microchannel heat exchangers could be manufactured. Successful deployment of the LP-DED technology requires critical understanding of the resulting features from the build process. Surface roughness is one of the critical areas that effects the friction factor and pressure drop within a heat exchanger and must be well understood. This presentation will provide an overview of the characterization work completed to understand the LP-DED process for thin-walls representative of hydrogen resistant alloy NASA HR-1 that would be used in a Hydrogen-environment application. An overview of the experiments conducted with varying LP-DED parameters, evaluation of various geometric channel and geometric build features and resulting surface finish will be provided along with a summary of conclusions from these experiments. Results indicate that powder size is a major contributor and surface roughness varies between closed channel shapes and external surfaces.

Directed Energy Deposition↗
NASA NTRSDate not supplied

Instabilities in the Wake of a Pseudorandom Roughness on a Supersonic Flat Plate

A roughness patch defined by a two-dimensional Fourier series was installed in a highly-polished flat plate model immersed in a supersonic quiet flow. The NASA Langley Research Center Supersonic Low Disturbance Tunnel is capable of providing a Mach 3.5 flow with low freestream acoustic noise and low turbulence levels. Measurements made with a hot-wire probe downstream of the roughness element showed the presence of instability modes that did not cause transition in the measurement region at a freestream unit Reynolds number of Re =12.6×10^6/m. The largest amplitude mode appeared to be similar to a symmetric mode. The same roughness patch immersed in a higher freestream acoustic noise environment showed little to no evidence of the instability modes. Instead, disturbances found in the boundary layer matched closely to the freestream spectra measured in the empty tunnel under the same conditions, indicating some evidence of freestream forcing in the boundary layer.

Roughness induced transition↗
NASA NTRS2022

Instabilities in the Wake of a Pseudorandom Roughness on a Supersonic Flat Plate

A roughness patch defined by a two-dimensional Fourier series was installed in a highly-polished flat plate model immersed in a supersonic quiet flow. The NASA Langley Research Center Supersonic Low Disturbance Tunnel is capable of providing a Mach 3.5 flow with low freestream acoustic noise and low turbulence levels. Measurements made with a hot-wire probe downstream of the roughness element showed the presence of instability modes that did not cause transition in the measurement region at a freestream unit Reynolds number of Re =12.6×10^6/m. The largest amplitude mode appeared to be similar to a symmetric mode. The same roughness patch immersed in a higher freestream acoustic noise environment showed little to no evidence of the instability modes. Instead, disturbances found in the boundary layer matched closely to the freestream spectra measured in the empty tunnel under the same conditions, indicating some evidence of freestream forcing in the boundary layer.

Roughness induced transition↗
NASA NTRS2022

Instabilities in the Wake of a Pseudorandom Roughness on a Supersonic Flat Plate

A roughness patch defined by a two-dimensional Fourier series was installed in a highly-polished flat plate model immersed in a supersonic quiet flow. The NASA Langley Research Center Supersonic Low Disturbance Tunnel is capable of providing a Mach 3.5 flow with low freestream acoustic noise and low turbulence levels. Measurements made with a hot-wire probe downstream of the roughness element showed the presence of instability modes that did not cause transition in the measurement region at a freestream unit Reynolds number of Re =12.6×10^6/m. The largest amplitude mode appeared to be similar to a symmetric mode. The same roughness patch immersed in a higher freestream acoustic noise environment showed little to no evidence of the instability modes. Instead, disturbances found in the boundary layer matched closely to the freestream spectra measured in the empty tunnel under the same conditions, indicating some evidence of freestream forcing in the boundary layer.

Roughness induced transition↗
NASA NTRS1971

Effects of surface roughness on the photometric properties of Mars

Starting with a macroscopically flat surface, which at each point scatters according to the Minnaert law with exponent k, the photometric effects of increasing the large-scale roughness of the Martian surface were investigated. The photometric properties of macroscopically rough surfaces can still be described by the Minnaert law, but with an exponent k star, in general not equal to k. Therefore observed values of the Minnaert exponent cannot be used to infer the small-scale surface properties of Mars (or of any planet) unless proper allowance is made for the photometric effects of large-scale roughness. The azimuthal dependence of k star provides a sensitive test for the importance of these effects.

Veverka, J.↗
NASA NTRS1971

Lunar terrain mapping and relative-roughness analysis

Terrain maps of the equatorial zone were prepared at scales of 1:2,000,000 and 1:1,000,000 to classify lunar terrain with respect to roughness and to provide a basis for selecting sites for Surveyor and Apollo landings, as well as for Ranger and Lunar Orbiter photographs. Lunar terrain was described by qualitative and quantitative methods and divided into four fundamental classes: maria, terrae, craters, and linear features. Some 35 subdivisions were defined and mapped throughout the equatorial zone, and, in addition, most of the map units were illustrated by photographs. The terrain types were analyzed quantitatively to characterize and order their relative roughness characteristics. For some morphologically homogeneous mare areas, relative roughness can be extrapolated to the large scales from measurements at small scales.

Rowan, L. C.↗
NASA NTRS1972

Surface roughness effects on equilibrium temperature.

An analysis is presented for evaluation of equilibrium temperature distribution on radiatively adiabatic, adjoint planes which are uniformly irradiated by a collimated solar flux. The analysis employs a semigrey spectral model. Radiation properties for surface emitted radiation are obtained from the expressions of electromagnetic theory for smooth surfaces. Rough surface properties for solar radiation are given by the Beckmann bidirectional reflectance model. Numerical solutions to the governing equations yield equilibrium temperature distributions for a range of the influencing parameters. Surface roughness has little influence on equilibrium temperature for materials with high values for solar absorptance. However, for low or intermediate values of solar absorptance, roughness effects on the spatial distribution of reflected solar radiation can significantly alter equilibrium temperature particularly at surface elements where radiant interaction is small.

Houchens, A. F.↗
NASA NTRS1972

Effects of surface roughness on the photometric properties of Mars.

Starting with a macroscopically flat surface which at each point scatters according to the Minnaert law with exponent k, we investigate the photometric effects of increasing the large scale roughness of the surface. It is found that the photometric properties of macroscopically rough surfaces can still be described by the Minnaert law, but with an exponent k1, in general not equal to k. In fact, examples are given where k1 differs considerably from k. Therefore observed values of the Minnaert exponent cannot be used to infer the small scale surface properties of Mars (or of any planet) unless proper allowance is made for the photometric effects of large scale roughness. The azimuthal dependence of k1 provides a sensitive test for the importance of these effects.

Veverka, J.↗
NASA NTRS1970

A rough earth scattering model for multipath prediction

The most important phenomena to be considered in a model of radio wave communication between earth satellites are scattering from the surface of the earth. A model is derived and implemented on a computer to predict the field received after reflection from a rough, spherical earth. The scattering integrals are computed numerically; the domain of integration is the appropriate region on the surface of the earth. Calculations have been performed at VHF frequencies and for terrain which could be described as marshy land. Rough surface scattering calculations must be performed over a spherical earth when satellites are involved. There is a definite dependence on the values of the roughness, and the correlation length.

Page, L. J.↗
NASA NTRS1973

Numerical comparisons of a modified Rayleigh approach with other rough surface EM scattering solutions.

A modification of the classical Rayleigh technique utilizing the fast Fourier transform, developed into the so-called Rayleigh-FFT method for electromagnetic scattering, is shown to obtain valid scattering results from arbitrary irregular periodic structures composed of a rough layer over a rough half-space. The considerable usefulness of the Rayleigh approach is thus established, and Beckmann's (1968) contention (that the convergence of the method is reasonably rapid only if the roughness is very slight) is refuted.

Jiracek, G. R.↗
NASA NTRS1974

Lubricant pressure rippling in dynamic Hertzian contacts induced by surface roughness

A numerical analysis of the pressure distribution of a lubricant in contact with a rough surface was conducted. The magnitude of the pressures was determined by their root mean square value for the contact of two dimensional cylinders. The pressure was found to vary in the following manner: (1) the location in the contact, (2) the spectrum or frequency content of the surface roughness, (3) the mean plateau film thickness, and (4) the root mean square value of the surface roughness. Mathematical models are developed to show the relationships of the parameters.

Tallian, T. E.↗
NASA NTRS1974

Dual frequency microwave radiometer measurements of soil moisture for bare and vegetated rough surfaces

Controlled ground-based passive microwave radiometric measurements on soil moisture were conducted to determine the effects of terrain surface roughness and vegetation on microwave emission. Theoretical predictions were compared with the experimental results and with some recent airborne radiometric measurements. The relationship of soil moisture to the permittivity for the soil was obtained in the laboratory. A dual frequency radiometer, 1.41356 GHz and 10.69 GHz, took measurements at angles between 0 and 50 degrees from an altitude of about fifty feet. Distinct surface roughnesses were studied. With the roughness undisturbed, oats were later planted and vegetated and bare field measurements were compared. The 1.4 GHz radiometer was less affected than the 10.6 GHz radiometer, which under vegetated conditions was incapable of detecting soil moisture. The bare surface theoretical model was inadequate, although the vegetation model appeared to be valid. Moisture parameters to correlate apparent temperature with soil moisture were compared.

Lee, S. L.↗