This study explores the potential of the naphthalene sublimation technique to be used to analyze the manner in which surfaces regress, specifically focusing on the effect that skin friction has on regression and vice versa. For this experimentation, a flat steel plate installed with three skin friction sensors was coated with naphthalene via a mechanized sprayer and was installed in the wall of the Ahmic Aerospace Turbulent Boundary Layer Research supersonic wind tunnel. The plate and sensor configuration was subjected to three subsequent tunnel runs at Mach 2.31. This process was repeated at plenum pressures of 0.35 MPa and 0.69 MPa, which correspond to Reynolds Numbers of 1.4x10^7/m and 2.8x10^7/m. Between the first and final run, a -4.7% and -3.7% percent change in the coefficient of friction was seen at the 0.35 MPa and 0.69 MPa plenum conditions, respectively. Images of the plate taken before and after each run qualitatively indicate continual naphthalene regression with each subsequent tunnel run. This decrease in the coefficient of friction was attributed in part to the regression of the naphthalene coating, indicating that this method has the potential to be used to study the interaction between skin friction and regressing surfaces. Additionally, this study showed that it is certainly possible to measure skin friction with sensors where both the head of the movable sensor element and the surrounding wall is coated with sprayed naphthalene. / Master of Science / This study explores the potential of using an experimental method to better understand how surfaces recede when subjected to moving air. A chemical called naphthalene was used because naphthalene naturally recedes very easily, unlike many other materials which a researcher would want to study the receding behavior of, making it an ideal choice for this study. Sensors made to detect the amount of force generated by flowing air over a surface were installed into a flat metal plate. The plate was coated with naphthalene and installed into the wall of a wind tunnel, which generates airflow over the plate to simulate flight conditions. Three separate and sequential airflow runs over the plate were conducted. This process was repeated under two different air conditions. In both conditions, a decrease in the amount of force due to the air moving over the plate was detected as subsequent runs were conducted and the naphthalene receded. This indicates the method has the potential to be used to study how other surfaces recede when subjected to moving air.

This investigation concerns the design, build, and testing of a new class of skin friction sensor capable of performing favorably in high-speed, high-enthalpy flow conditions, such as that found in atmospheric re-entry vehicles, scramjets, jet engines, material testing, and industrial processes. Fully understanding and optimizing these complex flows requires an understanding of aerodynamic properties at high enthalpies, which, in turn, requires numerical and analytical modeling as well as reliable diagnostic instrumentation. Skin friction is a key quantity in assessing the overall flight and engine performance, and also plays an important role in identifying and correcting problem areas. The sensor design is founded on a direct-measuring, cantilever arrangement. The design incorporates two fundamental types of materials in regards to thermal conductivity and voltage resistivity properties. The non-conducting material distinction greatly deters the effect of heat soak and prevents EMI transmission throughout the sensor. Four custom fabricated metal-foil strain gauges are arranged in a Wheatstone bridge configuration to increase sensitivity and to provide further compensation for sensitivity effects. The sensor is actively cooled via a copper water channel to minimize the temperature gradient across the electronic systems. The design offers a unique immunity to many of the interfering influences found in complex, high-speed, high-enthalpy flows that would otherwise overshadow the desired wall shear measurement. The need to develop an encompassing design methodology was recognized and became a principal focus of this research effort. The sensor design was developed through a refined, multi-disciplinary approach. Concepts were matured through an extensive and iterative program of evolving key performance parameters. Extensive use of finite element analysis (FEA) was critical to the design and analysis of the sensor. A software package was developed to utilize the powerful advantage of FEA methods and optimization techniques over the traditional trial and error methods. Each sensor endured a thorough series of calibrations designed to systematically evaluate individual aspects of its functionality in static, dynamic, pressure, and thermal responses. Bench-test facilities at Virginia Tech (VT) and Air Force Research Laboratory (AFRL) further characterized the design vibrational effects and electromagnetic interference countermeasure effectiveness. Through iterations of past designs, sources of error have been identified, controlled, and minimized. The total uncertainty of the skin friction sensor measurement capability was determined to be ±8.7% at 95% confidence and remained fairly independent of each test facility. A rigorous, multi-step approach was developed to systematically test the skin friction sensor in various facilities, where flow enthalpy and run duration were progressively increased. Initial validation testing was conducted at the VT Hypersonic Tunnel. Testing at AFRL was first performed in the RC-19 facility under high-temperature, mixing flow conditions. Final testing was conducted under simulated scramjet flight conditions in the AFRL RC-18 facility. Performance of the skin friction sensors was thoroughly analyzed across all three facilities. The flow stagnation enthalpies upward of 1053 kJ/kg (453 Btu/lbm) were tested. A nominal Mach 2.0 to 3.0 flow speed range was studied and stagnation pressure ranged from 172 to 995 kPa (25 to 144 psia). Wall shear was measured between 94 and 750 Pa (1.96 and 15.7 psf). Multiple entries were conducted at each condition with good repeatability at ±5% variation. The sensor was also able to clearly indicate the transient flow conditions of a full scramjet combustion operability cycle to include shock train movement and backflow along the isolator wall. The measured experimental wall shear data demonstrated good agreement with simple, flat-plate analytical estimations and historic data (where available). Numerical CFD predictions of the scramjet flow path gave favorable results for steady cold and hot flow conditions, but had to be refined to handle the various fueling injection schemes with burning in the downstream combustor and surface roughness models. In comparing CFD wall shear predictions to the experimental measurements, in a few cases, the sensor measurement was adversely affected by shock and complex flow interaction. This made comparisons difficult for these cases. The sensor maintained full functionality under sustained high-enthalpy conditions. No degradation in performance was noted over the course of the tests. This dissertation research and development program has proven successful in advancing the development of a skin friction sensor for applications in high-speed, high-enthalpy flows. The sensor was systematically tested in relevant, high-fidelity laboratory environments to demonstrate its technology readiness and to successfully achieve a technology readiness level (TRL) 6 milestone. The instrumentation technology is currently being transitioned from laboratory development to the end users in the hypersonic test community. / Ph. D.

An instrument for the direct measurement of skin friction in complex supersonic and hypersonic flows was developed. The flows were complex because they were of very short duration, with high temperature and shocks and often injection, mixing, and combustion. A wall-mounted, miniature cantilevered beam device measured the small tangential shear force on the non-intrusive floating element. Semiconductor strain gages mounted at the beam’s base measure the small strains that are generated. By modifying the geometry of the sensing unit, this design can be adapted for a variety of test flows. The use of engineering plastics and short beam length provide high frequency response and make the beam stiff so that the floating head’s deflection due to the shear is negligible, allowing for a non-nulling design. Measurements were made in scramjet models at the NASA Ames 16-inch Shock Tunnel and the General Applied Science Laboratory HYPULSE facility. Test flow conditions were harsh with the facilities simulating Mach 14 enthalpy conditions (320 atm and 10000 R total temperatures) for 0.3-2 milliseconds. The use of engineering plastics reduces heat transfer, so that measurements can be made in these very hot impulsive flows without thermal contamination of the data. Skin friction data in agreement with other correlations and measurements were obtained at both facilities. Mach 2.4 cold flow tests were also performed in the Virginia Tech Supersonic Tunnel. These helped verify the concept and to establish pressure gradient sensitivity in the case of a shock wave impacting directly on the sensing head. Analysis of the measurement uncertainty in the cold supersonic flow tests showed that an uncertainty of approximately 10 percent is achievable. An uncertainty of 15-20% is estimated for the most severe hot cases. An assortment of variations were applied to the gage to extend gage life. The most significant was the replacement of the oil in the sensing gap with a silicon rubber, eliminating service requirements. Tests at all of the facilities confirmed that the rubber-filled gages provided approximately the same level of accuracy as was achieved with the original oil-filled gage design, except when shocks impacted the gage head. / Ph. D.

Solo grampeado é uma alternativa eficiente utilizada em obras de reforço de solos. É resultante da inclusão de reforços, denominados grampos, em um maciço em corte. A resistência ao cisalhamento da interface solo-grampo é um dos parâmetros mais importantes para fins de projeto. Este parâmetro é determinado a partir da experiência dos projetistas e se baseia principalmente no tipo de solo e em ensaios de campo (arrancamento, sondagens a percussão e pressiométricos). Neste contexto, a realização de ensaios de arrancamento in situ é extremamente importante para a quantificação deste parâmetro e, conseqüentemente, para a elaboração de projetos mais econômicos e seguros. A execução de ensaios de arrancamento em laboratório permite verificar condições muitas vezes não encontradas em campo. Este trabalho apresenta os resultados de ensaios de arrancamento de grampos realizados em laboratório. Também foi analisada a evolução da força nos grampos e dos deslocamentos do solo. Os grampos foram instalados em um protótipo de solo grampeado sobre o qual se aplicou uma sobrecarga de 50 kPa através de uma bolsa de ar comprimido. Os ensaios de arrancamento permitiram quantificar valores de resistência ao cisalhamento de interface da ordem de 145 kPa, mobilizados com pequenos deslocamentos dos grampos. O arrancamento de grampos instrumentados indicou que cerca de 90% do comprimento total dos grampos foi solicitado. Ao final dos ensaios de arrancamento, os grampos foram extraídos completamente do maciço de solo o que permitiu comprovar a sua integridade física. Os deslocamentos do solo foram máximos próximo ao topo do protótipo e na direção horizontal. Os resultados demonstram a viabilidade de estudar o comportamento do maciço reforçado a partir do comportamento do protótipo de solo grampeado construído em laboratório. / Soil nailing is an efficient soil reinforcement technique which uses inclusions, namely nails, in soil slopes. Unit skin friction is one of the most important parameters used in soil nailing design. The definition of this parameter is commonly based on local experience and correlations to some in situ tests. This work presents the results obtained from the pullout test carried out in a soil nailed wall prototype built in laboratory. Forces acting in nails were measured by strain gage instrumentation. Soil displacement was measured in short and long terms. The pullout tests were carried out after the application of a uniform surcharge given by a compressed air bag. The results showed that unit skin friction was about 145 kPa, mobilized with little nail displacements. About 90% of nail length were solicited during pullout tests, according to tests performed in strain gage instrumented nails. Nail extraction showed a high level of nail integrity. Soil displacements were higher close to the wall top, near the face. These results demonstrate the feasibility of using of laboratory prototype studies to investigate the geotechnical behavior of soil nailing structures.

Within the confines of this study, numerical simulations of time dependent variation of downdrag forces on the diaphragm walls are analyzed for a generic soil site, where consolidation is not completed. As part of the first generic scenario, consolidation of a clayey site due to the application of the embankment is assessed. Then two sets of diaphragm walls, with and without bitumen coating, are analyzed. For comparison purposes, conventional analytical calculation methods (i.e., rigid-plastic and elastic-plastic soil models) are also used, the results of which, establish a good basis of comparison with finite-element based simulation results. Additionaly, the same generic cases are also analyzed during the stages of excavation, when diaphragm walls are laterally loaded. As the concluding remark, on the basis of time dependent stress and displacement responses of bitumen coated and uncoated diaphragm walls, it was observed that negative skin friction is a rather complex time-dependent soil-structure and loading interaction problem. This problem needs to be assessed through methods capable of modeling the complex nature of the interaction. Current analytical methods may significantly over-estimate the amount of negative skin friction applied on the system, hence they are judged to be over-conservative. However, if negative skin friction is accompanied by partial unloading as expected in diaphragm walls or piles used for deep excavations, then they may be subject to adverse combinations of axial load and moment, which may produce critical combinations expressed in interaction diagrams. Neglecting the axial force and moment interaction may produce unconservative results.

Thesis (Ph. D.)--Mississippi State University. Department of Mechanical Engineering. / Title from title screen. Includes bibliographical references.

Inlet-injection was motivated by the possibility for skin-friction reduction in the combustion chamber of a flight style, three-dimensional, scramjet engine. High Mach number flight, where skin friction in the combustion chamber is a significant proportion of the overall drag, is the regime of interest for this type of reduction. This is a result of high Mach number supersonic flow within the combustion chamber, coupled with high densities due to the compression process. The flight condition of interest was chosen to be Mach 8.0 at an altitude of 30km. This choice was dictated by near-term flight-testing capabilities. The approach was to design an inlet with a reduced contraction ratio. This would produce a relatively low-density combustion-chamber flow, that would, in turn, lead to lower viscous drag. Due to low temperatures in the combustion chamber, as a result of the reduced compression, a novel method of ignition was required. This fluid-dynamic ignition technique made use of inlet injection together with flow non-uniformities generated by the inlet. The inlet chosen for this purpose was a rectangular-to-elliptical-shape-transition inlet or REST inlet. The focus of the investigation, was therefore, to determine the potential for performance improvement using inlet injection of fuel. The general approach to the investigation was experimental, using a scramjet model consisting of inlet, combustion chamber and a truncated nozzle. Flow-path thrust-potential was used as the primary performance parameter, where the term `thrust-potential' is used to indicate the lack of full expansion. A secondary performance metric was combustion efficiency, determined by matching one-dimensional analysis to experimental pressure distributions. In addition to inlet-injection, conventional injection into the combustion-chamber was tested as the performance baseline. Based on findings from these tests, two additional methods of injection were investigated both having a combination of inlet and combustion-chamber injection. The general findings showed that inlet injection, in comparison to combustion-chamber injection, produced an increase in performance in terms of thrust-potential and combustion efficiency for supersonic combustion. This occurred over a range of equivalence ratios up to 1.0. However, the maximum thrust developed by inlet injection was limited by engine unstart. In terms of the maximum thrust-potential, combustion-chamber injection exceeded that of inlet injection but significantly higher fuelling was required and poor combustion efficiency persisted. In order to offset the limit in thrust production due to unstart, an alternative fuelling method was implemented. This took the form of partial injection of the fuel in the combustion chamber in combination with inlet injection. An increase in thrust-potential and combustion efficiency as a result of increased fuel coverage in areas of the combustion chamber, which were fuel lean under inlet-injection. A thrust potential level similar to that of combustion-chamber injection was achieved with significantly higher combustion efficiency and consequently a lower fuelling level. This type of combined-injection is an attractive option for fuel delivery at the nominal flight condition. An additional finding for combustion-chamber and combined injection was that very high equivalence ratios led to separated flow in the combustion chamber and isolator. This was a result of excessive heat release producing an adverse pressure gradient in the engine. This mode of operation showed high levels of thrust-potential at equivalence ratios in excess of 1.0. Although interesting, these findings were outside the scope of the investigation since the flow within the combustion chamber is no longer purely supersonic.

One of the major challenges of scramjet propulsion remains the generation of sufficient thrust to overcome the large drag of hypersonic vehicles. Since the viscous drag constitutes a large portion of the overall drag, its mitigation offers potential for performance improvement. Viscous drag is generated on all wetted surfaces of the vehicle but is largest in the scramjet combustion chamber, where the fluid not only has a high flow speed, but also a high density. Reduction of the skin friction drag in the combustor hence promises large improvements to the efficiency of the propulsive system. Stalker (2005) proposed a novel approach to skin friction reduction that is based on the combustion of hydrogen within the turbulent boundary layer of supersonic or hypersonic flow. An extension to the theory of van Driest II was developed that suggests that the effectiveness of this method is significantly superior to that of film cooling without combustion effects. In essence, the combustion heat release reduces the velocity gradient at the wall and the density in the boundary layer so that the momentum transfer to the wall is decreased. This work investigates the applicability of this skin friction reduction method to scramjet combustors that would operate at flight Mach numbers between 8 and 13 at altitudes between 34 and 39 km. The corresponding combustor Mach number is approximately 4.5 and the total enthalpies are between 3.6 and 7.8 MJ/kg. Experiments that directly measured the skin friction drag on the internal scramjet combustor surface were conducted in the T4 Stalker tube at The University of Queensland. A constant area, axisymmetric combustor was tested with a matching constant area, axisymmetric inlet that did not compress the oncoming flow. Therefore, the experiments were of a quasi-direct-connect nature where the inlet was used to condition the wall layer of the flow that enters the combustion chamber. The start of the combustor was formed by a step at the end of the inlet which contained an annular slot for the injection of the gaseous hydrogen fuel. Fuel was injected tangentially to the main stream flow into the circular combustor as a uniform layer underneath the established boundary layer from this annular slot. Combustion was monitored via the measurement of the axial pressure distribution in the combustor and viscous forces on the combustor were measured with a stress wave force balance. Two different inlet lengths were tested to assess the effect of the boundary layer state and thickness on the ignition and combustion of the injected hydrogen. The leading edge of the inlet was either sharp or blunt to investigate the effect of the hot gas that is contained in an entropy layer that is generated by a blunt leading edge. Finally, the diameter of the duct was varied to ensure that the experimental data was not subject to duct scaling effects. The effect on skin friction of the combustion of fuel in the boundary layer was assessed directly by measurement as well as analytically with several prediction methods. The experimental data show reductions of skin friction drag of up to 77% when stable combustion was established. A thick, turbulent boundary layer results in ignition for lower enthalpy conditions than a thin, laminar layer. The blunted leading edge configuration creates conditions that results in ignition of the injected fuel at all tested flow enthalpies and when a sharp leading edge configuration does not. Analytical predictions of the skin friction drag are in close agreement with the experimental data for fuel-off, film cooling and boundary layer combustion cases. It is demonstrated that the characteristics of boundary layer combustion do not change when the duct diameter is increased and the hydrogen mass flow rate per unit circumferential length is kept constant.

Thesis (M. Eng.), Memorial University of Newfoundland, 1998. / Bibliography: leaves 118-122.

Thesis (Ph.D.)--Memorial University of Newfoundland, 2003. / Bibliography: leaves 215-232.