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.

It is the responsibility of pavement engineers to design pavements that provide safe and smooth riding surfaces over their entire life cycle. Each year many people around the world lose their lives in vehicle crashes, which are one of the leading causes of death in the United States (US). One of the contributing factors in many of these crashes is inappropriate friction between tires and the pavement. To minimize the impact of this factor, state Departments of Transportation (DOTs) must monitor the friction of their pavement networks systematically and regularly. Several devices are used around the world for measuring friction. Locked-wheel skid trailers are the predominant technology for roadways in the U.S. However, Continues Friction Measuring Equipment (CFME) is emerging as a practical alternative, especially for network-level monitoring. This type of technology has been used for monitoring runway friction for many years and is starting to be used also for measuring roadway friction. This thesis evaluates the different operational characteristics of CFME to provide guidelines for highway agencies interested in using this technology for supporting their friction management programs. It follows a manuscript format and is composed of two papers. The first part of the thesis presents a methodology to objectively synchronize and compare CFME measurements using cross-correlation. This methodology allows for comparing the “shape” of the friction profiles, instead of only the average friction values. The methodology is used for synchronizing friction measurements and assessing the repeatability and reproducibility of the CFME using friction measurements taken on a wide range of surfaces at the Virginia Smart Road. The proposed approach provides highway agencies with a rigorous method to process CFME measurements. The second part of the thesis evaluates the impact of several operational characteristics on the CFME measurements using a field experiment. The results of the experiment confirmed that the measurements are significantly affected by (1) the direction of testing while testing on sections of road with a significant grade, (2) water film thickness, and (3) testing speed. The experiment showed that measurements taken downhill on a 6% grade were significantly higher than those taken uphill. The analysis also verified that, consistent with previous studies, the measured friction decreases with higher water depth and testing speeds. It also showed that the change of friction with speed is approximately linear over the range of speeds used in the experiment. In general, the thesis results suggest that CFME can provide repeatable and reproducible friction profiles that can be used to support friction management programs and other asset management business functions. However, care should be taken with regard to the operational conditions during testing since the measurements are affected by several factors. Further research is needed to (1) quantify the effect of these, and potentially other, operational factors; and (2) establish standard testing condition and approaches for correcting measurements taken under other conditions. / Master of Science

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.

This study investigated the dry sliding of Acrylonitrile Butadiene Styrene (ABS) in a polymer-on-polymer configuration. The effect of several material and system parameters were investigated to determine their influence on the frictional behavior of the ABS materials. These parameters included: the time dependence of friction under static load, friction-normal load relationships, surface roughness, internal lubrication, friction-velocity relationships, and system stiffness. Four basic types of frictional behavior were observed - steady sliding, a single stick followed by steady sliding, non-sustained stick-slip oscillations that decayed after several oscillations, and stick-slip oscillations lasting several minutes. Several observations were made regarding the experimental results. It was observed that increasing surface roughness decreases the unstable behavior of the materials. Also, decreasing normal load decreased unstable behavior. A relationship between the friction-velocity curve, stick-slip, and the critical velocity at which stick slip disappeared was observed. Finally, an attempt was made to explain the observed behavior in light of existing theories. / M.S.

Surface temperatures generated by dry sliding friction were measured with the use of an infrared radiometric microscope system developed at Virginia Polytechnic Institute and State University. Silver was used as the test material because of its resistance to oxide formation. The effects of load, velocity, geometry, and environment on surface temperatures, friction, and wear were investigated for the system of silver sliding on sapphire. Results of this investigation indicate that load, velocity, and geometry had significant effects on surface temperature rise, with load having the strongest effect. Measured values of surface temperature rise agreed reasonably well with the theoretical predictions of Archard and Jaeger. Load and environment were found to have a significant effect on the coefficient of friction. The average value of the coefficient of friction for each test was lower in nitrogen than in air. Load, geometry, velocity, and environment were significant to the wear rate. A striking difference was noticed in the structure that formed adjacent to the contact region for the different environments. / M.S.

No / Market structure affects the informational and real frictions faced by traders in equity markets. Using bid-ask spreads, we present evidence which suggests that while real frictions associated with the costs of supplying immediacy are less in order-driven systems, informational frictions resulting from increased adverse selection risk are considerably higher in these markets. Firm value, transaction size and order location are all major determinants of the trading costs borne by investors. Consistent with the stealth trading hypothesis of Barclay and Warner (1993), we report that informational frictions are at their highest for medium size trades that go through the order book. Finally, while there is no doubt that the total costs of trading on order-driven systems are lower for very liquid securities, the inherent informational inefficiencies of the trading format should not be ignored. This is particularly true for the vast majority of small to mid-size stocks that experience infrequent trading and low transaction volume.

In order to deliver statistical and qualitative backing to latent fingerprint evidence, algorithms are proposed (1) to perform fingerprint matching to aid in quality assessment, and (2) to discover statistically rare features or patterns in fingerprints. These features would help establish an objective minimum-quality baseline for latent prints as well as aid in the latent examination process in making a matching comparison. The proposed methodologies use minutiae triplet-based features in a hierarchical fashion, where not only minutia points are used, but ridge information is used to help establish relations between minutiae. Results show (1) that our triplet-based descriptor is useful in eliminating false matches in the matching algorithm, and (2) that a set of distinctive features can be found that have sufficient discriminatory power to aid in quality assessment. / Master of Science

No / Given that most of the working life of a brake pad life is spent in the bedded condition, it is important to examine the conditions of a fully bedded contact interface. An experimental and a numerical method are combined. Contact pressure and its effects (heat generation/partition and temperature rise) for the drag braking process with and without bedding are compared. The real contact area is also measured and found to be increased for the bedded interface. This results to the contact pressure being ¿more¿ evenly distributed than before. Spreading the contact pressure also results in increasing the total heat transfer between the disc and pad since now more heat can be transferred from the pad (where it is generated) to the disc. It is concluded that in order to have a reliable simulation it is recommended that the bedding-in effects are introduced in the simulations. / IMechE, RAE Travel Grant

Most tilting pad journal bearing dynamic characteristics estimation methodologies assume perfect shaft tracking by the pads. In other words, they neglect pivot friction. In case of pads having point or line contact that operate under most normal load conditions, the pad tilting is due to a rocking motion which is not greatly influenced by friction. Hence this simplifying assumption might be acceptable. Heavier loading conditions, such as those typically encountered in gearboxes, demand the use of spherical pivots to avoid pivot failure. The spherical pivot is very attractive for this reason, but the tilting motion is rather a sliding action that must occur in the precision ball socket. A valid concern exists for verifying the soundness of assumed shaft tracking by the pads of such bearings. A "fixed test bearing, floating shaft" type of test rig previously built for determining the dynamic characteristics of bearings was accordingly modified to facilitate the testing of shaft tracking for a spherical pivot bearing. This thesis describes the modifications carried out on the rig. The special instrumentation and data acquisition systems implemented to observe the minute pad motion are also discussed. Some preliminary results of the tests are presented for various loading conditions. They show excellent shaft tracking by the pads. More detailed testing and analysis of data is required to fully understand the pad motion and tracking ability of the spherical pivot design. / Master of Science