• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 471
  • 143
  • 113
  • 60
  • 58
  • 25
  • 12
  • 10
  • 10
  • 5
  • 5
  • 4
  • 2
  • 2
  • 1
  • Tagged with
  • 1201
  • 257
  • 133
  • 133
  • 126
  • 113
  • 101
  • 89
  • 85
  • 82
  • 81
  • 72
  • 66
  • 65
  • 64
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Measurements of leakage, power loss and rotordynamic force coefficients in a hybrid brush seal

Baker, Jose Enrique 15 May 2009 (has links)
This thesis presents measurements of power loss and leakage in a hybrid brush seal (HBS) for increasing pressure differentials and over a range of rotor speeds. The test HBS, Haynes-25 bristle pack [~850 bristles/cm] and 45o lay angle, is 166.4 mm in diameter and integrates 20-arcuate pads connected with thin EDM-webs to the seal casing. The measured drag power at low rotor speeds (< 11 m/s at 1,300 rpm) decreases as the pressure differential across the seal increases. At a fixed rotor speed, a significant drop in drag torque (and drag power) ensues as the supply pressure increases, thus demonstrating a gas film separates the rotor from the seal pads. A constant operating temperature (~24oC) at the rotor/seal interface during tests with shaft rotation also indicates the absence of intermittent contact between the seal pads and rotor. Flow rate measurements at room temperature (25oC) show an improved sealing ability with a leakage reduction of about 36%, when compared to a 1st generation shoedbrush seal. The HBS predicted effective clearance (~50 μm) is a small fraction of that in an equivalent one-tooth labyrinth seal. Identified HBS direct stiffness coefficients decrease (~15%) as function of rotor speed for an increasing supply pressure condition (Pr = 1.7 and 2.4). The identified cross-coupled stiffness is at least one or two orders of magnitude smaller than the direct stiffness coefficient. The cross-coupled mass is negligible for all tested rotor speeds and supply pressures. The HBS energy dissipation mechanism is characterized in terms of a loss factor ( γ) and dry friction coefficient ( μ). The direct HBS viscous damping coefficient is strongly dependent on the excitation frequency, while showing minimal dependence on rotor speed or supply pressure. The HBS novel configuration incorporates pads contacting on assembly the shaft; and which under rotor spinning; lift off due to the generation of a hydrodynamic pressure. Experimental results obtained show that hybrid brush seals (HBS) are a viable alternative to overcoming the major drawbacks of labyrinth seals; namely excessive leakage and potential for rotordynamic instability. Additionally, during operation a gas film in HBS eliminates rotor and bristle wear, as well as thermal distortions; which are commonly known limitations of conventional brush seals.
2

Aspects of the equine rhabdomyolysis syndrome

Harris, Patricia Ann January 1988 (has links)
No description available.
3

The study of contact phenomena using ultrasound

Quinn, Amy May January 2002 (has links)
No description available.
4

Structural modelling of adhesive joints in automotive bodies

Steidler, Silvana M. January 2000 (has links)
No description available.
5

Evaluation of Laboratory Conditioning Protocols for Warm-Mix Asphalt

Yin, Fan 1990- 14 March 2013 (has links)
Warm-Mix Asphalt (WMA) refers to the asphalt concrete paving material produced and placed at temperatures approximately 50°F lower than those used for Hot-Mix Asphalt (HMA). Economic, environmental and engineering benefits have boosted the use of WMA technology across the world during the past decade. While WMA technology has been successfully utilized as a paving material, several specifications and mix design protocols remain under development. For example, currently, there is no consistent laboratory conditioning procedure for preparing WMA specimens for performance tests, despite being essential for mix performance. Based on previous studies, several candidate conditioning protocols for WMA Laboratory Mixed Laboratory Compacted (LMLC) and off-site Plant Mixed Laboratory Compacted (PMLC) specimens were selected, and their effects on mixture properties were evaluated. Mixture stiffness evaluated in a dry condition using the Resilient Modulus (MR) test (ASTM D-7369) was the main parameter used to select a conditioning protocol to simulate pavement stiffness in its early life. The number of Superpave Gyratory Compactor (SGC) gyrations to get 7±0.5% air voids (AV) was the alternative parameter. Extracted binder stiffness and aggregate orientation of field cores and on-site PMLC specimens were evaluated using the Dynamic Shear Rheometer (DSR) (AASHTO T315) and image analysis techniques, respectively. In addition, mixture stiffness in a wet condition was evaluated using the Hamburg Wheel-Track Test (HWTT) (AASHTO T324) stripping inflection point (SIP) and rutting depth at a certain number of passes. Several conclusions are made based on test results. LMLC specimens conditioned for 2 hours at 240°F (116°C) for WMA and 275°F (135°C) for HMA had similar stiffnesses as cores collected during the early life of field pavements. For off-site PMLC specimens, different conditioning protocols are recommended to simulate stiffnesses of on-site PMLC specimens: reheat to 240°F (116°C) for WMA with additives and reheat to 275°F (135°C) for HMA and foamed WMA. Additionally, binder stiffness, aggregate orientation, and overall AV had significant effects on mixture stiffness. Mixture stiffness results for PMFC cores and on-site PMLC specimens in a wet condition as indicated by HWTT agree with those in a dry condition in MR testing.
6

Examining the Neuromuscular and Mechanical Characteristics of the Abdominal Musculature and Connective Tissues: Implications for Stiffening the Lumbar Spine

Brown, Stephen Hadley Morgan 24 April 2008 (has links)
Research has uncovered an essential role of proper abdominal muscle function in ensuring the health and integrity of the lumbar spine. The anatomical arrangement of the abdominal musculature (rectus abdominis, external oblique, internal oblique, transverse abdominis) and intervening connective tissues is unique in the human body. Despite the hypothesized importance and uniqueness of the abdominal muscles, very little research has been directed to understanding their role from a neuro-mechanical standpoint. Thus, this thesis was designed to study the neuro-activation and mechanical characteristics of the abdominal musculature and connective tissues, with a specific focus on torso stiffening mechanisms. Several experiments were performed and unified around this theme. The first study explored the fundamental relationship between EMG muscle activation recordings and the moments generated by the trunk musculature. This study was novel in that investigation of the abdominal musculature was augmented with consideration of antagonist muscle co-activation. The main finding was that the EMG-moment relationships were quite similar in both the abdominal and extensor muscle groups; however, the form of this relationship differed from that often reported in the literature. Specifically, consideration of antagonist muscle moments linearized the EMG-moment relationship of the agonist muscle groups. Once this activation-moment relationship had been established, the next line of questioning explored the association between torso muscle activation, driven through the abdominals, and torso stiffness. Two studies addressed this issue: the first examined the intrinsic resistance of the torso to bending in the flexion, extension, and lateral bend directions, while varying the levels of torso muscle activation; the second examined the response of the trunk to perturbations while varying the levels of torso muscle activation under the presence of limited reflexes. The first of these two studies demonstrated a rise in trunk stiffness as muscle activation increased over the lower 40% of range of motion. At greater ranges of motion in flexion and lateral bend the trunk appeared to become less stiff as the musculature contracted to higher levels. The latter study revealed substantial spinal displacements in response to trunk perturbations, indicating that in the absence of reflex activity, the stiffness produced by muscular contraction may be inadequate to stiffen the torso to prevent damage to spinal tissues. The fourth study was designed to enable in-vivo observation of abdominal muscle and connective tissue deformation using ultrasound imaging. During relatively simple abdominal contractions, the oblique aponeurosis demonstrated surprising deformation patterns that often exhibited the characteristic of a negative Poisson’s ratio. This was hypothesized to be facilitated by the composite laminate arrangement of the abdominal wall, whereby the loose connective tissues separating layers of collagen fibres may allow for separation of adjacent layers, giving the appearance of structural volume expansion. Further, a lateral displacement of the rectus abdominis muscle was noted in a majority of contractions, highlighting the dominance of the laterally oriented forces generated by the oblique muscles. The final study questioned, at a basic level, the nature of the anatomical arrangement of the abdominal muscle-connective tissue network. Examining the contraction of the rat abdominal wall uncovered the transfer of muscularly generated force and stiffness through the connective tissues binding the layered muscles. This suggests a functionality of the abdominal wall as a composite laminate structure, allowing substantial multi-directional stiffness to be generated and transmitted around the torso, thereby enhancing the ability to effectively stabilize the spine.
7

Examining the Neuromuscular and Mechanical Characteristics of the Abdominal Musculature and Connective Tissues: Implications for Stiffening the Lumbar Spine

Brown, Stephen Hadley Morgan 24 April 2008 (has links)
Research has uncovered an essential role of proper abdominal muscle function in ensuring the health and integrity of the lumbar spine. The anatomical arrangement of the abdominal musculature (rectus abdominis, external oblique, internal oblique, transverse abdominis) and intervening connective tissues is unique in the human body. Despite the hypothesized importance and uniqueness of the abdominal muscles, very little research has been directed to understanding their role from a neuro-mechanical standpoint. Thus, this thesis was designed to study the neuro-activation and mechanical characteristics of the abdominal musculature and connective tissues, with a specific focus on torso stiffening mechanisms. Several experiments were performed and unified around this theme. The first study explored the fundamental relationship between EMG muscle activation recordings and the moments generated by the trunk musculature. This study was novel in that investigation of the abdominal musculature was augmented with consideration of antagonist muscle co-activation. The main finding was that the EMG-moment relationships were quite similar in both the abdominal and extensor muscle groups; however, the form of this relationship differed from that often reported in the literature. Specifically, consideration of antagonist muscle moments linearized the EMG-moment relationship of the agonist muscle groups. Once this activation-moment relationship had been established, the next line of questioning explored the association between torso muscle activation, driven through the abdominals, and torso stiffness. Two studies addressed this issue: the first examined the intrinsic resistance of the torso to bending in the flexion, extension, and lateral bend directions, while varying the levels of torso muscle activation; the second examined the response of the trunk to perturbations while varying the levels of torso muscle activation under the presence of limited reflexes. The first of these two studies demonstrated a rise in trunk stiffness as muscle activation increased over the lower 40% of range of motion. At greater ranges of motion in flexion and lateral bend the trunk appeared to become less stiff as the musculature contracted to higher levels. The latter study revealed substantial spinal displacements in response to trunk perturbations, indicating that in the absence of reflex activity, the stiffness produced by muscular contraction may be inadequate to stiffen the torso to prevent damage to spinal tissues. The fourth study was designed to enable in-vivo observation of abdominal muscle and connective tissue deformation using ultrasound imaging. During relatively simple abdominal contractions, the oblique aponeurosis demonstrated surprising deformation patterns that often exhibited the characteristic of a negative Poisson’s ratio. This was hypothesized to be facilitated by the composite laminate arrangement of the abdominal wall, whereby the loose connective tissues separating layers of collagen fibres may allow for separation of adjacent layers, giving the appearance of structural volume expansion. Further, a lateral displacement of the rectus abdominis muscle was noted in a majority of contractions, highlighting the dominance of the laterally oriented forces generated by the oblique muscles. The final study questioned, at a basic level, the nature of the anatomical arrangement of the abdominal muscle-connective tissue network. Examining the contraction of the rat abdominal wall uncovered the transfer of muscularly generated force and stiffness through the connective tissues binding the layered muscles. This suggests a functionality of the abdominal wall as a composite laminate structure, allowing substantial multi-directional stiffness to be generated and transmitted around the torso, thereby enhancing the ability to effectively stabilize the spine.
8

Characterization of bending stiffness and spontaneous buckling of alpha-helices and coiled coils

Lakkaraju, Sirish Kaushik 15 May 2009 (has links)
Elasticity of α-helices and coiled coils have often been described by a linear response to local bending with bending stiffness (Kb) and persistence length (Lp) describing their flexibility. However, we observed that the non-bonded forces along the length of these structures are not screened at physiological conditions and introduce a buckling instability. For α-helical systems of same composition, but different lengths, this is identified by a drop in Kb for longer helices and the length where this drop is triggered is referred to as the critical buckling length. When shorter than their critical buckling length they behave linearly, and Kb calculated using normal mode analysis in this regime is about (3.0−3.4)×10-28 Nm2 for α-helices with varying compositions, and about (1.9 − 2.1) × 10−27 Nm2 for coiled coils with leucine zipper periodicity. Beyond the critical buckling length, normal mode solutions turn imaginary, leading to an eventual disappearance of bending modes. Investigations with one dimensional (1-D) linear chains of beads (a simplistic representation of bio-filaments) show that non-bonded forces have a reciprocal relation with the critical buckling length (no buckling instability existed in the absence of non-bonded forces). Critical buckling length is 115.3 ± 2.9 °A for α-helices and 695.1 ± 44.8 Å for coiled coils with leucine zipper periodicity, which is much smaller than their Lp (~ 800 Å for α-helices and ~ 3000 Å for coiled coils).
9

The effect of mechanical stress on the stiffness of articular cartilage and its role in the aetiology of osteoarthrosis

Swann, Anthony Charles January 1988 (has links)
Although a substantial amount is known about the pathogenesis of osteoarthrosis, its aetiology and in particular the role that mechanical factors play, remains unclear. One particular hypothesis suggests that cartilage adapts mechanically so that it may transmit, without sustaining damage, the stresses to which it is predominantly subjected, and that damage to the cartilage is caused by infrequent high stresses in excess of the predominant level. As a corollary, it was suggested that highly stressed cartilage should be stiffer than lowly stressed cartilage. A survey of the mechanical properties of normal articular cartilage from unembalmed cadaveric knee and ankle joints was undertaken to test this hypothesis. For this purpose, a specially developed indentation test apparatus was commissioned. Tests of the machine's measurement capabilities indicate that coefficients of variation of 2.14% and 1.20% for indentation and cartilage thickness measurement could be expected. The maximum percentage errors in the calculated creep modulus value which could result from these typical measurement errors, were 4.2% and 2.9% respectively. Creep modulus values, calculated from these measurements, were used in topographical comparisons of cartilage stiffness. The stiffest areas of cartilage in the knee joint were the femoral condyles and areas of the tibia covered by the menisci. Cartilage on the patellar surfaces of the femur and in areas exposed by the menisci was significantly softer. Cartilage from the ankle joint was considerably stiffer than cartilage from the knee. Comparisons between the cartilage stiffness and levels of stress which act in the knee and ankle joints during normal ambulatory activity, showed the stiffest areas of cartilage to be subjected to the greater stresses. Correlations of averaged data values indicated a significant (p < 0.01) direct relationship between cartilage stiffness and stress. This relationship and the consistency with which osteoarthrotic lesions were found in areas subjected to damaging patterns of stress supported the hypothesis under examination. The lack of correlation found between the proteoglycan content and cartilage stiffness suggested that structural rather than compositional factors may be more important in influencing the compressive stiffness of normal articular cartilage.
10

The association between physical activity and arterial stiffness in youth

Walker, Darolyn 10 September 2009 (has links)
Physical activity is a powerful modifiable lifestyle factor that reduces the risk of cardiovascular disease (CVD) in adults through favorable changes in conventional risk factors including serum lipids, blood pressure and glycemia. Recent evidence suggests that the cardioprotective effects of physical activity may also be mediated through beneficial effects on vascular function, in particular arterial stiffness. While the beneficial effects of physical activity in CVD risk in adults are irrefutable, data in youth are limited, especially for arterial stiffness. Purpose: The purpose of this project is to explore the continuous association between physical activity and arterial stiffness in youth. Hypothesis: We hypothesized that physical activity is negatively associated with arterial stiffness, whereby highly active youth would display lesser degrees of arterial stiffness than their less active (sedentary) peers. Methods: 485 youth (12-13 yrs) were recruited from the 1995 Manitoba birth cohort involved in the GreatICE asthma and allergy study. Youth were stratified into tertiles (high, medium, low) of self-reported physical activity. Global cardiometabolic risk was determined from a composite score of conventional risk factors including, LDL, SBP, Insulin, Glucose and Triglycerides. Arterial stiffness was assessed non-invasively using conventional pulse wave analysis and velocity. Results: Of the 485 youth who participated in this wave of the study, measures of PWV and PWA were available on 357 and 335 youth respectively. Cardiometabolic risk decreased with increasing levels of vigorous physical activity. Neither measure of arterial stiffness was associated with physical activity. Conclusion: Increased vigorous physical activity is associated with reduced cardiometabolic risk in youth independent of arterial stiffness.

Page generated in 0.0523 seconds