Browsing by Subject "roughness"
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Item Effect of Surface Roughness on Wind Turbine Performance(2014-06-25) Ehrmann, Robert SchaeferWind farm operators observe production deficits as machines age. Quantifying deterioration on individual components is difficult, but one potential explanation is accumulation of blade surface roughness. Historically, wind turbine airfoils were designed for lift to be insensitive to roughness by simulating roughness with trip strips. However, roughness was still shown to negatively affect performance. Furthermore, experiments illustrated distributed roughness is not properly simulated by trip strips. To understand how real-world roughness affects performance, field measurements of turbine-blade roughness were made and simulated on a NACA 63_(3)-418 airfoil in a wind tunnel. Insect roughness and paint chips were characterized and recreated as distributed roughness and a forward-facing step. Distributed roughness was tested in three heights and five density configurations. The model chord Reynolds number was varied between 0:8 to 4:8 ? 10^(6). Measurements of lift, drag, pitching moment, and boundary-layer transition were completed. Results indicate minimal effect from paint-chip roughness. As distributed roughness height and density increase, lift-curve slope, maximum lift, and lift-to-drag ratio decrease. As Reynolds number increases, bypass transition occurs earlier. The critical roughness Reynolds number varies between 178 to 318, within the historical range. Little sensitivity to pressure gradient is observed. At a chord Reynolds number of 3:2?10^(6), the maximum lift-to-drag ratio decreases 40% for 140 ?m roughness, corresponding to a 2.3% loss in annual energy production. Simulated performance loss compares well to measured performance loss on an in-service wind turbine.Item Laboratory experiments and numerical modeling of wave attenuation through artificial vegetation(2009-05-15) Augustin, Lauren NicoleIt is commonly known that coastal vegetation dissipates energy and aids in shoreline protection by damping incoming waves and depositing sediment in vegetated regions. However, this critical role of vegetation to dampen wave forces is not fully understood at present. A series of laboratory experiments were conducted in the Haynes Coastal Laboratory and 2-D flume at Texas A&M University to examine different vegetative scenarios and analyze the wave damping effects of incident wave height, stem density, wave period, plant type, and water depth with respect to stem length. In wetland regions vegetation is one of the main factors influencing hydraulic roughness. Traditional open-channel flow equations, including the Manning and Darcy- Weisbach friction factor approach, have been successfully applied to determine bottom friction coefficients for flows in the presence of vegetation. There have been numerous relationships derived relating the friction factor to different flow regime boundary layers to try and derive a wave friction factor for estimating energy dissipation due to bottom bed roughness. The boundary layer problem is fairly complex, and studies relating the wave friction factor to vegetation roughness elements are sparse. In this thesis the friction factor is being applied to estimate the energy dissipation under waves due to artificial vegetation. The friction factor is tuned to the laboratory experiments through the use of the numerical model COULWAVE so that the pipe flow formulation can be reasonably applied to wave problems. A numerical friction factor is found for each case through an iterative process and empirical relationships are derived relating the friction factor for submerged and emergent plant conditions to the Ursell number. These relationships can be used to reasonably estimate a wave friction factor for practical engineering purposes. This thesis quantitatively analyzes wave damping due to the effects of wave period, incident wave height, horizontal stem density, water depth relative to stem length, and plant type for a 6 m plant bed length. A friction factor is then determined numerically for each of the laboratory experiments, and a set of equations is derived for predicting a roughness coefficient for vegetation densities ranging between 97 stems/m2 and 162 stems/m2.Item Supersonic turbulent boundary layers with periodic mechanical non-equilibrium(Texas A&M University, 2007-04-25) Ekoto, Isaac WesleyPrevious studies have shown that favorable pressure gradients reduce the turbulence levels and length scales in supersonic flow. Wall roughness has been shown to reduce the large-scales in wall bounded flow. Based on these previous observations new questions have been raised. The fundamental questions this dissertation addressed are: (1) What are the effects of wall topology with sharp versus blunt leading edges? and (2) Is it possible that a further reduction of turbulent scales can occur if surface roughness and favorable pressure gradients are combined? To answer these questions and to enhance the current experimental database, an experimental analysis was performed to provide high fidelity documentation of the mean and turbulent flow properties along with surface and flow visualizations of a high-speed ( 2.86 M = ), high Reynolds number (Re 60,000 q ???? ) supersonic turbulent boundary layer distorted by curvature-induced favorable pressure gradients and large-scale ( 300 s k + ???? ) uniform surface roughness. Nine models were tested at three separate locations. Three pressure gradient models strengths (a nominally zero, a weak, and a strong favorable pressure gradient) and three roughness topologies (aerodynamically smooth, square, and diamond shaped roughness elements) were used. Highly resolved planar measurements of mean and fluctuating velocity components were accomplished using particle image velocimetry. Stagnation pressure profiles were acquired with a traversing Pitot probe. Surface pressure distributions were characterized using pressure sensitive paint. Finally flow visualization was accomplished using schlieren photographs. Roughness topology had a significant effect on the boundary layer mean and turbulent properties due to shock boundary layer interactions. Favorable pressure gradients had the expected stabilizing effect on turbulent properties, but the improvements were less significant for models with surface roughness near the wall due to increased tendency towards flow separation. It was documented that proper roughness selection coupled with a sufficiently strong favorable pressure gradient produced regions of ??????negative?????? production in the transport of turbulent stress. This led to localized areas of significant turbulence stress reduction. With proper roughness selection and sufficient favorable pressure gradient strength, it is believed that localized relaminarization of the boundary layer is possible.Item The Effect of Proppant Size and Concentration on Hydraulic Fracture Conductivity in Shale Reservoirs(2013-04-11) Kamenov, AntonHydraulic fracture conductivity in ultra-low permeability shale reservoirs is directly related to well productivity. The main goal of hydraulic fracturing in shale formations is to create a network of conductive pathways in the rock which increase the surface area of the formation that is connected to the wellbore. These highly conductive fractures significantly increase the production rates of petroleum fluids. During the process of hydraulic fracturing proppant is pumped and distributed in the fractures to keep them open after closure. Economic considerations have driven the industry to find ways to determine the optimal type, size and concentration of proppant that would enhance fracture conductivity and improve well performance. Therefore, direct laboratory conductivity measurements using real shale samples under realistic experimental conditions are needed for reliable hydraulic fracturing design optimization. A series of laboratory experiments was conducted to measure the conductivity of propped and unpropped fractures of Barnett shale using a modified API conductivity cell at room temperature for both natural fractures and induced fractures. The induced fractures were artificially created along the bedding plane to account for the effect of fracture face roughness on conductivity. The cementing material present on the surface of the natural fractures was preserved only for the initial unpropped conductivity tests. Natural proppants of difference sizes were manually placed and evenly distributed along the fracture face. The effect of proppant monolayer was also studied.Item Tribological Properties of Nanoparticle-Based Lubrication Systems(2013-08-02) Kheireddin, BassemNew nanomaterials and nanoparticles are currently under investigation as lubricants or lubricant additives due to their unusual properties compared to traditional materials. One of the objectives of this work is to investigate the tribological properties of these materials in relation to surface topography. Chemical etching and metal evaporation methods were employed to prepare surfaces with various topographies. Surfaces were sheared with the use of a nanotribometer and characterized with an atomic force and scanning electron microscopes. For a system consisting of ZnS nanowires dispersed in dodecane sheared across ductile surfaces, it was found that the geometry of the nanowire relative to the surface topography plays a significant role. Moreover, for brittle surfaces, it was found that beyond a certain roughness the frictional properties remain unchanged. In addition, this work is also intended to explore novel lubricants with nanoparticle additives in efforts to control friction and wear. A system consisting of silica nanoparticles dispersed in ionic liquids was examined at various concentrations. It was found that an optimum concentration of nanoparticles exists and yields the best tribological properties. Such work represents an important step in understanding the tribological properties of nanoparticle lubricant additives in general; one that may ultimately provide the guidelines necessary for designing novel, low-friction, and wear-controlling nanoparticle-based lubrication systems that minimize energy and material losses due to friction.