Surface pressure fluctuations developed by turbulent flow within a boundary layer is a major cause of flow noise from a body and an issue which reveals itself over a wide range of engineering applications. Modified boundary layers (MBLs) inspired by the down coat of an owl’s wing has shown to reduce the acoustic effects caused by flow noise. This thesis investigates the mechanisms that modified boundary layers can provide for reducing the surface pressure fluctuations in a boundary layer. This study analyzes various types of MBLs in a wall jet wind tunnel through computational fluid dynamics and numerical surface pressure spectrum predictions. A novel surface pressure fluctuation spectrum model is developed for use in a wall jet boundary layer and demonstrates high accuracy over a range of Reynolds numbers. Non-dimensional parameters which define the MBL’s geometry and flow environment were found to have a key role in optimizing the acoustic performance. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2019. / FAU Electronic Theses and Dissertations Collection

A dynamic hybrid Reynolds-averaged Navier-Stokes (RANS)-Large Eddy Simulation (LES) modeling framework has been investigated and further developed to improve the Computational Fluid Dynamics (CFD) prediction of turbulent flow features along with laminar-to-turbulent transitional phenomena. In recent years, the use of hybrid RANS/LES (HRL) models has become more common in CFD simulations, since HRL models offer more accuracy than RANS in regions of flow separation at a reduced cost relative to LES in attached boundary layers. The first part of this research includes evaluation and validation of a dynamic HRL (DHRL) model that aims to address issues regarding the RANS-to-LES zonal transition and explicit grid dependence, both of which are inherent to most current HRL models. Simulations of two test cases—flow over a backward facing step and flow over a wing with leading-edge ice accretion—were performed to assess the potential of the DHRL model for predicting turbulent features involved in mainly unsteady separated flow. The DHRL simulation results are compared with experimental data, along with the computational results for other HRL and RANS models. In summary, these comparisons demonstrate that the DHRL framework does address many of the weaknesses inherent in most current HRL models. Although HRL models are widely used in turbulent flow simulations, they have limitations for transitional flow predictions. Most HRL models include a fully turbulent RANS component for attached boundary layer regions. The small number of HRL models that do include transition-sensitive RANS models have issues related to the RANS model itself and to the zonal transition between RANS and LES. In order to address those issues, a new transition-sensitive HRL modeling methodology has been developed that includes the DHRL methodology and a physics-based transition-sensitive RANS model. The feasibility of the transition-sensitive dynamic HRL (TDHRL) model has been investigated by performing numerical simulations of the flows over a circular cylinder and a PAK-B airfoil. Comparisons with experimental data along with computational results from other HRL and RANS models illustrate the potential of TDHRL model for accurately capturing the physics of complex transitional flow phenomena.

The discrete-element method for predicting skin friction for turbulent flow over rough surfaces considers the drag on the surface to be the sum of the skin friction on the flat part of the surface and the drag on the individual roughness elements that protrude into the boundary layer. The discrete-element method considers heat transfer from a rough surface to be the sum of convection through the fluid on the flat part of the surface and the convection from each of the roughness elements. The discrete-element method has been widely used and validated for roughness composed of sparse, ordered, and deterministic elements. Modifications made to the discrete-element roughness method to extend the validation to real surface roughness are detailed. These modifications include accounting for the deviation of the roughness element cross sections from circular configurations, determining the location of the computational "surface" that differs from the physical surface, and accounting for temperature changes along the height of the roughness elements. Two randomly-rough surfaces found on high-hour gas-turbine blades were characterized using a Taylor-Hobson Form Talysurf Series 2 profilometer. A method for using the three-dimensional profilometer output to determine the geometry input required in the discrete-element method for randomly-rough surfaces is presented. Two randomly-rough surfaces, two elliptical-analog surfaces, and two cone surfaces were generated for wind-tunnel testing using a three-dimensional printer. The analog surfaces were created by replacing each random roughness element from the original randomly-rough surface with an elliptical roughness element with the equivalent planorm area and eccentricity. The cone surfaces were generated by placing conical roughness elements on a flat plate to create surfaces with equivalent values of centerline-averaged height or root-mean-square (RMS) height as the randomly-rough surfaces. The results of the wind tunnel skin friction coefficient and Stanton number measurements and the discrete-element method predictions for each of the six surfaces are presented and discussed. For the randomly-rough surfaces studied, the discrete-element method predictions are within 7% of the experimentally measured skin friction coefficients. The discrete-element predictions are within 16% of the experimentally measured Stanton numbers for the randomly-rough surfaces.

No description available.

This study concerns the design, analysis, and initial testing of a novel skin friction gage for applications in three-dimensional, high-speed, high-enthalpy flows. Design conditions required favorable gage performance in the Arc-Heated Facilities at Arnold Engineering Development Center. Flow conditions are expected to be at Mach 3.4, with convective heat properties of h= 1,500 W/(m°·K) (264 Btu/(hr·ft°·°R)) and T_aw= 3,900 K (7,000 °R). The wall shear stress is expected to be as high as τ_w= 2,750 Pa (0.40 psi) with a correlating coefficient of skin friction value around C_f= 0.0035. Through finite element model and analytical analyses, a generic gage design is predicted to remain fully functional and within reasonable factors of safety for short duration tests. The deflection of the sensing head does not exceed 0.025 mm (0.0001 in). Surfaces exposed to the flow reach a maximum temperatures of 960 K (1,720 °R) and the region near the sensitive electronic components experience a negligible rise in temperature after a one second test run. The gage is a direct-measuring, non-nulling design in a cantilever beam arrangement. The sensing head is flush with the surrounding surface of the wall and is separated by a small gap, approximately 0.127 mm (0.005 in). A dual-axis, semi-conductor strain gage unit measures the strain in the beam resulting from the shear stress experienced by the head due to the flow. The gage design incorporates a unique bellows system as a shroud to contain the oil filling and protect the strain gages. Oil filling provides dynamic and thermal damping while eliminating uniform pressure loading. An active water-cooling system is routed externally around the housing in order to control the temperature of the gage system and electronic components. Each gage is wired in a full-bridge Wheatstone configuration and is calibrated for temperature compensation to minimize temperature effects. Design verification was conducted in the Virginia Tech Hypersonic Tunnel. The gage was tested in well-documented Mach 3.0, cold and hot flow environments. The tunnel provided stagnation temperatures and pressures of up to T₀= 655 K (1,180 °R) and P₀= 1,020 kPa (148 psi) respectively. The local wall temperatures ranged from T_w= 292 to 320 K (525 to 576 °R). The skin friction coefficient measurements were between 0.00118 and 0.00134 with an uncertainty of less than 5%. Results were shown to be repeatable and in good concurrence with analytical predictions. The design concept of the gage proved to be very sound in heated, supersonic flow. When it worked, it did so very effectively. Unfortunately, the implementation of the concept is still not robust enough for routine use. The strain gage units in general were often unstable and proved to be insufficiently reliable. The detailed gage design as built was subject to many potential sources of assembly misalignment and machining tolerances, and was susceptible to pre-loading. Further recommendations are provided for a better implementation of this design concept to make a fully functional gage test ready for Arnold Engineering Development Center. / Master of Science

The present study examines the role of turbulence on the incipient motion of sediment. For this purpose, well-controlled experiments are performed at the laboratory in a tilting flume. In these tests glass beads of the same size and density are used as the testing material to isolate the role of turbulence. State of the art equipment are used during the course of this study. Specifically, a 3-D Laser Doppler Velocimetry system is employed to measure the instantaneous velocity components at different points near the vicinity of a ball while the ball motion is monitored with a video camera. An image analysis program is developed here to analyze the motion of the particles within a test area. To examine the importance of the different stress components in the entrainment of sediment, five tests of different packing configuration are performed. Specifically three different roughness regimes are examined namely, the isolated, the wake interference, and the skimming flow. The results reveal that the instantaneous normal stress in the streamwise direction is the most dominant component of the instantaneous stress tensor. The backbone of this study is the development of a methodology to link the effects of turbulence with the commencement of sediment motion. It is considered that the metastable bursting cycle (i.e. sweeps, ejections, inward and outward interactions) is responsible for the sediment entrainment. And that the sediment entrainment, if any, occurs within a bursting period. The main concept behind the determination of the critical conditions is that the probability of the entrainment of sediment (effect) is equal to the probability of occurrence of these highly energetic turbulent events that have magnitude greater than the critical (cause). The probability of sediment entrainment is computed by means of the image analysis tool. The balance of moments is obtained here to determine the minimum moment that is required for the commencement of sediment motion. The balance of moments yields the deduction of a new variable that is used to describe the probability of occurrence of the different turbulent events. This variable is the summation of the instantaneous normal stresses in the streamwise and vertical direction. It is shown here that a two-parameter gamma density function describes quite well the statistical behavior of this variable. The results that are obtained from the existing model suggest that the present methodology can adequately describe the commencement of sediment motion. It is shown here that the traditionally used shear stress term uw may not be the appropriate measure for the determination of the critical conditions. / Ph. D.

This research produced a new class of skin friction gages that measures wall shear even in shock environments. One test specimen separately measured wall shear and variable-pressure induced moment. Through the investigation of available computational modeling methods, techniques for accurately predicting gage physical responses were developed. The culmination of these model combinations was a design optimization procedure. This procedure was applied to three disparate test conditions: 1) short-duration, high-enthalpy testing, 2) blow-down testing, and 3) flight testing. The resulting optimized gage designs were virtually tested against each set of nominal load conditions. The finalized designs each successfully met their respective test condition constraints while maximizing strain output due to wall shear. These gages limit sources of apparent strain: inertia, temperature gradient, and uniform pressure. A unique use of bellows provided a protective shroud for surface strain gages. Oil fill provided thermal and dynamic damping while eliminating uniform pressure as a source of output voltage. Two Wheatstone bridge configurations were developed to minimize temperature effects first from temperature gradient and then from spatially varying heat flux induced gradient. An inertia limiting technique was developed that parametrically investigated mass and center of gravity impact on strain output. Multiple disciplinary computational simulations of thermal, dynamic, shear, moment, inertia, and instrumentation interaction were developed. Examinations of instrumentation error, settling time, filtering, multiple input dynamic response, and strain gage placement to avoid thermal gradient were conducted. Detailed mechanical drawings for several gages were produced for fabrication and future testing. / Ph. D.

Scalar (heat or mass) transfer plays an important role in many industrial and engineering applications. Difficulties in experimental measurements means that there is limited detailed information available, especially in the near-wall region. Prediction in simple flows is well documented and the basis for development of many Computational Fluid Dynamics (CFD) models. This is, however, not the case for scalar transfer, especially when the Prandtl (Pr) or Schmidt number (Sc) is much greater than unity. In complex flows that involve separation and reattachment, the scalar transfer coefficient is significantly different to that of fully developed turbulent flow. The purpose of this Thesis is to investigate high Prandtl number (Pr ≥ 10) scalar transfer in fully developed (pipe) and disturbed (sudden pipe expansion) turbulent flow using CFD. Direct Numerical Simulation (DNS) is the most straight-forward approach to the solution of turbulent flows with scalar transfer. However, this technique is computationally intensive because all turbulent scales need to be resolved by the simulation. Large eddy simulation (LES) is a compromise compared to DNS. Instead of resolving all spatial scales, LES resolves only the large-scales with the small-scales being accounted for by a subgrid-scale model. Chapter 2 details the mathematical, numerical and computational details of LES with scalar transfer. From this, an optimized and highly scalable parallel LES solver was developed based on state-of-the-art LES subgrid-scale models and numerical techniques. Chapter 3 provides a verification of the LES solver for fully developed turbulent pipe flow. Reynolds numbers between Re = 180 and 1050 were simulated with a single Prandtl number of Pr = 0.71. Detailed turbulent statistics are provided for Re = 180, 395 and 590 with varying grid resolution for each Reynolds number. The results from these simulations were compared to established experimental and numerical databases of fully developed turbulent pipe and channel flows. The LES solver was shown to be in good agreement with the prior work with most discrepancies being accounted for by only reporting the resolved (large-scale) component directly reported from the LES results. For a Prandtl number close to unity, the mechanisms of turbulent transport and scalar transfer are similar. The near-wall region was shown to be dominated by large-scale sweeping structures that bring high momentum and scalar concentrations to the near-wall region. These are convected parallel to the wall as diffusion mechanisms act to transfer this to the wall where dissipation takes effect. An ejection structure then acts to transport the resultant low momentum, scalar depleted fluid back to the bulk to be replenished and continue the cycle. As the Prandtl number increases, molecular diffusivity decreases relative to viscosity, and the mechanisms of scalar transfer differ to those at Pr = 0.71. This is investigated in Chapter 4 using simulations at Re = 180, 395 and 590, with detailed statistics at Re = 395 for Pr = 0.71, 5, 10, 100 and 200. Where possible the results are compared to other numerical work and the LES solver was shown to accurately resolve the higher Prandtl number flows. There are marked variations in the scalar transfer with increasing Prandtl number as the turbulent scalar transfer becomes concentrated closer to the wall and dominated by large-scale turbulent structures. Sweeping structures are still responsible for bringing the high scalar concentrations towards the wall, however, high Prandtl number scalars are unable to completely diffuse to the wall in the time that the structure is convected parallel to the wall adjacent to the diffusive sublayer. Therefore, most of the high Prandtl number scalar is returned to the bulk via the ejection structure rather than being dissipated at the wall. Chapter 5 uses the sudden pipe expansion (SPE) to investigate disturbed turbulent flow for an inlet Reynolds numbers of Reb = 15600 and a diameter ratio of E = 1.6. These simulation parameters were chosen to match the experimental LDA measurements of Stieglmeier et al. (1989). The LES results for a range of grid resolutions were shown to be in very good agreement with the experimental work. From the LES results it was determined that the fluctuations in the wall shear stress are important in the near-wall turbulent transport. These are the result of eddies originating from the free shear layer down-washing and impinging upon the wall. This is a more effective sweeping mechanism than that observed for the fully developed turbulent pipe flow. Despite the down-wash structures impinging upon the wall, a viscous sublayer still exists in the reattachment region, albeit much thinner than the fully developed turbulent pipe flow further downstream. Using the same Reynolds number and diameter ratio, scalar transfer simulations were also undertaken in the SPE with Prandtl numbers of Pr = 0.71, 5, 10, 100 and 200. An applied scalar flux was used to heat the expanded pipe wall. The LES results are in agreement with experimental Nusselt numbers from Baughn et al. (1984) for Pr = 0.71. The disturbed turbulent flow enhances the scalar transfer and this is the result of down wash events transporting low (cold) scalar from the inlet pipe to the near-wall of the expanded pipe. This cools the heated wall and enhances localized scalar transfer downstream of the expansion. A diffusive sublayer still exists in the reattachment region within the viscous sublayer for Prandtl numbers greater than unity. As the Prandtl number increases the diffusivity decreases relative to viscosity and near-wall scalar transfer enhancement decreases as the diffusion time-scales increase.

This thesis deals with the study of hybrid RANS/LES methods for modeling of turbulent flow with a focus on the DES method and its modifications. The theoretical part focuses on the description of turbulent flow and classical methods for its modeling. The following describes the hybrid RANS/LES methods, their principles and categories. Finally, the DES method is described in detail together with its improvement in form of DDES and IDDES methods. The practical part is devoted to the testing of DES and DDES on benchmark problems. We describe here used software OpenFOAM and numerical methods used to discretize the equations. One part is devoted to grid generation. The DES and the DDES methods are tested on two benchmarks: flat plate with zero pressure gradient and backward facing step. The simulaton results are compared with experimental data, with a focus on good modeling of the velocity profile near wall, turbulent viscosity and skin friction coefficient. Powered by TCPDF (www.tcpdf.org)

As variações de temperatura influenciam nas propriedades da água, especialmente na viscosidade. Este pode ser um fator significativo, que afeta a vazão dos emissores e consequentemente a uniformidade de aplicação. A necessidade de estudos que considerem o material das fitas gotejadoras, com diferentes tipos de emissores integrados e diferentes características construtivas, são importantes para entender a sensibilidade desses materiais quando submetidos a temperaturas de água distintas. Este experimento foi conduzido no Laboratório de Ensaios de Material de Irrigação (LEMI) do Departamento de Engenharia de Biossistemas da Escola Superior de Agricultura \"Luiz de Queiroz\" - ESALQ/USP. Foram avaliados cinco tipos de fitas gotejadoras, com diferentes emissores integrados e espessuras de parede. As curvas vazão-pressão para os diferentes materiais e espessuras de parede, apresentaram a mesma tendência, porém, com valores distintos dos parâmetros K e x para cada temperatura. Os valores de CVf para todos os materiais nas diferentes temperaturas apresentaram valores em conformidade com o estipulado por norma técnica. Não houve uma tendência específica dos valores de CVf e de IQR em relação à temperatura da água para os materiais avaliados. Para os emissores planos de fluxo turbulento, a vazão tende a diminuir com o incremento da temperatura, porém não significativamente (p>0,05). Para os emissores contínuos de fluxo turbulento, respostas diferentes foram obtidas, sendo que no emissor SilverDrip&#174; a vazão aumentou com o incremento da temperatura (p<0,05), enquanto no emissor Turbo Tape&#174;, a vazão diminuiu e as maiores variações de vazão ocorreram a partir de 60 kPa (p<0,05). Para o emissor moldado, a vazão aumentou (p<0,05) em função da temperatura, porém a maior variação ocorreu nas pressões mais baixas. Para nenhum dos materiais houve diferença significativa (p>0,05) na variação de vazão entre as espessuras de parede, indicando para os emissores estudados, que a espessura de parede não influencia na sensibilidade do emissor às variações de temperatura. Os resultados obtidos indicam que a sensibilidade do emissor em função da temperatura da água está associada ao valor do expoente de fluxo do emissor. / Temperature variations influence the properties of water, especially viscosity. This can be a significant factor, which affects the emitters\' discharge and consequently the uniformity of application. Studies analyzing drip tape material, types of integrated emitters and manufacturing characteristics are important to understand the sensitivity of these materials when operated in a range of values of water temperature. This experiment was carry out at the Laboratory of Tests of Irrigation Material (LEMI) of the Department of Engineering of Biosystems of the \"Luiz de Queiroz\" School of Agriculture - ESALQ / USP. Five types of drip tapes were evaluated, with various integrated emitters and wall thicknesses. The pressure-flow curves presented the same trend for the evaluated material, however, different values of the parameters K and x were found for each temperature. The CVf for all materials evaluating in a range of temperatures presented values in accordance with technical standards. There was no specific trend of the values of CVf neither IQR in relation to the water temperature. For the turbulent flow emitters, the discharge tends to decrease with increasing temperature, but not significantly (p>0.05). For the continuous emitters of turbulent flow, different responses were obtained, the emitter SilverDrip&#174; the discharge increased with the increase of the temperature (p<0,05), while for the emitter Turbo Tape&#174;, the discharge decreased and the greater variations of flow occurred from 60 kPa (p<0.05). For the emitter molded, the discharge increased (p<0.05) as a function of temperature, but the greater variation occurred in the lower pressures. For any of the materials, there was a significant difference (p>0.05) in the discharge variation comparing the wall thicknesses, indicating for the emitters studied, the wall thickness does not influence the discharge sensitivity to temperature variations. The results indicate that the sensitivity of the emitter as a function of the water temperature is associated with the exponent of emitter flow.