The effect of freestream turbulence on generation of crossflow disturbances over swept wings is investigated through direct numerical simulations.  The set up follows  the  experiments  performed  by Downs  et  al.  in their  TAMU  experi- ment.  In this experiment the authors use ASU(67)-0315 wing geometry which promotes  growth  of crossflow  disturbances.   Distributed  roughness  elements are locally placed near the leading edge with a span-wise wavenumber, to ex- cite the corresponding crossflow vortices.  The response of boundary layer to external disturbances such as roughness heights, span-wise wavenumbers, Rey- nolds numbers and freestream turbulence characteristics are studied.  It must be noted that the experiments were conducted at a very low level of freestream turbulence  intensity  (T u).   In this  study,  we fully  reproduce the  freestream isotropic homogenous turbulence through a DNS code using detailed freestream spectrum data provided by the experiment. The generated freestream fields are then applied as the inflow boundary condition for direct numerical simulation of the wing. The geometrical set up is the same as the experiment along with application of distributed roughness elements near the leading edge to precipi- tate stationary crossflow disturbances.  The effects of the generated freestream turbulence are then studied on the initial amplitudes and growth of the bound- ary layer perturbations.  It appears that the freestream turbulence damps out the dominant stationary crossflow vortices. / <p>QC 20130604</p>

Laminar flow control has been studied for several decades in an effort to achieve higher efficiencies for aircraft. Successful implementation of laminar flow control technology on transport aircraft could significantly reduce drag and increase operating efficiency and range. However, the crossflow instability present on swept-wing boundary layers has been a chief hurdle in the design of laminar wings. The use of spanwise-periodic discrete roughness elements (DREs) applied near the leading edge of a swept-wing typical of a transport aircraft represents a promising technique able to control crossflow and delay transition to accomplish the goal of increased laminar flow. Recently, the Flight Research Laboratory at Texas A&M University conducted an extensive flight test study using DREs on a swept-wing model at chord Reynolds numbers in the range of eight million. The results of this study indicated DREs were able to double the laminar flow on the model, pushing transition back to 60 percent chord. With the successful demonstration of DRE technology at these lower chord Reynolds numbers, the next logical step is to extend the technology to higher Reynolds numbers in the range of 15 to 20 million typical of smaller transport aircraft. To conduct the flight tests at the higher Reynolds numbers, DREs will be placed on a wing glove attached to the aircraft wing. However, a feasibility study was necessary before initiating the flight-testing. First, a suitable aircraft able to achieve the Reynolds numbers and accommodate a wing glove was identified. Next, a full CFD analysis of the aircraft was performed to determine any adverse effects on the wing flow-field from the aircraft engines. This required an accurate CAD model of the selected aircraft. Proper modeling techniques were needed to represent the effects of the aircraft engine. Once sufficient CFD results were obtained, they were used as guidance for the placement of the glove. The attainable chord Reynolds numbers based on the recommendations for the wing glove placement then determined if the selected aircraft was suitable for the flight-testing.

The jet-in-crossflow problem has been extensively studied, mainly because of its applications in film cooling and injector designs. It has been established that in low-speed flows, pulsing the jet significantly enhances mixing and jet penetration. This work investigates the effects of pulsing on mixing and jet trajectory in high speed (compressible) flow, using Large Eddy Simulation. Jets with different density ratios, velocity ratios and momentum ratios are pulsed from an injector into a crossflow. Density ratios used are 0.55 (CH4/air), 1.0 (air/air) and 1.5 (CO2/air). Results are compared with the low speed cases studied in the past and then analyzed for high speed scaling. The simulations show that the lower density jet develops faster than a higher density jet. This results in more jet spread for the lower density jet. Scaling for jet spread and the decay of centerline jet concentration for these cases are established, and variable density scaling law is developed and used to predict jet penetration in the far field. In most non-premixed combustor systems, the fuel and air being mixed are at different initial temperatures and densities. To account for these effects, heated jets at temperatures equal to 540K and 3000K have been run. It has been observed that, in addition to the lower density of heated jets, the higher kinematic viscosity effects the jet penetration. This effect has been included and validated in the scaling law for the heated jet trajectory.

During the past decade, new advanced combustion systems that share the same basic concept of using a substantially diluted and high-temperature oxidizer in the reaction volume have gained a great deal of interest regarding their application in industrial and power systems. These novel combustion technologies have proved to offer significant benefits compared to traditional combustion techniques. These benefits include reductions in pollutant emissions and energy consumption, as well as a higher and more uniformly distributed heat flux. This entails the potential to, for example, reduce the size of equipment in industrial units or increase production rates while fuel consumption and the subsequent CO2 emissions are decreased or maintained at the same level. Although the development of these new combustion technologies has occurred fairly recently, it has gained worldwide recognition. During the past few years the technique has been used commercially with several different types of burners. Despite its widespread use, the basic understanding of the chemical-physical phenomena involved is limited, and a better understanding of the combustion phenomena is required for more effective utilization of the technology. The objectives of this work have been to obtain fuel-jet characteristics in combustion under high-temperature, low-oxygen conditions and to develop some theoretical considerations of the phenomena. The effect of the preheat temperature of the combustion air, combustion stoichiometry and the fuel-jet calorific value on flame behavior was investigated. Temperature and heat-flux distribution were also studied using a semi-industrial test furnace to see if similar flame features would be found for the small- and large-scale experiments. Particle Image Velocimetry (PIV) was used for the first time to obtain information on the flow dynamics of a fuel jet injected into a crossflow of oxidizer at either a normal temperature or a very high temperature. Light emission spectroscopy was used to collect information on time-averaged radical distributions in the combustion jet. Jet turbulence, time-averaged velocity distribution, fuel-jet mixing, the distribution of radicals such as CH, OH and C2, and flame photographs were investigated. The results showed delayed mixing and combustion under high-temperature low-oxygen-concentration conditions. The combustion air preheat temperature and oxygen concentration were found to have a significant effect on the burning fuel-jet behavior. The results of the semi-industrial-scale tests also showed the features of even flame temperature and heat flux. / QC 20100610

Concerning management of smoke following an accidental fire within a building it is desirable to be able to estimate, within some understood, acceptable magnitude of error, the volume of smoke resulting from the combustion process of a predefined design fire scenario. Traditionally a range of first principle-based and empirically derived correlations are used to estimate the mass flow of smoke at a height of interest within the fire plume and are based upon the understanding that the mass flow of smoke at that height is a function only of the gravitational vector within the fire system, that is to say, that induced by the pressure differential between the naturally occurring hot plume gases and the surrounding quiescent bulk fluid. The statement that the fire plume is surrounded by a quiescent bulk fluid is in itself a significant simplification and is a key assumption required to facilitate the relative simplicity of the Froude-based entrainment correlations. It is of course quite intuitive to imagine that in real accidental fire scenarios in the built environment and across an array modern infrastructure, rarely does a fire exist submerged in a passive, quiescent atmosphere. This disconnect between the natural mechanics of the buoyant fire mechanism and the surrounding fluid in which it exists was necessary when the problem of entrainment by the fire plume was first described in the mainstream engineering literature around the middle of the twentieth century. Some 25 years later as ideal entrainment mechanics were beginning to be discussed specifically for application by a field of engineering in its infancy, a few researchers in the field of fire safety engineering published data that suggested that the addition of a relatively weak cross flow to the fire plume could have a significant impact upon the rate of air entrained by the plume, and by extension, the resultant smoke mass flow rate. The data published appeared more as a brief comment on an observation made during testing. It would be easily missed, nuzzled away in the middle of a lengthy doctoral thesis. Said thesis however happens to be one of the primary pieces of work that may be cited in reference to the formulation of perhaps the best known form of the axis-symmetric fire plume entrainment correlation, that of the so-called Zukoski correlation. It is perhaps curious then that the mention of a 3-fold increase in entrainment measurements following “small disturbances” in the atmosphere during the experimental work has seemingly been ignored by researchers, probably never-learned by students, and apparently forgotten by an industry. In a fire situation smoke can limit way-finding ability, severely irritate critical soft tissue like the eyes, trachea and oesophagus, impair cognitive function, contribute to significant property damage, facilitate the transfer of heat and carcinogens to locations remote to the fire source and it is well understood that most deaths due to fire are caused by asphyxiation following smoke inhalation. Significant portions of project budgets may be spent on designing, validating, installing and maintaining smoke management systems including the use of active systems such as extraction and pressurisation, passive curtains/reservoirs and detection such aspirating, video and beam detectors. Turbulent atmospheres may arise in any manner of situations such as modern buildings with large open spaces (airports, museums), hotel foyers and those with atriums spanning many floors, hangars and storage facilities/warehouses. Strong winds are normal on offshore oil platforms, outside the window on most floors of super-tall buildings or quite simply, anywhere on a blustery day. In specific cases the extraction systems designed to remove smoke and even normal HVAC systems can cause substantial air flow over large areas. In fact, a simple compartment with an uneven distribution of ventilation points (windows/doorways) has been shown to result in a directional fire flow that results in a significantly tilted flame, essentially inducing a cross flow scenario using the natural fire alone. With the coming-of-age of computational fluid dynamics models which are now a standard tool in all commercial fire engineering design offices, and probably in every smoke modelling report, it might be argued that there is little need to revisit the hand calculations from the ground up. Accepting, however, that a cross flow may increase the rate of entrainment of a fire plume and that this challenges the fundamental principles that all previous entrainment correlation knowledge is based on, and demonstrating the outcome (in terms of plume mass flow rate) with the use of a computational model, is an entirely different thing to understanding why this happens. Smoke management is one of the core design criteria, or questions at least, in practically all fire engineering design projects. In the literature there appears to be; no work quantitatively investigating cross flow fire plume entrainment rates; no work qualitatively describing the behaviour of the flame / fire plume under the influence of a cross flow (with respect to entrainment); and certainly no work framing this paradigm in the theoretical or practical context of the impact upon modern smoke control systems. This work aims to venture into these areas in the hope of beginning to piece together the overarching story of entrainment in the cross flow fire plume. The fundamental paradigm here is the addition of cross flow inertia (a horizontal pressure differential) to the axis-symmetric case where buoyancy (a zero initial momentum, vertical pressure differential) is the sole driver of the fluid flow system. How these flows then interact in a mixed convection sequence is investigated and described in terms that are useful for practical consideration by fire safety engineers. It is hoped that the concepts postulated and the questions raised will inspire further investigation into this poorly understood, but fundamental fire safety problem.

abstract: The atomization of a liquid jet by a high speed cross-flowing gas has many applications such as gas turbines and augmentors. The mechanisms by which the liquid jet initially breaks up, however, are not well understood. Experimental studies suggest the dependence of spray properties on operating conditions and nozzle geom- etry. Detailed numerical simulations can offer better understanding of the underlying physical mechanisms that lead to the breakup of the injected liquid jet. In this work, detailed numerical simulation results of turbulent liquid jets injected into turbulent gaseous cross flows for different density ratios is presented. A finite volume, balanced force fractional step flow solver to solve the Navier-Stokes equations is employed and coupled to a Refined Level Set Grid method to follow the phase interface. To enable the simulation of atomization of high density ratio fluids, we ensure discrete consistency between the solution of the conservative momentum equation and the level set based continuity equation by employing the Consistent Rescaled Momentum Transport (CRMT) method. The impact of different inflow jet boundary conditions on different jet properties including jet penetration is analyzed and results are compared to those obtained experimentally by Brown & McDonell(2006). In addition, instability analysis is performed to find the most dominant insta- bility mechanism that causes the liquid jet to breakup. Linear instability analysis is achieved using linear theories for Rayleigh-Taylor and Kelvin- Helmholtz instabilities and non-linear analysis is performed using our flow solver with different inflow jet boundary conditions. / Dissertation/Thesis / Ph.D. Mechanical Engineering 2013

<div>Experiments were performed on a sharp-nosed 7° half-angle cone at a 6° angle of attack in the Boeing/AFOSR Mach-6 Quiet Tunnel (BAM6QT) to study the stationary crossflow instability and its receptivity to small surface roughness. Heat transfer measurements were obtained using temperature sensitive paint (TSP) and Schmidt Boelter (SB) heat transfer gauges. Great care was taken to obtain repeatable, quantitative measurements from TSP.</div><div></div><div>Consecutive runs were performed at a 0° angle of attack, and the heat transfer measured by the SB was found to drop as the initial model temperature increased, while other initial conditions such as stagnation pressure were held constant. This agreed with calculations done using a similarity solution. It was found that repeatable measurements at a 6° angle of attack could be made if the initial model temperature was controlled and the patch location that was used to calibrate the TSP was picked in a reasonable and consistent manner.</div><div></div><div>The Rod Insertion Method (RIM) roughness, which was used to excite the stationary crossflow instability, was found to be responsible for the appearance of the streaks that were analyzed. The signal-to-noise ratio in the TSP was too low to properly measure the streaks directly downstream of the roughness insert. The heat transfer along the streak experienced linear growth, peaked, and then slightly decayed. It is possible this peak was saturation. The general trend was that the growth of the streaks moved farther upstream as the roughness element height increased, which agreed with past computations and low speed experiments. The growth of the streak also moved farther upstream as the freestream Reynolds number increased. The amplitude of the streaks was calculated by non-dimensionalizing the heat transfer using the laminar theoretical mean-flow solution for a 7° half-angle cone at a 6° angle of attack. The relationship between the amplitude and the non-dimensional roughness height was approximately linear in the growth region of the streaks.</div>

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<div>The atomization of jets and droplets undergoing breakup in high-speed flows has been experimentally measured and theoretically modeled. Systems for producing individual droplet breakup and full jet breakup were designed, and a wide range of diagnostics were developed and adapted to measure the results with reduced uncertainty.</div><div><br></div><div>A detailed methodology for investigating high-speed sprays in the Purdue Experimental Turbine Aerothermal Lab is presented. Optical diagnostic techniques were carefully selected and optimized for the test section geometries and flow features, such that images could be collected at high frequencies of 20 kHz with high resolutions. Developed image processing routines are outlined to demonstrate how backlit imaging with specialized lenses allowed for more accurate spray depth measurements in supersonic conditions, which were then used in regression modeling routines to derive empirical correlations that factored in test section geometry, flow conditions, and injector design. A Mie scattering imaging technique was used for quantitative analysis of the supersonic spray plume profile and measurement of the spray width. 20 kHz shadowgraphy provided sufficient gradients for analysis of the unsteadiness of the spray and surrounding supersonic flow at the point of injection. Droplet sizes and velocities were measured in subsonic conditions using digital in-line holography, in which recent advancements to the reconstruction algorithm were implemented to reduce out-of-plane measurement uncertainty, and phase Doppler particle analysis.</div><div><br></div><div>The breakup of a single drop undergoing multi-mode breakup was analytically characterized, with the proposal of a new breakup criterion in the Taylor analogy breakup model. Hill vortices within the drop were proposed as a new flow mechanism promoting multi-mode breakup. Product drop sizes from the ring breakup were predicted and compared with experimental results.</div>

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