Signie Cant Research Articles

Observations of Swirling Flows Behind Circular Disks

Thee owpatternsofaswirlingjetsubjecttotheine uencesofvariouscentralblockagesarestudiedexperimentally via the smoke-wire e ow visualization method. Details of the observed e ow structures are described and discussed. Thee owstructuresdisplay multiplemodes,depending on theblockageratio and swirlnumber. When theblockage ratio is less than about 0.1, swirling wake, vortex breakdown, and three-dimensional vortex shedding are observed for different ranges of swirl number. When the blockage ratio is greater than about 0.1, the bluff-body effect becomes signie cant. In this case, a pair of counter-rotating vortex rings and a central swirling jet characterize the e owe eld. Axisymmetric vortex breakdown and three-dimensional vortex shedding may appear on the central swirling jetwhen theswirlnumberorblockageratio islargerthan critical values.Somecharacteristicquantities of thee owe eld aremeasuredandcorrelated,includingthefreeseparationlines,recirculationlengths,andfrequencies of the vortex shedding.

Random Walk Models for Particle Diffusion in Free-Shear Flows

The main objectives were to establish and investigate discontinuous and continuous random walk models appropriate for free-shear e ows with regard to turbulent particle diffusion. The models were designed to capture the crossing trajectories effect, the continuity effect, and the inertial-limit effect, all for the case of heavy particles whose densities are much greater than that of the surrounding e uid. In addition, both techniques included an isotropic drift velocity to account for inhomogeneous turbulence. The computational efe ciency of the continuous random walk models is improved by utilizing local time stepping, which effectively e lters out high-frequency velocity e uctuations that do not have a signie cant ine uence on particle diffusion. The predictive performances of these two random walk models were examined through comparison with experimental data and idealized test conditions. The results indicate that both models agree well with experimental data for a nearly homogeneous turbulent wake and an inhomogeneous turbulent axisymmetric jet (although the continuous random walk model performs somewhat better for the inhomogeneous e ows ). It was also found that the proposed drift velocity models are important to ensure continuity when simulating particle diffusion with inhomogeneous turbulence.

Influence of Compressor Exit Conditions on Combustor Annular Diffusers Part II: Flow Redistribution

In gas-turbine engines the velocity of air, issuing from the compressor, must be reduced in order to permit effective operation of the downstream combustor. This is partly achieved by locating an annular diffuser behind the compressor outlet guide vanes (OGVs) and, in modern systems, the inlet of this diffuser is usually located at the trailing edge of the blade row. This paper is concerned with some of the interactions that occur between these components and, in particular, the e ow redistribution that occurs along the diffuser length due to the e ows generatedbytheOGVbladepassageandupstreamrotor.Amainlyexperimentalinvestigationhasbeenundertaken, on a fully annular facility, which incorporates a single-stage axial e ow compressor and simulated e ame tube. In addition, immediately downstream of the OGV row a constant-area passage, ordiffusers of area ratio 1.45 or 1.60, can be incorporated. The OGV blade row produces a proe le that, as a result mainly of the blade wakes, contains an excess of kinetic energy relative to that of a uniform proe le. The mixing out of these wakes therefore enhances the pressure rise within the downstream diffuser. Measured mean velocity data are used to determine the path of streamlinesalongeachdiffuserandindicateregionswherehigh-energye uidisbeingconvected,towardeachcasing, and low-energy boundary layere uidisbeing removed.Thisisbecauseoftheremnantsofthee owsgeneratedwithin each OGV passage. The mean momentum equation along each diffuser is then used to indicate that such e ows signie cantly offset the changes in momentum, within each boundary layer, that are associated with the applied pressure gradient. Such effects are therefore thought signie cant in terms of reducing the boundary-layer growth and delaying e ow separation from the casings.

Propulsive Performance of Hypersonic Oblique Detonation Wave and Shock-Induced Combustion Ramjets

A comparative study is presented of the propulsive characteristics of hypersonic detonation wave and shockinduced combustion ramjets. The same ramjet design methodology is used to assess the propulsive performance of both types of ramjets. The lower ‐upper symmetric Gauss ‐Seidel scheme combined with a symmetric shockcapturing total variation diminishing scheme are used to solve the Euler equations describing thetwo-dimensional hydrogen/air combustible e owe eld with nonequilibrium chemical reactions including 13 species (H2, O2, H, O, OH, H2O, HO2, H2O2, N, NO, HNO, N 2, and NO2). Results obtained for e ight Mach numbers 12< ‐ M1 < ‐ 16 and for a e ight dynamic pressure of 67,032 Pa (1400 psf) show that combustor entrance temperatures Tce (or inlet compression ratios ) substantially lower than the near ignition values of hydrogen/air mixture, adopted for the generation of a near Chapman ‐Jouguet oblique detonation wave in the combustor, entail signie cant performance augmentation. For the considered range of e ight Mach numbers and value of e ight dynamic pressure, the thrust of a shock-induced combustion ramjet is maximum for 650 <‐ Tce < ‐ 700 K, and at this point the combustion is entirely shock induced. The thrust generation can be enhanced by more than 10% and the fuel specie c impulses improved by more than one-third over their magnitudes corresponding to maximum thrust detonation wave ramjets.

Measurement-based aerosol forcing calculations: The influence of model complexity

On the basis of ground-based microphysical and chemical aerosol measurements a simple ‘ two-layer-singlewavelength’ and a complex ‘ multiple-layer-multiple-wavelength’ radiative transfer model are used to calculate the local solar radiative forcing of black carbon (BC) and (NH 4)2SO4 (ammonium sulfate) particles and mixtures (external and internal) of both materials. The focal points of our approach are (a) that the radiative forcing calculations are based on detailed aerosol measurements with special emphasis of particle absorption, and (b) the results of the radiative forcing calculations with two different types of models (with regards to model complexity) are compared using identical input data. The sensitivity of the radiative forcing due to key input parameters (type of particle mixture, particle growth due to humidity, surface albedo, solar zenith angle, boundary layer height) is investigated. It is shown that the model results for external particle mixtures (wet and dry) only slightly differ from those of the corresponding internal mixture. This conclusion is valid for the results of both model types and for both surface albedo scenarios considered (grass and snow). Furthermore, it is concluded that the results of the two model types approximately agree if it is assumed that the aerosol particles are composed of pure BC. As soon as a mainly scattering substance is included alone or in (internal or external) mixture with BC, the differences between the radiative forcings of both models become signie cant. This discrepancy results from neglecting multiple scattering effects in the simple radiative transfer model. Zusammenfassung

Dynamics of the Makassar Strait

Data collected as part of the Arlindo Project (“ Arlindo” is an acronym for Arus Lintas Indonesia, meaning “ throughe ow” in Bahasa Indonesia) from October 1996 to March 1998 are analyzed to study the characteristics of the e ow through the Makassar Strait. Analysis of inverted echo sounders (IES) and bottom pressure data (PIES) combined with TOPEX/POSEIDON satellite-derived sea height anomaly suggest that a minimum of three-layer approximation is necessary to explain the dynamics of the e ow in the Makassar Strait. The simple two-layer model used in several studies of the throughe ow is rejected based on total incompatibility with the data sets. A three-layer model with signie cant contributions by the middle layer provides a consistent interpretation of PIES and satellite data. Results are interpreted in the framework of the large-scale circulation.

Fuel Regression Rate in Hydroxyl-Terminated-Polybutadiene/Gaseous-Oxygen Hybrid Rocket Motors

The results of a systematic experimental investigation on the methods of enhancing the regression rate in hydroxyl-terminated polybutadiene (HTPB) fuel used in an HTPB/gaseous oxygen hybrid motor are presented. The effects of the addition of ammonium perchlorate (AP) or aluminum in the fuel, the variation of oxidizer ‐fuel ratio, and the variation of characteristic dimensions of fuel grain are presented. For the extents of the parametric variations considered, while the addition of AP and/or Al and the reduction of grain port diameter enhance the regression rate, the effect due to the latter is the most signie cant one. Furthermore, the regression rate increases along theaxis, and it becomes essentially constant in the port region corresponding to a fuel-rich composition. The possible physical processes for all of these behaviors are discussed. The experimentally obtained exponents of the variables for regression rate (oxidizer mass e ux and port diameter ) are found to be signie cantly different from those of the conventional theory. The similarity between the fuel regression rate equation used in solid-fuel ramjet and that obtained in hybrid motor is discussed.

Upstream Wake Influences on the Measured Performance of a Transonic Compressor Stage

The ine uence of an upstream wake on the performance of a downstream compressor stage with transonic inlet conditionsisstudied.Experimentalresultsfrome xedplanecompressorexitinstrumentationshowthatdeepwakes, representative of heavily loaded stator wakes, persist further downstream than anticipated. The ine uences of the upstreamwakesarealiasedintoatypicalstatorpitche eld,thusmagnifyingtheir“ true” ine uence.Resultsalsoshow that reducing the axial spacing between the upstream stator blade-row and downstream rotor blade-row reduced the overall performance suggesting that mechanisms other than wake recovery are present for this compressor. This change in performance was determined to be associated with the upstream stator wakes. The work presented herein shows that the stator/rotor interaction is signie cant and should be accounted for in the design, testing, and analysis of transonic axial compressors.

Estimation Methods for Two-Dimensional Conduction Effects of Shock-Shock Heat Fluxes

Highly localized heating rates occur in extreme thermal environments, such as shock ‐shock interaction at hypersonic speeds. Experimental estimation of heat e ux usually involves measurement of discrete unsteady surface temperatures on low conductivity materials that are chosen to reduce conduction effects. Typically, temperature measurementsarereducedtoheate uxesusingone-dimensionalconductiontechniques.However,lateralconduction from localized high heat e ux regions into low heat e ux regions is signie cant and ine uences the one-dimensional solution. The one-dimensional solution also suffers mathematical instabilities. To evaluate the nonuniform, unsteady surface e ux from measured temperatures, an inverse technique was devised that damps instabilities in the temporal direction and resolves large and sudden e ux changes in the spatial direction. Based on previous work, a simple inverse method was used in time, and a function specie cation method was used in space. Furthermore, a technique was devised to expedite the solution by marching in space as well as in time. The new multidimensional inversemethodwasfound toresolvesteep spatialgradientsmoreaccuratelyine uxthana one-dimensionalmethod. Furthermore, the inverse procedure exhibits better stability than a multidimensional forward technique.

Quantitative Effects of Projectile-Launch Tube Wall Friction on Ballistic Range Operation

Theoretical and experimental studies are conducted on the quantitative evaluation of a friction force at the interface between a projectile and the launch tube wall in a ballistic range. From a mechanical balance relation applied to the projectile, the friction force is expressed theoretically as a function of the net acceleration of the projectile. With this formulation, experimentally measured in-bore projectile velocity proe les obtained using a velocity interferometer system for any ree ector are well interpreted. If the initial projectile motion is signie cantly suppressed by the friction force while the burning rate of the propellant remains signie cant, the base pressure becomes much higher than that with a negligibly small friction force, thereby enhancing the net acceleration. However, if the projectile-tube e t is excessively high, the main function of the friction force is to decrease the muzzle velocity.

Unsteady Viscous Flow Causing Rotor-Stator Interaction in Turbines, Part 2: Simulation, Integrated Flowfield, and Interpretation

Numerical simulation has been performed to investigate the unsteady e ow physics associated with the nozzle ‐ rotorwakeinteractioninturbines.Acomparison between thepredictedand theexperimentalunsteadypressureon the blade surface presented by Lakshminarayana et al. (Lakshminarayana, B., Chernobrovkin, A., and Ristic, D., “ Unsteady Viscous Flow Causing Rotor-Stator Interaction in Turbines, Part 1: Data, Code Pressure,” Journal of Propulsion and Power , Vol. 16, No. 5, 2000, pp. 744 ‐750) shows good agreement. The predicted e owe eld also correlates well with the blade-to-blade laser Doppler velocimetry data. Results of the numerical prediction in conjunction with the experimental data have been used to gain a better understanding of the unsteady e ow physics. An assessment of the viscous and the inviscid contribution to the nozzle wake decay and the unsteady loss distribution in the rotor passage reveals the dominant effect of the viscous decay upstream of the leading edge. Inside the passage, the inviscid effects have a signie cant ine uence. The numerical solver has been able to predict mostof the features associated with theunsteady transition on a turbineblade. The predicted e ow at theoff-design conditionhasbeeninterpreted tounderstandthenatureoftheunsteadye owe eldata high negativeincidenceangle.

Unsteady Blade-Row Flow Calculations Using a Low-Reynolds-Number Turbulence Model

Acomputational study onunsteady compressiblee owshasbeenperformed by adoptinga low-Reynolds-number k‐!turbulencemodelinconjunctionwithdual-time-steppingandmultigridschemes.Anexplicitfour-stageRunge ‐ Kutta scheme for the Navier ‐Stokes equations and an approximate factorization scheme for the k‐! turbulence model equations are adopted for dual-time stepping. Computational results for unsteady shock-wave boundarylayer interaction in the case of shock buffeting phenomena around a bicircular-arc airfoil are in good agreement with extant experimental data, as well as those for blade surface-pressure distributions in the case of rotor-stator interaction in a turbine stage. Comparisons of the results obtained by the k‐! and algebraic turbulence models indicate that theeffect of turbulence models on the surface-pressure distributions is not so signie cant in the case of rotor-stator interaction as in the case of shock buffeting phenomena, which depend on the extent of the separated region involved. However, based on the results obtained by the k‐! model, the effects of the wake from the stator on the boundary-layer transition over the rotor blade surface can be explained by showing that high-intensity turbulence of the stator wake induces an early transition.

The Ballykissangelization of Ireland

It is ironic that a country which established its national identity largely through recourse to historical and mythical narratives has now, with equal fervour, adopted a policy of commodifying those narratives for the purposes of developing its second largest industry, tourism. It is a further irony that the major consumers of Irish heritage tourism are the former colonizers, the British. Irish culture e nds itself in a double bind, promoting itself through its past, whilst simultaneously denying much of what is signie cant in that past. This cultural imperative has given rise in turn to a specie c cycle of e lms made in and about Ireland, an Irish heritage cinema. A parallel discourse has emerged in television representations, typie ed by the highly successful series Ballykissangel. The regressive discourses of these heritage productions trade on a very specie c image of Irishness which has resonances for the way in which areas such as community, gender and history/the past are depicted. The evolution of the stereotype of the ‘Irish Paddy’ has already been widely discussed; this article focuses less on how that particular e gure has transferred into recent e lm and television works than on other aspects of comic representations of Irishness, in particular how themes of interdependency have been treated historically and in the present. This article emphasizes comedy and its centrality to the heritage cinema and related television programmes. The kind of pleasures offered by comedies have tended to obscure their value as cultural documents. For example, no serious academic work has been carried out on Ballykissangel or the earlier British comedies set in Ireland, which function simultaneously as post-colonial texts and as pleasurable outings for the vicarious heritage tourist. My emphasis is on how the texts circulate between English and Irish audiences and e lm makers and to speculate on what these heritage e lms can tell us about the complex relationship between former colonizer and formerly colonized. Before dee ning in greater detail what is meant by the term heritage cinema, I want briee y to trace its origins both in tourist narratives and earlier comic treatments of the themes of interdependency. Finally, I want to question to what extent we, the Irish, collude in this image-making process and for what reasons.

Modeling the Propagation of a Shock Wave Through a Glow Discharge

Several well-known analytical solutions of the equations of gasdynamics and magnetogasdynamics were used to study the relative importance of exothermic reactions, axial temperature variation, and the magnetic e eld in the glow discharge tube experiments of Ganguly et al. (Physics Letters A , Vol. 230, 1997, pp. 218 ‐222). In these experiments a spark was generated at one end of a tube of low-pressure argon gas, and the resulting shock pulse was allowed to propagate through a glow discharge. With the presence of the weakly ionized, nonequilibrium plasma, an acceleration and weakening of the shock pulse were observed, along with an apparent splitting of the shock. Of the three mechanisms addressed here thermal nonuniformity appears to have the most ine uence on the experimental results. A detonation model can probably be ruled out for two reasons. First, insufe cient energy is available from electron-ion recombination reactions to drive the detonation. Second, the detonation model predicts an increase in shock density ratio with increasing heat release, in contrast to the apparent drop seen in the experiments. In a similar manner an ideal magnetohydrodynamic shock model can probably be ruled out for lack of adequate electrical conductivity and of a sufe ciently strong magnetic e eld. This conclusion does not, however, exclude other electromagnetic phenomena, and the issue of the apparent shock splitting has not been addressed here. A combination of careful temperature measurements and numerical simulations is required to determine whether the experimental observations can be explained completely by thermal effects or physics inherent to the plasma are signie cant in these experiments.

Thermal Destruction of Cellulose and Surrogate Solid Wastes

The experimental and numerical studies presented here are on the thermal destruction behavior of cellulose and surrogate solid waste. An experimental study was conducted in a controlled mixing history reactor with plasma gas as the heating device. The effects of pyrolysis temperature, waste properties, residence time, and gaseous environment surrounding the waste have been examined. Gas-generation rate, chemical composition, and heating value of evolved gases as well as the solid residue remaining after the waste was exposed to prescribed environmentduringpyrolysishavealsobeen examined. Equilibrium thermochemical calculationswerecarriedout with STANJAN and SOLGASMIX computer codes to provide information on the thermal destruction behavior of samples of surrogate solid waste and its excursions at different temperatures. Calculated results show good trends with theexperimental data and can thereforebeused as a guideline to describe the experimentally observed results on the thermal destruction behavior of the wastes. The results show that the temperature and chemical composition surrounding the waste are important parameters for the pyrolysis process. These parameters affect destruction rate, solid residue remaining after pyrolysis, gas yield and its chemical properties, and pollutants and metals emission. The results also show a signie cant ine uence of waste properties and operational conditions on the wastedestruction characteristics andproductsformation.Celluloseand chosen surrogatewasteshowed signie cant differences in the thermal decomposition behavior and products formation. NVIRONMENTAL catastrophes resulting from improper treatment or disposal of different types of wastes have caused increased public awareness of the growing problem of waste generated in all sectors of public, industrial, and government. Waste minimizationand recycling canprovideonlyapartialsolutiontothe growing problem. The United States generates approximately 200 million tons of solid waste every year (» 4 lb/person/day ), and this amount is projected to increase at a rate of 1% annually. 1 Therefore stringent measures must be taken to provide a better and permanent solution to the problem. The thermal destruction of wastes can provide an increasingly important role in this area. This includes the applicationofthree fundamentalreacting processes:pyrolysis,gasie cation, and combustion. The existing destruction technologies that have been used include mass burn-type incineration, e uidized bed, rotary kiln, molten salt bed, low- or high-temperature oxygen/airenrichedsystems,andlow-orhigh-temperaturestarved-airsystems. More recently, electric heating, microwave, and plasma-assisted systems have also appeared. 2 Thermal destruction offers distinct advantages over the other methods as it provides maximum volume reduction, permanent disposal, and energy recovery, and the byproducts can be used in several ways, such as building material and roadbed construction. 3 For certain waste streams under certain conditions the byproduct material can be very hard. For example, the titanium and nitrates present in the waste material can form titanium nitrate at high temperatures, which is a very tough and strong material. This can be possible only with a controlled process so that the compound formed may be isolated fromthe other compoundsin the byproducts. Of all the permanent treatment technologies, thermal destruction provides the highest overall degree of destruction. In addition, it provides maximum volume and mass reduction and maximumenergy recovery and thebyproductscan benonleachable. The disposal of municipal solid wastes has traditionally been by means of lande lls (» 83% of the waste generated ) as the method is most convenient. Some of the gases released by this method, e.g., greenhouse gases, and volatile organic compounds are high and un

Space Environment Effects on Damping of Polymer Matrix Carbon Fiber Composites

Theeffectsof thelow-Earth-orbitenvironmentonthemechanical propertiesof somecarbon-reinforcedpolymer matrix composites are reported. Material damping loss factors and glass transition temperatures of control and e ightspecimensweremeasured. Flight specimenswereattached totheleading edgeoftheLongDurationExposure Facility, and control specimens were maintained at the NASA Marshall Space Flight Center. Changes in the glass transition temperatures and damping were due to combined exposure to 1 )atomic oxygen, 2 )UV radiation, and 3 ) thermal cycling. High atomic oxygen exposure during the e nal year of the Long Duration Exposure Facility e ight, however, eroded away much of the material that would have been affected by UV radiation. The glass transition temperatures of the e ight samples were slightly greater than those of the controls, whereas the material loss factors were nearly the same. From a practical point of view, these changes were not very signie cant. However, the relaxation data presented herein may beuseful forunderstandingthedetailsoftheeffectsofthespaceenvironment on composite materials and may aid researchers in developing composites that maintain their properties for extended periods in the harsh environment of low Earth orbit.

High Reynolds Number, Multielement Airfoil Flowfield Measurements

A unique set of high Reynolds number experimental results are presented that illustrate some of the crucial e ow physics issues controlling the aerodynamic performance characteristics of transport-aircraft-type high-lift geometries. Data include static-pressure distributions, lift and drag measurements, and boundary-layer and wake surveys. Results presented provide important insights into the underlying causes of differences in performance associated with various geometry modie cations. The two most signie cant and instructive examples involve the effects of a relatively small e ap gap change at a e xed e ap dee ection and the effects of an increase in e ap dee ection. For the former, decreasing the e ap gap eliminates an observed e ow separation on the e ap at (lower) approach angles of attack, but also results in a reduction in maximum lift caused by enhanced merging and spreading of the wakes above the e ap. For the latter, it is observed that the wake spreading is increased substantially at the higher e ap dee ection, thereby minimizing improvement in maximum lift. These data have been used for computational e uid dynamics (CFD) code calibration and should provide a means to identify the shortcomings of existing CFD methods, thereby identifying areas where improvements are required.

Characteristics of Transonic Rectangular Cavity Flows

Experiments are performed to study the characteristics of rectangular cavity e ows. Mean and e uctuating surfacepressurein a Mach-1.28 turbulente owpastrectangularcavitiesareobtained. Thecavity length-to-depth ratio L/D varies from 2.43 to 43.00, and the length-to-width ratio L/W is 0.5, 1.0, and 2.0. The effect of cavity depth is studied, in which L/W or both L/D and L/W are held constant. The results indicate that L/D is the most important parameter in determining the characteristics inside the cavity and in its vicinity. The effect of L/W is signie cant only at the aft half of the cavity. Lower static pressure aft of the front face and higher static pressure at the aft half of the cavity are observed with decreasing depth-to-incoming boundary-layer thickness D/±0 at constant L/W. At constantL/D and L/W, the results show an opposite trend. The amplitude of surfacepressure e uctuation decreases at larger L/D. The parameter L/W mainly affects the peak surface pressure e uctuation ahead of the rear face.

Optimizing Subcritical-Flow Thrust-Vectoring of Converging-Diverging Nozzles

Engine thrust vectoring (TV) is a potential technology for military and future civil aircraft in which the Technion— Israel Institute of Technology has made signie cant contributions. This paper provides realistic predictions of steady-state engine performance during steady-state pitch vectoring. The results obtained comprise a required fundamental step for advanced aircraft/TV implementation. This work is a part of the Lockheed Martin yaw‐pitch TV F-16/F-100 research study conducted here at the Jet Laboratory. To this end, a unique TV-engine computer algorithm has been developed that expands the conventional steady-state modeling capabilities of onand off-design as well as the conventional transients (via throttle changes ) to create steady-state and dynamic TV‐engine simulations at various altitudes and Mach numbers. This paper reviews the steady-state performances and the optimization observations initially obtained. The subcritical e ow realm of nozzle performance provides trends aiding in the prediction of thrust benee ts beyond the conventional nozzle design of the F100 model are available as the effective nozzle throat area is allowed to contract through vectoring.

Three-Dimensional Flow Analysis of Turbine Blade Cascades with Leading-Edge Ejection

The results of two different cooling cone gurations in a turbine cascade, one with slot ejection and one with shower-head ejection, are presented. Experimental and numerical results of the pressure distribution and the three-dimensional e owe eld are compared. Thenumerical results correspond well with the experimental data. The investigations are performed for e lm-cooled cases with a blowing ratio of M = 1.1. It can be shown that complete discretization of the cooling channels and the plenum is necessary to obtain good aerodynamic results of the main e ow because the complex e ow structure in the ejection channels and the plenum has a signie cant ine uence on the calculated aerodynamic behavior of the blades. In the leading-edge area, the interaction of the cooling e uid jets and the main e ow leads to mixture vortices and e ow separations. Furthermore, the ine uence of secondary e ow phenomena on the distribution of the cooling e uid along the blade surface is presented. On the suction side, the passage vortex leads to a displacement of cooling e uid in the direction of the midspan.