Showing papers in "AIAA Journal in 2006"

Robust Design: An Overview

Abstract: Robust design has been developed with the expectation that an insensitive design can be obtained. That is, a product designed by robust design should be insensitive to external noises or tolerances. An insensitive design has more probability to obtain a target value, although there are uncertain noises. Theories of robust design have been developed by adopting the theories of other fields. Based on the theories, robust design can be classified into three methods: 1) the Taguchi method, 2) robust optimization, and 3) robust design with the axiomatic approach. Each method is reviewed and investigated. The methods are examined from a theoretical viewpoint and are discussed from an application viewpoint. The advantages and drawbacks of each method are discussed, and future directions for development are proposed.

Plasma Actuators for Separation Control of Low-Pressure Turbine Blades

Abstract: This work involves the documentation and control of flow separation that occurs over turbine blades in the low-pressure-turbine (LPT) stage at the low Reynolds numbers typical of high-altitude cruise. We utilize a specially constructed linear cascade that is designed to study the flowfield over a generic LPT cascade consisting of Pratt and Whitney Pak B shaped blades. The center blade in the cascade is instrumented to measure the surface-pressure coefficient distribution. Optical access allows laser-Doppler-velocimetry measurements for boundary-layer profiles. Experimental conditions were chosen to give a range of chord Reynolds numbers from 10 4 to 10 5 , and a range of freestream turbulence levels from u'/U∞ = 0.08 to 2.85%. The surface-pressure measurements were used to define a region of separation and reattachment that depends on the freestream conditions

Separation Control Using Plasma Actuators: Dynamic Stall Vortex Control on Oscillating Airfoil

Abstract: A plasma actuator was used to control leading-edge flow separation and dynamic stall vortex on a periodically oscillated NACA 0015 airfoil. The effectiveness of the actuator was documented through phase-conditioned surface pressure measurements and smoke flow visualization records. The airfoil was driven in a periodic cycle corresponding to α = 15 deg+10deg sinwt. The results presented here are for a reduced frequency of k = ωc/2U ∞ = 0.08. Three cases of control with the plasma actuator were investigated: open-loop control with steady plasma actuation, open-loop control with unsteady plasma actuation, and closed-loop control with steady plasma actuation. For closed-loop control, the actuator was activated in selected portions of the oscillatory cycle based on angle-of-attack feedback. All of the cases investigated exhibited an increase in cycle-integrated lift with improvements in the lift-cycle hysteresis. In two cases, the pitch-moment stall angle was delayed and in one of these, the adverse negative moment peak was significantly reduced.

Statistical Improvement Criteria for Use in Multiobjective Design Optimization

Abstract: Design of experiment and response surface modeling methods are applied to the problem of constructing Pareto fronts for computationally expensive multiobjective design optimization problems. The work presented combines design of experiment methods with kriging (Gaussian process) models to enable the parallel evolution of multiobjective Pareto sets. This is achieved via the use of updating schemes based on new extensions of the expected improvement criterion commonly applied in single-objective searches. The approaches described provide a statistically coherent means of solving expensive multiobjective design problems using single-objective search tools. They are compared to the use of nondominated sorting genetic algorithm (NSGA-ii) based multiobjective searches, both with and without response surface support. The new approaches are shown to give more exact, wider ranging, and more evenly populated Pareto fronts than the genetic algorithm based searches at reduced or similar cost.

Flight Data Analysis of the HyShot 2 Scramjet Flight Experiment

Abstract: The development of scramjet propulsion for alternative launch and payload delivery capabilities has been composed largely of ground experiments for the last 40 years. With the goal of validating the use of short duration ground test facilities, a ballistic reentry vehicle experiment called HyShot was devised to achieve supersonic combustion in flight above Mach 7.5. It consisted of a double wedge intake and two back-to-back constant area combustors; one supplied with hydrogen fuel at an equivalence ratio of 0.34 and the other unfueled. Of the two flights conducted, HyShot 1 failed to reach the desired altitude due to booster failure, whereas HyShot 2 successfully accomplished both the desired trajectory and satisfactory scramjet operation. Postflight data analysis of HyShot 2 confirmed the presence of supersonic combustion during the approximately 3 s test window at altitudes between 35 and 29 km. Reasonable correlation between flight and some preflight shock tunnel tests was observed.

Design and analysis of 'noisy' computer experiments

Abstract: of functions calculated by long running computer codes. The literature in this area commonly assumes that the objective function is a smooth, deterministic function of the inputs. Yet it is well known that many computer simulations,especiallythoseofcomputational fluidandstructuraldynamicscodes,oftendisplaywhatonemightcall numerical noise: rather than lying on a smooth curve, results appear to contain a random scatter about a smooth trend. This paper extends previous optimization methods based on the interpolating method ofkriging to the case of such noisy computer experiments. Firstly, we review how the kriging interpolation can be modified to filter out numerical noise. We then show how to adjust the estimate of the error in a kriging prediction so that previous approaches to optimization, such as the method of maximizing the expected improvement, continue to work effectively. We introduce the problems associated with noise and demonstrate our approach using computational fluid dynamics based problems.

Large-Eddy Simulation of Realistic Gas Turbine Combustors

Abstract: Large-eddy simulation is a promising technique for accurate prediction of reacting multiphase flows in practical gas-turbine combustion chambers involving complex physical phenomena of turbulent mixing and combustion dynamics. Development of advanced models for liquid fuel atomization, droplet evaporation, droplet deformation and drag, and turbulent combustion is discussed specifically for gas-turbine applications. The nondissipative, yet robust numerical scheme for arbitrary shaped unstructured grids developed by Mahesh et al. (Mahesh, K., Constantinescu, G., and Moin, P., "A New Time-Accurate Finite-Volume Fractional-Step Algorithm for Prediction of Turbulent Flows on Unstructured Hybrid Meshes," Journal of Computational Physics, Vol. 197, No. 1, 2004, pp. 215-240) is modified to account for density variations due to chemical reactions. A systematic validation and verification study of the individual spray models and the numerical scheme is performed in canonical and complex combustor geometries. Finally, a multiscale, multi physics, turbulent reacting flow simulation in a real gas-turbine combustor is performed to assess the predictive capability of the solver.

Wall Pressure and Shear Stress Spectra from Direct Simulations of Channel Flow

Abstract: Wall pressure and shear stress spectra from direct numerical simulations of turbulent plane channel flow are presented in this paper. Simulations have been carried out at a series of Reynolds numbers up to Re? = 1440, which corresponds to Re = 6:92 x 10(4) based on channel width and centerline velocity. Single-point and two-point statistics for velocity, pressure, and their derivatives have been collected, including velocity moments up to fourth order.§ The results have been used to study the Reynolds number dependence of wall pressure and shear stress spectra. It is found that the point spectrum of wall pressure collapses for Re? ? 360 under a mixed scaling for frequencies lower than the peak frequency of the frequency-weighted spectrum, and under viscous scaling for frequencies higher than the peak. Point spectra of wall shear stress components are found to collapse for Re? ? 360 under viscous scaling. The normalized mean square wall pressure increases linearly with the logarithm of Reynolds number. The rms wall shear stresses also increase with Reynolds number over the present range, but suggest some leveling off at high Reynolds number.

Supersonic Cavity Flows and Their Control

Abstract: A detailed experimental study of supersonic, Mach 2, flow over a three-dimensional cavity was conducted using shadowgraph visualization, unsteady surface pressure measurements, and particle image velocimetry. Large-scale structures in the cavity shear layer and visible disturbances inside the cavity were clearly observed. A large recirculation zone and high-speed reverse flow was revealed in the cavity. In addition, supersonic microjets were used at the leading edge to suppress flow unsteadiness within the cavity. With a minimal mass flux (blowing coefficient B c = 0.0015), the activation of microjets led to reductions of up to 20 dB in the amplitudes of cavity tones and of more than 9 dB in the overall sound pressure levels. The microjet injection also modified the cavity mixing layer and resulted in a significant reduction in the flow unsteadiness inside the cavity as revealed by the shadowgraphs and the velocity-field measurements.

Adaptive Closed-Loop Separation Control on a High-Lift Configuration Using Extremum Seeking

Abstract: We present experimental results on adaptive closed-loop separation control on a 2-D generic high-lift configuration. Because model-based closed-loop flow control suffers from the lack of sufficient simple physical models for this configuration, a non-model-based control strategy, namely, the gradient-based extremum-seeking scheme, is used here. The controller exploits spanwise distributed pressure measurements and adjusts pulsed jets near the leading edge of the single-slotted flap. The jets are used for flow excitation to suppress separation over the flap at high angles of attack, high deflection angles, or to reattach an already separated flow. Starting from a single-input/single-output design, the extremum-seeking scheme is extended to both a single-input/single-output slope-seeking approach and a multi-input/multi -output approach. Multi-input/multi -output control accounts for spanwise-distributed, small-scale separation phenomena and shows the best performance. Additionally, this case even improves lift gain compared to preliminary open-loop studies. A lift increase is not only observed for angles of attack for which the unactuated flow obviously separates, but as well for smaller angles, which were assumed before to lead to an unseparated flow. Hence, closed-loop results demonstrate the capability of slope-seeking control to adjust the control signal automatically in an energy-efficient sense such that separation is minimized even in the presence of disturbances.

Atmospheric winds and their implications for microair vehicles

Abstract: Major challenges to low speed microllight are the transient and time-averaged velocities arising from the atmospheric boundary layer, particularly turbulence a few meters above the ground. In this paper, prior work on the temporal and spatial characteristics of the atmospheric boundary layer, close to the ground, and the relative turbulence as perceived by a moving craft, are considered. New measurements are described that document the time-, averaged and transient velocities at a height of 4 in above the ground. These were made using a bank of four multihole pressure probes laterally separated by 150 and 50 mm on a mast above a test car. Transient How pitch angles were investigated and it was found that the overall variation with lateral separation decreased relatively slowly with reducing separation, but that both this and the pitch angle coherence may be described nondimensionally. As the slow decrease in pitch variation with lateral spacing implies that the roll inputs arising from vertical fluctuations would increase with reducing span, it is speculated that increasingly active and authoritative control systems are required.

ADjoint: An Approach for the Rapid Development of Discrete Adjoint Solvers

Abstract: An automatic differentiation tool is used to develop the adjoint code for a three-dimensional computational fluid dynamics solver. Rather than using automatic differentiation to differentiate the entire source code of the computational fluid dynamics solver, we have applied it selectively to produce code that computes the flux Jacobian matrix and the other partial derivatives that are necessary to compute total derivatives using an adjoint method. The resulting linear discrete adjoint system is then solved using the portable, extensible toolkit for scientific computation. This selective application of automatic differentiation is the central idea behind the automatic differentiation adjoint (ADjoint) approach. This approach has the advantage that it is applicable to arbitrary sets of governing equations and cost functions, and that it is exactly consistent with the gradients that would be computed by exact numerical differentiation of the original solver. Furthermore, the approach is largely automatic, thus avoiding the lengthy development times usually required to develop adjoint solvers for partial differential equations. These significant advantages come at the cost of increased memory requirements for the adjoint solver. Derivatives of drag and lift coefficients are validated, and the low computational cost and ease of implementation of the method are shown.

Filtered Density Function for Subgrid Scale Modeling of Turbulent Combustion

Abstract: : This research was concentrated primarily on developments and applications of the filtered density function (FDF) for subgrid scale (SGS) modeling of turbulent reacting flows. During the past three years, this work addressed: (1) development of the joint velocity-scalar filtered mass density function (VSFMDF), (2) development of the joint frequency-velocity-scalar filtered mass density function (FVS-FMDF), and (3) implementation of the scalar filtered mass density function (SFMDF) and VSFMDF for large eddy simulation of complex turbulent flames. This is a final report of our activities sponsored by AFOSR under Grant FA9550-06-1-0015.

Unsteady Plasma Actuators for Separation Control of Low-Pressure Turbine Blades

Abstract: This is a continuation of our work on the use of single dielectric barrier plasma actuators for controlling flow separation on turbine blades in the low-pressure turbine stage at low Reynolds numbers typical of high-altitude cruise. This used a linear cascade of Pratt & Whitney "PakB" shaped blades to provide generic low-pressure turbine conditions. The flow over one of the blades was documented through surface pressure, laser-Doppler velocimetry, and hot-wire measurements. These were used to define the location and size of the separated flow region on the suction side of the blade. Both steady and unsteady plasma actuators were implemented and found to be effective in separation control. For the unsteady actuators, there was an optimum excitation frequency to reattach the flow that corresponded to a Strouhal number, based on the length of the separated zone and the local freestream velocity, equal to unity. The unsteady actuator was more effective than the steady actuator in reattaching the flow while also requiring less power. It was suggested by the experimental results that the mechanism for the steady actuators was turbulence tripping, whereas the mechanism for the unsteady actuators was to generate a train of spanwise structures that promoted mixing.

Skin Friction and Heat Flux Measurements in Shock Boundary Layer Interaction Flows

Abstract: Modern nonintrusive techniques are used to make skin-friction and heat transfer measurements in two shockwave/turbulent boundary-layer interactions (SWTBLIs). The two-dimensional SWTBLI is generated by impingement of an incident oblique shock wave on a flat-plate boundary layer. The three-dimensional SWTBLI results from the interaction of the swept shock generated by a fin with a flat-plate boundary layer. The measurements are made using the global interferometry skin-friction technique for the skin friction and the quantitative infrared thermography technique for the heat transfer rate. The results show that, for the two- and three-dimensional interactions, there is a clear difference in the behavior of skin friction and heat transfer as the strength of the shock is changed. This observation suggests that the analogy between momentum and heat transfer, which is the basis of many simplified physical models, is not valid in SWTBLIs. These new data supplement the previous measurements that include boundary-layer properties, surface pressure distributions, and patterns of the limiting streamlines. Taken together, these data complete a data set that is suited for computational fluid dynamics validation.

Combustion enhancement via stabilized piecewise nonequilibrium gliding arc plasma discharge

Abstract: A new piecewise nonequilibrium gliding arc plasma discharge integrated with a counterflow flame burner was developed and validated to study the effect of a plasma discharge on the combustion enhancement of methane-air diffusion flames. The results showed that the new system provided a well-defined flame geometry for the understanding of the basic mechanism of the plasma-flame interaction. It was shown that with a plasma discharge of the airstream, up to a 220% increase in the extinction strain rate was possible at low-power inputs. The impacts of thermal and nonthermal mechanisms on the combustion enhancement was examined by direct comparison of measured temperature profiles via Rayleigh scattering thermometry and OH number density profiles via planar laser-induced fluorescence (calibrated with absorption) with detailed numerical simulations at elevated air temperatures and radical addition. It was shown that the predicted extinction limits and temperature and OH distributions of the diffusion flames, with only an increase in air temperature, agreed well with the experimental results. These results suggested that the effect of a stabilized piecewise nonequilibrium gliding arc plasma discharge of air at low air temperatures on a diffusion flame was dominated by thermal effects.

Experimental Investigation of Separation Control Part 1: Baseline and Steady Suction

Abstract: Low-speed flow separation over a wall-mounted hump, and its control using steady suction, were studied experimentally in order to generate a data set for the development and evaluation of computational methods. The baseline and controlled data sets comprised time-mean and unsteady surface pressure measurements, flowfield measurements using particle image velocimetry, and wall shear stress obtained via oil-film interferometry. In addition to the specific test cases studied, surface pressures for a wide variety of conditions were acquired for different Reynolds numbers and suction rates. Stereoscopic particle image velocimetry and oil-film flow visualization indicated that the baseline time-averaged separated flowfield was two-dimensional. With the application of control, mild three-dimensionality was evident in the spanwise variation of pressure recovery, reattachment location, and spanwise pressure fluctuations.

Parameterization of Temporal Structure in the Single Dielectric Barrier Aerodynamic Plasma Actuator

Abstract: We present the results of two measurements that describe the interaction of an aerodynamic plasma actuator (a dielectric barrier discharge plasma in which an asymmetric arrangement of electrodes leads to momentum coupling into neutral air) with the surrounding atmosphere. We show that the presence of oxygen in the Earth's atmosphere plays a substantial role in the efficiency of the actuator. We measure the time-resolved neutral air density in the vicinity of the actuator using a laser beam probe. We show that the effect of the actuator is to establish a region of increased neutral density in the vicinity of its exposed electrode's edge, and we show that the actuator couples directed momentum into the air by pulling air up this density gradient, against the corresponding pressure, and releasing it in the downstream direction when the plasma quenches, replacing it with air from the volume above the actuator. These measurements harmonize previously contradictory measurements of the actuator's behavior.

Unsteady and Transitional Flows Behind Roughness Elements

Abstract: The role of surface roughness in boundary layers continues to be a topic of significant interest, especially with regard to how controlled roughness might be used to delay laminar-to-turbulent transition. Although it may be useful for control, large-amplitude roughness may itself lead to transition. In an effort to understand the breakdown mechanics associated with large-amplitude surface roughness, experiments are conducted to investigate the steady and unsteady disturbances generated by three-dimensional roughness elements whose amplitudes are close to the critical roughness-based Reynolds number Re k for roughness-induced transition. Measurements are obtained in a flat-plate boundary layer downstream of a spanwise array of cylindrical roughness elements at both subcritical and supercritical values of Re k . The steady disturbance field has strong shear in the wall-normal and spanwise directions, and the unsteady streamwise velocities in the roughness elements' wake show evidence of hairpin vortices. The locations of maximum fluctuation intensity correspond to the locations of inflection points in the steady flow streamwise velocity, and this suggests that the fluctuations may result from a Kelvin-Helmholtz-type instability. Temporal power spectra indicate an unstable band of frequencies from 300 to 800 Hz. The Strouhal number associated with the 650-Hz fluctuations that are often observed to be the strongest give Sr = 0.15, a value that is in good agreement with previous findings. At supercritical Re k , rapid transition takes place when the unsteady disturbances reach nonlinear amplitudes. The disturbance growth rates indicate that in this situation transition can be understood as a competition between the unsteady disturbance growth and the rapid relaxation of the steady flow that tends to stabilize these disturbances.

Subgrid Modeling for Simulation of Spray Combustion in Large-Scale Combustors

Abstract: Simulations of spray combustion in full-scale combustors under different operating conditions are conducted using large-eddy simulations (LES). The current methodology attempts to capture not only spray-turbulence interactions but also subgrid fuel-air mixing and finite-rate kinetics occurring at scales below the LES resolution. Reduced finite-rate kinetics for n-heptane and kerosene fuels are used in these studies to predict pollutant emission. Comparison of LES predictions with measurements for a single-cup swirl combustor shows reasonably good agreement. Results for spray combustion in a realistic two-cup combustor sector show a complex vortex breakdown process that creates multiple recirculation regions in the combustor. These regions of recirculation provide multiple sites to stabilize the spray and the flame. Because of the shape of the combustor, significant three-dimensional effects are apparent with no similarity between flame structures, vortex breakdown bubbles, and outflow between the two cups. Spray combustion is quite efficient during full power operation because of the distributed injection process. It is also shown that in the current subgrid mixing and combustion approach flame stabilization is more physical, and the flame anchors downstream of the dump plane. In contrast, a conventional LES study using a subgrid eddy breakup model shows a flame anchored inside the inlet, Immediately downstream of the spray injector, which is unphysical.

Summary of the 2004 Computational Fluid Dynamics Validation Workshop on Synthetic Jets

Abstract: A computational-fluid-dynamics (CFD) validation workshop for synthetic jets and turbulent separation control (CFDVAL2004) was held in Williamsburg, Virginia, in March 2004. Three cases were investigated: a synthetic jet into quiescent air, a synthetic jet into a turbulent boundary-layer crossflow, and the flow over a hump model with no-flow-control, steady suction, and oscillatory control. This is a summary of the CFD results from the workshop. Although some detailed results are shown, the CFD state of the art for predicting these types of flows is mostly evaluated from a general point of view. Overall, for synthetic jets, CFD can only qualitatively predict the flow physics, but there is some uncertainty regarding how to best model the unsteady boundary conditions from the experiment consistently. As a result, there is wide variation among CFD results. For the hump flow, CFD is capable of predicting many of the particulars of this flow, provided that it accounts for tunnel blockage, but it consistently overpredicts the length of the separated region compared to the experimental results.

Avian Wing Geometry and Kinematics

Abstract: The avian wing geometry of a seagull, merganser, teal, and owl extracted from noncontact surface measurements using a three-dimensional laser scanner is presented. The geometrical quantities, including the camber line and thickness distribution of the airfoil, wing planform, chord distribution, and twist distribution, are given in convenient analytical expressions. The avian wing kinematics is recovered from videos of a level-flying seagull, crane, and goose based on a two-jointed arm model in which three characteristic angles are expressed in the Fourier series as a function of time. Therefore, the flapping avian wing with the correct kinematics can be computationally generated for the aerodynamic study of flapping flight.

Boundary Layer Control with Atmospheric Plasma Discharges

Abstract: Recent studies have investigated the use of plasma actuators for the active control of the boundary layer on turbine blades. Although the overall effects have been quantified through experiments, the exact nature of the plasma and its momentum-transfer mechanisms have not been well characterized. In this study, particle-in-cell and Monte Carlo methods are used to computationally explore the plasma discharge and its interaction with the flow. The plasma composition, its methods of momentum addition, and the physics of its generation are quantified. Comparisons with experiments are made in order to support the findings. Simulations indicate that the plasma is generated through an electron avalanche in a dielectric barrier discharge. The plasma is created as the electrons stream to the dielectric on the first half of the electrode bias cycle and stream back on the second half. Momentum is imparted to the flow on both half-cycles, but the ionization is not equal during both half-cycles. This results in the plasma actuator producing a net force in one direction.

Flux corrected finite volume scheme for preserving scalar boundedness in reacting large-eddy simulations

Abstract: Preserving scalar boundedness is an important prerequisite to performing large-eddy simulations of turbulent reacting flows. A number of popular combustion models use a conserved-scalar, mixture-fraction to parameterize reactions that, by definition, is bound between zero and one. To avoid unphysical clipping, the numerical scheme solving the conserved-scalar transport equation must preserve these bounds, while minimizing the amount of numerical diffusivity. To this end, a flux correction method is presented and applied to the quadratic-upwind biased interpolative convective scheme that ensures preservation of the scalar's physical bounds while retaining the low numerical diffusivity of the original quadratic-upwind biased interpolative convective scheme. It is demonstrated that this bounded quadratic-upwind biased interpolative convective scheme outperforms the third-order weighted essentially nonoscillatory scheme in maintaining spatial accuracy and reducing numerical dissipation errors both in generic test cases as well as direct numerical simulation of canonical flows.

Shell Buckling Design Criteria Based on Manufacturing Imperfection Signatures

Abstract: An analysis-based approach for developing shell-buckling design criteria for laminated-composite cylindrical shells that accurately account for the effects of initial geometric imperfections is presented. With this approach, measured initial geometric imperfection data from six graphite-epoxy shells are used to determine a manufacturing-process-specific imperfection signature for these shells. This imperfection signature is then used as input into nonlinear finite element analyses. The imperfection signature represents a first-approximation mean imperfection shape that is suitable for developing preliminary-design data. Comparisons of test data and analytical results obtained by using several different imperfection shapes are presented for selected shells. These shapes include the actual measured imperfection shape of the test specimens, a first-approximation mean imperfection shape, with and without plus or minus one standard deviation, and the linear-bifurcation-mode imperfection shape. In addition, buckling interaction curves for composite shells subjected to combined axial compression and torsion loading are presented that were obtained by using the various imperfection shapes in the analyses. A discussion of the nonlinear finite element analyses is also presented. Overall, the results indicate that the analysis-based approach presented for developing reliable preliminary-design criteria has the potential to provide improved, less conservative buckling-load estimates and to reduce the weight and cost of developing buckling-resistant shell structures.

Large-eddy simulation and acoustic analysis of a swirled staged turbulent combustor

Abstract: The analysis of self-excited combustion instabilies encountered in a laboratory-scale, swirl-stabilized combustion system is presented. The instability is successfully captured by reactive large-eddy simulation (LES) and analyzed by using a global acoustic energy equation. This energy equation shows how the source term due to combustion (equivalent to the Rayleigh criterion) is balanced by the acoustic fluxes at the boundaries when reaching the limit cycle. Additionally, an Helmholtz-equation solver including flame-acoustics interaction modeling is used to predict the stability characteristics of the system. Feeding the flame-transfer function from the LES into this solver allows to predict an amplification rate for each mode. The unstable mode encountered in the LES compares well with the mode of the highest amplification factor in the Helmholtz-equation solver, in terms of mode shape as well as in frequency.

Experimental Study of Supersonic Inlet Buzz

Abstract: An experimental study of supersonic inlet buzz is presented. This study was carried out on a mixed compression rectangular inlet model; tests were done at Mach numbers ranging from 1.8 to 3, with and without bleed. Inlet flows were analyzed thanks to Schlieren videos and signal processing of unsteady pressure recordings. Two kinds of buzz were observed: "little buzz," which is thought to correspond to an acoustic resonance phenomenon excited by the presence of a shear layer under the cowl lip (Ferri criterion), and "big buzz," which seems to be triggered by a boundary layer separation on the compression ramps (Dailey criterion). In some cases, little buzz could be virtually suppressed by the introduction of a bleed. The frequency of big buzz is shown to be already present in the flow before the onset of large-amplitude oscillations, which suggests that the underlying mechanism of big buzz, probably linked to acoustics, already exists before buzz starts.

Effect of Approximations of the Discrete Adjoint on Gradient-Based Optimization

Abstract: An exact discrete adjoint of an unstructured nite-volume solver for the RANS equations has been developed. The adjoint is exact in the sense of being based on the full linearization of all terms in the solver, including all turbulence model contributions. From this starting point various approximations to the adjoint are derived with the intention of simplifying the development and memory requirements of the method; considered are many approximations already seen in the literature. The eect of these approximations on the accuracy of the resulting design gradients, and the convergence and nal solution of optimizations is studied, as it applies to a two-dimensional high-lift conguration.

Temperature Measurements Using a Microoptical Sensor Based on Whispering Gallery Modes

Abstract: Temperature measurements were made using a novel microoptical sensor based on dielectric microspheres that are excited by coupling light from optical fibers. The technique exploits the morphology-dependent shifts in resonant frequencies that are commonly referred to as the whispering gallery modes. A change in the temperature of the microsphere leads to a change in both the size and the index of refraction of the sphere which results in a shift of the resonant frequency. By monitoring this shift, the temperature of the environment surrounding the sphere can be determined. The whispering gallery mode shifts are observed by scanning a tunable diode laser that is coupled into the optical fiber on one end and monitoring the transmission spectrum by a photodiode on the other. When the microsphere is in contact with a bare section of the fiber, the optical modes are observed as dips in the intensity of the light transmitted through the fiber. Temperature measurements were made in both air and water using this novel technique. Measurements by the microoptical sensor were compared to those by thermocouples with good agreement between the two sets of results.