Owe Eld Research Articles

Transonic Flow of Moist Air Around a Thin Airfoil with Equilibrium Condensation

The two-dimensional and steady transonic e ow of atmospheric moist air with equilibrium condensation around a thin airfoil is investigated. The study is based on an asymptotic analysis and numerical simulations. A smalldisturbance model is developed to explore the nonlinear interactions between the near-sonic speed of the e ow, the small thickness ratio and angle of attack of the airfoil, and the small amount of mass of water vapor in the air. The condensation process of water vapor in the air is assumed to be isentropic. The similarity parameters that govern the e ow problem are provided. The e owe eld may be described by a modie ed transonic small-disturbance (TSD)equation that includes parameters that are related to the condensation process. Murman and Cole’ smethod (Murman, E. M., and Cole, J. D., “ Calculation of Plane Study Transonic Flows,” AIAA Journal , Vol. 9, No. 1, 1971, pp. 114 ‐121.)is used for the numerical solution of the modie ed TSD problem. The results show that the e ow of moist air is similar to the e ow of dry air with an effective freestream Mach number that is greater than the freestream Mach number of moist air. The present approach is used to study the aerodynamic performances of airfoils in atmospheric transonic e ight with humidity.

Numerical Simulation of Three-Dimensional Augmented Burnett Equations for Hypersonic Flow

For computation of hypersonic e owe elds about space vehicles in low Earth orbits, where the local Knudsen numbers Kn lie in the continuum-transition regime, a set of extended three-dimensional hydrodynamic equations is required that is more accurate than the Navier ‐Stokes equations and computationally more efe cient than the direct simulation Monte Carlo (DSMC) computations. The three-dimensional augmented Burnett equations are derived from the Chapman ‐Enskog expansion of the Boltzmann equation to O(Kn 2) and adding the augmented terms (linear third-order super Burnett terms with coefe cients determined from linearized stability analysis to ensure stability of the augmented Burnett equations to small wavelength disturbances ). The three-dimensional augmentedBurnettequationsareappliedtocomputethehypersonicblunt-bodye owsforvariousrangeofKnudsen numbers (0:0884 < ‐ Kn < ‐ 0:227) and Mach numbers (10 < ‐ M < ‐ 25:3). The computational results are compared with the Navier ‐Stokes solutions, the existing augmented Burnett solutions, and the available DSMC results. The comparisons show that the difference between the Navier ‐Stokes and the augmented Burnett solutions is very small (less than 3% for the shock layer thickness ) at Knudsen numbers less than 0.01; the difference becomes signie cant as the Knudsen number increases. The comparisons also show that the augmented Burnett solutions are signie cantly closer to the DSMC results for the temperature distributions in the continuum-transition regime than the Navier ‐Stokes calculations.

Investigation of Hypersonic Compression Ramp Heating at High Angles of Attack

To clarify the relationship of the e ow structure and the heating characteristics of blunt-nose e at plates with a compression ramp, an experimental and analytical study of the hypersonic aerodynamic heating characteristics at high angles of attack has been conducted. The effect of nose bluntness and angle of attack on the heating pattern of the compression ramp has been explored, and the relationship between the ramp heating and the shock/shock interaction of the bow, separation, and compression corner shocks were examined. Triple-shock interaction plays an important role in determining the state of the e ow downstream of the bow/separation shock interaction point and the resulting ramp heat transfer distribution. The interaction around theramp may beidentie ed as variations ofan Edney typeI, V, orVI interaction (Edney, B., “ AnomalousHeat Transferand PressureDistributionson Blunt Bodies at Hypersonic Speeds in the Presence of an Impinging Shock,” Aeronautical Research Inst. of Sweden, FFA Rept. 115, Stockholm, Feb. 1968 ). Numerical simulation of the interacting e owe eld using two-dimensional laminar calculations showed discrepancy with an experimental heat transfer distribution for cases of large bluntnosee at platesathigh anglesofattack.Exploratory studiesoftwo-dimensionalandthree-dimensionalcalculations were conducted for fully laminar, fully turbulent, and transitional e ow conditions to examine the reason for this discrepancy. The three-dimensional calculation showed satisfactory agreement with the experimental shock shape and separation length.

Experimental Study of Major Species and Temperature Profiles of Liquid Oxygen/Gaseous Hydrogen Rocket Combustion

Understanding thefundamentalsofhigh-pressure, high-temperaturecombustion of practical rocketpropellants is key to improving rocket engine performance and efe ciency. The application of Raman spectroscopy for making detailed species concentration measurements of oxygen, hydrogen, and water vapor in the elevated-pressure, combusting e owe eld downstream of a liquid-oxygen/gaseous-hydrogen swirl-coaxial injector element at fuel-rich conditionsispresented.Detailsregardingmeasurementerroranalysisandrecommendationsforfurtherree nement ofthediagnostictechniquearealsopresented.To theauthors’ knowledge,theseexperimentsrepresentthee rsttime that instantaneous temperature and major species proe les have been successfully made in such an environment. Theseresults,whichincludebothinstantaneousand time-averagedtemperatureand speciesconcentrationproe les, impact both rocket fuel preburner design and computational e uid dynamic code validation activities. Nomenclature f .T/ = temperature dependent function that relates the spectral bandwidth strength to the signal strength = focal length =# = focal length divided by the diameter of the lens K = constant that accounts for the laser pulse energy, species Raman cross section, optical collection efe ciency, and optical solid angle N = measurement uncertainty n = number density S = Raman signal intensity aeB = background shot noise due to e ame luminosity aeD = dark current noise of the intensie er aeR = Readout noise aeS = signal shot noise due to Raman signal

Rotordynamic Effects of Compressor Rotor-Tip Clearance Asymmetry

Rotordynamic effects caused by nonaxisymmetric rotor-tip clearance in an axial compressor stage have been analyzed. Two coupled actuator discanalyses are carried out at blade (meridional plane )andradius(radial plane ) scales. Continuity, axial, and tangential momentum equations are used to connect upstream and downstream e ows across the compressor stage, and a small perturbation analysis is used to obtain e owe eld nonuniformities. Predictions indicate the following. Relative to the passage e ow, the e ow associated with the rotor-tip clearance is axially retarded and tangentially underturned. Furthermore, the compressor blades near the smaller tip gap are more highly loaded. Thus, the tangential force asymmetry promotes a backward whirl. However, the pressure asymmetry causedbyazimuthal e ow redistribution inducesa forwardwhirl. Thenetresultis abackward-whirling force whose magnitude is much smaller than that of the forward-whirling force found in turbines.

Cavity Flame-Holders for Ignition and Flame Stabilization in Scramjets: An Overview

This paper describes ongoing research efforts in the scramjet community on cavity e ame holders, a concept for e ame holding and stabilization in supersonic combustors. During the last few years, cavities have gained the attention of the scramjet community as a promising e ame-holding device, owing to results obtained in e ight tests and to feasibility demonstrations in laboratory-scale supersonic combustors. However, comprehensive studies are needed to determine the optimal cone guration that will yield the most effective e ame-holding capability with minimum losses. The e owe eld characteristics of cavities and research efforts related to cavities employed in lowand high-speed e ows are summarized. Open questions impacting the effectiveness of the cavities as e ame holders in supersonic combustors are discussed.

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.

Numerical Study of Cylindrical Blast-Wave Propagation and Reflection

The problem of an unsteady cylindrical blast-wave interaction with a e at plate is numerically investigated. The numerical simulation aims at the understanding of blast-wave propagation, ree ection, and its transition phenomenonaswellasthee owfeaturesinducedbytheblastwave.Ae fth-orderweightedessentiallynonoscillatory scheme for spatial discretization associated with a fourth-order Runge ‐Kutta method for time integration is employed for solving the two-dimensional Euler/Navier ‐Stokes equations in a e nite volume fashion. To verify the accuracy of the numerical solver developed, several problems were tested. The computed results for the test problems are shown to be accurate in comparison with experimental data. To study the e owe eld of a blast wave, theproblemofblast-wavepropagation in a freee eld isexplorede rst.Twotypesofinitial conditionsareconsidered. A contact-surface instability that developed around a contact surface was found in the problem with the e rst kind of initial condition. Next, the problem of the blast-wave interaction with the e at plate is investigated. The e ow structure of a shock-shock interaction induced by the unsteady blast wave and its evolution are studied in detail. It was found that the lower the height of burst or the higher the shock Mach number, the earlier the occurrence of the transition from a regular ree ection to a Mach ree ection and the higher the triple points.

Laser Doppler Anemometry Measurements in an Axial Compressor Stage

The aim is to provide unsteady experimental data of the e owe eld in a benchmark one-stage axial compressor, to improvetheunderstanding ofthee owfeatures.Theinvestigationisconductedwitha two-componentlaserDoppler anemometer, and 32 axial locations are surveyed, from the rotor inlet to the stator outlet. Because laser Doppler anemometry is a nonintrusive device, data are obtained from the rotor row, providing information in a region dife cult to explore with other techniques. Limitation of blade shadowing is addressed with an adjustment of the laser head orientation, which permits reaching any point inside the rotor. Furthermore, a dual probe orientation gives two sets of measurements yielding three velocity components. The dynamics of the rotor tip clearance vortex is discussed in the interrow gap. There is a strong interaction between this vortex and the rotor bladewake. Phaseaveraged results show the unsteady e owe eld and the rotor ‐stator interaction. The rotor wakes are cut up by the stator row and are visible until the exit of the axial zone of measurements. Their interaction with the stator blades boundary layers creates strong time-dependent e ow structures resulting in phase lags between the rotor wakes at the stator outlet.

Numerical Prediction of Interference Drag of Strut-Surface Intersection in Transonic Flow

The numerical evaluation of the interference drag produced by a streamlined strut intersecting a e at wall in transonic e ow is presented. The calculations were performed on unstructured grids using the FUN2D and FUN3D codes and the Spalart ‐Allmaras turbulence model (Spalart, P. R., and Allmaras, S. R.,“ A One-Equation Turbulence Model for Aerodynamic Flows,” AIAA Paper 92-0439, Jan. 1992 ). The grids were generated with AFLR2 and VGRIDns. The impact of the thickness-to-chord ratio of the strut, theReynoldsnumber, and theeffect of the dihedral angle made by the strut with the wall are studied for both inviscid and viscous e ows. To better understand the e ow in the region where the strut and the wall intersect, the e owe eld is analyzed in a detailed fashion for the thicker strut with pressure contours and stream traces. When the thickness-to-chord ratio of the strutisreduced, the e owe eld is disturbed only locally at theintersection of thestrut with thewall. Theinterference drag is calculated as the drag increment of the arrangement compared to an equivalent two-dimensional strut of the same cross section. The results show a rapid increase of the interference drag as the angle of the strut deviates from a position perpendicular to the wall. Separation regions appear for low intersection angles, but the viscosity generally provides a positive effect in alleviating the strength of the shock near the junction and, thus, the drag penalty. This study provides an equation to estimate the interference drag of simpleintersections in transonic e ow.

A Study of Rotor Tip Vortex Structure Alteration Techniques

Numerical studies of the tip-vortex structure from a hovering rotor with blowing are presented, and compared with the tip-vortex structure from a clean rotor, and from a rotorusing a passive trailing-edgetip device. A hybrid, Navier‐Stokes potential e ow method is used to model the e owe eld. A scheme that is e fth-order accurate in space is used to accurately capture the tip vortices. Velocity and vorticity data in the core of the vortex are studied at various planes behind the blade trailing edge. These data for the clean rotor are e rst compared with experimental results obtained for the same rotor. Previously published results for the tip-vortex structure from the same rotor employing a passive tip device are discussed next. Finally, results for a rotor blade with upper and lower surface blowing are presented. It is concluded that the tip-vortex strength may be modie ed through blowing. Blowing is found to be just as effective as a spoiler in altering the tip-vortex strength, but does not have the high drag and torque penalty associated with spoilers.

Flow Control: The Future

The subject of e ow control, particularly reactive e ow control, is broadly introduced, leaving some of the details to other papers in this special volume of the Journal of Aircraft . The ability to manipulate a e owe eld actively or passively to effect a desired change is of immense technological importance. In general, methods of control to achieve transition delay, separation postponement, lift enhancement, drag reduction, turbulence augmentation, and noise suppression are sought for both wall-bounded and free-shear e ows. An attempt is made to present a unie ed view of the means by which different methods of control achieve a variety of end results. The important advances in the e eld of e ow control that took place during the past few years are discussed. Spurred by the recent developments in chaos control, microfabrication and neural networks, reactive control of turbulent e ows is now in the realm of the possible for future practical devices. HEability to manipulate a e owe eld actively or passively to effect a desired change is of immense technological importance, and this undoubtedly accounts for the subject being more hotly pursuedbyscientistsandengineersthananyothertopicine uidmechanics.The potential benee ts of realizingefe cient e ow-controlsystems range from saving billions of dollars in annual fuel costs for land, air,and seavehiclesto achieving economically andenvironmentally more competitive industrial processes involving e uid e ows. Methodsofcontroltoeffecttransitiondelay,separationpostponement,lift enhancement, drag reduction, turbulence augmentation, and noise suppression are considered. Prandtl 1 pioneered the modern use of e ow control in his epoch-making presentation to the Third International Congress of Mathematicians held at Heidelberg, Germany. In just eight pages, Prandtl introduced the boundary-layer theory, explained the mechanics of steady separation, opened the way for understanding the motion of real e uids, and described several experiments in which the boundary layer was controlled. He used active control of the boundary layer to show the great ine uence such control can exert on the e owpattern. Specie cally, Prandtl used suction to delay boundary-layer separation from the surface of a cylinder. NotwithstandingPrandtl’ s 1 success,aircraftdesignersinthethree decades following his convincing demonstration were accepting lift anddragofairfoilsaspredestinedcharacteristicswithwhichnoman could or should tamper. 2 This predicament changed mostly due to the German research in boundary-layer control pursued vigorously shortly before and during World War II. In the two decades following the war, extensive research on laminar e ow control, where the boundary layer formed along the external surfaces of an aircraft is kept in the low-drag laminar state, was conducted in Europe and the UnitedStates,culminatinginthesuccessfule ighttestprogramofthe X‐21,wheresuctionwasusedtodelaytransitiononasweptwingup to a chord Reynolds number of 4 :7£10 7 . The oil crisis of the early 1970s brought renewed interest in novel methods of e ow control to reduce skin-friction drag even in turbulent boundary layers. In the 1990s, the need to reduce the emissions of greenhouse gases and to constructsupermaneuverablee ghterplanes,faster/quieterunderwater vehicles, and hypersonic transport aircraft, for example, the U.S. National Aerospace Plane, provides new challenges for researchers in the e eld of e ow control.

Computational Analysis of Centrifugal Compressor Surge Control Using Air Injection

A Navier-Stokes solver for simulating unsteady viscous e uid e ow in turbomachinery components has been developed and used to study e uid dynamic phenomena that lead to instabilities in centrifugal compressors. These studies indicate that large e ow incidence angles, at reduced e ow rates, can cause boundary-layer separation near the blade leading edge. High-pressure jets upstream of the compressor face are studied as a means of controlling compressor instabilities. Steady jets are found to alter the leading-edge e ow pattern and effectively suppress compressorinstabilities.Itisalsoobservedthatyawedjetsaremoreeffectivethanparalleljetsandthatanoptimum yaw angle exists for each compression system. Pulsed jets can yield additional performance enhancements and lead to a reduction in external air requirements for operating the jets. Jets pulsed at higher frequencies perform better than low-frequency jets. HE stable operating regime of current generation centrifugal compressors is limited at reduced mass e ow rates by two e uid dynamic phenomena: rotating stall and surge. The occurrence of these phenomena dee nes the surge limit, also referred to as stall line. When operating a compression system under these conditions, stronge uctuationsandlimitcycleoscillationscancauselargevibra- tions, leading to fatigue and damage of the entire compression unit. Unsteady e uctuations, caused by rotating stall or surge, may lead to excessive heating of the impeller blades and can produce additional periodicloadson theimpeller,resultinginincreasedoperating noise levels. If the e uctuations are left unchecked, even a complete e ow re- versal is possible, a situation that a compressor cannot tolerate. Because of the severity of these hazardous conditions, centrifugal compressors are conservatively designed to operate well below the peak pressure rise point. A safety margin of 10 -20% is generally introduced between the surge line and the design operating condi- tion. Most compressor control systems currently used in industry are based on this strategy, called avoidance control. Signie cant progress has been made in the past several years to develop reliable compressor control schemes. These schemes em- ploy fast-response-stall-detection devices, in most cases circumfer- entially distributed over thecasing walls thatmeasure e owe eld data and send e ltered signals to a controller unit. When a compressor approaches a stall-like condition, the presence of growing insta- bilities, sometimes called stall precursor waves, is measured and signaled back to the controller unit before the actual stall process begins. The control mechanism is then activated by a direct link be- tween the controller and a set of high-bandwidth actuation devices. The type of control system is often categorized by the choice of a particular actuator.

Predictions of Ablating Hypersonic Vehicles Using an Iterative Coupled Fluid/Thermal Approach

A new iterative approach for the prediction of the ablation of hypersonic vehicles is presented. The solution technique is achieved by an iterative coupling of a computational e uid dynamics code and a material thermal response code through mass and energy balances at a common interface. The iterative approach proved necessary due to severe numerical instabilities that resulted when an explicit technique was employed involving a loose coupling approach between the two codes. The results presented are for an axisymmetric carbon ‐carbon nosetip on a reentry vehicle e ying a ballistic trajectory. Details of the computational technique and the computed e owe eld and materialthermalresponsearepresented.Comparisonsweremadewith thewidelyusedABRES ShapeChange Code (ASCC), and favorable results were obtained.

Computation of Rarefied Gas Flows Around a NACA 0012 Airfoil

Raree ed gas e ows around a NACA 0012 airfoil are simulated using both particle and continuum approaches. Three different conditions are considered: supersonic, transonic, and low subsonic. In all three cases, the continuum approach solves the Navier ‐Stokes equations with a slip boundary condition on the airfoil surface. For the supersonic and transonic cases, the particle method employed is the direct simulation Monte Carlo method. Because of problems with this method at the low subsonic condition, caused by excessive statistical e uctuations, a new particle method called the information preservation technique is applied. The computed density and velocity e owe elds are compared with experimental data and found to be in generally good agreement. Some interesting features in the surface pressure distributions along the airfoil are found for these low-Reynolds-number e ows.

Temperature- and Pressure-Sensitive Paint Measurements in Short-Duration Hypersonic Flow

Surface temperatures and pressures were measured on an elliptic cone lifting body in a hypersonic e owe eld using thin-e lm (» 5πm) temperature- and pressure-sensitive paints (TSPs and PSPs ). The tests were conducted in the 48-inch hypersonic shock tunnel (48-inch HST) at Calspan‐University of Buffalo Research Center and were part of a more comprehensive experimental study examining the three-dimensional characteristics of laminar, transitional, and turbulent e ow over the model. Measurement opportunity in the 48-inch HST was limited by the short duration of steady freestream conditions of the driven gas; image acquisition times were » 3 ms. Images of the coatings applied to the broad side of the symmetric elliptic cone were calibrated with in situ static pressure and surface-e lm temperature measurements. The TSP results illustrate the higher heat transfer rates and change in boundary-layer transition over the model surface caused by the nose geometry, and the PSP results show a mild pressure gradient over the interrogated surface region. Submillisecond TSP acquisition using a high-speed imager demonstrated the feasibility of measuring the surface temperature rise.

Euler Solutions for Airfoils in Inhomogeneous Atmospheric Flows

Inhomogeneous e ows occuring naturally in the atmosphere and their ine uence on the e owe eld around wing proe les are examined. Inhomogeneous, in this context, means that at least one e ow variable has a spatial gradient, and, in particular, a gradient in the velocity component normal to the velocity vector is considered. Well-known examples for such e ows are shear winds, jet streams, low-level jet streams, the e ow situation near the ground, and microbursts. Numerical simulations areperformed fora NACA 0012 airfoil using a e nite volume Euler code. They focus on two cases,namely,an idealized shearwind forparameterstudies anda microburst.For theidealized shear wind, a linear velocity change is assumed. In the subsonic case, the additional velocities on the lower and upper side of the airfoil result in a positive additional lift and a negative pitching moment around the 25% axis. In the transonic regime, the effect on shock strength and shock position is dominant. For simulation of the e ight through a microburst, a potential model for the velocity e eld, consisting of a vortex ring parallel to the ground and a vortex ring of same strength mirrored at the ground, is applied. The chosen parameters simulate the Dallas ‐Fort Worth microburst. The analysis shows that thecharacteristics of the lift and moment coefe cient follow that of the vertical velocity component, induced by the microburst. Rapid changes in the pitching moment with severe consequences on longitudinal stability occur.

Vorticity Dynamics in Isobarically Closed Porous Channels Part I: Standard Perturbations

When acoustic pressure oscillations are induced in a porous channel of the closed ‐open type, the linearized Navier‐Stokes equations can be solved analytically to obtain an accurate description of the temporal e owe eld corresponding to laminar conditions. The channel considered herehas a rectangularcross section, has two equally permeablewalls,andisopenatthedownstreamend.Thecurrentmethodologyparallelstheclosed ‐closedboundary analysis carried out previously for a channel with both ends closed. Limiting our scope to laminar conditions, we apply standard perturbation tools to present a closed-form solution that becomes asymptotically exact for large kinetic Reynolds numbers. Verie cations are materialized by way of numerical simulations and quantie cation of the maximum absolute error entailed in the e nal formulations. Note that the error is found to exhibit a clear asymptotic behavior. Furthermore, the analytical formulation reveals vortical structures and explains the direct ine uence of acoustic pressure oscillations on the rotational waves generated in the closed ‐open cone guration. Finally, the explicit roles of variable injection, viscosity, and oscillation frequency are explained.

Scaling Nonequilibrium-Reacting Flows: The Legacy of Gerhard Damko-yen;hler

Damks personal background is e rst outlined, followed by an examination of his early pioneering work in the 1930s on reacting e ows, which led to the basic nondimensional similitude parameters that now bear his name. We then survey the role that these parameters have played in dealing with e nite reaction-rate effects in acoustics, combustion, and the aerothermodynamics of hypervelocity e ight vehicles and their propulsion systems. Emphasis is placed on the aerospace sciences and the value of the Damk ¨ ohler number not only in understanding and correlating experimental data, but also in conducting efe cient numerical studies and analyses of reacting-gas e owe elds.

Lateral-Directional Coupling and Unsteady Aerodynamic Effects of Hypersonic Vehicles

Results from a joint research program of the Institutes of Fluid Dynamics and of Flight Mechanics and Flight Control of the Technical University of Munich on a two-stage space transportation system are presented. The unsteady aerodynamics resulting from an orbital stage at stage separation e ow conditions at Mach number 6.8 are e rst discussed. Unsteady e owe elds associated with yaw and roll oscillations of the orbital stage at a certain distanceabovethecarrierstageareinvestigated.Thecalculationsarebased ona e nitevolumemethodforreal-time solutions of the unsteady Euler equations. Theresultsfocus on pressuredistributions and aerodynamic coefe cients providing an essentialmeansforcarrying outstabilityand control investigations.Inthesecond part, e ightmechanics stability and control problems are addressed considering key issues of lateral ‐directional dynamics. Inherent vehicle characteristics can show specie c stability and e ying quality dee ciencies in hypersonic e ight, concerning a partially unstable dutch roll mode with high roll ‐yaw coupling and weak roll damping. There is also a coupling of the roll and spiral poles for certain cone gurations, yielding a slow, eventually unstable oscillation called lateral phugoid. Reasons and effects for the stability dee ciencies are discussed, including conditions of existence for the lateral phugoid.