Ine Ow Research Articles

European Unemployment and Turbulence Revisited in a Matching Model

We recalibrate den Haan, Haefke, and Ramey’ s matching model to incorporate our preferred specie cation of “ turbulence” as causing distinct dynamics of human capital after voluntary and involuntary job losses. Under our calibration, with high unemployment benee ts, an increase in turbulence increases the unemployment rate and the duration of unemployment while leaving the ine ow rate into unemployment roughly unchanged, mirroring features of European data in the 1980s and 1990s. The essential issue is that den Haan, Haefke, and Ramey specify that in turbulent times workers experiencing layoffs and quits are both subject to instantaneous skill losses, while we restrict instantaneous skill losses to laid-off workers. (JEL: E24, J64)

Rotor-Wake Modeling for Simulation of Helicopter Flight Mechanics in Autorotation

Rotorcrafte ightmechanicssimulationstypicallyusee nite-stateinducedvelocitymodelstomimictheeffectofthe rotorwakeon bladeloads. However, with recent developmentsin computational methods coupled e ight mechanics and comprehensive rotor-wake analyses have become more tractable. This paper compares results from a e nitestate induced velocity model with a vorticity transport wakemodel that computes thee owe eld in a domain around the aircraft. The study focuses on e ight mechanics in autorotation. Simple parameter estimation using three- and e ve-state induced velocity model structures facilitates interpretation of the vorticity transport induced velocity e eld as well as providing a direct comparison with the e nite-state model. Comparisons of performance, trim, and control response indicate that there is little difference between the two models where the autorotative descent is shallow. However, as airspeed reduces the descent angle steepens, and clear discrepancies become apparent. These are directly related to a single parameter in the e nite-state model. It is concluded that e nite-state models are adequate for much of the autorotation regime, but for low speeds in steep descents the improved resolution of the rotor ine ow provided by more comprehensive rotor-wake models might be essential.

The diurnal circulation of Zugspitzplatt: observations and modeling

Zugspitzplatt is a gently sloping plateau in the Wetterstein massif with a steep eastward oriented slope which leads down to the narrow and deep Reintal. Such combinations of plateaus and valleys are ubiquitous in the Alps. A e eld campaign has been conducted and a surface station has been installed to observe the diurnal circulation of this plateau. Nocturnal oute ow as well as an ine ow of a few hundred meters depth during the day are found at the eastern edge of the plateau on fair weather days. A rather extreme decrease of the dew point tends to occur during oute ow late in the night. This indicates that air from relatively large heights descends down to the plateau to e ow over the edge. Simulations with a mesoscale model have been performed for the days of the e eld campaign. The model is capable of simulating successfully the trajectories of the pilot balloons released near the edge. The model calculations suggest that the ine ow towards the plateau during the day ascends from Reintal and includes air from outside the Wetterstein massif. This ine ow normally reaches the western end of Zugspitzplatt. Zusammenfassung

Hybrid Reynolds-Averaged Navier-Stokes/Large-Eddy Simulations of Supersonic Turbulent Mixing

Aturbulentmixinglayerformedfromtwosupersonicstreamsinitially separatedbyasplitterplateisinvestigated withahybridcomputationalmethodusingaReynolds-averagedNavier ‐Stokes(RANS)approachforwall-bounded regions and a large-eddy simulation (LES) approach for the turbulent mixing region. Simulations of the mixing layer predict a vortex shedding originating from the splitter base and then a rapid transition to turbulence, which is in agreement with experimental observations. As a result, the potential of the hybrid method is demonstrated for e ows in which a geometric feature, such as the blunt splitter plate considered here, provides the dominant unsteady feature leading to turbulence, without imposing additional ine ow disturbances. Parametric studies of LES subgrid modeling settings, variations in the spanwise computational domain, and wall temperature settings in the RANS region were conducted. The subgrid model cases, using a baseline computational grid with small spanwise computational domain, overpredicted the mixing layer turbulencelevels but only showed small variation in the mixing layer predictions with large changes in the model parameters. The cases examining wider spanwise domains enabled more turbulent energy to be released in the spanwise direction, which in turn reduced axial and vertical turbulence levels. Finally, prescribing the wall temperatures in the RANS regions instead of using the more traditional adiabatic wall boundary conditions further reduced turbulence levels and enabled reasonable agreement with experimental data.

Prediction of Rod-Airfoil Interaction Noise Using the Ffowcs-Williams-Hawkings Analogy

SoundgeneratedatlowMachnumberbyanairfoilin thewakeofarodisinvestigatednumerically.TheGaussian spanwise loss of coherence of the vortex shedding is shown to have a signie cant ine uence on the broadband noise. Spanwise effects are successfully introduced into a time-domain formulation of the Ffowcs-Williams ‐Hawkings analogy, which is applied to aerodynamicdata computed on various contoursaround the sourceregion. Itisshown that a careful choice of these contours is required. The e owe eld is obtained from a two-dimensional Reynolds averaged Navier ‐Stokes calculation. Computed far-e eld spectra compare very well to measurements obtained in an accompanying experiment. Nomenclature Cp = pressure coefe cient c = airfoil chord d = rod diameter f = frequency g = integration surface, 0 k = turbulent kinetic energy Lg = Gaussian correlation length l = span length Mi = Mach number of g D0 Moi = observer Mach number vector Mref = reference Mach number M1 = ine ow Mach number

Thermodynamic Limits of Work and Pressure Gain in Combustion and Evaporation Processes

Combustion and evaporation processes occurring in a closed chamber can result in signie cant pressurerise and direct work transfer. The pressure and volumetric changes that accompany such processes allow substantial work potential to be achieved in cyclic nonsteady devices, such as internal combustion engines and pulsed combustion or detonation engines. The ideal pressure gain or work production is a function of the prescribed ine ow and oute ow conditions, volumetric cone nement, e uid properties, and other parameters. The generalized thermodynamic limits of pressure gain and work production in such devices are investigated. Analytic and iterative methods are provided to evaluate cyclic combustion and evaporation processes for enhancing airbreathing combustion engine performance.

Assessment of Rotor Blade Angle of Attack from Experimental Inflow Data

A series of experiments was conducted by NASA Langley Research Center and the U.S. Army, over a 10-year period beginning in the mid-1980s, to provide some insight into the nature of helicopter rotor-system-induced velocity and to provide calibration data for many promising computational methods. Rotor-induced velocities were measured above the rotor system for several forward e ight-test conditions in the NASA Langley Research Center 14 by 22 Foot Subsonic Tunnel using a 15%-scale, fully articulated, stiff-in-torsion rotor and in a joint government/industry partnershipwith Bell HelicopterTextron,Inc.,foralarger,aeroelastically scaled, bearingless rotor. Atwo-componentlaservelocimeterwasused to makethesemeasurements in thefacility.Thedata fromthese tests have been published previously as NASA quick release reports and conference papers, and they are available electronically. Further analyses of the data to assess the local angle of attack of the rotor blades as a function of azimuth, span, and test condition aredocumented. The results indicatethat assessing ine owusing thetime average ofa rotorrevolutiondoesnotalwayscaptureadequately thelocaleffectsofbladepassageontheine owdistribution. The ine ow distribution should be assessed using the average of the blade azimuth-dependent ine ow. At advance ratioslessthan0.30, theeffectsof theindividual trailing vorticeswereevidentintheine owmeasurements, andthey have a signie cant impact on the local blade e ow angle. For higher advance ratios, the blade azimuth-dependent ine ow velocities differed very little from the time-averaged ine ow characteristics. The analyses demonstrated that dynamic twist is signie cant for aeroelastically scaled rotors and must be measured or modeled to assess the rotor performance accurately.

Helicopter Flight Dynamic Simulation with Refined Aerodynamics and Flexible Blade Modeling

Acoupled rotor ‐fuselagee ight dynamicmodel thatincludesa maneuveringfreewakemodel and acouplede ap ‐ lag‐torsione exibleblademodel,aswellastheresultsofaninvestigationonsomeeffectsofine owandblademodeling onthefree-e ightresponseofahingelessrotorhelicoptertopilotinputs,aredescribed.Thewakemodelisarelaxation typefreewake, capableofmodelingthewakegeometry changesdueto maneuvers; no assumptionsaremadeonthe wakegeometry,whichisfreetoevolvebased on themaneuver.Theoreticalpredictionsarecomparedwith e ight-test data. The results show that the free-e ight, on-axis response to pilot pitch and roll inputs can be predicted with good accuracy with a relatively unsophisticated model. It is possible to predict the off-axis response from e rst principles, that is, without empirically derived correction factors and without assumptions on the wake geometry. To do so, however, requires sophisticated modeling. Both a free wake model that includes the wake distortions caused by the maneuver and a ree ned e exible blade model must be used. Most features of the off-axis response can be captured using a dynamic ine ow theory extended to account for maneuver-induced wake distortions, for a fraction of the cost of using a free wake model. The most cost-effective strategy, for typical e ight dynamic analyses and if vibratory loads are not required, is probably to calibrate such a theory using the more accurate free wake based model, and then use it in all calculations.

Tip Leakage Flow in a Radial Inflow Turbine with Varying Gap Height

Tip leakage e ow in a radial ine ow turbineisinvestigated for two different uniform clearances of 0.6 and 1.2% of span. The clearances are typical of radial turbines for industrial applications. Measurements were also performed for increased axial clearances (6 and 7% of span ), while the radial clearance was kept constant. Static pressure and hot-wire measurements in the gap region cone rmed an earlier e nding by the authors that the tip leakage e ow behaviour in a radial turbine is interdependent with the scraping e ow. Changes in the nature of tip leakage e ow with increasing tip clearance height seem mainly to occur in regions where the scraping effect is important. The mass e ow over the tip of the present radial ine ow turbine was quantie ed for all four clearance cone gurations. The measurements revealed that the relative casing motion drags an approximately constant amount of scraping e uid from the suction side to the pressure side irrespective of the gap height, if the tip gap height to width ratio is above a critical value. The modeling of the effect of scraping in the inducer seems necessary if a successful prediction of the mass e ow and, hence, loss is attempted in the future.

Comparison of Induced Velocity Models for Helicopter Flight Mechanics

Modeling of rotor-induced velocity receives continued attention in the literature as the rotorcraft community addresses limitations in thee delity of simulations of helicopterstability, control,andhandling qualities. A comparison is presented of results obtained using a rigid-blade rotor-fuselage model cone gured with two induced velocity models: a conventional, e rst-order, e nite state, dynamic ine ow model and a wake model that solves a vorticitytransport equation on a computational mesh enclosing the rotorcraft. Differences between the two models are quantie ed by comparing predictions of trimmed rotor blade e ap, lag and feather angles, airframe pitch and roll attitudes, cross-coupling derivatives, response to control inputs, and airframe vibration. Results are presented in the context of measurements taken on a Puma aircraft in steady e ight from hover to high speed. More accurate predictions of the cross-coupling derivatives, response to control, and airframe vibration obtained using the vorticity transport model suggest that incorporation of real e owe eld effects is important to extending the bandwidth of applicability of helicopter simulation models. Unexpectedly small differences in some of the trim predictions obtainedusing thetwo wakemodelssuggest thatanoverall improvement in simulatione delity may notbeachieved without equivalent attention to the rotor dynamic model.

Modeling of Three-Dimensional Viscous Compressible Turbomachinery Flows Using Unstructured Hybrid Grids

An advanced numerical model for the simulation of steady and unsteady viscous compressible e ows for turbomachinery applications is described. The compressible Favre-averaged Navier ‐Stokes equations are used together with a one-equation turbulence model. The e ow domain is discretized using unstructured hybrid grids that can contain a mixture of hexahedral, pentahedral, tetrahedral, and triangular prismatic cells. The e ow equations are discretized using a node-centered e nite volume scheme that relies on representing the mesh using an edge-based data structure. A dual time stepping technique is applied to a point implicit formulation so that time accuracy can be maintained with large Courant ‐Friedrichs‐Lewy numbers. Nonree ecting boundary conditions are applied at the ine ow and oute ow boundaries to prevent any spurious ree ections of the outgoing waves. The model was validated against measured data for two cases. Radial proe les of pressure and temperature rise were determined from the steady e ow analysis of a rig fan blade, and these were found to be in very good agreement with the measured quantities. A rotor/stator interaction was studied next. Detailed comparisons were carried out against measured steady and unsteady e ow data and good agreement was obtained in all cases.

Circulation and water masses of the southwest Pacific: WOCE Section P11, Papua New Guinea to Tasmania

The circulation near the western boundary of the South Pacie c is described on the basis of water properties and geostrophic velocities measured on a meridional section along 155E through the East Australian and Coral Basins. The section was occupied in winter 1993 as part of the World Ocean Circulation Experiment (WOCE, Section P11S). The primary objective of P11 was to quantify the zonal e ows entering and leaving the western boundary of the basin. The primary ine ow to the Tasman and Coral seas is supplied by the South Equatorial Current (SEC), which crosses the P11 section as a wide band of westward e ow between 14 and 18S with a total geostrophic transport of 55 Sv (1 Sv 5 10 6 m 3 /s) relative to the bottom. The SEC bifurcates at theAustralian coast near 18S: 29 Sv turns south to feed the EastAustralian Current (EAC), and 26 Sv recirculates in the Gulf of Papua New Guinea as a low-latitude western boundary current (the Great Barrier Reef and New Guinea Coastal Undercurrents, GBRUC/NGCUC). The NGCUC closes the tropical gyre, and carries South Pacie c water around the Louisiade Archipelago and through the Solomon Sea to the equator. The core of the EAC lies over the continental slope between 18S and 30S. A system of deep-reaching, recirculating eddies or gyres is located offshore of the EAC. At 30S the EAC separates from the coast, and the steeply sloping isopycnals associated with the current persist from the surface to the bottom.The total geostrophic transport of the EAC after separation is 57 Sv relative to the bottom. After separation from the coast, more than half of the EAC (33 Sv) recirculates north and then west, crossing P11 again at 28S. The remainder (24 Sv) continues east as a meandering jet across the Tasman Sea (the Tasman Front). The circulation in the southern part of the Tasman Sea is dominated by transient eddies and standing gyres.An anticyclonic circulation facilitates the exchange of water between the Tasman Sea and the Southern Ocean. About 10 Sv of subantarctic water spreads north to 36‐ 38S, then recirculates back to the west to merge with a weak southward e ow of modie ed subtopical water near the Tasmanian coast. The circulation in the deeper layers consists of a weak northward deep western boundary current, a cyclonic recirculation e lling the Tasman Basin, and a net export of about 3 Sv of deep water to the Coral Sea. The transport of mode and intermediate water in the low-latitude western boundary current crossing P11 is similar to the transport in these density classes further ‘ ‘ upstream’ ’ in the subtropical gyre at 32S. This suggests that the mode and intermediate waters entering the Pacie c from the south to compensate the export through the Indonesian passages are carried north to the tropical western Pacie c primarily along isopycnals.

Numerical Investigation of Supersonic Injection Using a Reynolds-Stress Turbulence Model

Thefull,three-dimensionalFavre-averaged Navier -Stokesequations,coupledwiththesecond-orderZhang etal. (Zhang, H., So, R., Gatski, T., and Speziale, C., A Near-Wall Second-Order Closure for Compressible Turbulent Flows,Near-WallTurbulentFlows ,editedbyR. So,C.Speziale, andB.Launder,Elsevier,NewYork,1993,pp.209 - 218) Reynolds-stress turbulence and K-≤ models, were used to numerically simulate a 25-deg, Mach 1.8 injection into a Mach3.0 crosse ow.Detailed comparisonswith experimental data wereperformed.Analysis of theReynolds- stress turbulence model simulation results revealed physically consistent and accurate predictions for mean e ow and turbulent quantities, whereas the simulations with the K-≤ model resulted in nonphysical and inconsistent turbulence predictions. Analysis of the three-dimensional e owe eld simulation with the Reynolds-stress turbulence model shows that the shock structure downstream of the oblique barrel shock wasa mirrored image of theleeward side of the oblique barrel shock. Furthermore, the downstream location where vortical motion was initiated in the jet plume was caused by the recompression shock-induced vortices. These vortices were generated through the combined effects of the ine ow air upwash behind the plume and the mirrored oblique barrel shock.

Flowfield and Acoustic Measurements from a Model Tiltrotor in Hover

Rotor ine ow aerodynamics and noise measurements were made on three cone gurations of an 8% scale model of the XV-15 tiltrotor in hover: 1 ) single isolated rotor, 2 ) two rotors with no fuselage, and 3 ) complete tiltrotor aircraft. For the tiltrotor aircraft cone guration and for the cone guration of two rotors without the fuselage, the mean ine ow velocity was higher at c = 270 deg compared with the rest of the rotor disk, leading to reduced blade angle of attack and blade loading in this region. This azimuthally varying blade loading caused an impulsive noise that radiated preferentially behind the model. For the complete tiltrotor cone guration, the turbulence ingested by the rotors was intermittent and depended on the instantaneous position of the fountain e ow, which shifted from side to side across the longitudinal plane of the model. The fountain turbulence had a higher velocity scale, smaller length scale, and was closer to isotropic than the ingested ambient turbulence. The tiltrotor cone guration radiated less harmonic noise, but more broadband noise than the cone guration with two rotors and no fuselage. Diagonal fences on the wings of the tiltrotor reduced the ine ow turbulence intensity in the fountain region by a factor of about 3, and reduced the noise by 4.1 dBA behind the model. Scaling relations were derived to extrapolate the model measurements to the full-scale XV-15.

Transition Characteristics of Flowfield in a Simulated Solid-Rocket Motor

The e ow characteristics in a two-dimensional porous-walled duct simulating a solid-propellant rocket motor are numerically computed to investigate the effects of viscosity, compressibility, and ine ow turbulence ( w) on the e ow transitions. The e nite volume technique is used to solve the time-dependent compressible Navier‐ Stokes equations with a subgrid-scale turbulence model, and the numerical e uxes are computed using a modie ed Godunov scheme. In addition to computed axial mean velocity and turbulence intensity proe les, the axial variations of skin friction coefe cient and the transverse location of peak turbulence intensity are used to identify the mean-e ow transition and turbulence-intensity transition, respectively. In particular, a new way of identifying turbulence-intensity transition by the use of the power spectrum of velocity e uctuations is presented for the e rst time in the present study. The minimum centerline Mach number for the onset of mean-e ow transition is obtained as the compressibility is considered alone. The critical values of w for the onset of turbulence-intensity transition and mean-velocity transition advance as well as for the concurrence and delay between the two transitions are also determined to illustrate why some researchers could observe only a single transition whereas others observed two transitions.

Computations of High-Speed, High Knudsen Number Microchannel Flows

The effect of varying the Knudsen number Kn in microchannel e ows was simulated using the direct simulation Monte Carlo method (DSMC) combined with the monotonic Lagrangian grid (MLG). The DSMC‐ MLG, a method that provides automatic grid ree nement according to number density, has been optimized for massively parallel computation and provides a fast, highly resolved description of the e ow. New oute ow boundary conditions, consistent with the DSMC ‐ MLG algorithm, were developed to allow the user to specify the oute ow pressures. The effect of varying Kn was examined for three different values of Kn (0.07, 0.14, and 0.19 ) for a high-speed ine ow by varying the channel height. A Navier ‐ Stokes computation was also performed to show continuum regime e ow. The computations provide contours of pressure, temperature, and Mach number to show complex interactions among oblique shocks and boundary layers, and how these change with the Kn. Temperature jumps and slip velocities as functions of position along the wall are compared for all cases. The computations show that the velocity slip is approximately constant behind the shock, while the temperature jump is reduced.

Influence of Shaft Angle of Attack on Sound Radiation by Subsonic Propellers

The formulation to account for effects of propeller angle of attack on acoustic radiation by propellers is presented. A propeller operating at an angle of attack presents a nonuniform ine ow situation that is known to have a pronounced ine uence on noise. Noise predictions, based on this new formulation, are compared with measured wind-tunnel data. Predictions for observers e xed in the propeller plane are also included to assess the ine uence of shaft angle of attack. Nomenclature c0 = ambient speed of sound f = function dee ning blade surface l = local force per unit area of blade acting on e uid lr = component of loading vector in direction of radiation vector, liri M = source Mach number Mr = component of source Mach vector in direction of