Showing papers on "Freestream published in 2002"

Boundary-layer receptivity to freestream disturbances

Abstract: The current understanding of boundary-layer receptivity to external acoustic and vortical disturbances is reviewed. Recent advances in theoretical modeling, numerical simulations, and experiments are discussed. It is shown that aspects of the theory have been validated and that the mechanisms by which freestream disturbances provide the initial conditions for unstable waves are better understood. Challenges remain, however, particularly with respect to freestream turbulence

Control of Low-Pressure Turbine Separation Using Vortex-Generator Jets

Abstract: The application of vortex-generator jets to control separation on the suction surface of a low-pressure turbine blade is reported. Blade Reynolds numbers in the experimental, linear turbine cascade match those for high-altitude operation of many aircraft gas-turbine engines, as well as the last stages of industrial ground-based gas turbines. Results are presented for steady blowing at jet blowing ratios from zero to four and at several chordwise positions and two freestream turbulence levels. Findings show that above a minimum blowing ratio, which is dependant on the injection location, the pressure loss in the modified blade's wake is reduced by a factor of between two and three. Boundary-layer traverses show that separation is almost completely eliminated with the application of blowing. No significant deleterious effects of vortex-generator jets are observed at higher (nonseparating) Reynolds numbers. The addition of 4% freestream turbulence to the cascade freestream lowers the separation Reynolds number of the turbine blade studied, but does not eliminate the effectiveness of the control technique. The vortex-generator jet control strategy is demonstrated to be a viable technique for low-pressure turbine separation control.

Film Cooling Effectiveness Due to Discrete Holes Within a Transverse Surface Slot

Abstract: The goal of many turbine airfoil film cooling schemes is the achievement of a tangentially injected 2D layer of protective film over the surface. In common nomenclature, this is referred to as 2D slot film cooling, which can achieve adiabatic effectiveness levels approaching unity at the injection location. Since continuous and uninterrupted slots are not structurally feasible in the high pressure turbine components, other approximate film cooling geometries have been sought. The present study examines two film cooling geometries which are formed by the combination of internal discrete film holes feeding continuous 2D surface slots. Experiments have been performed within a flat plate wind tunnel test section, which includes an accelerating freestream condition to model the surface of a turbine airfoil. As suggested by the experiments of Wang et al. [1], a normal 2D surface slot is located transverse to the mainstream flow direction. The slot is fed by a row of discrete coolant supply holes oriented in the spanwise direction with inclination angle of 30-degrees, pitch-to-diameter ratio of 3.57, and length-to-diameter ratio of 5.7. The slot depth-to-hole diameter ratio is S/D of 3. Two such slots were tested, one with axial width-to-hole diameter ratio of 1.13, and the other with ratio of 1.5. Tests were conducted for supply hole blowing ratios of 0.75 to 4, density ratios of 1.8, and a freestream approach turbulence intensity of 4.5%. The holes-within-slot film effectiveness data are compared with both axial and radial film data, ie. S/D equal to zero, obtained in the same test section. The holes-in-slot geometries demonstrate two important characteristics, (1) a relative insensitivity of the adiabatic film effectiveness to blowing rate, and (2) no observed film blow-off at high blowing rates. In addition, a novel film cooling arrangement is demonstrated in which the surface slot is very shallow, forming a narrow trench with S/D of only 0.43. It is shown that this novel surface geometry yields the best film effectiveness of all cases tested.Copyright © 2002 by ASME

Application of Large-Eddy Simulation to Supersonic Compression Ramps

Abstract: Large-eddy simulations of supersonic compression-ramp flowfields were performed by a high-order numerical method, utilizing the Smagorinsky dynamic subgrid-scale model to account for spatially underresolved stresses. Computations were carried out at a freestream Mach number of 3.0 for ramp angles of 8, 16, 20, and 24 deg. Extensive comparisons are made between the respective solutions and available experimental data that were collected at higher Reynolds numbers. These include surface pressure, skin friction, and both mean and fluctuating velocity profiles. For the 24-deg case, a number of experimentally measured statistical quantities are compared to the simulation

St and cf Augmentation for Real Turbine Roughness With Elevated Freestream Turbulence

Abstract: Experimental measurements of skin friction (c f ) and heat transfer (St) augmentation are reported for low speed flow over turbine roughness models. The models were scaled from surface measurements taken on actual, in-service land-based turbine hardware. Model scaling factors ranged from 25 to 63, preserving the roughness height to boundary layer momentum thickness ratio for each case. The roughness models include samples of deposits, TBC spallation, erosion, and pitting. Measurements were made in a zero pressure gradient turbulent boundary layer at two Reynolds numbers (Re x =500,000 and 900,000) and three freestream turbulence levels (Tu=1%, 5%, and 11%). Measurements at low freestream turbulence indicate augmentation factors ranging from 1.1-1.5 for St/St o and from 1.3-3.0 for c f /c fo (St o and c fo are smooth plate values). For the range of roughness studied (average roughness height, k, less than 1/3rd the boundary layer thickness) the level of c f augmentation agrees well with accepted equivalent sandgrain (k s ) correlations when k s is determined from a roughness shape/density parameter. This finding is not repeated with heat transfer, in which case the k s -based St correlations overpredict the measurements. Both c f and St correlations severely underpredict the effect of roughness for k + < 70 (when k s , as determined by the roughness shape/density parameter, is small). A new k s correlation based on the rms surface slope angle overcomes this limitation. Comparison of data from real roughness and simulated (ordered cones or hemispheres) roughness suggests that simulated roughness is fundamentally different from real roughness. Specifically, k s values that correlate c f for both simulated and real roughness are found to correlate St for simulated roughness but overpredict St for real roughness. These findings expose limitations in the traditional equivalent sandgrain roughness model and the common use of ordered arrays of roughness elements to simulate real roughness surfaces. The elevated freestream turbulence levels produce augmentation ratios of 1.24 and 1.5 (St/St o ) and 1.07 and 1.16 (c f /c fo ) compared to the Tu=1% flow over the smooth reference plate. The combined effects of roughness and elevated freestream turbulence are greater than their added effects suggesting that some synergy occurs between the two mechanisms. Specifically, skin friction augmentation for combined turbulence and roughness is up to 20% greater than that estimated by adding their separate effects and 8% greater than compounding (multiplying) their separate effects. For heat transfer augmentation, the combined effect of turbulence and roughness is 5% higher than that estimated by compounding their separate effects at high freestream turbulence (Tu=11%). At low turbulence (Tu=5%), there is a negative synergy between the two augmentation mechanisms as the combined effect is now 13% lower than that estimated by compounding their separate effects.

Influence of a Counterflow Plasma Jet on Supersonic Blunt-Body Pressures

Abstract: Aerodynamic augmentation in the presence of a thin high-temperature onboard plasma jet directed upstream of a slightly blunted cone was studied experimentally and numerically. The flow around a truncated cone cylinder at zero incidence was considered for Mach numbers M∞ = 2.0, 2.5, and 4.0. For the first time, computationally validated experimental pressure distributions over the model surface in the presence of the plasma jet were obtained. As in the conventional (nonplasma) counterflow jet, two stable operational regimes of the plasma jet were found. These were a short penetration mode and a long penetration mode (LPM) aerospike into the opposing supersonic freestream. The greatest drag reduction occurred in the moderate LPM regime. LPM strong overblowing reduces the benefits. The experimental pressure results were approximately validated against an Euler computational fluid dynamics simulation, modeling a perfect gas hot jet, counterflowing against a perfect gas supersonic freestream. Plasma effects such as electron pressure, radiation, electric field interactions, Joule heating, and induced vorticity, streamers, and plasmoids have been identified that, if accounted for, may improve the comparison. Procedures for the use of these experimental results have been outlined as a baseline that will be useful in separating fluid dynamic/thermal effects from plasma processes in understanding the physics of onboard plasma jets for aerodynamic augmentation.

Prediction of transition for attached and separated shear layers in turbomachinery

Abstract: A new transition model is developed to predict the onset of transition under the influence of freestream turbulence intensity, pressure gradient and flow separation. The model is based on Van Driest and Blumer’s concept of vorticity Reynolds number and has been calibrated for use with the Menter SST turbulence model. The new transition model has been validated against a large number of diverse and challenging experiments which include the T3 series of test cases as well as the PAK-B low-pressure turbine blade at very low Reynolds numbers. In all cases very good agreement is obtained and it would appear that the new transition model could be a very useful tool for the design of highly-loaded and efficient turbomachinery.

Modeling of the interaction of a side jet with a rarefied atmosphere

Abstract: The interaction of a jet from a 3000-N-class thruster positioned on the side of a small rocket, with the rarefied atmosphere at 100 and 80 km, is studied numerically. The direct simulation Monte Carlo method was applied to model the three-dimensional jet-atmosphere interaction. Chemical reactions between freestream and plume species were included in the simulations. A two-stage numerical strategy was used, with sequential computations of an axisymmetric plume core flow and three-dimensional plume-freestream interaction. The impact of altitude, angle of attack, rocket velocity, and thrust on flowfields and surface mass fluxes is examined.

Reducing Low-Pressure Turbine Stage Blade Count Using Vortex Generator Jet Separation Control

Abstract: An experimental investigation has been conducted into the feasibility of increasing blade spacing (pitch) at constant chord in a linear turbine cascade. Vortex generator jets (VGJs) located on the suction surface of each blade in the cascade are employed to maintain attached boundary layers despite the increasing tendency to separate due to the increased uncovered turning. Tests were performed at low Mach numbers and at blade Reynolds numbers between 25,000 and 75,000 (based on axial chord and inlet velocity). The vortex generator jets (30 degree injection angle and 90 degree skew angle) were operated with steady flow with momentum blowing ratios between zero and five, and from two spanwise rows of holes located at 45% and 63% axial chord. In the absence of control, pitch-averaged wake losses increase up to 600% as the blade pitch is increased from its design value to twice the design value. With the application of VGJs, these losses were driven down to or below the losses at the design pitch. The effectiveness of VGJs was found to increase modestly with increasing Reynolds number up to the highest value tested, Re = 75,000. The fluid phenomenon responsible for this remarkable range of effectiveness is clearly more than a simple boundary layer transition effect, as boundary layer trips installed on the same blades without VGJ blowing had no beneficial effect on blade losses. Also, tests conducted at elevated levels of freestream turbulence (4% at the cascade inlet) where the suction surface boundary layer is generally turbulent, showed wake loss reduction comparable to tests conducted at the nominal 1% freestream turbulence. For all configurations, blowing from the upstream row had the greatest wake influence. These findings open the possibility that future LPT designs could take advantage of active separation control using integrated VGJs to reduce the turbine part count and stage weight without significant increase in pressure losses.© 2002 ASME

Structures of Aerated-Liquid Jets in High-Speed Crossflows

Abstract: d0 = injector orifice diameter GLR = aerating gas-to-liquid mass ratio The structures of aerated-liquid jets injected into subsonic crossflows were studied experimentally. An aerated-liquid injector with a diameter of 0.5 mm was flush mounted on the bottom plate of a subsonic wind tunnel to provide normal injection. Freestream Mach numbers, M, of 0.2 and 0.3 were tested. Water at room temperature was used as the test injectant. Wide ranges of test conditions for jet-to-air momentum flux ratios, q0, and aeration levels, GLR, were tested. A phase Doppler particle analyzer (PDPA) was utilized to quantitatively measure droplet and spray plume properties. The obtained data was used to develop correlations for the properties of droplet and spray plume for aerated-liquid jets, using the least squares method. It was found that the atomization processes of a typical aerated-liquid jet are completed at a relatively short x/d0. The droplet size decreases as the GLR and M increase. The droplet velocity increases with GLR, x/d0, and M and has no significant dependence on q0. The cross-sectional area of the spray plume increases with GLR, q0, and x/d0 and decreases with M. As GLR increases, the increase in the cross-sectional area of spray plume, Aj, mainly comes from the increase in spray penetration height. The spray width, however, is fairly independent of GLR. The aspect ratio of the spray plume increases with GLR. It was also found that A70%/Aj and A30%/Aj of aerated-liquid jets increase with GLR and x/d0 and have values of 32.5 and 10.6, respectively. For the present study, the values of A70%/Aj and A30%/Aj for the pure-liquid jets are 29.3 and 8.5, respectively. h = spray penetration height L = nozzle passage length M = freestream Mach number Oh = Ohnesorge number, μL/(ρL(SMD) σ) Q = volumetric flow rate q0 = jet-to-freestream momentum flux ratio at GLR=0, ρLw0/ρ∞u∞ SMD = Sauter mean diameter, Σ di/Σ di, i for all droplets T = temperature u = velocity component in the x direction w = velocity component in the z direction; also spray width We = Weber number, ρL(SMD)(u∞-up)/σ x = distance in the freestream direction y = distance in the cross stream direction z = distance in the direction of liquid injection θ = jet injection angle

Dynamics of Slender Bodies Separating from Rectangular Cavities

Abstract: Vertical and pitching motions (two degrees of freedom) of a thin body of revolution separating from a rectangular cavity in a subsonic stream are investigated using combined asymptotic and numerical methods. The analysis is based on explicit analytical solutions for the lift force and pitching moment obtained in our previous studies. Body trajectory dependencies on initial conditions, body parameters, and freestream velocity are studied. The problem is divided into three phases of the motion. In phase 1, the body is inside the cavity. In phase 2, the body crosses the shear layer, and in phase 3, the body is outside the cavity. For phases 1 and 3, analytical solutions of the body dynamics are obtained for typical cases. This analysis provides insight into the separation process and identifies governing lumped nondimensional parameters relevant to the body dynamics as well providing a model that can provide quick, computationally non-intensive estimates of store separation with a personal computer. The role of the nondimensional parameters in the dynamic stability eigenvalues is identified and found particularly useful in this connection. These parameters implicitly contain the effect of the shear layer

The slowest soap-film tunnel in the Southwest

Abstract: Gravity-driven soap-film channels offer a convenient way to study two-dimensional hydrodynamics in the laboratory. With recently developed quantitative soap-film diagnostic techniques, velocity and thickness field information can be acquired to provide a complete description of the flow. We present a study of soap-film flow in a state-of-the-art tilted soap-film tunnel which can sustain a mean freestream velocity about 0.5 m/s with velocity fluctuations on the order of 1%. Our investigation concentrates on the evolution of the velocity and thickness profiles with downstream distance. We observe a hydrodynamically fully developed flow within a substantial range of distances and compare our results with the analytical solution based on the assumptions of linear air drag and constant film thickness. We find air drag to be the dominant dissipative mechanism in the flow, although the role of internal viscous dissipation is also apparent. Direct measurements of the air drag coefficient are in good agreement with the values inferred from the analytical solution.

High Intensity, Large Length-Scale Freestream Turbulence Generation in a Transonic Turbine Cascade

Abstract: Heat transfer predictions in gas turbine engines have focused on cooling techniques and on the effects of various flow phenomena. The effects of wakes, passing shock waves and freestream turbulence have all been of primary interest to researchers. The focus of the work presented in this paper is to develop a turbulence grid capable of generating high intensity, large-scale turbulence for use in experimental heat transfer measurements in a transonic facility. The grid is desired to produce freestream turbulence characteristic of the flow exiting the combustor of advanced gas turbine engines. A number of techniques are discussed in this paper to generate high intensity, large length-scale turbulence for a transonic facility. Ultimately, the passive grid design chosen is capable of producing freestream turbulence with intensity of approximately 10–12% near the entrance of the cascade passages with an integral length-scale of 2 cm.Copyright © 2002 by ASME

Supersonic boundary-layer response to optically generated freestream disturbances

Abstract: Controlled, localized disturbances were introduced into the supersonic freestream upstream of a 4:1 elliptic cross-section cone. The response of the initially laminar boundary layer to the laser-generated freestream perturbation was measured above the cone minor axis. The experiment was conducted in the Mach-4 Purdue Quiet-flow Ludwieg tube at a freestream unit Reynolds number of 4.5 million/m. The focused beam from a frequency-doubled Nd:YAG laser was used to generate the disturbance. The perturbation existed in the flowfield as a region of locally heated air, referred to here as the thermal spot. Constant-temperature anemometry was used to characterize the boundary-layer response to the introduction of the thermal spot. The response was largest and most complex near the boundary-layer edge. The duration of the measured boundary-layer response was an order of magnitude greater than the measured duration of the disturbance in the freestream. Within the boundary layer, the mass-flux deviation introduced by the thermal spot was of the same magnitude as the local mean mass flux. The optically generated disturbance is potentially useful as a perturbation source in future boundary-layer receptivity experiments.

Measurements of the Effects of Freestream Turbulence on Separation-Bubble Transition

Abstract: In this paper, measurements are presented on the effects of freestream turbulence on laminar-to-turbulent transition in separation bubbles, and correlations are proposed for the locations of transition and reattachment on the basis of this data. The boundary layer development is measured on a smooth, flat plate upon which streamwise pressure gradients are imposed by a flexible, contoured wall opposite to the test plate. Two variations in the streamwise pressure distribution are investigated, and two Reynolds numbers are considered for each pressure-gradient setting. For each combination of pressure distribution and Reynolds number, the freestream turbulence intensity and length scale are adjusted systematically by varying the open-area-ratio and cell size of the grid installed at the test-section inlet. Measured quantities consist of velocity obtained with a single-hot wire probe and surface pressures measured through pressure taps.Copyright © 2002 by ASME

Investigation on transonic convex-corner flows

Abstract: An experimental investigation was conducted to study the transonic flow past convex corners. The mean surface pressure indicated strong inviscid-viscous interactions, and the interaction region increased with the freestream Mach number and the convex-corner angle. Unsteadiness of the interaction was characterized by an intermittent region and a local peak pressure fluctuation. The peak pressure fluctuations and the tentative boundary between the attached and separated flows could be scaled with the convex-corner angle and a second-order freestream Mach number (M 2 ∞η)

Receptivity and linear stability of stetson's mach 8 blunt cone stability experiments

Abstract: Currently, the mechanisms leading to hypersonic boundary layer transition are still poorly understood. The transition in the boundary layer depends on the receptivity process, which is the process of environmental disturbances initially entering the boundary layers and generating disturbance waves. The receptivity of hypersonic boundary layers to free stream disturbances is altered considerably by the presence of bow shocks in hypersonic flow fields and by the entropy layers created by the blunt nose. This paper conducts a numerical simulation study of the receptivity to freestream acoustic disturban& waves for Mach 7.99 axisymmetric flow over a io half-angle blunt cone, and compares the numerical results with experimental results by Stetson et al. (1984) and with those obtained from linear stability. Both steady and unsteady flow solutions of the receptivity problem are obtained by computing the full Navier-Stokes equations using a high-order accurate shock-fitting finite difference scheme, which can accurately account for the effects of bow-sho&/free-streamsound interactions on the receptivity process. In addition, a normal-mode linear stability analysis is also used to study the stability and receptivity properties of the boundary layer affected by the entropy layer. The main focus of this study is on the excitation of the second mode waves in the receptivity process in the presense of freestream acoustic waves. One of the major findings of this study is that, for the case of receptivity to fast freestream acoustic waves with a blunt nose, the second mode waves are not excited in the early region dong the cone surface where the second modes are pre dicted to be unstable by the linear stability analysis. It is shown that the delay of the second mode excitation is due to the unique stability characteristics of the current flow which are affected by the entropy layer produced by the nose bluntness. The understanding of such receptivity processes may lead to a better understanding of nose bluntness effects on hypersonic boundary layer transition and the accuracy of the LST analysis.

Direct Numerical Simulation of Turbulent Trailing-Edge Flow with Base Flow Control

Abstract: Direct numericalsimulationhas been carried out for turbulent flow over a rectangular trailing edge at a Reynolds number of 1x10(3) (based on the freestream quantities and the trailing-edge thickness) and ratio of boundarylayerdisplacement thickness to trailing-edge thickness close to unity. Two types of flow control were studied: base transpiration and secondary splitter plate. Simulation of base transpiration was performed using different slit heights and volume flow rates. It was found that even small flow rates could produce significant changes in overallaerodynamic performance, measured, for example, by the base pressure coefficient. It was also found that for the same volume flow rate, a greater increase in base pressure (drag reduction) was obtained by blowing slowly through a wide slit rather than quickly through a narrowslit. The effectiveness of a secondary splitter plate located on the trailing-edge centerline was investigated by varying the plate length from one to five times the trailing-edge thickness. A significant increase in the base pressure coefficient (about 25%) was achieved, even with the shortest splitter plate equal to the trailing-edge thickness. The base pressure coeffi cient increased monotonically with the splitter plate length, and no intermediate maximum value was found.

Numerical Investigation of High-Enthalpy Flows Generated by Expansion Tube

Abstract: We have investigated by numerical simulation the hypersonic and high-enthalpy flows around a reentry body, which Sasoh et al. examined experimentally in an expansion tube facility. For the experiment, the evaluation of the timing at which the test flow is attained and the freestream condition ofthe test flow are the most crucial issues. Therefore, it was necessary to validate these issues in another way, rather than the simple estimation by Sasoh et al. based on the pressure and spectral emission measurements. Thus, the experimental result was intensively investigated compared to the numerical result based on the experimentally determined freestream condition

Mean and fluctuating pressure in boat-tail separated flows at transonic speeds

Abstract: Experiments were carried out investigating the features of mean and unsteady surface pressure fluctuations in boat-tail separated flows relevant to launch vehicle configurations at transonic speeds. The tests were performed on a generic axisymmetric body in the Mach-number range of 0.7-1.2, and the important geometrical parameters, namely, the boat-tail angle and diameter ratio, were varied systematically. The measurements made included primarily the mean and unsteady surface-pressure fluctuations on nine different model configurations. Flow-visualization studies employing a surface oil flow, and schlieren techniques were carried out to infer features like boundary-layer separation, reattachment, and shock waves in the flow. The features of mean and fluctuating surface pressures are discussed in detail including aspects of similarity. It has been observed that, on a generic configuration employed in the present study, the maximum levels of surface-pressure fluctuations in the reattachment zone are appreciably lower than those found on launch vehicle configurations having a bulbous or hammerhead nose shape. A simple correlation is suggested for the maximum value of rms pressure fluctuations in the reattachment zone at different freestream Mach numbers.

Gas Turbine Engine Durability Impacts of High Fuel-Air Ratio Combustors: Part 2 — Near Wall Reaction Effects on Film-Cooled Heat Transfer

Abstract: As commercial and military aircraft engines approach higher total temperatures and increasing overall fuel-to-air ratios, the potential for significant chemical reactions on a film-cooled surface is enhanced. Currently there is little basis for understanding the effects on aero-performance and durability due to such secondary reactions. A shock tube experiment was employed to generate short duration, high temperature (1000–2800 K) and pressure (6 atm.) flows over a film-cooled flat plate. The test plate contained two sets of 35° film cooling holes that could be supplied with different gases, one side using air and the other nitrogen. A mixture of ethylene and argon provided a fuel rich freestream that reacted with the air film resulting in near wall reactions. The relative increase in surface heat flux due to near wall reactions was investigated over a range of fuel levels, momentum blowing ratios (0.5–2.0), and Damkohler numbers (ratio of flow to chemical time scales) from near zero to 30. For high Damkohler numbers, reactions had sufficient time to occur and increased the surface heat flux by 30 percent over the inert cooling side. When these results are appropriately scaled, it is shown that in some situations of interest for gas turbine engine environments significant increases in surface heat flux can be produced due to chemical reactions in the film-cooling layer. It is also shown that the non-dimensional parameters Damkohler number (Da), blowing ratio (B), heat release potential (H* ), and scaled heat flux (Qs ) are the appropriate quantities to predict the augmentation in surface heat flux that arises due to secondary reactions.Copyright © 2002 by ASME

Particle turbulence interaction

Abstract: The interaction between the carrier and the dispersed phases is bi-directional: the carrier-phase turbulence influences the dispersion and preferential accumulation of particles and bubbles, and particles and bubbles in turn modulate the fluid turbulence. At the level of a single particle, the effect of freestream turbulence is to modify the drag force compared to that in a steady uniform flow. On the other hand, the particle can modify freestream turbulence by the formation of a wake, periodic shedding of vortices, and wake turbulence. The collective effect of a distribution of particles can further modify the effective drag force on a particle due to the screening effect and thereby influence the mean settling and dispersion characteristics. Similarly, the collective effect of the dispersion of particles will determine the attenuation or augmentation of the turbulence intensity. Thus, many of the mechanisms of multiphase flow turbulence are quite distinct from those that have been well established in single-phase turbulence and therefore deserve detailed investigation. Here we present results from fully-resolved direct numerical simulations (DNS) of turbulent multiphase flow. In addition to resolving the wide range of length and time scales associated with turbulence, which would exist even in the absence of the particles, we now need to resolve all the length scales associated with the particles and the small-scale flow features generated by them. Thus, direct numerical simulations of multiphase flow turbulence pose greater challenge than the DNS of corresponding single-phase turbulence. Here we plan to present results for the interaction of a single spherical with ambient turbulence. We will consider the cases of both isotropic freestream turbulence and wall-bounded channel flow turbulence. For the case of isotropic turbulence the particle Reynolds number is varied from about 50 to 600, and the particle diameter is about 1.5 to 10 times the Kolmogorov scale of the undisturbed turbulent flow. The Taylor microscale of the freestream turbulent flow is 164 and the relative intensity of freestream turbulence to mean crossflow is varies from 10% to 25%. The DNS technique employed here resolves both the smallest scales in freestream turbulence and the thin shear layers and complex vortical structures associated the particle wake. The present DNS study is similar to the experimental study by Wu & Faeth (1994a, 1994b), and agreement between the DNS results and the experimental measurement are presented. Figure 1a summarizes the drag coefficient for a sphere in the presence of freestream isotropic turbulence compiled from many different experimental measurements. Also plotted for reference as the solid line is the standard Schiller-Neumann drag law corresponding to a turbulence-free uniform flow. The scatter in the experimental data clearly illustrates the large discrepancy between the different results. Also plotted in the figure are the DNS results which show that freestream isotropic turbulence does not have a substantial and systematic effect on the time-averaged mean drag on the particle. Standard drag correlation based on instantaneous relative velocity between the particle and the undisturbed fluid velocity at the center of the particle results in a reasonable prediction of the mean drag. However, the accuracy of prediction of the instantaneous drag decreases with increasing particle size. For the smaller particles, the low frequency oscillations in the DNS drag are well captured by the standard drag, but for the larger particles significant differences exist even for the low frequency components. Inclusion of the added-mass and history terms, computed based on the undisturbed ambient flow at the center of the particle, does not improve the prediction of instantaneous forces. Fluctuations in the drag and lift forces are shown to scale with the mean drag as well as the freestream turbulence intensity (see figure 1b). The effect of freestream turbulence on the mean and instantaneous wake structure is studied. The mean wake in a turbulent flow shows reduced velocity deficit and a flatter profile. However, the mean wake in a turbulent flow behaves like a self‐ preserving laminar wake. At low Reynolds numbers the wake in a turbulent flow oscillates strongly without any vortex shedding, but at higher Reynolds numbers vortex shedding starts. The nature of the vortices are very different from that in a uniform flow. Increasing the freestream turbulence intensity suppresses the process of vortex shedding, and only marginally increases the wake oscillation. The modulation of freestream turbulence in the wake is studied in terms of the distribution of the distribution of the kinetic energy and RMS of velocity fluctuation. The freestream energy lost in the wake is recovered faster in a turbulent flow than in a uniform flow. The energy of the velocity fluctuation is enhanced in the wake at low freestream intensities, and is damped or marginally increased at higher intensities. The fluctuation energy is not equi-partitioned among the streamwise and cross-stream components. The RMS of the streamwise fluctuation is always enhanced, whereas the RMS of the cross-stream fluctuation is enhanced only at low freestream intensities, and damped at higher intensities.

The Effect of Freestream Turbulence on Film Cooling Adiabatic Effectiveness

Abstract: The film-cooling performance of a flat plate in the presence of low and high freestream turbulence is investigated using liquid crystal thermography. High-resolution distributions of the adiabatic effectiveness are determined over the film-cooled surface of the flat plate using the hue method and image processing. Three blowing rates are investigated for a model with three straight holes spaced three diameters apart, with density ratio near unity. High freestream turbulence is shown to increase the area-averaged effectiveness at high blowing rates, but decrease it at low blowing rates. At low blowing ratio, freestream turbulence clearly reduces the coverage area of the cooling air due to increased mixing with the main flow. However, at high blowing ratio, when much of the jet has lifted off in the low turbulence case, high freestream turbulence turns its increased mixing into an asset, entraining some of the coolant that penetrates into the main flow and mixing it with the air near the surface.Copyright © 2002 by ASME

Turbulent boundary layers on axially inclined cylinders. Part 1. Surface-pressure/velocity correlations

Abstract: The boundary layer on a cylinder with its axis at small inclinations of 0–6° to the freestream (an idealisation of `streamers' used in underwater seismic surveys) has been studied experimentally by measurements involving surface pressure fluctuations and their correlation with the axial velocity. There is no evidence of vortex shedding at Reynolds numbers typical of streamers at operating conditions. The behaviour of the wall-pressure field is substantially altered by small incidence: correlation length scales decrease on the upstream side, but remain relatively unaltered on the downstream side. Attention is also paid to the axisymmetry of the flow by reference to axial velocity statistics of up to fourth order.

Nonintrusive Temperature and Velocity Measurements in a Hypersonic Nozzle Flow

Abstract: Distributions of nitric oxide vibrational temperature, rotational temperature and velocity have been measured in the hypersonic freestream at the exit of a conical nozzle, using planar laser-induced fluorescence. Particular attention has been devoted to reducing the major sources of systematic error that can affect fluorescence tempera- ture measurements, including beam attenuation, transition saturation effects, laser mode fluctuations and transition choice. Visualization experiments have been performed to improve the uniformity of the nozzle flow. Comparisons of measured quantities with a simple one-dimensional computation are made, showing good agreement between measurements and theory given the uncertainty of the nozzle reservoir conditions and the vibrational relaxation rate.

St and c

Abstract: Experimental measurements of skin friction (cf ) and heat transfer (St) augmentation are reported for low speed flow over turbine roughness models. The models were scaled from surface measurements taken on actual, in-service land-based turbine hardware. Model scaling factors ranged from 25 to 63, preserving the roughness height to boundary layer momentum thickness ratio for each case. The roughness models include samples of deposits, TBC spallation, erosion, and pitting. Measurements were made in a zero pressure gradient turbulent boundary layer at two Reynolds numbers (Rex = 500,000 and 900,000) and three freestream turbulence levels (Tu = 1%, 5%, and 11%). Measurements at low freestream turbulence indicate augmentation factors ranging from 1.1–1.5 for St/Sto and from 1.3–3.0 for cf /cfo (Sto and cfo are smooth plate values). For the range of roughness studied (average roughness height, k, less than 1/3rd the boundary layer thickness) the level of cf augmentation agrees well with accepted equivalent sandgrain (ks ) correlations when ks is determined from a roughness shape/density parameter. This finding is not repeated with heat transfer, in which case the ks -based St correlations overpredict the measurements. Both cf and St correlations severely underpredict the effect of roughness for k+ < 70 (when ks , as determined by the roughness shape/density parameter, is small). A new ks correlation based on the rms surface slope angle overcomes this limitation. Comparison of data from real roughness and simulated (ordered cones or hemispheres) roughness suggests that simulated roughness is fundamentally different from real roughness. Specifically, ks values that correlate cf for both simulated and real roughness are found to correlate St for simulated roughness but overpredict St for real roughness. These findings expose limitations in the traditional equivalent sandgrain roughness model and the common use of ordered arrays of roughness elements to simulate real roughness surfaces. The elevated freestream turbulence levels produce augmentation ratios of 1.24 & 1.5 (St/Sto ) and 1.07 & 1.16 (cf /cfo ) compared to the Tu = 1% flow over the smooth reference plate. The combined effects of roughness and elevated freestream turbulence are greater than their added effects suggesting that some synergy occurs between the two mechanisms. Specifically, skin friction augmentation for combined turbulence and roughness is up to 20% greater than that estimated by adding their separate effects and 8% greater than compounding (multiplying) their separate effects. For heat transfer augmentation, the combined effect of turbulence and roughness is 5% higher than that estimated by compounding their separate effects at high freestream turbulence (Tu = 11%). At low turbulence (Tu = 5%), there is a negative synergy between the two augmentation mechanisms as the combined effect is now 13% lower than that estimated by compounding their separate effects.Copyright © 2002 by ASME

Predicting transition without empiricism or DNS

Abstract: A numerical procedure for predicting the receptivity of laminar boundary layers to freestream turbulence consisting of vortex arrays with a rbitrary orientation ha s been developed. Results s how that t he bounda ry layer is most receptivity to those vortices which ha ve their axes approximately in the streamwise direction and vortex wavelengths of approximately 1.2 . The computed near wall gains for isotropic turbulence are similar in magnitude to p reviously published experimental values used to predict transition. The new procedure is therefore capable of predicting the development of the fluctuations in the laminar boundary layer from values of t he freestream turbulence intensity and length scale and hence determining the start of t ransition without resorting to an y empirical correlation.