Showing papers on "Freestream published in 2017"

Experimental and Numerical Investigation of Sweeping Jet Film Cooling

Abstract: A companion experimental and numerical study was conducted of the performance of a row of 5 sweeping jet (SJ) film cooling holes consisting of conventional curved fluidic oscillators with an aspect ratio (AR) of unity and a hole spacing of P/D = 8.5. Adiabatic film effectiveness (η), thermal field (θ), convective heat transfer coefficient (h) and discharge coefficient (CD) were measured at two different freestream turbulence levels (Tu = 0.4% and 10.1%) and four blowing ratios (M = 0.98, 1.97, 2.94 and 3.96) at a density ratio (DR) of 1.04 and hole Reynolds number of ReD = 2800. Adiabatic film effectiveness and thermal field data were also acquired for a baseline 777-shaped hole. The sweeping jet film cooling hole showed significant improvement in cooling effectiveness in the lateral direction due to the sweeping action of the fluidic oscillator. An unsteady RANS simulation was performed to evaluate the flow field at the exit of the hole. Time resolved flow fields revealed two alternating streamwise vortices at all blowing ratios. The sense of rotation of these alternating vortices is opposite to the traditional counter rotating vortex pair (CRVP) found in a ‘jet in crossflow’ and serves to spread the film coolant laterally.Copyright © 2017 by ASME

Large-Eddy Simulation of the Gust Index in an Urban Area Using the Lattice Boltzmann Method

Abstract: We used numerical simulations to investigate the general relationship between urban morphology and the intensity of wind gusts in built-up areas at the pedestrian level. The simulated urban boundary layer developed over a 19.2 km (length) $$\times $$ 4.8 km (width) $$\times $$ 1.0 km (height) simulation domain, with 2-m resolution in all directions, to explicitly resolve the detailed shapes of buildings and the flow at the pedestrian level. This complex computation was accomplished using the lattice Boltzmann method and by implementing a large-eddy simulation model. To generalize the results, a new parameter that expresses the intensity of gusts (the gust index, $${\tilde{U}}_{ max})$$ was defined as the local maximum wind speed divided by the freestream velocity. In addition, this parameter was decomposed into the mean wind-speed ratio, $${\tilde{U}} $$ and turbulent gust ratio, $${\tilde{U}}^{{\prime }}$$ to evaluate the qualities of gusts. These parameters were useful for quantitatively comparing the gust intensities within urban canopies at different locations or even among different experiments. In addition, the entire horizontal domain was subdivided into homogeneous square patches, in which both the simulated gust parameters and the morphological characteristics of building geometries were averaged. This procedure masked the detailed structure of individual buildings but retained the bulk characteristics of the urban morphology. At the pedestrian level, the gust index decreased with increasing building cover. Compared to $${\tilde{U}} $$ , the quantity $${\tilde{U}}^{{\prime }}$$ notably contributed to the index throughout the range of plan area index $$(\lambda _p)$$ values. The dependences of all normalized wind-speed ratios transiently changed at $$\lambda _p =~0.28$$ . In cases where $$\lambda _p < 0.28, {\tilde{U}} $$ decreased with increasing $$\lambda _p $$ , although $${\tilde{U}}^{{\prime }}$$ was almost constant. In cases where $$\lambda _p > 0.28, {\tilde{U}}$$ was almost constant and $${\tilde{U}}^{{\prime }}$$ decreased with increasing $$\lambda _p $$ . This was explained by the change in flow regimes within the building canyon. At a higher elevation above the canopy layer, $$\lambda _p $$ becomes less relevant to normalized wind-speed ratios, and instead the aerodynamic roughness length became important.

An experimental study on the characteristics of wind-driven surface water film flows by using a multi-transducer ultrasonic pulse-echo technique

Abstract: An experimental study was conducted to investigate the characteristics of surface water film flows driven by boundary layer winds over a test plate in order to elucidate the underlying physics pertinent to dynamic water runback processes over ice accreting surfaces of aircraft wings. A multi-transducer ultrasonic pulse-echo (MTUPE) technique was developed and applied to achieve non-intrusive measurements of water film thickness as a function of time and space to quantify the transient behaviors of wind-driven surface water film flows. The effects of key controlling parameters, including freestream velocity of the airflow and flow rate of the water film, on the dynamics of the surface water runback process were examined in great details based on the quantitative MTUPE measurements. While the thickness of the wind-driven surface water film was found to decrease rapidly with the increasing airflow velocity, various surface wave structures were also found to be generated at the air/water interface as the surf...

Turbulence Intensity Effects on Laminar Separation Bubbles Formed over an Airfoil

Abstract: The effects of freestream turbulence intensity on the mean topology and transition characteristics of laminar separation bubbles forming over the suction side of a NACA 0018 airfoil are investigate...

An analytical model for a full wind turbine wake

Abstract: An analytical wind turbine wake model is proposed to predict the wind velocity distribution for all distances downwind of a wind turbine, including the near-wake. This wake model augments the Jensen model and subsequent derivations thereof, and is a direct generalization of that recently proposed by Bastankhah and Porte-Agel. The model is derived by applying conservation of mass and momentum in the context of actuator disk theory, and assuming a distribution of the double-Gaussian type for the velocity deficit in the wake. The physical solutions are obtained by appropriate mixing of the waked- and freestream velocity deficit solutions, reflecting the fact that only a portion of the fluid particles passing through the rotor disk will interact with a blade.

Nonintrusive Freestream Velocity Measurement in a Large-Scale Hypersonic Wind Tunnel

Abstract: H IGH-SPEED wind tunnels typically rely on pressure and/or temperature measurement and nozzle-flow calculations to determine the freestream conditions. This practice can require a complex treatment of the thermochemical state of the gas. The calorically perfect gas assumption begins to break down when producing air or N2 flows from a stagnated reservoir to freestream Mach number M∞ > 6. Rapid expansion in the nozzle can require modeling thermodynamic nonequilibrium processes, and if the gas is stagnated to high enthalpy, nonequilibrium chemistry must also be considered [1].Moreover, an excluded-volume equation of state may need to be used for high reservoir densities [2,3]. Although the modeling framework of these flows is tractable, some of the fundamentals pertaining to the thermochemical rate processes continue to be an ongoing topic of research [1]. One means of validating these run condition and nozzle-flow calculations is direct measurement in the freestream. Particle-based methods of velocimetry, such as particle image velocimetry, can produce high-quality multicomponent velocity data [4]. However, the engineering challenges associated with implementing particlebased techniques in large-scale high-speed facilities include timing, particle-seeding density and uniformity, and minimizing flow disturbances when injecting particles [5]. More importantly, there is the fundamental limitation of reduced particle response at Knudsen and Reynolds numbers [6] typical of high-speed wind tunnels, which can compromise the resolution of fine time and length scales. In contrast to the limitations of particle-based techniques, implementation of tagging velocimetry is not constrained by the aforementioned issues in large-scale high-speed facilities. Noted methods and tracers of tagging velocimetry include VENOM [7], APART[8],RELIEF [9], FLEET [10], STARFLEET [11], PLEET [12], nitrogen oxides [13–15], iodine [16], acetone [17], and the hydroxyl group techniques [18–21]. Continually advancing laser and imaging technology has enabled tagging velocimetry to be used in large-scale facilities where other approaches are difficult to implement. This technical note reports the direct measurement of freestreamvelocity profiles in AEDC Hypervelocity Tunnel 9 (Tunnel 9) with krypton tagging velocimetry (KTV). The KTV experimental setup is described, followed by an explanation of Tunnel 9 and the conventional method of run-condition calculation. KTV exposures are presented for four different Tunnel 9 conditions. Then, for two conditions, instantaneous velocity profiles and a comparison of the freestream velocity as calculated by conventional methods and KTV are presented.

Investigation of strut-ramp injector in a Scramjet combustor: Effect of strut geometry, fuel and jet diameter on mixing characteristics

Abstract: The strut-based injector has been found to be one of the most promising injector designs for a supersonic combustor, offering enhanced mixing of fuel and air. The mixing and flow field characteristics of the straight (SS) and Tapered strut (TS), with fixed ramp angle and height at freestream Mach number 2 in conjunction with fuel injection at Mach 2.3 have been investigated numerically and reported. In the present investigation, hydrogen (H2) and ethylene (C2H4) are injected in oncoming supersonic flow from the back of the strut, where jet to freestream momentum ratio is maintained at 0.79 and 0.69 for H2 and C2H4, respectively. The predicted wall static pressure and species mole fractions at various downstream locations are compared with the experimental data for TS case with 0.6 mm jet diameter and found to be in good agreement. Further, the effect of jet diameter and strut geometry on the near field mixing in strut ramp configuration is discussed for both the fuels. The numerical results are assessed based on various parameters for the performance evaluation of different strut ramp configurations. The SS configuration for both the injectant has been found to be an optimum candidate; also it is observed that for higher jet diameter larger combustor length is required to achieve satisfactory near field mixing.

Flow speed has little impact on propulsive characteristics of oscillating foils

Abstract: Experiments are reported on the performance of a pitching and heaving two-dimensional foil in a water channel in either continuous or intermittent motion. We find that the thrust and power are independent of the mean freestream velocity for two-fold changes in the mean velocity (four-fold in the dynamic pressure), and for oscillations in the velocity up to 38\% of the mean, where the oscillations are intended to mimic those of freely swimming motions where the thrust varies during the flapping cycle. We demonstrate that the correct velocity scale is not the flow velocity but the mean velocity of the trailing edge. We also find little or no impact of streamwise velocity change on the wake characteristics such as vortex organization, vortex strength, and time-averaged velocity profile development---the wake is both qualitatively and quantitatively unchanged. Our results suggest that constant velocity studies can be used to make robust conclusions about swimming performance without a need to explore the free-swimming condition.

Interaction between plasma synthetic jet and subsonic turbulent boundary layer

Abstract: This paper experimentally investigates the interaction between a plasma synthetic jet (PSJ) and a subsonic turbulent boundary layer (TBL) using a hotwire anemometer and phase-locked particle imaging velocimetry. The PSJ is interacting with a fully developed turbulent boundary layer developing on the flat wall of a square wind tunnel section of 1.7 m length. The Reynolds number based on the freestream velocity (U∞ = 20 m/s) and the boundary layer thickness (δ99 = 34.5 mm) at the location of interaction is 44 400. A large-volume (1696 mm3) three-electrode plasma synthetic jet actuator (PSJA) with a round exit orifice (D = 2 mm) is adopted to produce high-speed (92 m/s) and short-duration (Tjet = 1 ms) pulsed jets. The exit velocity variation of the adopted PSJA in a crossflow is shown to remain almost identical to that in quiescent conditions. However, the flow structures emanating from the interaction between the PSJ and the TBL are significantly different from what were observed in quiescent conditions. I...

Minimum Induced Drag Theorems for Multiwing Systems

Abstract: Under the assumption of a rigid wake aligned with the freestream velocity, a computationally efficient induced drag minimization procedure, tailored for the preliminary design phases of generic mul...

Buzz Flows in an External-Compression Inlet with Partially Isentropic Compression

Abstract: A rectangular external-compression supersonic inlet with partially isentropic compression is experimentally studied at a freestream Mach number of 2.0. During the test, the inlet operates at the li...

Study of the Afterbody Radiation during Mars Entry in an Expansion Tube

Abstract: Recent work has shown that a significant contributor to the afterbody aeroheating during Mars entry is radiation1-3. However, relevant ground test data is not available to help assess the uncertainty associated with prediction of the radiation when designing the thermal protection system for the aeroshell afterbody. The present work is aimed at studying the afterbody radiation experienced during Mars entry through experiments. The X2 expansion tube at the University of Queensland is used to generate the relevant experimental freestream flow conditions. Analysis is carried out to characterize the generated experimental freestream conditions. A two dimensional wedge model is used to produce the expanding flow which simulates aspects of the afterbody flow around Mars entry vehicles. This generated expanding flow has a test time of about 50-110 μs, thus, allowing flow measurements to be conducted. Horizontal emission spectroscopy measurements at 3.25 mm above the test model was taken for the carbon dioxide 4.3 μm band at freestream velocities of 2.8, 3.4, and 4.0 km/s. The measurements were compared to numerical calculations of the radiation emission generated from three-dimensional flowfield calculations. The comparison showed that the numerical result was significantly overpredicted for the 2.8 km/s condition and underpredicted for the 4.0 km/s condition.

Dissipation scaling in constant-pressure turbulent boundary layers

Abstract: Using results from previous direct numerical simulations and experiments in the outer region of spatially evolving turbulent boundary layers, we compute the streamwise evolution and the wall-normal variation of the dissipation parameter CE, namely, the turbulent kinetic energy dissipation rate, normalized by appropriate powers of the local turbulent kinetic energy and integral length scale. For Reθ3 10000 (Reθ is a Reynolds number on the freestream velocity and the local momentum thickness), CE is essentially constant in the streamwise direction, but varies by up to 50% in the wall-normal direction. For Reθ<10000, CE is additionally found to vary in the streamwise direction and is inversely proportional to the local turbulence Reynolds number Reλ.

Investigation of reduced frequency and freestream turbulence effects on dynamic stall of a pitching airfoil

Abstract: In this study, the dynamic stall evolutions were investigated using particle image velocimetry (PIV) in a water channel with Reynolds number Re = 4.5 × 103 based on the chord length. The airfoil pitching waveform was performed under the condition calculated from the angle of attack histogram of a vertical axis wind turbine (VAWT). Using PIV, the instantaneous vorticity contours and streamlines can be revealed. Based on the formation of the leading edge vortex, the stall angle can be explored at reduced frequency k = 0.09, 0.18, and 0.27. It was found that the stall angle was delayed from the angle of attack α = 16° to α = 30° as reduced frequency increased from k = 0.09 to 0.27. The hysteresis effect of stall angle delay was more pronounced for high reduced frequency. Moreover, the freestream turbulence effect on the pitching airfoil was investigated with turbulence intensity TI = 0.5 and 6.9 %. As found, the stall angles were postponed to higher angles of attack for the high turbulence intensity. The phase difference between TI = 0.5 and 6.9 % were ∆α = 8°, 4°, and 4° for k = 0.09, 0.18, and 0.27, respectively. For TI = 6.9 %, enhanced turbulence mixing reduces the velocity deficit (u/U < 1) and flow reversal (u/U < 0). In addition, the maximum velocity is reduced from u/U = 1.8 to 1.2 and the S-shaped velocity profile is diminished or weakened for TI = 6.9 %. Thus, the dynamic stall is further delayed to the downstroke. The circulation values increase rapidly to maximum and then drop quickly after dynamic stall for k = 0.18 and 0.27.

Swept-wing boundary-layer transition at various external perturbations: Scenarios, criteria, and problems of prediction

Abstract: This experimental work is devoted to a parametric study of characteristics of the laminar-turbulent transition occurred in a model of a crossflow-dominated swept-wing boundary layer. The experiments are performed in 39 different regimes with low and elevated freestream turbulence levels both in the absence and in the presence of steady freestream vortices of various spanwise scales. The measurements are performed at several values of freestream velocity for two types of distributed surface roughness. Transition scenarios and criteria of turbulence onset are investigated and the problem of transition prediction is examined. It is shown that in all studied cases, the turbulence onset starts with the appearance of local high-frequency secondary instability of the base flow perturbed by primary instability modes. It is found that the 30% level of the threshold zero-to-peak amplitude of combined boundary-layer disturbances (i.e., the steady ones plus unsteady ones) can be used in all studied cases as a simplif...

Unthrottled Flows with Complex Background Waves in Curved Isolators

Abstract: The unthrottled flows in curved isolators are studied experimentally and numerically. Wind-tunnel tests are conducted at a freestream Mach number of 4.92. The internal flowfield is visualized and m...

Computing Measured Spectra from Hypersonic Pitot Probes with Flow-Parallel Freestream Disturbances

Abstract: The relationship between freestream disturbances and measured pitot pressure spectra, defined as the transfer function, is investigated. Pitot probes used to characterize hypersonic facilities are ...

Influences of Freestream Turbulence on Flame Dynamics in a Supersonic Combustor

Abstract: Turbulent flame dynamics in an ethylene-fueled model scramjet is experimentally investigated in Mach 4.5 freestream flows with a total temperature of 2600 K using a pulsed arc-heated hypersonic wind-tunnel facility at the University of Notre Dame. Gaseous ethylene fuel is injected into the combustor through a supersonic nozzle, and is autoignited by high-enthalpy flows that are compressed and decelerated by a train of shock waves through the scramjet isolator/combustor. The turbulence level of the freestream is manipulated to investigate the influence of freestream turbulence on the flame dynamics in the supersonic combustor. General flame behavior is observed in time-averaged ethylene-flame-chemiluminescence images, whereas the detailed turbulent flame structures are resolved by imaging instantaneous ground-state hydroxyl radical (OH) distributions using the planar-laser-induced-fluorescence technique, wherein a planar laser sheet is projected into the scramjet in the counterstreamwise direction to illum...

Reduction of freestream turbulence at low velocities

Abstract: Controlling freestream turbulence (FST) in low-turbulence wind or water channels is a common challenge and often difficult to achieve. Particularly at low velocities, design guidelines from literature may not fulfill their purpose and thus require alternative strategies. In this study, we propose the installation of a fine-meshed screen downstream of the contraction and not in the settling chamber as typically advised in literature. With this strategy, the lower operational limitation of our facility could be extended below $$U_{\infty }= 0.06\,\text {ms}^{-1}$$ and the turbulence intensity reduced by $$60 \%$$ at 0.04 ms $$^{-1}$$ . This is not only an improvement of freestream conditions but also the key to experiments in the laminar boundary layer, which is highly sensitive to FST. In fact, two fundamentally different transition mechanism can be distinguished with this approach.

Blockage and Three-Dimensional Effects in Wind-Tunnel Testing of Ice Accretion over Wings

Abstract: Wind-tunnel testing of ice accretion over aircraft wings is simulated numerically to investigate the effects of tunnel blockage and of the wall–wing interference at the wall–model juncture. The open-source OpenFOAM software is used to compute the flowfield and the trajectories of water droplets. The PoliMIce ice-prediction software is applied to solve the multiphase flow around the body surface and to compute the thickness of the iced layer. Blockage effects are investigated for diverse operating conditions. Similar to dry testing, wind-tunnel blockage is found to produce a variation of the apparent angle of attack of the airfoil with respect to the freestream. However, different from dry testing, the blockage correction is found to be a function of time, due to the continuous modification of the airfoil shape in time resulting from ice accretion. The wall–wing interference results in the occurrence of a shadow region where water droplets do not impinge. In the rime-ice regime, the shadow region is theref...

Effect of Impinging Wake Turbulence on the Dynamic Stall of a Pitching Airfoil

Abstract: Dynamically moving airfoils are encountered in helicopter rotors, wind-turbine blades, and maneuvering aircraft. A clearer understanding of how freestream disturbances affect the aerodynamic forces...