Showing papers on "Freestream published in 2016"

On the reduction of aerofoil-turbulence interaction noise associated with wavy leading edges

Abstract: An aerofoil leading-edge prole based on wavy (sinusoidal) protuberances/tubercles is investigated to understand the mechanisms by which they are able to reduce the noise produced through the interaction with turbulent mean flow. Numerical simulations are performed for non-lifting at-plate aerofoils with straight and wavy leading edges (de- noted by SLE and WLE, respectively) subjected to impinging turbulence that is synthetically generated in the upstream zone (freestream Mach number of 0.24). Full three-dimensional Euler (inviscid) solutions are computed for this study thereby eliminating self-noise components. A high-order accurate nite-dierence method and artefact-free boundary conditions are used in the current simulations. Various statistical analysis methods, including frequency spectra, are implemented to aid the understanding of the noise-reduction mechanisms. It is found with WLEs, unlike the SLE, that the surface pressure fluctuations along the leading edge exhibit a signicant source cut-o eect due to geometric obliqueness which leads to reduced levels of radiated sound pressure. It is also found that there exists a phase interference eect particularly prevalent between the peak and the hill centre of the WLE geometry, which contributes to the noise reduction in the mid- to high-frequency range.

Pressure Fluctuations induced by a Hypersonic Turbulent Boundary Layer.

Abstract: Direct numerical simulations (DNS) are used to examine the pressure fluctuations generated by a spatially-developed Mach 5.86 turbulent boundary layer. The unsteady pressure field is analyzed at multiple wall-normal locations, including those at the wall, within the boundary layer (including inner layer, the log layer, and the outer layer), and in the free stream. The statistical and structural variations of pressure fluctuations as a function of wall-normal distance are highlighted. Computational predictions for mean velocity profiles and surface pressure spectrum are in good agreement with experimental measurements, providing a first ever comparison of this type at hypersonic Mach numbers. The simulation shows that the dominant frequency of boundary-layer-induced pressure fluctuations shifts to lower frequencies as the location of interest moves away from the wall. The pressure wave propagates with a speed nearly equal to the local mean velocity within the boundary layer (except in the immediate vicinity of the wall) while the propagation speed deviates from the Taylor's hypothesis in the free stream. Compared with the surface pressure fluctuations, which are primarily vortical, the acoustic pressure fluctuations in the free stream exhibit a significantly lower dominant frequency, a greater spatial extent, and a smaller bulk propagation speed. The freestream pressure structures are found to have similar Lagrangian time and spatial scales as the acoustic sources near the wall. As the Mach number increases, the freestream acoustic fluctuations exhibit increased radiation intensity, enhanced energy content at high frequencies, shallower orientation of wave fronts with respect to the flow direction, and larger propagation velocity.

Characterization of wind velocities in the upstream induction zone of a wind turbine using scanning continuous-wave lidars

Abstract: As a wind turbine generates power, induced velocities, lower than the freestream velocity, will be present upstream of the turbine due to perturbation of the flow by the rotor. In this study, the upstream induction zone of a 225 kW horizontal axis Vestas V27 wind turbine located at the Danish Technical University's Riso campus is investigated using a scanning Light Detection and Ranging (lidar) system. Three short-range continuous-wave “WindScanner” lidars are positioned in the field around the V27 turbine allowing detection of all three components of the wind velocity vectors within the induction zone. The time-averaged mean wind speeds at different locations in the upstream induction zone are measured by scanning a horizontal plane at hub height and a vertical plane centered at the middle of the rotor extending roughly 1.5 rotor diameters (D) upstream of the rotor. Turbulence statistics in the induction zone are studied by more rapidly scanning along individual lines perpendicular to the rotor at differ...

Experimental study of transonic buffet phenomenon on a 3D swept wing

Abstract: The objective of this paper is to analyze the 3D buffet phenomenon which appears on a swept wing at a high Mach number and/or high angle of attack. This aerodynamic instability induces strong wall pressure fluctuations and as such limits aircraft envelope. Consequently, it is interesting to understand this phenomenon in order to not only improve aircraft performance but also to provide more flexibility during the design phase. Results from two wind tunnel tests on a 3D half wing-body configuration are presented for several freestream Mach numbers (0.78–0.86) and Reynolds numbers (2.83 × 106–8.49 × 106, based on the aerodynamic mean chord). The buffet phenomenon is characterized using steady and unsteady wall pressure measurements. By opposition to the 2D buffet which exhibits rather a well marked peak in the pressure spectra, the 3D buffet is characterized by a broadband bump at a much higher Strouhal number (between 4 and 7 times higher). It is also observed that two different instabilities coexist on th...

Hypersonic Wind-Tunnel Measurements of Boundary-Layer Transition on a Slender Cone

Abstract: Boundary-layer transition was studied on a sharp 7 deg cone in two hypersonic wind tunnels at Mach numbers of 5, 6, 8, and 14 over a range of freestream Reynolds numbers between 3.3 and 15.4×106/m. High-speed schlieren measurements visualized the intermittent formation of instabilities and turbulent spots within the transitional boundary layer. Surface pressure and heat-transfer measurements revealed how the intermittent behavior of the boundary layer produces the mean character of these quantities. Transition at Mach 5 appeared to be initiated by a combination of first- and second-mode instabilities. These disturbances were isolated and surrounded by an otherwise smooth boundary layer. At higher Mach numbers, the boundary layer was dominated by second-mode instabilities, which covered most of the model before breakdown into turbulent spots. The spots remain surrounded by second-mode waves throughout the transitional region. These differences alter the pressure fluctuations and heat transfer profiles duri...

Unsteady Low-Speed Wind Tunnels

Abstract: A theory based upon linearized governing equations is developed that describes the operation principles and design parameters for low-speed wind tunnels with longitudinal freestream oscillations. Existing measurements made in unsteady wind tunnels are shown to be consistent with the theory and targeted validation experiments performed in a variable-geometry blowdown-type wind tunnel revealed excellent correspondence with the theoretical results. In particular, the tunnel frequency bandwidth is proportional to the mean tunnel freestream velocity and inversely proportional to the test-section length and the square of the exit area to test-section area ratio. The acoustics equations reveal a “Helmholtz damping ratio” that is not only dependent on the tunnel geometry and exit area but also proportional to the freestream Mach number. At appreciable reduced frequencies, dynamic stall on the louver vanes increases pressure losses, thereby reducing the mean flow speeds. Varying the exit area results in louver-van...

Neurofuzzy-Model-Based Unsteady Aerodynamic Computations Across Varying Freestream Conditions

Abstract: This paper presents a reduced-order modeling approach based on recurrent local linear neurofuzzy models for predicting generalized aerodynamic forces in the time domain. Regarding aeroelastic applications, the unsteady aerodynamic loads are modeled as a nonlinear function of structural eigenmode-based disturbances. In contrast to established aerodynamic input/output model approaches trained by high-fidelity flow simulations, the Mach number is considered as an additional model input to account for varying freestream conditions. To train the relationship between the input parameters and the corresponding flow-induced forces, the local linear model tree algorithm is adopted in this work. The proposed method is tested exemplarily with respect to the AGARD 445.6 configuration in the subsonic, transonic, and supersonic flight regimes. It is shown that good conformity is obtained between the reduced-order model results and the respective full-order computational-fluid-dynamics solution. A further comparative an...

Influence of the bluff body shear layers on the wake of a square prism in a turbulent flow

Abstract: The influence of freestream turbulence on the transitional characteristics of a 2D square prism shear layer was considered experimentally. Bypass transition was observed to cause intermittent shear layer reattachment; this event has a cascading influence downstream, altering the formation of the von-K\'arm\'an street.

Effects of Freestream Turbulence in a Model Wind Turbine Wake

Abstract: The flow structure in the wake of a model wind turbine is explored under negligible and high turbulence in the freestream region of a wind tunnel at R e ∼ 7 × 10 4 . Attention is placed on the evolution of the integral scale and the contribution of the large-scale motions from the background flow. Hotwire anemometry was used to obtain the streamwise velocity at various streamwise and spanwise locations. The pre-multiplied spectral difference of the velocity fluctuations between the two cases shows a significant energy contribution from the background turbulence on scales larger than the rotor diameter. The integral scale along the rotor axis is found to grow linearly with distance, independent of the incoming turbulence levels. This scale appears to reach that of the incoming flow in the high turbulence case at x / d ∼ 35–40. The energy contribution from the turbine to the large-scale flow structures in the low turbulence case increases monotonically with distance. Its growth rate is reduced past x / d ∼ 6–7. There, motions larger than the rotor contribute ∼ 50 % of the total energy, suggesting that the population of large-scale motions is more intense in the intermediate field. In contrast, the wake in the high incoming turbulence is quickly populated with large-scale motions and plateau at x / d ∼ 3 .

Characteristics of Cascaded Fuel Injectors Within an Accelerating Scramjet Combustor

Abstract: Performances of cascaded hydrogen injectors within an accelerating scramjet combustor were characterized via a three-dimensional numerical study. Two streamwise-aligned jets are employed, with the upstream injector half the diameter of the rear jet. Each are inclined at 45 deg to the freestream and achieve a jet-to-freestream momentum ratio of unity. Performance was evaluated over a range of freestream Mach numbers, modeling combustor entrance conditions on an accelerating access-to-space scramjet trajectory. Distance between injectors was varied to estimate the optimum jet-to-jet spacing to achieve robust performance across the Mach number range. It is shown that the downstream injector benefits jointly from shielding effects induced by the smaller upstream injector, as well as its increased diameter. Injector spacings greater than two total jet diameters Dt displayed improved absolute jet penetration, spanwise spread, and entrainment rates over an equivalent single injector across all Mach numbers. Jet ...

Prediction of Nonequilibrium Air Plasma Radiation Behind a Shock Wave

Abstract: The absolute radiation measurements obtained in the electric arc-driven shock-tube facility at NASA Ames Research Center were analyzed to test the collisional-radiative model developed at Ecole Centrale Paris. Two conditions representative of Earth reentry at 10.54 and 11.17 km/s were investigated in the vacuum ultraviolet and infrared spectral ranges. For each of the conditions, the corresponding charge-coupled device images were analyzed. The electron number density was inferred from Stark-broadened nitrogen and Hα lines. Comparisons with the predicted electron number density profiles enabled us to validate the ionization rate constant model implemented in the flowfield solver and to accurately locate the shock front. For both freestream conditions and all the spectral ranges, the predictions of the initial intensity rises were improved when the total spatial smearing (due to the shock motion, the optics, and the camera) was taken into account. The nonequilibrium intensities observed in the vacuum ultr...

Comparison of Passive Flow Control Methods for a Cavity in Transonic Flow

Abstract: A comparative study of different passive control techniques was conducted on a cavity with a length of 320 mm with length-to-depth and length-to-width ratios of five and two, respectively. The tests were conducted at a freestream Mach number of 0.71. Both leading-edge and trailing-edge modifications were included in the studies. Results from surface pressure measurements showed that leading-edge control techniques were more effective at suppressing cavity tone amplitudes than trailing-edge modifications. A square-tooth spoiler showed the greatest reduction in tonal amplitude (8.8 dB); however, a sawtooth spoiler showed the greatest reduction in overall sound pressure level (8.13 dB). Velocity measurements inside the cavity were made using particle image velocimetry for the clean cavity and the cavity with sawtooth spoilers. The results showed a reduction in momentum exchange between the freestream flow and the cavity when spoilers were used. This is proposed to be the main reason for the reduced tonal amp...

Simultaneous Infrared And Pressure Measurements Of Crossflow Instability Modes For HIFiRE 5 (POSTPRINT)

Abstract: A 2:1 elliptic cone model was tested in a Mach-6 quiet wind tunnel with both low and elevated freestream noise levels. Simultaneous measurements were made using an infrared camera and 22 pressure sensors mounted flush with the model surface. Infrared measurements confirmed the presence of stationary crossflow vortices in quiet flow. This is the first time infrared imaging has identified stationary crossflow vortices in a quiet wind tunnel. Power spectral densities of the signals from the pressure transducers also revealed the presence of traveling crossflow waves much farther upstream than had previously been measured. Stationary crossflow wavelengths and traveling crossflow wave properties matched those measured on a previous model of the same geometry. Neither stationary nor traveling crossflow vortices were observed in noisy flow, even though the boundary layer was seen to transition from laminar to turbulent as the freestream Reynolds number was increased. The primary transition mechanism in noisy flow is unknown.

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.

Optical Characterization of Nozzle-Wall Mach-6 Boundary Layers

Abstract: This paper presents the results of experimental investigation of aero-optical distortions caused by turbulent and laminar boundary layers in the Boeing/AFOSR Mach-6 Quiet Tunnel at Purdue University. Using optical window inserts installed in the test section, aerooptical distortions of the turbulent boundary layer at different freestream pressures were measured with a high-speed wavefront sensor at sampling speeds up to 1 MHz. Temporal and spatial statistics of the related aero-optical structure were measured and found to be quite similar to the ones at subsonic and low supersonic speeds. The existing model was found to under-predict the experimentally-measured levels of aero-optical distortions for turbulent boundary layers by 20%; possible reasons for this discrepancy are discussed. Intermittent turbulent spots and single-mode transitional events in the laminar boundary layers were observed and briefly analyzed.

OH PLIF Visualization of a Premixed Ethylene-fueled Dual-Mode Scramjet Combustor

Abstract: Hydroxyl radical (OH) planar induced laser fluorescence (PLIF) measurements have been performed in a small-scale scramjet combustor at the University of Virginia Aerospace Research Laboratory at nominal simulated Mach 5 enthalpy. OH lines were carefully chosen to have fluorescent signal that is independent of pressure and temperature but linear with mole fraction. The OH PLIF signal was imaged in planes orthogonal to and parallel to the freestream flow at different equivalence ratios. Flameout limits were tested and identified. Instantaneous planar images were recorded and analyzed to compare the results with width increased dual-pump enhanced coherent anti-Stokes Raman spectroscopy (WIDECARS) measurements in the same facility and large eddy simulation/Reynolds average Navier-Stokes (LES/RANS) numerical simulation. The flame angle was found to be approximately 10 degrees for several different conditions, which is in agreement with numerical predictions and measurements using WIDECARS. Finally, a comparison between NO PLIF non-combustion cases and OH PLIF combustion cases is provided: the comparison reveals that the dominant effect of flame propagation is freestream turbulence rather than heat release and concentration gradients.

Visualization of Simulated Fuel–Air Mixing in a Dual-Mode Scramjet

Abstract: Nitric oxide planar laser-induced fluorescence measurements have been performed in a small-scale scramjet combustor at the University of Virginia Aerospace Research Laboratory at nominal simulated Mach 5 flight. A mixture of nitric oxide and nitrogen was injected at the upstream end of the inlet isolator as a surrogate for ethylene fuel, and the mixing of this fuel simulant was studied with and without a shock train. The shock train was produced by an air throttle, which simulated the blockage effects of combustion downstream of the cavity flameholder. Nitric oxide planar laser-induced fluorescence signal was imaged in a plane orthogonal to the freestream at the leading edge of the cavity. Instantaneous planar images were recorded and analyzed to identify the most uniform cases, which were achieved by varying the location of the fuel injection and shock train. This method was used to screen different possible fueling configurations to provide optimized test conditions for follow-on combustion measurements...

Numerical Simulations of Transition due to Isolated Roughness Elements at Mach 6

Abstract: An accurate prediction of transition onset behind an isolated roughness element has not yet been established. This is particularly important in hypersonic flow, where transition is accompanied by increased surface heating. In the present contribution, a number of direct numerical simulations have been performed of a Mach 6 boundary layer over a flat plate with isolated roughness elements. The effects of roughness shape, planform, ramps, and freestream disturbance levels on instability growth and transition onset are investigated. It is found that the frontal shape has a large effect on the transition onset, which is in agreement with previous studies, whereas the roughness element planform has a marginal influence. A new result is that the roughness shape in the streamwise direction (in particular, the aft section) is also an important characteristic, since an element with a ramped-down aft section allows the detached shear layer to spread out and weaken, leading to a lower instability growth rate. Above ...

Streamwise Oscillation of Airfoils into Reverse Flow

Abstract: A NACA 0012 airfoil is oscillated in streamwise direction in a constant freestream and at a fixed incidence angle such that reverse flow occurs cyclically. Force measurements reveal that lift is close to unsteady theory while advancing into the freestream, if the angle of attack permits attached flow. Lift is augmented at large angles of attack, where the flow is separated under steady conditions, and does not become appreciatively negative in flow reversal for either attached or separated flow, contrary to one unsteady theory but supported by another. Dye flow visualization reveals a coherent vortical structure upstream of the leading edge before flow reversal, which is believed to attenuate negative lift.

Numerical simulation of the evolution of unstable disturbances of various modes and initial stages of the laminar-turbulent transition in the boundary layer at the freestream Mach number М = 6

Abstract: Direct numerical simulations of linear and nonlinear stages of the evolution of unstable disturbances of various modes and initial stages of the laminar-turbulent transition in the boundary layer on a flat plate at the freestream Mach number M = 6 are performed on the basis of full unsteady Navier–Stokes equations for a compressible gas. A considerable effect of three-dimensional unstable disturbances on initiation of the laminar-turbulent transition is demonstrated.

Local Burning Rates and Heat Flux for Forced Flow Boundary-Layer Diffusion Flames

Abstract: A methodology based on the Reynolds analogy was developed earlier that allowed for the estimation of local mass burning rates and heat fluxes in free-convection laminar boundary-layer diffusion flames In this study, the relationship was examined in a forced-convective environment using methanol as a liquid fuel The gas-phase temperature profiles across the laminar boundary layer with a methanol diffusion flame established over it were measured with the freestream air flowing parallel to the condensed fuel surface Local and averaged mass burning rates were measured along with shear stresses at the fuel surface The fuel consumption rate and flame lengths were observed to increase monotonically with an increase in the freestream velocity Although the initial study was taken in the laminar regime, further extensions of the technique could be applicable to turbulent boundary-layer combustion in propulsion-oriented research