Showing papers on "Freestream published in 2018"

Quantification of airborne odor plumes using planar laser-induced fluorescence

Abstract: Planar laser-induced fluorescence (PLIF) is used to quantify spatiotemporal structure of gaseous scalar plumes (Sc $$\approx 1.5$$ ) in a benchtop-scale low-speed wind tunnel. The study is motivated by a desire to understand variations in information content in odor plumes used by animals for navigation. Acetone vapor is used as a fluorescent odor surrogate and is released isokinetically to form a neutrally buoyant plume. Three cases are investigated: a near-bed scalar release at 10 cm/s, a freestream scalar release at 10 cm/s, and a second freestream scalar release at 20 cm/s. PLIF image data are collected at 15 Hz and processed to provide distributions of instantaneous concentrations. Spatial distributions of mean concentration, root-mean-square fluctuations, and concentration intermittency are presented, along with probability density functions of concentrations at select locations. The results demonstrate significant differences in spatiotemporal structure of the scalar plumes across the three tested cases. In particular, the near-bed plume is markedly different from those released in the freestream. The results have important implications for understanding gaseous plume structure. Specifically, the results suggest that different flow and release conditions control constraints and opportunities for animals using odor plumes for navigational purposes.

Transition Prediction in Hypersonic Boundary Layers Using Receptivity and Freestream Spectra.

Abstract: Boundary-layer transition in hypersonic flows over a straight cone can be predicted using measured freestream spectra, receptivity, and threshold values for the wall-pressure fluctuations at the tr...

Behavior of Shock Train in Curved Isolators with Complex Background Waves

Abstract: To study the duct curvature effects on a shock train in rectangular hypersonic inlet/isolator models, four curved isolators are designed and tested at a freestream Mach number of 4.92. Strong inter...

Throttling Process and Buzz Mechanism of a Supersonic Inlet at Overspeed Mode

Abstract: A rectangular external-compression supersonic inlet with a shock-on-lip Mach number of 2.0 is carefully studied at a freestream Mach number of 2.5. In the throttling process before the onset of buz...

Predicting Roof Pressures on a Low-Rise Structure From Freestream Turbulence Using Artificial Neural Networks

Abstract: This paper presents a generalized approach for predicting (i.e., interpolating) the magnitude and distribution of roof pressures near separated flow regions on a low-rise structure based on freestream turbulent flow conditions. A feed-forward multilayer artificial neural network (ANN) using a backpropagation (BP) training algorithm is employed to predict the mean, root-mean-square (RMS), and peak pressure coefficients on three geometrically scaled (1:50, 1:30, and 1:20) low-rise building models for a family of upwind approach flow conditions. A comprehensive dataset of recently published boundary layer wind tunnel (BLWT) pressure measurements was utilized for training, validation, and evaluation of the ANN model. On average, predicted ANN peak pressure coefficients for a group of pressure taps located near the roof corner were within 5.1, 6.9, and 7.7% of BLWT observations for the 1:50, 1:30, and 1:20 models, respectively. Further, very good agreement was found between predicted ANN mean and RMS pressure coefficients and BLWT data.

Investigation of shock–acoustic-wave interaction in transonic flow

Abstract: The buffet flow field around supercritical airfoils is dominated by self-sustained shock wave oscillations on the suction side of the wing. Theories assume that this unsteadiness is driven by an acoustic feedback loop of disturbances in the flow field downstream of the shock wave whose upstream propagating part is generated by acoustic waves. Therefore, in this study, first variations in the sound pressure level of the airfoil’s trailing-edge noise during a buffet cycle, which force the shock wave to move upstream and downstream, are detected, and then, the sensitivity of the shock wave oscillation during buffet to external acoustic forcing is analyzed. Time-resolved standard and tomographic particle-image velocimetry (PIV) measurements are applied to investigate the transonic buffet flow field over a supercritical DRA 2303 airfoil. The freestream Mach number is $$M_{\infty } = 0.73$$ , the angle of attack is $$\alpha = {3.5}^{\circ }$$ , and the chord-based Reynolds number is $$Re_c = 1.9\times 10^6$$ . The perturbed Lamb vector field, which describes the major acoustic source term of trailing-edge noise, is determined from the tomographic PIV data. Subsequently, the buffet flow field is disturbed by an artificially generated acoustic field, the acoustic intensity of which is comparable to the Lamb vector that is determined from the PIV data. The results confirm the hypothesis that buffet is driven by an acoustic feedback loop and show the shock wave oscillation to directly respond to external acoustic forcing. That is, the amplitude modulation frequency of the artificial acoustic perturbation determines the shock oscillation.

Experimental Analysis of a Small-Scale Rotor at Various Inflow Angles

Abstract: The performance characteristics of a rotor that is typically used for small unmanned aircraft were analyzed in a series of wind-tunnel experiments. Wind-tunnel measurements were conducted with the rotor at various inflow angles in order to investigate the effects on the rotor performance of partially or fully edgewise flow as they are typically encountered with small multirotor vehicles. Rotor tests were also performed under static and fully axial flow conditions in order to investigate the aerodynamic performance during hover as well as vertical climb and descent. The wind-tunnel data were corrected to account for the interference of wind-tunnel walls with the rotor wake and the blockage due to the presence of the rotor test stand in the wind-tunnel test section. The results are presented in terms of thrust, power, and roll moment coefficients under different rotor rotational speeds for a T-motor 18x6.1. Additionally, the measured thrust and power coefficients of Master Airscrew Electric 11x7 are compared with available propeller data under static and axial flow conditions for verification purposes. It is shown that the rotor performance characteristics are strongly affected by the freestream advance ratio and the freestream inflow angles. For example, at inflow angles that are typical for multirotor vehicles between about 15° and 0° with respect to the rotor disc, thrust coefficients stay constant or grow with increasing advance ratio, whereas power coefficients remain relatively constant with changing advance ratio.

Characterization of Freestream Disturbances in Conventional Hypersonic Wind Tunnels.

Abstract: Although low-disturbance (“quiet”) hypersonic wind tunnels are believed to provide more reliable extrapolation of boundary-layer transition behavior from ground to flight, the presently available q...

Effect of fineness ratios of 0.75–2.0 on aerodynamic drag of freestream-aligned circular cylinders measured using a magnetic suspension and balance system

Abstract: The drag coefficients of freestream-aligned circular cylinders of fineness ratios of 0.75–2.0 were investigated with a magnetic suspension and balance system (MSBS). The objective was to find the critical geometry, that is, the fineness ratio at which the drag coefficient becomes the local maximum within this ratio range. The experiments were conducted using the 1-m MSBS at the low turbulence wind tunnel at the Institute of Fluid Science, Tohoku University. The drag and base pressure coefficients of various cylinders were measured. The freestream velocity was varied to produce flows with Reynolds numbers ranging from $$0.6\times 10^5$$ to $$1.0\times 10^5$$ . The drag coefficient monotonically decreases as the fineness ratio increases and no critical geometry or local maximum of the drag coefficient is found in the range we investigated. The base pressure coefficient decreases as the fineness ratio increases. The temporal fluctuations of the base pressure of the models with fineness ratios of 0.75, 1.0, and 1.2 are approximately twice as large as that of the model with a ratio of 2.0. The relationship between the fineness ratio and the drag coefficient is similar to that between the fineness ratio and the base pressure coefficient, similar to the findings of previous studies of two-dimensional bodies.

Measurements of Freestream Fluctuations in the NASA Langley 20-Inch Mach 6 Tunnel

Abstract: An experimental campaign was conducted to measure and to characterize the freestream disturbance levels in the NASA Langley Research Center 20-Inch Mach 6 Wind Tunnel. A pitot rake was instrumented with fast pressure transducers, hot wires, and an atomic layer thermopile to quantify the fluctuation levels of pressure, mass flux, and heat flux, respectively. In conjunction with these probe-based measurements, focused laser differential interferometry was used to optically measure density fluctuations. Measurements were made at five nominal different unit Reynolds numbers ranging from (3.28 to 26.5) times 10 (sup 6) per meter. The rake was positioned at two different stream-wise locations and several different roll angles to measure flow uniformity within the test section. In general, noise levels were spatially consistent within the tested region. Pitot pressure fluctuation levels ranged from 0.84 percent at the highest Reynolds number tested to 1.89 percent at the lowest Reynolds number tested. Freestream mass-flux fluctuations remained relatively constant between 1.8-2.5 percent of the freestream. The pressure transducers were also used to determine the dominant disturbance speed and angle of propagation. The disturbances were estimated to travel at approximately 54-81 percent of the freestream speed at an angle of approximately 21-44 degrees from the freestream direction, but these measurements had a significant amount of uncertainty. A comparison to previous measurements of pressure made in 2012 and of mass flux made in 1994 show almost no change in the RMS (Root Mean Square) fluctuation of these flow quantities.

Systematic study of distributed excitation of unsteady Görtler modes by freestream vortices

Abstract: The paper is devoted to experimental investigation of an efficient mechanism of distributed excitation of nonstationary Gortler vortices in a boundary layer on concave wall due to scattering of three-dimensional, streamwise oriented freestream vortices on natural two-dimensional base-flow nonuniformity associated with the boundary layer growth. The investigations are performed in a broad range of disturbance frequencies and spanwise wavenumbers. The measurements have shown that the distributed receptivity mechanism is able to modify significantly the disturbance growth rates. This mechanism is found to be able to compete successfully with the linear instability mechanism. It is found that at relatively low frequencies and not too large freestream disturbance amplitudes, the Gortler modes’ development is dominated basically by the corresponding linear instability mechanism. Meanwhile, this mechanism becomes weaker with growth of frequency and, simultaneously, the distributed receptivity mechanism gets stronger. This leads to enhancement of role of the latter in amplification of Gortler instability modes, and at high frequencies the role of the distributed receptivity mechanism becomes very important and often even decisive one. Based on deep processing of the experimental data, quantitative values of the distributed linear receptivity coefficients are obtained for the case of excitation of unsteady Gortler modes and estimated for the case of steady Gortler vortices by means of extrapolation to the zero frequency. A complementary experiment devoted to investigation of the problem of linear unsteady Gortler instability is performed for the particular conditions of the distributed receptivity experiments. All main stability characteristics are obtained and compared with calculated ones. In general, the experimental information presented in this paper can be used for verification of various instability and vortex receptivitytheories.

Comparative Study of Dynamic Stall under Pitch Oscillation and Oscillating Freestream on Wind Turbine Airfoil and Blade

Abstract: This study aims to assess the dynamic stall of the wind turbine blade undergoing pitch oscillation (PO) and oscillating freestream (OF), respectively. Firstly, a thin-airfoil theoretical analysis was performed to differentiate between these two dynamic effects. During upstroke, PO results in a positive effective airfoil camber, while OF has an additional negative effective airfoil camber, and yet in contrast during downstroke, PO decreases the effective camber, while OF increases the effective camber. Secondly, the equivalence relation between PO and OF is investigated by numerically solving the unsteady Reynolds-averaged Navier-Stokes equations. The difference between PO and OF mainly exists in the linear part of the aerodynamic loads. Because the difference is great at high reduced frequencies or angle of attack (AOA) amplitudes, PO and OF should be treated separately for dynamic stall from different aerodynamic sources. Thirdly, the Beddoes-Leishman dynamic model coupled with Bak’s rotational stall delay model was used to predict the yawed responses of the blade section. The obtained results show different aerodynamic responses between PO and OF, although consideration of rotational augmentation can greatly improve the accuracy of the lift and drag coefficients. To improve the understanding and coupling modeling of rotational augmentation and dynamic stall, an extended analysis of the coupled effect was performed as well.

Unit Reynolds number, Mach number and pressure gradient effects on laminar–turbulent transition in two-dimensional boundary layers

Abstract: The influence of unit Reynolds number ( $$Re_1=17.5\times 10^{6}$$ – $$80\times 10^{6}\,\text {m}^{-1}$$ ), Mach number ( $$M= 0.35$$ –0.77) and incompressible shape factor ( $$H_{12} = 2.50$$ –2.66) on laminar–turbulent boundary layer transition was systematically investigated in the Cryogenic Ludwieg-Tube Gottingen (DNW-KRG). For this investigation the existing two-dimensional wind tunnel model, PaLASTra, which offers a quasi-uniform streamwise pressure gradient, was modified to reduce the size of the flow separation region at its trailing edge. The streamwise temperature distribution and the location of laminar–turbulent transition were measured by means of temperature-sensitive paint (TSP) with a higher accuracy than attained in earlier measurements. It was found that for the modified PaLASTra model the transition Reynolds number ( $$Re_{\text {tr}}$$ ) exhibits a linear dependence on the pressure gradient, characterized by $$H_{12}$$ . Due to this linear relation it was possible to quantify the so-called ‘unit Reynolds number effect’, which is an increase of $$Re_{\text {tr}}$$ with $$Re_1$$ . By a systematic variation of M, $$Re_1$$ and $$H_{12}$$ in combination with a spectral analysis of freestream disturbances, a stabilizing effect of compressibility on boundary layer transition, as predicted by linear stability theory, was detected (‘Mach number effect’). Furthermore, two expressions were derived which can be used to calculate the transition Reynolds number as a function of the amplitude of total pressure fluctuations, $$Re_1$$ and $$H_{12}$$ . To determine critical N-factors, the measured transition locations were correlated with amplification rates, calculated by incompressible and compressible linear stability theory. By taking into account the spectral level of total pressure fluctuations at the frequency of the most amplified Tollmien–Schlichting wave at transition location, the scatter in the determined critical N-factors was reduced. Furthermore, the receptivity coefficients dependence on incidence angle of acoustic waves was used to correct the determined critical N-factors. Thereby, a found dependency of the determined critical N-factors on $$H_{12}$$ decreased, leading to an average critical N-factor of about 9.5 with a standard deviation of $$\sigma \approx 0.8$$ .

Mach Wave Effect on Laminar-Turbulent Transition in Supersonic Flow over a Flat Plate

Abstract: The effect of a Mach wave (N wave) on laminar-turbulent transition induced by the first instability mode (Tollmien–Schlichting wave) in the flat-plate boundary layer is investigated on the basis of the numerical solution of Navier–Stokes equations at the freestream Mach number of 2.5. In accordance with the experiment, the N wave is generated by a two-dimensional roughness at the computation domain boundary corresponding to the side wall of the test section of a wind tunnel. It is shown that the disturbance induced by the backward front of the N wave in the boundary layer has no effect on the beginning of transition but displaces downstream the nonlinear stage of the first mode development. The disturbance induced by the forward front of the N wave displaces the beginning of transition upstream.

Influence of Propulsion Type on the Stratified Near Wake of an Axisymmetric Self-Propelled Body

Abstract: To better understand the influence of swirl on the thermally-stratified near wake of a self-propelled axisymmetric vehicle, three propulsor schemes were considered: a single propeller, contra-rotating propellers (CRP), and a zero-swirl, uniform-velocity jet. The propellers were modeled using an Actuator-Line model in an unsteady Reynolds-Averaged Navier–Stokes simulation, where the Reynolds number is R e L = 3.1 × 10 8 using the freestream velocity and body length. The authors previously showed good comparison to experimental data with this approach. Visualization of vortical structures shows the helical paths of blade-tip vortices from the single propeller as well as the complicated vortical interaction between contra-rotating blades. Comparison of instantaneous and time-averaged fields shows that temporally stationary fields emerge by half of a body length downstream. Circumferentially-averaged axial velocity profiles show similarities between the single propeller and CRP in contrast to the jet configuration. Swirl velocity of the CRP, however, was attenuated in comparison to that of the single propeller case. Mixed-patch contour maps illustrate the unique temperature distribution of each configuration as a consequence of their respective swirl profiles. Finally, kinetic and potential energy is integrated along downstream axial planes to reveal key differences between the configurations. The CRP configuration creates less potential energy by reducing swirl that would otherwise persist in the near wake of a single-propeller wake.

Aerodynamic drag determination of a full-scale cyclist mannequin from largescale PTV measurements

Abstract: Large-scale Particle Tracking Velocimetry (PTV) measurements are cond ucted in the wake of a fu ll-scale cyclist model in time-trial position at freestream velocities between 12.5 and 15 m/ s, corresponding to Reynolds numbers of the order of 5×105. Lagrangian particle tracking is employed to determine the velocity and static pressure statistics in the wake plane, showing good agreement with previous results reported in literature. The aerodynamic drag is estimated from the large-scale PTV measurements invoking the conservation of momentum in a control volume enclosing the model (PIV wake rake approach). The estimated drag follows the expected quadratic increase with increasing freestream velocity. The accuracy of the drag estimate is evaluated by comparison to state-of-the-art force balance measurements, resulting in a resolution of the PIV wake rake approach of 30 drag counts. The three terms composing the overall drag force, associated with the time-average streamwise velocity, its fluctuations and the time-averaged pressure, respectively, are evaluated separately, demonstrating that the contribution of the pressure term is negligible and the resistive force is dominated by the time-average streamwise momentum deficit.

Effects of Freestream Turbulence on the Pressure Acting on a Low-Rise Building Roof in the Separated Flow Region

Abstract: This paper presents the experimental design and subsequent findings from a comprehensive series of experiments in a large BLWT to investigate the variation of surface pressures with increasing upwind terrain roughness on low-rise buildings. Geometrically scaled models of the Wind Engineering Research Field Laboratory (WERFL) experimental building were subjected to a wide range of turbulent boundary layer flows, through precise adjustment of a computer control terrain generator called the Terraformer. The study offers an in-depth examination of the effects of freestream turbulence on extreme pressures under the separation “bubble” for the case of the wind traveling perpendicular to wall surfaces, independently confirming previous findings that the spatial distribution of the peaks is heavily influenced by the mean reattachment length. Further, the study shows that the observed peak pressures collapse if data are normalized by the mean reattachment length and a non-Gaussian estimator for peak velocity pressure.

Time-resolved PIV measurements of the effect of freestream turbulence on horseshoe vortex dynamics

Abstract: The horseshoe vortex system is a common flow feature in many natural and industrial flows occurring near the junction of a blunt obstacle with the endwall surface. In industrial settings, such as in high temperature gas turbine engines, the dynamic behavior of the horseshoe vortex has been shown to contribute significantly to the pressure loading and heat transfer behavior on surfaces near the leading edge of the obstacle. Fundamental studies of the horseshoe vortex have characterized its time mean and dynamic behavior at low freestream turbulence conditions, and studies using industry relevant geometries, such as cylindrical pin fin arrays common to cooling applications, have captured dynamic behavior of the vortex at high freestream turbulence. The isolated effect of high freestream turbulence on the dynamic behavior of the vortex, independent of upstream wake effects found in pin fin arrays and other industry geometries, however, is not well understood. This study seeks use high-speed time resolved stereo particle image velocimetry (SPIV) measurements of the horseshoe vortex system taken at varied freestream turbulence levels in front of a single Rood wing obstacle to better understand the isolated effect of freestream turbulence on the vortex position and vortex breakdown dynamics.