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Artykuły w czasopismach na temat "Convecting Freestream Vortex"
1
Green, R. B., i R. A. McD Galbraith. "Dynamic stall vortex convection: thoughts on compressibility effects". Aeronautical Journal 100, nr 999 (listopad 1996): 367–72. http://dx.doi.org/10.1017/s0001924000066902.
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AbstractThis paper considers the convection of the dynamic stall vortex. A brief discussion of existing data is given. A comparison of two sets of data from different experimental facilities is then presented, and it is indicated that an important anomaly exists concerning the behaviour of the dynamic stall vortex. It emerges that freestream Mach number is the only parameter that can account for the observed differences. The importance of this parameter is then discussed in the context of vorticity flux from the aerofoil surface.
2
Zhang, X. "Turbulence Measurements of a Longitudinal Vortex Generated by an Inclined Jet in a Turbulent Boundary Layer". Journal of Fluids Engineering 120, nr 4 (1.12.1998): 765–71. http://dx.doi.org/10.1115/1.2820736.
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A longitudinal vortex in a flat-plate turbulent boundary layer was examined in a wind tunnel experiment using Laser Doppler Anemometry. The vortex was produced by an inclined round jet (D = 14 mm) in the turbulent boundary layer (δ0.99 ≈ 25 mm). The jet nozzle was positioned at pitch and skew angles of 45 deg to the oncoming stream, and the jet speed ratios (jet speed/freestream flow speed) were 0.5, 1.0, and 1.5. The flow was characterized by embedded vortices, induced high turbulent kinetic energy peak, local areas of high primary shear stress, and negative shear stress. Two types of normal stress evolution were observed: (a) low normal stress beneath the vortex on the upwash side and high normal stress above the center of the vortex, caused by spanwise momentum transfer and local turbulent production; (b) high normal stress beneath the vortex on the upwash side and high normal stress coinciding with the center of the vortex, produced by spanwise and transverse momentum transfer of a vortex away from the wall with turbulent convection playing an important role. The study provided a database for numerical modeling effort.
3
Lin, San-Yin, Sheng-Chang Shih i Jen-Jiun Hu. "Dissipation Improvement of MUSCL Scheme for Computational Aeroacoustics". Journal of Mechanics 17, nr 1 (marzec 2001): 39–47. http://dx.doi.org/10.1017/s1727719100002409.
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ABSTRACTAn upwind finite-volume scheme is studied for solving the solutions of two dimensional Euler equations. It based on the MUSCL (Monotone Upstream Scheme for Conservation Laws) approach with the Roe approximate Riemann solver for the numerical flux evaluation. First, dissipation and dispersion relation, and group velocity of the scheme are derived to analyze the capability of the proposed scheme for capturing physical waves, such as acoustic, entropy, and vorticity waves. Then the scheme is greatly enhanced through a strategy on the numerical dissipation to effectively handle aeroacoustic computations. The numerical results indicate that the numerical dissipation strategy allows that the scheme simulates the continuous waves, such as sound and sine waves, at fourth-order accuracy and captures the discontinuous waves, such a shock wave, sharply as well as most of upwind schemes do. The tested problems include linear wave convection, propagation of a sine-wave packet, propagation of discontinuous and sine waves, shock and sine wave interaction, propagation of acoustic, vorticity, and density pulses in an uniform freestream, and two-dimensional traveling vortex in a low-speed freestream.
4
Chung, Kung-Ming, Kao-Chun Su i Keh-Chin Chang. "The Effect of Vortex Generators on Shock-Induced Boundary Layer Separation in a Transonic Convex-Corner Flow". Aerospace 8, nr 6 (2.06.2021): 157. http://dx.doi.org/10.3390/aerospace8060157.
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Deflected control surfaces can be used as variable camber control in different flight conditions, and a convex corner resembles a simplified configuration for the upper surface. This experimental study determines the presence of passive vortex generators, VGs (counter-rotating vane type), on shock-induced boundary layer separation for transonic convex-corner flow. The mean surface pressure distributions in the presence of VGs for h/δ = 0.2 and 0.5 are similar to those for no flow control. If h/δ = 1.0 and 1.5, there is an increase in the amplitude of the mean surface pressure upstream of the corner’s apex, which corresponds to greater device drag and less downstream expansion. There is a decrease in peak pressure fluctuations as the value of h/δ increases, because there is a decrease in separation length and the frequency of shock oscillation. The effectiveness of VGs also depends on the freestream Mach number. For M = 0.89, there is an extension in the low-pressure region downstream of a convex corner, because there is greater convection and induced streamwise vorticity. VGs with h/δ ≤ 0.5 are preferred if deflected control surfaces are used to produce lift.
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Yao, Hua-Dong, Zhongjie Huang, Lars Davidson, Jiqiang Niu i Zheng-Wei Chen. "Blade-Tip Vortex Noise Mitigation Traded-Off against Aerodynamic Design for Propellers of Future Electric Aircraft". Aerospace 9, nr 12 (15.12.2022): 825. http://dx.doi.org/10.3390/aerospace9120825.
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We study noise generation at the blade tips of propellers designed for future electric aircraft propulsion and, furthermore, analyze the interrelationship between noise mitigation and aerodynamics improvement in terms of propeller geometric designs. Classical propellers with three or six blades and a conceptual propeller with three joined dual-blades are compared to understand the effects of blade tip vortices on the noise generation and aerodynamics. The dual blade of the conceptual propeller is constructed by joining the tips of two sub-blades. These propellers are designed to operate under the same freestream flow conditions and similar electric power consumption. The Improved Delayed Detached Eddy Simulation (IDDES) is adopted for the flow simulation to identify high-resolution time-dependent noise sources around the blade tips. The acoustic computations use a time-domain method based on the convective Ffowcs Williams–Hawkings (FW-H) equation. The thrust of the 3-blade conceptual propeller is 4% larger than the 3-blade classical propeller and 8% more than the 6-blade one, given that they have similar efficiencies. Blade tip vortices are found emitting broadband noise. Since the classical and conceptual 3-blade propellers have different geometries, especially at the blade tips, they introduce deviations in the vortex development. However, the differences are small regarding the broadband noise generation. As compared to the 6-blade classical propeller, both 3-blade propellers produce much larger noise. The reason is that the increased number of blades leads to the reduced strength of tip vortices. The findings indicate that the noise mitigation through the modification of the blade design and number can be traded-off by the changed aerodynamic performance.
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Yokota, Sho, Keisuke Asai i Taku Nonomura. "Instability of Separated Shear Layer around Levitated Freestream-Aligned Circular Cylinder". Physics of Fluids, 11.05.2022. http://dx.doi.org/10.1063/5.0091044.
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In the present study, the characteristics of the shear layer around a freestream-aligned circular cylinder and the relationship between the shear layer motion and the aerodynamic force were investigated under supportless condition. The 0.3-m magnetic suspension and balance system (MSBS) was employed and experiments were conducted without a mechanical supporting device. Velocity fields were measured using particle image velocimetry with a sufficient temporal and spatial resolution and high-frequency velocity fluctuations caused by small Kelvin-Helmholtz (KH) vortices were captured. The power spectral densities of velocity fluctuations represent phenomena such as KH vortex convection, vortex pairing, and convection of multiple vortices. Furthermore, fluctuations of the shear layer position were investigated. The results illustrates that the dominant frequency of the shear layer position is lower than the frequency of velocity and it shows good agreement with the characteristic frequency of lift force fluctuations. The present results together with the report in the previous study illustrate that the pressure fluctuations are considered to drive both fluctuations of the shear layer position and lateral aerodynamic force.
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McAuliffe, Brian R., i Metin I. Yaras. "Numerical Study of Turbulent-Spot Development in a Separated Shear Layer". Journal of Turbomachinery 130, nr 4 (4.08.2008). http://dx.doi.org/10.1115/1.2812948.
Pełny tekst źródłaStreszczenie:
The development of turbulent spots in a separation bubble under elevated freestream turbulence levels is examined through direct numerical simulation. The flow Reynolds number, freestream turbulence level, and streamwise pressure distribution are typical of the conditions encountered on the suction side of low-pressure turbine blades of gas-turbine engines. Based on the simulation results, the spreading and propagation rates of the turbulent spots and their internal structure are documented, and comparisons are made to empirical correlations that are used for predicting the transverse growth and streamwise propagation characteristics of turbulent spots. The internal structure of the spots is identified as a series of vortex loops that develop as a result of low-velocity streaks generated in the shear layer. A frequency that is approximately 50% higher than that of the Kelvin–Helmholtz instability is identified in the separated shear layer, which is shown to be associated with the convection of these vortex loops through the separated shear layer. While freestream turbulence is noted to promote breakdown of the laminar separated shear layer into turbulence through the generation of turbulent spots, evidence is found to suggest coexistence of the Kelvin–Helmholtz instability, including the possibility of breakdown to turbulence through this mechanism.
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Vikramaditya, N. S., i M. Viji. "Mach Number Effect on Symmetric and Antisymmetric Modes of Base Pressure Fluctuations". Journal of Fluids Engineering 141, nr 2 (16.08.2018). http://dx.doi.org/10.1115/1.4040928.
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An experimental study aimed at evaluating the influence of Mach number on the base pressure fluctuations of a cylindrical afterbody was performed over a wide range of Mach numbers from subsonic to supersonic speeds. Time-averaged results indicate that the coefficient of base pressure drops with the increase in the freestream Mach number at subsonic speeds and increases at supersonic Mach numbers. The coefficient of root-mean-square of the pressure fluctuations follows a decreasing trend with the increase in the Mach number. Examination of the spectra reveals different mechanisms dominate the pressure fluctuations from the center to the periphery of the base as well as with the change in the Mach number. Analysis of the azimuthal coherence indicates that all the dominant tones in the spectra can be classified either into a symmetric or an antisymmetric mode at subsonic Mach numbers. However, at supersonic Mach numbers, all the dominant tones in the spectra are symmetric in nature. The results from the cross-correlation suggest that two possible mechanisms of recirculation bubble pulsing and convective motions/vortex shedding are driving the dynamics on the base at subsonic Mach numbers. However, at supersonic Mach numbers, only single mechanism of the recirculation bubble pulsing dominates. Moreover, it indicates that the symmetric mode is associated with the dynamics of the recirculation bubble and the antisymmetric mode is related to the convective motions/vortex shedding.
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Kamrani Fard, Kiana, Vickie Ngo, Deborah Pence i Jim A. Liburdy. "Energy Harvesting Performance of Thick Oscillating Airfoils Using a Discrete Vortex Model". Journal of Fluids Engineering, 29.11.2022, 1–33. http://dx.doi.org/10.1115/1.4056339.
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Abstract The energy harvesting performance of thick oscillating airfoils is predicted using an inviscid discrete vortex model (DVM). NACA airfoils with different leading-edge geometries are modeled that undergo sinusoidal heaving and pitching with reduced frequencies, k = f c/U∞, in the range 0.06–0.14, where f is the heaving frequency of the foil, c the chord length, and U the freestream velocity. The airfoil pitches about the mid-chord with heaving and pitching amplitudes of h0 = 0.5c and θ0 = 70°, respectively, known to be in the range of peak energy harvesting efficiencies. A vortex shedding initiation criteria is proposed based on the transient local wall stress distribution determined from computational fluid dynamics (CFD) simulations and incorporates both timing and location of leading-edge separation. The scaled shedding times are shown to be predicted over the range of reduced frequencies using a timescale based on the leading-edge shear velocity and radius of curvature. The convection velocity of the shed vortices is also modeled based on the reduced frequency to better capture the dynamics of the leading-edge vortex. An empirical trailing-edge separation correction is applied to the transient force results using the effective angle of attack modified to include the pitching component. Impulse theory is applied to the DVM to calculate the transient lift force and compares well with the CFD simulations. Results show that the power output increases with increasing airfoil thickness and is most notable at higher reduced frequencies where the power output efficiency is highest.
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Hossain, Mohammad A., Robin Prenter, Ryan K. Lundgreen, Ali Ameri, James W. Gregory i Jeffrey P. Bons. "Experimental and Numerical Investigation of Sweeping Jet Film Cooling". Journal of Turbomachinery 140, nr 3 (28.12.2017). http://dx.doi.org/10.1115/1.4038690.
Pełny tekst źródłaStreszczenie:
A companion experimental and numerical study was conducted for the performance of a row of five 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 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 SJ film cooling hole showed significant improvement in cooling effectiveness in the lateral direction due to the sweeping action of the fluidic oscillator. An unsteady Reynolds-averaged Navier–Stokes (URANS) 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.
Streszczenia konferencji na temat "Convecting Freestream Vortex"
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Morse, Daniel R., i James A. Liburdy. "Dynamic Characteristics of Flow Separation From a Low Reynolds Number Airfoil". W ASME/JSME 2007 5th Joint Fluids Engineering Conference. ASMEDC, 2007. http://dx.doi.org/10.1115/fedsm2007-37083.
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This study examines the generation of large scale vortices caused by flow separation from a flat wing at various angles of attack. Time-resolved particle image velocimetry is used to determine the evolution and convective characteristics of the large scale structures. A rectangular airfoil with aspect ratio of 0.5 is used and data are collected at a Reynolds number of 23,500, for angles of attack from 0° to 20°. Data consists of two dimensional velocity fields obtained at 500 Hz located at the airfoil centerline. The region of interest is near the separation point but fields of view extend over approximately one half of the chord length from the leading edge to document the downstream progression of the large scale vortical flow elements. The velocity data were processed to identify the vorticity field dynamics in terms of the Kelvin-Helmholtz instability occurring near the leading edge. The vortical structures are identified using vortex detection based on local circulation. The convective nature of the vortex elements are shown to consist of merging, stalling and convecting, with convective velocities on the order of 20% of the freestream velocity with an associated Stouhal number based on chord length and freestream velocity of approximately 1.0.
2
Morse, Daniel R., i James A. Liburdy. "Vortex Detection and Characterization in Low Reynolds Number Separation". W ASME 2007 International Mechanical Engineering Congress and Exposition. ASMEDC, 2007. http://dx.doi.org/10.1115/imece2007-43011.
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This study focuses on the detection and characterization of vortices in low Reynolds number separation flow over the elliptical leading edge of a flat plate airfoil. Velocity fields were obtained using Time Resolved Particle Image Velocimetry (TRPIV). The Reynolds number based on chord length ranged from 14,700 to 66,700. Experiments were performed for velocities of 1.1, 2.0 and 5.0 m/s and angles of attack of 14°, 16°, 18° and 20°. These velocities correspond to chord length Reynolds numbers of 1.47×104, 2.68×104, and 6.70×104, respectively. A local swirl calculation was used to determine regions of high circulation, and the convection of the centers of these regions was used to determine convective velocities of these vortical structures. The streamwise convective velocity normalized by the freestream velocity is observed to range from approximately 0.4 to 0.65 over the range of angles of attack, with slightly increasing values as the angle of attack increases.
3
Radomsky, R. W., i K. A. Thole. "High Freestream Turbulence Effects on Endwall Heat Transfer for a Gas Turbine Stator Vane". W ASME Turbo Expo 2000: Power for Land, Sea, and Air. American Society of Mechanical Engineers, 2000. http://dx.doi.org/10.1115/2000-gt-0201.
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High freestream turbulence along a gas turbine airfoil and strong secondary flows along the endwall have both been reported to significantly increase convective heat transfer. This study superimposes high freestream turbulence on the naturally occurring secondary flow vortices to determine the effects on the flowfield and the endwall convective heat transfer. Measured flowfield and heat transfer data were compared between low freestream turbulence levels (0.6%) and combustor simulated turbulence levels (19.5%) that were generated using an active grid. These experiments were conducted using a scaled-up, first stage stator vane geometry. Infrared thermography was used to measure surface temperatures on a constant heat flux plate placed on the endwall surface. Laser Doppler velocimeter (LDV) measurements were performed of all three components of the mean and fluctuating velocities of the leading edge horse-shoe vortex. The results indicate that the mean flowfields for the leading edge horseshoe vortex were similar between the low and high freestream turbulence cases. High turbulence levels in the leading edge-endwall juncture were attributed to a vortex unsteadiness for both the low and high freestream tubulence cases. While, in general, the high freestream turbulence increased the endwall heat transfer, low augmentations were found to coincide with the regions having the most intense vortex motions.
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McAuliffe, B. R., i M. I. Yaras. "Numerical Study of Turbulent Spot Development in a Separated Shear Layer". W ASME Turbo Expo 2007: Power for Land, Sea, and Air. ASMEDC, 2007. http://dx.doi.org/10.1115/gt2007-27604.
Pełny tekst źródłaStreszczenie:
The development of turbulent spots in a separation bubble under elevated freestream turbulence levels is examined through direct numerical simulation. The flow Reynolds number, freestream turbulence level, and streamwise pressure distribution are typical of the conditions encountered on the suction side of low-pressure turbine blades of gas-turbine engines. Based on the simulation results, the spreading and propagation rates of the turbulent spots and their internal structure are documented, and comparisons are made to empirical correlations that are used for predicting the transverse growth and streamwise propagation characteristics of turbulent spots. The internal structure of the spots is identified as a series of vortex loops that develop as a result of low-velocity streaks generated in the shear layer. A frequency that is approximately 50% higher than that of the Kelvin-Helmholtz instability is identified in the separated shear layer, which is shown to be associated with convection of these vortex loops through the separated shear layer. While freestream turbulence is noted to promote breakdown of the laminar separated shear layer into turbulence through the generation of turbulent spots, evidence is found to suggest co-existence of the Kelvin-Helmholtz instability, including the possibility of breakdown to turbulence through this mechanism.
5
Hossain, Mohammad A., Robin Prenter, Ryan K. Lundgreen, Ali Ameri, James W. Gregory i Jeffrey P. Bons. "Experimental and Numerical Investigation of Sweeping Jet Film Cooling". W ASME Turbo Expo 2017: Turbomachinery Technical Conference and Exposition. American Society of Mechanical Engineers, 2017. http://dx.doi.org/10.1115/gt2017-64479.
Pełny tekst źródłaStreszczenie:
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.
6
Khalatov, A. "Improved Approach to an Endwall Heat Transfer Analysis: A Linear Guide Vane and a Curved Duct". W ASME 1998 International Gas Turbine and Aeroengine Congress and Exhibition. American Society of Mechanical Engineers, 1998. http://dx.doi.org/10.1115/98-gt-293.
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This paper consists of two sections. The first section of the paper illustrates successful application of the improved approach developed by author to the endwall heat transfer data analysis in a low speed linear guide vane and in a curved duct. Effects of a three dimensional turbulent flow, a horseshoe vortex, a passage vortex, as well as an entry boundary layer thickness have been considered in both passages and as a result the common experimental correlation on a local heat transfer have been derived for the H/t = 1.0 ratio. All affected factors are presented as a superposition of the linear correction functions in the basic experimental correlation for a flat plate heat transfer. In the second section the common correlation is used as the reference correlation to establish effect of the span-to-pitch ratio on the endwall heat transfer in both passages. It was found that variation in the H/t ratio affects slightly the freestream velocity; the most important result which came from the heat transfer study is that in contrast to a curved duct a heat transfer rate in a blade passage is reduced while the H/t ratio decreases. Comparison of the experimental data obtained by the author with results of the two-dimensional heat transfer prediction confirms that it is very important to take a three-dimensional heat transfer nature into account in design of the endwall convective cooling system. It has been demonstrated that distinction between the results of two- and three dimensional approach to the endwall heat transfer can achieve up to 70% at the passage’s inlet area.