Wake Properties of an Oscillating Airfoil Undergoing Small Amplitude Asymmetric Oscillation

2020 ◽

Author(s):

Colin Stutz ◽

Douglas Bohl ◽

Melissa Green

Keyword(s):

Small Amplitude ◽

Single Vortex ◽

Pitching Motion ◽

Vortical Structures ◽

Naca0012 Airfoil ◽

Reduced Frequency ◽

Wake Patterns ◽

Asymmetric Oscillation ◽

Secondary Vortices ◽

Equivalent Strength

Abstract The flow around, and in the wake of, pitching airfoils has received renewed interest due to its potential for thrust production at low Reynolds numbers. Past work has centered on the flow fields generated by symmetric pitching of the airfoil. Studies investigating the effects of asymmetric motion are more limited. This work focuses on the wake patterns developed due to asymmetric pitching. Particle Image Velocimetry (PIV) is used to quantify the flow field around a NACA0012 airfoil undergoing small amplitude, high frequency asymmetric pitching. The airfoil is pitched about the quarter chord point with an amplitude of ±4° at reduced frequencies of k = 2.6–5.8 at a Rec = 12000. Pitching symmetries of 50/50, 40/60 and 30/70 are studied, where the symmetry is defined by the fraction of the cycle spent in the pitch down versus pitch up motion. The data show that for the 50/50 (symmetric) motions two alternating sign vortices, with equivalent strength, are formed as expected. The asymmetric cases show that a single vortex is formed during the “fast” portion of the pitching motion. Multiple vortices are formed during the “slow” portion of the pitching motion. The number of secondary vortices and the downstream evolution of the vortices depends on the symmetry value. In some cases they remain isolated but orbit other vortical structures, while in other cases they pair with other vortical structures, and finally when the reduced frequency and asymmetry values are high enough the vortex array shows interaction between cycles.

Download Full-text

On nearly-commensurable periods in the restricted problem of three bodies, with calculations of the long-period variations in the interior 2:1 case

Symposium - International Astronomical Union ◽

10.1017/s0074180900105480 ◽

1966 ◽

Vol 25 ◽

pp. 197-222 ◽

Cited By ~ 4

Author(s):

P. J. Message

Keyword(s):

Periodic Solution ◽

Periodic Solutions ◽

Large Amplitude ◽

Restricted Problem ◽

Small Amplitude ◽

Minor Planets ◽

Long Period ◽

Numerical Integrations ◽

Period Variations ◽

The Mean

An analytical discussion of that case of motion in the restricted problem, in which the mean motions of the infinitesimal, and smaller-massed, bodies about the larger one are nearly in the ratio of two small integers displays the existence of a series of periodic solutions which, for commensurabilities of the typep+ 1:p, includes solutions of Poincaré'sdeuxième sortewhen the commensurability is very close, and of thepremière sortewhen it is less close. A linear treatment of the long-period variations of the elements, valid for motions in which the elements remain close to a particular periodic solution of this type, shows the continuity of near-commensurable motion with other motion, and some of the properties of long-period librations of small amplitude.To extend the investigation to other types of motion near commensurability, numerical integrations of the equations for the long-period variations of the elements were carried out for the 2:1 interior case (of which the planet 108 “Hecuba” is an example) to survey those motions in which the eccentricity takes values less than 0·1. An investigation of the effect of the large amplitude perturbations near commensurability on a distribution of minor planets, which is originally uniform over mean motion, shows a “draining off” effect from the vicinity of exact commensurability of a magnitude large enough to account for the observed gap in the distribution at the 2:1 commensurability.

Download Full-text

RSCVn Stars in the Southern Hemisphere

International Astronomical Union Colloquium ◽

10.1017/s0252921100073644 ◽

1979 ◽

Vol 46 ◽

pp. 371-384 ◽

Cited By ~ 3

Author(s):

J.B. Hearnshaw

Keyword(s):

Light Curve ◽

Southern Hemisphere ◽

Orbital Period ◽

Binary Stars ◽

Small Amplitude ◽

Light Variation ◽

Orbital Phase

RSCVn stars are fully detached binary stars which show intrinsic small amplitude (up to 0.3 amplitude peak-to-peak) light variations, as well as, in most of the known cases, eclipses. The spectra are F to G, IV to V for the hotter component and usually KOIV for the cooler. They are also characterised by abnormally strong H and K emission from the cooler star, or, occasionally, from both components. The orbital and light curve periods are in the range 1 day to 2 weeks. An interesting feature is the migration of the light variations to earlier orbital phase, as the light variation period is shorter than the orbital period by a few parts in 10+4to a few parts in 10+3.

Download Full-text

Amplitude Distributions of Cochlear Microphonic Response to an Acoustic Sinusoid in Noise

Journal of Speech and Hearing Research ◽

10.1044/jshr.1101.63 ◽

1968 ◽

Vol 11 (1) ◽

pp. 63-76

Author(s):

Donald C. Teas ◽

Gretchen B. Henry

Keyword(s):

Small Amplitude ◽

Basilar Membrane ◽

Spectral Composition ◽

Pass Band ◽

Cochlear Microphonic ◽

Filter Output ◽

Electronic Filter ◽

Low Pass ◽

Band Pass ◽

Instantaneous Voltage

The distributions of instantaneous voltage amplitudes in the cochlear microphonic response recorded from a small segment along the basilar membrane are described by computing amplitude histograms. Comparisons are made between the distributions for noise and for those after the addition to the noise of successively stronger sinusoids. The amplitudes of the cochlear microphonic response to 5000 Hz low-pass noise are normally distributed in both Turn I and Turn III of the guinea pig’s cochlea. The spectral composition of the microphonic from Turn I and from Turn III resembles the output of band-pass filters set at about 4000 Hz, and about 500 Hz, respectively. The normal distribution of cochlear microphonic amplitudes for noise is systematically altered by increasing the strength of the added sinusoid. A decrease of three percent in the number of small amplitude events (±1 standard deviation) in the cochlear microphonic from Turn III is seen when the rms voltage of a 500 Hz sinusoid is at −18 dB re the rms voltage of the noise (at the earphone). When the rms of the sinusoid and noise are equal, the decrease in small voltages is about 25%, but there is also an increase in the number of large voltage amplitudes. Histograms were also computed for the output of an electronic filter with a pass-band similar to Turn III of the cochlea. Strong 500 Hz sinusoids showed a greater proportion of large amplitudes in the filter output than in CM III . The data are interpreted in terms of an anatomical substrate.

Download Full-text