9,066 research outputs found
Comparative jet wake structure and swimming performance of salps
Salps are barrel-shaped marine invertebrates that swim by jet propulsion. Morphological variations among species and life-cycle
stages are accompanied by differences in swimming mode. The goal of this investigation was to compare propulsive jet wakes
and swimming performance variables among morphologically distinct salp species (Pegea confoederata, Weelia (Salpa)
cylindrica, Cyclosalpa sp.) and relate swimming patterns to ecological function. Using a combination of in situ dye visualization
and particle image velocimetry (PIV) measurements, we describe properties of the jet wake and swimming performance variables
including thrust, drag and propulsive efficiency. Locomotion by all species investigated was achieved via vortex ring propulsion.
The slow-swimming P. confoederata produced the highest weight-specific thrust (T =53 N kg^(–1)) and swam with the highest wholecycle
propulsive efficiency (η_wc= 55%). The fast-swimming W. cylindrica had the most streamlined body shape but produced an
intermediate weight-specific thrust (T=30 N kg^(–1)) and swam with an intermediate whole-cycle propulsive efficiency (η_wc =52%).
Weak swimming performance variables in the slow-swimming C. affinis, including the lowest weight-specific thrust (T=25 N kg^(–1))
and lowest whole-cycle propulsive efficiency (η_wc=47%), may be compensated by low energetic requirements. Swimming
performance variables are considered in the context of ecological roles and evolutionary relationships
Observed Non-Steady State Cooling and the Moderate Cluster Cooling Flow Model
We examine recent developments in the cluster cooling flow scenario following
recent observations by Chandra and XMM-Newton. We show that the distribution of
gas emissivity verses temperature determined by XMM-Newton gratings
observations demonstrates that the central gas in cooling flow clusters cannot
be in simple steady-state. Combining this result with the lack of spectroscopic
evidence for gas below one-third of the ambient cluster temperature is strong
evidence that the gas is heated intermittently. While the old steady-state
isobaric cooling flow model is incompatible with recent observations, a
"moderate cooling flow model", in which the gas undergoes intermittent heating
that effectively reduces the age of a cooling flow is consistent with
observations. Most of the gas within cooling flows resides in the hottest gas,
which is prevented from cooling continuously and attaining a steady-state
configuration. This results in a mass cooling rate that decreases with
decreasing temperature, with a much lower mass cooling rate at the lowest
temperatures. The present paper strengthens the moderate cooling flow model,
which can accommodate the unique activities observed in cooling flow clusters.Comment: ApJ, in pres
Unsteady blade pressure measurements on a model counterrotation propeller
In an exploratory effort an advanced counterrotation propeller instrumented with blade-mounted pressure transducers was tested in the NASA Lewis 9- by 15-Foot Anechoic Wind Tunnel at a simulated takeoff and landing speed of Mach 0.20. The propeller's aft diameter was reduced to investigate possible noise reductions resulting from reduced blade row interaction with the tip vortex. The propeller was tested at three blade row spacings at fixed blade setting angles, at the maximum blade row spacing at higher blade setting angles and at propeller axis angles attack to the flow up to + or - 16 deg. A limited number of unsteady blade surface pressure measurements were made on both rotors of the model counterrotation propeller. Emphasis was placed on determining the effects of rotor-rotor interactions on the blade surface pressures. A unique method of processing the pressure signals was developed that enables even weak interaction waveforms and spectra to be separated from the total signal. The interaction on the aft rotor was many times stronger than that on the forward rotor. The fundamental rotor interaction tone exhibited complicated behavior but generally increased with rotational speed and blade setting angle and decreased with rotor spacing. With the propeller axis at an angle to the flow, the phase response of the aft rotor appeared to be significantly affected by the presence of the forward rotor
Advanced turboprop wing installation effects measured by unsteady blade pressure and noise
A single rotation model propeller (SR-7A) was tested at simulated takeoff/approach conditions (Mach 0.2), in the NASA Lewis 9- by 15-Ft Anechoic Wind Tunnel. Both unsteady blade surface pressures and noise measurements were made for a tractor configuration with propeller/straight wing and propeller alone configurations. The angle between the wing chord and propeller axis (droop angle) was varied along with the wing angle of attack to determine the effects on noise and unsteady loading. A method was developed that uses unsteady blade pressure measurements to provide a quantitative indication of propeller inflow conditions, at least for a uniform (across the propeller disk) inflow angle. The wing installation caused a nearly uniform upwash at the propeller inlet as evidenced by the domination of the pressure spectra by the first shaft order. This inflow angle increased at a rate of almost 150 percent of that of the wing angle-of-attack for a propeller-wing spacing of 0.54 wing chords at a constant droop angle. The flyover noise, as measured by the maximum blade passing frequency level, correlates closely with the propeller inflow angle (approx. 0.6 dB per degree of inflow angle) for all droop angles and wing angles of attack tested, including the propeller alone data. Large changes in the unsteady pressure responses on the suction surface of the blade were observed as the advance ratio was varied. The presence of a leading edge vortex may explain this behavior since changes in the location of this vortex would change with loading (advance ratio)
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