Jet Characteristics of a Plunging Airfoil

Presented as Paper 98-0101 at the AIAA 36th Aerospace Sciences Meeting, Reno, NV, 12–15 January 1998The article of record as published may be found at https://doi.org/10.2514/2.641Water-tunnel tests of a NACA 0012 airfoil that was oscillated sinusoidally in plunge are described. The flowfield downstream of the airfoil was explored by dye flow visualization and single-component laser Doppler velocimetry (LDV) measurements for a range of freestream speeds, frequencies, and amplitudes of oscillation. The dye visualizations show that the vortex patterns generated by the plunging airfoil change from drag-producing wake flows to thrust-producing jet flows as soon as the ratio of maximum plunge velocity to freestream speed, i.e., the nondimensional plunge velocity, exceeds approximately 0.4. The LDV measurements show that the nondimensional plunge velocity is the appropriate parameter to collapse the maximum streamwise velocity data covering a nondimensional plunge velocity range from 0.18 to 9.3. The maximum streamwise velocity at a given streamwise distance downstream starts to exceed the freestream speed as soon as the nondimensional plunge velocity exceeds 0.25. Furthermore, this maximum jet velocity has been shown to be a linear function of the nondimensional plunge velocity

Computational and Experimental Investigation of Flapping-Foil Propulsion

1st International Symposium on Aqua Bio-Mechanisms / International Seminar on Aqua Bio-Mechanisms, Tokai University Pacific Center, Honolulu, Hawaii, Aug. 2000.It is the objective of this paper to summarize the authors' recent work on flapping foils, including the application of flapping foils for boundary layer and flow separation control. Water tunnel experiments on sinusoidally plunging foils are described which elucidate the change in vortical wake pattern shed from the foil's trailing edge. These experiments were carried out using dye flow visualization and laser-Doppler velocimetry. It is found that the wake pattern is a strong function of the non-dimensional plunge velocity, with the wake topology changing from a typical Kármán vortex street to an inverse Kármán vortex street and to an asymmetric wake structure as the non-dimensional plunge velocity increases. The transition points between the various structures is dependent on scaling effects and wing and flow quality. These results are partly reproducible with inviscid panel code and Navier-Stokes code predictions. Additional interesting features are obtained if two degrees of freedom are permitted (pitch-plunge motions) or two airfoils in close proximity to each other are studied. Finally, experimental and computational results are presented which demonstrate the use of flapping foils for boundary layer and flow separation control

Acoustic noise prediction in a vector controlled induction machine

The deterministic approach for the prediction of noise in electrical machines is generally based on the use of FEM/BEM models. Mechanical and electromagnetic properties included in these models, such as the damping or the BH curve, are difficult to estimate, compromising the accuracy in the noise prediction. Simulations showed that a 10% increase in the saturated part of the BH curve results in an increase of up to 3 dB in the sound power level, and that the sound power level could be reduced by up to 10 dB with a 5 times increase in the damping coefficient. The accuracy of the noise prediction was tested using a vector controlled drive with a very high PWM switching frequency. This system allowed the structure to be excited with the radial and tangential forces independently. Discrepancies between predictions and measurements of the sound power level at low frequencies have been attributed to the rotor and some stator/casing modes not being modeled, while at high frequencies, discrepancies are due to skewing of the rotor not being included in the mode

INSTANTANEOUS VELOCITY MEASUREMENTS IN A VANE-EXCITED PLANE JET.

The effects of oscillating a vane in the potential core of a turbulent jet have been investigated for the low Strouhal number range of 0.0016-0.0048 by making instantaneous velocity measurements with phase-averaging techniques. Significant unsteady effects, in the form of jet flapping, in the presence of harmonics, and nonsimilar profiles, are described. At the measured stream wise stations, entrainment and jet spreading increases significantly with increasing frequency and amplitude of vane oscillation compared with the corresponding steady jet values

Control of corrosion-erosion of tube inlets of shell and tube heat exchangers

Earlier investigations showed that excessive corrosion of the first few diameters of standard shell and tube heat exchangers is the consequence of an unfavourable flow pattern in the inlet channel. Flow correction devices were incorporated in a model shell and tube heat exchanger to alter the existing flow pattern to give an axial flow into the tubes in order to reduce tube inlet corrosion. The resulting modified flow characteristics were studied by flow visualization and velocity distribution measurements. Recommendations regarding optimum dimensions and positioning of the various flow correction devices are made