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A user's guide to the COSMIC/NASTRAN free field input for the Bulk Data section of the NASTRAN program is proposed. The free field input is designed to be user friendly and the user is not forced out of the computer system due to input errors. It is easy to use, with only a few simple rules to follow. A stand alone version of the COSMIC/NASTRAN free field input is also available. The use of free field input is illustrated by a number of examples.
Over the past several years, significant effort has been expended at NASA Langley developing new Micro-Electro-Mechanical System (MEMS)-based microphone directional array instrumentation for high-frequency aeroacoustic measurements in wind tunnels. This new type of array construction solves two challenges which have limited the widespread use of large channel-count arrays, namely by providing a lower cost-per-channel and a simpler method for mounting microphones in wind tunnels and in field-deployable arrays. The current generation of array instrumentation is capable of extracting accurate noise source location and directivity on a variety of airframe components using sophisticated data reduction algorithms [1-2]. Commercially-available MEMS microphones are condenser-type devices and have some desirable characteristics when compared with conventional condenser-type microphones. The most important advantages of MEMS microphones are their size, price, and power consumption. However, the commercially-available units suffer from certain important shortcomings. Based on experiments with array prototypes, it was found that both the bandwidth and the sound pressure limit of the microphones should be increased significantly to improve the performance and flexibility of the microphone array [3]. It was also desired to modify the packaging to eliminate unwanted Helmholtz resonance s exhibited by the commercial devices. Thus, new requirements were defined as follows: Frequency response: 100 Hz to 100 KHz (+/-3dB) Upper sound pressure limit: Design 1: 130 dB SPL (THD less than 5%) Design 2: 150-160 dB SPL (THD less than 5%) Packaging: 3.73 x 6.13 x 1.3 mm can with laser-etched lid. In collaboration with Novusonic Acoustic Innovation, NASA modified a Knowles SiSonic MEMS design to meet these new requirements. Coupled with the design of the enhanced MEMS microphones was the development of a new calibration method for simultaneously obtaining the sensitivity and phase response of the devices over their entire broadband frequency range. Traditionally, electrostatic actuators (EA) have been used to characterize air-condenser microphones; however, MEMS microphones are not adaptable to the EA method due to their construction and very small diaphragm size [4]. Hence a substitution based, free-field method was developed to calibrate these microphones at frequencies up to 80 kHz. The technique relied on the use of a random, ultrasonic broadband centrifugal sound source located in a small anechoic chamber. The free-field sensitivity (voltage per unit sound pressure) was obtained using the procedure outlined in reference 4. Phase calibrations of the MEMS microphones were derived from cross spectral phase comparisons between the reference and test substitution microphones and an adjacent and invariant grazing-incidence 1/8-inch standard microphone. The free-field calibration procedure along with representative sensitivity and phase responses for the new high-frequency MEMS microphones are presented here.
Consideration is given to the exterior source surface for force-free fields. The spherical harmonic expansion is presented for boundary values on two concentric spheres. An upper limit on a constant which measures the strength of coronal currents is found to be a function of the lowest multipole moment of the prescribed boundary values. The solar atmosphere is in the class of magnetic fields for which the study is applicable.
Delayed sea urchin egg mitosis by high magnetic field - testing methods for magnetic field-free environment
A free-field (FF) substitution method for calibrating the pressure sensitivity of microphones at frequencies up to 80 kHz is demonstrated with both grazing and normal incidence geometries. The substitution-based method, as opposed to a simultaneous method, avoids problems associated with the non-uniformity of the sound field and, as applied here, uses a 1/2 -inch air-condenser pressure microphone as a known reference. Best results were obtained with a centrifugal fan, which is used as a random, broadband sound source. A broadband source minimizes reflection-related interferences that often plague FF measurements. Calibrations were performed on 1/4-inch FF air-condenser, electret, and micro-electromechanical systems (MEMS) microphones in an anechoic chamber. The accuracy of this FF method is estimated by comparing the pressure sensitivity of an air-condenser microphone, as derived from the FF measurement, with that of an electrostatic actuator calibration and is typically 0.3 dB (95% confidence), over the range 2-80 kHz.
Effects of strong magnetic fields and of magnetic field free environments on man and animals
This paper presents particularly simple mathematical formulas for the calculation of force-free fields of constant alpha from the distribution of discrete sources on a flat surface. The advantage of these formulas lies in their physical simplicity and the fact that they can be easily used in practice to calculate the fields. The disadvantage is that they are limited to fields of 'sufficiently small alpha'. These formulas may be useful in the study of chromospheric magnetic fields by the comparison of high-resolution H-alpha photographs and photospheric magnetograms.
Techniques for studying aerodynamic noise generating mechanisms without disturbing the flow in a free field, and in the reverberation environment of the ARC wind tunnel were investigated along with the design and testing of an acoustic antenna with an electronic steering control. The acoustic characteristics of turbojet as a noise source, detection of direct sound from a source in a reverberant background, optical diagnostic methods, and the design characteristics of a high directivity acoustic antenna. Recommendations for further studies are included.
Some new results are given showing the possible evolution of a two-dimensional force-free field in the half-space z greater than 0 toward an open field. This evolution is driven by shearing motions applied to the feet of the field lines on the boundary z = 0. The consequences of these results for a model of the two-ribbon solar flare are discussed.
The scattering and diffraction of spherical waves by an infinite rigid cylinder is analytically studied for monopole and dipole acoustic sources. A correction factor at the cylinder surface for the change in the free-field radiation levels due to the presence of the cylinder is calculated. The results indicate that the correction factor is sensitive to source frequency and location. Regions of negative correction factor, due to shadowing and creeping wave behavior are identified.
This report describes test results from an assessment of the acoustically treated 9x15 Foot Low Speed Wind Tunnel at the NASA Glenn Research Center in Cleveland, Ohio in July of 2016. The tests were conducted in accordance with the recently adopted international standard ISO 26101-2012 on Qualification of Free Field Test Environments. This method involves moving a microphone relative to a source and comparing the sound pressure level versus distance measurements with theoretical inverse square law spreading.
This report describes test results from an assessment of the acoustically treated 9- by 15-Foot Low Speed Wind Tunnel at the NASA Glenn Research Center in Cleveland, Ohio in June of 2019. The tests were conducted in accordance with the recently adopted international standard ISO 26101-2012 on Qualification of Free Field Test Environments. This method involves moving a microphone relative to a source and comparing the sound pressure level versus distance measurements with theoretical inverse square law spreading.
Effect of very strong magnetic fields & magnetic- free environments on man & animals
We solve the force-free equation J x B = 0 for fields which are toroidally symmetric. The technique utilizes an expansion about a cylindrical field and is therefore valid or tori with a large aspect ratio such as long, thin, coronal loops. The calculation is performed in spatial toroidal coordinates, rather than in the flux coordinates used by previous authors; this allows direct calculation of the loci of flux surfaces and of surfaces of constant magnetic pressure. Our solutions differ significantly from toroidal fields in laboratories, which are in general not force-free. They are characterized by field lines whose projections in the poloidal planes are circles with centers displaced by varying distances from the axis of the torus. In general, flux surfaces do not correspond to surfaces of constant magnetic pressure. We have examined solutions corresponding to simple analytic zero-order cylindrical fields. For moderate twists in the zero-order (cylindrical) field, the magnetic pressure is larger on the inner toroidal radius. However, this effect diminishes with twist angle and in fact, for extreme initial twists, the magnetic pressure can be larger on the outer radius. We compare our results with previous work utilizing flux coordinates.
Magnetic clouds observed at 1 AU are modeled as cylindrically symmetric, constant alpha force-free magnetic fields. The model satisfactorily explains the types of variations of the magnetic field direction that are observed as a magnetic cloud moves past a spacecraft in terms of the possible orientations of the axis of a magnetic cloud. The model also explains why the magnetic field strength is observed to be higher inside a magnetic cloud than near its boundaries. However, the model predicts that the magnetic field strength profile should be symmetric with respect to the axis of the magnetic cloud, whereas observations show that this is not generally the case.
A large-scale potential field with an embedded smaller-scale force-free structure gradient x B equals alpha B is studied in cylindrical geometry. Cases in which alpha goes continuously from a constant value alpha 0 on the axis to zero at large r are considered. Such a choice of alpha (r) produces fields which are realistic (few field reversals) but not completely stable. The MHD-unstable wavenumber regime is found. Since the considered equilibrium field exhibits a certain amount of magnetic shear, resistive instabilities can arise. The growth rates of the tearing mode in the limited MHD-stable region of k space are calculated, showing time-scales much shorter than the resistive decay time.
Boundary conditions are imposed on the normal component of B on two parallel planes which delineate the force-free volume of linear magnetic fields in low coronal levels of the solar atmosphere. Green's function is employed to ensure that equations for the magnetic field energy remain bounded and smooth. Field line tracings of a simple bipolar source distribution are developed and compared with empirical data obtained from visual observations in H(alpha), EUV, and X-rays.
A test was conducted in the Boeing Large Anechoic Chamber to determine static jet noise source locations of six baseline and suppressor nozzle models, and establish a technique for extrapolating near field data into the far field. The test covered nozzle pressure ratios from 1.44 to 2.25 and jet velocities from 412 to 594 m/s at a total temperature of 844 K.