Engineering topics

Hardin, J. C.

Publications and source records attributed to Hardin, J. C..

At least 37 records · Page 2

NASA NTRS1983

Aeroacoustic computation of cylinder wake flow

Sound generation by uniform flow over a cylinder at the Reynolds number of 200 is calculated. The incompressible, time dependent flow field is first computed by standard computational fluid dynamics techniques using a stream function/vorticity formulation. The sound radiation is then obtained by integrating over the flow field with the Coriolis acceleration as the source term and a low frequency Green's function technique. Time histories, spectra, and directivity of the radiated sound are obtained. Comparisons with experimental data, where possible, indicate the feasibility of this computational aeroacoustics approach.

Hardin, J. C.↗

NASA NTRS1982

The velocity field induced by a helical vortex filament

An exact analytical solution for the velocity field, both interior and exterior, induced by an infinite right-handed helical vortex filament is derived. Due to the way the variables combine in this solution, the paper also shows that it is possible to derive a stream function for this nonaxisymmetric flow. Sample calculations of these expressions are included.

Hardin, J. C.↗

NASA NTRS1981

The role of the helical jet mode in aerodynamic noise generation

The helical jet mode is modeled by an infinite helical vortex filament, and an exact analytical solution for the induced velocity field interior to the filament is derived. These expressions are then used to model the jet flow field and to analyze the high subsonic and supersonic jet noise generation. It is shown that noise will be generated directly if the helical mode convects or rotates. Reducing this noise by stabilizing the mode is difficult, since the self-induced velocities of the mode itself cause it to convect and rotate; the use of noncircular nozzles is suggested to eliminate the symmetry of the boundary condition. The helical mode can also generate noise indirectly by inducing time-dependent Coriolis accelerations on small-scale turbulence propagating in its induced velocity field. This suggests that any upstream turbulence generation has an enhanced potential for noise propagation when propagating in the presence of the helical mode.

Hardin, J. C.↗

NASA NTRS1981

Stochastic analysis of spectral broadening by a free turbulent shear layer

The effect of the time-varying shear layer between a harmonic acoustic source and an observer on the frequency content of the observed sound is considered. Experimental data show that the spectral content of the acoustic signal is considerably broadened upon passing through such a shear layer. Theoretical analysis is presented which shows that such spectral broadening is entirely consistent with amplitude modulation of the acoustic signal by the time-varying shear layer. Thus, no actual frequency shift need be hypothesized to explain the spectral phenomenon. Experimental tests were conducted at 2, 4, and 6 kHz and at free jet flow velocities of 10, 20, and 30 m/s. Analysis of acoustic pressure time histories obtained from these tests confirms the above conclusion, at least for the low Mach numbers considered.

Hardin, J. C.↗

NASA NTRS1980

Genesis of breath sounds-Preliminary verification of theory

Experimental results are presented which tend to validate a previously developed theory of sound production in the human lung over a particular Reynolds number range. In addition, a new, presently nonunderstood, phenomenon was observed at higher Reynolds number. These results, which show how sound generation in the lung depends upon the physiologically important variables of volume flow rate and bronchial diameter, have potentially important application in noninvasive lung examination and the diagnosis of lung disease.

Patterson, J. L.↗

NASA NTRS1980

Noise generation at the side edges of flaps

The recently observed phenomenon of high noise radiation from the side edges of flaps is investigated by way of a two-dimensional model based upon a physical picture of chordwise boundary layer vorticity being swept around the edge by spanwise flow on the flap. The trajectory and resulting noise radiation for a discrete vortex in such a flow is obtained. Further, a mathematical condition for the vortex to be captured by the flow and swept around the edge is derived. The sound generation depends strongly upon the vortex strength and distance from the edge and can be more intense than trailing edge noise in agreement with experimental observations.

Hardin, J. C.↗

NASA NTRS1980

Noise radiation from the side edges of flaps

The recently observed phenomenon of high noise radiation from the side edges of flaps in flow is investigated by way of a simple two-dimensional model problem. The model is based upon a physical picture of boundary layer vorticity being swept around the edge by spanwise flow on the flap. The model problem is developed and solved and the resulting noise radiation calculated. Further, a mathematical condition for the vortex to be captured by the potential flow and swept around the edge is derived. The results show that the sound generation depends strongly upon the strength of the vorticity and distance from the edge and that it can be more intense than the more common trailing edge noise source in agreement with the experimental observations.

Hardin, J. C.↗

NASA NTRS1979

Evaluation of a vortex model of turbulent cavity flow

A two dimensional discrete vortex model is applied to turbulent flow over a cavity at a Reynolds number based on cavity half length of approximately 500,000. These model predictions are compared with experimental data on a time average and on a spectral basis. The comparisons indicate that the vortex model reproduces the mean velocity profiles of this complex flow, while the turbulent intensity and Reynolds stress profiles are overpredicted in magnitude. Spectral analyses show that the vortex model introduces unrealistic low frequency power which becomes less dominant as the model becomes more disordered. The spectral power distribution of the model in this state is nearly in agreement with that of the data, somewhat too low at the higher frequencies and too high at the lower frequencies. The model is shown to be capable of reproducing the cavity feedback oscillation phenomenon.

Hardin, J. C.↗

NASA NTRS1979

Monitoring the state of the human airways by analysis of respiratory sound

A mechanism whereby sound is generated by the motion of vortices in the human lung is described. This mechanism is believed to be responsible for most of the sound which is generated both on inspiration and expiration in normal lungs. Mathematical expressions for the frequencies of sound generated, which depend only upon the axial flow velocity and diameters of the bronchi, are derived. This theory allows the location within the bronchial tree from which particular sounds emanate to be determined. Redistribution of pulmonary blood volume following transition from Earth gravity to the weightless state probably alters the caliber of certain airways and doubtless alters sound transmission properties of the lung. We believe that these changes can be monitored effectively and non-invasively by spectral analysis of pulmonary sound.

Lung/anatomy & histology/physiology↗

NASA NTRS1978

Monitoring the state of the human airways by analysis of respiratory sound

A mechanism whereby sound is generated by the motion of vortices in the human lung is described. This mechanism is believed to be responsible for most of the sound which is generated both on inspiration and expiration in normal lungs. Mathematical expressions for the frequencies of sound generated, which depend only upon the axial flow velocity and diameters of the bronchi, are derived. This theory allows the location within the bronchial tree from which particular sounds emanate to be determined. Redistribution of pulmonary blood volume following transition from earth gravity to the weightless state probably alters the caliber of certain airways and doubtless alters sound transmission properties of the lung. We believe that these changes can be monitored effectively and non-invasively by spectral analysis of pulmonary sound.

Hardin, J. C.↗

NASA NTRS1978

Noise calculation on the basis of vortex flow models

Flow-modeling technique yields relatively simple method for calculating sound radiation involving planar, cylindrical, or spherical surfaces. Model employs potential flow theory with action of viscosity on flowfield described in terms of point vortices. Surface presence in flow is analyzed, using classical image method; sound is calculated through sound generation theory reformulation.

Hardin, J. C.↗

NASA NTRS1977

Airplane self-noise - four years of research

A critical assessment of the state of the art in airframe self-noise is presented. Full-scale data on the intensity, spectra, and directivity of this noise source are evaluated. Vibration of panels on the aircraft is identified as a possible additional source of airframe noise. The present understanding and methods for prediction of other component sources (airfoils, struts, and cavities) are discussed, and areas for further research as well as potential methods for airframe noise reduction are identified. Various experimental methods which have been developed for airframe noise research are discussed, and sample results are presented.

Hardin, J. C.↗

NASA NTRS1977

Noise calculation on the basis of vortex flow models

A technique for noise calculation on the basis of vortex flow models is described. The 'reflection principle' is first extended to the whole class of potential flows which may be solved by the method of images. This allows the sound radiation to be computed solely through a volume integral over both the exterior and interior of any surfaces which may be present. The source distribution is then rewritten in terms of the vorticity within the flow which yields a highly computationally efficient formulation of the aeroacoustic theory. Several examples of such noise calculations are included.

Hardin, J. C.↗

NASA NTRS1976

Airframe self-noise: Four years of research

A critical assessment of the state of the art in airframe self-noise is presented. Full-scale data on the intensity, spectra and directivity of this noise source are evaluated in the light of the comprehensive theory developed by Ffowcs-Williams and Hawkins. Vibration of panels on commercial aircraft is identified as a possible additional source of airframe noise. The present understanding and methods for prediction of other component sources - airfoils, struts, and cavities - are discussed, and areas for further research as well as potential methods for airframe noise reduction are identified. Finally, the various experimental methods which have been developed for airframe noise research are discussed and sample results are presented.

Hardin, J. C.↗

NASA NTRS1976

A vortex model of cavity flow

The paper presents a model of two-dimensional cavity flow in which the shear layer over the cavity is represented by discrete rectilinear vortices which are free to move as the flow progresses. Although the model is initially started impulsively, the computation is continued until a statistically steady flow is attained. The broadband noise generation of the cavity is calculated by first running the model until the steady state is reached and then computing a stationary record of far-field density fluctuation through an equation that is suitable for calculating the quadrupole noise generation by the model. The discrete time series obtained can be analyzed by ordinary digital spectral techniques to determine the spectra and overall levels of the noise in airframe noise testing of real aircraft.

Hardin, J. C.↗

NASA NTRS1976

Airframe noise: A design and operating problem

A critical assessment of the state of the art in airframe noise is presented. Full-scale data on the intensity, spectra, and directivity of this noise source are evaluated in light of the comprehensive theory developed by Ffowcs Williams and Hawkings. Vibration of panels on the aircraft is identified as a possible additional source of airframe noise. The present understanding and methods for prediction of other component sources - airfoils, struts, and cavities - are discussed. Operating problems associated with airframe noise as well as potential design methods for airframe noise reduction are identified.

Hardin, J. C.↗

NASA NTRS1975

A potential flow model for calculation of jet noise

The development of the large-scale eddy structure of a circular jet is calculated from a potential flow model which includes the vorticity distribution. The model exhibits all the observed general features of high Reynolds number jet flows although it is constrained to remain axisymmetric. The far acoustic field is calculated from the predicted unsteady vorticity distribution.

Davies, P. O. A. L.↗

NASA NTRS1975

Prediction of airframe noise

Methods of predicting airframe noise generated by aircraft in flight under nonpowered conditions are discussed. Approaches to predictions relying on flyover data and component theoretical analyses are developed. A nondimensional airframe noise spectrum of various aircraft is presented. The spectrum was obtained by smoothing all the measured spectra to remove any peculiarities due to airframe protrusions, normalizing each spectra by its overall sound pressure level and a characteristics frequency, and averaging the spectra together. A chart of airframe noise sources is included.

Hardin, J. C.↗