Experiments Concerning the Hartmann Whistle

Abstract

The Hartmann whistle, in its most basic configuration, consists of a flat-bottomed, cylindrical cavity which is axially aligned with a supersonic air jet of the same diameter. Discrete-frequency oscillations of the enclosed air column are driven at large amplitudes when the cavity is located within certain regions of the cellular structure of the jet. An optical and acoustical study of the phenomenon is described, together with that of the Hartmann 'pulsator'. In the latter form the whistle has the small cavity replaced by a large Helmholtz-type resonator with the same orifice diameter, resulting in a large-amplitude aeroacoustic oscillator with a periodic time of several orders of magnitude greater than for the regular whistle. The underlying cause of the newly discovered bistable condition of the normal 'shock-disc' located in the alrstream between the nozzle and the cavity orifice is an important aspect which makes possible a (presently qualitative) theory of operation which accounts for the principal features of the Hartmann whistle and its direct derivatives. Some other aspects still requiring further elucidation and which are the subject of continuing effort are mentioned. The Report includes a brief review of the currently available literature pertaining to the phenomenon.

Self-oscillatory flow in the Hartmann resonator – Numerical simulation

“…Thus, the numerical simulation confirms the main features of the self-oscillatory flow in the Hartmann resonator observed in experimental investigations, such as, Smith and Powell 8 and Sarohia and Back. 47 At the same time, it allows one to obtain an extensive sequence of flow snapshots which makes it possible to discern some fine flow structures which would hardly be observable in the physical experiments.…”

“…We will now consider the flow pattern inside and outside the tube with reference to one oscillation cycle from t = t0 = 688.3 to t = t0+T = 715 , where T = 26.7 is the oscillation period. We recall that this flow pattern was qualitatively described in Smith and Powell 8 and, later, in Sarohia and Back 47 on the basis of the experimental data obtained by spark Schlieren shadowgraphy of the flow and the pressure measurements at the tube endwall. It includes the stages of tube filling by the gas from the jet incident on the cavity (inflow) and gas evacuation from it (outflow).…”

“…It is in exploring the flow patterns in imperfectly expanding, or off-design, jets issuing from convergent nozzles that Hartmann detected violent fluctuations of the jet flow parameters and oscillations of the jet itself, when certain zones within the jet, referred to as instability zones, were perturbed by placing the Pitot tube along the jet axis for the purpose of measuring the total pressure in the jet. As suggested in Smith and Powell, 8 his discovery dates from 1916. Apparently, his subsequent years were devoted to the thorough analysis of the phenomenon and the development of the device named later the Hartmann resonator, siren, whistle, tube, or cavity.…”

“…The results of the studies performed during the first half-century after Hartmann’s discovery were summarized in the extensive overview by Smith and Powell. 8 They considered the Hartmann devices of different types, described the qualitative shock-wave-driven flow patterns in the cavities, and established certain dependences of the flow pattern on the control parameters. In particular, it was noticed that the oscillation frequency in the tube flows is only slightly affected by the nozzle/tube spacing.…”

“…The recent review of Raman and Srivanasan 15 surveys the entire history of investigations concerned with the Hartmann resonator till recently, with emphasis placed on the period after Smith & Powell review. Thus, the three above-mentioned overviews 8,13,15 can be regarded as the milestones on the hard road of exploration in the field of self-oscillatory impinging jet flows.…”