Characteristics of Noise Produced during Impingement of a Compressible Vortex Ring on a Wall

Abstract

Noise produced during normal impingement of a compressible vortex ring on a flat surface is studied in the shock-Mach number (M) range of 1.31 to 1.55. The compressible vortex ring is generated at the open end of a short driver section shock tube. The far-field noise is decomposed into three major components; (i) sound field due to formation and evolution of the vortex ring, (ii) reflected shock and vortex ring interaction noise and (iii) noise due to impingement of the ring on the wall. The impingement noise consists of fluctuating pressure due to deformation and stretching of the vortex ring, formation and growth of a secondary wall vortex ring, lifting-off of the primary-secondary vortex ring pair. All these events are identified using discrete wavelet transform (DWT) of the sound pressure signal and verified with the laser sheet based flow visualizations. Acoustics fluctuations measured at different angular location shows that the noise due to wall-vortex ring interaction is dominant at 10°≤ θ ≤ 40°.

Characteristics of shock tube generated compressible vortex rings at very high shock Mach numbers

“…The embedded shock-free vortex ring (for PR = 3) diameter is found to increase initially during the formation time and then attain constant value at approximately t* = 1.24 (t = 600 μs), similar to an incompressible vortex rings free of stopping vortex (Das et al 2017). After reaching a constant diameter in PR = 8 case, a marginal increase in ring diameter is observed at t* = 4.40 (t = 1198 μs) due to the induced effect of CRVR as it moves ahead of the primary core (Murugan & Das 2010). The Primary ring diameter reduces marginally when the CRVR moves behind the core-centre of the primary ring due to the same induced effect, which also accelerates the flow (Murugan & Das 2010).…”

“…After reaching a constant diameter in PR = 8 case, a marginal increase in ring diameter is observed at t* = 4.40 (t = 1198 μs) due to the induced effect of CRVR as it moves ahead of the primary core (Murugan & Das 2010). The Primary ring diameter reduces marginally when the CRVR moves behind the core-centre of the primary ring due to the same induced effect, which also accelerates the flow (Murugan & Das 2010). The leapfrogging (Maxworthy 1972, Riley Stevens 1993 action of the shear layer vortices has the opposite effect and is evident from the vorticity field and numerical shadowgraphs of PR = 50 (see figure 8f).…”

“…Brouillette & Hebert (1997) first observed the appearance of a tiny vortex ring of opposite circulation to that of the primary vortex ring ahead of it and termed it as CRVR. The primary vortex ring's propagation and dynamics are severely affected by the CRVRs in a highly under-expanded flow regime (Murugan & Das 2010).…”