JOURNAL OF SOUND AND VIBRATION Journal of Sound and Vibration 313 (2008) 1–6 Rapid Communication Isom’s thickness noise for axial and centrifugal subsonic fans S. Khelladi à , S. Kouidri, R. Rey Ecole Nationale Supe´rieur d’Arts et Me´tiers, Paris 75013, France Received 27 September 2007; received in revised form 14 January 2008; accepted 17 January 2008 Handling Editor: C. Morfey Available online 5 March 2008 Abstract The thickness noise predicted by the Ffowcs Williams and Hawkings (FW&H) equation depends on the normal velocity v n which is very sensitive to the meshing size. Isom showed that in a far field a monopolar source is equivalent to a dipolar source induced by a uniform distribution of the load on the entire moving surface. Consequently, the calculation of the thickness noise becomes completely independent of the normal velocity v n . Its expression, as suggested by Farassat, is for any moving surface. The main objective of this work is to determine a specific expression of Isom’s thickness noise in time and frequency domains for axial and centrifugal subsonic fans. The proposed form of the thickness noise enables to highlight the effect of each geometrical parameter of the fan on the overall thickness noise, on the one hand, and presents a fast computational mean and low memory storage capability since the acoustic pressure in the frequency domain is calculated for only one blade, on the other. r 2008 Elsevier Ltd. All rights reserved. 1. Introduction In a free field, Isom showed in Ref. [1] that the acoustic pressure generated by a mopole is equivalent to these generated by a dipole induced by a uniform load equal to r 0 c 2 over a moving surface. Consequently, the calculation of the thickness noise becomes completely independent of the normal velocity v n over the surface. Farassat shows in Refs. [2,3] that it is more suitable to determine the thickness noise using Isom’s formulation than methods based on the resolution of Ffowcs Williams and Hawkings (FW&H) equation [4] using fluctuating velocity. The latter presents an important variation on the surface which makes the numerical integration very sensitive to the meshing resolution. On the other hand, Isom’s thickness noise formula can present some difficulties for rotating machinery because of its sensitivity to the geometry of the blade tips [5]. Isom’s thickness noise formula was used by Ghorbaniasl and Hirsch [6] as a consistency benchmark to show that a discrepancy should not exist between FW&H’s thickness noise and Isom’s noise when Farassat 1A formulation presented by Farassat and Succi in Ref. [7] is numerically solved in the time domain. The main objective of this paper is to define specific formulas of thickness noise in time and frequency domains for axial and centrifugal subsonic fans based on initial Isom’s formulation. This approach has the ARTICLE IN PRESS www.elsevier.com/locate/jsvi 0022-460X/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsv.2008.01.044 à Corresponding author. E-mail address: sofiane.khelladi@paris.ensam.fr (S. Khelladi).