41 COSMOS-V, an Aerodynamic Noise Simulator Nariaki Horinouchi COSMOS-V COSMOS-V COSMOS-V 3 The present and future computational problems of the aerodynamic noise analysis using COSMOS-V, our in-house CFD software, are explained by focusing on the wind noise and the wind-throb phenomenon. In addition, the sideview mirror surface vibration is equivalently treated as an aerodynamic noise problem because of the similarity of their mechanisms in the sense that both phenomena are caused by flow fluctuations around an automobile body. In general, pressure fluctuations due to the aerodynamic noise are minimal compared to those of the flow field itself which generates the sound. To date, however, the present computational techniques cannot directly resolve the noise. Instead, in the present approach, the noise characteristics are often indirectly predicted by measuring the resolvable-scale fluctuation of the unsteady pressure field. Thus, the accurate computation of the unsteady flow field is indispensable for a reliable aerodynamic noise analysis. In this regard, this paper presents three key computational techniques to attain accurate results using COSMOS-V. These include: 1. the overset grid method to generate the appropriate structured computational grid system in a complicated geometry; 2. the finite volume method (FVM) on the collocated grid system to conserve the mass and the momentum on the discretized fundamental equations; and 3. the weak compressible flow model derived through the assumption of a slight nominal density fluctuation to simulate the wind-throb phenomenon. Two computational results from COSMOS-V are shown for the side-view mirror surface vibration and the wind-throb phenomenon. Keywords Aerodynamic noise, Wind noise, Wind-throb, COSMOS-V, Unsteady flow, Overset grid, Collocated grid, Weak compressible flow model
42 (CFD : Computational Fluid Dynamics) (A) (B) (C) (D) 1) (A) COSMOS-V 3 100Hz khz ( 10 50Hz ) ( ) ( ) 10 5 2, 3) Lighthill-Curle
43 u j = 0 (1) u i t + u iu j τ ij = 1 u i + u j Re x i (2) (3) (1) (2) Navier-Stokes ( ) u, p, Re Reynolds Lighthill-Curle P a P a = 4πc 1 x i r 2 t = p x i + τ ij s n i PdS (4) cx i r P (1)(2) (4) 2 4) (4) COSMOS-V ( ) ( ) COSMOS-V 5) ( ) 2 COSMOS-V ( ) Example of overset grid system. (For simulation of flow around sedan type automobile)
44 COSMOS-V 6) u i p COSMOS-V ( ) p COSMOS-V u i p u i JU i (1)(2) α ji = ξ ξ x ξ y ξ z i dξ j = α ji dx i, = η x x η y η z (5) j ζ x ζ y ζ z η ξ JU p v JV u 1 (JU j ) = 0 J ξ j u i t (6) = α ik p + 1 (Jα mj )α mk u i ξ i Re J ξ j ψ k (7) J = 1 JU i = (Jα ki )u k α, ji (8) Navier-Stokes (7) u i (6) JU i (7) ( 2 ) JU i (7)QUICK 2 Crank-Nicolson COSMOS-V Mach 8) M 2 p (9) t + u p j + u i = 0 u i t + 1 J + u iu j ξ j (JU j u i ) u i u j = p x i + τ ij (10) M Mach 0.1 (1)(2) (9) 1 Collocated grid on two dimensional plane. R&D Review of Toyota CRDL Vol. 36 No. 4 ( 2001.12 )
45 COSMOS-V2 7, 8) 2 COSMOS-V ( Cp: ) 3 Three-dimensional open cavity. Sound pressure levels. Time averaged velocity vectors and pressure maps. Frequencies of pressure fluctuations.
46 U U SPL f Fig. 5 (calc.) (exp.) Fig. 6 ( n = 1, 2, 3 ) 30m/s (1) (2) COSMOS-V (1) (2) Lighthill-Curle COSMOS-V 1kHz QUICK LES (Large Eddy Simulation) COSMOS-V (2)Lighthill-Curle 9) 3 2GFLOPS 1 100 CPU 1), :, (1996), 2) :, -1(1998), 17 3) :, (1980), 4), 2 :, -1(1996), 2 5), 2 : R&D, -2(1997), 23 6), : 10, (1996), 410-411 7), 4 : 2001, (2001), 79-82 8), 3 : R&D, -2 (2001), 31 9), 4 : 2000 Vol.1(2000), 951-952 (2001 10 4 1961 R&D Review of Toyota CRDL Vol. 36 No. 4 ( 2001.12 )