• 동력기계공학회지
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• 제11권1호
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• pp.82-91
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• 2007

기존의 2차원 FDLB 모델(D2Q21)에서 비열비 ${\gamma}$는 공간의 차원수(D)에 의존한다. 즉, 2차원 공간의 계산에서는 ${\gamma}=(D+2)/D=2.0$밖에 취할 수 없으며, 공기와 같은 실체기체를 전산모사 하기에는 여러 어려움이 있다. 이러한 이유 때문에 문헌[1]의 LBM에서 제안된 조정 가능한 비열비 모델을 2차원 FDLB모델에 적용하여 자려발생 에지톤(edgetone)의 수치계산이 수행되었다. wedge의 선단각도가 ${\alpha}=23^{\circ}$(Case I) 및 $20^{\circ}$(Case II)를 갖는 2가지 모델이 설정되었으며, 노즐출구에서 wedge선단까지의 거리 w/d는 $3d{\sim}12d$사이에서 주어졌다. edgetone은 노즐로부터 나온 분류와 edge의 상호작용으로 이난 음압(sound pressure)의 차에 의해서 소음이 발생하며, 이 음압은 다시 상류의 분류에 영향을 미쳐 분류의 변동을 가져온다. w/d가 ??9d이하인 경우, 피드백(feedback) 메커니즘에 기인한 주기적인 운동이 발생하지만, w/d가 큰 ??9d이상인 경우에는 분류의 불안정성 때문에 규칙적인 분류의 운동은 보이질 않으며, 이는 기존의 연구결과들과 잘 일치함을 보였다. 본 연구에서 적용된 모델을 이용하여 공기와 같은 2원자 기체의 비열비 ??${\gamma}=1.4$를 갖는 유체에 있어서 공력 소음의 수치예측이 가능하다는 것을 확인하였다.

• Journal of Ship and Ocean Technology
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• 제12권1호
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• pp.1-15
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• 2008

An edgetone is the discrete tone or narrow-band sound produced by an oscillating free shear layer, impinging on a rigid surface. In this paper, 2-dimensional edgetone to predict the frequency characteristics of the discrete oscillations of a jet-edge feedback cycle is presented using lattice Boltmznan model with 21 bits, which is introduced a flexible specific heat ratio y to simulate diatomic gases like air. The blown jet is given a parabolic inflow profile for the velocity, and the edges consist of wedges with angle 20 degree (for symmetric wedge) and 23 degree (for inclined wedge), respectively. At a stand-off distance w, the edge is inserted along the centerline of the jet, and a sinuous instability wave with real frequency is assumed to be created in the vicinity of the nozzle exit and to propagate towards the downward. Present results presented have shown in capturing small pressure fluctuating resulting from periodic oscillation of the jet around the edge. The pressure fluctuations propagate with the speed of sound. Their interaction with the wedge produces an irrotational feedback field which, near the nozzle exit, is a periodic transverse flow producing the singularities at the nozzle lips. It is found that, as the numerical example, satisfactory simulation results on the edgetone can be obtained for the complex flow-edge interaction mechanism, demonstrating the capability of the lattice Boltzmann model with flexible specific heat ratio to predict flow-induced noises in the ventilating systems of ship.

• Journal of Advanced Marine Engineering and Technology
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• 제32권1호
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• pp.82-93
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• 2008

The flowfield behind two cylinders and flow-induced noise generated from the cylinders in various arrangement are numerically investigated based on the finite difference lattice Boltzmann model with 21 velocity bits. which is introduced a flexible specific heat ${\gamma}$ to simulate diatomic gases like air. In an isolated cylinder with two type of mesh. some flow parameters such as Strouhal number $S_t$ and acoustic pressure ${\Delta}p$ simulated from the solution are given and quantitatively compared with those provided the previous works. The effects of the center-to-center pitch ratio $L_{cc}/d=2.0$ in staggered circular cylinders as shown in Fig. 1 and angles of incidence ${\alpha}=30^{\circ}(T_{cc}/d=0.5)$, $45^{\circ}(T_{cc}/d =0.707)$ and $60^{\circ}\;(T_{cc}/d=0.866)$, respectively, are studied. Our analysis focuses on the small-scale instabilities of vortex shedding, which occurs in staggered arrangement. With the results of drag $C_d$ and lift $C_l$ coefficients and vorticity contours. the mechanisms of the interference phenomenon and its interaction with the two-dimensional vortical structures are present in the flowfields under $Re\;{\le}\;200$. The results show that we successively capture very small pressure fluctuations, with the same frequency of vortex shedding, much smaller than the whole pressure fluctuation around pairs of circular cylinders. The upstream cylinder behaves like an isolated single cylinder, while the downstream one experiences wake-induced flutter. It is expected that, therefore, the relative position of the downstream cylinder has significant effects on the flow-induce noise, hydrodynamic force and vortex shedding characteristics of the cylinders.