Direct numerical simulation (DNS) and large eddy simulation (LES) have been used as a fundamental tool in the study of turbulence physics and turbulent flow control. Many findings have been observed from numerical simulations for the last two decades. Due to the limitation of available computer resources, however, most direct numerical simulations and large-eddy simulations have been restricted to simple flows, where periodic boundary conditions can be employed in the streamwise direction.
The objective of this treatise is to perform spatially-evolving simulations with DNS and LES. In order to simulate spatially-evolving flows correctly, implementation of suitable inflow and outflow conditions is of prime importance. An inflow condition should supply turbulent kinetic energy continuously to maintain turbulence while a proper outflow condition makes the flow pass through the exit boundary with little distortion, so that the interior solution is not polluted by errors from the exit boundary. Implementation of an appropriate inflow condition is more difficult to deal with than the outflow condition, because the influence of the inflow condition persists over large distances downstream. This present study is composed of four parts.
In part I, a comparative study is made of inflow conditions for spatially-evolving flows. A new spatio-temporal inflow condition is devised and evaluated with other methods, i.e., temporal, phase jittering and amplitude jittering, and random noise. These methods are validated by testing a large-eddy simulation of turbulent channel flow. Computational results are presented to disclose the ability of inflow conditions and to capture the turbulent statistics with correct phase information and dynamics. The present spatio-temporal inflow condition is found to be generally satisfactory in CPU time and data management.
In part II, direct numerical simulations are made of instability in a spatially-evolving channel flow. A local surface suction/blowing is imposed at the upper wall. A Tollmien-Schlichting wave is superimposed on the laminar channel flow at the inflow. At the outflow, the buffer domain technique is applied to suppress the reflection of outgoing waves. The influence of the local suction/blowing on the linear and nonlinear instabilities of the flow is examined. It was found that the local suction/blowing increases the disturbance energy significantly in the interaction zone for subcritical (Re=5000) and supercritical (Re=10,000) cases. The effects of the blowing strength and the initial T-S wave amplitude on the subcritical channel flow are scrutinized. Two regimes of the wave/flow interaction were found by varing the blowing strength, i.e., 'monotonic' and 'vortex splitting' regimes.
In part III, large eddy simulations have been performed to investigate the effect of the local wall suction and blowing on the downstream development of turbulent structure in the spatially-evolving channel flow. Steady wall suction and blowing were imposed at the lower wall of the channel flow through a spanwise slot. Unsteady flow data from the inflow simulation with the identical grid spacings and time step was employed at inflow boundary. Downstream evolution of the perturbed turbulent channel flow was examined. The wall perturbation was found to modify the time-mean velocity, Reynolds stresses, and vorticity significantly. The wall blowing increased the turbulent kinetic energy and vorticity near the blowing slot, while suction resulted in opposite behavior. The effects of the suction/blowing strength on the turbulent channel flow were scrutinized.
Finally, in part IV, the Karhunen-Loeve (K-L) expansion is used to extract coherent structures from a leading-edge separation bubble with local forcing. A leading-edge separation bubble is simulated using a discrete vortex method, where a time-dependent source forcing is perturbed near the separation point. The K-L procedure is applied in a range of the forcing amplitude and forcing frequency. Application of K-L procedure reveals that the eigenstructures are changed notice ably by local forcings. In an effort to investigate the mechanism of decreasing reattachment length, dynamic behavior of the expansion coefficients and contributions of the eigenfunctions are scrutinized. As the forcing amplitude increases, the large-scale vortex structures are formed near the separation point. Furthermore, the flow becomes more organized, which results in the reduction of $x_R$. Two distinctive regimes are classified: the regime of increasing $x_R$ and the regime of increasing $x_R$. The K-L global entropy indicates that $x_R$ is closely linked to the organization of the flow structure.
유동장의 3차원, 비정상 수치 해석은 난류 구조와 난류 유동 제어 연구에 있어서 기본적인 연구 분야로 인식되고 있다. 지난 20여년 동안 난류 유동장의 많은 물리적 현상이 수치해석을 통해서 규명되어왔다. 그러나, 이제까지 대부분의 연구는 기하학적 형상이 단순한 유동장에 대해 주로 이루어졌으며, 주기적 경계조건을 흐름방향에 적용하였다. 이러한 경계조건은 공학적으로 중요한 대부분의 유동장에 적용이 적당치 않으며, 이를 위해서는 적절한 입구 및 출구 조건을 사용하여야 한다. 본 연구에서는 무한 평판 사이의 채널 유동을 다룬다. 유동장은 채널 벽에서의 분사/흡입을 통해 국소 교란이 가해진다. 벽면에서의 분사/흡입을 이용한 유동 제어는 가장 간단하면서도 널리 사용되고 있는 유동 제어 방법이다.
Part I에서는 이러한 계산에 필요한 입구 경계 조건에 대해 문헌에 나타난 방법들의 상호 비교를 통해서 타당성을 검토하고 이를 바탕으로 새로운 입구 경계 조건을 제안한다. Part II에서는 본 유동장의 안정성 해석을 수행하고, 벽면 분사/흡입이 유동장의 안정성에 미치는 영향을 알아보고 유동 천이의 제어에 대한 가능성을 살펴본다. Part III에서는 Part I의 연구를 바탕으로 벽면 교란이 있는 난류 채널 유동의 3차원 비정상 수치해석을 수행한다. 벽면 교란이 난류 구조에 미치는 영향을 살펴보고, 난류 제어에 대한 적용 가능성을 검토한다. 마지막으로, Part IV에서는 수치 계산 결과의 해석을 위해 POD를 적용한다. 이를 통해 난류 유동의 응집구조를 추출하여 난류 유동에 대한 이해를 돕는다.
