tailieunhanh - MODERN HELICOPTER AERODYNAMICS phần 2

Nhiều kỹ thuật thực nghiệm và tính toán phức tạp đã được sử dụng trong một nỗ lực để dự đoán các thông số hiệu suất. Quan tâm đặc biệt là cấu trúc sau cánh quạt, mà là ba chiều và không ổn định, và phân phối áp lực cánh quạt phiến, mà đáng kể là bị ảnh hưởng bởi sức mạnh và vị trí của đánh thức. Chúng tôi mô tả các phương pháp hiện đại khác nhau của | HELICOPTER AERODYNAMICS 525 Mach number. Consequently modeling efforts have been designed to predict sectional lift and drag in order to calculate the thrust and power coefficients. However in blade element theory the effects of dynamic stall compressibility and blade-vortex interactions are usually omitted. The velocity at the inflow boundary to the rotor crucially depends on the structure of the wake flow. Consider the rotation of the blades started from rest. In the initial stages of the motion the velocity at the inflow rotor-disk depends on the local flow near the rotor blades. As time passes the vortex sheet and the tip-vortex shed from the rotor blades begin to extend far below the rotordisk forming the rotor wake. At this point the inflow velocity at the rotor-disk becomes critically dependent on the precise placement of the wake. This is why the calculation of the rotor wake is crucial in calculating the loads on the rotor blades. Vortex methods can predict the unsteady effects of the rotor wake on the blades and on the airframe and this is discussed next. 4. MODERN THEORETICAL AND COMPUTATIONAL APPROACHES TO THE ROTOR WAKE The last fifteen or twenty years have seen rapid development of computational and experimental efforts to calculate rotor wake problems. Nevertheless the accurate and efficient computation of the three-dimensional and unsteady flow around the rotor blades and the rotor wake as a solution of the Navier-Stokes equations remains elusive for reasons that will be explained. In what follows we outline the basic methods employed in calculating the wake of an isolated rotor. Gray 1955 1956 conducted experiments that led to the characterization of the rotor wake as being composed of high-strength tip-vortices and an inboard vortex sheet. This situation is depicted in Figure 3. In modeling the rotor wake by systems of vortices of this type three approaches have generally been used rigid wake models prescribed wake models and free wake models. In