Vortical flows over a LEX-delta wing at high angles of attack

The vortical flows over sharp-edged delta wings with and without a leading edge extension have been investigated using a computational method. Three-dimensional compressible Rey-nolds-averaged Navier-Stokes equations are solved to provide an understanding of the effects of the angle of attack and the angle of yaw on the development and interaction of vortices and the aerodynamic characteristics of the delta wing at a freestream velocity of 20 m/s. The present computations provide qualitatively reasonable predictions of vortical flow characteristics, compared with past wind tunnel measurements. In the presence of a leading edge extension, a significant change in the suction pressure peak in the chordwise direction is much reduced at a given angle of attack. The leading edge extension can also stabilize the wing vortex on the windward side at angles of yaw, which dominates the vortical flows over yawed delta wings.     

Nonlinear adaptive control for high angle of attack flight

This paper presents a nonlinear methodology for the control of a high angle of attack aircraft, in particular, a modified F-18 aircraft. As a modern combat aircraft demands better maneuverability and performance over domains which include high angles of attack, research in high angle of attack is presently at an advanced stage. An adaptive controller is developed to maneuver an aircraft at a high angle of attack even if the aircraft is required to fly over a highly nonlinear flight regime. The adaptive, controller presented in this paper is based on nonlinear prediction models, and can be constructed to minimize the given cost function or the difference of a described Lyapunov function with respect to the control input at each step. A controller uses system identification parameters to calculate a command signal so that the output of system follows the reference trajectory. The control is calculated to let system follow the reference trajectory under some constraints. This paper shows that nonlinear adaptive control can be utilized effectively to control high performance aircraft such as the F-18 aircraft for rapid maneuvers with large changes in angle of attack.     

Numerical study on flow characteristics at blade passage and tip clearance in a linear cascade of high performance turbine blade

A numerical analysis has been conducted in order to simulate the characteristics of complex flow through linear cascades of high performance turbine blade with/without tip clearance by using a pressure-correction based, generalized 3D incompressible Navier-Stokes CFD code. The development and generation of horseshoe vortex, passage vortex, leakage vortex, tip vortex within tip clearance, etc. are clearly identified through the present simulation which uses the RNG k-ε turbulent model with wall function method and a second-order linear upwind scheme for convective terms. The present simulation results are consistent with the generally known tendency that occurs in the blade passage and tip clearance. A 3D model for secondary and leakage flows through turbine cascades with/without tip clearance is also suggested from the present simulation results, including the effects of tip clearance height.     

A computational study of annular cascade flows using aq-ω turbulence model with a wall function

A computational code has been developed for steady viscous flows in three dimensional annular cascades. This code solves a special form of the thin-layer Navier-Stokes equations with a two-equationq-ω turbulence model in curvilinear coordinates using a time asymptotic method for steady state solutions. It employs a scalar implicit approximate factorization in time and a finite volume formulation with second-order upwind-differencing in space. A wall function treatment is implemented at solid boundaries for turbulence equations instead of integration to the wall to relieve gridding requirements. In order to validate the effectiveness of this code, computational studies have been made to access modeling capability for complex turbulent flow fields in three dimensional annular cascade geometries which typically include laminar-turbulent boundary layer transition. The results have been compared with both the computational studies with integration to the wall and the experimental studies. The wall function treatment was found to be reliable by predicting secondary flows and loss contours reasonably well.     

Steady aerodynamic characteristics of a wing flying over a nonplanar ground surface Part I: Rail

The aerodynamic interaction between a wing and a rail is investigated using a boundary-element method. The source and doublet singularities are distributed on the wing and its guideway rail surface. The unknown strengths of the singularities are determined by inverting the aerodynamic influence coefficient matrices. Present method is validated by comparing computed results with the other numerical data. Rail width and rail height affect the aerodynamic characteristics of the wing only if the rail is narrower than the wing span. Although the present results are limited to the inviscid, irrotational flows, it is believed that the present method can be applied to the conceptual design of the high speed ground transporters moving over the rail.     

Steady aerodynamic characteristics of a wing flying over a nonplanar ground surface Part II: Channel

The steady aerodynamic characteristics of a wing flying over a channel are investigated using a boundary-element method. The present method is validated by comparing the computed results with the measured data. Compared with a flat ground surface, the channel fence augmented the lift increase and induced drag reduction. When the fence is lower than the wing height, the gap between the wingtip and the fence does not affect the aerodynamic characteristics of the wing much. When the fence is higher than the wing height, the close gap increased the lift. The induced drag is reduced when the wing is placed near the ground or at the same height as the fence. It is believed that present results can be used in the conceptual design of the high-speed ground transporters flying over the channel.     

A new model for combined Couette and Poiseuille flows in the transverse groove of a plane inclined slider bearing

Although extensive research has been performed on the grooving of hydrodynamic bearings, there is much to be done on the flow interactions near to and in axial grooving. This work was initiated when the pressure boundary conditions were unknown for the case of a journal bearing with multiple axial grooves each being fed by an external lubricant source. This work is a forerunner to a more extensive research programme using journal bearings. A new technique has been devised to calculate the pressure profile across and along an axial groove. There is a strong pressure flow along the groove and Couette and pressure flow in the direction of sliding. Good correlation between experimental results and theoretical predictions for a wide range of operating conditions has been achieved.