Effects of tip perturbation and wing locations on rolling oscillation induced by forebody vortices

The wing rock motion is frequently suffered by a wing-body configuration with low swept wing at high angle of attack. It is found from our experimental study that the tip perturbation and wing longitudinal locations affect significantly the wing rock motion of a wing-body. The natural tip perturbation would make the wing rock motion of a nondeterministic nature and an artificial mini-tip perturbation would make the wing rock motion deterministic. The artificial tip perturbation would, as its circumferential location is varied, generate three different types of motion patterns： （1） limit cycle oscillation, （2） irregular oscillation, （3） equilibrium state with tiny oscillation. The amplitude of rolling oscillation corresponding to the limit cycle oscillatory motion pattern is decreased with the wing location shifting downstream along the body axis.

Low-diffusion preconditioning scheme with linear cut-off parameter

A low-diffusion preconditioning Roe scheme with an adjustable parameter to control the numerical dissipation is proposed. This scheme reflects the real physical dissipation in the extremely low-speed region. The preconditioning parameter in scheme is improved by linear cut-off and correction factor. The numerical results of low-Mach-number/low-Reynolds-number steady solutions of viscous flows past a circular cylinder and past a NACA0012 airfoil show the efficiency of the new scheme.

Numerical simulation of gurney flaps lift-enhancement on a low reynolds number airfoil

Two-dimensional steady Reynolds-averaged Navier-Stokes （RANS） equations with transition shear stress transport （SST） model were solved to investigate the effects of Gumey flaps on the aerodynamic performance of a low Reynolds number airfoil. This airfoil was designed for flight vehicles operating at 20 km altitude with freestream velocity of 25 rn/s. The chord length （C） of this airfoil is 5 m and the corresponding Reynolds number is 7.76× 10^5. Gurney flaps with the heights ranging from 0.25%C to 3%C were investigated. It has been shown that Gurney flaps can enhance not only the prestall lift but also lift-to-drag ratio in a certain range of angles of attack. Specially, at cruise angle of attack （3°）, Gurney flap with the height of 0.5%C can increase lift-to-drag ratio and lift coefficient by 1.6% and 12.8%, respectively. Furthermore, the mechanisms of Gumey flaps to improve the aerodynamic performance were illustrated by analyzing the surface pressure distribution, streamlines and trailing-edge flow structure for this low Reynolds number airfoil. Specially, distinguished from some other numerical researches, the flow details such as the laminar separation bubble and transition phenomena for low Reynolds number airfoil with Gumey flaps were investigated and it was found that Gurney flaps can delay the transition onset position at small angles of attack （≤2°）. However, with the increase of angles of attack, Gurney flaps will promote the boundary layer transition.

Flow-structure interaction of an inverted flag in a water tunnel

Herein, the dynamics and flow fields of an inverted flag are studied using hydrogen bubble flow visualization and particle image velocimetry technologies at different height-to-length ratios and flow velocities in a water tunnel. Results show that the heightto-length ratio of the inverted flag at which the critical flow velocity remains nearly constant is approximately 1.4. Moreover, a nonperiodic flapping phenomenon is observed under various height-to-length ratios. This phenomenon may be attributed to the existence of multiple equilibrium solutions to the self-excited vibration system, thus engendering chaos in the system comprising an inverted flag and surrounding fluid. Other indications that the system has entered chaos include multiple frequencies, nonoverlapping phase diagram, and positive Lyapunov exponent. Further discussion of the flow fields around the inverted flag reveals that the large-amplitude oscillation is due to the flow separation, while the flapping instability is a static divergence instability. In the large flapping mode, the starting leading-edge vortex(LEV) is wrapped by Kelvin-Helmholtz instabilities,which are arranged at almost uniform spacing along a circular path. In addition, the variation in position, circulation, and radius of the starting LEV are discussed in detail.

Mechanism of perturbation-combined active control technique for asymmetric vortex flow over slender body at high angle of attack

Based on the determinability of asymmetric vortices flow over slender body under changeless round grain at high angle of attack,the effect of microblowing set in special position on the behaviors of asymmetric flow is discussed in this paper,including blowing momentum and circumferential locations of the microblowing hole of 0.5 mm in diameter on nose tip.A new kind of active control technique,named perturbation-combined active control technique,which combines the micro-grain and micro-blowing perturbation,was proposed on the basis of the above.This control technique can not only change the sign of side force of slender body arbitrarily through changing the vortices positions between yaw-left and yaw-right configuration,but also can make the magnitude of side force variable gradually even at bistable state of asymmetric vortex.Finally,the interferential mechanism of the perturbation-combined active control technique has also been concluded from this paper.The tests have been conducted at low speed wind tunnel with subcritical Reynolds number of 1.05×10~5 at angle of attack α=50° in Beihang University,Beijing,China.

Generation of spatial atmospheric turbulence field in aircraft motion simulation based on refined similarity hypothesis

It is necessary to build turbulence model to study the response of aircraft to atmospheric turbulence for high resolution earth observation. The conventional method is on the basis of Dryden’s model with the assumption that individual patches are Gaussian. In this paper,based on Kraichnan’s refined similarity idea,a new 1D atmospheric turbulence model is set up by introducing the energy transfer rate as an intermittency disturbance to a Gaussian process. Our results show that the turbulent fields generated by our new method exhibit an anomalous scaling described by the She-Leveque (SL) formula,which is now well accepted for homogenous and isotropic turbulence.

Attractive fixed-point solution study of a shell model for turbulent convection

Different scaling behaviors, such as Kolmogorov (K41) scaling and Bolgiano and Obukhov (BO) scaling, have been reported in various shell models proposed for turbulent thermal convection. However, two coexistent subranges with K41 and BO scaling are not set up with Bolgiano scale interlaying between the largest scale and the dissipation scale. In this paper, we summarize fixed-point solution study of the Brandenburg model with small perturbation theory by introducing a small disturbance term as the impact of buoyancy. Three groups of fixed-point solutions with different locations of the so-called buoyancy scale, above/below which buoyancy is significant/insignifant. Both theoretical and numerical results show that a modified K41 scaling, instead of K41 and BO coexistent scaling, is set up even though buoyancy may be significant over the scaling range, which suggests that the buoyancy scale is not related exactly to the Bolgiano scale. Thus, a K41 and BO coexistent scaling behavior is not setup for the Brandenburg model.