Effect of anisotropic resistance characteristic on boundary-layer transitional flow

It is observed that the feather surface exhibits anisotropic resistances for the streamwise and spanwise flows.To obtain a qualitative understanding about the effect of this anisotropic resistance feature of surface on the boundary-layer transitional flow over a flat plate,a simple phenomenological model for the anisotropic resistance is established in this paper.By means of the large eddy simulation(LES)with high-order accurate finite difference method,the numerical investigations are conducted.The numerical results show that with the spanwise resistance hindering the formation of vortexes,the transition from laminar flow to turbulent flow can be delayed,and turbulence is weakened when the flow becomes fully turbulent,which leads to significant drag reduction for the plate.On the contrary,the streamwise resistance renders the flow less stable,which leads to the earlier transition and enhances turbulence in the turbulent region,causing a drag increase for the plate.Thus,it is indicated that a surface with large resistance for spanwise flow and small resistance for streamwise flow can achieve significant drag reduction.The present results highlight the anisotropic resistance characteristic near the feather surface for drag reduction,and shed a light on the study of bird’s efficient flight.

Side force control on slender body by self-excited oscillation flag

Strong asymmetrical vortices appear on the leeward of slender body at high angles of attack,which has very unfavorable effect on the stability and control of the aircraft.A method is developed to control the side force of slender body at high angles of attack,and is verified in wind tunnel.A thin-film triangular self-excited oscillation flag is fixed at the tip of the slender body model whose semi-apex angle is 10°.Side force is approximately linearly proportional to roll-setting angle of self-excited oscillation flag at high angles of attack,and the slop of fitting straight line obtained by the least square method is-0.158.The linear relationship between side force and roll-setting angle provides convenience for developing side force control law of slender body at high angles of attack.Experimental data shows that the side force coefficients vary linearly with roll-setting angles when a specific plastic self-excited oscillation flag is used as the control flag.The range of side force coefficient and roll-setting angle are,respectively,-3.2to 3.0 and-20° to 20°.The device is simple,effective,and is of great potential in engineering application.

Aeroelastic stability analysis of heated flexible panel subjected to an oblique shock

The critical conditions for aeroelastic stability and the stability boundaries of a flexible two-dimensional heated panel subjected to an impinging oblique shock are considered using theoretical analysis and numerical computations, respectively. The von-Karman large deflection theory of isotropic flat plates is used to account for the geometrical nonlinearity of the heated panel, and local first-order piston theory is employed in the region before and after shock waves to estimate the aerodynamic pressure. The coupled partial differential governing equations, according to the Hamilton principle, are established with thermal effect based on quasi-steady thermal stress theory.The Galerkin discrete method is employed to truncate the partial differential equations into a set of ordinary differential equations, which are then solved by the fourth-order Runge-Kutta numerical integration method. Lyapunov indirect method is applied to evaluate the stability of the heated panel. The results show that a new aeroelastic instability(distinct from regular panel flutter) arises from the complex interaction of the incident and reflected wave system with the panel flexural modes and thermal loads. What's more, stability of the panel is reduced in the presence of the oblique shock. In other words, the heated panel becomes aeroelastically unstable at relatively small flight aerodynamic pressure.

Sparse grid-based polynomial chaos expansion for aerodynamics of an airfoil with uncertainties

The uncertainties can generate fluctuations with aerodynamic characteristics. Uncertainty Quantification(UQ) is applied to compute its impact on the aerodynamic characteristics.In addition, the contribution of each uncertainty to aerodynamic characteristics should be computed by uncertainty sensitivity analysis. Non-Intrusive Polynomial Chaos(NIPC) has been successfully applied to uncertainty quantification and uncertainty sensitivity analysis. However, the non-intrusive polynomial chaos method becomes inefficient as the number of random variables adopted to describe uncertainties increases. This deficiency becomes significant in stochastic aerodynamic analysis considering the geometric uncertainty because the description of geometric uncertainty generally needs many parameters. To solve the deficiency, a Sparse Grid-based Polynomial Chaos(SGPC) expansion is used to do uncertainty quantification and sensitivity analysis for stochastic aerodynamic analysis considering geometric and operational uncertainties. It is proved that the method is more efficient than non-intrusive polynomial chaos and Monte Carlo Simulation(MSC) method for the stochastic aerodynamic analysis. By uncertainty quantification, it can be learnt that the flow characteristics of shock wave and boundary layer separation are sensitive to the geometric uncertainty in transonic region. The uncertainty sensitivity analysis reveals the individual and coupled effects among the uncertainty parameters.

An improved WENO-Z scheme with symmetry-preserving mapping

Since the classical weighted essentially non-oscillatory(WENO)scheme is proposed,various improved versions have been developed,and a typical one is the WENO-Z scheme.Although better resolution is achieved,it is shown in this article that,the result of WENO-Z scheme suffers evident distortion in the long-time simulation of the linear advection equation.In order to fix the problem of WENO-Z,a symmetrypreserving mapping method is proposed in this article.In the original mapping method,the weight of each sub-stencil is used to map,which is demonstrated to cause asymmetric improvement about a discontinuity.This asymmetric improvement will lead to a distorted solution,more severe with longer output time.In the symmetry-preserving mapping method,a new variable related to the smoothness indicator is selected to map,which has the same ideal value for each sub-stencil.Using the new mapping method can not only fix the distortion problem of WENO-Z,but also improve the numerical resolution.Several benchmark problems are conducted to show the improved performance of the resultant scheme.