Chuen-Yen Chow

Numerical simulation of transonic flow over porous airfoils

A numerical study was made to examine the effect of a porous surface on the aerodynamic performance of a transonic airfoil. The pressure jump across the normal shock wave on the upper surface of the airfoil was reduced by making the surface below the shock porous. The weakened shock is preceded by an oblique shock at the upstream end of the porous surface where air is blown out of the cavity. The lambda shock structure shown in the numerical result qualitatively agrees with that observed in the wind tunnel. According to the present analysis, the porous airfoil has a smaller drag and a higher lift than the solid airfoil.

Unsteady loading on airfoil due to vortices released intermittently from its surface

An unsteady flow analysis is made of the flow past a symmetric airfoil with identical vortices released intermittently from its upper surface. The vortex train is used to simulate the flow observed in the laboratory, which was perturbed by an oscillating spoiler or a rotating cam embedded in the airfoil surface. Based on numerical computations, the airfoil lift generally increases oscillationally with time and seems to approach an asymptotic value as time increases indefinitely. The asymptotic lift is enhanced with increasing frequency and is only slightly influenced by changing the vortex-releasing position along the chord. The behavior of the drag is similar to that of the lift, but its magnitude is two orders smaller. Our study also indicates that it is more efficient to implement the vortex-augmented unsteady lift at higher angles of attack of the airfoil.

Transonic aeroelasticity analysis for rotor blades

A numerical method is presented for calculating the unsteady transonic rotor flow with aeroelasticity effects. The blade structural dynamic equations based on beam theory were formulated by the finite element method and were solved in the time domain instead of the frequency domain. A global-local coordinate-transformation matrix was used to reduce the inaccuracy caused by large blade deformations. A new structure code was developed and was validated by comparing the computed natural frequencies with experimental data of a model rotor blade. For different combinations of precone, droop, and pitch, the correlations are very good in the first three flapping modes and the first twisting mode. However, the predicted frequencies are too high for the first lagging mode at high rotational speeds. This new structure code has been coupled into a transonic rotor flow code, TFAR2, to demonstrate the capability of treating elastic blades in transonic rotor flow calculations. The flowfields for a model-scale rotor in both hover and forward flight are calculated. Results show that the blade elasticity significantly affects the flow characteristics in forward flight.

Flight test and numerical simulation of transonic flow around YAV-8B Harrier II wing

A computational fluid dynamics (CFD) method is used to study the aerodynamics of the YAV-8B Harrier II wing in the transonic region. A numerical procedure is developed to compute the flow field around the complicated wing-pylon-fairing geometry. The surface definition of the wing and pylons were obtained from direct measurement using theodolite triangulation. A thin-layer Navier-Stokes code with the Chimera technique is used to compute flow solutions. The computed pressure distributions at several span stations are compared with flight test data and show good agreement. Computed results are correlated with flight test data that show the flow is severely separated in the vicinity of the wing-pylon junction. Analysis shows that shock waves are induced by pylon swaybrace fairings, that the flow separation is much stronger at the outboard pylon and that the separation is caused mainly by the crossflow passing the geometry of wing-pylon junction.

Transonic Inviscid-Viscous Interactions over Porous Surfaces

The viscous effects on transonic flow past an airfoil which contains a shallow cavity beneath a porous surface are studied numerically. The porous region occupies a small portion of the airfoil surface, and is near the shock. A thin-layer Navier-Stokes algorithm in combination with a modified Baldwin-Lomax turbulence model is used in computing the outer flow, whereas a stream-function formulation is used to model the inner flow in the cavity. The two flow regions are coupled at the porous surface through Darcy’s law. Shock structure and flow patterns have been computed to show the influence of shock/boundary-layer interactions. In agreement with experiments, numerical results show that the total drag of a porous airfoil is reduced at higher transonic speeds but is increased at lower Mach numbers.