Local Separation Zones Research Articles

Computational and experimental study on the aerodynamic performance of NACA 4412 airfoil with slot and groove

Since the Wright Brothers’ first flight in 1903, extensive research has been dedicated to improving the aerodynamic performance of aircraft. This study investigates the effect of two distinct wing geometric modifications on airfoil performance at high angles of attack (AOAs). These two modifications are slot, specifically the NACA 4412 with only a slot, and groove, specifically the NACA 4412 with both a slot and a groove. The investigation combines numerical simulation using ANSYS fluent with experimental evaluations conducted in the VDAS AF1300 subsonic wind tunnel. Since turbulent airflow often results in early stall, the primary objective of this research is to delay the stall angle of the normal NACA 4412 airfoil by mitigating local separation zones, and boundary layer transitions. Numerical simulations are performed at airspeeds of 20 m/s and 43.9 m/s, while experimental investigations are conducted at a speed of 20 m/s. The results indicate that both modified airfoils have higher lift-to-drag ratio than the normal airfoil at high AOAs. Specifically, the NACA 4412 airfoil with only a slot demonstrates the highest lift-to-drag ratio among the modified airfoils at high AOAs. Moreover, the NACA 4412 airfoil equipped with a slot and a groove demonstrates the highest stall angle, measured at 18°, compared to the normal NACA 4412 airfoil with a stall angle of 14°. At high AOA, the NACA 4412 airfoil with a slot generates a nearly 35 % higher lift coefficient than the normal NACA 4412 airfoil, while the NACA 4412 with a slot and a groove achieves almost a 16 % higher lift coefficient than the normal NACA 4412 airfoil.

Evolution of unsteady secondary flow structures near the onset of stall in a tip-critical transonic axial flow compressor stage

The aerodynamic stability of an axial compressor stage depends on the rotor and stator. However, due to the specific design requirements, the evolving flow field and the resulting secondary flow structures are different in both components. This study investigates the evolution of dominant secondary flow structures occurring in the rotor and stator of a tip-critical transonic compressor stage at the near-stall condition using unsteady numerical analysis and validates performance characteristics with the experimental data. The investigation reveals that the presence of rotor tip shock creates a large difference in the pressure gradient across the pressure surface and the suction surface, intensifying tip leakage flow and shock-induced boundary layer separation. The higher incidence angle near the hub leading edge creates a local separation and reattachment zone. The radial pressure gradient causes the low momentum flow from this local separation zone to migrate radially upward. This migrated flow interacts with the tip leakage flow and separated blade boundary layer, eventually creating a colossal recirculation zone and subsequent rotor blockage of around 46%. The increasing streamwise adverse pressure gradient pushes the tip shock upstream, and at the onset of the stall, the flow directly separates from the rotor leading edge avoiding the shock formation. The stator flow field is dominated by the asymmetric hub corner separation induced by the streamwise adverse pressure gradient and the tip corner separation caused by the vortex structures convected downstream from the rotor tip region leading to stator blockage of around 48%. Along with the blade passing frequency, four other dominating frequencies (0.06×BPF, 0.12×BPF, 0.44×BPF, and 0.84×BPF) related to the aerodynamic instabilities are observed at the inception of stall.

Formation of local separation zones on the wings with a rigid and varioform wavy surface

In the present paper, a method for choosing the parameters of a wing-surface waviness intended for use on small aircraft is introduced. We propose choosing the waviness geometry based on the parameters of the boundary layer for the given regime of the flow around the classical wing installed at zero angle of attack, α = 0°. We propose to check the result of the choice of the waviness parameters by the change of the flow structure in the boundary layer of the wing. For the properly chosen waviness, the continuous separation zone characteristic of the classical wing should transform into local separation zones. To substantiate and verify the proposed method, we investigate the flow around two wing models, one model being a wavy wing with a rigid surface and the other, a wavy wing with a varioform surface characterized by variable hump height. The study was carried out at wing-chord-based Reynolds numbers Re ranging from 0.35·105 to 1.4105 at α = 0°. Using oil-film visualization, a transition of local separation zones into a continuous separation zone on the rigid-surface wing model with a decrease of Reynolds number implemented by decreasing the flow velocity was revealed. By measuring the velocity profile in the boundary layer on the varioform wing, the transformation of the continuous separation zone into local separation zones yet at a constant velocity with a change in waviness height has been also demonstrated. It is experimentally shown that the formation of local separation zones is observed provided that the waviness height f is one order of magnitude greater than the displacement thickness σl in the boundary-layer pre-separation region, and the waviness coefficient Kw is greater than unity. A distinct formation of local separation zones is revealed by oil-film visualization at Kw = 2.8.

Subsonic slip waves along the interface between two piezoelectric solids in sliding contact with local separation

The Stroh formalism of piezoelectric crystals and singular integral equation technique are applied to study the propagation of possible slip waves in presence of local separation at the interface between two frictionless contact piezoelectric solids, which are pressed together by uniaxial pressure and laid in the electric field. The problem is cast into a set of singular integral equations of which the closed solutions are obtained. Discussion on the existence of such slip waves is presented. The results show that such slip waves, which have square-root singularities at both ends of the local separation zones, can propagate in some special material combinations. And the existence of such slip waves is related with the applied mechanical and electric fields.

Propagation of slip pulse along frictionless contact interface with local separation between two piezoelectric solids

The Stroh formalism of piezoelectric materials, Fourier analysis and singular integral equation technique were used to investigate the existence of a pulse at the frictionless interface in presence of local separation between two contact piezoelectric solids. The two solids were combined together by uniaxial tractions and laid in the electric field. The problem was cast into a set of Cauchy singular integral equations, from which the closed-form solutions were derived. The numerical discussion on the existence of such a slip pulse was presented. The results show that such a slip pulse, which has square root singularities at both ends of the local separation zone, can propagate in most material combinations. And the existence of such a slip pulse will not be affected by the applied mechanical and electric fields in some special material combinations.

Formation of supersonic zones and local separation zones in transonic steady-state flow past surface roughness in the free interaction regime

Within the framework of free interaction theory numerical methods are used to investigate the occurrence of supersonic zones with shocks in the outer inviscid region for flow past roughness in the lower viscous sublayer, with and without the formation of local separation zones.

Supersonic Flow Past a Circular Cylinder with an Isothermal Surface

Supersonic perfect-gas flow past a circular cylinder with an isothermal surface is investigated at the Mach number 5 and Reynolds numbers ranging from 30 to 500,000. It is shown that two branches of the numerical solution of the problem can exist. On the first branch the following flow patterns are successively realized as Re is increased: separationless flow, flow with formation of a local separation zone, and flow with formation of a global separation zone. On the second branch the flow pattern with a local separation zone is observed at all Reynolds numbers; at a certain value of Re this solution jumps to the first branch.

A Robust High-Resolution Split-Type Compact FD Scheme for Spatial Direct Numerical Simulation of Boundary-Layer Transition

In this paper an efficient split-type Finite-Difference (FD) scheme with high modal resolu- tion - most important for the streamwise convection terms that cause wave transport and interaction - is derived for a mixed Fourier-spectral/FD method that is designed for the spatial direct numerical simulation (DNS) of boundary-layer transition and turbulence. Using a relatively simple but thor- ough and instructive modal analysis we discuss some principal trouble sources of the related FD discretization. The new scheme is based on a 6th-order compact FD discretization in streamwise and wall-normal direction and the classical 4th-order Runge-Kutta time-integration scheme with sym- metrical final corrector step. Exemplary results of a fundamental-(K-) type breakdown simulation of a strongly decelerated Falkner-Skan boundary layer (Hartree parameterH D 0.18) using 70 mega grid points in space are presented up to the early turbulent regime (Re;turb 820). The adverse pressure gradient gives rise to local separation zones during the breakdown stage and intensifies final breakdown by strong amplification of (background) disturbances thus enabling rapid transition at moderate Reynolds number. The appearance and dynamics of small-scale vortical structures in early turbulence basically similar to the large-scale structures at transition can be observed corroborating Kachanov's hypothesis on the importance of the K-regime of breakdown for coherent structures in turbulence.

Interaction and separation in internal flows with low skin friction

High-Reynolds-number plane steady flow of a viscous incompressible fluid past small distortions on straight-line walls intersecting at a point source is considered. The flow regimes are investigated in detail for a near-critical diffuser opening half-angle, when the skin friction is small. Solutions with local separation zones are obtained. The dependence of the skin friction on the Reynolds number is determined for transition to streams with a large region of reversed flow.

Three-dimensional separation in the neighborhood of roughness on the surface of an axisymmetric body

Steady-state viscous incompressible fluid flow past an axisymmetric slender body is considered at high Reynolds numbers in the regime with vanishing surface friction in a certain cross-section. In a small neighborhood of this cross-section interaction between the boundary layer flow and the external irrotational stream develops. In order to study the structure of the three-dimensional flow with local separation zones it is assumed that there is three-dimensional roughness on the surface of the body with the scale of the interaction zone. For this zone a numerical solution of the problem is obtained and its nonuniqueness is established. The surface friction line (limiting streamline) patterns with their inherent features are constructed.

Numerical analysis of three-dimensional flow past a high-speed electric train with turbulent boundary layer separation

An approach to the numerical study of three-dimensional flow past a high-speed electric train is considered, including the case of turbulent boundary layer separation. The method of viscous-inviscid interaction is used to compute the aerodynamic characteristics. The results of calculating the 3D flow past two configurations of a high-speed electric train, taking into account the close proximity of the ground surface, are presented for a train speedV∞=300 km/h and the per meter Reynolds number Re=5.6·106 m−1. One of these configurations is shown to have the advantage of separationless flow past a front locomotive and less intense diffuser separations on the afterbody of a rear locomotive. A local separation zone on the sides of the front locomotive's nose was detected in one of the cases considered.

Measurement of Rotating Stall in a Centrifugal Blower by Semiconductor Laser 2-Focus Velocimeter. (2nd Report, Rotating Stall Analysis Based on Flow-Fluctuation Ellipse).

Fundamental knowledge for detecting and predicting the inception of rotating stall was investigated. The measured blade-to-blade variation of velocity fluctuation was analyzed on the conception of the flow-fluctuation ellipse which indicates the fluctuation ranges of velocity vector and correlation coefficient based on the covariance between the tangential and radial components of velocity fluctuation. It was cleanly indicated that, (1) the inception of rotating stall was closely related to the three-dimensional separation of boundary layer on the diffuser wall, and (2) the rotating stall inception could be predicted experimentally by detecting the variation of correlation coefficient at the impeller exit. That is, it is presumed that the rotating stall will occur if the value of correlation coefficient becomes close to zero and then larger than zero, on if the local separation zone originated in the vaneless diffuser reaches the impeller exit.

Control of convective thermal exchange in a laval nozzle during turbulent flow with a local separation zone

Control of convective thermal exchange in a laval nozzle during turbulent flow with a local separation zone