Analysis of Laminar Boundary-Layer Separation in Retarded Flow over Bodies of Revolution

Laminar boundary-layer separation phenomenon is one of the interesting and important aspects of boundary-layer flows. It occurs in various physical situations because of decreasing wall shear stress. Retarded flow velocities are one of the reasons to happen this event. Flow separation can be prevented or delayed by utilizing bodies of revolution as surface transverse curvature produces the effects of the nature of favorable pressure-gradient which in turn increases wall shear stress that keeps the flow attached to the surface. Bodies of revolution whose body contour follows power-law form also play a vital role to delay flow separation. Bodies of revolution of varying cross-sections and involving surface transverse curvature (TVC) are utilized to examine their effects on flow separation. Particularly, a convex transverse curvature has been considered due to its effects of the nature of favorable pressure-gradient which causes to delay the flow separation. A retarded flow velocity of Görtler’s type is considered in this study to investigate flow separation process. A detailed analysis is provided to understand the flow separation by calculating separation points under various assumptions. It has been observed that the body contours exponent n and the convex transverse curvature parameter k play an assistive role in the delaying of boundary-layer separation even under the influence of strong retardation. Results are presented through various Tables and graphs in order to highlight the role of the power-law exponent of external velocity m, the convex transverse curvature parameter k, and the body contours exponent n on separation points.

Micro-Vortex Generators on Transonic Convex-Corner Flow

A convex corner models the upper surface of a deflected flap and shock-induced boundary layer separation occurs at transonic speeds. This study uses micro-vortex generators (MVGs) for flow control. An array of MVGs (counter-rotating vane type, ramp type and co-rotating vane type) with a height of 20% of the thickness of the incoming boundary layer is installed upstream of a convex corner. The surface pressure distributions are similar regardless of the presence of MVGs. They show mild upstream expansion, a strong favorable pressure gradient near the corner’s apex and downstream compression. A corrugated surface oil flow pattern is observed in the presence of MVGs and there is an onset of compression moving downstream. The counter-rotating vane type MVGs produce a greater reduction in peak pressure fluctuations and the ramp type decreases the separation length. The presence of MVGs stabilizes the shock and shock oscillation is damped.

Wake Structures and Effect of Hydrofoil Shapes in Efficient Flapping Propulsion

In this work, direct numerical simulation (DNS) is used to investigate how airfoil shape affects wake structure and performance during a pitching-heaving motion. First, a class-shape transformation (CST) method is used to generate airfoil shapes. CST coefficients are then varied in a parametric study to create geometries that are simulated in a pitching and heaving motion via an immersed boundary method-based numerical solver. The results show that most coefficients have little effect on the propulsive efficiency, but the second coefficient does have a very large effect. Looking at the CST basis functions shows that the effect of this coefficient is concentrated near the 25% mark of the foils chord length. By observing the thrust force and hydrodynamic power through a period of motion it is shown that the effect of the foil shape change is realized near the middle of each flapping motion. Through further inspection of the wake structures, we conclude that this is due to the leading-edge vortex attaching better to the foil shapes with a larger thickness around 25% of the chord length. This is verified by the pressure contours, which show a lower pressure along the leading edge of the better performing foils. The more favorable pressure gradient generated allows for higher efficiency motion.

Passive Control of Base Pressure: A Review

In the present world, passive control finds application in various areas like flow over blunt projectiles, missiles, supersonic parallel diffusers (for cruise correction), the engine of jets, static testbeds of rockets, the ports of internal combustion engines, vernier rockets, and single expansion ramp nozzle (SERN) rockets. In this review, various passive control techniques to control the base pressure and regulate the drag force are discussed. In the study, papers ranging from subsonic, sonic, and supersonic flow are discussed. Different types of passive control management techniques like cavity, ribs, dimple, static cylinder, spikes, etc., are discussed in this review article. This study found that the passive control device can control the base pressure, resulting in an enhancement in the base pressure and reducing the base drag. Also, passive control is very efficient whenever there is a favorable pressure gradient at the nozzle exit.

Control of Supersonic Elliptic Jet with Ventilated Tabs

Control of Mach 1.5 elliptic jet with ventilated triangular tabs is studied experimentally, in the presence of different levels of pressure gradient at the nozzle exit. Three different sets of ventilated tabs with circular, triangular and trapezoidal ventilations were studied. Two tabs were placed, at the ends of major and minor axes, at the exit of the elliptic nozzle of aspect ratio 3.37. The mixing enhancement caused by these tabs was studied in the presence of adverse and favorable pressure gradients, corresponding to nozzle pressure ratio (NPR) from 3 to 8. For Mach 1.5 jet NPR 3 corresponds to 18 % adverse pressure gradient and NPR 8 corresponds to 118 % favorable pressure gradient. The results of ventilated tabs are compared with unventilated truncated triangular tabs of identical geometry. The difference between the mixing promoting efficiency of the unventilated and ventilated tabs is only marginal (around 5–6 %). All tabs cause jet bifurcation and weaken the waves in the jet core. The tab with trapezoidal ventilation, at NPR 3, promotes mixing to an extent of reducing the core to about 92 %. At higher NPRs the mixing caused by unventilated tab is slightly better than the ventilated tabs.