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Flow separation
About: Flow separation is a research topic. Over the lifetime, 16708 publications have been published within this topic receiving 386926 citations.
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TL;DR: In this paper, the steady laminar flow is studied by means of a three-dimensional extension of triple-deck theory, so that there is the prospect of separation in the nonlinear motion.
Abstract: A shallow three-dimensional hump disturbs the two-dimensional incompressible boundary layer developed on an otherwise flat surface. The steady laminar flow is studied by means of a three-dimensional extension of triple-deck theory, so that there is the prospect of separation in the nonlinear motion. As a first step, however, a linearized analysis valid for certain shallow obstacles gives some insight into the flow properties. The most striking features then are the reversal of the secondary vortex motions and the emergence of a ‘corridor’ in the wake of the hump. The corridor stays of constant width downstream and within it the boundary-layer displacement and skin-friction perturbation are much greater than outside. Extending outside the corridor, there is a zone where the surface fluid is accelerated, in contrast with the deceleration near the centre of the corridor. The downstream decay (e.g. of displacement) here is much slower than in two-dimensional flows.
95 citations
01 Jan 1958
TL;DR: In this article, the authors considered the problem of estimating the energy balance of the airfoil boundary layer in the case of a rotating stall in an axial flow compressor and stalling flutter of an air-foil and showed that any essential unsteadiness in the boundary layer tends to produce a hysteresis in the relation between lift and angle of attack, thus affecting the flow oscillation.
Abstract: When a boundary layer is subjected to unsteady conditions, it strongly tends, if it is thin, to be quasi-steady; that is, to be described at each instant by the appropriate steady equations of motion. In many cases of practical interest a thin boundary layer is nearly quasi-steady, but, owing to some imposed unsteadiness, its departures from quasi-steadiness require estimation. Such is the case during rotating stall in an axial flow compressor and stalling flutter of an airfoil. In these problems, any essential unsteadiness in the boundary layer tends to produce a hysteresis in the relation between lift and angle of attack, thus affecting the energy balance of the flow oscillation. In this connection, of course, the unsteady features of separation must be accounted for, at least approximately, in any attempt to predict consequences of unsteadiness of the airfoil boundary layer. It is the particular problem of separation to which the present paper is addressed.
95 citations
TL;DR: In this paper, the authors used random-textured hydrophobic surfaces (fabricated using large-length scalable thermal spray processes) on a flat plate geometry to demonstrate skin-friction reduction in the turbulent regime.
Abstract: Technologies for reducing hydrodynamic skin-friction drag have a huge potential for energy-savings in applications ranging from propulsion of marine vessels to transporting liquids through pipes. The majority of previous experimental studies using hydrophobic surfaces have successfully shown skin-friction drag reduction in the laminar and transitional flow regimes (typically Reynolds numbers less than ≃106 for external flows). However, this hydrophobicity induced drag reduction is known to diminish with increasing Reynolds numbers in experiments involving wall bounded turbulent flows. Using random-textured hydrophobic surfaces (fabricated using large-length scalable thermal spray processes) on a flat plate geometry, we present water-tunnel test data with Reynolds numbers ranging from 106 to 9 × 106 that show sustained skin-friction drag reduction of 20%–30% in such turbulent flow regimes. Furthermore, we provide evidence that apart from the formation of a Cassie state and hydrophobicity, we also need a low surface roughness and an enhanced ability of the textured surface to retain trapped air, for sustained drag reduction in turbulent flow regimes. Specifically, for the hydrophobic test surfaces of the present and previous studies, we show that drag reduction seen at lower Reynolds numbers diminishes with increasing Reynolds number when the surface roughness of the underlying texture becomes comparable to the viscous sublayer thickness. Conversely, test data show that textures with surface roughness significantly smaller than the viscous sublayer thickness and textures with high porosity show sustained drag reduction in the turbulent flow regime. The present experiments represent a significant technological advancement and one of the very few demonstrations of skin-friction reduction in the turbulent regime using random-textured hydrophobic surfaces in an external flow configuration. The scalability of the fabrication method, the passive nature of this surface technology, and the obtained results in the turbulent regime make such hydrophobic surfaces a potentially attractive option for hydrodynamic skin-friction drag reduction.
95 citations
TL;DR: Asymptotic and numerical solutions of the unsteady boundary-layer equations are obtained for a main stream velocity given by equation (1.1) in this article, which is attributed to exponentially small oscillatory eigenfunctions, which account for different initial conditions upstream.
Abstract: Asymptotic and numerical solutions of the unsteady boundary-layer equations are obtained for a main stream velocity given by equation (1.1). Far downstream the flow develops into a double boundary layer. The inside layer is a Stokes shear-wave motion, which oscillates with zero mean flow, while the outer layer is a modified Blasius motion, which convects the mean flow downstream. The numerical results indicate that most flow quantities approach their asymptotic values far downstream through damped oscillations. This behaviour is attributed to exponentially small oscillatory eigenfunctions, which account for different initial conditions upstream.
95 citations
TL;DR: In this article, a single dielectric barrier discharge plasma actuator for controlling turbulent boundary-layer separation from the deflected flap of a high-lift airfoil is investigated between Reynolds numbers of 240,000 (15 m/s) and 750,000(45 m/S).
Abstract: The efficacy of a single dielectric barrier discharge plasma actuator for controlling turbulent boundary-layer separation from the deflected flap of a high-lift airfoil is investigated between Reynolds numbers of 240,000 (15 m/s) and 750,000 (45 m/s). Momentum coefficients for the dielectric barrier discharge plasma actuator are approximately an order of magnitude lower than those usually employed for such studies, yet control authority is still realized through amplification of natural vortex shedding from the flap shoulder, which promotes momentum transfer between the freestream and separated region. This increases dynamic loading on the flap and further organizes turbulent fluctuations in the wake. The measured lift enhancement is primarily due to upstream effects from increased circulation around the entire model, rather than full reattachment to the deflected flap surface. Lift enhancement via instability amplification is found to be relatively insensitive to changes in angle of attack, provided that the separation location and underlying dynamics do not change. The modulation waveform used to excite low-frequency perturbations with a high-frequency plasma-carrier signal has a considerable effect on the actuator performance. Control authority decreases with increasing Reynolds number and flap deflection, highlighting the necessity for further improvement of plasma actuators for use in realistic takeoff and landing transport aircraft applications. These findings are compared to studies on a similar high-lift platform using piezoelectric-driven zero-net-mass flux actuation.
95 citations