The behaviour of a turbulent boundary layer on a flat surface as it encounters the nose of a cylindrical wing mounted normal to that surface is being investigated. A three-component laser anemometer has been developed to measure this highly turbulent three-dimensional flow. Measurements of all the non-zero mean-velocity and Reynolds-stress components have been made with this instrument in the plane of symmetry upstream of the wing. These data have been used to estimate some of the component terms of the turbulence kinetic energy equation. Histograms of velocity fluctuations and short-time cross-correlations between the laser anemometer and a hot-wire probe have also been measured in the plane of symmetry. In all, these results reveal much of the time-dependent and time-averaged turbulence structure of the flow here.Separation occurs in the plane of symmetry because of the adverse pressure gradient imposed by the wing. In the time mean the resulting separated flow consists of two fairly distinct regions: a thin upstream region characterized by low mean backflow velocities and a relatively thick downstream region dominated by the intense recirculation of the mean junction vortex. In the upstream region the turbulence stresses develop in a manner qualitatively similar to those of a two-dimensional boundary layer separating in an adverse pressure gradient. In the vicinity of the junction vortex, though, the turbulence stresses are much greater and reach’ values many times larger than those normally observed in turbulent flows. These large stresses are associated with bimodal (double-peaked) histograms of velocity fluctuations produced by a velocity variation that is bistable. These observations are consistent with large-scale low-frequency unsteadiness of the instantaneous flow structure associated with the junction vortex. This unsteadiness seems to be produced by fluctuations in the momentum and vorticity of fluid from the outer part of the boundary layer which is recirculated as it impinges on the leading edge of the wing. Though we would expect these fluctuations to be produced by coherent structures in the boundary layer, frequencies of the large-scale unsteadiness are substantially lower than the passage frequency of such structures. It therefore seems that only a fraction of the turbulent structures are recirculated in this way.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112090001215

Time-dependent and time-averaged turbulence structure near the nose of a wing-body junction

Aspects of turbulent boundary-layer separation

With the desire for increased power output for a gas turbine engine comes the continual push to achieve higher turbine inlet temperatures. Higher temperatures result in large thermal and mechanical stresses particularly along the nozzle guide vane. One critical region along a vane is the leading edge-endwall juncture. Based on the assumption that the approaching flow to this juncture is similar to a two-dimensional boundary layer, previous studies have shown that a horseshoe vortex forms. This vortex forms because of a radial total pressure gradient from the approaching boundary layer.This paper documents the computational design and experimental validation of a fillet placed at the leading edge-endwall juncture of a guide vane to eliminate the horseshoe vortex. The fillet design effectively accelerated the incoming boundary layer thereby mitigating the effect of the total pressure gradient. To verify the CFD studies used to design the leading edge fillet, flow field measurements were performed in a large-scale, linear, vane cascade. The flow field measurements were performed with a laser Doppler velocimeter in four planes orientated orthogonal to the vane. Good agreement between the CFD predictions and the experimental measurements verified the effectiveness of the leading edge fillet at eliminating the horseshoe vortex. The flowfield results showed that the turbulent kinetic energy levels were significantly reduced in the endwall region because of the absence of the unsteady horseshoe vortex.Copyright © 2001 by ASME

/pdf/computational-design-and-experimental-evaluation-of-using-a-2oi9ftuyis.pdf

Computational Design and Experimental Evaluation of Using a Leading Edge Fillet on a Gas Turbine Vane

Junction flows may suffer from secondary flows such as horseshoe vortices and corner separations that can dramatically impair the performances of aircrafts. The present article brings into focus the unsteady aspects of the flow at the intersection of a wing and a flat plate. The simplified junction flow test case is designed according to a literature review to favor the onset of a corner separation. The salient statistical and fluctuating properties of the flow are scrutinized using large eddy simulation and wind tunnel tests, which are carried out at a Reynolds number based on the wing chord c and the free stream velocity U∞ of Rec=2.8×105. As the incoming boundary layer at Reθ=2100 (θ being the boundary layer momentum thickness one-half chord upstream the junction) experiences the adverse pressure gradient created by the wing, a three dimensional separation occurs at the nose of the junction leading to the formation of a horseshoe vortex. The low frequency, large scale bimodal behavior of the horseshoe vortex at the nose of the junction is characterized by multiple frequencies within f.δ/U∞=[0.05−0.1] (where δ is the boundary layer thickness one-half chord upstream the wing). Downstream of the bimodal region, the meandering of the core of the horseshoe vortex legs in the crossflow planes is scrutinized. It is found that the horseshoe vortex oscillates around a mean location over an area covering almost 10% of the wing chord in the tranverse plane at the trailing edge at normalized frequencies around f.δ/U∞=0.2–0.3. This so-called meandering is found to be part of a global dynamics of the horseshoe vortex initiated by the bimodal behavior. Within the corner, no separation is observed and it is shown that a high level of anisotropy (according to Lumley’s formalism) is reached at the intersection of the wing and the flat plate, which makes the investigated test case challenging for numerical methods. The conditions of apparition of a corner separation are eventually discussed and we assume that the vicinity of the horseshoe vortex suction side leg might prevent the corner separation. It is also anticipated that higher Reynolds number junction flows are more likely to suffer from such separations.

Flow dynamics past a simplified wing body junction

A flow directing device of a gas turbine engine, comprising: an airfoil having a leading edge, trailing edge, suction side and pressure side; a wall abutting the airfoil; and a fillet between the airfoil and wall. The fillet has an enlarged section at the leading edge, along the suction and pressure sides, and towards the trailing edge. The device could be part of a vane segment. In addition to eliminating a horseshoe vortex, the device also reduces heat load on the airfoil by directing the cooler gas from the proximal end of the airfoil to the hotter gas at the medial section of the airfoil.

Flow directing device

Measurements are presented to demonstrate the effects of a single fillet on the flow of a turbulent boundary layer past an idealized wing-body junction. The time-averaged flow structure in the vicinity of the wing and in the wake of the wing-body junction is considered, as well as the unsteadiness of the horseshoe vortex. The effects of angle of attack and approach boundary-layer thickness are also examined.

Effects of a Fillet on the Flow Past a Wing-Body Junction

Effects of a fillet on the flow past a wing body junction

Abstract Three-dimensionalities in the wake of flow past a circular cylinder executing sinusoidal rotary oscillations about its axis is studied experimentally. The results of water tunnel experiments on a rotationally oscillating cylinder for Reynolds number of 250 with varying amplitude and forcing frequency are discussed. Qualitative studies using hydrogen bubble and laser-induced fluorescence flow visualisation techniques are performed. Observation made for oscillating amplitude, $\theta _{0} = {\rm \pi}/4$ and $\theta _{0}=3{\rm \pi} /4$, and a low normalised forcing frequency, $FR$, of 0.75 and 0.5, respectively, confirmed a mode having a spanwise non-dimensional wavelength of $\sim$1.8 which is also observed for a rotating cylinder. On increasing forcing frequency this mode disappears and a new mode with a bean-shaped structure and a much smaller spanwise normalised wavelength of $\sim$0.8 appears at an $FR$ of 1 and an oscillation amplitude of ${\rm \pi} /2$. This mode remains almost stable until a forcing frequency of $FR=1.4$. At higher forcing frequency, $FR=2.75$, and oscillation amplitude of $3{\rm \pi} /4$, a mode with cellular structure and a normalised spanwise wavelength of $\sim$1.6 is identified. The cells in this mode flatten up with increasing downstream distance and are shed alternately with respect to the adjacent cell. Certain combinations of forcing parameters resulted in a forced two-dimensionality of the wake. Quantitative studies using hot-wire measurements and particle image velocimetry confirm the presence of these modes and wake characteristics. Wake mode map in the forcing frequency and amplitude plane is presented showing regions of newly discovered modes and wake lock-on boundaries.

pdf/experimental-investigation-of-three-dimensional-modes-in-the-15gzqahf.pdf

Experimental investigation of three-dimensional modes in the wake of a rotationally oscillating cylinder

The transition of flow from laminar to turbulent has been a topic of considerable interest for fluid dynamicists. Evolution of spanwise three-dimensionalities in the wake of an elongated bluff body occurs before the wake becomes turbulent. Several engineering structures such as industrial chimneys, marine risers, and off-shore structures have tapered geometry. In this study, we show the three-dimensional (spanwise) modes in the wake of a rotationally oscillating tapered cylinder. 

Experimental investigation of flow past a rotationally oscillating tapered cylinder

This paper proposes Fault Detection Isolation and Recovery (FDIR) techniques for in-built hard IPs of Phase Locked Loop (PLL) or Digital Clock Manager (DCM) of Field Programmable Gate Array (FPGA). PLLs inside the FPGAs are susceptible to Single Event Effects (SEE) even for space-qualified FPGAs. Under the radiation environment, loss of lock, perturbation in the clocks or no clocks are the common phenomena observed. Thus, we developed an Intellectual Property (IP) core based on FDIR, which adds redundancy to the PLL output and improves the Mean Time to Failures (MTTF). The generic IP core is technology independent and can be configured in dual or triple use of PLL mode. All the PLLs are kept as hot redundant mode, and their health is continuously monitored. SEE-affected PLL is immediately isolated, and the output is switched to a healthy PLL (if required) with or without a few cycle gaps. Recovery of faulty PLL is carried out in parallel. Telemetry signals notify every PLL switching and reset. For triple redundant PLL, minimum switching is ensured by logic. Redundancy bypass and selection of PLL make it work in case of complete failure of one or two PLL. 

/pdf/radiation-tolerant-pll-for-onboard-fpgas-1oq8do8i.pdf

Radiation Tolerant PLL for Onboard FPGAs

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