The Aeroelastic Instability of an Elevator Balance Horn in a Shear Layer Wake Flow

1986 ◽  
Author(s):  
Ralph Tate ◽  
Ronald O. Stearman
Keyword(s):  
Shear Layer ◽  
Wake Flow ◽  
Aeroelastic Instability

Interaction Between a Subsonic Air Jet and a Sharp Edge

1984 ◽  
Vol 8 (3) ◽  
pp. 126-132
Author(s):  
N.W.M. Ko
Keyword(s):  
Experimental Investigation ◽  
Mach Number ◽  
Shear Layer ◽  
Coherent Structure ◽  
Wake Flow ◽  
Sharp Edge ◽  
Spectral Measurements ◽  
Acoustic Feedback ◽  
Air Jet ◽  
Set Up

This paper describes an experimental investigation of a jet of Mach number 0.5 which is partially interrupted by an 180° sharp edge. Detailed Schlieren and pressure spectral measurements of the jet with the sharp edge located at different locations inside the jet have indicated the presence of the basic jet coherent structure, the axisymmetrical and azimuthal constituents and the resonances set up by the interaction of the jet flow and sharp edge. The resonances arc due not only to the interaction of the initial shear layer with the acoustic feedback from the basic coherent structure but also with the acoustic feedback from the wake vortices set up in the wake flow behind the sharp edge. For the former, dependence of the level of resonance on location of the sharp edge has also been found.

Some features of steady separated flow from low speed to hypersonic

2008 ◽  
Vol 112 (1128) ◽  
pp. 109-113
Author(s):  
S. L. Gai
Keyword(s):  
Shear Layer ◽  
Bluff Body ◽  
Separated Flow ◽  
Base Flow ◽  
The Body ◽  
Wake Flow ◽  
Recirculating Flow ◽  
Flow Features ◽  
Hypersonic Speeds ◽  
Low Speeds

Steady non-vortex shedding base flow behind a bluff body is considered. Such a flow is characterised by the flow separation at the trailing edge of the body with an emerging shear layer which reattaches on the axis with strong recompression and recirculating flow bounded by the base, the shear layer, and the axis. Steady wake flows behind a bluff body at low speeds have been studied for more than a century (for example, Kirchhoff; Riabouchinsky). Recently, research on steady bluff body wake flow at low speeds has been reviewed and reinterpreted by Roshko. Roshko has also commented on some basic aspects of steady supersonic base flow following on from Chapman and Korst analyses. In the present paper, we examine the steady base flow features both at low speeds and supersonic speeds in the light of Roshko’s model and expand on some further aspects of base flows at supersonic and hypersonic speeds, not covered by Roshko.

scholarly journals Decomposition of wake dynamics in fluid–structure interaction via low-dimensional models

2019 ◽  
Vol 867 ◽  
pp. 723-764 ◽  
Author(s):  
T. P. Miyanawala ◽  
R. K. Jaiman
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
High Amplitude ◽  
Wake Flow ◽  
High Dimensional ◽  
Near Wake ◽  
Structure Interaction ◽  
Moderate Reynolds Number ◽  
Low Dimensional ◽  
Lock In

We present a dynamic decomposition analysis of the wake flow in fluid–structure interaction (FSI) systems under both laminar and turbulent flow conditions. Of particular interest is to provide the significance of low-dimensional wake flow features and their interaction dynamics to sustain the free vibration of a square cylinder at a relatively low mass ratio. To obtain the high-dimensional data, we employ a body-conforming variational FSI solver based on the recently developed partitioned iterative scheme and the dynamic subgrid-scale turbulence model for a moderate Reynolds number ($Re$). The snapshot data from high-dimensional FSI simulations are projected to a low-dimensional subspace using the proper orthogonal decomposition (POD). We utilize each corresponding POD mode to detect features of the organized motions, namely, the vortex street, the shear layer and the near-wake bubble. We find that the vortex shedding modes contribute solely to the lift force, while the near-wake and shear layer modes play a dominant role in the drag force. We further examine the fundamental mechanism of this dynamical behaviour and propose a force decomposition technique via low-dimensional approximation. To elucidate the frequency lock-in, we systematically analyse the decomposed modes and their dynamical contributions to the force fluctuations for a range of reduced velocity at low Reynolds number laminar flow. These quantitative mode energy contributions demonstrate that the shear layer feeds the vorticity flux to the wake vortices and the near-wake bubble during the wake–body synchronization. Based on the decomposition of wake dynamics, we suggest an interaction cycle for the frequency lock-in during the wake–body interaction, which provides the interrelationship between the high-amplitude motion and the dominating wake features. Through our investigation of wake–body synchronization below critical $Re$ range, we discover that the bluff body can undergo a synchronized high-amplitude vibration due to flexibility-induced unsteadiness. Owing to the wake turbulence at a moderate Reynolds number of $Re=22\,000$, a distorted set of POD modes and the broadband energy distribution are observed, while the interaction cycle for the wake synchronization is found to be valid for the turbulent wake flow.

Theoretical and Numerical Study of Vortex-Wake Flow Generated From Stack of Rectangular Bodies

2007 ◽  
Author(s):  
Khaled Alhussan
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Vortex Shedding ◽  
Numerical Study ◽  
The Body ◽  
Fluid Particle ◽  
Wake Flow ◽  
Free Shear Layer ◽  
Complex Flow ◽  
Discrete Vortex

Flow over external bodies has been studied extensively because of their many practical applications. For example, flow past a rectangular bodies, usually experiences strong flow oscillations and boundary layer separation in the wake region behind the body. As a fluid particle flows toward the leading edge of a rectangular body, the pressure of the fluid particle increases from the free stream pressure to the stagnation pressure. The boundary layer separates from the surface forms a free shear layer and is highly unstable. This shear layer will eventually roll into a discrete vortex and detach from the surface. A periodic flow motion will develop in the wake as a result of boundary layer vortices being shed alternatively from either side of the rectangular shapes. The periodic nature of the vortex shedding phenomenon can sometimes lead to unwanted structural vibrations, especially when the shedding frequency matches one of the resonant frequencies of the structure. The work to be presented herein is a theoretical and numerical analysis of the complex fluid mechanism that occurs over stack of rectangular bodies for different number of rectangular bodies, specifically with regard to the vortex shedding and generation of wake. A number of important conclusions follow from the current research. First, study of the actual flow configuration over rectangular bodies offers some insight into the complex flow phenomena. Second, the characteristics of the vortex and wakes change considerably with the number of bodies.

Quantification of wake unsteadiness for low-Re flow across two staggered cylinders

2019 ◽  
Vol 233 (19-20) ◽  
pp. 6892-6909 ◽  
Author(s):  
Wei Zhang ◽  
Xiaojun Li ◽  
Zuchao Zhu
Keyword(s):  
Sensitivity Analysis ◽  
Shear Layer ◽  
Unstable Mode ◽  
Flow Patterns ◽  
Temporal Variations ◽  
Wake Flow ◽  
Circular Cylinders ◽  
Flow Unsteadiness ◽  
Far Wake ◽  
Perturbation Growth

This work performs a numerical investigation on the two-dimensional flow across two circular cylinders in staggered arrangement at Re = 100. The seaparting distances between the centers of the cylinders are D/ d = 4–10 with Δ D/ d = 2 and T/ d = 0.0–2.0 with Δ T/ d = 0.5 in the streamwise and transverse directions, respectively, in which d is the cylinder diameter. Although the low- Re flow across staggered cylinders has been studied in a number of works, the authors mainly concerned about the identification and transition of various flow patterns. In this work, our objective is to quantitatively reveal the characteristics of flow unsteadiness as affected by the two separating distances. The flow unsteadiness is assessed from several aspects, including the spatial distributions and temporal variations of instantaneous flow patterns, fluctuating characteristic quantities, and fluctuating flow in the gap and in the near- and far-wake regions. To investigate the inherent instability of the flow, the global linear stability and sensitivity analysis is further carried out to demonstrate the unstable mode of perturbation growth and the critical flow patterns that destabilize the flow. The numerical results reveal that the wake flow between the two centerlines and beside the upstream cylinder is the most intensely perturbed. The flow around the downstream cylinder exhibits great fluctuation as perturbed by the destabilized shear layer of the upstream cylinder. The flow downstream of both cylinders shows multiple peak fluctuation of velocity because of the complex interactions between the destabilized shear layer and the wake vortices, resulting in the bidirectional transverse propagation of fluctuation. The stability analysis demonstrates that the unstable mode of perturbation growth is more significant in the far-wake region as the two cylinders are placed in proximity; the sensitivity analysis shows that the gap flow is crucial for the flow destabilization at small D, while the wake flow of cylinder- B is more significant for large D.

Flow and Dispersion Characteristics of a Stack-Issued Backward Inclined Jet in Crossflow

2016 ◽  
Vol 33 (6) ◽  
pp. 841-852 ◽  
Author(s):  
M. G. Khouygani ◽  
R.-F. Huang ◽  
C.-M. Hsu
Keyword(s):  
Shear Layer ◽  
Inclination Angle ◽  
Flow Characteristics ◽  
Flux Ratio ◽  
Wake Flow ◽  
Flow Structures ◽  
Wake Vortex ◽  
Near Wake ◽  
Jet In Crossflow ◽  
Inclination Angles

AbstractThe effects of backward inclination angle on flow characteristics and jet dispersion properties of a stack-issued jet in crossflow were studied by means of instantaneous and long-exposure photography, particle image velocimetry (PIV), and tracer-gas concentration detections at a Reynolds number of 2,400, a jet-to-crossflow momentum flux ratio of 1.0, and the backward inclination angles θ = 0° - 60°. Three characteristic flow patterns featured by different near-wake flow structures were found within the surveyed span of the backward inclination angle: low (θ ≤ 25°), mediate (25° < θ < 50°), and high (θ ≥ 50°). In the range of low backward inclination angle, mushroom vortices appeared in the upwind shear layer. Jet fluids were entrained into the jet- and tube-wakes because the near wake region was characterized by a jet-wake vortex and a downwash flow. In the range of mediate backward inclination angle, forward-rolling vortices were formed in the upwind shear layer. Jet fluids were entrained into the jet wake but not appearing in the tube wake because the near wake was characterized by an isolated tube wake and up-going flows. In the range of high backward inclination angle, small-sized forward-rolling vortices were observed in the upwind shear layer. Jet fluids were not observed in both the jet- and tube-wakes because all flows went forward without reversal or vortex, which was similar to that in a jet in co-flow. Large turbulence intensities occurred around the jet-wake vortex and along sides of the tube wake bifurcation line, therefore the mixing at the low backward inclination angles presented better properties than those at mediate and high backward inclination angles owing to the featured flow structures and turbulence intensities.

A computational investigation of the instability of the detached shear layers in the wake of a circular cylinder

2010 ◽  
Vol 659 ◽  
pp. 375-404 ◽  
Author(s):  
MAN MOHAN RAI
Keyword(s):  
Shear Layer ◽  
Shear Layers ◽  
Wake Flow ◽  
Recirculation Region ◽  
Computational Investigation ◽  
Near Wake ◽  
Layer Transition ◽  
Flow Phenomena ◽  
Considerable Impact ◽  
Velocity Spectra

Cylinder wakes have been studied extensively over several decades to better understand the basic flow phenomena encountered in such flows. The physics of the very near wake of the cylinder is perhaps the most challenging of them all. This region comprises the two detached shear layers, the recirculation region and wake flow. A study of the instability of the detached shear layers is important because these shear layers have a considerable impact on the dynamics of the very near wake. It has been observed experimentally that during certain periods of time that are randomly distributed, the measured fluctuating velocity component near the shear layers shows considerable amplification and it subsequently returns to its normal level (intermittency). Here, direct numerical simulations are used to accomplish a number of objectives such as confirming the presence of intermittency (computationally) and shedding light on processes that contribute significantly to intermittency and shear-layer transition/breakdown. Velocity time traces together with corresponding instantaneous vorticity contours are used in deciphering the fundamental processes underlying intermittency and shear-layer transition. The computed velocity spectra at three locations along the shear layer are provided. The computed shear-layer frequency agrees well with a power-law fit to experimental data.

A freely yawing axisymmetric bluff body controlled by near-wake flow coupling

2019 ◽  
Vol 863 ◽  
pp. 1123-1156 ◽  
Author(s):  
Thomas J. Lambert ◽  
Bojan Vukasinovic ◽  
Ari Glezer
Keyword(s):  
Shear Layer ◽  
Time Scales ◽  
Attitude Control ◽  
Bluff Body ◽  
Cross Flow ◽  
The Body ◽  
Wake Flow ◽  
Small Scale ◽  
Near Wake ◽  
Yaw Attitude

Flow-induced oscillations of a wire-mounted, freely yawing axisymmetric round bluff body and the induced loads are regulated in wind tunnel experiments (Reynolds number $60\,000<Re_{D}<200\,000$) by altering the reciprocal coupling between the body and its near wake. This coupling is controlled by exploiting the receptivity of the azimuthal separating shear layer at the body’s aft end to controlled pulsed perturbations effected by two diametrically opposed and independently controlled aft-facing rectangular synthetic jets. The model is supported by a thin vertical wire upstream of its centre of pressure, and prescribed modification of the time-dependent flow-induced loads enables active control of its yaw attitude. The dynamics of the interactions and coupling between the actuation and the cross-flow are investigated using simultaneous, time-resolved measurements of the body’s position and phase-locked particle image velocimetry measurements in the yawing plane. It is shown that the interactions between trains of small-scale actuation vortices and the local segment of the aft-separating azimuthal shear layer lead to partial attachment, and the ensuing asymmetric modifications of the near-wake vorticity field occur within 15 actuation cycles (approximately three convective time scales), which is in agreement with measurements of the flow loads in an earlier study. Open- and closed-loop actuation can be coupled to the natural, unstable motion of the body and thereby affect desired attitude control within 100 convective time scales, as is demonstrated by suppression or enhancement of the lateral motion.

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