Multi-Time-Delay LSE-POD Complementary Approach Applied to Wake Flow Behind a Bluff Body

2007 ◽  
Author(s):  
Vibhav Durgesh ◽  
Jonathan W. Naughton
Keyword(s):  
Time Delay ◽  
Surface Pressure ◽  
Bluff Body ◽  
Wake Flow ◽  
Time Varying ◽  
Pressure Measurements ◽  
Near Wake ◽  
Time Resolved ◽  
Complementary Approach ◽  
The Time Domain

An understanding of the near wake dynamics of a bluff body is desired to better link base drag reduction observed on these bodies with the coherent structures in the wake. This investigation explores different Linear Stochastic Estimation-Proper Orthogonal Decomposition (LSE-POD) methods that can be employed to estimate the dynamics of the energy containing structure. Statistically independent two-dimensional PIV measurements and time-resolved surface pressure measurements are used to determine spatial POD modes and LSE coefficients for estimating the time-varying POD coefficients using measured surface pressures. These results are used with the time-resolved surface pressure measurements to estimate the time-varying POD coefficients that may be used for a low-order, time-resolved reconstruction of the flow field. The multi-time LSE approach formulated in the time domain (multi-time-delay LSE) is found to be successful in capturing the important near wake dynamics.

The Interaction of Porous Metal Coating With the Near Wake of Bluff Body

2012 ◽  
Author(s):  
Hanru Liu ◽  
Jinjia Wei ◽  
Zhiguo Qu
Keyword(s):  
Circular Cylinder ◽  
Bluff Body ◽  
Porous Metal ◽  
Metal Coating ◽  
Wake Flow ◽  
Navier Stokes ◽  
Aerodynamic Forces ◽  
Near Wake ◽  
Drag Forces ◽  
Flow Induced Noise

The flow around a circular cylinder with porous metal coating (PMC) is numerically investigated based on an approach of unsteady Reynolds Averaged Navier-Stokes (URANS) at subcritical Reynolds number. The model validation is carried out through comparison with some available experimental results in the literatures. It is found that the simulated results in the present work coincide well with the experimental data. The interaction of PMC with the near wake of circular cylinder such as streamline, vorticity and shear stress are studied in detail. The result reveals that PMC has capability of manipulating the wake flow so that the near wake of PMC cylinder is substantially different from that of smooth one. In addition, the fluctuations of aerodynamic forces are mitigated effectively. Varying the thickness of porous metal coating causes various velocity distributions and aerodynamic performance of bluff body. When the thickness is appropriate, the drag forces can be reduced to a certain extent. It is expected that the modification of flow characteristic and aerodynamic forces also produces the suppression of flow-induced noise generated by bluff body. These studies on wake flow and analysis of its relationship to flow-induced noise will be useful to understand the mechanism of controlling bluff body flow-induced noise by using PMC and to optimize the PMC for controlling flow and flow-induced noise.

Analysis of the Near Wake of Bluff Bodies in Ground Proximity

2002 ◽  
Author(s):  
Szabolcs R. Balkanyi ◽  
Luis P. Bernal ◽  
Bahram Khalighi
Keyword(s):  
Vector Fields ◽  
Aerodynamic Drag ◽  
Base Pressure ◽  
Wake Flow ◽  
Vehicle Body ◽  
Recirculation Region ◽  
Bluff Bodies ◽  
Pressure Measurements ◽  
Near Wake ◽  
Mean Velocity

The effect of several drag reducing devices on the near wake of a generic ground vehicle body was investigated. Drag and base pressure measurements were conducted to identify the effects of the devices on the base drag. A Particle Image Velocimetry (PIV) study was conducted to determine changes of the near wake flow field. Averages of more than 200 PIV velocity vector fields were used to compute the mean velocity and turbulent stresses at several cross section planes. The results of the drag and base pressure measurements show that significant reductions of the total aerodynamic drag (as high as 48%) can be achieved with relatively simple devices. The results also indicated that models with base cavity have lower drag than their counter parts without it. The base pressure distributions showed a strong effect of the ground, resulting in decrease of pressure towards the lower half of the base. The PIV study showed that the extent of the recirculation region is not strongly affected by the drag reducing devices. The tested devices however, were found to have a strong effect on the underbody flow. A rapid upward deflection of the underbody flow in the near wake was observed. The devices were also found to reduce the turbulent stresses in the near wake. The turbulent stresses were found to decrease in magnitude with increasing drag reduction.

Global dynamics and aerodynamic flow vectoring of wakes

1997 ◽  
Vol 338 ◽  
pp. 231-248 ◽  
Author(s):  
D. A. HAMMOND ◽  
L. G. REDEKOPP
Keyword(s):  
Bluff Body ◽  
Wake Flow ◽  
Global Dynamics ◽  
Control Input ◽  
Near Wake ◽  
Directional Control ◽  
Fixed Base ◽  
Mechanical Movement ◽  
Streaming Flow

A methodology for vectoring the near-wake flow behind a bluff body without any mechanical movement of the physical boundaries of the generating body is described. The sole control input is suction applied at the fixed base of the forebody. Once the suction volume flux exceeds a critical value needed to suppress the global dynamics associated with vortex shedding, local directional control of the near wake can be achieved. The distribution of suction velocities across the base can be varied to obtain proportional directional control. The role of symmetries in stimulating aerodynamic vectoring of a streaming flow is emphasized and illustrated.

scholarly journals Turbulent Wake-Flow Characteristics in the Near Wake of Freely Flying Raptors: A Comparative Analysis Between an Owl and a Hawk

2020 ◽  
Vol 60 (5) ◽  
pp. 1109-1122 ◽  
Author(s):  
Krishnamoorthy Krishnan ◽  
Hadar Ben-Gida ◽  
Gareth Morgan ◽  
Gregory A Kopp ◽  
Christopher G Guglielmo ◽  
...  
Keyword(s):  
High Speed ◽  
Large Scale ◽  
Flow Characteristics ◽  
Wake Flow ◽  
Flight Behavior ◽  
Aerodynamic Noise ◽  
Particle Image Velocimetry System ◽  
Near Wake ◽  
Time Resolved ◽  
Turbulent Characteristics

Synopsis Owl flight has been studied over multiple decades associated with bio-inspiration for silent flight. However, their aerodynamics has been less researched. The aerodynamic noise generated during flight depends on the turbulent state of the flow. In order to document the turbulent characteristics of the owl during flapping flight, we measured the wake flow behind a freely flying great horned owl (Bubo virginianus). For comparison purposes, we chose to fly a similar-sized raptor a Harris’s hawk (Parabuteo unicinctus): one is nocturnal and the other is a diurnal bird of prey. Here, we focus on the wake turbulent aspects and their impact on the birds’ flight performances. The birds were trained to fly inside a large-scale wind tunnel in a perch-to-perch flight mode. The near wake of the freely flying birds was characterized using a long duration time-resolved particle image velocimetry system. The velocity fields in the near wake were acquired simultaneously with the birds’ motion during flight which was sampled using multiple high-speed cameras. The turbulent momentum fluxes, turbulent kinetic energy production, and dissipation profiles are examined in the wake and compared. The near wake of the owl exhibited significantly higher turbulent activity than the hawk in all cases, though both birds are similar in size and followed similar flight behavior. It is suggested that owls modulate the turbulence activity of the near wake in the vicinity of the wing, resulting in rapid decay before radiating into the far-field; thus, suppressing the aerodynamic noise at the far wake.

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.

Flow behind castellated blunt-trailing-edge aerofoils at supersonic speeds

1998 ◽  
Vol 375 ◽  
pp. 85-111 ◽  
Author(s):  
E. C. MAGI ◽  
S. L. GAI
Keyword(s):  
Quantitative Data ◽  
Base Pressure ◽  
Qualitative Description ◽  
Wake Flow ◽  
Spanwise Direction ◽  
Pressure Measurements ◽  
Near Wake ◽  
Trailing Edge ◽  
Blunt Trailing Edge ◽  
The Mean

A study of the near-wake flow of castellated blunt-trailing-edge aerofoils at a Mach number of 2 was conducted to understand the nature of the flow and the mechanisms of base pressure recovery. The investigation has shown that strong gradients exist in the spanwise direction and that the formation of the wake recompression shock occurs further away from the wake axis. Also, the wake neck is broader and diffused. Detailed quantitative data involving pressure measurements, schlieren and holographic interferometry, and laser transit velocimetry, are presented. A theoretical model to predict the mean base pressure on a castellated base is also proposed. Comparison with experimental data shows that the model provides a qualitative description of the flow behind a castellated base at supersonic speeds.

scholarly journals Flow pattern similarities in the near wake of three bird species suggest a common role for unsteady aerodynamic effects in lift generation

2017 ◽  
Vol 7 (1) ◽  
pp. 20160090 ◽  
Author(s):  
Roi Gurka ◽  
Krishnamoorthy Krishnan ◽  
Hadar Ben-Gida ◽  
Adam J. Kirchhefer ◽  
Gregory A. Kopp ◽  
...  
Keyword(s):  
Unsteady Flow ◽  
Bird Species ◽  
Unsteady Aerodynamics ◽  
Flow Patterns ◽  
Flapping Flight ◽  
Wake Flow ◽  
Flapping Wings ◽  
Flow Structures ◽  
Near Wake ◽  
Time Resolved

Analysis of the aerodynamics of flapping wings has yielded a general understanding of how birds generate lift and thrust during flight. However, the role of unsteady aerodynamics in avian flight due to the flapping motion still holds open questions in respect to performance and efficiency. We studied the flight of three distinctive bird species: western sandpiper ( Calidris mauri ), European starling ( Sturnus vulgaris ) and American robin ( Turdus migratorius ) using long-duration, time-resolved particle image velocimetry, to better characterize and advance our understanding of how birds use unsteady flow features to enhance their aerodynamic performances during flapping flight. We show that during transitions between downstroke and upstroke phases of the wing cycle, the near wake-flow structures vary and generate unique sets of vortices. These structures appear as quadruple layers of concentrated vorticity aligned at an angle with respect to the horizon (named ‘double branch’). They occur where the circulation gradient changes sign, which implies that the forces exerted by the flapping wings of birds are modified during the transition phases. The flow patterns are similar in (non-dimensional) size and magnitude for the different birds suggesting that there are common mechanisms operating during flapping flight across species. These flow patterns occur at the same phase where drag reduction of about 5% per cycle and lift enhancement were observed in our prior studies. We propose that these flow structures should be considered in wake flow models that seek to account for the contribution of unsteady flow to lift and drag.

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