scholarly journals The disappearance of laminar and turbulent wakes in complex flows

2002 ◽  
Vol 457 ◽  
pp. 111-132 ◽  
Author(s):  
J. C. R. HUNT ◽  
I. EAMES
Keyword(s):  
Flow Field ◽  
Rigid Body ◽  
The Body ◽  
Viscous Drag ◽  
Total Force ◽  
Far Field ◽  
Volume Flux ◽  
Wake Region ◽  
Complex Flows ◽  
Turbulent Wakes

The singular effects of steady large-scale external strain on the viscous wake generated by a rigid body and the overall flow field are analysed. In an accelerating flow strained at a positive rate, the vorticity field is annihilated owing to positive and negative vorticity either side of the wake centreline diffusing into one another and the volume flux in the wake decreases with downwind distance. Since the wake disappears, the far-field flow changes from monopolar to dipolar. In this case, the force on the body is no longer proportional to the strength of the monopole, but is proportional to the strength of the far field dipole. These results are extended to the case of strained turbulent wakes and this is verified against experimental wind tunnel measurements of Keffer (1965) and Elliott & Townsend (1981) for positive and negative strains. The analysis demonstrates why the total force acting on a body may be estimated by adding the viscous drag and inviscid force due to the irrotational straining field.Applying the analysis to the wake region of a rigid body or a bubble shows that the wake volume flux decreases even in uniform flows owing to the local straining flow in the near-wake region. While the wake volume flux decreases by a small amount for the flow over streamline and bluff bodies, for the case of a clean bubble the decrease is so large as to render Betz's (1925) drag formula invalid.To show how these results may be applied to complex flows, the effects of a sequence of positive and negative strains on the wake are considered. The average wake width is much larger than in the absence of a strain field and this leads to diffusion of vorticity between wakes and the cancellation of vorticity. The latter mechanism leads to a net reduction in the volume flux deficit downstream which explains why in calculations of the flow through groups of moving or stationary bodies the wakes of upstream bodies may be ignored even though their drag and lift forces have a significant effect on the overall flow field.

On Detached Shocks for Blunt Bodies at Small Angles of Attack

1965 ◽  
Vol 32 (2) ◽  
pp. 271-276
Author(s):  
Hsuan Yeh ◽  
Raymond Doby
Keyword(s):  
Flow Field ◽  
Rigid Body ◽  
Blunt Body ◽  
Subsonic Flow ◽  
The Body ◽  
Body Rotation ◽  
Numerical Computations ◽  
Primary Flow ◽  
Perturbation Procedure ◽  
Blunt Bodies

This paper is addressed to the problem of determining the subsonic flow field that exists between the shock and the associated blunt body at small angles of attack. An inverse perturbation procedure is used whereby the shock itself is caused to rotate and the body that supports the perturbed shock is determined. It is found that the perturbed body does not possess a rigid-body-rotation relation relative to the primary flow body. Curves and tables are presented which represent the results of the numerical computations.

scholarly journals Time-dependent lift and drag on a rigid body in a viscous steady linear flow

2019 ◽  
Vol 864 ◽  
pp. 554-595 ◽  
Author(s):  
Fabien Candelier ◽  
Bernhard Mehlig ◽  
Jacques Magnaudet
Keyword(s):  
Reynolds Number ◽  
Flow Field ◽  
Rigid Body ◽  
Slip Velocity ◽  
Fluid Velocity ◽  
The Body ◽  
Time Dependent ◽  
Leading Order ◽  
Linear Flow ◽  
Force Components

We compute the leading-order inertial corrections to the instantaneous force acting on a rigid body moving with a time-dependent slip velocity in a linear flow field, assuming that the square root of the Reynolds number based on the fluid-velocity gradient is much larger than the Reynolds number based on the slip velocity between the body and the fluid. As a first step towards applications to dilute sheared suspensions and turbulent particle-laden flows, we seek a formulation allowing this force to be determined for an arbitrarily shaped body moving in a general linear flow. We express the equations governing the flow disturbance in a non-orthogonal coordinate system moving with the undisturbed flow and solve the problem using matched asymptotic expansions. The use of the co-moving coordinates enables the leading-order inertial corrections to the force to be obtained at any time in an arbitrary linear flow field. We then specialize this approach to compute the time-dependent force components for a sphere moving in three canonical flows: solid-body rotation, planar elongation, and uniform shear. We discuss the behaviour and physical origin of the different force components in the short-time and quasi-steady limits. Last, we illustrate the influence of time-dependent and quasi-steady inertial effects by examining the sedimentation of prolate and oblate spheroids in a pure shear flow.

Far-field drag decomposition using hybrid formulas and vorticity based area sensors

2020 ◽  
pp. 095441002097390
Author(s):  
L Qiao ◽  
XL He ◽  
Y Sun ◽  
JQ Bai ◽  
L Li
Keyword(s):  
Flow Field ◽  
Axial Velocity ◽  
Near Field ◽  
Three Dimensional ◽  
Field Method ◽  
Wave Drag ◽  
Viscous Drag ◽  
Far Field ◽  
Velocity Defect ◽  
Detection Failure

Numerical simulation of flow-field has become an indispensable tool for aerodynamic design. Usually, wall surface integration is a tool used to calculate values of pressure drag and skin friction drag, but the aerodynamic mechanism of drag production is still confusing. In present work, in order to decompose the total drag into viscous drag, wave drag, induced drag, and spurious drag, a far-field drag decomposition (FDD) method is developed. This method depends on axial velocity defect and area sensor functions. The present work proposes three hybrid formulas for velocity defect to tackle the negative square root issue by analyzing the existing axial velocity defect formulas. For dealing with the issue of detection failure for near-wall cells, a novel vorticity based viscous area sensor function is proposed. The new area sensor function is also independent of the turbulence model, which ensures easy application to general simulation methods for flow-field. Three tests cases are there to validate the proposed FDD method. The three dimensional transonic CRM test case shows that the present improvement is crucial for accurate drag decomposition. Excellent agreement between total decomposed drags and results from the near-field method or experimental data further confirms the correctness.

Generalized Kirchhoff equations for a deformable body moving in a weakly non-uniform flow field

1994 ◽  
Vol 446 (1926) ◽  
pp. 169-193 ◽  
Keyword(s):  
Flow Field ◽  
Rigid Body ◽  
Degrees Of Freedom ◽  
Surface Deformation ◽  
Nonlinear Differential Equations ◽  
Deformable Body ◽  
Equations Of Motion ◽  
First Integral ◽  
Uniform Flow ◽  
The Body

The classical Kirchhoff’s method provides an efficient way of calculating the hydrodynamical loads (forces and moments) acting on a rigid body moving with six-degrees of freedom in an otherwise quiescent ideal fluid in terms of the body’s added-mass tensor. In this paper we provide a versatile extension of such a formulation to account for both the presence of an imposed ambient non-uniform flow field and the effect of surface deformation of a non-rigid body. The flow inhomogeneity is assumed to be weak when compared against the size of the body. The corresponding expressions for the force and moment are given in a moving body-fixed coordinate system and are obtained using the Lagally theorem. The newly derived system of nonlinear differential equations of motion is shown to possess a first integral. This can be interpreted as an energy-type conservation law and is a consequence of an anti-symmetry property of the coefficient matrix reported here for the first time. A few applications of the proposed formulation are presented including comparison with some existing limiting cases.

The flow field due to a body in impulsive motion

1996 ◽  
Vol 325 ◽  
pp. 79-111 ◽  
Author(s):  
Renwei Mei ◽  
Christopher J. Lawrence
Keyword(s):  
Finite Difference ◽  
Flow Field ◽  
Transition Zone ◽  
Bluff Body ◽  
The Body ◽  
Matched Asymptotic Expansion ◽  
Volume Flux ◽  
Global Flow ◽  
Unsteady Wake ◽  
Long Time

An asymptotic analysis for the long-time unsteady laminar far wake of a bluff body due to a step change in its travelling velocity from U1 to U2 is presented. For U1 [ges ] 0 and U2 > 0, the laminar wake consists of a new wake of volume flux Q2 corresponding to the current velocity U2, an old wake of volume flux Q1 corresponding to the original velocity U1, and a transition zone that connects these two wakes. The transition zone acts as a sink (or a source) of volume flux (Q2 – Q1) and is moving away from the body at speed U2. Streamwise diffusion is negligible in the new and old wakes but a matched asymptotic expansion that retains the streamwise diffusion is required to determine the vorticity transport in the transition zone. A source of volume flux Q2 located near the body needs to be superposed on the unsteady wake to form the global flow field around the body. The asymptotic predictions for the unsteady wake velocity, unsteady wake vorticity, and the global flow field around the body agree well with finite difference solutions for flow over a sphere at finite Reynolds numbers. The long-time unsteady flow structures due to a sudden stop (U2 = 0) and an impulsive reverse (U1U2 < 0) of the body are analysed in detail based on the asymptotic solutions for the unsteady wakes and the finite difference solutions. The elucidation of the long-time behaviour of such unsteady flows provides a framework for understanding the long-time particle dynamics at finite Reynolds number.

Swimming in the electric eels and knifefishes

1983 ◽  
Vol 61 (6) ◽  
pp. 1432-1441 ◽  
Author(s):  
R. W. Blake
Keyword(s):  
Rigid Body ◽  
Body Wave ◽  
The Body ◽  
Viscous Drag ◽  
Entire Body ◽  
Number Range ◽  
Large Amplitude Motions ◽  
Burst Swimming ◽  
Experimental Values ◽  
Notopterus Notopterus

The kinematics of steady forward swimming in six species of Gymnotidae and three species of Notopteridae are described. All the gymnotids and one notopterid (Xenomystis nigri) are propelled by the action of an undulatory anal fin (gymnotiform mode). Notopterus notopterus and Notopterus chilata employ the body and anal fin as a single propulsive unit and generate a body wave. Rapid bouts of burst swimming activity (e.g., escape responses) are generated by large amplitude motions of the entire body in all species studied. Experimentally determined drag coefficients exceed the theoretical rigid body predicted minimum values for the case of a laminar boundary layer and reasons for this are suggested. Values of the drag coefficient inferred from hydromechanical theory are within 20% of the experimental values. It is concluded that fish swimming in the gymnotiform mode may be subject to significantly less viscous drag than fish of equivalent size swimming at the same speed in the subcarangiform mode. Hydromechanical theory indicates a significant enhancement of fin added mass (due to the presence of a rigid body) over that of an isolated fin. The hydromechanical efficiency of the undulating fin system ranged from about 0.6 to 0.9 over a speed range of 0.2–5.0 lengths∙s−1 (corresponding to a Reynolds number range of about 103–105). It is suggested that undulatory median fin propulsion in the electric eels and knifefishes is an adaptation to swimming with high hydromechanical efficiency at low absolute forward speeds. Similarities and differences in body form between the species and the evolution of gymnotiform swimming are discussed.

scholarly journals Investigation on Wireless Link for Medical Telemetry Including Impedance Matching of Implanted Antennas

Sensors ◽  
2021 ◽  
Vol 21 (4) ◽  
pp. 1431
Author(s):  
Ilkyu Kim ◽  
Sun-Gyu Lee ◽  
Yong-Hyun Nam ◽  
Jeong-Hae Lee
Keyword(s):  
Real Time ◽  
Human Body ◽  
Near Field ◽  
Impedance Matching ◽  
The Body ◽  
Field Pattern ◽  
Communication Link ◽  
Far Field ◽  
Antenna Coupling ◽  
Medical Telemetry

The development of biomedical devices benefits patients by offering real-time healthcare. In particular, pacemakers have gained a great deal of attention because they offer opportunities for monitoring the patient’s vitals and biological statics in real time. One of the important factors in realizing real-time body-centric sensing is to establish a robust wireless communication link among the medical devices. In this paper, radio transmission and the optimal characteristics for impedance matching the medical telemetry of an implant are investigated. For radio transmission, an integral coupling formula based on 3D vector far-field patterns was firstly applied to compute the antenna coupling between two antennas placed inside and outside of the body. The formula provides the capability for computing the antenna coupling in the near-field and far-field region. In order to include the effects of human implantation, the far-field pattern was characterized taking into account a sphere enclosing an antenna made of human tissue. Furthermore, the characteristics of impedance matching inside the human body were studied by means of inherent wave impedances of electrical and magnetic dipoles. Here, we demonstrate that the implantation of a magnetic dipole is advantageous because it provides similar impedance characteristics to those of the human body.

Survey on the Indirect Methods of Potential Flow Used for the Optimization of Airships Profile

2014 ◽  
Vol 554 ◽  
pp. 717-723
Author(s):  
Reza Abbasabadi Hassanzadeh ◽  
Shahab Shariatmadari ◽  
Ali Chegeni ◽  
Seyed Alireza Ghazanfari ◽  
Mahdi Nakisa
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Flow Field ◽  
Drag Coefficient ◽  
Body Surface ◽  
Potential Flow ◽  
The Body ◽  
Indirect Methods ◽  
Singularity Method ◽  
Singularity Distribution

The present study aims to investigate the optimized profile of the body through minimizing the Drag coefficient in certain Reynolds regime. For this purpose, effective aerodynamic computations are required to find the Drag coefficient. Then, the computations should be coupled thorough an optimization process to obtain the optimized profile. The aerodynamic computations include calculating the surrounding potential flow field of an object, calculating the laminar and turbulent boundary layer close to the object, and calculating the Drag coefficient of the object’s body surface. To optimize the profile, indirect methods are used to calculate the potential flow since the object profile is initially amorphous. In addition to the indirect methods, the present study has also used axial singularity method which is more precise and efficient compared to other methods. In this method, the body profile is not optimized directly. Instead, a sink-and-source singularity distribution is used on the axis to model the body profile and calculate the relevant viscose flow field.

Boundaries Influence on the Flow Field Around an Oscillating Sphere

2013 ◽  
Author(s):  
Domenica Mirauda ◽  
Antonio Volpe Plantamura ◽  
Stefano Malavasi
Keyword(s):  
Free Surface ◽  
Flow Field ◽  
Boundary Conditions ◽  
Damping Ratio ◽  
Water Channel ◽  
Open Water ◽  
Free Surface Flows ◽  
The Body ◽  
Sphere Diameter ◽  
Oscillating Sphere

This work analyzes the effects of the interaction between an oscillating sphere and free surface flows through the reconstruction of the flow field around the body and the analysis of the displacements. The experiments were performed in an open water channel, where the sphere had three different boundary conditions in respect to the flow, defined as h* (the ratio between the distance of the sphere upper surface from the free surface and the sphere diameter). A quasi-symmetric condition at h* = 2, with the sphere equally distant from the free surface and the channel bottom, and two conditions of asymmetric bounded flow, one with the sphere located at a distance of 0.003m from the bottom at h* = 3.97 and the other with the sphere close to the free surface at h* = 0, were considered. The sphere was free to move in two directions, streamwise (x) and transverse to the flow (y), and was characterized by values of mass ratio, m* = 1.34 (ratio between the system mass and the displaced fluid mass), and damping ratio, ζ = 0.004. The comparison between the results of the analyzed boundary conditions has shown the strong influence of the free surface on the evolution of the vortex structures downstream the obstacle.

Fractures in the northern plains, stream patterns, and the midcontinent stress field

1987 ◽  
Vol 24 (6) ◽  
pp. 1086-1097 ◽  
Author(s):  
Mel R. Stauffer ◽  
Don J. Gendzwill
Keyword(s):  
Stress Field ◽  
Horizontal Stress ◽  
The Body ◽  
North American Plate ◽  
Plate Motion ◽  
Viscous Drag ◽  
Near Surface ◽  
The North ◽  
Elastic Rebound ◽  
Western North Dakota

Fractures in Late Cretaceous to Late Pleistocene sediments in Saskatchewan, eastern Montana, and western North Dakota form two vertical, orthogonal sets trending northeast–southwest and northwest–southeast. The pattern is consistent, regardless of rock type or age (except for concretionary sandstone). Both sets appear to be extensional in origin and are similar in character to joints in Alberta. Modem stream valleys also trend in the same two dominant directions and may be controlled by the underlying fractures.Elevation variations on the sub-Mannville (Early Cretaceous) unconformity form a rectilinear pattern also parallel to the fracture sets, suggesting that fracturing was initiated at least as early as Late Jurassic. It may have begun earlier, but there are insufficient data at present to extend the time of initiation.We interpret the fractures as the result of vertical uplift together with plate motion: the westward drift of North America. The northeast–southwest-directed maximum principal horizontal stress of the midcontinent stress field is generated by viscous drag effects between the North American plate and the mantle. Vertical uplift, erosion, or both together produce a horizontal tensile state in near-surface materials, and with the addition of a directed horizontal stress through plate motion, vertical tension cracks are generated parallel to that horizontal stress (northeast–southwest). Nearly instantaneous elastic rebound results in the production of second-order joints (northwest–southeast) perpendicular to the first. In this manner, the body of rock is being subjected with time to complex alternation of northeast–southwest and northwest–southeast horizontal stresses, resulting in the continuous and contemporaneous production of two perpendicular extensional joint sets.

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