Vortex Shedding in Fluid Dynamics
Vortex Shedding
•
In fluid dynamics, vortex shedding is an
oscillating flow that takes place when a fluid
such as air or water flows past a bluff body at
certain velocities, depending on the size and
shape of the body.
•
In this flow, vortices are created at the back of
the body and detach periodically from either
side of the body.
•
The rate at which this happens is the vortex
shedding frequency. The Strouhal Number
characterizes the frequency of oscillation.
•
The
‘additional lift’
hypothesis represents the
traditional view. It presumes that an additional
suction/circulation from the LEV increases the lift
above that of a potential flow solution.
•
The
‘absence of stall’
hypothesis is a more recent
contender that presumes that the LEV prevents stall
at high angles of attack where flow separation
would normally occur. This behaviour is
represented through the so-called ‘normal force’
model.
Given the two hypotheses above,
why does it matter which is more
representative of the physics if the
final outcome is the same?
The reason is that the role of the LEV fundamentally
affects the way we understand how insect like wings
at low Reynolds number work and hence our
experimental approach to understanding insect flight
and the design of engineered flapping wing vehicles.
•
While the context for the work is flapping wing
aerodynamics, it is important to stress we
explicitly focus here on
quasi-steady revolving
wing aerodynamics
.
•
It is understood that there will be
unsteady
effects
due to stroke reversal. However, for the
most important case of normal hovering with
symmetric half-strokes, these
effects are
relatively small compared with the quasi-steady
effects
.
1. Potential flow mode
•
The classical potential flow model is based on
assumption of fully attached flow to the wing up to
90 degree angle of attack.
•
In an idealized two-dimensional (2D) flow, the lift
coefficient, CL, for a flat plate is expressed as:
Where CLalpha,2D is the 2D aerofoil lift curve slope
and alpha is the angle of attack.
The potential flow
model is
useful at low
angles of attack
, but
clearly non-physical at
high angles of attack
approaching 90 degree,
where geometry dictates
that the lift must tend
towards zero when the
wing is perpendicular to
the flow.
LEV topologies
varies from a conical form with a substantial
spanwise flow at the vortex core (as that observed on model hawkmoth
wings at Reynolds number from 103 to 104) to a cylindrical form with a
substantially weaker corewise vortex flow(as that observed on fruit fly
and thrip wings at Reynolds number of the order 102 and 101]).
However, these differences in the LEV flow topologies were not
reflected in differences in the measured lift, suggesting that the LEV is
playing an aerodynamic role that it is to some extent independent of its
shape under quasi-steady conditions.
Vortex shedding in fluid dynamics occurs when a fluid flows past a bluff body, creating oscillating vortices at certain velocities. The two hypotheses, additional lift and absence of stall, impact our understanding of insect flight and flapping wing vehicles. While focusing on quasi-steady revolving wing aerodynamics, the classical potential flow model aids in analyzing lift coefficients at low angles of attack. These concepts are crucial for studying and designing aerodynamic systems.
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Presentation Transcript
Vortex Shedding In fluid dynamics, vortex shedding is an oscillating flow that takes place when a fluid such as air or water flows past a bluff body at certain velocities, depending on the size and shape of the body. In this flow, vortices are created at the back of the body and detach periodically from either side of the body. The rate at which this happens is the vortex shedding frequency. characterizes the frequency of oscillation. The Strouhal Number
The additional lift hypothesis represents the traditional view. It presumes that an additional suction/circulation from the LEV increases the lift above that of a potential flow solution. The absence of stall hypothesis is a more recent contender that presumes that the LEV prevents stall at high angles of attack where flow separation would normally occur. represented through the so-called normal force model. This behaviour is
Given the two hypotheses above, why does it matter which is more representative of the physics if the final outcome is the same? The reason is that the role of the LEV fundamentally affects the way we understand how insect like wings at low Reynolds number work and hence our experimental approach to understanding insect flight and the design of engineered flapping wing vehicles.
While the context for the work is flapping wing aerodynamics, it is important to stress we explicitly focus here on quasi-steady revolving wing aerodynamics. It is understood that there will be unsteady effects due to stroke reversal. However, for the most important case of normal hovering with symmetric half-strokes, relatively small compared with the quasi-steady effects. these effects are
1. Potential flow mode The classical potential flow model is based on assumption of fully attached flow to the wing up to 90 degree angle of attack. In an idealized two-dimensional (2D) flow, the lift coefficient, CL, for a flat plate is expressed as: Where CLalpha,2D is the 2D aerofoil lift curve slope and alpha is the angle of attack.
The model is useful at low angles of clearly non-physical at high angles of attack approaching 90 degree, where geometry dictates that the lift must tend towards zero when the wing is perpendicular to the flow. potential flow attack, but
LEV topologies varies from a conical form with a substantial spanwise flow at the vortex core (as that observed on model hawkmoth wings at Reynolds number from 103 to 104) to a cylindrical form with a substantially weaker corewise vortex flow(as that observed on fruit fly and thrip wings at Reynolds number of the order 102 and 101]). However, these differences in the LEV flow topologies were not reflected in differences in the measured lift, suggesting that the LEV is playing an aerodynamic role that it is to some extent independent of its shape under quasi-steady conditions.
