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Flow Separation and Pressure Gradients

An adverse pressure gradient can cause flow separation over a curved surface. As the pressure increases downstream, the velocity near the wall decreases and the boundary layer thickens. At the point of separation, the wall shear stress reduces to zero and the flow reverses direction, forming a recirculating region. A separation bubble may occur if the flow reattaches downstream due to a favorable pressure gradient or transition to turbulence. Large separation bubbles near the trailing edge can cause stall on airfoils.

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Dinku Seyoum Zeleke
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123 views2 pages

Flow Separation and Pressure Gradients

An adverse pressure gradient can cause flow separation over a curved surface. As the pressure increases downstream, the velocity near the wall decreases and the boundary layer thickens. At the point of separation, the wall shear stress reduces to zero and the flow reverses direction, forming a recirculating region. A separation bubble may occur if the flow reattaches downstream due to a favorable pressure gradient or transition to turbulence. Large separation bubbles near the trailing edge can cause stall on airfoils.

Uploaded by

Dinku Seyoum Zeleke
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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Next: Drag
Up: Viscous Effects in External
Previous: Laminar and Turbulent
Boundary

Separation of Flow
Pressure gradient is an is one of the factors that influences a
flow immensely. It is
easy to see that the shear stress caused by
viscosity has a retarding effect upon the
flow. This effect can
however be overcome if there is a negative pressure gradient
offered to the flow. A negative pressure gradient is termed a
Favourable pressure
gradient. Such a gradient enables the
flow. A positive pressure gradient has the
opposite effect and is
termed the Adverse Pressure Gradient. Fluid might find it
difficult to negotiate an adverse pressure gradient. Sometimes, we
say the the fluid
has to climb the pressure hill.

Figure 6.4: Separation of flow over a curved surface

One of the severe effects of an adverse pressure gradient is to


separate the flow.
Consider flow past a curved surface as shown in
Fig.6.4. The geometry of the surface
is such that we have
a favourable gradient in pressure to start with and up to a point
P. The negative pressure gradient will counteract the retarding
effect of the shear
stress (which is due to viscosity) in the
boundary layer. For the geometry considered
we have a an adverse
pressure gradient downstream of P.

Now the adverse pressure gradient begins to retard. This effect is felt more strongly
in the regions close to the wall where the momentum is lower than
in the regions
near the free stream. As indicated in the figure,
the velocity near the wall reduces
and the boundary layer
thickens. A continuous retardation of flow brings the wall
shear
stress at the point S on the wall to zero. From this point onwards
the shear
stress becomes negative and the flow reverses and
a region of recirculating flow
develops. We see that the flow no
longer follows the contour of the body. We say that
the flow has
separated. The point S where the shear stress is zero is called
the Point
of Separation.

Depending on the flow conditions the recirculating flow terminate


and the flow may
become reattached to the body. A separation
bubble is formed. There are a variety of
factors that could
influence this reattachment. The pressure gradient may be now
favourable due to body geometry and other reasons. The other
factor is that the flow
initially laminar may undergo transition
within the bubble and may become turbulent.
A turbulent flow has
more energy and momentum than a laminar flow. This can kill
separation and the flow may reattach. A short bubble may not be
of much
consequence.
Figure 6.5: Separation bubble over an aerofoil

On aerofoils sometimes the separation occurs near the leading


edge and gives rise to
a short bubble. What can be dangerous is
the separation occurring more towards the
trailing edge and the
flow not reattaching. In this situation the separated region
merges with the wake and may result in stall of the aerofoil (loss of lift).

Next: Drag
Up: Viscous Effects in External
Previous: Laminar and Turbulent
Boundary
(c) Aerospace, Mechanical & Mechatronic Engg. 2005
University of Sydney

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