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Flow direction problem

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Old   January 16, 2020, 13:32
Default Flow direction problem
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I know from Fluid Mechanic course that flow direction should be from high pressure to low pressure.

In the attached image, the right area has lower pressure than the left area.

Why doesn't the flow direction in this image obey this rule?

Absolute pressure and meridional velocity vectors are time-averaged.

Flow direction is from left to right.
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Old   January 16, 2020, 13:56
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Flow doesn't always flow from simply just higher pressure to lower pressure. There is much more involved. Shear forces are included in the Navier stokes equations as well.

Think about a rotating vortex, or any recirculating flow pattern. It would be impossible for this to always flow from higher to lower pressure, ad the flow comes back to its starting point.
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Old   January 16, 2020, 14:12
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Also, be careful interpreting time averaged flows. Context is very important.

It we setup two parallel plates with an inlet and outlet and impose a sinusoidal inlet condition with zero mean flow, the time average plot in the domain will show 0 velocity everywhere. Does it mean there is no flow throughout the simulation?
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Old   January 17, 2020, 07:48
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Quote:
Originally Posted by Opaque View Post
Also, be careful interpreting time averaged flows. Context is very important.

It we setup two parallel plates with an inlet and outlet and impose a sinusoidal inlet condition with zero mean flow, the time average plot in the domain will show 0 velocity everywhere. Does it mean there is no flow throughout the simulation?
Sounds it shouldn't.
How does CFX calculate time-averaged velocity vectors or pressure contours for a transient flow?
Does CFX calculate time-averaged result as instantaneous value minus fluctuation or perturbation part or it is found by evaluating the integral over a dt?
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Old   January 17, 2020, 09:39
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There is only one definition for time average

Time averaged value of < > = Integral over time interval of ( < > dt) / (time interval)

The special case of equal time step for the simulation, the equation simplifies to

Time averaged value of < > = sum of ( weights * time values)

if the weights are the ones for the trapezoidal rule,

Time averaged values of < > = Sum of time values / Number of time values.

i.e.

arithmetic average.
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Old   January 17, 2020, 13:13
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Can I ask in what situations a time-averaged result should not be used?

Last edited by Julian121; Yesterday at 00:43.
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