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What is vortex shedding?
Source: http://hmf.enseeiht.fr/travaux/CD0102/travaux/optmfn/gpfmho/01-02/grp1/index.htm
More Specifically Vortex Shedding is an effect of:
ViscosityReynolds Number*
What is Reynolds Number? Reynolds Number (Re) is a
dimensionless quantity Relates the inertial forces to the viscous
forces in a flow. Low Re: viscous forces
dominate flow High Re: inertial forces
dominate flow
viscosityabsolute
velocity
diameter
density
Re
u
D
uD
What Happens Next? As the flow starts around the cylinder, the
local Re increases as a function of the local velocity and distance from the Leading Edge (LE).
Smooth (Laminar) flow for Re < Recrit
(~500,000)
Flow transitions around Recrit
Turbulent flow for Re > Recrit
After the flow separates due to viscous effects, a re-circulation bubble forms at the Trailing Edge (TE) of the cylinder.
Low-pressure region forms in the re-circulation bubble.
A vortex forms in the re-circulation to equalize the pressure.
Disturbances in the flow cause the vortex to be shed creating a new vortex to equalize the pressure from the preceding vortex
Change in velocity affects local pressure. INCREASE in velocity = DECREASE in
pressureDECREASE in velocity = INCREASE in
pressure At Low Re (< 400), the pressure oscillations
are strong. Shed vortices will form the Von Karman Vortex
Street At higher Re (>400), the Von Karman Vortex
Street disappears due to turbulence
Source: http://chaos.usc.es/uploads/Galego/flow_visualizations.pdf
Vortex Shedding and Strouhal Number The Strouhal number relates the velocity
of the freestream velocity, diameter of the cylinder and the frequency at which vortices are shed into the Von Karman Vortex Street.
u
fdSt
Strouhal Number As shown below, the Strouhal Number varies
with Reynolds number
Commonly accepted value for the Strouhal Number for most flows is 0.21
Source: http://hmf.enseeiht.fr/travaux/CD0102/travaux/optmfn/gpfmho/01-02/grp1/index.htm
Our Difficulties Pressure Sensor Locations:
Recommendations: Far enough from the ends of the rocket to simulate an “infinite cylinder.”
Flow will not always “see” a cylinder.Source: http://chaos.usc.es/uploads/Galego/flow_visualizations.pdf
Must develop Strouhal Numbers for use with “infinite ellipses.”Relate major axis and velocity to frequency of
vortex shedding.CFD computing Time
○ Must run Navier-Stokes Solutions to capture vortex shedding Time Intensive
○ Major diameter of the ellipse varies with the angle of the cross-flow (with respect to the horizontal - β
)horizontal the(W.R.T. angle flow-cross
body tube ofdiameter
sec Diameter Major
d
d