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