Sticky Water Taiwan Hsieh TsungLin Question When a

Question When a horizontal cylinder is placed in a vertical stream of water, the

Coanda Effect The tendency for a fluid jet to be attracted to a nearby

A Simple Argument From a book “Fluid Dynamics for Physicists” by T. E. Faber,

At the impact point, tangential momentum is conserved, so the backward jet is

Some Subtle Differences We should consider Adhesive force Cohesive force Gravity Viscosity Vorticity The

Experimental Setup Water tank Faucet Cylinder Reservoir

At the impact point Water spread out in every direction. There is more momentum

Spreading Out The water jet flows down due to gravity. The velocity will be

Convergence The stream converges because: The velocity is greater, Evidently, the cohesion wins over

Instability Convergence results in instability. Do not necessary flows tangentially.

Detachment • Fluid instability results in detachment.

Parameters Initial speed at the point of impact- 220, 200, 170, 150 cm/s Cylinder

Side Width- Initial Speed Higher velocity, larger is the spread. Water spreads wider on

Summary The “sticky water” effect is real. The leaving angle have large error due

Reference Fluid Dynamics for Physicists by T. E. Faber Flying Circus of Physics by

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Sticky Water Taiwan Hsieh, Tsung-Lin

Question When a horizontal cylinder is placed in a vertical stream of water, the stream can follow the cylinder's circumference along the bottom and continue up the other side before it detaches. Explain this phenomenon and investigate the relevant parameters.

Coanda Effect The tendency for a fluid jet to be attracted to a nearby surface. Theoretical explanation is difficult because we have to deal with fluid mechanics, not particle mechanics.

A Simple Argument From a book “Fluid Dynamics for Physicists” by T. E. Faber, we have a simple argument but not realistic. 2 -d flow (the 3 rd dimension is infinite) A perfect imcompressible fluid (No viscosity, no vorticity) No gravity Conservation of energy Conservation of tangential momentum

At the impact point, tangential momentum is conserved, so the backward jet is thinner, less mass. The two jets move half a circle in equal time and collide again. The jet detaches because of collision.

Some Subtle Differences We should consider Adhesive force Cohesive force Gravity Viscosity Vorticity The cross section of the jet will be subjected to a free surface boundary condition by the air pressure and the cylinder.

Experimental Setup Water tank Faucet Cylinder Reservoir

Experimental Setup Front view Side view

At the impact point Water spread out in every direction. There is more momentum (mv) in the forward direction. Water flow

Spreading Out The water jet flows down due to gravity. The velocity will be different at different point. Wetting Surface tension

Convergence The stream converges because: The velocity is greater, Evidently, the cohesion wins over the wetting. Wetting Surface tension

Instability Convergence results in instability. Do not necessary flows tangentially.

Detachment • Fluid instability results in detachment.

Parameters Initial speed at the point of impact- 220, 200, 170, 150 cm/s Cylinder material- acrylic, steel, wax paper Impact angle- 20~75 deg Cylinder diameter Faucet bore diameter

Definitions

Water Speed

Impact Angle

Cylinder Diameter

Cylinder Material

Side Width- Initial Speed Higher velocity, larger is the spread. Water spreads wider on acrylic cylinder. Maximum width

Summary The “sticky water” effect is real. The leaving angle have large error due to instability of the stream. No significant relation is found. Stream spreads out when the speed is higher, and wider on acrylic cylinder.

Reference Fluid Dynamics for Physicists by T. E. Faber Flying Circus of Physics by … Teapot effect Halliday…

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