Sticky Water Taiwan Hsieh, Tsung-Lin
Jan 15, 2016
Sticky Water
TaiwanHsieh, 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 3rd 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
FaucetCylinder
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.
Surface tension
Wetting
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
-500
50100150
0 2 4 6
speed
deta
chm
ent a
ngle
Impact Angle
0
50
100
150
0 20 40 60 80
impact angle
deta
chm
ent a
ngle
Cylinder Diameter
-500
50100150200
0 1 2 3 4 5
cylinder diameter
deta
chm
ent a
ngle
Cylinder Material
-50
0
50
100
acrylic paper steel
deta
chm
ent a
ngle
17cm
20cm
Higher velocity, larger is the spread. Water spreads wider on acrylic cylinder.
Side Width- Initial Speed
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…
Thank you!