# boundary layer

```INVERTEBRATES
AND FLUID
DYNAMICS:
HOLDING ON
BOUNDARY LAYER
Distance from substrate
Mainstream Velocity (U)
Boundary layer thickness (90%)
Boundary layer thickness (99%)
Velocity
BOUNDARY LAYER
Using the boundary layer – water pennies (Psephenidae)
BOUNDARY LAYER
Distance from substrate
Mainstream Velocity (U)
Velocity
BOUNDARY LAYER
Using the boundary layer – water pennies (Psephenidae)
Other stream insects
Trichoptera
Plecoptera
Ephemeroptera
Stream animals– Strategies for holding on
Baetis
Rhithrogena
Ancylus
Stream animals– Strategies for holding on
Psephenus
Bibliocephala
Neothremma
Donax
Swash Surfing
Donax
Density
Weight distribution
Shape
Density
2.0
Density
(103 kg/m3)
1.0
0
Weight Distribution
Pivot point
Dorsal
Posterior
Anterior
Ventral
Behaviour in a Swash Zone
WAVE STRESS
a. Limitation of size
Water flow
100%
90%
Boundary layer
WAVE STRESS
a. Limitation of size
Water flow
WAVE STRESS
b. Holding on – flow tolerance
3
Flow rate
(m/s)
0
Time to dislodge
WAVE STRESS
c. Holding on - orientation
Keyhole limpet
WAVE STRESS
c. Holding on - orientation
Water flow
WAVE STRESS
c. Holding on - orientation
&lt;.5 m/s
&gt;.5 m/s
Freq
-90
0
90
-90
Orientation (&ordm; to flow)
0
90
WAVE STRESS
d. Holding on - tenacity
What is “tenacity”?
1. Testing holding power
WAVE STRESS
d. Holding on - tenacity
What is “tenacity”?
1. Testing holding power
100
Kg required to
dislodge
0
Foot area
(cm2)
15
WAVE STRESS
d. Holding on - tenacity
What is “tenacity”?
1. Suction?
Atmospheric pressure ≈ 1 kg/cm2
Patella ≈ 3 - 7 kg/cm2
-can’t generate a force &gt; atmospheric pressure
- No negative pressure under foot
WAVE STRESS
d. Holding on - tenacity
What is “tenacity”?
Patella
Mucous layer
WAVE STRESS
d. Holding on - tenacity
What is “tenacity”?
area
2AS
F=
d
surface tension
Thickness of fluid
3. WAVE STRESS
d. Holding on - tenacity
What is “tenacity”?
1
F &micro;
d
Tenacity
(kg/cm2 to
detach)
Weight of mucous
3. WAVE STRESS
d. Holding on - tenacity
What is “tenacity”?
Foot rigidity
\ Foot rigidity &micro; Tenacity
3. WAVE STRESS
d. Holding on - tenacity
What is “tenacity”?
Foot rigidity
l
d
Flexibility &micro; l:d ratio
3. WAVE STRESS
d. Holding on - tenacity
What is “tenacity”?
Foot rigidity
High
In field Tenacity
Tenacity &micro; severity of wave action
Low
Low
High
Flexibility
3. WAVE STRESS
d. Holding on - tenacity
What is “tenacity”?
Drag
Resistance to water movement
depends on:
FLOW
1) Size
2) Shape
3) Texture
3. WAVE STRESS
d. Holding on - tenacity
What is “tenacity”?
Drag
Drag – not well correlated with density
Drag
Side
Front
Rear
Types of Limpets
Non-Migratory
Migratory
-don’t move far
-move up shore
-often fixed and territorial
-high r
-low r
-high growth
-low growth
(need high food intake)
-react to predators by clamping
-flee from predators
LESS FLEXIBLE
MORE FLEXIBLE
A COMPROMISE OF SEVERAL FACTORS
Sea Urchins - Echinoidea
lunules
WHAT IS THE FUNCTION OF LUNULES?
1) Aid in burrowing
2) Removal of feces
3) Maintain a “communication” with the surface if buried
4) Maintain inclined posture
5) Feeding
If lunules have a hydrodynamic function
-they should grow with the animal
observed
isometric
Log lunule
length
p &lt; .05
Log test diameter
Burrowing speed
230 sec
231 sec
Time to
burrow
Plugged
Not plugged
Flow through a sand dollar
Weight
Lift
Weight
Lift
Weight
Sand Dollar reorientation - Dendraster
Reducing lift and drag
Skin drag – important?
Pressure drag – depends on shape
Reducing lift and drag
Skin drag – important?
Pressure drag – depends on shape
Very flat
Reducing lift and drag
Skin drag – important?
Pressure drag – depends on shape
Rounded
Area of lower pressure
How do you reduce lift
Reduce pressure
differential between
upper and lower
surfaces
Can sand dollars tell the direction of orientation?
Posterior
Anterior
Can sand dollars tell the direction of orientation?
-inverted sand dollars – can flip over more easily with posterior edge facing upstream
(i.e critical velocity to re-orient is lower)
Initial orientation
Final orientation
(1 hr)
Hardy &amp; Merz. 2013. Invert. Bio 132:52
```