Physical Context and Life in Flow

Vocabulary flashcards covering physical factors, fluid dynamics, Reynolds numbers, drag, lift, shear, and boundary layers in marine ecosystems based on the lecture transcript.

Reynolds number ($Re$)

A derived dimensionless property describing the relative importance of inertial forces to viscous forces in a fluid, defined by $Re = \frac{\rho U l}{\mu}$.

Drag

The hydrodynamic force acting opposite to the direction of motion for a moving organism, or pushing a stationary organism in the direction of fluid flow.

Drag force equation

The mathematical equation $F_{drag} = \frac{1}{2} \rho A C_d U^2$, where $\rho$ is fluid density, $A$ is projected area, $C_d$ is drag coefficient, and $U$ is fluid velocity.

Lift

The force acting in a direction perpendicular to fluid flow, typically directed upward away from the substrate.

Lift force equation

The mathematical formula $F_L = \frac{1}{2} \rho v^2 C_l A$, where $\rho$ is fluid density, $v$ is fluid velocity, $C_l$ is lift coefficient, and $A$ is projected area into flow.

Shear

The variation or change in force experienced by a body along a spatial dimension, such as along the height of an organism.

Bernoulli's principle

A fluid dynamics principle stating that an increase in fluid speed occurs simultaneously with a decrease in pressure, contributing to lift.

Newton's 3rd law in fluid lift

The mechanism where lift force is generated on an object as a reaction force to the downward deflection of fluid flow.

No-slip condition

The physical condition where fluid in direct contact with a solid surface adheres to it, resulting in fluid velocity relative to the surface being zero.

Free stream velocity ($U_\infty$)

The fluid velocity far enough from a solid boundary where the presence of the substrate or object no longer slows down fluid motion.

Skin friction drag

Drag generated by the friction of fluid passing over an object's surface, which is directly proportional to its total surface area.

Pressure drag

Drag resulting from fluid flow separation that creates a low-pressure, turbulent wake behind an object.

Attached flow

Fluid flow that closely follows the continuous surface contour of an object.

Separation point

The specific boundary location on an object's surface where fluid flow detaches from the surface.

Separated flow

The region downstream of the separation point characterized by low pressure, unstable recirculating vortices, and wake formation.

Turbulisors

Physical surface structures such as fins, denticles, or riblets located near a body's widest point that induce turbulent flow to delay boundary layer separation.

Benthic boundary layer (BBL)

The broad, large-scale fluid boundary layer that forms over the sea floor.

Momentum boundary layer (MBL)

The localized boundary layer surrounding an individual organism containing eddies where compounds move slowly via advection.

Diffusion boundary layer (DBL)

A extremely thin, viscous fluid layer directly adjacent to an organism where fluid motion is minimal and transport occurs primarily via diffusion and conduction.

Mass Transfer

The movement of chemical species such as $H_2O$, $O_2$, $CO_2$, or nutrients between pools driven by concentration gradients.

Postelsia

A genus of intertidal kelp (sea palm) with a short, thick stipe that bends over during heavy waves and surge to minimize drag, lift, and shear forces.

Low vs High Reynolds Number Flow

At low $Re$, flow streamlines remain parallel and viscous forces dominate; at high $Re$, streamlines become chaotic, creating turbulence, wakes, and thin boundary layers.