Low-Reynolds-number swimming at pycnoclines
Amin Doostmohammadi1, Roman Stocker, Arezoo M Ardekani
1Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA.
Summary
Density stratification at aquatic pycnoclines hinders microbial swimmers by increasing energy costs and reducing nutrient uptake. This fundamental effect of buoyancy impacts many organisms in lakes and oceans.
Area of Science:
- Aquatic microbial ecology
- Biogeochemistry
- Fluid dynamics
Background:
- Microorganisms are crucial for aquatic ecosystems.
- Pycnoclines are hotspots with unique conditions like reduced turbulence and high particle concentrations.
- Density stratification may fundamentally impact microbial life at pycnoclines.
Purpose of the Study:
- To investigate how density stratification affects microbial swimmers at pycnoclines.
- To identify the key parameter quantifying stratification effects on motility.
Main Methods:
- Numerical simulations of a low-Reynolds-number swimmer (squirmer).
- Analysis of the Richardson number (ratio of buoyancy to viscous forces).
Main Results:
- Density stratification at pycnoclines alters microbial ecology.
- Stratification increases energetic expenditure for swimmers.
- Nutrient uptake is stifled for motile organisms in stratified layers.
- The Richardson number quantifies stratification's impact.
Conclusions:
- Buoyancy has an unexpected effect on low-Reynolds-number swimming.
- Stratification impacts microbial ecology at pycnoclines.
- Findings are relevant to numerous aquatic organisms in oceans and lakes.
Related Concept Videos
01:10Poiseuille's Law and Reynolds Number
Any fluid in a horizontal tube can flow due to pressure differences—fluid flows from high to low pressure. The flow rate (Q) is the ratio of pressure difference and resistance through a horizontal tube. The greater the pressure difference, the higher the flow rate. The flow resistance is expressed as:
01:24Reynolds Transport Theorem
The Reynolds transport theorem provides a framework to relate the time rate of change of an extensive property within a system to that in a control volume, which is crucial for analyzing fluid dynamics. Extensive properties, such as mass, velocity, acceleration, temperature, and momentum, can be expressed in terms of the mass of a fluid portion. These properties are called extensive because they depend on the system's size, while intensive properties are their corresponding values per unit mass.
01:30Bernoulli's Equation for Flow Along a Streamline
Bernoulli's equation relates the energy conservation in a fluid moving along a streamline. The equation applies to incompressible and inviscid fluids under steady flow. For such a flow, Newton's second law is applied to a small fluid element, which experiences forces due to pressure differences, gravity, and velocity variations. The force balance leads to the following form of Bernoulli's equation:
01:16Bernoulli's Equation for Flow Normal to a Streamline
Bernoulli's equation for flow normal to a streamline explains how pressure varies across curved streamlines due to the outward centrifugal forces induced by the fluid's curvature. The pressure is higher on the inner side of the curve, near the center of curvature, and decreases outward to balance these centrifugal forces.
The pressure difference depends on the fluid's velocity and radius of curvature. The pressure variation is minimal in flows with nearly straight streamlines. However, the...
The pressure difference depends on the fluid's velocity and radius of curvature. The pressure variation is minimal in flows with nearly straight streamlines. However, the...
01:15Dimensionless Groups in Fluid Mechanics
Dimensionless groups in fluid mechanics provide simplified ratios that help analyze fluid behavior without relying on specific units. The Reynolds number (Re), which represents the ratio of inertial to viscous forces, distinguishes between laminar and turbulent flows, making it essential in the design of pipelines and aerodynamic surfaces. The Froude number (Fr), the ratio of inertial to gravitational forces, is particularly useful in predicting wave formation and hydraulic jumps in...
01:27Laminar Flow
Laminar flow represents a smooth, orderly fluid motion where particles move along parallel paths, resulting in minimal mixing between layers. Streamlined particle paths characterize this flow regime and occur under conditions where viscous forces dominate over inertial forces. The distinction between laminar, transitional, and turbulent flow is primarily determined by the Reynolds number, a dimensionless quantity calculated as: