Fluid Dynamics 101: Basics to Understanding How ...

Fluid dynamics is one of the two categories of fluid mechanics, a branch in physics concerned with fluids in either a rested state or in motion. In this article, we will dive into the basics of fluid dynamics, the subdiscipline of fluid mechanics focused on fluid in motion.

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Fluid dynamics is a complex field of study, with a wide range of applications. Engineers and scientists use fluid dynamics to solve critical problems, improve existing technologies, and innovate new solutions. Understanding these principles led to advancements in areas such as aerodynamics, hydrodynamics, meteorology, and technologies in our everyday lives.

What is Fluid Dynamics?

Fluid dynamics is a branch of physics and mathematics that focuses on the behavior of fluids (liquids and gasses) in motion. It examines the forces and interactions that influence how fluids flow and change. In layman's terms, fluid dynamics looks at how fuel moves through an engine, how air moves around an airplane wing, and how blood circulates through our bodies. It answers questions like how submarines navigate underwater, why weather systems form, and how to make pipelines more efficient.

Fluids and Their Properties

A fluid is a substance that can flow and take the shape of the container that it is in. Liquids and gasses are both considered &#;fluids&#; with each of them having their own distinct properties. The main difference between the two types of fluids is their molecular structure and behavior.

Difference Between Liquids and Gasses

Liquids are made up of tightly packed molecules that are attracted to each other. The compact molecules give liquids a definite volume. They will take the shape of the container they are in, but they don&#;t easily compress. Think of a glass of water. The water takes the shape of the glass, filling it from the bottom up and spreading out to the sides. If you pour the water into a different container, it will take the shape of the new container with the volume remaining the same.

Gasses are made up of molecules that are widely spaced from one another and constantly moving and colliding with each other. Unlike liquids, gasses do not have a definite shape or volume and will expand to fill any container they're in. They are also highly compressible. Think of a balloon filled with air. The molecules inside spread out to fill the entire space of the balloon. When you squeeze the balloon, the gas inside gets compressed, pushing the molecules closer together and reducing the volume of the balloon. If you let the air out, the gas escapes and spreads out to fill the room, showing how gasses can expand to occupy any available space.

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Key Properties of Fluids

Fluids possess different properties which are classified as kinematic properties (motion of fluids without considering the forces causing the motion), thermodynamic properties (state of the fluid and its energy content), and physical properties (physical state and appearance of the fluid).

Kinematic Properties

• Velocity: Speed and direction the fluid particles are moving with respect to time
• Acceleration: Rate of change of the fluid's velocity with respect to time

Thermodynamic Properties

• Temperature: Average kinetic energy of the fluid molecules
• Pressure: Force exerted by the fluid per unit area on its surroundings
• Density: Mass of the fluid per unit volume

Physical Properties

• Viscosity: The internal resistance to flow
• Surface Tension: The elastic tendency of fluid surfaces
• Buoyancy: Ability of fluids to exert an upward force on objects

Types of Fluid Flow

Fluid flow is the movement of liquid or gas particles in a particular environment. The primary types of flow are laminar and turbulent. These two types are classified based on the nature of the flow itself.

Primary Types of Fluid Flow (Based on Behavior)

Image source - NASA Glenn Research Center

Laminar (Streamline) Flow

Liquid flows in smooth, parallel layers, with minimal mixing between the layers. This type of flow is characterized by low velocities and high viscosity.

Turbulent Flow

Whereas laminar flow is smooth and predictable, turbulent flow is chaotic and random. The fluid flow is irregular with eddies, swirls, and fluctuations in velocity and pressure. This type of flow is characterized by high velocities and low viscosity.

Additional Classifications (Based on Properties)

There are additional fluid flow types which are classified based on specific characteristics such as steadiness, compressibility, and viscosity.

Steady vs. Unsteady Flow

• Steady Flow: Fluid properties (velocity, pressure, etc.) remain constant at any given point over time.
• Unsteady Flow: Fluid properties vary with time at a given point.

Compressible vs. Incompressible Flow

• Compressible Flow: Fluid density varies significantly with pressure (common in gasses)
• Incompressible Flow: Fluid density is constant (common in liquids)

Viscous vs. Inviscid Flow

• Viscous Flow: Fluid molecules transfer momentum between layers due to their viscous properties. Layers nearer the surface move slower than the layers further away.
• Inviscid Flow: An idealized model that assumes the fluid has zero viscosity and viscous forces are negligible. (ideal fluid flow)

Transitional Flow

Transitional Flow: Mixes characteristics of both laminar and turbulent flows. It often occurs during the transition between the two.

Beyond the Basics

Most of this article covers the basics of fluid dynamics, but there are a few terms and principles worth briefly touching on when discussing fluid dynamics.

Bernoulli's Principle

Image - Jacques and Jean Bernoulli working on geometrical problems

This principle describes the relationship between the pressure and velocity of a fluid in motion. Basically, as the speed of a fluid increases, its pressure decreases. This is how airplane wings generate lift, why shower curtains pull inward when water is running, and how a pitcher can get a curveball to drop down.

Reynold's Number

Reynold's Number (Re) is a dimensionless number used to predict the type of flow (laminar or turbulent) in different situations. Low Reynold's numbers indicate laminar flow, while high Reynold's numbers indicate turbulent flow. This is useful for determining things like the appropriate pipe diameter and pumping pressure for transporting fluids like oil or water and in aircraft design to make wings that minimize drag and maximize lift.

A dimensionless number is a number that represents a relationship between two or more things, without being tied to any specific units of measurement.

The Reynolds number is calculated using the following formula:

Re = (ρ * V * L) / μ

Where:

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• Re is the Reynolds number, which is dimensionless
• ρ (kg/m³) is the density of the fluid
• V (m/s) is the characteristic velocity of the flow
• L (m) is the characteristic length scale of the flow (e.g., diameter of a pipe)
• μ (Pas) is the dynamic viscosity of the fluid

Pascal's Law

Pascal's Law states that pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid and to the walls of the containing vessel. This principle is the basis for hydraulic systems, such as hydraulic lifts and brakes, which use fluids to transmit force and power.

Continuity Equation

What goes in must come out. The continuity equation states that, for any incompressible fluid, the mass flow rate must remain constant from one cross-section of a pipe to another. As a section narrows, the fluid must speed up to maintain the same flow rate. For example, when a river flows through a canyon, the water speeds up to maintain the same flow rate.

Computational Fluid Dynamics (CFD)

Computational Fluid Dynamics (CFD) is a powerful tool for predicting and visualizing fluid flow. It uses computers to simulate and analyze fluid flow using complex calculations and algorithms. This allows engineers and scientists to test and optimize designs virtually before building physical prototypes, saving time and resources. CFD plays a crucial role in various industries where designing more efficient and reliable products is paramount, including aerospace, automotive, and energy.

Examples - How are Fluid Dynamics Used in Real Life?

Fluid dynamics isn't just a theoretical concept that you&#;ll only find in college textbooks and labs. It's all around us, playing a major role in everyday life and cutting-edge technology:

• Transportation:
  • Airplanes: The shape of airplane wings are designed using principles of fluid dynamics to create lift. This is what allows these massive vehicles to soar through the air. The principles of fluid dynamics are also applied in the design of pneumatic systems in aircraft, where pneumatic solenoid valves play a crucial role in controlling airflow
  • Cars and Trucks: Fluid dynamics helps optimize the design of vehicles to reduce drag and improve fuel efficiency.
  • Ships and Submarines: The shape of a ship's hull and the design of submarines are carefully crafted to minimize resistance to move smoothly through water.
• Weather and Climate:
  • Weather Forecasting: Fluid dynamics models are used by meteorologists to predict weather patterns, including the formation and movement of storms.
  • Climate Modeling: Scientists use fluid dynamics to understand ocean currents, atmospheric circulation, and other complex phenomena that influence our planet's climate.
• Energy:
  • Wind Turbines: The design of wind turbine blades relies on fluid dynamics to maximize energy capture from the wind.
  • Hydroelectric Dams: The flow of water through turbines in dams generates electricity, and fluid dynamics principles play a major part in designing efficient hydroelectric systems.
• Medicine and Biology:
  • Blood Flow: Understanding how blood flows through our arteries and veins is crucial for diagnosing and treating cardiovascular diseases.
  • Drug Delivery: Fluid dynamics helps design drug delivery systems that ensure medications reach their target sites in the body effectively.
• Everyday Life:
  • Plumbing Systems: The flow of water through pipes in our homes and cities is governed by fluid dynamics. Understanding fluid dynamics allows engineers to design plumbing systems with the most efficient water flow that also prevents leaks and other problems.
  • Sports: Fluid dynamics plays a role in the design of sports equipment like golf balls and swimsuits, aiming to reduce drag and enhance performance.
  • Inkjet Printers: These printers use tiny nozzles to eject precise droplets of ink onto paper, a process governed by fluid dynamics.

Simple Experiments to Try at Home

Just for fun, here are some ways to see fluid dynamics in action at home.

Bernoulli&#;s Principle with a Piece of Paper

• Materials Needed: A piece of paper.
• Steps: Hold the paper by the edges and blow across the top surface. Watch how the paper rises.
• Explanation: Blowing across the top of the paper increases the airspeed above the paper, reducing pressure and causing the paper to lift. This demonstrates Bernoulli's Principle in action.

Laminar vs. Turbulent Flow with Food Coloring

• Materials Needed: Clear glass, water, food coloring, a spoon.
• Steps: Fill a clear glass with water. Add a drop of food coloring without stirring and observe how it moves through the water. Then, stir the water with a spoon and observe how the food coloring spreads.
• Explanation: Initially, the food coloring moves slowly and smoothly, showing laminar flow. When you stir, the movement becomes chaotic and mixes quickly, illustrating turbulent flow.

Surface Tension with a Needle

• Materials Needed: A bowl of water, a needle, tissue paper.
• Steps: Fill the bowl with water. Place a small piece of tissue paper on the water's surface, and then carefully place a needle on top of the tissue paper. Once the tissue sinks, the needle should stay afloat.
• Explanation: The needle floats due to the surface tension of the water, which creates a 'skin' strong enough to support the needle.

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From single-use valves for TFF to a redesigned sanitary safety relief valve, Equilibar and Steriflow have been exploring synergistic ways to combine two distinct technologies to create better fluid control solutions for the biopharmaceutical industry since .

As they prepare for the April INTERPHEX show in New York City, Single Use Technology Manager Ryan Heffner and Megan Cider Elm, Product Manager for Pharma Multi-Use, took a moment to share what they look forward to at this year&#;s global conference. Equilibar and Steriflow Valve are divisions of Richards Industrials.

Richards Industrials (RI) Q&A with Megan and Ryan:

RI: Ryan, you are presenting with colleagues from Endress & Hauser and Arcadis at the Interphex Tech Theater this year. Can you explain more about your topic, Improving WFI Point-of-use with single-use flow control, and give a sneak preview of what attendees can expect to learn?

Ryan: Certainly. This will be an exciting presentation to share with end users, since the focal point will highlight some new technological advancements in the single use space to improve Water-For-Injection (WFI) dispensing while saving floor space and improving clean room workflow efficiency. If you are at Interphex, please Join us at Tech Theater 1 on Tuesday afternoon at 4pm to learn more. We will also have a live demonstration of this technology for visitors to see at our booth .

RI: Biopharmaceutical production facilities continue to expand, and processing techniques are evolving. What are some recent developments in biopharma processing where you have seen our technology offers solutions?

Megan: There are a lot of exciting new medicines coming to the market and many manufacturers have a more critical need to increase their output as demand for their products grows. Interest has grown in our Steriflow Valve solutions for adapting or future-proofing plants to increase output with high turn-down valves that limit the need for upsizing equipment.

Ryan: We are also seeing a lot of innovation in the biopharma market right now in process improvements. Several of our customers are inventing new filtration techniques and process flow paths to support growing interest in continuous manufacturing as well as new modalities like cell & gene therapies. A recent case study we wrote describes how an Equilibar® SDO single-use valve is used in a novel process for single-pass tangential filtration (SP-TFF). In the study, the SDO is automated to maintain precise trans-membrane pressure in a continuous TFF setup.

RI: Preventing unwanted pump push-through flow has been a topic of discussion recently. Can you explain what that is and how our back pressure regulators can help?

Ryan: As customers re-evaluate clean room floorspace management, we see them relocating buffer storage tanks to a floor level above the clean rooms and installing ceiling penetrations for piping down to the process skid. This increased elevation of the supply tank from the process delivery pump creates head pressure at the pump inlet and can result in uncontrolled push-through flow. The Equilibar SDO single use back pressure regulator is the industry&#;s first single use control valve and can prevent push-through flow. Read more about how the SDO can help here.

Megan: Push-through flow has been an accepted occurrence that creates significant buffer waste when those buffers are stored at higher elevations. Flow through a diaphragm pump occurs even when the pump is off due to the head pressure. Our FDO back pressure regulator, when installed downstream of the pump, is perfectly designed to eliminate push-through flow while also allowing operation of the pump at low RPM&#;s, which leads to reduction of waste and increased efficiency.

RI: Megan, Steriflow Valve introduced a sanitary safety relief valve (SSRV) into its portfolio last year. Please explain how it is different from what is already available.

Megan: Of course, safety is no less important in a sanitary system than an industrial one, but what often gets overlooked is the need to protect the process from the environment as much as the typical need of protecting the environment from the process. We work with customers who have had to sacrifice batches of product when rupture discs blow. When pressure is exceeded, the disc bursts, as intended, and the pressure is relieved, but the process is exposed. As an alternative, utilizing a sanitary safety relief valve can relieve pressure without exposing the process, thereby preserving the batch and preventing waste.

RI: What are some highlights that this year&#;s Interphex attendees can expect when they visit your booth?

Megan: This year the Steriflow and Equilibar booth will have more working displays of our products to show how our valves operate in real time and how they respond in different process scenarios. It will be a great opportunity for visitors to speak with our technical experts, watch our demonstrations and look for new ways to improve their process.

Ryan: I think our newest working demo will be very instructive for guests to come and witness. We&#;ve created a condensed version of a WFI distribution loop to demonstrate key performance advantages our single and multi-use product lines offer. For customers who don&#;t often dive deep into the fluid mechanics of valve design, it&#;s a great functional tool that can simply present complex control challenges. Watching will help scientists and engineers envision how these performance features can translate to system benefits.

RI: Are there any new products, solutions, or services that you have been hearing about in the industry that you are eager to learn more about while at Interphex this year?

Megan: There have been a lot of developments with oligonucleotide production due to the increase in demand. I am looking forward to learning more about new challenges manufacturers are facing and how our products may be helpful in these applications.

Ryan: I am excited to connect with customers to learn what process development is going on at their facility. Customers are constantly coming up with creative solutions to re-imagine bioprocessing. I like to learn how their process solutions synergize with the innovations we are designing in the pharmaceutical fluid control space to create the next generation of bioprocessing equipment.

RI: It&#;s been three years since Richards Industrials added Equilibar to our family of fluid technology brands. What have you seen evolve during this time in terms of access to biopharma fluid control options for Equilibar and Steriflow Valve customers? 

Megan:  The best part of the integration has been improving our capacity to offer customers a wider range of solutions to fit their needs for every application. The Equilibar FDO valve technology brings new capabilities to the Steriflow multi-use portfolio, making it easier to solve even more challenges that our customers are facing. The experienced Steriflow distributors have been able to share the unique FDO technology with many more customers through their network.

Equilibar and Steriflow Valve will once again be in Booth at  INTERPHEX in The Javits Center in New York City &#; April 16-18. In addition to Megan and Ryan, the booth will have other experienced engineers to discuss precision fluid control for pharmaceutical systems as well as hands-on demonstrations and sample devices. Please make a note to come by to say hello and ask questions. As always, feel free to contact Equilibar or Steriflow engineers to discuss your fluid control challenges.

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