Tag: Newton’s law of viscosity

🛠️ EPCLAND WORKSPACE CONTROL PANEL ⚠️ DELETE THIS ENTIRE CONTAINER BOX BEFORE PUBLISHING THE BLOG POST Hero Image: Purpose: To visually introduce the concept of fluid viscosity and resistance to flow using a clear, real-world comparison. Description: A high-quality close-up shot of two different liquids, such as thick honey and thin water, being poured simultaneously to demonstrate different levels of fluid resistance. SEO Alt Text: Comparison of high viscosity honey and low viscosity water pouring to demonstrate fluid resistance Image Slug: comparing-fluid-viscosity-levels Filename URL: https://epcland.com/wp-content/uploads/2026/06/comparing-fluid-viscosity-levels.jpg Technical Infographic: Purpose: To illustrate the mathematical and physical concept of Newton's Law of Viscosity using a fluid shear diagram. Description: A clear, labeled vector diagram showing fluid layers between a stationary plate and a moving plate, illustrating shear stress, velocity gradient, and laminar flow. SEO Alt Text: Diagram of Newton's law of viscosity showing fluid shear stress between parallel plates Image Slug: newtons-law-of-viscosity-diagram Filename URL: https://epcland.com/wp-content/uploads/2026/06/newtons-law-of-viscosity-diagram.jpg Meta Data: Focus Keyword: Newton's law of viscosity Title: What is Viscosity? Factors Affecting It and Newton's Law Explained Slug: newtons-law-of-viscosity Meta Description: Learn what viscosity is, explore the key factors affecting fluid flow, and understand Newton's law of viscosity with clear engineering examples. Tags: viscosity, fluid mechanics, Newton's law of viscosity, fluid flow, dynamic viscosity, kinematic viscosity, engineering physics Author: Atul Singla | Piping Engineering Expert | Updated: May 2026 Understanding Newton's Law of Viscosity and Key Fluid Flow Factors Fluid Viscosity Fundamentals: Viscosity represents the internal resistance of a fluid to shear deformation under applied stress, serving as a foundational parameter for piping design and hydraulic calculations governed by ASME B31.3 and API standards. In my 20 years of designing piping systems and troubleshooting fluid transport lines, I have seen many young engineers make the mistake of treating every fluid like water. When you are dealing with heavy crude oil, polymer solutions, or high-pressure chemical lines, ignoring how a fluid deforms under shear can lead to catastrophic pump failures, ruptured lines, and massive pressure drops. Viscosity is not just a theoretical number in a textbook; it is the single most critical physical property that dictates how your pumps, valves, and pipes will behave in the field. Whether you are sizing a centrifugal pump under API 610 guidelines or calculating line losses for a refinery expansion, you must understand how viscosity changes with temperature, pressure, and shear rate. Let us break down the core mechanics of fluid resistance, explore the factors that alter fluid behavior, and master the application of Newton's law of viscosity in real-world engineering scenarios. What You Will Learn in This Guide: The physical definition of dynamic and kinematic viscosity and how to convert between them. How temperature and pressure affect the viscosity of liquids versus gases. The mathematical formulation of Newton's law of viscosity and its practical limits. How to distinguish between Newtonian and Non-Newtonian fluids in piping design. Real-world field calculations and troubleshooting strategies for highly viscous systems. Interactive Engineering Quiz EPCLAND Portal Question 1 of 3 Under Newton's law of viscosity, the shear stress ($\tau$) in a fluid is directly proportional to the rate of shear deformation (velocity gradient). For a Newtonian fluid undergoing steady, laminar flow between two parallel plates (Couette flow) where the upper plate moves at a constant velocity $U$ and the lower plate is stationary, separated by a distance $h$ with no pressure gradient, which of the following statements correctly describes the velocity profile and the shear stress distribution? The velocity profile is parabolic, and the shear stress is maximum at the stationary plate and zero at the moving plate. The velocity profile is linear, and the shear stress is constant throughout the fluid gap. The velocity profile is logarithmic, and the shear stress varies linearly across the gap. The velocity profile is linear, and the shear stress varies linearly from zero at the stationary plate to a maximum at the moving plate. Next Question → Question 2 of 3 The dynamic viscosity of fluids is highly sensitive to temperature changes, but liquids and gases exhibit opposite behaviors. Which of the following statements accurately explains the physical mechanisms governing these temperature dependencies? In liquids, cohesive intermolecular forces dominate, which weaken as temperature increases, decreasing viscosity; in gases, molecular momentum transfer dominates, which increases with molecular thermal velocity, increasing viscosity. In liquids, molecular momentum transfer dominates, which decreases as temperature increases; in gases, cohesive forces dominate, which strengthen as temperature increases. Both liquids and gases experience a decrease in viscosity with temperature due to the expansion of intermolecular spacing, but gases recover viscosity at high pressures. In liquids, viscosity decreases because the activation energy for molecular shear increases; in gases, viscosity increases because the mean free path decreases with temperature. Next Question → Question 3 of 3 While temperature has a pronounced effect on viscosity, pressure also influences it, particularly at extreme ranges. Which of the following statements correctly describes the effect of high pressure on the dynamic viscosity of typical lubricating oils (liquids) and ideal gases? High pressure significantly decreases the viscosity of lubricating oils due to molecular compression, while having no effect on ideal gases. High pressure exponentially increases the viscosity of lubricating oils (described by the Barus equation), while the dynamic viscosity of an ideal gas remains independent of pressure. High pressure has negligible effect on the viscosity of lubricating oils, but significantly decreases the viscosity of ideal gases due to reduced mean free path. High pressure linearly decreases the viscosity of both lubricating oils and ideal gases due to the suppression of turbulent fluctuations. 🎉 Quiz Completed! You have passed the engineering review criteria. Fundamental Principles of Fluid Shear Resistance How Does Newton's Law of Viscosity Govern Fluids? Newtonian Shear Behavior: Newton's law of viscosity dictates that the shear stress between adjacent fluid layers is directly proportional to the negative velocity gradient perpendicular to the direction of flow. To understand how fluids behave inside a pipe, we must look at the molecular level. When a fluid flows, adjacent layers slide past one another. The friction between these layers is what we call viscosity. Isaac Newton formulated this relationship by observing fluid trapped between two parallel plates—one stationary and one moving at a constant velocity. According to Newton's law of viscosity, the shear stress (represented by the Greek letter tau) is directly proportional to the rate of shear strain (the velocity gradient, du/dy). Mathematically, we write this as: Shear Stress (tau) = Dynamic Viscosity (mu) * Velocity Gradient (du / dy) Where: Shear Stress (tau): The force applied per unit area parallel to the fluid layers, measured in Pascals (Pa) or Newtons per square meter. Dynamic Viscosity (mu): The proportionality constant representing the fluid's internal resistance, measured in Pascal-seconds (Pa-s) or Centipoise (cP). Velocity Gradient (du/dy): The change in velocity (du) over the distance between layers (dy), measured in reciprocal seconds (1/s). This is also known as the shear rate. Dynamic vs. Kinematic Viscosity In my design work, I constantly see engineers confuse dynamic viscosity with kinematic viscosity. Dynamic (or absolute) viscosity measures a fluid's internal resistance to flow under an external force. Kinematic viscosity (represented by the Greek letter nu) is the ratio of dynamic viscosity to the fluid's density (rho): Kinematic Viscosity (nu) = Dynamic Viscosity (mu) / Density (rho) Kinematic viscosity represents how fast a fluid spreads or flows under the influence of gravity alone. Its SI unit is square meters per second (m^2/s), but in the field, we almost always use Centistokes (cSt), where 1 cSt equals 1 square millimeter per second. Field Warning: Temperature-Viscosity Sensitivity In heavy fuel oil (HFO) transfer lines, a drop in temperature of just 10 degrees Celsius can cause the viscosity to spike by over 300 percent. If your heat tracing system fails, your transfer pumps will lock up due to high torque demand. Always design your piping systems for the worst-case minimum operating temperature, not just the nominal process temperature. Factors Affecting Fluid Viscosity Viscosity is not a static property. It changes dynamically based on several environmental and physical factors: Temperature: This is the most dominant factor. In liquids, cohesive forces between molecules decrease as temperature rises, causing viscosity to drop. In gases, molecular collisions increase with temperature, which actually increases gas viscosity. Pressure: For most liquids, viscosity is relatively independent of pressure at low to moderate ranges. However, at extreme pressures (such as in deep-well oil extraction or high-pressure hydraulics), liquid molecules are compressed closer together, increasing cohesive forces and raising viscosity. For gases, viscosity increases directly with high pressures. Shear Rate: For Newtonian fluids, viscosity remains constant regardless of the shear rate. For Non-Newtonian fluids, viscosity can decrease (shear-thinning/pseudoplastic) or increase (shear-thickening/dilatant) as the shear rate increases. Viscosity Values for Common Industrial Fluids The table below provides typical dynamic and kinematic viscosity values for common industrial fluids at standard atmospheric pressure. These values are critical when performing line sizing calculations under ASME B31.3. Fluid Name Temperature (C) Dynamic Viscosity (cP) Density (kg/m^3) Kinematic Viscosity (cSt) Flow Behavior Water 20 1.00 998.2 1.00 Newtonian Water 80 0.35 971.8 0.36 Newtonian Lube Oil (SAE 30) 20 290.00 890.0 325.80 Newtonian Glycerin 20 1490.00 1261.0 1181.60 Newtonian Heavy Crude Oil 15 450.00 920.0 489.10 Non-Newtonian (Shear-Thinning) Technical Mapping & Specifications Matrix This matrix maps fluid properties to their corresponding testing standards and engineering applications. Use this to select the correct testing protocol for your project specifications. Property / Parameter Standard Reference Primary Measurement Device Engineering Application Kinematic Viscosity ASTM D445 / ISO 3104 Glass Capillary Viscometer Piping pressure drop, Reynolds number calculations Dynamic Viscosity ASTM D2196 Rotational Viscometer (Brookfield) Pump shaft power, agitator torque sizing High-Shear Viscosity ASTM D4683 Tapered Bearing Simulator Engine oil lubrication, high-speed bearing design Viscosity Index (VI) ASTM D2270 Calculated from ASTM D445 data Lubricant selection for wide temperature ranges Field Verification Checklist for Viscous Fluids Applying Newton's Law of Viscosity in Design Viscosity Verification Protocol: Field verification of fluid viscosity parameters ensures that piping pressure drop calculations and pump selections comply with hydraulic design limits specified in API 610 and ASME B31.3. Before finalizing any piping isometric or pump datasheet, you must verify that your fluid properties match the actual field operating conditions. Use this checklist during the detailed engineering phase to prevent costly field modifications. Piping Design & Viscosity Checklist: Verify Fluid Classification: Confirm if the fluid is Newtonian or Non-Newtonian. If Non-Newtonian, obtain the shear-rate vs. viscosity curve from the lab. Check Minimum Ambient Temperature: Ensure the piping heat tracing system is sized to maintain the fluid viscosity below the maximum limit of the pump. Calculate Reynolds Number at Worst Case: Use the highest viscosity (at lowest operating temperature) to verify if the flow is laminar, transitional, or turbulent. Size Pump for Viscous Derating: Apply the Hydraulic Institute (HI) viscosity correction factors if using a centrifugal pump for fluids above 10 cSt. Review Valve Pressure Drops: Ensure control valves are sized using the correct viscosity correction factors to prevent cavitation and flashing. Confirm Lab Test Standards: Verify that the viscosity values on the process datasheet were measured using ASTM D445 or equivalent standards. Validate Pipe Wall Shear Stress: Ensure the calculated wall shear stress does not exceed the erosion-corrosion limits of the selected pipe material. Industrial Case Study: Viscous Fluid Transport Field Case Study: Real-World Application The Problem: A chemical processing plant in northern Canada experienced repeated motor trips on their heavy polymer transfer pumps during winter. The original design team had calculated the line pressure drop using the polymer's nominal viscosity at 25 degrees Celsius (150 cSt). However, during winter, the uninsulated piping dropped to 5 degrees Celsius, causing the polymer's viscosity to skyrocket to 1,200 cSt. The centrifugal pumps could not handle the massive shear resistance, leading to motor overload and zero flow. The Solution: I was brought in to audit the system. We first replaced the centrifugal pumps with positive displacement screw pumps, which handle high-viscosity fluids much more efficiently. Next, we installed electric heat tracing and polyurethane insulation on the entire 300-meter piping run to maintain a constant fluid temperature of 40 degrees Celsius. This kept the viscosity stable at 80 cSt, well within the new pump's optimal operating envelope. Direct Recommendation: Never assume your process fluid will always remain at its design temperature. Always perform a sensitivity analysis on your viscosity values. If your fluid is highly temperature-sensitive, insulate your lines and consider positive displacement pumps instead of centrifugal pumps. Frequently Asked Engineering Questions What is the difference between Newtonian and Non-Newtonian fluids? A Newtonian fluid has a constant viscosity that does not change with the shear rate (e.g., water, mineral oil). A Non-Newtonian fluid's viscosity changes as the shear rate changes (e.g., polymer solutions, ketchup, drilling muds). For Non-Newtonian fluids, you cannot use standard hydraulic formulas without adjusting for the specific shear rate of your piping system. How does temperature affect gas viscosity compared to liquid viscosity? As temperature increases, liquid viscosity decreases because the cohesive intermolecular forces weaken. In contrast, gas viscosity increases with temperature because the gas molecules move faster and collide more frequently, increasing momentum transfer between fluid layers. Why is kinematic viscosity used more often in piping design than dynamic viscosity? Kinematic viscosity is preferred because it directly incorporates the fluid's density, which is required for calculating the Reynolds number and friction factors in pipe flow. Most industrial viscometers, like those specified in ASTM D445, measure kinematic viscosity directly by timing the gravity-driven flow of fluid through a capillary tube. What are the units of viscosity in the SI and Imperial systems? In the SI system, dynamic viscosity is measured in Pascal-seconds (Pa-s) or milliPascal-seconds (mPa-s), which is equivalent to Centipoise (cP). Kinematic viscosity is measured in square meters per second (m^2/s) or Centistokes (cSt). In the Imperial system, dynamic viscosity is measured in pound-force seconds per square foot (lb-s/ft^2), and kinematic viscosity is measured in square feet per second (ft^2/s). How does high viscosity affect centrifugal pump performance? High viscosity significantly reduces a centrifugal pump's flow rate, head capacity, and efficiency while dramatically increasing the required shaft horsepower. When pumping fluids with a viscosity greater than 10 cSt, you must apply correction factors from the Hydraulic Institute standards to prevent motor overload. What is the Viscosity Index (VI) and why does it matter? The Viscosity Index is an empirical, unitless measure of how much a fluid's viscosity changes with temperature. A high VI indicates that the fluid's viscosity remains relatively stable over a wide temperature range, which is highly desirable for lubricants and hydraulic fluids operating in extreme outdoor environments.