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In simple words, a Newtonian fluid is a liquid or gas whose thickness (viscosity) stays the same when you stir it slowly or fast, as long as temperature and pressure do not change.
If you double the shear rate (how fast layers of fluid slide over each other),
Everyday Newtonian fluids:
These fluids behave in a simple, linear, and predictable way, so they are very important in fluid mechanics, pipe flow, and engineering exam numericals.
Think of a glass of water and a bottle of ketchup.
When you stir water slowly or fast, it always feels the same. Its thickness does not change.
Such a liquid is called a Newtonian fluid.
Its (गाढ़ापन) (viscosity) stays the same even if you apply more force.
But ketchup at home behaves differently.
In school life terms:
Engineers use these ideas to design water pipelines, pumps, machines and hydraulic systems that you see everywhere in India.
| Language | Word or phrase used | Simple explanation in that language | What it relates to |
|---|---|---|---|
| Hindi | न्यूटनियन द्रव (Newtonian drav) | ऐसा द्रव जिसकी गाढ़ापन एक जैसा रहता है, चाहे आप उसे तेज हिलाएं या धीरे हिलाएं | द्रव यांत्रिकी, इंजीनियरिंग, पानी की पाइपलाइन |
| Marathi | न्यूटनियन द्रव | असा द्रव ज्याची घट्टपणा (viscosity) बदलत नाही, गती काहीही असो | फ्लुइड मेकॅनिक्स, पाणीपुरवठा, हायड्रॉलिक सिस्टम |
| Tamil | நியூட்டனிய திரவம் (Newtonian thiravam) | எவ்வளவு வேகமாகக் கிளறினாலும் அதன் கனம் மாறாத திரவம் | திரவ இயந்திரவியல், குழாய் ஓட்டம் |
| Kannada | ನ್ಯೂಟೋನಿಯನ್ ದ್ರವ (Newtonian drava) | ಎಷ್ಟು ಬಲದಿಂದ ಕೆದಕಿದರೂ ದ್ರವದ ಗಟ್ಟಿತನ ಬದಲಾಗದ ದ್ರವ | ಇಂಜಿನಿಯರಿಂಗ್, ಹೈಡ್ರಾಲಿಕ್, ನೀರಿನ ಸರಬರಾಜು |
| Bengali | নিউটনীয় তরল (Newtoniyo torol) | যতই নাড়ুন, যার ঘনত্ব একই রকম থাকে, সেই তরল | ফ্লুইড মেকানিক্স, পাইপে জল প্রবাহ |
| Gujarati | ન્યુટોનિયન પ્રવાહી (Newtonian pravāhī) | જેને તમે ધીમે કે તેજ હલાવો તો પણ એની ચીકાશ એકસરખી રહે | પ્રવાહી ગતિશાસ્ત્ર, પાણીની લાઇન, તેલ પ્રવાહ |
| Telugu | న్యూటోనియన్ ద్రవం (Newtonian dravam) | ఎంత బలంగా కలిపినా దాని సాంద్రత మారని ద్రవం | ద్రవ గతిక శాస్త్రం, పైప్ లైన్, హైడ్రాలిక్స్ |
| Malayalam | ന്യൂട്ടോണിയൻ ദ്രാവകം (Newtonian drāvakam) | എത്ര ശക്തിയായി കലക്കിയാലും കട്ടിപ്പ് മാറാത്ത ദ്രാവകം | ദ്രാവ ഗതിശാസ്ത്രം, ജലവിതരണ പൈപ്പ്, എഞ്ചിനീയറിംഗ് |
Technically, a Newtonian fluid is defined using Newton's law of viscosity:
τ = μ (du/dy)
Where:
For a Newtonian viscous fluid, the relation between shear stress (τ) and shear rate (du/dy) is:
This simple relation was first proposed by Sir Isaac Newton, so such fluids are called Newtonian fluids.
Before going further, remember three basic terms:
1. Viscosity (μ)
2. Shear stress (τ)
3. Shear rate (du/dy)
For a Newtonian fluid, viscosity is constant, and:
Shear stress ∝ shear rate
In industrial mixing, the idea of a Newtonian fluid is directly used when designing an industrial agitator, because most standard formulas for power, Reynolds number and flow patterns assume that viscosity stays constant. For common Newtonian liquids like water, thin oils and many solvents, engineers can reliably use these Newtonian based correlations to choose impeller type, size, speed and motor power, and to scale up from a lab mixer to a large industrial agitator. However, when the liquid is non Newtonian, such as thick slurries, creams, gels or ketchup type fluids, viscosity changes with shear rate near the impeller, so a single constant μ is no longer valid. In those cases the designer must use rheological data, apparent viscosity and often special low speed, high torque impellers to achieve proper mixing, heat transfer and suspension, instead of directly applying simple Newtonian fluid design rules.
Consider two large parallel plates with a fluid between them:
If the flow is steady and laminar, the velocity between plates changes linearly from 0 to U. There is internal friction between the layers. Because of this friction:
Newton proposed that:
This is Newton's law of viscosity. For a Newtonian fluid, μ is a constant at a given temperature and pressure.
τ = μ (du/dy)
τ (shear stress)
μ (dynamic viscosity)
du/dy (shear rate)
Example numerical (very exam-like):
Water flows between two horizontal plates 1 mm apart.
Velocity at lower plate = 0 m/s, velocity at upper plate = 1 m/s.
Take μ for water ≈ 0.001 Pa·s.
Shear rate:
Shear stress:
So the shear stress in the water is 1 Pa.
This same concept is extended to pipe flow of Newtonian fluids (like water in pipelines), which is a very important topic for SSC JE, RRB JE, GATE and university exams.
Isaac Newton developed the idea that shear stress is proportional to shear rate for certain fluids.
Fluids that obey this simple linear law are called Newtonian fluids.
He laid the foundation for Newtonian mechanics, which includes:
Because of him, we now have Newtonian fluid models and Navier–Stokes equations for fluid flow.
Common Newtonian fluid examples you see daily:
If you stir any of these slowly or very fast, the fluid does not suddenly become extra thick or super thin. Its viscosity stays nearly constant, so it behaves as a Newtonian liquid.
In engineering and industry, many working fluids are approximated as Newtonian:
These behave nearly as Newtonian viscous fluids under normal operating conditions, which makes calculations and design easier.
Some fluids are not perfectly Newtonian, but in a certain range of shear rates they behave almost like Newtonian. For engineering design, we often treat them as Newtonian:
This approximation is widely used in pipe flow, pump sizing, and heat exchanger design.
Newtonian fluid:
Non Newtonian fluid:
Important types of non Newtonian fluid (very exam-relevant):
1. Shear thinning / pseudoplastic fluids
2. Shear thickening / dilatant fluids
3. Bingham plastic / ideal plastic fluid
4. Thixotropic fluids
5. Rheopectic (rheopexic) fluids
An ideal plastic fluid (or Bingham plastic fluid) is not Newtonian. Its behaviour:
Examples:
In exams, remember: Bingham plastic = non Newtonian fluid with yield stress.
When bubbles, droplets or solid particles move through non Newtonian fluids, their motion is more complex than in Newtonian fluids because:
This is important in:
For exam level, just remember: non Newtonian behaviour affects bubbles, drops and particles significantly, making analysis more complicated than for Newtonian fluids.
For Newtonian fluid flow in a circular pipe under laminar conditions:
This is the standard model for water flow and other Newtonian fluids in small pipes at low velocities.
For non Newtonian fluids:
Therefore:
Because viscosity behaviour is different:
For Newtonian fluids, calculations are simpler and more standard, which is why water and other Newtonian fluids are often preferred where possible.
Many low-viscosity process fluids are Newtonian or nearly Newtonian:
In India, these appear in:
Newtonian behaviour makes pump and pipeline design, filling and mixing operations more predictable.
Newtonian hydraulic oils and lubricants are used in:
Because viscosity is reasonably constant:
For Indian students, Newtonian fluid is a very common topic in:
Typical questions:
Common exam patterns:
In jobs related to:
Engineers routinely assume some fluids as Newtonian, especially water and light oils, and design systems based on Newtonian fluid mechanics.
Understanding Newtonian behaviour helps in:
| Property | Newtonian fluid | Non Newtonian fluid | Simple example |
|---|---|---|---|
| Viscosity behaviour | Constant with shear rate | Changes with shear rate or time | Water (Newtonian), ketchup (non Newtonian) |
| Shear stress vs shear rate relation | Linear (straight line) | Non-linear (curved) | Graph passes through origin for Newtonian |
| Formula | τ = μ (du/dy), where μ is constant | Complex models needed | Newton's law vs power law, Bingham model |
| Design calculations | Simple and predictable | Complex, needs rheological data | Easy pump selection vs specialized analysis |
| Fluid | State (liquid/gas) | Approximate viscosity at room temperature | Typical use in India |
|---|---|---|---|
| Water | Liquid | 0.001 Pa·s | Drinking water, cooling, irrigation |
| Air | Gas | ~0.000018 Pa·s | Ventilation, AC systems, wind engineering |
| Thin engine oil | Liquid | 0.05–0.1 Pa·s (approx.) | Vehicle lubrication, machine tools |
| Ethanol (alcohol) | Liquid | ~0.0012 Pa·s | Solvents, pharmaceutical industry |
| Kerosene | Liquid | ~0.002 Pa·s | Fuel for lamps, stoves, jet engines |
| Type | Behaviour of viscosity | Simple everyday example | Typical application |
|---|---|---|---|
| Shear thinning (pseudoplastic) | Viscosity decreases with increasing shear rate | Ketchup, shampoo | Food processing, cosmetics |
| Shear thickening (dilatant) | Viscosity increases with increasing shear rate | Cornstarch + water (oobleck) | Body armor, impact protection |
| Bingham plastic | Has yield stress, then flows with constant viscosity | Toothpaste, drilling mud | Drilling operations, consumer products |
| Thixotropic | Viscosity decreases with time at constant shear | Some paints, gels | Paints, printing inks |
| Rheopectic | Viscosity increases with time at constant shear | Some lubricants | Specialized industrial applications |
| Application area | Newtonian fluid used | Real life Indian example | Why Newtonian behaviour helps |
|---|---|---|---|
| Hydraulic systems | Hydraulic oil | JCB, excavators at construction sites | Predictable fluid behaviour makes design and control easier |
| Water supply | Water | Municipal pipelines, irrigation canals | Simple calculation of head loss and pump requirements |
| Industrial processing | Solvents, light oils | Chemical plants, refineries | Standard formulas work for mixing, pumping, heat transfer |
| Lubrication | Machine oils | Gearboxes, bearings in factories | Consistent performance and maintenance scheduling |
| Cooling systems | Water, coolants | Thermal power plants, AC systems | Reliable heat transfer calculations |
| Formula | Symbol | Meaning | SI unit | Example value |
|---|---|---|---|---|
| τ = μ (du/dy) | τ | Shear stress | Pa (N/m²) | 1 Pa in the example calculation |
| τ = μ (du/dy) | μ | Dynamic viscosity | Pa·s | 0.001 Pa·s for water at 20°C |
| τ = μ (du/dy) | du/dy | Shear rate (velocity gradient) | s⁻¹ | 1000 s⁻¹ in the example calculation |
Ask yourself:
If yes → Most likely Newtonian (for that operating range).
A Newtonian fluid is a liquid or gas whose viscosity stays constant, so the shear stress is directly proportional to shear rate. Water and air are common examples of Newtonian fluids.
In a Newtonian fluid, viscosity is constant and the relation between shear stress and shear rate is linear. In non Newtonian fluids, viscosity changes with shear rate or time, so the relation is non-linear.
Common non Newtonian fluids are ketchup, toothpaste, paint and blood. Their viscosity changes when you shake, press or stir them, so they are not Newtonian.
An ideal plastic fluid (Bingham plastic) behaves like a solid until a certain yield stress is crossed, and then flows like a fluid. It is a type of non Newtonian fluid, not a Newtonian one.
For Newtonian fluid flow, velocity profile and pressure drop can be calculated using simple formulas because viscosity is constant. For non Newtonian fluids, viscosity changes with shear rate, so special rheological models must be used to calculate velocity profile and pressure drop.
Newtonian fluids like water and low-viscosity oils are used in water supply pipelines, cooling systems, hydraulic systems, lubrication in machines, and many process industries across India.
Blood is usually treated as a non Newtonian fluid because its viscosity changes with shear rate and it shows shear thinning behaviour.
Informally, some people say anti Newtonian fluid for fluids that show behaviour strongly opposite to simple Newtonian fluids, for example strongly shear thickening suspensions, where viscosity increases sharply with shear rate.
Newtonian fluids directly follow Newtonian physics and Newton's law of viscosity, so they are easier to model mathematically. Questions on Newtonian fluid meaning, Newtonian fluid flow, and comparison of Newtonian and non Newtonian fluids are very common in mechanical and civil engineering exams in India.