# Fluids Definition and 520 Threads

In physics, a fluid is a liquid or gas or other material that continually deforms (flows) under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shear force applied to them.
Liquids form a free surface (that is, a surface not created by the container) while gases do not.
Although the term "fluid" generally includes both the liquid and gas phases, its definition varies among branches. Definition of "solid" vary as well, and depending on field, some substances can be both "fluid" and "solid". Viscoelastic fluids like Silly Putty appear to behave similar to a solid when a sudden force is applied. Also substances with a very high viscosity such as pitch appear to behave like a solid (see pitch drop experiment). In particle physics, the concept is extended to include fluidic matters other than liquids or gases. "Fluid" in medicine and biology specifically means any liquid constituent of the body (body fluid), whereas "liquid" is not used in this sense. Sometimes liquids to be taken for fluid replacement, either by drinking or by injection, are also called "fluid" ("drink plenty of fluids"). In hydraulics, "fluid" is a term which refers to liquids of certain properties, and is a broader concept than "(hydraulic) oils".

View More On Wikipedia.org
1. ### A cylinder has two sides containing gas and water separately

I think this problem is somehow weird since fluid has different pressures at different depths.
2. ### B Fully submerged block in a pool of water

Let’s say we let a block of iron sink into the water and it reaches the bottom. Would the force pushing on body towards the bottom be: ##F_{downwards}=mg+F_P## where ##F_P## is the force caused by hydrostatic + atmospheric pressure. The force acting upwards should be then...
3. ### B What is my yogurt doing?

Hello, I don't know anything about fluid dynamics and hope someone can tell me what it is I saw the other day. I work in yoghurt factory and had to throw out a whole vat full of product (the culture died). So i pumped the product thru a say 70mm hose and poured it into a waste bin (which was...
4. ### Why does this question make this simplifying assumption?

The question says to "neglect the change of volume of the water". This confuses me. Why do we neglect the change of the water's volume? Let's say we didn't. A = Q/It, where A is area, Q is heat, I is intensity, t is time. Q = mc(deltatheta), mass is constant, specific heat is constant (as mass...
5. ### Pressure in fluids and Archimedes' principle

Hello. A: If we put this piece of wood in the emty container, the volume of displaced water will not change (because Buoyant Force has not changed), so the pressure at the bottom of the water container doesn't change and it remains constant. B: If we put the metal in the emty container, the...
6. ### Fluids: Concept about Fluid Pressure in a relation with Force and Area

I've already got the correct answer in letter (a), which is 17140.2 Pascals. My question will be focusing about the letter b of the question and here is my solution: (b) FORMULA: P = F/A F = P*A My understanding about this problem is I have to use the pressure that I got in letter (a) to...
7. ### B Pressure in a viscous liquid versus non-viscous fluids

Hc verma, concepts of Physics, vol 1 pg 258 "We define pressure of fluid at the point A as : ##P= F/\Delta S## For a homogeneous and non-viscous fluid, this quantity does not depend on orientation of ##\Delta S## and hence we talk of pressure at a point". Why did the author stress that the...
8. ### I do not understand stress energy tensor for fluids

I do private studies on my own for fun and right now I read about relativistic field theory as a preparation for later studies of quantum field theory. I simply do not understand where equation 13.78 in Goldstein's "Classical Mechanics" third edition comes from. Please explain. Please also...
9. ### Measuring the Speed of Sound in Water, Using Faraday's Law

Hi! This project involves both mechanical and electrical elements, so I'm discussing it in this forum since I'm not sure which one it would fit better into. I'm working on an experiment in which I'm trying to measure the speed of sound through water. The approach is simple: I have a long...
10. ### Why does less dense air rise and more dense air come down?

My answer given below seems incomplete. Since warm air causes the air to expand in volume, so its density becomes less as compared to the colder air at the top of the room. After this, I generally find all books saying the less dense air rises and more dense air from top comes down and...
11. ### I Collision-induced free-free absorption in fluids and superfluids

Lone homonuclear diatomic molecules have vibrational excitations and rotational excitations. However, due to lack of transitional dipole moment, these are strongly forbidden to absorb IR. Lone atoms don´t have the above excitations in the first place. Now, when a diatomic molecule collides with...
12. ### I Ideal Fluids & General Relativity: Why Needed?

Why in general relativity do we need the physics of perfect fluids?
13. ### Fluid Heights in an Open U-Tube

Pressure on side with liquid 1 = ##p_0 + \rho_1gh_1## Pressure on side with liquid 2 = ##p_0 + \rho_2gh_2## The solution sets the two pressures equal. I am confused how the two sides have the same amount of pressure, one would have more than the other.
14. ### B Questions regarding Magnetorheological fluids

Hello, has anyone worked with magnetorheological fluids before? I plan on creating one for a project by using carbonyl iron powder, a surfactant, and some oil. I have heard that when subject to a current, these liquids basically become solids. However, I am not sure if they become complete...
15. ### Non-Newtonian Fluids: Understanding Strength for Armor Experiments

Hello, It's been some time since I've visited and the nature of my question could cover engineering, physics or chemistry so I'm unsure where exactly to post it. I hope here is ok. I don't have the education to understand any published research on the topic and I hope someone here can provide a...
16. ### Speed/Velocity and Volume Flow Rate of Viscous Fluids

In my first attempt, I started off converting the radii of all three sections from centimeters (10, 8, 6) to meters (0.10 , 0.08 , 0.06), then used the VFR=Av formula to find the speed/velocity of section one. VFR== 0.063 m^3/s A== pi*r^2=pi*(10cm)^2=pi*(0.10)^2=pi*0.01 == 0.031415927 VFR/A=v...
17. ### Calculating Hydraulic Jump with Unknown Cross Sectional Area: Help Needed

I have 90% of this done. When i looked in my manual i noticed an odd jump they made and i don't get why. that is where i need the help V1(b1h1) = V1(b2h2) Im solving for h2. What i can't figure out is what to use for the base measurement of the cross section of the water. i have no idea how...
18. ### Tubing Pressure drops with Compressible Fluids

I am a poor, dumb EE often stuck with the odd plumbing calculation. I am often asked questions like: "what size tubing do I need to convey 10 SLPM of 20 PSIG oxygen 200' with no more than 2 PSI pressure drop?" I generally treat the fluid as in-compressible and use Darcy-Weisbach (I like Bellos...
19. ### B How Does Viscosity and Turbulence Affect Fluid Flow Between Plates?

https://openstax.org/books/university-physics-volume-1/pages/14-7-viscosity-and-turbulence I am trying to understand the derivation from Figure 14.36 which starts with " The fluid to be measured is placed between two parallel plates. The bottom plate is held fixed, while the top plate is moved...
20. ### I Fluids: Bernoulli's Equation Derivation Question

I figure that either the Force F2 is applied in the opposite direction because of some kind of resistance, but I'm not sure. Thanks!
21. ### Does Poiseuille's Law apply for vertical motion of fluids down a tube?

I investigated the flow rate of differing dilutions of glycerol through an orifice of a vertical tube and obtained the following: I'm looking for a way to quantify these results so looked to Poiseuille's Law; I'm pretty sure my graph does not show inverse proportion? Could anyone advise me as...

23. ### Frame indifference and stress tensor in Newtonian fluids

During lecture today, we were given the constitutive equation for the Newtonian fluids, i.e. ##T= - \pi I + 2 \mu D## where ##D=\frac{L + L^T}{2}## is the symmetric part of the velocity gradient ##L##. Dimensionally speaking, this makes sense to me: indeed the units are the one of a pressure...
24. ### Slip Conditions for flow between Parallel Plates

The problem states: Two parallel plates separated by distance h, the plate at the top moves with velocity V, while the one at the bottom remains stationary. My initial approach was: I considered, ##du/dy = V/h## and for the shear stress ##\tau = \mu \frac{\partial u}{\partial y}## For...
25. ### Solving the Mystery of the Ball & Beakers: Determining the Layers of Fluids

a) I think in the beaker with A and B, A must be the top fluid and B must be the bottom. The ball sinks through the top layer but not through the bottom. In the second beaker (B and C), it does not sink through either layer. Therefore, both B and C are more dense than the ball. So B must be...
26. M

### Is a Static Contact Angle Model Appropriate for Microgravity Fluid Simulations?

Hi PF! I have an experiment where a wedge about 160mm long is in microgravity. I withdraw silicone oil from the wedge at a relatively slow rate (no turbulence). Since the wedge angle is small, a lubrication approximation is made. Inertia is shown to be low. I want to simulate this flow. When...
27. ### Has Interprocess Communication Been Used for Multi-Phase Macroscopic Analyses?

I am aware that a lot of software exists to conduct such multi-phase macroscopic analyses. But has anyone ever used the tools of interprocess communication to do this? Consider, for example, these three: A Finite element code (For deformation) on a parallel machine A Dynamcis code (for...
28. ### Weight of fluids on a submerged person?

https://www.asi.edu.au/wp-content/uploads/2015/08/NQE_2009_Physics_solutions.pdf Pg 9 Q 11 A) Was wondering why there isn't a weight of water in the force diagram? The boy is submerged, as I understand. Do we normally include the weight of a fluid on someone or how does this work? For...
29. ### Engineering Fluids mechanics homework, feel like I have messed up the equations

Edit: So I have written out equation 3 again and taken a picture to show my working out, the 2 things that I am confused about is how do I find the coefficient of viscosity? I know that syrup is high viscosity and water is low viscosity but I don't know the equation to get the actual value. The...
30. M

### Fluids: Kistler contact angle model

Hi PF! I keep seeing Kistler's contact angle model wrote like this $$\theta_d = f(Ca+f^{-1}(\theta_e)) : f = \arccos\left( 1-2\tanh\left( 5.15 \left( \frac{x}{1+1.31x^{0.99}} \right)^{0.706} \right) \right)$$, where the plot then has a sort of break, something like this: But this is not the...
31. ### Explaining Pressure & Depth Relationships in Fluids Using Halliday & Resnick

Homework Statement:: Not a homework problem. I need a conceptual explanation. Relevant Equations:: ##p_2 = p_1 + \rho g(y_1 - y_2)## (1) ## p = p_0 + \rho gh## (2) When deriving equation (1) we use the example of a submerged cylinder in static equilibrium with its top at position ##y_1## and...
32. ### Classical Analyze Dampened Oscillations in Fluids: Math & Physics

I am a layman with very little experience in math and physics and recently I became curious about how to analyze dampened oscillations occurring in fluid mediums, such as those following a disturbance in a pool of water. What sort of math and physics is required to understand this phenomenon and...
33. ### What Is the Correct Equation for Equilibrium Temperature of Two Fluids?

Not really sure how to start this one.
34. ### Fluid Mechanics: Fluid Transfer Between Two Reservoirs at Different Heights

A reservoir supply tunnel transfers water from a high level storage reservoir via a 200mm diameter pipe to a lower intermediate reservoir. The total height difference between the free surfaces of each reservoir is 76 m. The pipe is 700 m long and the coefficient of friction, Cf is 0.009. The...
35. ### Three doubts in a paragraph on the equilibrium of Incompressible fluids

These are the images from Sommerfeld’s Lectures on Theoretical Physics, Vol 2 chapter 2, section 6, Equilibrium of Incompressible Fluids. Image 1 Image 2 Doubt 1 : What does it mean for a force to act on a fluid volume? Force acts on a point, force may act on a surface but I’m unable to...
36. ### Fluid dynamics and incompressible fluids

Hi sorry about the way I've posted I'm new to this site. Anyway basically I've been set this question which should be attached to this post, I have attempted to do this question but I'm having trouble in forming an equation in the first place. I'm unsure where to start, I understand I need to...
37. ### Engineering Find out the percent exergy loss

Problem, with state values, and pie chart (Fig 4.20) showing answers: ^ This shows the system in question (Kapitza Liquefaction System). Methane gas enters into the compressor (c), then goes through the first heat exchanger (HX1). Some of it (z) gets routed to the expander (exp). Afterwards...
38. ### Engineering Kinematic and geometric similarity (fluids)

My attempt at a solution is to start off first denoting V_a to be the automobile an V_e to be the economy version. Same goes with l_a and l_e. To try and relate the two I have tried: V_a I_a = V_l L_e, however I am really not sure how they got the square root. The answer is: v = V sqrt(l/L)...
39. ### A question about non-Newtonian fluids in a "Transport Phenomena" book

In this topic >> https://www.physicsforums.com/threads/stress-tensor-for-non-Newtonian-fluid.860240/ Mr. Chestermiller replies that "For a purely viscous non-Newtonian fluid (not viscoelastic), you use exactly the same form of equation as for a Newtonian fluid (see Bird, Stewart, and...
40. M

### How can I pin the fluid-solid contact line in OpenFOAM?

Hi PF! Does anyone know how to pin the fluid-solid contact line in OpenFOAM? There has to be a way to do this, and I've tried the cfd-online community but they're largely inactive and consequently no one has replied to my question. Any help or guidance is greatly appreciated!
41. ### Intro Physics Which Fluid Mechanics Textbook is Best for High School Students?

Can anyone suggest me a Fluid Mechanics textbook? I am currently in High School in my Junior Year. I am looking for a entry-level textbook so I will be able to solve I.E. Irodov's Hydrodynamics section. (My Mathematics is quite strong)
42. ### Force on side of tank filled with liquid of variable specific weight

Homework Statement: An open tank with a rectangular side 1 m wide and 4 m high is filled with a liquid of variable specific weight, γ, with γ = 50 + 2y (N/m3), where y is measured vertically downward from the free surface. Find the magnitude of the force on the side of the tank. Homework...
43. ### Dynamic Viscosity in Reynolds Number

I am trying to run a calculation to work out the overall heat transfer coefficient of an arbitrary ten plate exchanger where the fluid is not determined using Re=puD/mu where mu is dynamic viscosity. When mu is such a strong function of temperature how should I determine which valye to use, say...
44. ### Calculating the force from a pump + nozzle system

I am attempting to calculate the force (in KgF) of a water pump + nozzle system. I have the pump curve (head vs. flow rate) for the pump, the diameter of the connection hose (2 inches) and the nozzle diameter (1 inch). The connection hose between the pump and nozzle is very short (approx 10 cm)...
45. ### Gauge pressure in Momentum conservation of fluids.

I want to ask why is it that we use gauge pressure instead of absolute pressure in CV analysis for momentum conservation of fluids. I did read that because P(atm) would be present everywhere so it won't have a net effect on the CV but it's highly non intuitive as I can't apply force balance on...
46. M

### What Characteristic Length Should Be Used for Fluids in Low Gravity?

Hi PF! Fluids in low gravity have a natural oscillation frequency ##\lambda = \sqrt{\sigma / (\rho L^3)}##, where ##\sigma## is surface tension, ##\rho## density, ##L## characteristic length. Then given a particular object, say a sphere, is ##L=D## or ##L=r##? How about a channel; would ##L##...
47. ### Understanding Shearing Stress & Tangential Forces on Fluids

what does it mean that fluid cannot sustain a force that is tangential to its surface.which surface fluids's surface or container? and what does it mean it cannot withstand shearing stress.what's shearing stress and what does it mean here.and why exert force only in direction perpendicular to...
48. ### Understand Pressure in Fluids: Conceptual Guide

I'm looking for a conceptual understanding of pressure in fluid. According to what I've gathered, in static fluid, the pressure at any point in the fluid depends on the depth within the fluid, because there is more fluid weighing down on an object the deeper into the fluid it is. However, for...
49. ### Mixing Fluids in Flow: How to Achieve a Fixed Concentration | Sanatan

Hello, I am trying to mix two fluids which are in flow. To be exact, I am trying to mix a concentrated mineral solution to an outlet of my RO filter. Is there a method to mix the two fluids to get a fixed concentration of the mixed solution ? Regards, Sanatan