What is Fluids: Definition and 518 Discussions

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".

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  1. S

    Which Phase Can Withstand a Force?

    Homework Statement Which of the following phases is capable of permanently withstanding a force perpendicular to its surface? A. Gas B. Liquid C. Solid D. All of the above Homework Equations The Attempt at a Solution I understand that the answer is D. However, what does...
  2. U

    Mean Occupation Number of Non-interacting quantum fluids

    Hi, I get two contradicting sections of the book "Concepts in Thermal Physics": Earlier in the section they used the grand partition function to derive the mean occupation number ##<n_i> = -\frac{1}{\beta}\frac{\partial ln Z}{\partial E_i}## Later in the section, they said ##<n_k> =...
  3. N

    Euler equations for ideal fluids, approximations

    Homework Statement The Euler equations for ideal compressible flow are given by \partial_t v + (v\cdot \nabla)v = g-\frac{1}{\rho}\nabla p \\ \partial_t \rho + \nabla \cdot(\rho v) = 0 In my book these are written in terms of the small-value expansions \rho = \rho_0 + \delta \rho, p = p_0 +...
  4. M

    MHB Torricelli's Law of Fluids: Solving a Differential Equation for Drainage Time

    I am working on this project for Diff eq's and I seem to be stuck. i think I am messing something up but I need a fresh pair of eyes to show me where it is. imgur: the simple image sharer imgur: the simple image sharer (this is the same page as ^ just scrolled down) imgur: the simple image...
  5. P

    How does the viscosity of a fluid change with temperature in a rotating system?

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  6. R

    Question about fluids and Bernoulli's Equation

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  7. C

    Good resources for more advanced fluids?

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  8. L

    Ideal fluids, bernouilli's law

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  9. S

    Dynamics vs Thermo Fluids for EE

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  10. N

    Why do free-falling fluids contract?

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  11. E

    Simulating the Behavior of Rotating Fluids: CFD vs ALE

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  12. E

    Fluids question: U Shaped Tube.

    Homework Statement (1.1.10) The diagram below shows a U-shaped tube. The fluids in the tube are glycerine (blue shading) and mercury (gray shading). The height of the glycerine column on the left side of the tube is h1 = 5.00 cm. The density of the mercury is 13.53 g/cm3 . (a) What is h2...
  13. E

    Engineering Master in Maths seeking engineering job in Fluids Dynamics area

    I did BA Maths at Cambridge with a First Class back in 2009-12, worked in i-banking for one year (partially because I need to fund my graduate study) and will continue with a Master in Maths at Cam this coming autumn. My courses had been (and will continue to be) concentrated in Fluid Dynamics...
  14. E

    Fluids Problem - Dimensional analysis, pipe flow, propeller

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  15. D

    Fluids Problem - Atmospheric Tank Pressure Vent

    Homework Statement Theres a atmospheric tank with a goose neck vent on the top and also an inlet pipe [a diameter 2 inch schedule 40] on the top. There is a compressed air line [from a 50 psi compressor] going into the inlet. The pressure inside the tank should not be above 2 psi. Size the...
  16. L

    Pressure on manometer with different fluids

    Homework Statement The fluid in a manometer tube is 40% water and 60% alcohol (specific gravity = 0.8). What is the manometer fluid height difference if a 50kPa pressure difference is applied across the two ends of manometer? Homework Equations P1-P2=50 kPa P3-P2=(ρgh)alc P3-P1=(ρgh)h2o...
  17. T

    Fluids: potential flow, calculating gauge pressure from two sources

    Homework Statement In a plane onset flow of 4 m/s, a source of strength 13 m2/s per metre is located at the origin (x=0, y=0) and another source of strength 18 m2/s per metre is located at (x=3m, y=0). Calculate the gauge pressure at the point (0, 2m), Take ρ =1.2kg/m3 Homework...
  18. N

    Refractive index of two fluids when mixed

    Hi all! I was wondering what will happen if you have two aqueous solutions with the same refractive index and you mix them? Would you end up with the same refractive index as the both solutions or will the refractive index change? In my case I'm using a aqueous 40% wt. potassium...
  19. E

    FLUIDS: Potential Flow Question - HELP NEEDED

    FLUIDS: Potential Flow Question -- URGENT HELP NEEDED I need to submit this soon and I am way behind in this topic. I would appreciate it a lot if you can help me out figure the answer. Homework Statement In a uniform onset flow of 7 m/s parallel with the x axis, a source of strength 19...
  20. M

    Question about formulas in fluids

    When using bernoullis and momentum, gauge pressure is only used in calculations, any atmospheric pressure is generally omitted, i could understand if, for example, using bernoullis, pressure 1 and pressure 2 were both atmospheric, therefore they would cancel out, but when one pressure is...
  21. E

    Are there multiple pressures within the human body?

    I recently learned about the pressure of the atmosphere against the human body. It just so happens that this force is in equilibrium with the human body, so we do not feel any pressure. Although if we fly an airplane, we do. This got me to thinking, what of the blood in our veins? Does...
  22. C

    Why is rigid body rotational energy not exactly applicable to fluids?

    I was thinking about the rotational kinetic energy of fluids the other day and I realized that I have a huge gap in my knowledge of physics. Why doesn't rigid body rotational kinetic energy (KE = 1/2*I*ω^2) not apply to fluids or deformable bodies (it should at least be proportional to that...
  23. R

    Fluid Mechanics: Compressible Fluids, density function

    Homework Statement "Suppose that a liquid has an appreciable compressibility. Its density therefore varies with depth and pressure. The density at the surface is ρ0 (a) Show that the density varies with pressure according to ρ=ρ0ekp where P is gauge pressure at any depth and k is the...
  24. S

    AP PHYSICS ~ FLUIDS ~ A cube of osmium with a volume of

    A cube of osmium with a volume of 166 cm³ is placed in fresh water. The cube's apparent weight is 35.0 N. What is the density of osmium?This is what I did: I converted force to mass by : 35 N / 9.8 = 3.57 kg then changed the 166 cm³ to m³ which is : .000166 m³ After that i divided the mass...
  25. B

    Perfect fluids and the stress energy tensor

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  26. C

    Velocity of Water in 5 cm Tube: Solve Equation

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  27. H

    Pressure in fluids and Pascal's law.

    I have a lot of confusion and I hope someone can help me in clearing my doubts. considering a right cylindrical container with a liquid filled in ,if we take two points, y1 and y2(near the bottom) 1) first of all I thought that pressure inside a fluid arose because of the weight of the fluid...
  28. B

    Fluids - maximum error - diameters

    1. Homework Statement : The expressions you obtained for acceleration and drainage time (equations 3 and 4 in the manual) both contain the following factor or something equivalent: (D/d)^4-1 In the lab, we will be dealing with a bottle with a diameter around 9 cm. A typical diameter of the...
  29. M

    Surface between 2 rotating fluids.

    Homework Statement Oil and water are put in a cylindrical container. They can rotate at various frequency around the rotational symmetry axis. Consider gravitational and radial forces to find the form of the surface between the fluids. no friction, no mixing the surface has the form of a...
  30. nukeman

    Electricity & Flies: Exploring the Safety of Charged Fluids

    Can anyone tell me some fluids that are electrically charged? And if it would be safe for, let's say, a fly to consume?
  31. M

    Surface of 2 rotating fluids in a cylindrical container

    Homework Statement In a cylindrical container (with radius R) there are 2 fluids (separated like water and oil, fluid 1 lies under fluid 2) with given volumes V_i, given densities ρ_i. You let them rotate with respective angular frequencies ω_i. There is no friction. Find the functions of...
  32. S

    Measuring Slipperiness of Fluids on Inclined Planes

    I want to test several different fluids on an inclined planes and figure out which one is the most slippery. What would be the most scientifically sound way to measure this?
  33. X

    Can a piece that expands when blood flows through it decrease pressure?

    If i have a syringe used to pump blood through a set of tubing that becomes smaller and smaller, the pressure will obviously increase until it gets to the smallest diameter of the tubing. However, if I create a piece that expands when the blood flows through it, will this decrease the pressure...
  34. S

    Fluids, flow velocity and flow rate.

    Homework Statement Water is flowing from a container through a 50mm diameter hole at 36 liters a minute. Calculate the flow rate in L/s and the flow velocity through the hole. Homework Equations The Attempt at a Solution flow rate all i did was 36L/60s to get 0.6 L/s flow...
  35. B

    Fluids Problem: Is there a Typo?

    1/2/3. Homework Statement and equations Hi everyone, So, I have a fluids problem to do and I think there may be a slight typo or maybe I am missing something. The image of the problem is attached. So, the only problem I have is how L is defined. Since L is defined from below the zero...
  36. N

    Why Does Pressure Decrease with More Outlets in a Fluid System?

    Hey, so my situation is a positive displacement pump churning out 30gpm. Theres a bunch of pipe losses that I can deal with, and then a long 1.5" pipe with 8 possible outlets (you can screw on a hose). When one hose is attached the pressure is a we will say P1 (read from a gauge on the 1.5"...
  37. denjay

    Explanation for moving fluids having lower pressure

    I've known this fact for a long time and have thought about an explanation for it for just as long. I've observed its effects such as in the shower: as the water flows down it moves the air with it causing the curtain to move inward into the shower. It just seems that with a moving fluid that...
  38. A

    How Does Pascal's Principle Affect Fluid Dynamics and Comfort in Bedding?

    Hey all, I apologize if this question has a really obvious, basic answer, but I've done a lot of searching and haven't come across anything that explains it satisfactorily for me. So, I know that Pascal's principle underlies the workings of the hydraulic lift, and I know that while force is...
  39. L

    Behavior of MR Fluids in Magnetic Fields

    Like to know the behavior of MR Fluids when rotated inside a hollow shaft subjected to magnetic fields.. Could anyone please help
  40. J

    How to reduce Einstein's equation for perfect fluids

    Using the metric $$ds^2=-V^2 dt^2 +g_{ab} dx^a dx^b$$, where $$g_{ij}$$ is the Riemannian metric of the constant t-surfaces, and V is the gravitational potential, show that Einstein's equation $$G_{ij}=8\pi T_{ij}$$ for perfect fluids reduces to the system $$D^a D_a V= 4 \pi V( \rho +3p)\\...
  41. D

    Fluids rate of flow homework help

    Homework Statement Two cylindrical pipes both carry an ideal fluid (zero viscosity) in laminar flow. Pipe A has radius Ra = 2 and flow velocity Va = 8cm/s. Pipe B has radius Rb = 4cm and flow velocity Vb = 4cm/s. A) Calculate the cross section of pipe A and pipe B. B) Calculate the rate of...
  42. A

    Calculating SCUBA Tank Air Consumption for Offshore Oil Rig Maintenance

    Homework Statement You are designing an underwater maintences schedule for an offsore oil rig. The Divers work at a depth of 15m and the seawater Denisty is 1027Kgm^-3. They Are know to Consume 130L of air at the ambient temperature of 10C over the time to do the job. Atmospheric pressure is...
  43. J

    Viscosity Problem using power law for non-Newtonian fluids

    Problem Statement: Blood is a pseudoplastic fluid that has a variable viscosity at 37 °C that depends on the percent composition of hematocrit and plasma. It will usually range between 3 x 10^-3 to 4 x 10^-3 Pa s. A small sample of blood is tested in a viscometer and the following results are...
  44. O

    Visualizing Flow Using IR Transparent Fluids

    Hi. I am working on visualization of flow with thermal infrared camera. As it is known, water is not transparent for IR spectrum of light, so you can`t watch phenomena that are occurring "inside". You can only see things that are occurring on the interface of water and wall. I have been...
  45. J

    Does the buoyant force change in different fluids?

    Given two objects of identical density and weight, one is floating on water and the other is floating on saltwater. Is the magnitude of buoyant force acting on the object the same in the two situations? From my thinking, it seems that the weight of the object in the the two situations should not...
  46. B

    Squirting fluids by driving Syringes (experiment)

    Hello, I'm new to PF and looking for some guidance! I'm currently investigating a simple experiment involving two different capacity syringes filled with water and then applying a constant force to see their maximum verticle squirting height; My basic apparatus is basically this; We have a...
  47. J

    Equation of State for Solids (Dense Fluids?)

    Stumbled across an old journal a few weeks ago that suggested the equation of state for a solid could be written: V[P,T] = Vi - (C1)P + (C2)T with an internal energy: U[P,T] = (C3)T - (C2)PT C1, C2, C3 being constants of course. Does this make logical sense? It seems awfully...
  48. S

    Calculate Density of Wood & Oil: Fluids & Density Homework

    Homework Statement A block of wood with volume V floats in water with 0.67V submerged. In oil the block floats with 0.88V of its volume submerged. (a) Find the density of the wood. g/cm3 (b) Find the density of the oil. g/cm3 I honestly have no idea where to begin. I...
  49. P

    Temperature problem in fluids mechanics

    Homework Statement Assume the temperature of the exhaust in an exhaust pipe can be approximated by T=T0(1+ae-bx)[1+c cos(\omegat)] T0=100oC, a=3, b=0.03m-1, c=0.05, \omega=100 rad/s. If the exhaust speed is a constant 3 m/s, determine the time rate of change of temperature of the...
  50. T

    Calculations for heat transfer between fluids

    Im working on a little project where I am attempting to generate steam from a hot gas pipe. I haven't measured the rate of flow or temperature of the fluid as of yet, and I haven't decided upon the choice of material, but I'm looking for some calculations to start. My plan is to wrap a water...
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