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. M

    Lattice Models for Fluids - Regular Solution Model

    1. Problem Statement: For the regular solution model, develop the equations for the compositions of the coexisting phases in a binary system and plot the phase boundary as a function of χ/RT.2. This question stems from Sandler's Introduction to Applied Statistical Thermodynamics. The Attempt...
  2. T

    How does the viscous force of the liquid affect momentum in this problem?

    Homework Statement Please see attached file for problem statement Homework Equations ΣM = Min - Mout = 0 The Attempt at a Solution I have tried using the equation of conservation of momentum on this problem but I'm getting stuck at how the viscous force of the liquid is playing a role in this...
  3. M

    Conservation of Momentum Fluids Question

    Homework Statement I am studying for an upcoming exam and stumbled upon a website with a bunch of practice problems. I would typically state the question but this one is so long and requires a picture so here is the hyperlink to it: http://web.mit.edu/2.25/www/5_10/5_10.html Homework Equations...
  4. DaTario

    I Pascal´s Principle in Compresssible Fluids

    Hi All, When explaining Pascal´s Principle I had some bad time related to its application in compressible fluids. Correlation functions (forces in two points separated in space at the same time) came to my mind but at the end the doubt persists. Is Pascal´s Principle valid for compressible...
  5. M

    Is my use of ##e_x## in my solution for Fluids Angular Momentum correct?

    I don't typically do this, but attached is a figure with a problem statement. This is not homework because I have already solved it! My question is a subtle one. Please see the attachments, one is the question and the other is my solution. I know my solution is correct, but at the end of...
  6. M

    Squid swimming in water fluids problem

    Homework Statement Some underwater fish use a jet to move forward. The body expands with water and contracts, expelling water and thus propelling forward. For these purposes, assume that a submerged fish experiences a friction drag proportional to its surface area ##A_s## and proportional to...
  7. artriant

    How do fluids behave in a space centrifugal structure (artificial gravity)

    How fluids would act around the circumference of a space centrifugal, where the habitable zones are. Given that we have a radial gradient of Artificial Gravity , plus Coriolis, inertia, frictions etc. Id expect some change. Will add a list of questions but this more like an open discussion...
  8. H

    Calculating Entrained Flow in Injector Design: Challenges and Considerations

    Hello, I am trying to analyze an injector and having trouble coming up with the final equation for the calculation. The apparatus can be seen below. Assuming we know the inlet flow rate and pressure (A) from the pump/fan and the pipe geometry. The pressure out of the outlet of the nozzle is...
  9. G

    When will the house be flooded?

    Homework Statement [/B] A simplified schematic of the rain drainage system for a barn is shown in the figure. Rain falling on the slanted roof runs off into gutters around the roof edge; it then drains through several downspouts (only one is shown) into a main drainage pipe M below the...
  10. D

    I Confusion about derivation for isotropic fluids

    In Woodhouse's 'General Relativity' he finds an expression for the energy-momentum tensor of an isotropic fluid. If W^a is the rest-velocity of the fluid and \rho is the rest density then the tensor can be written as T^{ab} = \rho W^aW^b - p(g^{ab} -W^aW^b) for a scalar field p. The...
  11. Benign Paradigm

    I Energy Conversion With Supercritical Fluids

    Technically, this isn't a question concerning supercritical fluids. It's more about converting thermal energy into velocity by playing "keep-away" with molecules that want to go supercritical. The idea is simple. In an fully contained system, we can more or less say volume is static...
  12. C

    Forces Due to Static Fluids

    Homework Statement Find height (h) of the water level which can result in failure of dam by tipping it over due to water pressure at U/S of dam. Also draw the free body diagram. Density of water to be = 1000 kg/m^3 and its specific weight=62.4 lb/ft^3 Hints: The specific weight of concrete is...
  13. S

    Density parameter against scale factor for perfect fluids

    Homework Statement This is a basic cosmology problem. The Friedmann equations are ##\Big( \frac{\dot{a}}{a}\Big)^{2}+\frac{k}{a^{2}}=\frac{8\pi}{3m_{Pl}^{2}}\rho## and ##\Big( \frac{\ddot{a}}{a} \Big) = - \frac{4\pi}{3m_{Pl}^{2}}(\rho + 3p)##. Using the density parameter ##\Omega \equiv...
  14. S

    Can inviscid fluids rightly be called Newtonian?

    I am studying an inviscid fluid. I am trying to characterise the fluid. Does it make sense to call it Newtonian or should I avoid this designation? What I mean is - if there are no viscous stresses then does it make sense to characterise it's response to viscous stresses? (That box that doesn't...
  15. jdawg

    Derivation of Hydrostatic Equations Fluids

    Homework Statement I'm having a little trouble understanding how to derive the hydrostatic equations for fluid mechanics! My professor showed the derivation with respect to y in class, and it kind of made sense to me. Now I'm trying to see if I can derive the equation with respect to z and x...
  16. Z

    Fluid Dynamics question -- compressibily of brake fluids "oil"

    As part of a racing community, there is a discussion going on that talks about compressibility of brake fluid. the Street car versions have a value of 6.5E-05 and the racing stuff is 4.5E-05 both at a pressure of near 5000psi. there is talk about the difference of the racing fluids which...
  17. ramzerimar

    Engineering Mechatronics or Thermal and Fluids for specialization?

    I'm a Mechanical Engineering student at the 4th semester, and I've been thinking about what specializations I should pursue after I get my bachelor's degree. I've always find mechatronics (control and automation) to be quite fun. I've developed some projects using Arduino and studied a little...
  18. H

    Question about pressure drops due to friction and valves

    If I'm designing an injector for a rocket engine, I have a specific mean flow rate (m/s) that must be met due to the pipe diameter and the required mass flow rate. I've calculated all the pressure drops due to lost work from friction, valves, and orifice geometries (not from change in area, but...
  19. E

    I Fun question: Can this fish swim?

    Imagine a fish in a tank that is completely filled with water and closed on all sides. Can this fish swim? Since water is extermely difficult to compress, when the fish moves its fin to propel itself forward, the water can't move anywhere, so the fish should not be able to move right? Then I...
  20. R

    B Understanding Supercritical Fluids: Properties, Risks, & Applications

    Hi, looking for help understanding the concept of SCF's. I understand that SCF'S are indistinguishable in their gas and liquid phases, and that they show properties of both. My question is, whose properties would they show in the event of a vessel failure? For example, if you took a 100 l...
  21. A

    I Can You Explain: Why Is It One Sec?

    can you help me with this, please?
  22. bill222

    Calculate position of plate between two fluids

    Homework Statement Problem is as shown in the picture. The answer is meant to be a 4.59mm. Homework Equations Tau=mu(du/dy)The Attempt at a Solution Force (tau) acting on plate from both fluids is equal, as is velocity (du). mu1 (du/dy1) = mu2 (du/dy2) mu1/dy1 = mu2/dy2 (mu2/mu1) *y1 = y2...
  23. A

    Topic: Hydraulic Fluids in Aviation

    Hello, We all know that, in Commercial aviation, we use Hyjet (SKydrol or 500B4). and in military we use MIL-H-83282 (red oil). what are the main reasons? MIL-H-83282 has a synthetic hydro carbon base and contains additives to provide the required viscosity and anti wear characteristics, which...
  24. Titan97

    Loss in energy of a ball dropped in a fluid

    Homework Statement Two identical balls A and B are dropped from rest into a tall column of viscous liquid. Ball A is dropped from the surface of the liquid while ball B is dropped from a small height ##h## above the liquid. Let ##E_A## and ##E_B## be the loss in mechanical energy of bal A and B...
  25. Tazerfish

    I Oscillations and Waves in Fluids

    I first wanted to ask a very specific question: There is something called the Brunt-Vaisala frequency. It describes the frequency of oscillation in a fluid with a density gradient. Because if a parcel of fluid is pushed up or down from its stable state it will oscillate around it. What i don't...
  26. M

    Fluids PDE Problem: Understanding the Elimination of c_1 in Boundary Condition

    Hi PF! So my book has boiled the problem down to $$\psi(r,\theta) = c_1 \ln \frac{r}{a}+\sum_{n=1}^\infty A_n\left(r^n-\frac{a^{2n}}{r^n}\right)\sin n\theta$$ subject to ##\psi \approx Ur\sin\theta## as ##r## get big. The book then writes $$\psi(r,\theta) = c_1 \ln...
  27. M

    Non-slip condition for inviscid fluids

    Hi, I would like to ask that whether or not ## non-slip condition## is valid for inviscid fluids. We know that inviscid fluids there is no friciton. If there is no friction in the inner surface of pipe (I do not name its technical name) so it seems that there cannot be the non-slip condition...
  28. R

    Fluids: Energy equation involving head loss

    Homework Statement See attached image: Homework Equations p/ρg + V^2/2g + z = constant head loss (major) = f * l/D * V^2/2gThe Attempt at a Solution To use the energy equation while incorporating head loss, I need to determine the velocity in each section of pipe. The problem is I don't know...
  29. The-Mad-Lisper

    An Open Cylindrical Tank of Acid....

    Homework Statement An open cylindrical tank of acid rests at the edge of a table 2.20\cdot 10^0\ m above the floor of the chemistry lab. If this tank springs a small hole in the side at its base, how far from the foot of the table will the acid hit the floor if the acid in the tank is...
  30. thebosonbreaker

    I Non-Newtonian fluids at the quantum level

    I am intrigued by the diversity of Non-Newtonian fluids and need to ask a question. Can anyone provide me with a breakdown of what goes on at the quantum level as the Non-Newtonian fluid bounces up and down on top of the speaker in the video I have embedded. What I would like to know is...
  31. R

    How can I calculate total drag on a multi-hull?

    I was given the task to create a hull which must be propelled by human and solar power; I have found it difficult to obtain existing hull designs with their respective coefficient of total resistance so I am looking for a way to calculate the total hull resistance for a hull of my own. I...
  32. vinamas

    Calculating Buoyancy Force & Length of Submerged Cube

    Homework Statement A layer of oil that has a density of 930kg/m^3 is floating on the surface of water in a container if a wooden cube with a length of 4 cm becomes submerged where it's lower half is in water and it's upper half is in oil the cube's density is 960kg/m^3 find A) the buoyancy...
  33. L

    Mixing Fluids: Calc Velocity & Temp of Final Mixture

    A stream of water at 300 Deg K flowing into a mixture at a velocity of 5m/s. Another stream of water at 400 Deg K is flowing in at 8 m/s.How to calculate the final velocity and temperature of the fluid after mixing? Please find the attached image? If it requires any c/s dimensions please assume..
  34. C

    Stress tensor for non-Newtonian fluid

    How does one setup the stress tensor for a non-Newtonian fluid? I know that for any fluid the normals should be the pressure and for a power law fluid the shear stress in the direction of flow is related by K(du/dy)^n. Does this mean that all other components are 0 for a symmetric pipe or...
  35. C

    Is the Moody Chart valid for non-Newtonian fluids?

    Is the Moody Chart valid for non-Newtonian fluids?
  36. B

    Cylindrical barrel Hydrostatic Pressure

    Homework Statement A circular cylindrical barrel is half full with oil. If the diameter of the base is 8.0 m, find the net force against each end if ρo = 800 kg/m3. The cylinder is on its side. Homework Equations F=P*A P=ρgdy The Attempt at a Solution P = ρo*g*h, where h is the radius which...
  37. DaTario

    Maximum number of layers of immiscible of fluids

    Hi All, What is the maximum number of layers of immiscible fluids in nature. Is seems intuitive to me that this limit exists. for the sake of definiteness, let' s consider usual laboratory"s condition of temperature and pressure. Best wishes, DaTario
  38. Titan97

    Bouyant force when there is no liquid beneath an object

    Homework Statement A block floating on water contained in a vessel is pushed inside until it touches the bottom of the vessel and then released. Will the block come back? Homework EquationsThe Attempt at a Solution [/B] Bouyant force acts because the pressure on the bottom of the block is...
  39. E

    Object in equilibrium between 2 fluids

    Homework Statement if an object is in equilibrium at the interface of 2 fluids in an open tank, why do both provide upthrust? Homework Equations P=P' +dgh The Attempt at a Solution The upper fluid is not in contact with the lower part of the object, so why would the fluid provide an upward...
  40. Z

    Calculate Flow Rate of a Syringe: Poiseuille's & Bernoulli's Laws

    I am trying to come up with a mathematical model so that, when the displacement of the plunger of a syringe is know, I can calculate the amount of a specific liquid in the barrel. Or the relationship between the speed of the plunger and flow rate at the tip of the needle (Again assuming that the...
  41. V

    Fluid Statics and Dynamics: P Equation

    in fluid statics we have learned that p =patm+density*g*h. is the same equation valid in fluid dynamics?
  42. T

    Effect of contamination and solutes on surface tension

    I was asked to explain how different factors affect surface tension. I can understand how temperature plays a role. 1) Contamination: Increase in contamination decreases surface tension. I tried thinking about it.I thought maybe the particles that contaminate the fluid get in the way and...
  43. T

    Pressure inside a soap bubble just under surface

    Homework Statement What should be the pressure inside a small air bubble of 0.1,, radius,situated just below the surface? st of water=7.2 *10-2 and atmospheric pressure=1.013*105. Homework EquationsThe Attempt at a Solution I am of the understanding that the pressure inside the...
  44. Kori Smith

    Clarification on derivations of specific heats in fluids

    Hello! I understand what specific heats are and how to derive them. I just feel that I'm missing a little something in the methodology. Consider the 1st law of thermodynamics and the definition of enthalpy: 1) dU = δQ -δW = δQ - PdV 2) H = Q - VP For the derivation of CV, dV = 0 and the...
  45. Greg Bernhardt

    Realistic fluids simulation and animations

    Great for those interested in animations
  46. L

    Fluid Mechanics: Pressure in Fluids

    Homework Statement The gauge pressure at the inlet of a horizontal venturi meter is 40kPa. The vacuum at the throat is 38 mm mercury. If a differential U-tube manometer containing mercury is the U-tube and the oil in the connecting tubes is connected to the venturi, what will the difference in...
  47. D

    Pressure and area relationship in fluids

    Hoping to clarify something about this... Is it fair to say that you should consider the relationship between pressure and area as a function of whether a fluid is moving or standing still? In other words, when a fluid is moving and you decrease the area, the pressure goes down because there...
  48. J

    Reynolds transport theorem (1st year undergrad fluids)

    hello all, I just wanted to check my worded interpretation of this otherwise messy result is ok:
  49. B

    [Fluids] Determine pressure change from industrial jet pump

    Homework Statement The apparatus or "jet pump" used in an industrial plant is constructed by placing the tube within the pipe. The velocity of the flow within the 200-mm-diameter pipe is 2 m/s, and the velocity of the flow through the 20-mm-diameter tube is V = 37 m/s . The fluid is ethyl...
  50. T

    Hydrostatic fluids question

    MODERATOR'S NOTE: THIS THREAD HAS BEEN MOVED FROM ANOTHER FORUM, SO THERE IS NO TEMPLATE. This is the problem here. I am having trouble making progress on this problem after getting the wrong answer for the vertical hydrostatic force. The equations I am using are: fx=...
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