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. J

    Filling a long tube by submerging it in fluid

    Hi everyone, I'm working on a first-year college assingment and need some help with it. I need to figure out how deep does a tube, initially filled with air, need to be submerged into water in order get fully filled, meaning in order for the air to be fully displaced by water. The tube is 1/2...
  2. S

    Help with Bernoulli's equation for a central heating system

    so far I have found the velocity 1 and 2 by dividing the volume flow rate over the area which I got from pi x dia squared/4 my v1 = 1.01859m/s and v2= 2.82942m/s i have then figured out a pressure for the 15mm pipe which i got an answer of 2.71 bar however i am stuck on the rest of the question...
  3. P

    Calculating Pressure Losses & Coefficients in Y-Junction w/Mixed Fluids

    i have a y - junction fitting (two inlets (1,2) and one outlet (3)). Two different fluids (with different densities and viscosities but same temperatures) let's say paint and other fluid comes from two inlets, mix and they will go through the outlet. and also laminar flow conditions. How to...
  4. M

    Same pressure points , 2 fluids u shaped tube

    Homework Statement see pic uploadedHomework Equations Pascal principal p = ρgh The Attempt at a Solution Pressure at C and D is the same . Pressure at A and B is the same ? I believe it is
  5. G

    I Pascal's law (fluids): Derivable from fundamental laws?

    Hi. Pascal's law states that static pressure in a confined incompressible fluid without gravity is the same everywhere. Is this law derivable from more fundamental laws? Some thoughts: Is Pascal's law part of the definition of the liquid state? If the liquid operates between two hydraulic...
  6. flamephoenix15

    How would you calculate how deep an object would go in water?

    Homework Statement So given a ball, radius r, mass m ,an a known height y from the surface the ball is dropped from, how would you calculate the depth the ball goes to in water (including the water it displaces), with the density of the ball less than the density of water. Ignore surface...
  7. wolram

    B Dark Energy Fluids: Read the Latest Research

    Although I am unable to judge this paper it should make an interesting read to toughs that can. https://arxiv.org/abs/1810.00269
  8. C

    Calculating Pressure from Air Displacement in Gasoline Tank

    I am struggling with what seemed to be a simple problem and any help would be greatly appreciated. I have to determine the amount of pressure created when air is displaced buy gasoline flowing into a tank. The gas enters the tank at 10gpm. The entrance has a diameter of 1.5" and the vent has a...
  9. C

    I Babies Upside Down: How Buoyancy Protects from Gravity

    Although this has biology in it, I still think it qualifies more as a physics question. If you take person and hang them upside-down long enough, blood will pool in their head and they will eventually die because of the pressure on their brain. I know that in the uterus, an unborn baby is...
  10. Lukeblackhill

    I Separation of Fluids in a Centrifuge

    Morning Mates, I have a question based in a statement take from Berkeley Physics Course, Vol.1 - Mechanics, pg. 102-103, where it reads, "Suspended molecules whose density (mass/volume) is different from that of the surrounding liquid will experience in the ultracentrifuge cell a strong force...
  11. V

    Buoyant force on a submerged body

    Homework Statement A hemispherical portion of radius R is removed from the bottom of a cylinder of radius R. The volume of the remaining cylinder is V and its mass is M. It is suspended by a string in a liquid of density ρ where it stays vertical. The upper surface of the cylinder is at a depth...
  12. V

    A vessel with fluids of two different refractive indices

    Homework Statement both filled to height h in the vessel of length 2h. one has refractive index root 2 and the bottom fluid has refractive index n. find the apparent depth of vessel. Homework Equations n = real depth/apparent depth The Attempt at a Solution what these guys have done ...
  13. Moayd Shagaf

    Equation for Water Flow Over Pipe Circular Hole

    suppose a water falling from pipe that above the ground with 5 meters, what is the equation that describe the level of water ( h) from pipe the hole at every moment of time? consider the pipe hole is circle .
  14. N

    Fluid Mechanics: Choosing a fuel line diameter for a required pressure drop

    Homework Statement [/B] Homework Equations - The pressure drop equation. [/B]The Attempt at a Solution
  15. Dane P

    The effect of pressure gradient across dissimilar fluids

    So my question is - would a strong enough negative pressure be able to pull a gas through a liquid? I can draw a diagram if anyone needs it but I'm trying to figure out what would happen in the following situation. Imagine you had a solid pipe that formed a large U shape with one end sealed...
  16. M

    Fluids: Continuity and Conservation of Mass

    Hi PF! In fluids I've noticed many authors use the continuity equation with an integral form of conservation of volume (assume density is constant). Is this double counting? Example: let fluid velocity inside an idle bubble be ##\vec u = \nabla \phi##. Conservation of mass implies ##\nabla u =...
  17. M

    A Fluids and the no-penetration condition

    Hi PF! Denote the velocity of a fluid ##\vec u## and define a potential ##\vec u = -\nabla \phi##. Let ##\hat n ## be an outward-oriented surface normal to a solid boundary. I would express no penetration at the boundary as $$ u \cdot \hat n = 0 \implies \nabla \phi \cdot \hat n = 0.$$ However...
  18. thepikminman

    Fluid Mechanics - Why? How? Why?

    Hi, I've been studying fluid mechanics for my mech. engineering degree for the past few months. Unlike my other subjects (mechanical analysis, thermodynamics, automated systems, mechanical vibrations), I can't seem to grasp the problem-solving technique. The annoying thing is, I find the...
  19. S

    Confusion about Bernoulli's Equation & Airplane Wing

    Hi, I'm a little confused about the theory behind this problem related to fluids/Bernoulli's equation: "An airplane wing is designed so that the speed of the air across the top of the wing is 251 m/s when the speed of the air below the wing is 225 m/s. The density of the air is 1.29 kg/m3...
  20. M

    A How Does Surface Energy Affect Thin Fluid Films?

    I'm studying thin fluid films, and the text writes free surface energy of a film (puddle) over domain ##(0,X)## can be expressed as $$E=\int_0^X \left[\frac{h_x^2}{2}+\omega(h)+G\frac{h^2}{2} \right]\, dx$$ where ##X## is a length that the thin film (puddle) rests on, ##h## is the height of the...
  21. M

    Fluids Bernoulli and a pressure field

    I have a text that writes a pressure balance for a cylindrical shape of fluid, where the linearized Bernoulli gives the pressure field ##p = p_0+\rho\partial_t \phi : \vec{v} = -\nabla \phi## where ##\vec{v}## is the velocity vector. ##p_0## is the static pressure required to maintain the...
  22. M

    Kinematic Boundary Condition Fluids

    Hi PF! Given a fluid/fluid interface ##F(t,x,y,z) = 0## the kinematic boundary condition states ##DF/Dt = 0##. Given ##y=F## the text states ##v = \partial_tF+u\partial_xF+w\partial_zF##. How is this possible? I thought ##DF/Dt = 0 = \partial_tF+u\partial_xF+v\partial_yF+w\partial_zF## and...
  23. S

    I Understanding Stress Energy Tensor of Fluids

    Hello! I am reading about stress energy tensor of a perfect fluid and I don't understand the ##T^{ij}## terms. They are defined to be the flux of i-th momentum through the j-th surface. Now you take a fluid element and in its momentary comoving reference frame (MCRF) you calculate these...
  24. H

    Fluid Statics- u-tube displacement

    Homework Statement A uniform U-tube is partially filled with water (ρH20 = 1000 kg/m3). Oil, of density 750 kg/m3, is poured into the right arm until the water level in the left arm rises 30 mm. The length of the oil column is then: (Answer: 40mm) Homework Equations P = pgh Pressure = density...
  25. G

    Conductive fluids and magnetism

    I've just started looking into MHD and am a bit confused.ive read the threads on moving charges,and special relativity. So I am thinking of this thought experiment.. we construct a test area shielded from all extraneous magnetic fields, in this area we have a torus shaped pipe and pump a...
  26. Idrees Afridi

    How to analyze heat transfer effects of viscoelastic fluids

    n the heat transfer analysis of boundary layer flow of viscoelastic fluids many researchers see the variation of temperature of fluid inside the boundary layer with Prandtl number, for which they use prandtl number 0.7, 2 , 3, 4 .. This is what generally the researchers do.. Currently i am...
  27. Gabriel Ulisses

    B What property of a fluid determinates rising and sinking Velocities

    If we have a bottle with a fluid A of X density, and in the bottom of this bottle it magically spawns the same amount of fluid B with X/2 density, fluid B should rise in fluid A until they both change positions and fluid B floats in A. Wich property of the fluids are the ones that determinates...
  28. T

    B What attribute makes some fluids more compressible than others?

    Are all fluids incompresible? If not, what is the attribute that allows any compressible fluid to do so while others, such as hydraulic fluid to not? tex
  29. M

    Conservation of Mass Fluids Wave

    Hi PF! Suppose we have a water wave with mean depth ##H## with disturbance ##\zeta## above/below ##H## propagating through a channel of thickness ##b##. The book parenthetically remarks that the continuity equation becomes $$\partial_t(b(H+\zeta))+\partial_x(bHu)=0.$$ However, when I try...
  30. P

    Friction between cart and a surface covered with a liquid?

    << Thread moved from the technical forums so no Homework Help Template is shown >> Hello, I'm attempting an experiment where I take a cart and slide it down a ramp. Each time, I will coat the ramp with a different liquid. I want to compare the effects of the dry ramp, a ramp with water poured...
  31. M

    Do Streamlines Change Shape Depending on Frame of Reference in Fluid Dynamics?

    Hi PF! I read that streamlines around a body change according to one's frame of reference. If we consider a sphere that is moving in a flow (low Re) would the streamlines change shape as we change our reference frame from being on the ball verses sitting aside and watching the ball move past...
  32. W

    Entrainment Coefficient - Dynamics of Fluids

    Hi all, i ask in advance sorry for my bad english, i hope you will understand my question. I'm studying the motion of a plume fluid in a fluid ambient, with ρ(plume) > ρ(ambient). Afterwards there si a transport of fluid ambient as an effect of the turbolence caused by the viscosity between...
  33. M

    A Linearized Continuity (Fluids)

    Hi PF! Continuity for incompressible flow independent of ##\theta## is ##\nabla\cdot u = \partial_ru_r+u_r/r+\partial_z u_z=0##. However, I'm following a problem in cylindrical coordinates, same assumptions as above, and the author states the linearized conservation of mass is...
  34. M

    A Understanding Fluids and Vortices: Exploring Circulation and Vortex Behavior

    Hi PF! For an irrotational vortex, the circulation is zero along any closed contour that does not enclose the vortex axis; and has a fixed value, ##\Gamma##, for any contour that does enclose the axis once. Why? Secondly, why must vortex tubes close on themselves or begin/end at fluid...
  35. Mike Dacre

    Speed and pressure relative to a piston (fluids)

    I am studying for the MCAT and I got an interesting question from a Kaplan book: 15. A water tower operator is interested in increasing the pressure of a column of water that is applied to a piston. She hopes that increasing the pressure will increase the force being applied to the piston. The...
  36. S

    Assumption of local thermodynamic equilibrium in a fluid

    Moving fluids are generally in a state of non-equilibrium. However, in fluid dynamics, people generally assume a state of local thermodynamic equilibrium and argue that in such a condition, equilibrium thermodynamic concepts such as pressure, temperature, entropy, internal energy etc. can be...
  37. M

    What is Pressure and surface tension in fluids

    Today I was speaking with a professor and he asked me, regarding fluids, what is pressure. I said force per unit area acting normal to a surface. He then asked what I meant by surface. I wasn't really sure how to respond. He also asked me what surface tension was. I said it is a force per unit...
  38. M

    Fluids and Ping Pong Ball Magnus and Coanda Effect

    Hi PF! Given a hairdryer blowing vertically against gravity toward a ping pong ball the ping pong ball is suspended. The ball is relatively stable in the jet because the high velocity of the jet causes a decrease in pressure, causing the ambient air to mix, therefore keeping the ball in the jet...
  39. M

    Torque and concentric cylinders fluids

    Hi PF! If we have two concentric cylinders with Newtonian fluid between them, and the small cylinder is at rest and the larger cylinder with radius ##R## rotates at some angular velocity ##\Omega##, how would you calculate torque ##\vec{T}## on the outer edge? My thoughts: ##\vec{T} =...
  40. M

    Can Different Curvatures Exist in Equilibrium for Fluid-Filled Pipe?

    Suppose we have a pipe of some length ##2L## and at the ends there is to spherical caps of fluid, as shown in the attachment. Is it possible for the two ends to have different curvatures in equilibrium? My initial thoughts are no, since pressure is proportionate to curvature (Young-Laplace) and...
  41. M

    Why can't a channel flow with two immiscible fluids have two maximum velocities?

    Hi PF! I was recently looking at two immiscible fluids in a 2-D Poiseuille Flow with two immiscible fluids of different densities. Let the total distance of the channel be ##L+\epsilon L##, where the dividing line between the two flows is ##L##. I was thinking of the possible flow profiles and...
  42. S

    Is Gas Pressure in a Weightless Environment Mainly Due to Particle Collisions?

    Is the pressure of gas at any point within mostly due to particles collisions rather than weight of gas above point considered?
  43. M

    How to Solve Hydraulic Jump Problems with Conservation of Momentum and Mass?

    Homework Statement The question is stated here, though I'm happy to repost but they include a picture. I should say this is not homework, I'm doing problems for practice. http://web.mit.edu/2.25/www/5_pdf/5_04.pdf Homework Equations Conservation of momentum/mass The Attempt at a Solution...
  44. L

    How Do You Calculate the Velocity of a Piston in a Viscous Fluid?

    1. piston having a diameter of D = 5.48 in. and a length of L = 9.50 in. slides downward with a velocity V through a vertical pipe. The downward motion is resisted by an oil film between the piston and the pipe wall. The film thickness is δ = 0.002 in., and the cylinder weighs W = 0.5 lb...
  45. john101

    What is the best DIY mix for cleaning with an ultrasonic cleaner?

    I bought an ultrasonic cleaner to clean my fuel injectors. It appears very difficult to get a recipie for the liquid to clean in. I think using distilled water is best. At about 60 degrees celsius. What fluids are good to use? I understand that petrol deposits a kind of 'varnish' that can...
  46. ryanuser

    Fluids Lab Report Homework: Venturi & Orifice Coefficients

    Homework Statement I have to write a report based on an experiment I conducted with the class in the school lab. The experiment was about orifice plate meter and venturi meter (both to measure rate of flow of fluids; this is an introductory fluids mechanics module I am studying). The...
  47. RMalt

    Evaluate Problems using fluid dynamics

    I have a diagram similar to the following. Water entering the larger end is at 20degreesC. The larger end has a diameter of 8cm and Area 50.26cm2. The small side has a diameter of 3cm and Area 7.0685cm2. The water jet exerts a force of 87N on a flat plate at an unknown distance. Assuming no...
  48. M

    Fluids, Streamlines and Pressure

    Hi PF! Can anyone help me understand the relationship between streamlines and pressure in an inviscid/incompressible flow? I know a streamline is everywhere parallel to flow. Is it ever possible for streamlines to cross each other? If you have a recommended reading please let me know, or if you...
  49. J

    Surface Tension and Fluids

    If you increase surface tension, the molecules at the surface are more strongly pulled into the center of mass due to the stronger attractive molecular forces. -Why is it that when surface tension increases in a liquid, its surface decreases in area, but its volume does NOT change? (Volume...
  50. EMP

    Pressure in a Fluid-Filled Glass with a Side-Hole Tube: Is Pa = Pb = Pc?

    Homework Statement It's not a homework exercise, it's more of a conceptual question to help my understanding of the subject. Imagine that you have a glass of height H filled with a fluid with density ro and viscosity eta. There is a hole somewhere on a side of the glass, at a height h relative...
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