What is Fluid flow: Definition and 213 Discussions

In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including aerodynamics (the study of air and other gases in motion) and hydrodynamics (the study of liquids in motion). Fluid dynamics has a wide range of applications, including calculating forces and moments on aircraft, determining the mass flow rate of petroleum through pipelines, predicting weather patterns, understanding nebulae in interstellar space and modelling fission weapon detonation.
Fluid dynamics offers a systematic structure—which underlies these practical disciplines—that embraces empirical and semi-empirical laws derived from flow measurement and used to solve practical problems. The solution to a fluid dynamics problem typically involves the calculation of various properties of the fluid, such as flow velocity, pressure, density, and temperature, as functions of space and time.
Before the twentieth century, hydrodynamics was synonymous with fluid dynamics. This is still reflected in names of some fluid dynamics topics, like magnetohydrodynamics and hydrodynamic stability, both of which can also be applied to gases.

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  1. L

    Why does fluid pressure decrease with higher density in narrow pipes?

    So I was playing around with Fluid Pressure and Flow over on PHET animations and noticed something odd. When a fluid with high flow rate is going from a large pipe to a small pipe a lower density increases the pressure. And a higher density lowers the pressure in the narrow part of the pipe. My...
  2. W

    Understanding Fluid Flow and Stress in Two Dimensions with LaTex Tutorial

    Homework Statement Homework Equations 0=viscous+gravitational+pressure I saw in the solutions that pressure=0 in this case, but why? I also knew that : accumulation= flow in - flow out+generation, why not use this one? The Attempt at a Solution (their solution) We are interested in...
  3. E

    In which direction a spherical particle moves under torque?

    Consider a small rigid spherical particle of radius a, fully immersed in a viscous incompressible Newtonian fluid of shear viscosity η above a hard-wall with stick (no-slip) boundary conditions, located at the plane z = 0. A constant positive (external) torque Tx is applied on the particle. My...
  4. sa1988

    Made a 2D fluid flow simulator

    I've made a fluid flow simulator, for anyone interested in fluid dynamics. The simulator models some basic flows which I learned on a 2nd year fluid dynamics course. It doesn't include viscosity, density or whatever else (hence for example the flow past a sphere in the simulator is a perfect...
  5. ScottAllenRauch

    Fluid Flow Loss Analysis Using Real Gas Properties

    I am trying to model fluid flow and losses through a component (e.g., pipe) using REAL GAS properties since 1) I have access to RefProp from NIST, and 2) I am dealing with refrigerants, which are far from ideal. I have seen nowhere an analysis of fluid flow losses (e.g., drag, friction...
  6. S

    Acoustics and Fluid Flow - COMSOL 4.2

    Hello everyone, First of all I am new here~ Second, I would like to "prove" on COMSOL that soundwaves (sound vibration) can alter/reshape the fluid flow Can someone tell me the physics i should use as well as all the other parameters to make it work? if someone is more experience can they...
  7. pistolpete

    External Fluid Flow -- Immersed Body Friction Drag

    Hello all, I have browsed this forum occasionally when I had questions and threads showed up on a search. I have been stumped by a problem, so I decided to create an account and see if anyone could help me out! 1. Homework Statement A coal barge 1000 ft long and 100 ft wide is submerged in a...
  8. O

    Bernoulli's Equation and pressure differences

    Homework Statement Problem in attached image Homework Equations $$P_1+\frac{\rho v_1^2}{2}=P_2+\frac{\rho v_2^2}{2}$$ The Attempt at a Solution I understand everything in the solution except why $$P_A-P_B=h(\rho_{Hg}-\rho)g$$ Why do we have to subtract the density of water from that of...
  9. S

    Understanding Fluid Flow in Physics Lab: Poiseuille Equation

    Hi everyone, I have to answer some question about a physics lab..here are the questions A tank is empty by a pipe and we measure the height (y-y0) of fluid in the tank as a function of time. We obtain this graph ( I join the graph) We can find the relaxation time with the relation : with...
  10. A

    Pitot Tubes and Static Pressure

    Hey so I am learning physics on Khan academy and they had a video on Pitot tubes: https://www.khanacademy.org/science/physics/fluids/fluid-dynamics/v/venturi-effect-and-pitot-tubes I had a question. I understand that the lower part of the pitot tube in the video is measuring the pressure...
  11. G

    Compressible air flow from a tank through a regulator

    This is probably a simple question but I am really struggling as to where to start. We have a tank of compressed air, Volume of tank (V1) = 0.2litres Pressure of tank (P1) = 206.8 bar Regulator Pressure (P2) = 58.6 bar I would like to find the volume of air that can pass through the regulator...
  12. A

    Boundary Layer Separation in Laminar and Turbulent Flows

    When compared to laminar flows, the fluid "sticks" with the solid surface longer in case of turbulent flows. For example, the angle of separation for flow over a circular cylinder is 80 degrees for laminar flows, and 140 degrees for turbulent flows. What is the reason?
  13. CivilSigma

    Interpreting negative pressure in pipe fluid flow

    Homework Statement (This is more of a discussion question) [/B] My question is, if we have a pipe between two reservoir A and reservoir B (Height of A > height of B) , then if flow in the pipe is to happen from A to B, what sign would the pressure sign be at any point in the pipe? Homework...
  14. W

    Bernoulli equation exercise from Fanning and Moody

    Homework Statement I would really appreciate it if you could give me a hand with this exercise, not sure on what I've done. Data: Moody: L=55*10-3m D=10-1m k=0.0002m Homework Equations ##Re=\frac{D*u*ρ} {μ}## ##Re=\frac{4*m} {pi*D*μ}## Relative roughness ##ξ=\frac k D## where k=rougness...
  15. W

    Fluid flow/heat transfer Area dilemma

    Homework Statement I have a confusion regarding areas. Usually in fluid flow I am using the A=(pi*D^2)/4 However in heat transfer we usually use A=pi*D*L Could you please explain this?I mean in first case(fluid flow) it is cross sectional area and in the 2nd case(heat transfer) is like the...
  16. W

    Parallel Flow Exercise Homework: Find QB Value

    Homework Statement I would really appreciate if you could give me some hints regarding what exactly to iii) Homework Equations we know they are in parallel so Q=Q1+Q2+...+Qn delta hloss=delta hloss1+deltahloss2+... delta P=delta P1=delta P2=...The Attempt at a Solution We know flow is...
  17. J

    Diameter of hole vs time to submerge

    Hey everyone, I'm new to physics forums and I would really like some help with my physics report. For my high school physics topic I have chosen an experiment where holes with differing diameters are drilled into the centre of the bottom of PVC pipe caps and the time taken for the each cap to...
  18. W

    Bernoulli equation with losses dilemma

    Homework Statement Hello,Could you please lend me a hand with this problem?I would really appreciate it. Question:[/B] Their Answer: KL=2*0.75(2 elbows) D=5*10^-2 m ;f=0.001(fanning friction factor) Assumptions I made: Point 1 which is at the top of the liquid in the tank: h1=23 m...
  19. W

    Heat transfer and Fluid Flow Struggling Problems

    Homework Statement Hello, I am struggling with this heat transfer and fluid flow problem.I would really appreciate if someone could have a look over what I have done on it. I am not sure if what I have done is correct ,I would really appreciate if you could tell me if it is correct or wrong.And...
  20. W

    Fluid Flow/Heat Transfer question struggle

    Homework Statement I would really appreciate if someone could help me out with this exercise in fluid flow/heat transfer ,because I really struggling with it.Below I am attaching photo of problem+table with values. Thank you in advance !Homework Equations Question link:[/B]...
  21. 1

    I Fluid flow: larger diameter vs. smaller diameter

    Hello everybody, It is normally said that fluid flow in a larger diameter pipe is easier (less energy losses) than in a smaller diameter pipe. This indeed appears to be true in relatively high flowrate applications however, I'm struggling to understand why drinking from a normal straw (small...
  22. Ravi Singh choudhary

    I What will be the flow direction?

    I guess there would be no flow in case 2; case 3 seems easy to me applying fluid mechanics principle; flow will be 1 to 2. But I doubt case 1; it is not in thermal equilibrium ? Pls help
  23. Ravi Singh choudhary

    Difference between Partial pressure and vapor pressure

    Partial pressure must be less than or equal to the vapor pressure if there is no liquid present. However, when both vapor and liquid are present and the system is in the phase equilibrium, the partial pressure of the vapor must be equal to the vapor pressure and system is said to be saturated...
  24. S

    Solve question using Excel or Matlab

    water moves over a stationary horizontal flat plate of one-meter length. if velocity of water is 1 m/s, draw boundary layer and friction coefficient versus x for this plate. hint: start your curves from x=1cm.Professor asked us to do it in Matlab or Excel and I'm not sure I understand how to...
  25. D

    Fluid speed in a straight pipe

    Hello everyone I have a simple question regarding how to calculate the speed of a fluid in a normal pipe with a constant cross-section If we are given the pressure difference at the two ends of the pipe (P1 and P2) is it possible to calculate the speed of the fluid? (I think I can imagine this...
  26. S

    Plotting pressure distribution of fluid flow in a pipe

    Hi all, I am analysing the flow of water in a subsea pipe, and have been getting stuck in trying to solve the energy equation to plot a temperature distribution across the pipe diameter I have derived the equation as such seen in my attachement. But my main question is how should/would I...
  27. S

    I Solving Navier Stokes & energy equations with different coordinates

    Hi all I am conducting a fluid analysis on water flowing through a subsea pipe. Having used navier stokes equation, i derived the equation for velocity in the r-direction (using cylindrical coordinates. But when initially solving the energy equation to determine temperature distribution I...
  28. S

    Fluid Flow Equations With Real Values

    Hi, all I am working on a project that involves analyzing fluid flow behavior in oil & gas pipelines. I am interested in calculating and plotting the velocity profile, and also(pressure distribution, temp distribution) Having studies fluid mechanics last year I am aware of the EOS for fluid...
  29. Alettix

    Choosing Reference Points in Bernoulli's Equation for Fluid Flow

    Hello! I have encountered some trouble with choosing the right reference points when using Bernoulli's equation and I would be glad if you could help me sort it out with this made up example. :) 1. Homework Statement There is a large, open, cylindrical water tank with a cross section area of...
  30. B

    Finding out Shear rate of a bent pipe which has different bend radius values

    which has different radiuses at both end. Its actually two pipes merging after the entrance going straight and somewhere close to middle of the pipe it's bent 90°. So for simple pipes it's =4*Q π*r^3 But what for the pipe I...
  31. C

    Water flow through a perforated plate

    Hi, I'm seeking the formula to calculate how water moves from one tank into another through a perforated plate driven by waterlevel compensation. I found the calculation for the discharge of water through submerged orifices, but I think the calculation does not include that holes, one above the...
  32. CivilSigma

    Separation of Particles in a fluid flow?

    Hello, I'm doing some online research on fluid flow in waste water treatment. One of the problem I'm trying to find information about is how can visible particulate matter be separated within a liquid. I saw the following picture: What fluid concepts are going here that allow the grit to...
  33. C

    Ideal fluid flow velocities & pressure

    1. Air at 0∘C flows through the tube shown in the figure. Assume that air is an ideal fluid. What is the air speed v1 at point 1?2. Formulas: A1V1=A2V2, ΔP + 1/2ρΔV2, ρgΔh3. My first through process was to calculate the change in pressure in the mercury as the change in pressure in the...
  34. K

    Navier-Stokes equation for parallel flow

    Homework Statement [/B] Find an equation for the flow velocity of a river that is parallel to the bottom as a function of the perpendicular distance from the surface. Apply the boundary conditions given and solve, and find the velocity at the surface. Note that the coordinates are: x is the...
  35. MdAsher

    Fluid flow equations for a frisbee

    Hi All, I'm hoping to work on deriving the governing fluid flow equations for a frisbee in flight theoretically and then to test it on a wind tunnel, and compare results. If u could please help on how do i apply/derive the necessary equations. Respectful Regards
  36. MdAsher

    Understanding Fluid Dynamics of a Flying Frisbee

    Hi All, I'm hoping to work on deriving the governing fluid flow equations for a frisbee in flight theoretically and then to test it on a wind tunnel, and compare results. If u could please help on how do i apply/derive the necessary equations. Respectful Regards
  37. M

    Mis-using Bernoulli's equation

    Homework Statement An incompressible heated gas of constant temperature and pressure flows along an infinitely long tube at an unspecified velocity v1; a pressure of P1; and a density of p1 into an unheated open area of infinite volume containing the same gas at a lower pressure of P2; a...
  38. mariem.makh

    Fortran FORTRAN Code for Control Volume Finite Element Method Fluid Flow Problems

    where i can found an example A FORTRAN control volume finite element method code FOR FLUID FLOW PROBLEMS thanks for help
  39. T

    I Confusion Over Hydraulic Gradient, L parameter

    I've come across two different approaches to quantifying what l is in the equation for hydraulic gradient Δh/L. In this first picture L is the parallel distance along the datum across the reference plane But in this second picture L is the length along the pipe Why are the two L's...
  40. P

    Fluid flow acceleration in a channel

    MODERATOR NOTE: THIS HOMEWORK WAS SUBMITTED TO A NON-HOMEWORK FORUM SO THERE IS NO TEMPLATE Hi everyone, I'm stuck on a fluid mechanics problem and maybe you could help me. As shown in figure, there is a two-dimensional converging channel, in which cross-section area varies linearly with x...
  41. Q

    Fluid flow across a cylinder at an angle

    Hello, I need some help with a problem in fluid mechanics. Consider a situation where a cylinder ( of a given radius and length) is kept at an angle theta to the vertical, and a fluid flows across this cylinder. I need to determine the forces and moments acting on this cylinder, for different...
  42. B

    Equations for fluid flowing down a slope

    I want to be able to calculate the change in velocity of a fluid flowing down a slope using the drag equation as the source of friction. However the drag equation uses velocity squared which is constantly changing as the fluid accelerates/decelerates. How do I integrate the drag equation into...
  43. Soumalya

    Flow work and kinetic energy in fluid flow

    Considering horizontal flow in a pipe (under the action of pressure forces only) if we focus on a fluid element we have a pressure gradient across the same i.e, a net pressure force will always act over the element that essentially maintains the flow.Now, in a region in the flow field if the...
  44. 9

    Power Loss or gain from a fluid flow

    Hi guys, I have quick question about calculating the power lost in fluid flow. So imagine there is a simple setup as in the attached image, where you know the mass flow in and out, pressures at inlet and outlet and the temperature at the inlet. Is the equation to find the power lost or...
  45. meyol99

    Describe movement of particles with one equation?

    Hello everybody, I have a new thread to post,it is very important to find a solution for this : -Imagine a box full of air particles.The particles are forced to move to a point A on the edge of the box.My question is now,how can I mathematicly describe the movement of these particles toward...
  46. P

    Calculating Pressure Loss in Compressible Fluid Flow: A Case Study

    I have a quick question regarding compressible fluid flow, specifically: for a given amount of time, how much pressure is lost when attaching a balloon valve to a small air tank. I decided to model my solution after the following problem (from an old fluid mechanics textbook of mine), Air is...
  47. C

    Solving Fluid Flow in Pipe: Mass Rate 4Kg/s, P1 700Pa, P2 600Pa

    1. I am trying to help my grandson with his homework and I have to determine the flow of a fluid in a pipe. I am told that mass flow rate is 4Kg/s; density p is 1012Kg/m3; Diameters D1 = 300mm, D2 = 400mm; Pressure P1 = 700Pa, P2 = 600Pa; Heights y1=0mm, y2=100mm 2. I have applied Bernoulli's...
  48. S

    Conformal Transformation: Fluid flow over surface waves

    I would like to obtain the conformal map from a uniform rectilinear fluid flowing in the x-direction, where the field is bounded below by the x-axis, to the flow in the w-plane. In the w-plane the flow is correspondingly bounded from below by a trochoid. (A trochoid is a continuous waveform...
  49. Sivaraman

    Homogeneous equilibrium model-fluid flow

    Hello,I am trying to calculate the velocity in a pipe with length L and Dia D, which is connected to bottom of a pressurized vessel (Vessel dimensions are known, Level of liquid inside the vessel is known). Now i need to figure out the velocity as a function of pressure inside the vessel.We can...
  50. Soumalya

    Accelerating and Non-accelerating Coordinates - Fluid flow

    Referring to the problem in the attachment, the author mentions that if we consider the coordinate system attached to the bicycle and the bicycle accelerates or decelerates, the flow past the bicycle becomes unsteady. For an unsteady flow, we know that nothing changes at a given location on a...
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