Fluid dynamics Definition and 641 Threads

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. KnightTheConqueror

    B Resnick Halliday Krane Venturimeter formula mistake?

    If this formula were to be correct, if we use the liquids in the Venturimeter and the tube to be of the same density, the velocity would come out to be zero which makes no sense. I calculated the formula and got a slightly different numerator. Am I correct?
  2. WhiteWolf98

    Transitional Reynolds Number When Analysing Flow Through Orifices

    Greetings, I need to work out the flow rate of of a flow through an orifice, where the size of the orifice and differential pressure are varied. The primary unknown in working out flow rate is the coefficient of discharge. A textbook I've been using to help me is "Hydraulic Control Systems" by...
  3. U

    Physics, energy and sustainability research

    I'm 64. My background prior to 2001 - Aviation, experimental aircraft and aerodynamics, robotics, mechanical and software engineering, disaster recovery. Since 2001 - Human sustainability and global recovery, researching every related sector in depth in order to understand what can now be proven...
  4. H

    Different forms of fluid energy conservation equations

    I asked this question about one year ago, but at that time I didn't really understand what I was doing. After spending a lot of time in this problem, I still fail to get the asked answer. Starting with ##\frac{De}{Dt} + (\gamma - 1)e \nabla \cdot \vec{u} = - \frac{1}{\rho} (\vec{u} \cdot...
  5. lost captain

    I Is shear stress at the pipe wall the same for turbulent and laminar flows?

    Hello everyone 😊, I know that shear stress in turbulent flow is a lot larger from shear stress in laminar flow. My question is about the shear stress at the walls of the pipe. So i was watching a video about shear stress in turbulent flow and the narrator pointed out that the shear stress in...
  6. lost captain

    I Shouldn't the pressure difference along a pipe be a negative number?

    Across a horizontal pipe with fluid flowing inside, the pressure will drop due to the friction between the fluid and the pipe walls So if for example the fluid flows from left to right and we take 2 points: P1 (pressure on the left) and P2 (pressure on the right) then P1 should be bigger than...
  7. lost captain

    I How does friction in a pipe occur, with the "no slip condition" ?

    Hello everyone 😊 Let's say, we are having laminar flow in a cylindrical pipe. The fluid in direct contact with the pipe doesn't move (no slip condition), so there is no sliding between the surface of the pipe and the surface of the water. The friction that occurs is actually between this...
  8. lost captain

    What's the pressure long before the fluid exits a pipe?

    TL;DR Summary: We know that the fluid pressure at the exit of the pipe is Patm but what about before the exit? How can we calculate that pressure using Bernoulli's equation. Hello everyone☺️. Please help, i'm seriously stuck 🫠🙇‍♀️ When learning about bernoulli's equation there's a classic...
  9. A

    Oil enters a flexible bend in a pipe -- calculate the force needed to hold the bend in place

    The pressure is easily calculated from equation of continuity and Bernoullis theorem: $$A_1v_1=A_2v_2\implies v_2=16ms^{-1}$$ SInce pipe is in horizontal plane, no difference in pressure because of height $$P_1+\frac{\rho v_1^2}{2}=P_2+\frac{\rho v_2^2}{2}$$ $$P_2=172\times 10^3$$ What I...
  10. H

    Boundary condition for a flow past a spherical obstacle

    I'm trying to find how the author finds the boundary condition at ##r\to\infty## is ## \Phi(r,\theta, \phi) = - V r cos \theta##. Using the spherical coordinates. ##- V \hat{z} = \nabla \Phi## ##- V ( cos \theta \hat{r} - sin \theta \hat{\theta}) = \frac{d \Phi}{dr}\hat{r} + 1/r \frac{d...
  11. Andy Resnick

    Q: Fluid drainage along the outside of an inclined cylinder

    Asking on behalf of a colleague who is studying rainwater transport/drainage in the context of plants (trees, mostly): There's a lot of published work analyzing the flow of fluid within a tube, or along an interior corner. I can't seem to find much of anything about the flow of water along the...
  12. Massimo

    I Express drag force/acceleration/velocity as function of time

    Let's say an object with a mass of 400,000 kg, a drag coefficient of C, and a reference area of A m^2 moves horizontally with a thrust of X N through a fluid with a density of Z kg/m^3. The object is initially stationary. I'm trying to figure out how I can express the drag force, acceleration...
  13. tworitdash

    A Continuity of a quantity in a conical system to determine the velocity

    My research is on radar images and the images are collected in several conical surfaces. These conical surfaces have the same origin, the same maximum length (max flare or max range), but different elevations angles. The images are collected on the surface of the cones only. I want to determine...
  14. VincentIsoz78

    I Static sphere with gravitating fluid

    Hi The rotating bucket problem with a fluid is well known as a homework. For the fun i wanted to adapt it to the case of a massive non-rotating sphere surrounded by a fluid. However i don't know if the calculations i made are correct or don't make sense at all (even if the result lead to an...
  15. printereater

    Formula derivation connecting vertical water flowrate & horizontal distance moved by a suspended sphere

    TL;DR Summary: I am Highschool student writing a 4000 word research paper on Bernoulli's principle and the coanda effect. I need help with derivation of a formula that connects flow rate of water and distance moved by the sphere in my experiment. I am a high school student writing a 4000 word...
  16. hya_t

    Calculating the inlet lengths of a fluid circulating in a pipe?

    TL;DR Summary: why do we need input length in fluid's dynamics What is the purpose of calculating the inlet lengths of a fluid circulating in a pipe? and how to calculate it correctly? why do we need it in engineering? This is for a class project where we are asked to calculate it and I think...
  17. N

    Where can I study this type of problem? (fluid dynamics)

    Hello everyone! I have to study for an exam, of which I have just retaken the subject (fluid dynamics) and I don't feel very good. In my language (Spanish) I find these similar problems, but in English I only find problems that do not include these types of arrangements. I know that the...
  18. H

    Coordinate transformation of the Navier Stokes equation

    i have successfully transformed the continuity equation using coordinate transform,but having trouble with the momentum equation . can someone kindly provide the transformation of the right hand sight of equation of the image i have attached.
  19. cyboman

    I G force and extreme aviation maneuvers

    I'm wondering what the potential benefit would be to having a fighter pilot surrounded by fluid in the cockpit on their ability to withstand high G maneuvers. How do the mechanics of fluid dynamics interact with the forces of gravity and the physiological stresses on the pilot during high G...
  20. G

    I Conservation of linear momentum in RH relations

    I am trying to follow a derivation of the Rankine-Hugoniot equations in a paper by Peter Krehl titled: The classical Rankine-Hugoniot jump conditions, an important cornerstone of modern shock wave physics: ideal assumptions vs. reality This paper talks about the RH equations which relate...
  21. H

    I Fluid dynamics: drag coefficient and pressure at the stagnation point.

    Hi, In my textbook the author say that the drag coefficient is the drag force divided by the pressure at the stagnation point time the area perpendicular to the stream. ##c_d = \frac{2F_d}{\rho v^2 A}## To get the pressure at the stagnation point I'm using Bernoulli for an incompressible fluid...
  22. jackcarroll434

    Spec'ing the power of heater for an atypical heat exchanger problem

    Homework Statement: Calculate the power of heating source required in a heat exchanger Relevant Equations: Heat transfer for LMTD heat exchanger. I have a real-world problem whereby: Water is flowing at a constant flow rate of 10 mL/min through a PVC tube, inner radius of 1.25mm and outer...
  23. H

    From fluid energy conservation equation to the continuity equation

    Hey there, First of all, all energy conservation equations for a fluid I found on google hadn't the ##\gamma## coefficient. What exactly is the difference? Secondly, by substituting e by ##e = \frac{1}{\gamma -1} \frac{p}{\rho}## in the following equation ##\frac{De}{Dt} + (\gamma - 1)e \nabla...
  24. tracker890 Source h

    Q: Confusing Conservation of Mass Flow Rate and Conservation of Flow Rate

    ref. answer Q： Why ρ1≠ρ2≠ρ3, but the answer says ρ1Q1+ρ2Q2=ρ3Q3 can be simplified into Q1+Q2=Q3?
  25. HelloCthulhu

    DIY electrowetting on a hydrophilic surface

    Even as a layman, I've been fascinated with the structure and activity of water for years. And after reading this paper, I thought testing the electrostatic repulsion of water on a hydrophilic surface would be an interesting experiment. I'd like to see if the water droplet starts dispersing into...
  26. RCH

    Which Pump Can Safely Circulate Fluid with Glass Spheres?

    Hello -- I am working on a project where I need to recirculate a fluid which has a large quantity of glass spheres of say, 1mm diameter suspended in it, but without the pump crushing the spheres. I would appreciate any advice. To give a simple description of the desired function: it will be a...
  27. I

    I Compressed air Pushing a Column of Water

    Greeting, I have been noodling on this problem for a bit and was hoping to have some input from this form. what volume of compressed air, at 200 PSI, will it take to push lets say 30 gallons of water 8 feet up a 3/4" pipe at a minimum of 8 gpm with a minimum pressure of 8psi at the top if...
  28. E

    Why is solid density used in this thermodynamics problem?

    Callen asks us the following question in his famous textbook: I have answered as follows: However, I get the wrong answer and, in fact, the correct answer obtains from using ##\Delta P = -\rho_s g h##; that is, using the solid density for the change in pressure. Now why on earth should this...
  29. turo_loler

    I Any free MHD software recomended?

    I'm currently searhing for a magnetohydrodynamics simulator to study how different molten metals interact with variating magnetic fields, is there any free (preferebly) simulator that someone recomends in particular? And also, is there any way to use Autodesk's CFD to create a MHD simulation?
  30. axelb

    I How is the Reynolds number derived (is my derivation wrong)?

    I'm a HS student so please dumb it down. I'm looking into the Reynolds number of a sphere sinking in a fluid, and I want to determine whether my results meet creeping flow or not Re<<1, here's what I got. **sorry if I misused the prefix, I'm not sure whether it's highschool or undergraduate**...
  31. dylanwalt

    Fluid Dynamics - Using the Manometer Equation

    I tried to use this equation, so I isolated the delta h because that is what im solving for and then I thought because the pressure on both ends of the reservoir is both atmospheric pressure the change in pressure is 0. This makes my entire equation 0 and thus height is 0 which is definitely not...
  32. Valour549

    Fluid Mechanics Force Body Diagrams

    Let me start off by saying that I have found (or is given) all of these: ρ, Q, V1, V2, P1, P2, A1, A2 (V being the velocity here). So no problem with Bernoulli or the Continuity equation calculations. I am just struggling with drawing the FBD in order to evaluate the axial force, Fx I know we...
  33. R

    A Converting this vector into polar form

    In the following%3A%20https://pubs.rsc.org/en/content/articlehtml/2013/sm/c3sm00140g?casa_token=3O_jwMdswQQAAAAA%3AaSRtvg3XUHSnUwFKEDo01etmudxmMm8lcU4dIUSkJ52Hzitv2c_RSQJYsoHE1Bm2ubZ3sdt6mq5S-w'] paper, the surface velocity for a moving, spherical particle is given as (eq 1)...
  34. C

    B Knudsen Flow: High School Student Q&A

    I am a high school student trying to carry out an experiment about fluid. Thus I am studying Knudsen flow and come up with following questions. 1. How can a Knudsen flow occurs? 2. Can I simply dig a small hole on a board and make Knudsen flow? 3. What the difference between viscous flow...
  35. S

    Engineering How to Approach the Prandtl Boundary Layer Equation for Steady Laminar Flow?

    I have tried to approach in the following way I am stuck. How should I approach this next.please help
  36. URIA

    How to Determine the Stability Condition in Couette Flow?

    Dear All, I tried to solve the attached question. it's about Couette flow, where the 2 plates move. in fact, I have to find the stability condition. is someone familiar with this and can help? many thanks, uria
  37. Valentina Chianese

    B Understanding the Rheopectic Properties of Blood: Fact or Fiction?

    Hello everyone, I need your help. A teacher once told me that blood is a non-Newtonian, rheopectic, pseudo plastic fluid. I get the non-Newtonian/pseudo plastic part, but I can't understand why blood is rheopectic...is it or not? Thanks
  38. P

    Need help coming up with ideas (bachelor's thesis)

    TL;DR Summary: Need help coming up with ideas to present a current thesis in terms of qualitative and quantitative ideas. Hey! I have a problem. I have picked up on a project for which I am planning on doing my bachelor's thesis on. The guy behind the project is not a physicist, per se, as he...
  39. jackkk_gatz

    Volumetric flow of a overflowing cone

    I solved the case where m=0.99999. Then the height at which it overflows can be obtained with the equation, when points on the liquid surface are chosen. Then the cross-sectional area is given by the circumference of the circle times the height that the parabola reaches, that cross-sectional...
  40. A

    I Fluid flow through a pin-hole of x diameter in a closed container

    Greetings, I've come across lots of exercises regarding Bernoulli's equation. However, never seen one where the top of the vessel is closed, and fluid flow exists via gas (air) going in. Has this problem been studied in the past? Assume a cylindrical vessel filled to the maximum with a D-sized...
  41. person123

    I Momentum of a Water Jet Impacting Plate

    Suppose you have a jet of fluid (say water) traveling vertically upward at a constant velocity. It impacts a stationary horizontal plate and so moves radially outward in all directions. Assume that there's no energy loss during the impact, so the speed of the fluid remains constant. Is momentum...
  42. A

    Bernoulli equation and parallel pipe branch

    Hello! I have a question regarding the application of the bernoulli equation and calculation of the flow through a parallel pipe branch. It's more the basic understanding how the flow will establish. You can find a sketch attached to follow my explanation. Let's assume I have a pipe with...
  43. N

    Fluid Dynamics Question -- Water flowing through a pipe into two cylinders

    Because my little work project involves fluids I thought this the best topic to post under. I took the route of biological sciences and computer science. This area is out of my league at the moment I'm not sure the amount of time that would be required to get the material applicable to this...
  44. casparov

    I Physics of paper absorbing Water -- Doesn't this decrease Entropy?

    Summary: doesn't this decrease entropy ? Cellulose is known for its hydrophilic quality, which can be explained from the polarity of its hydroxyl groups. We all know water can overcome the force of gravity through a piece of paper you put in the water. Correct me if I'm wrong but this is a...
  45. Delta2

    I Understanding Sound Waves in Fluids: Pressure and Velocity Fields

    When we talk about sound waves in a fluid (air, water e.t.c.) we mean that the pressure ##P(x,y,z,t)## satisfies the wave equation, the so called velocity field of the fluid ##v(x,y,z,t)## satisfies the wave equation or both?
  46. Ozen

    Bi-propellant Liquid Fuel Engine Pressures Before Thrust Chamber

    Been reading Rocket Propulsion Elements 9th Edition and got approval from my university to design a bi-propellant liquid fuel rocket engine for my senior design project, and I've been understanding everything so far but I haven't quite found an answer to how the pressure works throughout the...
  47. PaxFinnica96

    Engineering Fluid Dynamics: Proof of the Static Pressure Head equation

    I am trying to mathematically prove the Static Pressure Head equation: H = p/ρg How can I prove this equation and thus determine the nature of the relationship between these variables?
  48. jones1234

    A How can I interpret the 2D advection equation?

    I want to model the advection of debris rock layer with a thickness hd on top of a glacier through ice flow with velocity components u and v. Can anybody explain the physical difference between these 2 equations and which one I should take? Thanks
  49. jones1234

    I Estimating Vertical Wind Speeds: T_a, T_s, & P_a

    Hi all, I have some data from an automatic weather station, with recordings of both 2m air temperature and 2m air pressure and also the surface temperature. Is it possible to estimate vertical wind speeds between the AWS and the surface based upon this data? Imagine T_a = 15 degrees and T_s =...
  50. F

    Audio/Video Fluid Dynamics—Building a better MIDI Breath Controller

    Background A MIDI breath controller converts breathing to MIDI values, which are then used to control a MIDI instrument and produce sound. The ones I'm familiar with work through the use of a pressure chip. For example, the TEC Breath and Bite Controller 2 uses the MPCV5010GP...
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