# What is Fluid dynamics: Definition and 630 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. ### 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...
2. ### 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...
3. ### 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...
4. ### 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...
5. ### 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...
6. ### 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...
7. ### 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.
8. ### 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...
9. ### 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...
10. ### 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...
11. ### 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...
12. ### 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...
13. ### 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?
14. ### 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...
15. ### What kind of pump can do this?

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...
16. ### 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...
17. ### 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...
18. ### 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?
19. ### 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**...
20. ### 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...
21. ### 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...
22. ### 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)...
23. ### 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...
24. ### Engineering Fluid dynamics problem involving the Prandtl boundary layer equation for a two dimensional steady laminar flow

I have tried to approach in the following way I am stuck. How should I approach this next.please help
25. ### Help with fluid dynamics problem

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
26. ### 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
27. ### 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...
28. ### 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...
29. ### 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...
30. ### 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...
31. ### 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...
32. ### 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...
33. ### 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...
34. ### 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?
35. ### 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...
36. ### 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?
37. ### 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
38. ### 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 =...
39. ### 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...
40. ### B Propagation speed of movements of a fluid in a pipe

Assume that we have a 1.5 km x 100 cm^2 long straight pipe, totally inelastic and full of water. From time t = 0, a pressure of 300 000 Pa is continuously applied to the water with a piston at one extremity. This correspond to a force of 30 000 N on the pipe cross section in the direction of the...
41. ### I Jupiter atmosphere, turbulence and ocean fluid dynamics

Ocean physics explain cyclones on Jupiter https://phys.org/news/2022-01-ocean-physics-cyclones-jupiter.html Moist convection drives an upscale energy transfer at Jovian high latitudes https://www.nature.com/articles/s41567-021-01458-y...
42. ### A The kinematic equation'' of fluid flows

I saw this in a textbook and I thought it is a corollary of Reynold's transport theorem. Let \mathbf{F} be a smooth vector field Consider the surface integral: \int_{S}\mathbf{F}\cdot d\mathbf{S} and now take the derivative of it, then the expression can be written as...

44. ### B Beam of air: Extending the range of a fan

If one Googles for "laminar flow nozzle", one finds many interesting tutorials on creating a nozzle for a laminar water jet; a stream of water that remains coherent over a long distance without breaking up. These typically consist of a large-diameter tube with regions inside (like sponges and...
45. ### Dependence of the stress vector on surface orientation

According to Cauchy's stress theorem, the stress vector ##\mathbf{T}^{(\mathbf{n})}## at any point P in a continuum medium associated with a plane with normal unit vector n can be expressed as a function of the stress vectors on the planes perpendicular to the coordinate axes, i.e., in terms of...
46. ### Real-life phenomenon/application of this fluid dynamics model

Hi! Water is flowing in a converging duct, with the angle α, see the figure. My task is to find a real-life phenomenon / application of this model, and later solve it numerically / analytically where this fluidproblem occurs. However, my imagination is kinda slow today, what are some fun /...
47. ### Calculating force on a syringe plunger for a viscous fluid?

I'm currently working on a precise glue/resin dispenser, and I'm trying to derive an equation for the force one must exert on a syringe plunger as a function of the desired flow rate Q, and also accounting for the fluid viscosity and the syringe barrel and needle geometry. I've attached a scan...
48. ### Modelling of two phase flow in packed bed using conservation equations

Previously, I have seen the derivation of the energy conservation equations for simulation of single phase flow in a porous media (a packed bed). These are the energy equations for the solid and fluid respectively: I understand the derivation, however, these equations will only work when the...
49. ### Speed/Velocity and Volume Flow Rate of Viscous Fluids

In my first attempt, I started off converting the radii of all three sections from centimeters (10, 8, 6) to meters (0.10 , 0.08 , 0.06), then used the VFR=Av formula to find the speed/velocity of section one. VFR== 0.063 m^3/s A== pi*r^2=pi*(10cm)^2=pi*(0.10)^2=pi*0.01 == 0.031415927 VFR/A=v...
50. ### Notation in fluid dynamics: A circle with a horizontal bar inside

I came across this notation when reading the wiki article on Venturi effect, which baffled me. Can someone please tell me its meaning?