# Fluid flow Definition and 52 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. ### A Steady state confined flow field: Is it cyclic?

For a fluid that is confined to a finite region with no sources and sinks, are the only options for the flow field a) static, and b) cyclic? The example I have in mind is Rayleigh convection in a shallow dish heated from below, where convection cells are formed beyond a certain temperature...
2. ### I About Bernoulli's equation for fluid flow

Hello, I am currently studiying Bernoulli's equation and I have trubble understanding something , say we have a horizontal hose (no change in altitude of pressure ) Bernoulli's equation state that an ideal fluid can flow thought the hose with the same velocity , does an ideal fluid need a...
3. ### 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...
4. ### Heat Transfer in thin tubes

Hi, I am trying to understand how I can estimate the time it takes for a fluid at room temperature flowing through a thin capillary glass tube (2 mm ID) connected to an oven to reach the equilibrium temperature (oven temperature). Assuming the oven is preheated and the tube inside the oven is...
5. ### Fluid flow around a wind turbine blade -- help please

I have to estimate the Reynolds number of the fluid flowing around a blade but I only have one detail: the length of the blade is 0.66m. I have no idea how to do it and I got to submit the assignment in a week, I am desperate for help here.
6. ### Flow of a Non Viscous Fluid Through this Nozzle

My first post here! I have calculated everything i need, except the thickness of the second layer dZ*, therefore i can't solve V3/V2=(R2ˇ2)/(R3ˇ2) Trying for days now, i would appreciate any help.
7. ### Water flowing out of a tank through a hole

I assume the water to start flowing from rest at position 1, hence ##v_1 = 0##. Applying the continuity equation, ##A_1 v_1 = A_2 v_2##, we find the (wrong) result that the velocity at position 2 is ##v_2 = 0## also! (We assume that ##A_2## is small but finite) Hence, to answer the question...
8. ### Maximum height for water from a fire hose

Homework Statement Fire hose has diameter of 4.0 cm and flow rate of 10 L/s. There is pressure of 2.2 bar inside the hose. How high the water can go at best? Water density is 1.00E3 kg/m^3 and air pressure outside the hose is 1.0 bar. Homework Equations Flow rate $$Q = Av$$ Newtons...
9. ### Shear stress direction and the velocity gradient

1)In a flowing fluid in laminar fashion we know that it flows in planes which slides over each other, Let's take a fluid element (cylindrical) in a pipe(Radius=R) the resistive force is (stress)(cross section area of cylinder with radius 'r') acting in backward direction, now if I take Flow...
10. ### Effective Resistance (Fluid Mechanics)

Imagine you have a vertical pressure head 2m tall with water flowing down to height 0m and emptying. Through this length we have 3 identical tubes of resistance R and length L. Assuming I know the volumetric flow rate(Q) as a function of height, how can I calculate the resistance of each tube...
11. ### Hydraulic head as a function of flow rate

Hello! 1. Homework Statement Symbols and variables: ##\dot{V}## : flow rate ##H## : Hydraulic head ##g = 9.81 \frac{N}{kg}## ##P_{atmosphere} = 101325Pa## ##\rho = 1000\frac{kg}{m^3}## I am given a piping system, with serie and parallel pipes, they start from one point and go back into one...
12. ### I How does a fluid behave inside of a spherical cavity?

Suppose you have a spherical cavity, with a flow of a fluid ( in particular water), entering it from one side. What will happen to this flow? Will it create turbulences? Will speed increase? Will eventual waves entering the cavity be reflected, so creating waves with bigger amplitude? Suppose...
13. ### Air resistance in projectile motion

In lower-division physics classes, air resistance is usually ignored to make the mathematics of projectile motion easier to understand. When air resistance is included, it's often stated that at lower velocities, air resistance is proportional to the velocity of the object, Fair ∝ kv At...
14. ### I Variation of Torriceli's theorem?

Hi there. Everyone knows about Torricelli's theorem that says about , in a too big container (opened) the speed of the liquid is given by: v=√(2gh) This result is just for containers that have a hole in the side and the fluid goes out perpendicular to the gravity. And also this result is just...
15. ### Difficuilt and/or interesting problems in fluid mechanics

Homework Statement Can anyone please give me hard and interesting problems regarding fluid mechanics problems such as hydrostatic and air prssure bouyance force and Bernoullis equation(Bernoullis equation problems only for liquids)? I tried looking for them but the ones I found were quite...
16. P

### Fluid Dynamics -- a spherical particle immersed in water

Homework Statement Consider a spherical particle immersed in water. It will experience random collisions with the surrounding water molecules. Suppose there are such water molecules around the particle. Half (n/2) of the water molecules will push the particle to the right and the other half to...
17. ### Surface runoff

Homework Statement In my book , i was told that when the drainage of surface is high then the surface runoff will be low . However , i don't think it's correct . Homework Equations The Attempt at a Solution Definiton : Runoff can be described as the part of the water cycle that flows over...
18. ### Modeling a multiple inlet piping system

Hello, PF! I’m trying to model a real piping system, which has multiple inlets and one outlet, so I can’t use Bernoulli’s equation. Instead, I’m planning to use the generalized macroscopic energy balances as shown in BSL, which allow for any number of inlets and outlets. However, first I want...
19. ### Cellular Automaton, Random Walks, and Fluid Mechanics

Hi I'm trying to understand a paper that approximates the solution to Burger's equation (1D Navier Stokes) by a doing a one-dimensional cellular automaton simulation. I'm having a hard time understanding how all these topics connect. I have seen and walked through various demonstrations that...
20. ### Fluid pressure and flow

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...
21. ### Falling film momentum balance

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...
22. ### 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...
23. ### 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...
24. ### 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...
25. ### 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...
26. ### 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...
27. ### 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?
28. ### 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...
29. ### 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...
30. ### Parallel flow exercise

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...
31. ### 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...
32. ### 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...
33. ### 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...
34. ### 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]...
35. ### 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
36. ### 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...
37. ### 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...
38. ### 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...
39. ### 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...
40. ### Using Bernoulli's Equation

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...
41. ### 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...
42. ### 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...
43. ### 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...
44. ### 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...
45. ### 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...
46. ### 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...
47. ### 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...
48. ### 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...
49. ### Fluid flow/pressure in a pipe going uphill

Homework Statement Water is flowing along a pipe. Points B and C in the pipe are at the same height but point A is at a higher elevation.The pipe has the same cross-sectional area at points A and B but is wider at point C. Rank in order from largest to smallest, the pressures at the three...
50. ### Using Laminar Flow to measure Thermal Conductivity

So, there's a project I'm working on (no, it's not school related), where I'm trying to figure out how to measure thermal conductivity of a material using laminar flow. The idea is to integrate this into a larger system, and it's a measurement I'd like to take in the process. I know the...