# What is Fluid machanics: Definition and 51 Discussions

No Wikipedia entry exists for this tag
1. ### Confusion in using the continuity equation here

Q： Why does assuming "Properties in the tank are uniform, but time-dependent" lead to the validity of (DmDt)sys=0? Doesn't the mass within the system change over time? reference.

4. ### 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...
5. ### How far will the bottom gate of a reservoir open?

<Moderator's note: Upload images to PF. Do not use an external image server.> I got an expression for the angle taking that initially the gate is closed, so the gate has h, W and L as their dimensions. So there is a surface integral to find the resultant force applied on the gate, using the...
6. ### How did they get that the vorticity = 2##\omega##?

I'm learning Meteorology, and using the book Atmospheric Science by Wallace and Hobbs. We're discussing the kinematics of the winds (fluids). I shall post some images to say what I don't understand. This is how they define their natural coordinate system I know and understand the concepts...
7. ### Comparetto type Air Cleaner/Filter: What causes its increased airflow?

Typical box fan+air filter setups use axial consumer-grade fans designed for providing maximum air flow at low power consumption. These fans do not provide sufficient air flow at the high static pressure requirements typical of air filters. The air filters DIYers use are high Merv(e.g Merv 13)...
8. ### I What is a free surface?

I’m currently studying Fluid Mechanics, during an analysis I came across this We now consider an example that combines centrifugal force and gravity: a liquid in a drum (centrifuge) rotates with constant angular velocity ##\omega## about a vertical axis. The centrifugal force per unit of...
9. ### Three doubts in a paragraph on the equilibrium of Incompressible fluids

These are the images from Sommerfeld’s Lectures on Theoretical Physics, Vol 2 chapter 2, section 6, Equilibrium of Incompressible Fluids. Image 1 Image 2 Doubt 1 : What does it mean for a force to act on a fluid volume? Force acts on a point, force may act on a surface but I’m unable to...
10. ### (Fluid mechanics) How to find the flow rate of a gas from a vessel

I am working on a project where I have to calculate various results relating to the motion of a water bottle rocket being launched. I am currently stuck on trying to find how long the thrust period of the rocket is. The model for the rocket is as follows: It is a 2L (0.002m3 bottle filled with...
11. ### Flow Rate (Q) of Viscous Flow between Two Plates

I know how to derive the formula, but I have no idea how to actually use this. Where it says ##50~cm## wide, I'm not sure if that's in the x-direction or the z-direction. ##W##, the width in the equation, is in the z-direction. ##\mu## can be calculated, but I'm not sure what the pressure...
12. ### Trouble with fluid thermodynamics and nuclear thermal rockets

Summary:: In need of help determining the exhaust velocity of a rocket nozzle given temperature and propellant molar mass Greetings and salutations! My name is Robert DeVries, world builder extraordinaire. I have come with questions in search of answers. So for the last few days I've been...
13. ### Solving Impeller Analysis: 3 Unknowns & 0 Inlet Velocity?

Hello there, I am trying to analyze an impeller i found at home (small one) so i can find the torque-speed graph. The thing is that i get stuck in an equation with 3 unknowns instead of 2 and i don't know what else to assume. My approach is as follows: I take the usual equation: ## M = \dot{m} (...
14. ### Fluid Mechanics: Flow rate required to achieve a constant height

Some thoughts that I've had on the question are saying the volume flow rate (##Q##) in, must equal the volume flow rate out. If that's the case, then: ##Q_{in} = Q_{out}## ##A_1V_1=A_2V_2## But... no areas have been given. And height doesn't enter this equation at all. Then I thought it...
15. ### Thrust on an triagular lamina emersed in a liquid.

I feel there is a really obvious flaw in my logic but i can't pin it down So i have to find the thrust on the lamina which is basically force of bauyancy F(bouyancy)= Vρg Now volume of the triangular lamina is its rea into its hieght. v = Ah hence F = Ahρg some information i feel i didnt...
16. ### I want to understand this equation - Fluid Mechanics

Hi all! I have started reading Fluid Mechanics at my own pace (no university study) and really I would like to grasp the ideas behind it. So I have Fluid Mechanics by Cengel - 4th edition. At page 45 the coefficient of compressibility or bulk modulus of elasticity (κ) is introduced. ##κ =...
17. ### Energy equation for fluid mechanics

Homework Statement Water is supplied at 4.50 m3/s and 415 kPa (abs) to a hydraulic turbine through a 1.0-m inside diameter inlet pipe as indicated in the figure. The turbine discharge pipe has a 1.2-m inside diameter. The static pressure at section (2), 3 m below the turbine inlet, is 25 cm Hg...
18. ### Splash mechanics on Mars

They found an underground lake on Mars last week! Given that the lake exists and is actually liquid (and has a surface), two obvious questions come to mind: 1. Will I be able to beet my earthly current record in bouncing a pebble there? 2. If I release a pebble from a given height here and...
19. ### Injector pressure drop in a liquid rocket engine

Hello everybody, I hope you are doing fine. I'm currently designing an injector for a hydrogen peroxide - ethanol engine, and the following formula is confusing me: Q = Cd * A * sqrt( 2 * dP / rho); where Cd is the discharge coefficient, A the total area, dP the pressure drop, and rho the...
20. ### Relationship between volumetric flow rate and pressure

Hello everybody, This is my first post. I've been working on a liquid rocket engine for a while, but I have never gotten to work on its feed system (fluid mechanics), until recently. We are using a very simple pressure fed system, and its main goal is to deliver the propellants at a specific...
21. ### Control volume, Fluid mechanics

Homework Statement I have a situation as illustrated by the image above, where the red box illustrates the CV. The pressure on the left side of CV is P1, while the pressure on the right side of CV is P2. Here I´ve directed the pressure forces acting inwards on CV. The problem arises with...
22. ### Rising of a liquid during rotataion in a cylinder

Homework Statement [/B] A 25-cm-diameter, 100-cm-high cylindrical container, (as shown), is partially filled with 75-cm-high liquid (density = 900 kg/m3). Determine the rotational speed at which the liquid will start spilling. Calculate the gauge pressure at the centre of the bottom of the tank...
23. ### Area of common surface of two bubbles

What will be the area of common surface of two identical bubbles of radius R , i know there common surface will be flat as the radius of curvature of comman surface will tends to Infinity , but how do i relate with area of flat surface I tried to use Energy = Surface tension * area And then...
24. ### Mathematica Solve Perturbation problem with mathematica

u'(1-epsilon(u')^2)=-y uo(1)=0 u1(1)=0 I need solve this problem with mathematica.
25. ### 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...
26. ### A hollow sphere sinks some fraction f in water...

Homework Statement When completely hollow. To what fraction of volume should water be poured into it so that it is just enough to cause the sphere to sink. [/B] Homework Equations Archemedes : Buoyant fource equals weight of water displaced The Attempt at a Solution Let's first find the...
27. ### Why must we use integral to find the resultant force?

Homework Statement Hydrostatic force on a plane surface ex: Hydrostatic force on a gate: Homework Equations The Attempt at a Solution why can't we just use the formula in the red box above for problem 3.57? instead I have to use integral I am confused, how does this gate different from...
28. B

### Finding torque in rotating cylinder (viscosity )

Homework Statement Homework Equations τ= η* (dv/dy) The Attempt at a Solution so dA= 2π* r * dr T = ∫ τ * r * dA T = ∫((0.01* r* 2*π * 450 /60)/(5*10^-3) ) * r * 2π* r *dr = (592.1762)*(r^4/4) T = 0.11 Nm is this correct?
29. D

### E-Balance for Jet Ejector

I am working on problem that has to deal with a jet ejector. For the life of me, I cannot figure out the E-Balance equation for it. Below is a cut away image of the ejector. The fluid is the same so using the ideal gas law I do not think should be included. Any help is greatly appreciated...
30. ### Mean Section Method Homework: Cross Sectional Area Solution

Homework Statement I have problem of obtaining the cross sectional area . Homework Equations The Attempt at a Solution Taking section 2-3 as an example , my working is [ (3.3 +4.8) / 2 ]* 5 = 20.25 , but the ans provided is 29.28 , how to get the 29.28 ?
31. ### Surface Runoff - Explained and Debated

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...
32. ### Flow thru a channel constriction

Homework Statement When a part of the channel is constricted , why will the Emin = Eo ? Homework Equations The Attempt at a Solution I just have the figure showing in case 1 and case 2, Emin = Eo without any explanation ... Can someone explain about it ? I tried to search many online...
33. ### Possible Air Conditioning System?

Hello everyone! Summer is coming and instead of buying a $200-$400 air conditioner, I'd figure I could try a DIY version. I came up with a schematic (attached below) and how I think it would work. I feel like I'm missing something blatantly obvious that will make it not feasible. Wondering if...
34. ### Best hydraulic section

Homework Statement Determine the depth of the flow of channel if the best hydraulic channel is needd for composite channel in figure to convey flow rate of 6.5(m^3/s) , Manning coefficient n on bed slope are 0.015 and 0.0015 respectively . Homework Equations The Attempt at a Solution I...
35. ### Does entropy increase behind surface tension?

Is there any entropic gain when the surface of a liquid is minimised? Per example, molecules "enjoy" maximum entropy when they are at the interior. Is this valid?
36. ### Should I learn fluid mechanics or thermodynamics

I am graduate student from structural mechanics of solids and need to learn 1 course from fluids and thermal group which includes fluid mechanics or thermodynamics+heat transfer. I am unsure which one would be better to learn, especially which would be helpful in future for solid mechanics. Can...
37. ### Container of Liquid Accelerated Upward

Homework Statement A block floats partially submerged in a container of liquid. When the entire container is accelerated upward, which of the following happens? Assume that both the liquid and the block are incompressible. A) The block descends down lower into the liquid. B) The block does...
38. ### A problem about fluid mechanics

1. The problem statement, all variables and given/known d At what speed, the velocity head of water is equal to pressure head of 40 cm of Hg? (1) 10.32 m/s (2) 2.8 m/s (3) 5.6 m/s Homework Equations v^2/2g = p/rho*g (velocity head= pressure head). The Attempt at a Solution since pressure head...
39. ### Design a converging/diverging nozzle

I am intending to design a CD nozzle that produces 20mN thrust under atmospheric conditions. The reservoir pressure will be 1.4 bar, and the ambient pressure will be 1 bar. The other variables are adjustable. This will in the end be sent out for fabrication.My process is as follow, May I know...
40. ### Calculation of water flow rate in heat exchanger

I am not a mechanical engineer; but I have physics background. I want to lift about 250 watts of heat from a metal block. At the same time I have to maintain its temperature at 30 deg Cel. For this I want to use water-air heat exchanger (HE). Heat exchanger will have inlet & outlet pipes. Hot...
41. ### What research field is concerned with the mechanics of sand?

I am interested in conducting a research project involving the design of a tool to aid astronauts to perform experiments on lunar surfaces (for the NASA Micro-g NExT student challenge), but I do not know appropriate key words or questions to ask to begin my research. The specific scenario of...
42. ### I Strange Magnus Force in a Whirlpool

1. Imagine a big river with water flowing constant velocity , assume there is a whirlpool at center as shown in the figure. 2. Because of the magnus force the whirlpool should move towards left or right towards river bank depending on the spin, but it does't happens it just stays fine in its...
43. ### 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...
44. ### Pascal's Principle: Valid w/o Gravity?

Is Pascal's Principle valid with no gravity?
45. ### Doubt from fluid mechanics

for static fluids we have studied that p=patm +density*g*depth. and this equation is derived from Newton's laws,but in fbd why didn't we consider the force exerted by curved surface area of container,it doesn't cancel out when containers are in frustrum shape.and the vertical components add up...
46. ### Correction Of Transfer Function

Hello everyone; My question is regarding to velocity effect on transducer diaphragm. When fluid flow is acting to differential pressure transducer's diaphragm, the diaphragm is osculating(vibrating) and there will be fluid flow which is transverse(opposite flow,back flow) to normal fluid flow...
47. ### Contradictions in Irrotational Vortex?

So I was reading about Vortices in Wikipedia, when I stumbled upon the explanation about Irrotational Vortices. It states there that in Irrotational vortices, the fluid's flow velocity u, is inversely proportional to the distance r, from the vortex's axis. Then, at the end of the...
48. ### Help with torque calculation of underwater rotating disk?

Hello Guys, I have a disk of diameter of 150mm and thickness of 10mm which is rotating at the constant speed of 100rpm. The disk is inside water which is approximately 4-5 cm below the surface. Disk is horizontal (parallel to the water surface). Disk is attached with the shaft of the motor. At...
49. ### Bernoulli's equation and velocity

What happens when you have more than one hole in a vessel containing liquid? Can you help me finding the velocities at different holes if they are at some height from level of liquid at a particular instant? I know what happens when you put a small hole in a tank containing liquid at a height h...
50. ### Can applied physicists be CFD analysts

I am about to graduate with an aerospace engineering degree. I am planning to go into grad school for Applied physics(probably plasma physics), which I find very interesting. But the main reason I joined the aerospace degree was because I was interested in CFD. But since I could't land any...