Questions tagged [fluid-mechanics]

Questions relating to fluids at rest (fluid statics) or the forces and effects of fluids in motion (fluid dynamics).

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converting air flow rate between l/min to kg/s

I am calculating the air mass flow rate in kg/s for air in a numerical model, and i want to compare the results with an experimental investigation, the flowmeter unit is l/min. how can I convert l/min ...
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Inaccurate flow rate measurements of an engine diesel fuel due to change of its viscosity

I am measuring the volumetric flow rate of a diesel fuel using "Turbines" flow meter, both in the supply and return fuel lines of an industrial diesel engine (Caterpillar 3512B – coupled ...
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Glide Ratio of an unpowered "sub"

Flying gliders can have glide ratios (ratio of horizontal distance vs vertical distance) around 50 or 60. How would that change when the fluid density changes, in particular under water. Could a ...
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How to determine time taken for tyre to deflate using CFD analysis?

I am working on my final year project, and am at a point which requires us to deflate a tyre in 0.8 seconds. This needs to be simulated using CFD. We have drilled a hole into the rim and attached a ...
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Why is the exit loss $\frac{u^2}{2g}$ is small for laminar flow?

I have done some calculations: My lecturer just said that exit loss $\frac{u^2}{2g}$ is small for laminar flow so ignore it. But never explains why it is small?
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What aerodynamic equation should be used to determine the drop in air pressure over an airfoil embedded within a vertical pipe?

I have a conceptual idea for a thruster for a VTOL aircraft/drone which should produce lift by utilizing a upward-flowing airflow to produce low air pressure over the top surface of an airfoil that is ...
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Risk of operating hydraulic gear pumps below minimum speed

I'm looking at Marzocchi micro gear pumps, particularly the 2.00 model. Speed range is 800-200 rpm (although page 20 shows flow for 100-3500), but in my application I plan on using a bi-directional ...
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Increasing airflow of fan

I learned at at young age from a physics kit that one can blow up a large bag with your mouth by blowing into it from a distance and letting the air pick more air with it. Later in life when Dyson ...
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What is the thermal conducivity of Air between two plates?

What is a rough cut figure I could use to determine what the thermal conductivity of 10mil and/or 20mil of air sandwiched between two plates (perfectly conducting)? I understand that convection dies ...
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How do I find the amount of energy needed to evaporate water below its boiling point?

Let's say I have a container filled with water and its surface is in direct contact with air. How much heat/energy do I have to supply to the water such that it evaporates under its boiling point? is ...
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What are pressures, air-flow rates and ground clearance of some real-life hovercraft?

I'm trying to get a feel for the real life behavior of hover craft air cushions and the associated power consumption. To get a feel for relevant numbers, I'd like to know actual pressure used to lift ...
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Different equations for Hydropower

I was doing some reading in hydropower and I came across these two power equations: $$P = ρghQ $$ and $$ P = QHg $$ Q is flow rate, ρ is water density, g is gravity, h is height, and H is head. I do ...
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Different Values for Different Calculations of Specific Weight of Water in lbf/ft^3

I have been trying to wrap my head around this, but I really can't figure it out. I don't understand why the value for the specific weight of water differs when I directly convert it from $\frac{N}{m^...
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Is structural strength the only reason for the hyperboloid shape of natural draft cooling towers?

I have read that the main reason for the hyperboloid shape of cooling towers you find next to thermal power plants is because it allows more structural strength with less material. According to some ...
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Discharge at an angle in a channel concept question

Now I agree that the velocity of the discharge and the area is not at right angles, but I don't understand how using $\cos \theta$ helps? Shouldn't I use the area perpendicular to the channel? Like so:...
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Forces from momentum flux in pipe bend Confusion

I understand the general process of solving these kind of problems. First draw the control volume and then show the forces on the boundaries and draw the reactions. In this case, there will be sum of ...
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1 answer
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Difference between Front Tracking and Front Capturing Method

I am having difficulty understanding the difference between interface capturing and interface tracking schemes. It is important to track it for solving Navier Stokes equation and other fluid problems ...
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Bernouli equation in a glass of water

If a glass full of water has a hole in the bottom connected to a horizontal straw with a known diameter and length. How could you apply Bernoulli to calculate the height of the glass if you could ...
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1 answer
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Can airflow through an HVAC duct be considered inviscid? Does viscid flow ever occur for HVAC?

Can airflow through an HVAC duct be considered inviscid? Does viscid flow ever occur for HVAC? If the answer to (1) is yes, how do we know airflow in an HVAC pipe is inviscid? In general, what's a ...
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why the fluidity test for casting is made in spiral shape?

Why fluidity test uses a spiral shape of casting? Is it to reduce the heat transfer? make it compact? Or any other reason?
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Calculating pressure forces (vertical) on a curve surface

I am particularly confused about finding pressure forces (specifically vertical force) on a curve surface. Although I understand the general method is to use sum of forces in the x/y direction (ie by ...
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2 answers
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How to use moments to find the positions of the centre of buoyancy and the centre of gravity?

Here is the question: A pontoon of length L is ballasted so that the density in the bottom half is twice that in the top half. a) Find the maximum value, smax, of the specific density, s, for which ...
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Finding the weight of water required to float object

Find the weight of water required to float the pontoon in the casting dock below when the gap all around is 0.1m. The total weight the pontoon is 500 tons (1ton=8896N). Is this consistent with ...
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What is the difference between $\Delta h$ and $\Delta z$ in this diagram and in a manometer equation?

Derive an expression for the differential pressure (in m and Pa) between A and B shown by the oil-water manometer below. Solve for $Δh=0.7 \ \mathrm m$ and $dz=1.5 \ \mathrm m$, $s_{soil}=0.8$. Pipes ...
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The physical principle behind short recoil operation in guns

Introduction I'm interested in the underlying physics of the recoil-operated loading mechanism in a general rifle system (e.g. M107A1 rifle). A technical of the rifle can be found on wikipedia https://...
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Method for computing the pressure drop across a bank of tubes

I would like to determine the pressure drop of fluid in cross flow over a bank of tubes. (see figure 1). Previously, I've used the correlation listed in "Fundamentals of Heat and Mass Transfer&...
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1 answer
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Value of the Reynolds Number in constant velocity flows

Consider a fluid flowing such that all the particles have same velocity, what will be the Reynolds Number for such a flow? We know that Reynolds number, $$Re=\frac{inertia \,forces}{viscous \,forces}$$...
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1 answer
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Thrust force of a fan

If I have a fan with these attributes Category Value Air flow 108.2 CFM (3.03 $m^3/min$) RPM 3100 Dimensions LxHxW (mm) 120x120x25 Is it correct to calculate the thrust force at max rpm like this:...
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Can anyone help me solve this problem?

I am a bit confused with this because the channel is not infinite in the z direction. I know that I have to do some things with the Fourier expansion but I'm not sure how. Would be great if anyone ...
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A fluid element taken in the velocity boundary layer region

Consider a fluid flowing over a flat plate with a free stream velocity of $U_\infty$. We know that a thin boundary layer region will be formed. The shear stress acting at any distance y (measured ...
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Effect of density of a fluid on laminar boundary layer thickness in external flow

We know that laminar boundary layer thickness depends on the distance from the leading edge and also characteristics of the fluid such as approach velocity, dynamic viscosity and density. I understand ...
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2 votes
1 answer
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Fluid dynamics - Velocity required to turn wheel

I want to calculate the velocity of a fluid required to turn a water wheel at a specific angular velocity. The fluid enters the housing of the water wheel through a inlet channel. It turns the wheel ...
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1 answer
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Will water flow into a tank connected to a hole in a pipe?

Consider water flowing through a pipe from a pump to an outlet at atmospheric pressure. The pipe has a hole in the bottom which initially connects to an empty tank. The water fills the tank. Once ...
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Why is the fluidization pressure drop profile different when this one is measured while increasing and decreasing fluid flows?

why is it that when measuring pressure drop across a solid particle bed at different fluid flows the pressure drop profile looks different? When you measure pressure drop while increasing fluid flow, ...
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1 answer
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How can a fan keep spinning normally but suddenly move very little air?

I have a small portable fan, around 15 cm in diameter and lately it is behaving oddly. Basically I turn it on, the airflow is good but then after 10 minutes or 30 minutes or sometimes hours, the ...
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2 answers
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Bernouli equation

I was wondering whether it was possible to apply Bernoulli's equation to determine what pressure your hand would experience if you were to hold it out the window of a driving car? If so, how could you ...
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How does the equilibrium temperature of heat source depend on flow rate?

The scenario I'm considering is a heat source in a closed system which is initially at some temperature $T_{initial}$ and is in equilibrium with its surroundings, being cooled by air (at room ...
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2 answers
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Relief valve for "semi-closed" loop system (non-engineer warning :-)

My excuses in advance for the probably-bad wording of the question, I'm just an hobbyist without much knowledge in engineering. To the point I can't google for information on my problem since I don't ...
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Continuity / conservation of mass

Two different incompressible fluids a and b flow into a chamber through two circular ports of area 0.1 and 0.2 m2 respectively. Fluid a is of density 1,200 kgm−3 and flows directly into the chamber ...
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1 vote
1 answer
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Understanding the Navier-Stokes Equation

I was watching this video on the Navier-Stokes Equation (https://www.youtube.com/watch?v=XoefjJdFq6k) and had the following questions: @ 0:53 Prove that for any given condition, the Navier-Stokes ...
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-1 votes
2 answers
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Why does my coffee machine's water reservoir have two connections to the pump?

I'm trying to bypass the water reservoir and plug my Ninja CP307 directly into a reverse osmosis system.. This is a picture of a disassembled Ninja CP307 coffee maker. But, when I opened up the ...
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1 vote
1 answer
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What is the difference between a 100 USD professional pitot tube and a 5 USD hobbyist pitot tube?

I would like to measure air flow speeds in a duct system for dust collection in a workshop. Relevant air speeds in the ducts are around 10-30 m/s. The ducts are circular tubes with a diameter of 160 ...
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Deflection or expansion

In terms of shockwaves how can you tell if a certain region follows deflection or expansion. For example for the image below: I am told that for region 1-2 it is oblique shock, For region 2-3 it is ...
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1 answer
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Optimized water droplet formation

What is the best way to "shatter" a water drop falling purely under gravity if the objective is to break it into as many droplets and as fine (smallest diameter) as possible? Should the ...
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0 votes
1 answer
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Does a venturi cause strain on a pump

Excuse my terminology if it's wrong. I'd like to know if a venturi causes strain on a pump. For example, in my drawing. I have an air pump behind the house with a 50mm pipe fitting. On the other side ...
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Pressure variation through reduction (Bernoulli) and pipe wall design

Let's say I have a pipe followed by a reduction and then another smaller diameter pipe continues. Using conservation of mass, the flow through the smaller diameter pipe will have higher velocity. If I ...
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2 answers
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Interpretation of dx/dt and u in CFD

I am currently studying CFD and I have a question about how I should interpret the term u and dx/dt. I would like to find that out in the textbook, but I could not. When I solve 1D Euler Equation with ...
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1 vote
1 answer
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Axial compressor: How does it work that the stator vanes diffuse the flow to build up pressure, but also the flow area reduces?

I'm reading about how the stator vanes are diffusors, which diffuse the air flow in a axial compressor to a larger flow area to build up pressure BUT at the same time the compressor shows its overall ...
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1 answer
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Drag coefficient of a complex object made using simple objects (with known drag coefficient) in small Re?

Let say you have an object made of simple objects (figA1). You know the drag coefficients of simpler objects (here spheres and rods, figB1). Is there a way to estimating the drag coefficients of the ...
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1 vote
2 answers
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How to calculate the pressure applied in a compressible fluid?

If I am trying to figure the pressure needed contain a fluid like hydrogen in a cylindrical tank, how would calculating this go on? As Hydrogen is compressible to a differing amount than other ...
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