Questions tagged [fluid-dynamics]

The quantitative study of how fluids (gases and liquids) move.

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Is There a Theoretical Limit to the Magnitude of the (mass) Diffusive Constant?

There are two types of diffusivity: Eddy and Molecular. Eddy diffusion is generally the largest of the two. Is there a theoretical limit to how large the parameter "D" (diffusive constant) is? ...
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What is the argument against the Newtonian explanation for lift and flight by airplanes? [duplicate]

Newtonian and Fluid mechanics compete for acceptance as the true explanation of the physics of lift and how airplanes fly. If you could disprove Newtons laws here, then fluid mechanics is left as the ...
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Having difficulty trying to calculate the temperature difference between inside and outside of a hot air balloon for neutral buoyancy

From Wikipedia Density of air article: $\rho_{dry\_air} = {P \over {R_{specific} T}}$ where $R_{specific} = 287.058$ for dry air, $P = absolute\ pressure (Pa) $ and $T = absolute\ temperature (K)$. ...
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Why do we press our nostrils inward when we blow our nose?

Suppose my nose is runny and I want to blow mucus into a tissue. Scenario 1 (the typical scenario): I press my nostrils inward slightly with my hands, narrowing the passageway, and then blow ...
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Why is relative roughness for pipes a function of diameter rather than circumference? [closed]

From my understanding, the relative roughness of a pipe is known as the absolute roughness ɛ of a pipe divided by the inside diameter of a pipe. Intuitively it seems the "roughness" would be related ...
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Behavior of parachutes in Crew Dragon launch escape experiment

When watching deployment of parachutes in SpaceX Crew Dragon Escape experiment, I noticed that sometimes parachutes collides with each other and later- separates. (Btw, you can watch this movie here, ...
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What is the physical idea behind the mathematical definition of the dimensionless “surf similarity parameter”?

My question refers to a parameter called "iribarren number", whose value corresponds qualitatively to the type of breaking of water waves (spilling, plunging, collapsing and surging) on a uniformly ...
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What causes vortices at the billiard ball particle level?

In a fluid/gas model consisting of billiard ball hard particle collisions what are the key behaviours that are needed to manifest the larger scale vortices? I can only seem to find descriptions of ...
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Intuition behind Streamlines and Flow

Consider the example shown above. Ideal fluid is flowing along the flat tube of uniform cross section area located in the horizontal plane and bent as shown. The flow is steady. My text book says ...
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Which are the exact definitions of Lagrangian and Eulerian descriptions of motion?

Trying to reply to myself to the question 'Which are the exact definitions of Lagrangian and Eulerian descriptions of motion?' My attempts go in three directions. Lagrangian description of motion is ...
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Derivative expansion in hydrodynamics and the definition of the temperature

Thermodynamic quantities are well defined at equilibrium. a fluid at zeroth order can be approximated by a perfect fluid and conserved currents such as electric current and energy momentum tensor is ...
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Initial conditions in incompressible lattice Boltzmann method

I have very recently come to LB methods, and, after some reading, have implemented a little 2D code of my own. It uses some of the most basic assumptions : D2Q9 lattice BGK collision operator Zou-He ...
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How to calculate towing angle on submerged cable in the presence of water currents [closed]

I am trying to calculate the angle of a cable being towed underwater in the presence of some currents: The variables supplied are the speed of the boat (Sb), the direction the boat is sailing (Db), ...
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If I blow down with speed into a bottle which is half-filled with water, will Bernoulli's principle apply which would result in the water rising?

If I blow down with speed into a bottle which is half-filled with water, will Bernoulli's principle apply which would result in the water rising? If I had a straw in the bottle would the water go ...
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Rotational, Potential Flow

The short answer is "by definition, potential flow is irrotational", but please hear me out. I was working through "Fundamentals of Aerodynamics" by Anderson, and I noticed the following when he ...
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Fluid Dynamics (Air) Specifically Aeronautics

An airplane lifts off when the pressure of air pushing down on the wing is reduced due to the speed of the vehicle. Would it be possible to construct an airplane so heavy that it would be totally ...
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Molecular vs Eddy Diffusion

Let us assume that we have two boxes that are connected by a thin permeable barrier: If both boxes are non-convective and all motions are due to the brownian motion of atoms, then I assume that you ...
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Variation of compressor cylinder pressure with respect to time during discharge stroke

I have been stuck here from about a month now, I have formulated a simple compressor reed valve lift with respect to time equation by using newtons second law. On the Right hand side of the equation I ...
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Flow always increases diffusion flux

Reading Falkovich's Fluid Mechanics (Section 2.2.4 Mixing), I became stuck when tying to understand this piece of reasoning that is meant 'to show that incompressible flows can only increase the flux ...
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Why does water falling slowly from a tap bend inwards?

This is something anyone could easily verify. When we open a tap slowly, water bends inwards (towards the axis) while maintaining its laminar flow. After a certain height below the opening, the flow ...
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Lubrication theory with a cylinder approaching a wall

In my lecture notes, we went through a problem with a cylinder approaching a stationary wall with speed $v$. Since we are interested in the thin layer approximation, we utilised lubrication theory. ...
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1answer
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Difference between Pressure and Pressure Energy in fluids

For a fluid with viscosity to flow through a pipe that has the same cross-sectional area at both ends, at a constant velocity, there has to be a pressure difference according to Poiseuille's Law. Why ...
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Is there a height limit to the 'inflatable dancer'?

From the Bernoulli equation $$\frac{p}{\rho} + \frac{v^2}{2} + gh = \mathrm{const.}$$ along streamlines, it seems that there should be an ultimate limit to the height of an 'inflatable dancer' when ...
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1answer
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liquid flowing through tiny holes super slowly [closed]

How could I make a liquid in a container to flow out of several tiny holes super slowly, let's say that the whole container would empty in a month duration. Is it possible? does it only depend of ...
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Fluid dynamics between with 3 infinite planes

I've been studying the dynamic of a fluid trapped between two infinite plates at speeds $v_1$ and $v_2$. So far, I understand that between those two plates, the velocity of the particles in the fluid ...
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Effective viscosity in bubbly liquid

Motivation: I have screwed some crepes big time this morning and it made me think... Consider a bubbly liquid (such as a crepes batter if you whisk it too much). Is there an approximate measure of ...
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Why does a weather vane arrow point in the direction of the wind?

It seems that a weather vane will rotate in order to minimize energy and thus orient itself parallel to the wind. What I do not understand is why it is implied that the weather vane arrow should ...
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Osmosis ,Pascals and surface pressure

I am hoping to clarify the explanation of osmotic pressure. I understand that the presence of an semi-permeable membrane (permeable to a solvent but impermeable to a solute) where there is a ...
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1answer
42 views

Derivation of Burgers' equation in sonic boom

I am studyng Burgers equation. In many resources is said that this equation arise in sonic boom model. I' m look for derivation of this equation starting from Navier-Stokes or (Euler). I know that ...
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1answer
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$3$ immiscible liquids in a communicating vessel

We have $3$ immiscible liquids in a communicating vessel. I should determine which liquid has the smallest density. I think it's liquid $1$, but I cannot give an adequate explanation. Does ...
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Why surface tension occurs [duplicate]

I have this confusion of surface tension, in my books it is written that surface tension occurs due to cohesive forces acting on the particles which makes them to contract. (If this is the case then ...
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1answer
41 views

Understanding the relationship between $P=F/A$ in brain ventricles

Humans have a hollow space (see the colored part in the diagram below) inside our brain where cerebrospinal fluid is produced and circulated. A disease exists called "normal pressure hydrocephalus" ...
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Does the engine's thrust not contribute to the lift? [duplicate]

Does the engine's thrust not contribute to the lift? Here are my thoughts: There are two types of aircraft resistance, one is the resistance created to create lift, referred to as lift resistance, ...
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What does the Laplacian of a fluid velocity potential mean?

I was studying something related to fluid mechanics and then I found that $\nabla^2 \Phi = 0$ where $\Phi$ is the fluid velocity potential ($\vec{V}=\nabla \Phi$). So I was wondering what does it mean ...
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160 views

Why does this fan blade have a serrated edge? [closed]

The title speaks for itself. I got this fan for free and I’m wondering why it has a serrated edge at the end? For anyone curious, according to the sticker on it, it’s a Muffin XL MX2B3, 115V, 0.2A, ...
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For a rotating fluid in a beaker, why does the depth of the parabola increase as the angular velocity increases (in terms of forces)? [closed]

I am writing a highschool paper on this and wanted to know the relationship between the parabolic depth and the angular velocity of the rotating fluid in terms of the forces.
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Perfect gas relationship between pressure and density

I've been given the following equation for a perfect gas: \begin{equation} p = k \rho^\gamma \end{equation} Where $k$ and $\gamma$ are constants. I'm trying to find out where this comes from or a ...
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Control volume and momentum theorem

I'm studying fluid and propulsion mechanics by myself. I stumbled upon this website from MIT: http://web.mit.edu/16.unified/www/SPRING/propulsion/UnifiedPropulsion2/UnifiedPropulsion2.htm#...
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1answer
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Problem on mechanics of bodies [closed]

I am trying to solve this problem I thought using archimidis principle would be a way. the second body has more cavity than the first body(left body), i think, given that the 1st body will ...
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When can the relation $\frac{T2}{T1}= \left( \frac{P_2}{P_1}\right)^ {(r-1/r)}$ be used in case of a nozzle? [closed]

Air enters an adiabatic nozzle at 300 kPa, 500 K with a velocity of 10 m/s. It leaves the nozzle at 100 kPa with a velocity of 180 m/s. The inlet area is 80 cm2. The specific heat of air Cp is 1008 J/...
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Is the flow velocity above or below the inclined plane high? Why?

There is an inclined plane between the two plates. The fluid flows between the plates. Is the flow velocity above or below the inclined plane high? Why? As shown in the figure, the blue line segment ...
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Specific heat ratio in shockwaves

This is not a homework problem. I am looking for some direction: A normal shockwave occurs in a gas with an unknown specific heat ratio $\gamma$. The static pressure ratio across the shockwave is $10....
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3answers
51 views

How does one calculate the expected leak rate through a hole, and the pressure drop due to that leak?

Let's say we have a pressurized horizontal tube that has a hole along the way. What would be the flow rate through that hole, and what would the pressure be downstream of the hole? Or if you prefer ...
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1answer
50 views

What is the formula to calculate the velocity of an airstream based on pressure differential? [closed]

I have an air tank at 100 psi above ambient pressure (roughly 14 psi). If a small spigot is opened, what will the velocity of the core of the air stream be? I realize friction will slow the outside ...
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1answer
54 views

Confusion about the continuity equation for incompressible fluid

Consider that water, a perfect incompressible fluid, flow through the section $S_B$ of a cylindrical container with section $S$ and height $H$: figure (reference: "Jean Jacques Meister, exercices, ...
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103 views

How to determine effect of obstruction on pressure?

I am struggling to figure out what theory explains a situation I observed. A steam boiler had a pressure control which would turn off the boiler if it sensed 2 psi. The boiler would frequently switch ...
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Is it possible to explain the terms of the Navier-Stokes equation in a non mathematical way? [duplicate]

Is it possible to explain the terms of the N-S equation in a non mathematical way? The mathematical method is too professional for ordinary people to understand. I mean: I need an explanation that ...
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Flow through a pipe using Bernoulli equation - Real industrial application using pressure monitors

This is not a homework question or off topic so please do not delete as previously done. I'm working in industry and need assistance on the real application. I aim to calculate the flow through a ...
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Can you explain the Navier-Stokes equations vividly and easily? [duplicate]

Can you explain the Navier-Stokes equations vividly and easily? What do its items mean?