# What is Hydrodynamic: Definition and 45 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. ### B What actually is static pressure?

I’ve been confused about the term static pressure for quite some time. Different sources use very different definitions. From the problems perspective, it’s usually some external pressure. For example we are having a pool with tiny hole on the bottom which makes water level decrease and it flows...
2. ### I have a hydrodynamic loss head question

Hi! I have a loss head question. There is a difficulty in understanding the hydrodynamic loss head concept. Assuming that there is a one-sided tube with incompressibility, visibility, tube friction coefficient and cross-sectional area A.. There is friction in the loss head formula, but is the...
3. ### 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...
4. ### A Hydrodynamic stability- Rayleigh equation - Couette flow

Folks, I'm trying to formulate the stability problem in the incompressible inviscid limit and find the dispersion relation in the Couette flow regime. As shown, the 2 infinite plates move one against the other, unlike the "standard" case where one plate is static and the second moves. I'm trying...
5. ### Engineering Hydrodynamic force on hinged flap gate

I have attempted to use the law of conservation of momentum. The Areas are the lengths marked with the red striped line times the length into the paper, and the Forces F1 and F_hydro are the hydrostatic pressure and hydrodynamic forces respectively (acting on the plate) (The x y components is...
6. ### Advanced Hydrodynamic Problem - University level

TL;DR Summary: Advanced hydrodynamic calculation- University Level Calculate the necessary force of the hydraulic piston to empty the tank located at the bottom in the indicated time. Hi all guys I'm new on this forum, and I write this post to ask you for help on a mechanical engineering...
7. ### I Can the hydrodynamic formulation of QM be understood purely classically?

I was wondering this question for quite some time because this view seems to work surprisingly well and also coincides with how the state space must be setup in any attempt model QM within classical probability theory. So starting with the hydrodynamic formulation from...
8. ### Why Is Acetone's Pressure Relevant in Determining Air Pressure in a Tank?

Can you, please, help me with this exercise? I know the formulas for the required parameters, but I know neither how to use them here nor in what way to solve the exercise. Plus I don't understand what for I was given the temperature Thank you for your help and advice
9. ### I Hydrodynamic interpretation and consistency classic probability

So back in the other thread I asked about compatibility of classical probability theory (PT) and QM – and it turns out there is no inherent reason why they need to be incompatible. Therefore I was looking for something that makes them compatible, which wasn’t easy to search for. But there seems...
10. ### Hydrodynamic drag of a rotating paddle wheel

Hello, i have a problem that look's easy to solve but really is not. It involves a rotating paddle wheel submerged in water and i want to know the force exerted on the wheels and the torque required to rotate it. I have made a simple drawing to illustrate the problem below. I want to use the...
11. ### Torque generated by water flow

I know that I have to calculate force that is generated by water. However, I do it wrong way. ## A = \pi r^2 ## ## v = \frac Q A = \frac Q {\pi r^2}## ## F = P t = mvt = ...? ## - have problem with mass. and is it even correct way? if I can calculate F - its easy afterwards... ##\tau = F l ##...
12. ### Velocity of fluid flowing from a hole at the bottom of a conical shape

My approach was that I consider the pressure of cross section A first Pa= P + dgh Now by writting Bernoulli's Equation between the cross-section A and the opening : $$Pa + 2dgh + 2dV^2/2 = Pb + 2dV'^2/2$$ Where Pb is the pressure of the opening which is equal to the atmospheric pressure...
13. ### Comprehensive treatise on hydrodynamic bearings?

I am looking for a book which covers hydrodynamic bearings which must support both axial and radial loads. Despite having 10 ft of books on machine design, not one treats anything other than pure axial and pure radial bearing designs. Application is a lathe spindle, so this has been studied to...
14. ### Contradicting values of pressure for liquid cross-sections

Consider a fixed horizontal tube of uniform cross section with pressure being 1atm at one of it's end and 5atm at the other (former due to 'open to atmosphere' and latter due to force on a piston), then liquid would flow towards low pressure end. By equation of continuity all cross sections will...
15. ### How to approach the design for a boat step by step?

Two pontoon boat design approach and how to find center of gravity, center of buoyancy, buoyancy, volume of displacement, meta center, meta centric height. I need to know is there a different approach to designing dual hulled boat than a single hulled boat and all the formulas and examples I...
16. ### A Validity of Relativistic Hydrodynamic Equations

For a relativistic fluid, the equation of state is given by: $$\rho = \rho_0 + 3p/c^2$$ The above expression is nicely derived in Weinberg (1972). Although I was told that for a compressible fluid that is relativistically hot (i.e. ##p \gg \rho_0 c^2)## under a constant acceleration, ##g##...
17. ### Hydrodynamic forces on a foil, Reynolds number, viscosity

Hi! I know little about physics, but I am very well educated , with an inquisitive mind. First the context, then my specific question A. Context The practice of windsurfing has brought me to try and understand the principles of hydrodynamics, in order to make my sailing more efficient. As...
18. ### Continuous-time loop computer for NP problems?

In NP problems we can cheaply (polynomial time) test if a given input is satisfying, but the difficulty is that there is an exponential number of possible inputs - the question is if e.g. there is a satisfying input. Imagine there is a simple hardware implementation of verifier (e.g. in form of...
19. ### Charity engineering project water projections

Kia ora I'm a fist year engineering student in Christchurch NZ, and a few friend and I, are designing a simple engineering report for a water projection system for an organization called gap filler. Were currently thinking of projecting an image of music onto the river Avon so people can read...
20. ### Experience Hydrodynamic Levitation: A Simple yet Mesmerizing Effect

Here is a fun effect. I wonder if it has been described previously, given how simple it is to reproduce. I remember seeing similar effects with an empty plastic bag, spinning midair on the side of an air updraft.
21. ### Find velocity of air using Bernoulli principle

Homework Statement A horizontal stream of air is blown just above the open end of the hares apparatus with an initial speed "V". The density of air is 1.2 kgm-3.Then the water column rises to 6 cm in the respective limb.Density of water is 103 kgm-3 Using Bernoulli`s principle find "V"...
22. ### Explanation for hydrodynamic problem

Homework Statement See the photo of the problem in the attachment Homework Equations Bernoulli equation and continuity equation The Attempt at a Solution I don't know from where to begin , i only can get the velocity of fluid when it reaches the hole
23. ### Terminal Velocity of a Cylinder Freefalling Through a Fluid

Hello :) I am an engineer and I am trying to analyse a system which basically contains a cylindrical body free-falling through a body of static water beginning with zero velocity. I am ultimately trying to find what the velocity of the object would be at a depth of 20m. In order to do this I...
24. ### Python Why is my Hydrodynamics Simulation Code Producing a Noisy Signal?

Hi! I have this code which should simulate a sod shock tube like this: https://en.wikipedia.org/wiki/Sod_shock_tube But when i plot density versus distance I just get something like a noisy signal. Any idea where I am doing something wrong? from matplotlib import pyplot as plt import numpy as...
25. ### Thermal Boundary Layer vs. Hydrodynamic Boundary Layer

Hello Guys, Could someone explain to me the meaning of greater thermal boundary layer over hydrodynamic boundary layer over a flat plate surface? I know how to calculate both streams, but I don't understand the meaning of smaller thermal boundary vs. hydrodynamic boundary and vice-versa. What...
26. ### Is My Vehicle in Danger of Sinking if Water Enters?

I need to know approximately what percentage of a vehicle with an airtight passenger compartment must be filled with water before it sinks to the bottom. The fact that it is airtight may normally prevent it from sinking, but if a window was cracked from impact, and water slowly trickled in...
27. ### Hydrodynamic Entry Length of Flow Through Circular Annulus

I might be going crazy or searching at the wrong places. Is there an empirical formula for solving the hydrodynamic entry length of laminar flow through a circular annulus? Thanks!
28. ### What's the relation of the viscous hydrodynamic forces and swirl？

what's the relation of the viscous hydrodynamic forces and swirl？
29. ### Viscous Hydrodynamic Forces on Fixed Offshore Platforms

Are the viscous hydrodynamic forces and the nonlinear viscous hydrodynamic forces applied on offshore platform in the ocean？ The platform is fixed.
30. ### Viscous Hydrodynamic Forces and Ship Velocity: An Analysis

The viscous hydrodynamic forces will increase with increase of the velocity of ship？ Can the viscous hydrodynamic forces be neglected if the velocity of ship is small　？
31. ### Thermal boundary layer and hydrodynamic boundary layer

So I know individually how these form. Unfortunately I haven't found any sources that describe more detailed questions that pop up in my mind. Could someone help me answer a couple of questions? 1. So if a thermal boundary layer forms in a 'plug flow' model i.e. when there is no momentum...
32. ### What Factors Influence the Siphon Effect in Hydrodynamics?

Hi all, This is my first post. I was wondering what the effects of hydrodynamics are on water in regards to when you suck water out of a pipe leading to a barrel of water for example. When you then stop sucking the water from the pipe the water leaving the pipe then continues to pull water...
33. ### Hydrodynamic force of water droplet stated in psi at various mpg

I'm not a student of physics but do have puzzle I am trying to solve. Is there a way to calculate the force of a water droplet impacting a vertical surface stated in psi at various speeds of impact? A more practical way to state the problem would be; what psi will rain impact the chest of a...
34. ### The Hydrodynamic and Thermal boundary layers

Hi, I'm doing 'Heat and Mass transfer' at college and we're covering the topic on the hydrodynamic and thermal boundary layers. I have a couple of questions, the answers to which are not given explicitly in any of my textbooks. 1. During open flow, why does laminar flow eventually have...
35. ### Derivation of Hydrodynamic Equations from Interacting Particles

Hi there, does any of you know a place where I can find the derivation of the hydrodynamic equations (navier stokes, etc) starting from interacting particles? I need this done in a pedantic way as I have to deal with the case in which the free streaming length is of the same order of the...
36. ### Hydrodynamic interaction of particles

Homework Statement The flow of a single sphere translating at speed U in a fluid at rest far away from the sphere is given by the streamfunction, ψ(r,Θ) = (1/4)Ua2(3r/a - a/r)sin2Θ a=radius of sphere the origin is at the centre of the sphere and the axis Θ = 0 is...
37. ### Hydrodynamic entrance length: independent of Re?

Hi all, I have learned that the hydrodynamic entrance length of a channel (to form fully developed laminar flow) is correlated to the Reynolds number, because the shear effects have to propagate inwards from the walls of the channel. However recently I found out that there is a 'minimum' to the...
38. ### Fluids- hydrodynamic interactions

If we consider a system of N suspended particles diffusing through a narrow channel. What happened if the hydrodynamic interactions are neglected? Is there only flow? How can the particles diffusse without flow?
39. ### A sphere heads east from Los Angeles ( centre of hydrodynamic 'mass' problem)

A sphere heads east from Los Angeles... ("centre of hydrodynamic 'mass'" problem) Homework Statement This is actually from research, but it's one of those "this should be an easy problem and I'm wasting too much time making it too difficult" problems, so I thought it fit better under...
40. ### Hydrodynamic cavitation via an orifice plate

Hello all, I'm trying to induce hydrodynamic cavitation in a water filled vessel at STP using a high pressure piston pump and an orifice plate. Does anyone have any knowledge of estimation methods for the relationship between pressure drop, orifice area and cavitation intensity? Clearly the...
41. ### Calculating the hydrodynamic entry length of a cicular pipe.

Does anyone know how to calculate the entry region length of laminar incompressible flow in a pipe? There are several formulae available which give a ratio of the (entry length)/(pipe diameter)/(Reynolds number) = 0.02-0.06. I'm not looking for the final solution, but rather the approach...
42. ### Hydrodynamic drag force density velocity

Homework Statement A steel sphere in a tank of oil is given an initial velocity v = 2i (m/s) at the origin of the coordinate system shown (if you don't trust the attached image it is essenital). The radius of the sphere is 15 mm. The density of the steel is 8000 kg/m3 and the density of the...
43. ### Can Three Holes in a Tall Cylinder of Water Create a Hydrodynamic Coincidence?

Given a comparatively tall cylinder full of water, is it possible to punch in its side three similar holes so that their streams all intersect at one point?
44. ### Understanding Viscosity and Wear in Hydrodynamic Bearings

Is there a way to relate the viscosity of a liquid to the fatigue wear of a bearing. The bearings are lubricated with the flow of liquid through the metering devise. The flow of liquid creates a perpendicular force on a shaft with carbon bearings on each end. Is there a formula that I can use...
45. ### Magneto Hydrodynamic Propulsion for hypersonic aircraft?

I watched a "Black Aircraft" show on the science channel. In the show they mentioned a researcher working on old Russian technology that placed a strong beam in front of the aircraft to strip electrons from the air molecules so the plasma creates its own initial shockwave to lessen drag on...