What is Fluid: Definition and 1000 Discussions

FLUID (Fast Light User Interface Designer) is a graphical editor and GUI builder that is used to produce FLTK source code. FLUID edits and saves its state in text .fl files, which can be edited in a text editor for finer control over display and behavior.After designing the application, FLUID compiles the .fl file into a .cxx file, which defines all the objects from the .fl file, and an .h file, which declares all the global ones. FLUID also supports localization of label strings using message files and the GNU gettext or POSIX catgets interfaces.

A simple program can be made by putting all non-interface code (including a main function) into the .fl file, thus making the .cxx file a single source file to compile. Most programs are more complex than this, so other .cxx files can be written that call the FLUID functions. These .cxx files must #include the .h file, or they can #include the .cxx file so it still appears to be a single source file.
Normally the FLUID file defines one or more functions or classes, which output C++ code. Each function defines one or more FLTK windows, and all the widgets that go inside those windows.
Widgets created by FLUID are "named", "complex named", or "unnamed". A named widget has a legal C++ variable identifier as its name (i.e. only alphanumeric and underscore), and is defined by a global variable or class member that will point at the widget after the function defining it is called. A complex named object has punctuation such as '.' or '->' or any other symbols in its name. In this case, FLUID assigns a pointer to the widget to the name, but does not attempt to declare it. This can be used to get the widgets into structures. An unnamed widget has a blank name and no pointer is stored.
Widgets may either call a named callback function that one writes in another source file, or one can supply a small piece of C++ source and FLUID will write a private callback function into the .cxx file.

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  1. B

    I Derivation of Fluid Pressure In A Gravitational Field

    Given that ##P = ρgh##, there's obviously a problem with the following derivation of fluid pressure under gravity. Can someone spot the flaw? $$W = mgh$$ $$W = ρVgh$$ $$F \cdot dh = ρVgh$$ $$F \cdot dh = ρ(Ah)gh$$ $$F \cdot dh = ρgAh^{2}$$ $$\frac{d(F \cdot dh)}{dh} = \frac{d(ρgAh^{2})}{dh}$$...
  2. Martin Jediny

    B Too much energy -- thought experiment

    1/ Have a closed hydraulic circuit. Two columns of fluid. One hot, one cold, which are connected at the bottom and top. Let's have small heat loss and small hydrodynamic resistance to fluid flow and incompressible fluid. I heat the lower interconnection, I cool the upper interconnection. I have...
  3. H

    Potential flow around a sphere

    I tried to find a solution to the Laplace equation using spherical coordinates and the separable variable method. However, I found equations that I simply don't know how to find a solution. Thus, I tried in cylindrical coordinates with an invariance in ##\theta## but now I'm facing this...
  4. M

    Varying fluid (density) in a cylinder rolling along an inclined plane

    I am not sure if my report is complex enough as it should be at the undergraduate level preferably based on the requirements for it and it feels like it's all over the place as of now.
  5. brochesspro

    B Finding Angle Between Fluid & Horizontal: Clarified

    I have been taught that in order to find the angle made by a liquid in a container with the horizontal, we use the fact that the free surface is perpendicular to the net force vector acting on the fluid (including the pseudo force vector for an accelerating container), however, if say the...
  6. axelb

    I How is the Reynolds number derived (is my derivation wrong)?

    I'm a HS student so please dumb it down. I'm looking into the Reynolds number of a sphere sinking in a fluid, and I want to determine whether my results meet creeping flow or not Re<<1, here's what I got. **sorry if I misused the prefix, I'm not sure whether it's highschool or undergraduate**...
  7. mahdis

    Fluid flow rates in straight vs coiled tubes

    If you have 2 pipes of the same cross sectional area, same volume of fluid, and same length, the only difference is one pipe is coiled several times over a cylindrical object theoretically shortening the length of the pipe, while the other is a straight pipe. Will the flow rate be same in both...
  8. S

    A Conservation Laws from Continuity Equations in Fluid Flow

    Consider a fluid flow with density ##\rho=\rho(t,x)## and velocity vector ##v=v(t,x)##. Assume it satisfies the continuity equation $$ \partial_t \rho + \nabla \cdot (\rho v) = 0. $$ We now that, by Reynolds Transport Theorem (RTT), this implies that the total mass is conserved $$...
  9. D

    A Discretization of diffusion equation of a fluid in movement

    Hello, I want to model the thermal behaviour of a moving heat transfer fluid in 1D, with convective exchanges with the walls. I have obtained the following equation (1 on the figure). I have performed a second order spatial discretization with decentred schemes at the extremities (y = 0 and H)...
  10. C

    How does soap water membrane thickness affect surface tension?

    1.If the thickness of soap water surface decrease, whereas the concentration remain the same, how will the surface tension change? 2. If the membrane of soap water start to evaporate, its thickness become thinner and it’s concentration increase, How will the surface tension change? Thank you!
  11. Austin_Tx_Rockets

    Does the discharge coefficient of an injector depend on the fluid?

    The discharge coefficient is the ratio of the actual discharge to the ideal discharge and can be measured experimentally. If one were to experimentally measure the discharge coefficient using water, would this value be constant for all incompressible fluids? What does the discharge coefficient...
  12. URIA

    Help with fluid dynamics problem

    Dear All, I tried to solve the attached question. it's about Couette flow, where the 2 plates move. in fact, I have to find the stability condition. is someone familiar with this and can help? many thanks, uria
  13. Valentina Chianese

    B Understanding the Rheopectic Properties of Blood: Fact or Fiction?

    Hello everyone, I need your help. A teacher once told me that blood is a non-Newtonian, rheopectic, pseudo plastic fluid. I get the non-Newtonian/pseudo plastic part, but I can't understand why blood is rheopectic...is it or not? Thanks
  14. G

    I How can I integrate variable velocity in fluid mechanics?

    Do you know of any place where I can look up things about the momentum (linear momentum) in fluid mechanics? It's just that when I have a variable velocity and it has to be integrated, I don't quite understand how to do it. I have looked for videos and things and I can't find that they are...
  15. G

    Fluid mechanics: water jet impacting an inclined plane

    I was looking at an example of fluid mechanics and I don't understand this. Statement figures: CONTINUITY EQUATION $$\left. \dfrac{dm}{dt}\right]_{MC}=(\dot{m}_2+\dot{m}_3)-\dot{m}_1=0$$ $$\dot{m}_1=\dot{m}_2+\dot{m}_3$$ $$\rho c_1A_1=\rho c_2A_2+\rho c_3A_3$$ $$\rho c_1 h1=\rho c_2 a1+\rho...
  16. tracker890 Source h

    Q:Hydrostatic Pressure vs. Energy Conservation Equation

    Please help me to understand which ans is correct. To determine the ##P2##. $$ h_{LM}\ne 0 $$ Method 1: $$dP=\frac{\partial P}{\partial x}dx+\frac{\partial P}{\partial y}dy+\frac{\partial P}{\partial z}dz$$$$\phantom{\rule{0ex}{0ex}}\rho \overset\rightharpoonup{a}=-\triangledown p+\rho...
  17. ermia

    Krotov problem: how to write Energy conservation for this fluid?

    I wrote some potentials but they were wrong. I used the cm of all fluid parts and I used the radius which is $$ \sqrt S/ \pi $$ .
  18. C

    Solving Fluid Statics Problem: Accounting for Atmospheric Pressure

    Hi! For this fluid statics problem, One of the answers is: However, why did they assume the pressure at the top was zero? I thought the pressure at the top would be 1 atm? So tried to take atmospheric pressure into account putting 1 atm at the top, Do you please know how to get P_0A/2 so...
  19. T

    Initial Position Discrepancy Involving Fluid Resistance

    Good evening, I have a question on the problem I've provided in the homework statement. Essentially, the problem is asking us to compare the maximum height and time to max-height between a simple projectile motion case vs. a case using fluid resistance proportional to v. I've solved everything...
  20. A

    I Fluid flow through a pin-hole of x diameter in a closed container

    Greetings, I've come across lots of exercises regarding Bernoulli's equation. However, never seen one where the top of the vessel is closed, and fluid flow exists via gas (air) going in. Has this problem been studied in the past? Assume a cylindrical vessel filled to the maximum with a D-sized...
  21. samy4408

    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...
  22. samy4408

    I Relationship between viscosity and velocity in fluid mechanics

    hello, I read in a lecture paper about fluid mechanics that velocity is not related to viscosity, i found this odd and i think it is an error , can someone confirm that?
  23. G

    Tainter Damper Figure: Analyzing Forces

    Figure: Attempt at a solution: $$b=12\, \textrm{m},\quad H=8\, \textrm{m}$$ a) $$F_H=p_{CG}A=3767040\, \textrm{N}=\boxed{3767,04\, \textrm{kN}}$$ $$A=8\cdot 12=96\, \textrm{m}^2$$ $$p_{CG}=\rho_g h_{cg}=39240\, \textrm{Pa}$$ b) $$F_V=mg=\rho_g V$$ We calculate ##\theta \rightarrow 8=10\cdot...
  24. G

    Total force that a fluid exerts on a cylinder

    Figure: I have calculated the density of the cylinder: ##5479,0\, \textrm{kg}/\textrm{m}^3##. Attempt at a Solution: $$d=0,12,\,\, L=1,1,\,\, D=0,122,\,\, e=0,002,\,\, c=0,02,\,\, b=0,4,\,\, a=0,7$$ $$\omega =100\, \textrm{rpm}=10,472\, \textrm{rad}/\textrm{s}\quad e=0,122-0,12=0,002$$ We...
  25. G

    Reduce the pressure in an air reservoir

    TRANSITIONAL REGIME: ##\dot{V}=90\, \textrm{L}/\textrm{min}=0,0015\, \textrm{m}^3/\textrm{s}## $$\dfrac{d}{dt}\int_{VC}\rho\, dV=\dot{m}\rightarrow \dfrac{d}{dt}\rho V=\dot{m}\rightarrow $$ $$\boxed{\rho_{\textrm{air}}}\rightarrow pV=R'T\rightarrow \rho =\dfrac{p}{R'T}\rightarrow 287$$...
  26. G

    Resultant force and centre of pressure in a tank

    Figure: a) CASE A ##\rightarrow p_a=101300\, \textrm{Pa}## $$F_{\textrm{res}}?,\,\, y_c-y_{cg}=y_{cp}$$ We find ##h_{cg}\rightarrow h_{cg}=3+h## $$h=0,75\cdot \sin (30)=0,375\, \textrm{m}\rightarrow h_{cg}=3,375\, \textrm{m}$$ $$p_{cG}=\rho_{H2O}gh_{cg}=33108,75\, \textrm{Pa}$$ We calculate...
  27. G

    Manometer containing two liquids

    Figure: $$p_A^r=-0,11\, \textrm{at}\rightarrow p_A=90534\, \textrm{Pa}$$ $$\rho_A^r=1,6\rightarrow \rho_A=\rho_{AR}\cdot \rho_{H2O}=1600\, \textrm{kg}/\textrm{m}^3$$ $$\rho_1=1,225\, \textrm{kg}/\textrm{m}^3$$ $$\left. \begin{array}{l} \textrm{LOOP I}\rightarrow p_A-p_1=-\rho_A (Z_A-Z_1) \\...
  28. G

    Viscosity in lubricating fluid in the bench-guide of a machine

    Figure: bench (bancada), guide (guide), oil (aceite). My attempt at a solution: $$\textrm{extension } L=0,6\, \textrm{m}$$ $$F=1500\, \textrm{N},\,\, c=0,1\, \textrm{m}/\textrm{s},\,\, h=0,0001\, \textrm{m}$$ $$v=5\, \textrm{stoke}=0,0005\, \textrm{m}^2/\textrm{s},\,\, \rho_r=0,9$$ a)...
  29. N

    Fluid Dynamics Question -- Water flowing through a pipe into two cylinders

    Because my little work project involves fluids I thought this the best topic to post under. I took the route of biological sciences and computer science. This area is out of my league at the moment I'm not sure the amount of time that would be required to get the material applicable to this...
  30. P

    SYLTHERM XLT heat transfer fluid

    Hi, cannot get a definite answer anywhere on how heat transfer occurs with this fluid. All that the manufacturer gives is "excellent heat transfer". Water has excellent heat transfer due to convection currents. Need to know if this fluid also heats by convection rather than conduction...
  31. lavalite

    Thin-walled sphere and fluid mechanics question

    Suppose you had a thin-walled sphere fully submerged in a liquid. The sphere is filled to the equator with a liquid of sufficient density to reach buoyant equilibrium. Will the lateral cross-sectional areas of the thin-walled sphere experience tensile stresses in the longitudinal axis? Why or...
  32. AJSayad

    Engineering Experimental Fluid Mechanics Vs. Computational Fluid Mechanics

    Hi everyone, I just started graduate school and I've been working in an experimental fluid mechanics lab. We have a shock tube and run tests on studying supersonic and hypersonic airflow under different conditions. We use Schileren imaging and transducers to take measurements during tests. We...
  33. A

    Fluid mechanics - Additional liquid capacity due to compression

    A cylindrical tube (diameter = D, width = L) is completely filled with a liquid (density = ρ). A pump pressurizes the system with a pressure P. Consequently, 1) the solid tube is compressed and deformed according to Hooke's law (σ = ε.E), and 2) the liquid is compressed and deformed, following...
  34. wessholders

    I Water flowing through a funnel

    Will water flowing through a funnel be accelerated more by a "curve-walled" funnel rather than a "straight-walled" funnel?
  35. K

    Is the pressure everywhere on the inflated fluid pad the same?

    I guess the pressure would reach equilibrium after the weight is stable, that is when points X and Y are stable. So, the answer should be A?
  36. theycallmevirgo

    Intro text on building a small fluid pressure chamber?

    As stated in the summary, I'd like to design a chamber to store water under pressure. Nothing too powerful - basically at the level of a super soaker, maybe a little higher. The trick is, I want it to be powered and controlled electronically. But, I come from electronics land and know nothing...
  37. Delta2

    I Understanding Sound Waves in Fluids: Pressure and Velocity Fields

    When we talk about sound waves in a fluid (air, water e.t.c.) we mean that the pressure ##P(x,y,z,t)## satisfies the wave equation, the so called velocity field of the fluid ##v(x,y,z,t)## satisfies the wave equation or both?
  38. M

    A Model the pressure of a zero-gravity simple fluid system

    Hi PF! A circular cylinder contains an inviscid liquid, which is softly disturbed (i.e. velocities are small). There are no body forces, which implies the equilibrium free surface is that of a spherical cap with radius ##R## (see figure A). Inviscid implies we can assume potential flow, such...
  39. A

    Engineering Need Advice on Fundamentals of Fluid Mechanics Textbook

    I would like to buy a Fundamentals of Fluid Mechanics for Engineer textbook. I have 2 options, they are: 1. https://www.amazon.com/dp/0133521702/?tag=pfamazon01-20 2. https://www.amazon.com/dp/0132788128/?tag=pfamazon01-20 Can someone please tell me which one of above Fluid Mechanics textbook...
  40. Bruno Cardin

    A Relativistic Hydrodynamics Eqn for Perfect Fluid: Insight Needed

    Hello. Could anyone help me with some insight in an extra term appearing in the motion equations of a relativistic fluid? I say extra term, because it's not present on the motion for a test particle, as it follows: Let's propose Minkowski space-time, the motion equations for a fluid with zero...
  41. chemisthypnos

    Searching for Low Impact Fluid with Boiling Point of 150-180 F

    I am looking for a good working fluid whose boiling point is in the range of 150-180 F for a waste heat recovery system. The fluid should be minimally flammable, preferably nontoxic, and low in environmental impact. Does anyone know of any working fluids that match this description?
  42. leifh

    Question about fluid flowing into branching pipes

    For above pipe or duct flowing (where point A and C are higher diameter than B), same volume flow for point A,B & C. Velocity B > Velocity A & C ; Pressure energy A > Pressure energy B ; friction pressure B > friction pressure A. Question one: When pipe or duct sizing to be reducing even...
  43. N

    Pressure drop through volumes of compressible fluid

    Thanks for your response. I'm trying to estimate the pressure in the cavity, i.e. P2. I know the areas, A1 which in practicality is an annulus and A2 which is a 600m pipe with i/d 8mm. I would like to initially understand how to calculate the pressure drop across the annulus. At the moment I'm...
  44. PaxFinnica96

    Engineering Fluid Dynamics: Proof of the Static Pressure Head equation

    I am trying to mathematically prove the Static Pressure Head equation: H = p/ρg How can I prove this equation and thus determine the nature of the relationship between these variables?
  45. M

    A Experimental Studies of the Internal Energy of a real fluid

    https://www.nature.com/articles/s41598-022-11093-z In this new 2 May 2022 publication, an experimental effort was made to measure directly the internal energy changes of non-ideal CO2, from the decrease in temperature as the liquid-gas expanded from one cylinder into two. With the empirical...
  46. P

    Bernoulli equation / fluid flow / finding height

    Hi, everyone! Doing some fluid flow/Bernoulli tasks. Ok, so the task is: «A hose with a radius of 0,035m is connected to a nozzle, which reduces the radius of the hose to 0,018m (2). The hose carries (qv) 0,0075 m3/sec and the totalpressure (3) in the wide section (1) is 211 kPA. The density of...
  47. F

    Audio/Video Fluid Dynamics—Building a better MIDI Breath Controller

    Background A MIDI breath controller converts breathing to MIDI values, which are then used to control a MIDI instrument and produce sound. The ones I'm familiar with work through the use of a pressure chip. For example, the TEC Breath and Bite Controller 2 uses the MPCV5010GP...
  48. A

    Fluid Heights in an Open U-Tube

    Pressure on side with liquid 1 = ##p_0 + \rho_1gh_1## Pressure on side with liquid 2 = ##p_0 + \rho_2gh_2## The solution sets the two pressures equal. I am confused how the two sides have the same amount of pressure, one would have more than the other.