What is Compressible flow: Definition and 42 Discussions
Compressible flow (or gas dynamics) is the branch of fluid mechanics that deals with flows having significant changes in fluid density. While all flows are compressible, flows are usually treated as being incompressible when the Mach number (the ratio of the speed of the flow to the speed of sound) is smaller than 0.3 (since the density change due to velocity is about 5% in that case). The study of compressible flow is relevant to high-speed aircraft, jet engines, rocket motors, high-speed entry into a planetary atmosphere, gas pipelines, commercial applications such as abrasive blasting, and many other fields.
Been trying to find out what compressible flows would look like, to picture why you get the not so intuitive property changes.
Started with looking into what might be happening in Rayleigh flow. Theres plently of examples on how to use maths to calculate property changes when going from inlet...
I'm trying to figure out how the mathematics of conservation of mass in compressible flow works for a simple setup. I posted this problem in hw physics section, and the conversation turned to the physics model and appears to have gone kaput. This was supposed to be a mathematics question (but...
For a fluid that is confined to a finite region with no sources and sinks, are the only options for the flow field a) static, and b) cyclic? The example I have in mind is Rayleigh convection in a shallow dish heated from below, where convection cells are formed beyond a certain temperature...
The liquid-vapor mass transfer (evaporation and condensation) is governed by the vapor transport equation:
$$\frac{\partial}{\partial t} (\alpha_l \rho) + \nabla \cdot (\alpha_l \rho \vec v) = \dot m^{+} + \dot m^{-}$$
In the incompressible flow case (constant density), it reduces to
$$...
Hello, I am working on a project that involves air at high pressure and temperature flowing through an orifice (valve) from one container to another. For each container, the volume, temperatures and pressures are known. The valve diameter is also known, and the fluid is air (which can be...
Hello good people!
I'm studying the compressible flow and the book refers to this example. I can't understand how does it calculate the angle Θu?
In the previous example, there is a method to calculate it but it requires all the properties once the fluid is deflected, therefore, that's not the...
For compressible flow in a duct, mass conservation combined with reversibility (no entropy change) implies $$(1-\frac{u^2}{c^2})\frac{du}{u} = -\frac{dA}{A}$$
where u is the flow velocity of the gas, c is the speed of sound in the gas, and A is the area of the duct. I am assuming a calorically...
1. A CO2 canister with the following dimensions will be used.
2. A shell was designed to house the CO2 canister, and the coefficient of drag was calculated to be 0.33
3. The constant thrust required to move the torpedo at approximately 1ft/s was found by using a free body diagram to generate...
When dealing with a compressed air system I would like to know how to make some calculations to predict pressure and flow rate throughout a system.
e.g.
If I have a compressor that can supply 50 cfm @ 100 psi at the source, and the air flows through a system of pipes, filters, turns, etc. which...
Hi, there is no particular question that I need help on, just something my lecturer told us in lesson which I couldn't quite understand so i'd like to check my understanding on this. I know that the speed of a soundwave is 'c' in undisturbed flow. Suppose the flow velocity is 'U'. If the...
Hello everyone..
I am using 2-dimension Navier Stokes equation, but I confused that my problem is compressible or incompressible flow form, because if I have initial pressure and temperature and velocity for x-axis only in one grid that they are so very high but the others grid are zero. Can I...
Hi guys,
I'm trying to delve into compressible flow and in the various textbooks I'm reading I've found two school of thoughts when it comes to how to calculate the threshold (or error compared to incompressible flow) of 'M' when to consider a flow compressible.
In Fundamentals of Aerdnyamics...
G'Day All,
This is my first post so please let me know if I have completed this form incorrectly, or missed a point of etiquette etc...
1. Homework Statement
The problem is to determine the pressurisation rate of a tank being filled by a pipe connected to a compressor.
Assumptions:
Pipe...
I just learned that an incompressible fluid must have zero divergence within a given control volume. Given that the divergence of a fluid at a point(x,y,z) can be found by taking the scalar sum of the of the x, y, z acceleration vectors at the given point, wouldn't this mean that water flowing...
This started out as a thought experiment and then I realized I was stuck in how to figure it out.
If I have an airsoft rifle that uses compressed air in the tank at 3000 psi. It leaves the bottle regulated down to 800 psi. It then goes into a second regulator that reduces the pressure further...
Something that routinely comes up as a problem at work is when we size flow projects for piping systems. We calculate pressure drop across a length of fixed diameter for compressible gasses, but we do not calculate pressure changes through an expansion or contraction with gas (for example...
A body is reentering the Earth's atmosphere at a Mach number of 20. In front of the body is a shock
wave. Opposite the nose of the body, the shock can be seen to be normal to the flow direction. Determine the stagnation pressure and temperature to which the nose is subjected. Assume that the air...
So, a coworker approached me today with a 'simple problem' to solve, looking to sanity-check a design choice.
Much to my dismay, I had no idea how to approach it. Worse, google had no straight-forward answers either, and any formulae I did find seemed circular.
This seems too simple to...
Hey,
So I have a system where there is an air tank (0.2Litres @ 3000psi, 293K) and I have a convergent-divergent nozzle attached directly to it. What I want to know is an expression for how the mass flow rate varies with time as the mass in the tank decreases causing the pressure to decrease...
If I have a tank of volume V, pressure P, and temperature T, what is the most effective way to determine the volume flow rate if I were to suddenly allow the air to discharge through a hose?
My first thought was that I could use Boyle's Law to determine the compressed volume of air in the tank...
Hello, PF!
I'm currently brushing up my fluid mechanics and came across some questions while studying the compressible flow of an ideal gas using Bernoulli's equation. First, consider incompressible flow in the following system
Neglecting any changes in elevation, the Bernoulli equation for...
Hi y'all, thanks in advance to anyone who knows a bit more about compressible/isentropic flow than me for the help,
I have an interesting problem which I've been working on for a bit. Basically, I would like to measure species coming out of a reactor using molecular beam mass spectrometry...
is it correct modeling mathematical model for Compressed air engine,
As here air Flowing through Pipe in Kg/s and its not a continuous flow and flow through vent into the cylinder and pushes Piston and and Piston moves down.
As I created model like this
(P1*V1)^Y = (P2*V2)^Y
So,
P2 =...
I was reading a bit about comrpessible flow and I came upon a problem, there's a diffuser which has air at Ma=0.9 at its inlet and the air comes out at 0.65, how should I go about finding the ratio of the inlet and outlet areas?
I know It's related to this equation but I'm unsure about how...
When compressible effects are accounted for, viscosity should vary with temperature.
Doesn't this violate the concept of Newtonian fluids, where shear stress is linearly proportional to the strain rate?
I have a quick question regarding compressible fluid flow, specifically: for a given amount of time, how much pressure is lost when attaching a balloon valve to a small air tank. I decided to model my solution after the following problem (from an old fluid mechanics textbook of mine),
Air is...
I am doing a lab report and it is to do with compressible flow through rotameters. An upstream and downstream rotameter to measure flow rate with a needle valve between the two. The needle valve acts as a fixed orifice in one part of the experiment. Then the inlet pressure is altered.
Plotting...
I am confused about the relation of mass flow rate with different mach numbers in a compressible flow.
In wikipedia (http://en.wikipedia.org/wiki/Choked_flow), I read the following:
"Although the gas velocity reaches a maximum and becomes choked, the mass flow rate is not choked. The mass flow...
Homework Statement
Calculate the pressure increment required to isentropically compress air and water at 1 atm and 20°C to 50% of their volume. The isentropic bulk modulus for water at 20°C is 2.24 109 N/m2, and air can be considered an ideal gas with γ =1.4 .
Homework Equations...
Hey y'all,
Thanks in advance for any help on this. I'm kind of stumped by this problem I've come across in the lab.
We have a tube reactor with a nozzle, and we want to measure the combustion/pyrolysis products coming out the model using GC/MS and MBMS. Right now, we are just running...
Hi, could anyone tell me a reference on Navier-Stokes equation for the COMPRESSIBLE flow in CYLINDRICAL coordinate? Just can't find a good reference book.
Thanks in advance.
Jo
G'day!
This is my first post on this forum and I hope you find it usefull!
Attached are complete set of Excel functions for
*Isentropic Flow
*Prandlt Meyer Expansion
*Raleigh Flow
*Fanno Flow
*Plane Normal Shock
*Plane Oblique Shock
They are contained in the attached workbook (zipped as it...
Homework Statement
Suppose that a blast wave, which might have been initiated by an atomic bomb explosion, is traveling through air at standard atmospheric conditions with a speed of 60,000 m/s
find change in ( pressure , temperature , stagnation pressure , velocity ) produced by the wave...
Hi
I was wondering, if a flow is compressible, then does that mean that if the inlet and exit of the control volume (same C.S. area) have different densities, would that mean that the temperature and/or pressure of the exit be different than at the inlet (assuming ideal gas)?
If this were...
Suppose we have a flow of a fluid and let it flow (in x direction) into a stationary wall..the x velocity becomes zero...is this an example of stagnation point?
If not, please cite an example of when that is true.
Homework Statement
A compressor is required to drive acetylene gas at 1.85 kg/s through a horizontal pipe, 68.7 m long. The maximum pressure that may be developed by the compressor is to be found, and gas pressure at the delivery end of the pipeline must be 470 kPa. The system is to...
Hi,
I'm new here so I'd like to apologise in advance if this has been posted before. I tried searching first but nothing came up.
I'm doing a project on subsonic, compressible flow in curved ducts and have a question about the physics involved as the flow navigates a bend. Allow me to...
Homework Statement
Consider the flow with velocity vector v = xi.
Show that the individual particles have the position vectors r(t) = c1eti + c2j + c3k with constant c1, c2, c3.
Show that the particles that at t = 0 are in the cube whose faces are portions of the planes x = 0, x = 1, y = 0, y...
Hi! I have experimental data measuring losses across an obstruction in compressible flow which basically consists of static pressure, total pressure and total temperature upstream of the obstruction and the same variables downstream for a number of different mass flow rates. This data was...
Hi,
I'm currently a student and learning these two topics for a module. I've searched the net for many references but can't find anything that is really that helpful.
Can anyone reference me any good links to questions and solutions regarding these topics? Also any books you guys would...
I have air flowing through a gate valve with lengths of pipe of matching diameter upstream and downstream. At a certain distance from the valve upstream and downstream, I am measuring static temperatures and static and stagnation pressures. I can estimate the Mach numbers at the tapping points...
Homework Statement
I am currently studying for finals, and I am really confused about compressible flow in nozzles. Some questions I have:
1) What is exactly is the condition for choked flow for converging, diverging, and converging-diverging nozzles?
2) When can we assume that the back...