# 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.

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1. ### What does compressible flow look like in a duct undergoing heating/cooling?

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...
2. ### I Conservation of Mass for Compressible Flow

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...
3. ### A Steady state confined flow field: Is it cyclic?

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...

8. ### Optimizing Thrust and Operation Time for a CO2-Powered Rocket Torpedo Design

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...
9. ### Compressed air problem (pressure and flow rate calculations)

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...
10. ### Speed of a sound wave (compressibe flow)

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...
11. ### Compressible flow or incompressible flow equation?

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...
12. ### Threshhold compressible flow Ma = 0.3

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...
13. ### Pressure trace of a tank fed by a compressor

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...
14. ### Divergence of downhill flowing water

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...
15. ### Velocity of compressed air through pipe

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...
16. E

### Calculate Pressure Drop through Expansion/Reduction for Gas

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...
17. ### Stagnation pressure across a normal shock wave

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...
18. ### Pressure drop across a tube section - compressible flow

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...
19. ### Air Tank Discharging Through A Nozzle - Unsteady Flow

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...
20. ### Flow rate of air in discharging reservoir

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...
21. ### Modeling ideal gas flow using Bernoulli's equation

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...
22. ### Solving Long Mach Nozzle Shape Problem for MBMS

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...
23. ### Pressure on Piston due to Compressible flow

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 =...
24. ### Nozzle/Diffuser equation for compressible flow

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...
25. ### Newtonian fluid in compressible flow?

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?
26. ### Calculating Pressure Loss in Compressible Fluid Flow: A Case Study

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...
27. ### Compressible Flow, Pressure and Mass Flowrate

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...
28. ### Mass flow rate for compressible flow

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...
29. ### Help with compressible flow problems

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...
30. ### Compressible flow problem in heated tube reactor

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...
31. ### Compressible flow in cylindrical coordinate

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
32. ### VBA Excel Functions for Compressible Flow. Houghton and Brock Tables

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...
33. ### Gas Dynamics (Compressible Flow) Normal Shock Wave

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...
34. ### Compressible Flow Question

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...
35. ### Understanding what a stagnation point is in compressible flow

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.
36. ### Isothermal compressible flow in horizontal pipe

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...
37. ### Exploring Subsonic, Compressible Flow in Curved Ducts

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...
38. ### Compressible Flow Homework: Show Particles in Cube Occupying Volume e at t=1

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...
39. ### Comparing pressure losses in compressible flow between air and natural gas

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...
40. ### Combustion and Compressible Flow

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...
41. ### Energy loss in compressible flow

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...
42. ### Compressible Flow: Flow in nozzles

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...