What is Gas dynamics: Definition and 18 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. rogdal

    Deriving the kinetic energy flux in an effusion process

    I could not find any derivations in the litterature, except for the expected value of the energy flux expression itself: $$\overline{\Phi_{effusion,\epsilon}} = \overline{\dot{N_{ef}}}\overline{\epsilon_{ef}}=\frac{3Nl}{2A}\sqrt{\frac{(k_BT)^3}{2\pi m}}$$ I've started off by calculating the...
  2. rogdal

    Gas in a box with Maxwell-Boltzmann distribution

    I have considered two scenarios: 1) A particle that has just collided with the wall at ##z=L## is moving with a velocity ##v_z<0## moving away from the wall. Hence, the probability that this particle has of colliding again is ##0##, so its distribution is also ##0##. 2) A particle moving with...
  3. C

    I Resources to learn about particles on a grid/mesh

    Hello. I am looking to learn about averaging out a particle gas or any other type of organization of particles within a system or volume that can be approximated onto a grid or mesh where the particles are at a constant distance from each other: https://en.wikipedia.org/wiki/Particle_mesh. I...
  4. Eko

    Understanding Gas Dynamics in Piston-Crank Slider System

    I need help understanding what happens to the gas that is being worked by the piston, in the crank slider. My goal is to produce linear motion on an object up to a certain height and I'm wondering since the slider moves back during the rotation of the crank what happens to the object I'm trying...
  5. Limebat

    I Does combusted gas travel faster in a vacuum tube?

    Hey all, So if a gas is combusted, would it travel faster through a vacuum tube or a regular ole' tube? I would assume the vacuum tube, as there are less particulates collisions in the way of the fast-moving gas molecules. Yet this also implies pressure on the outside of the thin barrier _ 2...
  6. Limebat

    I What happens when gas is combusted while moving?

    If a combustible gas mixture (say O2 and H2 are combusted to form H2O) are already moving before combustion, what would happen? Intuition tells me the previous velocity (say the gas was moving 150 feet per second) would just add to the velocity achieved when standing still and then combusted...
  7. S

    Engineering Kinematic and geometric similarity (fluids)

    My attempt at a solution is to start off first denoting V_a to be the automobile an V_e to be the economy version. Same goes with l_a and l_e. To try and relate the two I have tried: V_a I_a = V_l L_e, however I am really not sure how they got the square root. The answer is: v = V sqrt(l/L)...
  8. N

    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...
  9. T

    I What is the difference between Mach cone and shock wave in aerodynamics?

    Is mach cone the area of the shock wave or the wave front of the Mach wave? Usually the diagram for mach cone is this, However I still see some author states that the Mach cone is the shock wave. Shouldn't the shock wave be at the intersection(between the circles) and if the Mach cone is about...
  10. A

    What is a very weak shock wave? (Gas Dynamics)

    I stumbled upon this concept when studying gas dynamics. What I get from my readings it must occur when Mach number is really close to 1. But I didn't understand how the property relations across a shock wave is affected by it.
  11. D

    I What happens if two moving shock waves collide?

    Say there were to be two normal shock waves that were moving towards each other at different speeds. What would happen when they collide with each other? Would the shock waves flow past each other or would they be reflected back from the direction they came from? Also how would you be able to...
  12. P

    Acoustic horns and impedance matching

    I found a fast solenoid valve lying around (~1ms rise time) so I thought it would be fun to hook up to a compressed air line and use to generate some really loud bass. I hooked it up to a function generator and air supply only to find that it was disappointingly quiet (well not quiet, but...
  13. A

    Conv-Div nozzle and Fanno flow (Gas Dynamics)

    Homework Statement A rocket nozzle is operating with a stretched out throat, (L = 50 cm and D = 10 cm). If the inlet stagnation conditions are po1 = 1 MPa and To1 = 1500 K, determine the nozzle exit velocity and mass flow for a back pressure of 30 kPa. The diameter of the nozzle at the exit...
  14. R

    How can I accurately calculate the required motive gas rate for an ejector?

    Does anyone have a good reference to calculate an ejectors required motive gas rate. As I understand it the pressure of the motive gas out of the nozzle and the pressure of the process gas are the same. A mass/volume flow can be guessed and the properties on the inlet can be calculated using...
  15. C

    Help with Gas dynamics problem

    Homework Statement for a weak shock wave (p2-p1)/p1=epsilon show that (s2-s1)/cv=(gamma^2-1)epsilon^3/(12*gamma^2)+higher order terms HINT: it is helpful to subtract this equation [p]/<p>+gamma[v]/<v>=0 from the equation [ln p(v)^gamma]=[s]/cv before expanding in terms of epsilon...
  16. P

    [Difficult] Gas Dynamics Situation

    Hoping someone finds this problem interesting, I'm looking for some ideas on how to solve a real world problem, not a complete and detailed solution. This topic is related to an astrophysical situation. However, I prefer to present it just as a question on gas dynamics since there's no need for...
  17. E

    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...
  18. U

    Formation of shock waves and gas dynamics of flow in a nozzle.

    Hi I have a few questions regarding gas dynamics... 1. when M=1, dA=0...what does this mean? Does it imply that if we have a converging (subsonic) nozzle with its exit mach no,say 0.8, and then direct the flow to a constant area tube, then we have to attain M=1? 2. When we know the length...
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