# compressible flow

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## Compressible flow

Flow in which density changes are significant. Pressure changes normally occur throughout a fluid flow, and these pressure changes, in general, induce a change in the fluid density. In a compressible flow, the density changes that result from these pressure changes have a significant influence on the flow. The changes in the flow that result from the density changes are often termed compressibility effects. All fluids are compressible. However, compressibility effects are more frequently encountered in gas flows than in liquid flows.

An important dimensionless parameter in compressible flows is the Mach number, M. This is defined by Eq. (1),

(1) where a is the speed of sound and V is the velocity of the flow. For a gas, the speed of sound is given by Eq. (2),
(2) where R is the gas constant, k = cp/cv, cp and cv being the specific heats at constant pressure and constant volume respectively, and T is the temperature. If M < 0.3 in a flow, the density changes in the flow will usually be negligible; that is, the flow can be treated as incompressible. Compressible flows are, therefore, as a rough guide, associated with Mach numbers greater than 0.3.

When M < 1, the flow is said to be subsonic; when M = 1, the flow is said to be sonic; when M varies from slightly below 1 to slightly above 1, the flow is said to be transonic; and if M > 1, the flow is said to be supersonic. When the Mach number is very high, this usually being taken to mean M > 5, the flow is said to be hypersonic.

Compressible flows can have features that do not occur in low-speed flows. For example, shock waves and expansion waves can occur in supersonic flows. Another important phenomenon that can occur due to compressibility is choking, where the mass flow rate through a duct system may be limited as a result of the Mach number being equal to 1 at some point in the flow. See Choked flow, Shock wave, Sonic boom

Another effect of compressibility is associated with the acceleration of a gas flow through a duct. In incompressible flow, an increase in velocity is associated with a decrease in the cross-sectional area of the duct, this in fact being true as long as M < 1. However, when M > 1, that is, when the flow is supersonic, the opposite is true; that is, an increase in the velocity is associated with an increase in the cross-sectional area. Therefore, in order to accelerate a gas flow from subsonic to supersonic velocities in a duct, it is necessary first to decrease the area and then, once the Mach number has reached 1, to increase the area, that is, to use a so-called convergent-divergent nozzle. An example is the nozzle fitted to a rocket engine. See Fluid flow, Mach number, Supersonic flow

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.

## compressible flow

[kəm′pres·ə·bəl ′flō]
(fluid mechanics)
Flow in which the fluid density varies.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

## compressible flow

The flow of fluid in which its density varies. Compressible flow is encountered in supersonic flight.
References in periodicals archive ?
A study of the compressible flow through a diffusing S-duct, NASA Lewis Research Center, 1993.
 a numerical investigation of the inviscid, axisymmetric, steady swirling flow in cylindrical pipe for low Mach number approximation is developed ([rho] = 1/T, where [rho], T are the dimensionless density and the temperature in compressible flow).
Fortunately, some researchers, trying to apply the POD method in the compressible flow, provide some excellent ideas.
While his research was classified during this time, his oral histories discuss the type of computer models for compressible flow with embedded radiation transport that he developed throughout the 1950s.
They considered a compressible flow and a steady state k-e turbulence model for their simulation.
Coanda-effect ejectors are subjected in most applications to conditions of turbulent and compressible flow. This study considers a compressible, isothermal, Newtonian fluid under stationary flow conditions and does not consider external forces that interact with the fluid.
[9.] Anderson, J.D., Jr., "Modern Compressible Flow: With Historical Perspective", 2nd Edition McGraw-Hill, New York.
Compared to the above research, some flow topology studies on compressible flow were performed as well.
In LBM, the nonlinear convection term in the macroscopic method is replaced by the linear transfer process, which is similar to the method of solving the compressible flow characteristics.
Among his topics are the kinematics of fluid flow, control volume analysis, compressible flow, boundary layer flow, and the computational fluid dynamics of turbulent flow.
(2014) have conducted an extensive numerical investigation of a double-circuit Ranque-Hilsch vortex tube by applying and testing multiple turbulence models (k-[epsilon], k-[epsilon] RNG, SAS-SST, RSM-LRR and LES) using a three-dimensional model for solving steady state compressible flow in Ansys CFX.
Compressible flow (gas dynamics) is the branch of fluid mechanics that deals with flows having significant changes in fluid density.

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