Only in the low-density realm of rarefied gas dynamics does the motion of individual molecules become important.Ī related assumption is the no-slip condition where the flow velocity at a solid surface is presumed equal to the velocity of the surface itself, which is a direct consequence of assuming continuum flow. This assumption provides a huge simplification which is accurate for most gas-dynamic problems. Instead, the continuum assumption allows us to consider a flowing gas as a continuous substance except at low densities. All fluids are composed of molecules, but tracking a huge number of individual molecules in a flow (for example at atmospheric pressure) is unnecessary. There are several important assumptions involved in the underlying theory of compressible flow. Introductory concepts Breakdown of fluid mechanics chart Much of basic gas dynamics is analytical, but in the modern era Computational fluid dynamics applies computing power to solve the otherwise-intractable nonlinear partial differential equations of compressible flow for specific geometries and flow characteristics. Theoretical gas dynamics considers the equations of motion applied to a variable-density gas, and their solutions. Experimental gas dynamics undertakes wind tunnel model experiments and experiments in shock tubes and ballistic ranges with the use of optical techniques to document the findings. Historically, two parallel paths of research have been followed in order to further gas dynamics knowledge. Piloted by Chuck Yeager, the X-1 officially achieved supersonic speed in October 1947. However, aircraft design progressed sufficiently to produce the Bell X-1. Overcoming the larger drag proved difficult with contemporary designs, thus the perception of a sound barrier. Amongst other factors, conventional aerofoils saw a dramatic increase in drag coefficient when the flow approached the speed of sound. Many others also contributed to this field.Īccompanying the improved conceptual understanding of gas dynamics in the early 20th century was a public misconception that there existed a barrier to the attainable speed of aircraft, commonly referred to as the " sound barrier." In truth, the barrier to supersonic flight was merely a technological one, although it was a stubborn barrier to overcome. Other notable figures ( Meyer, Luigi Crocco, and Ascher Shapiro) also contributed significantly to the principles considered fundamental to the study of modern gas dynamics. Theodore von Kármán, a student of Prandtl, continued to improve the understanding of supersonic flow. Ludwig Prandtl and his students proposed important concepts ranging from the boundary layer to supersonic shock waves, supersonic wind tunnels, and supersonic nozzle design. As the century progressed, inventors such as Gustaf de Laval advanced the field, while researchers such as Ernst Mach sought to understand the physical phenomena involved through experimentation.Īt the beginning of the 20th century, the focus of gas dynamics research shifted to what would eventually become the aerospace industry. At the beginning of the 19th century, investigation into the behaviour of fired bullets led to improvement in the accuracy and capabilities of guns and artillery. The study of gas dynamics is often associated with the flight of modern high-speed aircraft and atmospheric reentry of space-exploration vehicles however, its origins lie with simpler machines. 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. 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). Compressible flow (or gas dynamics) is the branch of fluid mechanics that deals with flows having significant changes in fluid density.
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