O Escoamentos Compressíveis

4 – Escoamento compressível externo

4.1 - Escoamento Transônico

É um tipo de escoamento caracterizado por ser uma mistura entre regiões de Mach maiores que 1 e menores que 1. Isso ocorre porque um número de Mach menor que 1 não garante que o escoamento sob o corpo seja subsônico, em um corpo aerodinâmico a velocidade do escoamento incidindo sob ele é maior que a velocidade do vento relativo, e se o Mach do vento relativo for perto de 1, o número de Mach sob o corpo pode ser maior que 1, ou seja, pode haver algumas regiões no corpo que escoamento seja supersônico. Através de vários testes práticos observou-se que para corpos esbeltos o Mach do vento relativo tem que ser menor que 0,8 para garantir que o escoamento seja subsônico. Para corpos rombudos o M deve ser menor ainda. Na figura 1 no esquema (b)

[pic 1]

Um escoamento sônico é caracterizado por um número de mach igual a um (M=1)

Transonic flow (mixed regions where M < 1 and M < 1). As stated above, if M∞ is subsonic but near unity, the flow can become locally supersonic (M < 1). This is sketched inFig. 4b, which shows pockets of supersonic flow over both the top and the bottom surfaces of the airfoil, terminated by weak shock waves behind which the flow becomes subsonic again. Moreover, if M∞ is increased slightly above unity, a bow shock wave is formed in front of the body; behind this shock wave the flow is locally subsonic, as shown inFig. 4c. This subsonic flow subsequently expands to a low supersonic value over the airfoil. Weak shock waves are usually generated at the trailing edge, sometimes in a “fishtail” pattern as shown inFig. 4c. The flow fields shown inFigs. 4b and4c are characterized by mixed subsonic–supersonic flows and are dominated by the physics of both types of flows. Hence, such flow fields are called transonic flows. Again, as a rule of thumb for slender bodies, transonic flows occur for freestream Mach numbers in the range 0.8 < M∞ < 1.2.

3.

Supersonic flow (M < 1 everywhere). A flow field is defined as supersonic if the Mach number is greater than 1 at every point. Supersonic flows are frequently characterized by the presence of shock waves across which the flow properties and streamlines change discontinuously (in contrast to the smooth, continuous variations in subsonic flows). This is illustrated inFig. 4d for supersonic flow over a sharp-nosed wedge; the flow remains supersonic behind the oblique shock wave from the tip. Also shown are distinct expansion waves, which are common in supersonic flow. (Again, the listing of M∞ > 1.2 is strictly a rule of thumb. For example, inFig. 4d, if θ is made large enough, the oblique shock wave will detach from the tip of the wedge and will form a strong, curved bow shock ahead of the wedge with a substantial region of subsonic flow behind the wave. Hence, the totally supersonic flow sketched inFig. 4d is destroyed if θ is too large for a given M∞. This shock detachment phenomenon can occur at any value of M∞ > 1, but the value of θ at which it occurs increases as M∞ increases. In turn, if θ is made infinitesimally small, the flow field inFig. 4d holds for M∞ ≥ 1.0. However, the above discussion clearly shows that the listing of M∞ > 1.2 inFig. 4d is a very tenuous rule of thumb and should not be taken literally. In a supersonic flow, because the local flow velocity is greater than the speed of sound, disturbances created at some point in the flow cannot work their way upstream (in contrast to subsonic flow). This property is one of the most significant physical differences between subsonic and supersonic flows. It is the basic reason why shock waves occur in supersonic flows but do not occur in steady subsonic flow.