رقم ماخ
رقم ماخ (الماخ Ma ) هومقياس سرعة عديم الوحدة سُمّي على اسم الفيزيائي والفيلسوف النمساويّ إرنست ماخ Ernst Mach.
يعرّف رقم ماخ على أنّه نسبة السرعة المحلّيّة لجسم مائع إلى سرعة الصوت في الجسم المائع ذاته. هذه السرعة المحلّة للجسم المائع تتأتّى من قذيفةٍ أوجسمٍ ما في الهواء أوالسوائل كما قلنا (الجسم المائع). فيستعمل هذا المصطلح للأجسام التي تسير بسرعة كبيرة كالطائرات النفاثة مثلاً أوسرعة الهواء داخل قنوات التجارب الهوائية ومطاحن الرياح.
حيث
- هوعدد ماخ
- هوسرعة المصدر (الغرض، بالنسبة للوسط) و
- سرعة الصوت في الوسط
تكون سرعة الماخ-واحد على مستوى سطح البحر 1225 كيلومتر/ساعة أي 765.6 ميل/ساعة أوبالأحرى 340 مترًا / ثانية تقريبًا، ويتغيّر هذا الرقم أيضًا باختلاف درجة الحرارة والضغط الجوّ. ولعلّ الملاحظة سليمة، بأنّ السوط هوأوّل جسم أنتجه الإنسان وتزيد سرعته عن سرعة الصوت.
لأن الماخ عديم الوحدة، توضع عدد المرّات لسرعة ماخ خلف الحدثة، فنقول مثلاً 1-ماخ و2-ماخ و3-ماخ... إلخ.
تصنيفات سرعات ماخ
While the terms "subsonic" and "supersonic" in the purest verbal sense refer to speeds below and above the local speed of sound respectively, aerodynamicists often use the same terms to talk about particular ranges of Mach values. This occurs because of the presence of a "transonic regime" around M = 1 where approximations of the Navier-Stokes equations used for subsonic design actually no longer apply, the simplest of many reasons being that the flow locally begins to exceed M = 1 even when the freestream Mach number is below this value.
Meanwhile, the "supersonic regime" is usually used to talk about the set of Mach numbers for which linearised theory may be used, where for example the (air) flow is not chemically reacting, and where heat-transfer between air and vehicle may be reasonably neglected in calculations.
In the following table, the "regimes" or "ranges of Mach values" are referred to, and not the "pure" meanings of the words "subsonic" and "supersonic".
Generally, NASA defines "high" hypersonic as any Mach number fromعشرة to 25, and re-entry speeds as anything greater than Mach 25. Aircraft operating in this regime include the Space Shuttle and various space planes in development.
| التوصيف | ماخ | ميل/س | كم/س | م/ث | سمات عامة للطائرة |
|---|---|---|---|---|---|
| Subsonic | <0.8 | <610 | <980 | <270 | Most often propeller-driven and commercial turbofan aircraft with high aspect-ratio (slender) wings, and rounded features like the nose and leading edges. |
| Transonic | 0.8-1.2 | 610-915 | 980-1,470 | 270-410 | Transonic aircraft nearly always have swept wings, delaying drag-divergence, and often feature design adhering to the principles of the Whitcomb Area rule. |
| Supersonic | 1.2–5.0 | 915-3,840 | 1,470–6,150 | 410–1,710 | Aircraft designed to fly at supersonic speeds show large differences in their aerodynamic design because of the radical differences in the behaviour of flows above Mach 1. Sharp edges, thin aerofoil-sections, and all-moving tailplane/canards are common. Modern combat aircraft must compromise in order to maintain low-speed handling; "true" supersonic designs include the F-104 Starfighter, SR-71 Blackbird and BAC/Aérospatiale Concorde. |
| Hypersonic | 5.0–10.0 | 3,840–7,680 | 6,150–12,300 | 1,710–3,415 | جـِلد من النيكل-تيتانيوم مبرّد؛ highly integrated (due to domination of interference effects: non-linear behaviour means that superposition of results for separate components is invalid), small wings, such as those on the X-51A ويڤرايدر، وو-14 |
| هائلة السرعة High-hypersonic |
10.0–25.0 | 7,680–16,250 | 12,300–30,740 | 3,415–8,465 | Thermal control becomes a dominant design consideration. Structure must either be designed to operate hot, or be protected by special silicate tiles or similar. Chemically reacting flow can also cause corrosion of the vehicle's skin, with free-atomic oxygen featuring in very high-speed flows. Hypersonic designs are often forced into blunt configurations because of the aerodynamic heating rising with a reduced radius of curvature. |
| سرعات Re-entry speeds |
>25.0 | >16,250 | >30,740 | >8,465 | Ablative heat shield; small or no wings; blunt shape |
التيار عالي السرعة حول الأجسام
Flight can be roughly classified in six categories:
| Regime | Subsonic | Transonic | Sonic | Supersonic | Hypersonic | High-hypersonic |
|---|---|---|---|---|---|---|
| ماخ | <1.0 | 0.8-1.2 | 1.0 | 1.0-5.0 | 5.0-10.0 | >10.0 |
| (a) | (b) |
Fig. 1. Mach number in transonic airflow around an airfoil; M<1 (a) and M>1 (b).
![{\displaystyle {M ={\sqrt {{\frac {2 {\gamma -1 \left[\left({\frac {q_{c {P +1\right)^{\frac {\gamma -1 {\gamma -1\right]](https://en.wikipedia.org/api/rest_v1/media/math/render/svg/cf735121af8d857ade924cb99e9b52cdacbe1f68)
حيث:
- هوعدد ماخ
- is the ratio of specific heat of a gas at a constant pressure to heat at a constant volume (1.4 for air).
The formula to compute Mach number in a supersonic compressible flow is derived from the رايلي Supersonic Pitot equation:
![{\displaystyle {M =0.88128485{\sqrt {\left[\left({\frac {q_{c {P +1\right)\left(1-{\frac {1 {[7M^{2 ] \right)^{2.5 \right]](https://en.wikipedia.org/api/rest_v1/media/math/render/svg/399ed68976e8121e2bff0abcbd17e1a632644a66)
where:
اقرأ أيضا
- رقم كنودسن
- ميكانيك الموائع
- Critical Mach number
- Machmeter
- Ramjet
- Scramjet
- سرعة الصوت
- True airspeed
الهامش
وصلات خارجية
- Gas Dynamics Toolbox Calculate Mach number and normal shock wave parameters for mixtures of perfect and imperfect gases.
- NASA's page on Mach Number Interactive calculator for Mach number.
- NewByte standard atmosphere calculator and speed converter
