Gelombang kejutan

Suatu gelombang kejutan (juga dipanggil sebagaidepan kejutan atau hanya "kejutan") adalah sejenis gangguan rambatan. Ia membawa tenaga dan boleh merambat menerusi suatu medium (pepejal, cecair, gas atau plasma) seperti gelombang biasa, atau di dalam beberapa kes di dalam ketiadaan medium bahan, menerusi suatu medan seperti medan elektromagnet. Gelombang kejutan boleh dicirikan oleh suatu peubahan mendadak yang terputus-putus di dalam ciri-ciri medium tersebut. Merentasi kejutan tersebut biasanya terdapat kenaikan mendadak yang melampau di dalam tekanan, suhu dan ketumpatan suatu aliran. Di dalam aliran supersonik, pengembangan dijayakan menerusi satu kipas pengembangan. Suatu gelombang kejutan bergerak menerusi kebanyakan media pada kelajuan yang lebih tinggi daripada gelombang biasa.
Tidak seperti soliton (jenis gelombang bukan linear yang lain), tenaga gelombang kejutan hilang secara relatifnya cepat dengan jarak.
Juga, pengiringan pelebaran gelombang mendekati dan akhirnya akan bersambung dengan gelombang kejutan, sebahagiannya akan membatalkannya. Akhirnya dentuman bunyi yang berkait rapat dengan laluan pesawat supersonik telah dihasilkan oleh degradasi dan menyambung kembali gelombang pengembangan telah dihasilkan oleh pesawat.
Apabila suatu gelombang kejutan melalui jisim, jumlah tenaga disimpan yang boleh dikeluarkan sebagai kerja berkurangan dan entropy meningkat. Ini, sebagai contoh, mencipta daya gelombang heretan pada pesawat dengan kejutan.

Kejutan normal

In elementary fluid mechanics utilizing ideal gasses, a shock wave is treated as a discontinuity where entropy increases over a nearly infinitesimal region. Since no fluid flow is discontinuous, a control volume is established around the shock wave. With the control surfaces the exit lines into and out of the region. At the control surfaces assuming these are parallel to the shock wave, momentum, mass flux, and energy are constant. It is assumed the system is adiabatic (no heat exits or enters the system) and no work is being done.
Taking into account the established assumptions, in a system where the downstream properties are becoming subsonic: the upstream and downstream flow properties of the fluid are considered isentropic. Since the total amount of energy within the system is constant, the stagnation enthalpy remains constant over both regions. Though, entropy is increasing this must be accounted for by a drop in stagnation pressure of the downstream fluid.

Disebabkan curaman bukan linear

Shock waves can form due to steepening of ordinary waves. The best-known example of this phenomenon is ocean waves that form breakers on the shore. In shallow water, the speed of surface waves is dependent on the depth of the water. An incoming ocean wave has a slightly higher wave speed near the crest of each wave than near the troughs between waves, because the wave height is not infinitesimal compared to the depth of the water. The crests overtake the troughs until the leading edge of the wave forms a vertical face and spills over to form a turbulent shock (a breaker) that dissipates the wave's energy as sound and heat.
Similar phenomena affect strong sound waves in gas or plasma, due to the dependence of thesound speed on temperature and pressure. Strong waves heat the medium near each pressure front, due to adiabatic compression of the air itself, so that high pressure fronts outrun the corresponding pressure troughs. While shock formation by this process does not normally happen to sound waves in Earth's atmosphere, it is thought to be one mechanism by which the solar chromosphere and corona are heated, via waves that propagate up from the solar interior.

Analogi-analogi

Gelombang kejutan boleh digambarkna sebagai titik paling jauh menentang arah objek bergerak yang "mengetahui" tujuan objek tersebut. In this description, the shock wave position is defined as the boundary between the zone having no information about the shock-driving event, and the zone aware of the shock-driving event, analogous with the light cone described in the theory of special relativity.
To get a shock wave something has to be travelling faster than the local speed of sound. In that case some parts of the air around the aircraft are travelling at exactly the speed of sound with the aircraft, so that the sound waves leaving the aircraft pile up on each other, similar to a tailback on a road, and a shock wave forms, the pressure increases, and then spreads out sideways. Because of this amplification effect, a shock wave is very intense, more like an explosion when heard (not coincidentally, since explosions create shock waves).
Analogous phenomena are known outside fluid mechanics. For example, particles accelerated beyond the speed of light in a refractive medium (where the speed of light is less than that in a vacuum, such as water) create visible shock effects, a phenomenon known as Cherenkov radiation.

Contoh-contoh

Contoh gelombang sebegini yang dikelaskan bersama fenomena kejutan yang sama seperti berikut:

Gelombang kejutan di dalam aliran bebutir pantas

Shock waves can also occur in rapid flows of dense granular materials down inclined channels or slopes. Strong shocks in rapid dense granular flows can be studied theoretically and analyzed to compare with experimental data. Consider a configuration in which the rapidly moving material down the chute impinges on an obstruction wall erected perpendicular at the end of a long and steep channel. Impact leads to a sudden change in the flow regime from a fast movingsupercritical thin layer to a stagnant thick heap. This flow configuration is particularly interesting because it is analogous to some hydraulic and aerodynamic situations associated with flow regime changes from supercritical to subcritical flows.

Gelombang kejutan di dalam astrofizik

Astrophysical environments feature many different types of shock waves. Some common examples are supernovae shock waves or blast waves traveling through the interstellar medium, thebow shock caused by the Earth's magnetic field colliding with the solar wind and shock waves caused by galaxies colliding with each other. Another interesting type of shock in astrophysics is the quasi-steady reverse shock or termination shock that terminates the ultra relativistic wind from young pulsars.

Gelombang kejutan oleh enjin pembakaran

The wave disk engine (also named "Radial Internal Combustion Wave Rotor") is a kind ofpistonless rotary engine that utilize the shock waves to transfer energy between a high-energy fluid to a low-energy fluid, thereby increasing both temperature and pressure of the low-energy fluid.