Editing Shock wave (section)

== Shock capturing and detection ==
[[File:F4 p4 red planedrop.jpg|alt=Interacting shockwaves from two supersonic planes|thumb|NASA took their first [[Schlieren photograph]] of shock waves interacting between two aircraft in 2019.]]
{{Anchor|structure2016-01-29}}Advanced techniques are needed to capture shock waves and to detect shock waves in both numerical computations and experimental observations.<ref>{{Citation | title=Review of shock wave detection method in CFD post-processing|journal=Chinese Journal of Aeronautics | volume=26 | issue=3 | year=2013 | pages= 501–513 | last1=Wu ZN, Xu YZ, etc|doi=10.1016/j.cja.2013.05.001 | doi-access=free |bibcode=2013ChJAn..26..501W }}</ref><ref>{{cite journal | last1=Solem | first1=J. C. | last2=Veeser | first2=L. | year=1977 | title=Exploratory laser-driven shock wave studies | journal=Los Alamos Scientific Laboratory Report LA-6997 | volume=79 | page=14376 | doi=10.2172/5313279 | bibcode=1977STIN...7914376S |osti=5313279| url=https://digital.library.unt.edu/ark:/67531/metadc1071555/m2/1/high_res_d/5313279.pdf }}</ref><ref>{{cite journal | last1=Veeser | first1=L. R. | last2=Solem | first2=J. C. | year=1978 | title=Studies of Laser-driven shock waves in aluminum | journal=Physical Review Letters | volume=40 | issue=21 | pages=1391 |bibcode = 1978PhRvL..40.1391V |doi = 10.1103/PhysRevLett.40.1391 }}</ref><ref>{{cite journal | last1=Solem | first1=J. C. | last2=Veeser | first2=L. R. | year=1978 | title=Laser-driven shock wave studies | journal=Proceedings of Symposium on the Behavior of Dense Media Under High Dynamic Pressure. (Éditions du Commissariat à l'Énergie Atomique, Centre d'Études Nucléaires de Saclay, Paris) | issue=Los Alamos Scientific Laboratory Report LA-UR-78-1039 | pages=463–476}}</ref><ref>{{cite journal | last1=Veeser | first1=L. | last2=Solem | first2=J. C. | last3=Lieber | first3=A. | year=1979 | title=Impedance-match experiments using laser-driven shock waves | journal=Applied Physics Letters | volume=35 | issue=10 | pages=761 |bibcode = 1979ApPhL..35..761V |doi = 10.1063/1.90961 }}</ref><ref>{{cite book | last1=Solem | first1=J. C. | last2=Veeser | first2=L. | last3=Lieber | first3=A. | journal=Applied Physics Letters | year=1979 | title=Impedance-match experiments using laser-driven shock waves | volume=35 | issue=10 | pages=761–763 | url= https://books.google.com/books?id=GE39BAAAQBAJ&q=Impedance-match+experiments+using+laser-driven+shock+waves+AIRAPT+france&pg=PR13| isbn=9781483148526 | bibcode=1979ApPhL..35..761V | doi=10.1063/1.90961 }}</ref><ref>{{cite journal | last1=Veeser | first1=L. | last2=Lieber | first2=A. | last3=Solem | first3=J. C. | year=1979 | title=Planar streak camera laser-driven shockwave studies | journal=Proceedings of International Conference on Lasers '79| volume=80 | pages=45 | bibcode=1979STIN...8024618V |osti=5806611}}</ref>

[[Computational fluid dynamics]] is commonly used to obtain the flow field with shock waves. Though shock waves are sharp discontinuities, in numerical solutions of fluid flow with discontinuities (shock wave, contact discontinuity or slip line), the shock wave can be smoothed out by low-order numerical method (due to numerical dissipation) or there are spurious oscillations near shock surface by high-order numerical method (due to Gibbs phenomena<ref>{{Cite book|last=Smith|first=Steven W.|url=https://www.dspguide.com/ch11/4.htm|title=Digital Signal Processing a Practical Guide for Engineers and Scientists|publisher=California Technical Publishing|year=2003|isbn=978-0966017632|location=San Diego, California|pages=209–224}}</ref>).

There exist some other discontinuities in fluid flow than the shock wave. The slip surface (3D) or slip line (2D) is a plane across which the tangent velocity is discontinuous, while pressure and normal velocity are continuous. Across the contact discontinuity, the pressure and velocity are continuous and the density is discontinuous. A strong expansion wave or shear layer may also contain high gradient regions which appear to be a discontinuity. Some common features of these flow structures and shock waves and the insufficient aspects of numerical and experimental tools lead to two important problems in practices:
(1) some shock waves can not be detected or their positions are detected wrong, (2) some flow structures which are not shock waves are wrongly detected to be shock waves.

In fact, correct capturing and detection of shock waves are important since shock waves have the following influences: 
(1) causing loss of total pressure, which may be a concern related to scramjet engine performance, 
(2) providing lift for wave-rider configuration, as the oblique shock wave at lower surface of the vehicle can produce high pressure to generate lift, 
(3) leading to wave drag of high-speed vehicle which is harmful to vehicle performance,
(4) inducing severe pressure load and heat flux, e.g. the Type IV shock–shock interference could yield a 17 times heating increase at vehicle surface, (5) interacting with other structures, such as boundary layers, to produce new flow structures such as flow separation, transition, etc.