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| [[File:One dimensional shock plain.svg|thumb|300px|One-dimensional shock wave in a medium. The shock moves with a velocity ''u''<sub>s</sub>. The coordinate system has been chosen to move with the shock so that the particle velocities outside the shock are zero. The shock front does not change with time in a stationary shock.]]
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| The '''Rankine–Hugoniot conditions''', also referred to as '''Rankine–Hugoniot jump conditions''' or '''Rankine–Hugoniot relations''', | |
| describe the relationship between the states on both sides of a [[shock wave]] in a one-dimensional flow in fluids or a one-dimensional deformation in solids. They are named in recognition of the work carried out by Scottish engineer and physicist [[William John Macquorn Rankine]]<ref name="[Ran-70]">{{Cite journal|last=Rankine |first=W. J. M. |authorlink=William John Macquorn Rankine |year=1870 |title=On the thermodynamic theory of waves of finite longitudinal disturbances |journal=Philosophical Transactions of the Royal Society of London |volume=160 |pages=277–288 |url=http://gallica.bnf.fr/ark:/12148/bpt6k559658/f328.image.r=Philosophical+Transactions+of+the+Royal+Society+of+London+1870.langEN |doi=10.1098/rstl.1870.0015 |format= }}</ref> and French engineer [[Pierre Henri Hugoniot]].<ref name="[Hug-87]">{{Cite journal|last=Hugoniot |first=H. |authorlink=Pierre Henri Hugoniot |year=1887 |language=French |title=Mémoire sur la propagation des mouvements dans les corps et spécialement dans les gaz parfaits (première partie) [Memoir on the propagation of movements in bodies, especially perfect gases (first part)] |url=http://books.google.com/books?id=wccAAAAAYAAJ&pg=PA3#v=onepage&q&f=false |journal=[[Journal de l'École Polytechnique]] |volume=57 |pages=3–97}} See also: Hugoniot, H. (1889) [http://gallica.bnf.fr/ark:/12148/bpt6k4337130/f5.image "Mémoire sur la propagation des mouvements dans les corps et spécialement dans les gaz parfaits (deuxième partie)"] [Memoir on the propagation of movements in bodies, especially perfect gases (second part)], ''Journal de l'École Polytechnique'', vol. 58, pages 1-125.</ref> See also Salas (2006)<ref name="[Sal-06]">{{Cite web|last=Salas |first=M. D. |year=2006 |title=The Curious Events Leading to the Theory of Shock Waves, Invited lecture,'' 17th Shock Interaction Symposium'', Rome'','' 4–8 September. |url=http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20060047586_2006228914.pdf}}</ref> for some historical background.
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| In a coordinate system that is moving with the shock, the Rankine–Hugoniot conditions can be expressed as:
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| :<math>
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| \begin{align}
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| & \rho_1\,u_s = \rho_2(u_s - u_2) &\qquad \text{Conservation of mass}\\
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| & p_2 - p_1 = \rho_2\,u_2\,(u_s - u_2) = \rho_1\,u_s\,u_2 &\qquad \text{Conservation of momentum}\\
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| & p_2\,u_2 = \rho_1\,u_s\,\left(\tfrac{1}{2}\,u_2^2 + E_2 - E_1\right) &\qquad \text{Conservation of energy}
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| \end{align}
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| </math>
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| where ''u''<sub>s</sub> is the shock wave speed, ''ρ''<sub>1</sub> and ''ρ''<sub>2</sub> are the [[mass density]] of the fluid behind and inside the shock, ''u''<sub>2</sub> is the particle velocity of the fluid inside the shock, ''p''<sub>1</sub> and p<sub>2</sub> are the pressures in the two regions, and ''E''<sub>1</sub> and ''E''<sub>2</sub> are the [[internal energy|internal energies]] per unit mass in the two regions. A schematic of the quantities used in the above equations is shown in the adjacent figure. These equations can be derived in a straightforward manner from equations (12), (13) and (14) below. Using the Rankine-Hugoniot equations for the conservation of mass and momentum to eliminate ''u''<sub>s</sub> and ''u''<sub>2</sub>, the equation for the conservation of energy can be expressed in the more popular form:
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| :<math>
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| E_2 - E_1 = \tfrac{1}{2}\,(p_2 + p_1)\,\left(\tfrac{1}{\rho_1}-\tfrac{1}{\rho_2}\right) = \tfrac{1}{2}\,(p_2 + p_1)\,(v_1-v_2)
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| </math>
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| where ''v''<sub>1</sub> and ''v''<sub>2</sub> are the uncompressed and compressed [[specific volume]]s per unit mass, respectively.
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| ==Basics: Euler equations in one dimension==
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| Consider gas in a one-dimensional container (e.g., a long thin tube).
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| Assume that the fluid is [[Inviscid flow|inviscid]]
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| (i.e., it shows no viscosity effects as for example
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| friction with the tube walls).
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| Furthermore, assume that there is no heat transfer by conduction or radiation and that gravitational acceleration can be neglected.
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| Such a system can be described by the following
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| system of [[conservation law]]s,
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| known as the 1D [[Euler equations (fluid dynamics)|Euler equations]]
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| ::<math>(\;1)\quad \quad\frac{\partial\rho}{\partial t} \;\; = -\frac{\partial}{\partial x}\left(\rho u\right)</math>
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| ::<math>(\;2)\quad \quad\frac{\partial\rho u}{\partial t} \, = -\frac{\partial}{\partial x}\left(\rho u^{2}+p\right)</math>
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| ::<math>(\;3)\quad \quad\frac{\partial\rho E}{\partial t} = -\frac{\partial}{\partial x}\left[\rho u\left(e+\frac{1}{2}u^{2}+p/\rho\right)\right],</math>
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| where
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| :<math>\rho = \,</math> fluid [[mass density]], [kg/m<sup>3</sup>]
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| :<math>u = \,</math> fluid [[velocity]], [m/s]
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| :<math>e = \,</math> specific [[internal energy]] of the fluid, [J/kg]
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| :<math>p = \,</math> fluid [[pressure]], [Pa]
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| :<math>t = \,</math> time, [s]
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| :<math>x = \,</math> distance, [m], and
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| :<math>E = e+\frac{1}{2}u^{2},</math> specific total energy of the fluid, [J/kg].
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| Assume further that the gas is calorically ideal and that therefore a polytropic [[equation of state|equation-of-state]] of the simple form
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| ::<math>(\;4)\quad\quad p=\left(\gamma-1\right)\rho e,</math>
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| is valid, where <math>\gamma</math> is the constant ratio of specific heats <math>c_p/c_v</math>.
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| This quantity also appears as the ''polytropic exponent''
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| of the polytropic process described by
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| ::<math>(\;5)\quad \quad \frac{p}{\rho^\gamma} = \text{constant}.</math>
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| For an extensive list of compressible flow equations, etc., refer to [[National Advisory Committee for Aeronautics|NACA]] Report 1135 (1953).<ref name="[Ame-53]">{{citation |author=Ames Research Staff |year=1953 |title=Equations, Tables and Charts for Compressible Flow |journal=Report 1135 of the National Advisory Committee for Aeronautics |url=http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091059_1993091059.pdf }}</ref>
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| Note: For a calorically ideal gas <math>\gamma \,</math> is a constant and for a thermally ideal gas <math>\gamma \,</math> is a function of temperature. In the latter case, the dependence of pressure
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| on mass density and internal energy might differ from that given by
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| equation (4).
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| ==The jump condition==
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| Before proceeding further it is necessary to introduce the concept of a ''jump condition'' – a condition that holds at a discontinuity or abrupt change.
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| Consider a 1D situation where there is a jump in the scalar conserved physical quantity <math>w</math>, which is governed by the hyperbolic conservation law
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| ::<math> (\;6) \quad\quad \frac{\partial w}{\partial t}+\frac{\partial}{\partial x}f\left(w\right)=0.</math>
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| Let the solution exhibit a jump (or shock) at <math>x=x_{s}(t)</math> and integrate over the partial domain, <math>x\in\left(x_{1},x_{2}\right)</math>, where <math>x_{1}<x_{s}(t)</math> and <math>x_{s}(t)<x_{2}</math>,
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| ::<math>(\;7) \quad\quad \frac{d}{dt}\left(\int_{x_1}^{x_s(t)}w \, dx + \int_{x_s(t)}^{x_2} w\,dx\right) = -\int_{x_1}^{x_2}\frac{\partial}{\partial x}f\left(w\right)\,dx</math>
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| ::<math>(\;8) \quad\quad \therefore w_1\frac{dx_s}{dt}-w_2 \frac{dx_s}{dt} + \int_{x_1}^{x_s(t)} w_t \, dx + \int_{x_s(t)}^{x_2}w_t \, dx = -\left.f\left(w\right)\right|_{x_1}^{x_2}</math>
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| The subscripts ''1'' and ''2'' indicate conditions ''just upstream'' and ''just downstream'' of the jump respectively. Note, to arrive at equation (8) we have used the fact that <math>{\displaystyle dx_1/dt=0}</math> and <math>{\displaystyle dx_2/dt=0}</math>.
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| Now, let <math>x_1\rightarrow x_s(t)</math> and <math>x_2\rightarrow x_s(t)</math>, when we have <math>\int_{x_1}^{x_s(t)}w_t \, dx\rightarrow 0</math> and <math>\int_{x_s(t)}^{x_2}w_t \, dx\rightarrow0</math>, and in the limit
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| ::<math> (\;9) \quad\quad u_s\left(w_1 - w_2\right) = f \left( w_1 \right) - f \left( w_2 \right),</math>
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| where we have defined <math>u_s=dx_s(t)/dt \,</math> (the system ''characteristic'' or ''shock speed''), which by simple division is given by
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| ::<math> (10) \quad\quad u_s = \frac{f\left(w_1\right) - f\left(w_2 \right)}{w_1 - w_2}.</math>
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| Equation (9) represents the jump condition for conservation equation (6). A shock situation arises in a system where its ''characteristics'' intersect, and under these conditions a requirement for a unique single-valued solution is that the solution should satisfy the ''admissibility condition'' or ''entropy condition''. For physically real applications this means that the solution should satisfy the ''Lax entropy condition''
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| ::<math> (11) \quad\quad f^{\prime} \left(w_1\right) > u_s > f^{\prime} \left( w_2 \right),</math>
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| where <math>f^{\prime}\left(w_1\right)</math> and <math>f^{\prime}\left(w_2\right)</math> represent ''characteristic speeds'' at upstream and downstream conditions respectively.
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| ==Euler equations shock condition==
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| In the case of the hyperbolic conservation equation (6), we have seen that the shock speed can be obtained by simple division. However, for the 1D Euler equations ( 1), ( 2) and ( 3), we have the vector state variable <math>\left[\rho,\rho u,\rho E\right]^T</math> and the jump conditions become
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| ::<math>(12)\quad\quad\quad\quad\; u_s\left(\rho_2 - \rho_1 \right) = \rho_2 u_2 - \rho_1 u_1 </math>
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| ::<math>(13)\quad\quad\; u_s\left(\rho_2 u_2 - \rho_1 u_1 \right) = \left( \rho_2 u_2^2 +p_ 2 \right) - \left(\rho_1 u_1^2 +p_1 \right)</math>
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| ::<math>(14)\quad\quad u_s\left(\rho_2 E_2 - \rho_1 E_1 \right) = \left[ \rho_ 2 u_ 2 \left( e_ 2 + \frac{1}{2} u_2^2 +p_2/\rho_2 \right)\right] - \left[\rho_1 u_1 \left( e_1 + \frac{1}{2} u_1^2 + p_1/\rho_1 \right)\right].</math>
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| [[Image:Rankine Hugoniot Shock.png|thumb|200px|A schematic diagram of a shock wave situation with the density <math>\rho</math>, velocity <math>u</math>, and temperature <math>T</math> indicated for each region.]]
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| Equations (12), (13) and (14) are known as the ''Rankine–Hugoniot conditions'' for the Euler equations and are derived by enforcing the conservation laws in integral form over a control volume that includes the shock. For this situation <math>s</math> cannot be obtained by simple division. However, it can be shown by transforming the problem to a moving co-ordinate system
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| (setting <math>u_s':=u_s-u_1</math>, <math>u'_1:=0</math>, <math>u'_2:=u_2-u_1</math>
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| to remove <math>u_1</math>)
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| and some algebraic manipulation
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| (involving the elimination of <math>u'_2</math>
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| from the transformed equation (13) using the transformed equation (12)),
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| that the shock speed is given by
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| ::<math>(15)\quad\quad u_s = u_1 + c_1 \sqrt{1 + \frac{\gamma+1}{2\gamma} \left( \frac{p_2}{p_1} - 1\right)},</math>
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| where <math>c_{1}=\sqrt{\gamma p_{1}/\rho_{1}}</math> is the speed of sound in the fluid at upstream conditions.
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| See Laney (1998),<ref name="[Lan-98]">{{Cite book|last= Laney |first= Culbert B. |year=1998 |title=Computational Gasdynamics |publisher=Cambridge University Press |isbn=978-0-521-62558-6}}</ref>
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| LeVeque (2002),<ref name="[LeV-02]">{{Cite book|last=LeVeque |first=Randall |authorlink=Randall J. LeVeque |year=2002 |title=Finite Volume Methods for Hyperbolic Problems |publisher=Cambridge University Press |isbn=978-0-521-00924-9}}</ref> Toro (1999),<ref name="[Tor-99]">{{Cite book|last=Toro |first=E. F. |year=1999 |title=Riemann Solvers and Numerical Methods for Fluid Dynamics |publisher=Springer-Verlag |isbn=978-3-540-65966-2}}</ref> Wesseling (2001),<ref name="[Wes-01]">{{Cite book|last=Wesseling |first=Pieter |year=2001 |title=Principles of Computational Fluid Dynamics |publisher=Springer-Verlag |isbn=978-3-540-67853-3}}</ref> and Whitham (1999)<ref name="[Whi-99]">{{Cite book|last=Whitham |first=G. B. |authorlink=Gerald B. Whitham|year=1999 |title=Linear and Nonlinear Waves |publisher=Wiley |isbn=978-0-471-94090-6}}</ref> for further discussion.
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| ===Stationary shock===
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| For a stationary shock <math>u_s=0</math>, and for the 1D Euler equations we have
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| ::<math>(16)\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\;\; \rho_1 u_1 = \rho_2 u_2 </math>
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| ::<math>(17)\quad\quad\quad\quad\quad\quad\quad\quad\;\; \rho_1 u_1^2 + p_1 = \rho_2 u_2^2 + p_2 </math>
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| ::<math>(18)\quad\quad \rho_1 u_1 \left( e_1 + \frac{1}{2} u_1^2 + p_1/\rho_1 \right) = \rho_2 u_2 \left(e_2 + \frac{1}{2} u_2^2 + p_2/\rho_2 \right).</math>
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| In view of equation (16) we can simplify equation (18) to
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| ::<math>(19)\quad\quad e_1 + \frac{1}{2} u_1^2 + p_1/\rho_1 = e_2 + \frac{1}{2} u_2^2 + p_2/\rho_2,</math>
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| which is a statement of [[Bernoulli's principle]], under conditions where gravitational effects can be neglected.
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| Substituting <math>u_{1}</math> and <math>u_{2}</math> from equations (16) and (17) into equation (19) yields the following relationship:
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| ::<math>(20)\quad\quad 2 \left( h_2 - h_1 \right) = \left( p_2 -p_1 \right) \cdot \left(\frac{1}{\rho_1} + \frac{1}{\rho_2}\right),</math>
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| where <math>h=\frac{p}{\rho}+e</math> represents [[Enthalpy|specific enthalpy]] of the fluid. Eliminating internal energy <math>e</math> in equation (19) by use of the equation-of-state, equation ( 4), yields
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| ::<math>(21)\quad\quad \frac{\rho_2}{\rho_1} = \frac{\frac{p_2}{p_1} (\gamma+1) + (\gamma-1)}{(\gamma+1) + \frac{p_2}{p_1}(\gamma-1)} = \frac{u_1}{u_2}</math>
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| ::<math>(22)\quad\quad \frac{p_{2}}{p_{1}} = \frac{\frac{\rho_2}{\rho_1} (\gamma+1) - (\gamma-1)} {(\gamma+1) - \frac{\rho_2}{\rho_1} (\gamma-1)}.</math>
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| From physical considerations it is clear that both the upstream and downstream pressures must be positive, and this imposes an upper limit on the density ratio in equations (21) and (22) such that <math>\rho_{2}/\rho_{1}<(\gamma+1)/(\gamma-1)\,</math>. As the strength of the shock increases, there is a corresponding increase in downstream gas temperature, but the density ratio <math>\rho_{2}/\rho_{1}</math> approaches a finite limit: 4 for an ideal [[monatomic]] gas <math>(\gamma=5/3)</math> and 6 for an ideal [[diatomic]] gas <math>(\gamma=1.4)</math>. Note: air is comprised predominately of diatomic molecules and therefore at [[standard conditions]] <math>\gamma_\mathrm{air}\simeq1.4</math>.
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| == Shock Hugoniot and Rayleigh line ==
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| [[File:HugoniotRaleigh.png|250px|thumb|Shock Hugoniot and Rayleigh line in the ''p''-''v'' plane. The curve represents a plot of equation (24) with ''p''<sub>1</sub>, ''v''<sub>1</sub>, ''c''<sub>0</sub>, and ''s'' known. If ''p''<sub>1</sub> = 0, the curve will intersect the specific volume axis at the point ''v''<sub>1</sub>.]]
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| [[File:HEL_plain.svg|250px|thumb|Hugoniot elastic limit in the ''p''-''v'' plane for a shock in an elastic-plastic material.]]
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| For shocks in solids, a closed form expression such as equation (15) cannot be derived from first principles. Instead, experimental observations <ref>{{Citation|last=Ahrens|first=T.J.|year=1993|title=Equation of state|journal=High Pressure Shock Compression of Solids, eds. J. R. Asay and M. Shahinpoor|location=Springer-Verlag, New York|url=http://www.fas.harvard.edu/~planets/sstewart/ahrens/Papers_pdf/Seismo_1656.pdf}}</ref> indicate that a linear relation can be used instead (called the '''shock Hugoniot''' in the ''u''<sub>s</sub>-''u''<sub>p</sub> plane) that has the form
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| :<math>
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| (23) \qquad u_s = c_0 + s\,u_p = c_0 + s \,u_2
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| </math>
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| where ''c''<sub>0</sub> is the bulk speed of sound in the material (in uniaxial compression), ''s'' is a parameter (the slope of the shock Hugoniot) obtained from fits to experimental data, and ''u''<sub>p</sub>=''u''<sub>2</sub> is the particle velocity inside the compressed region behind the shock front.
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| The above relation, when combined with the Hugoniot equations for the conservation of mass and momentum, can be used to determine the shock Hugoniot in the ''p''-''v'' plane, where ''v'' is the specific volume (per unit mass):<ref>Poirier, J-P. (2008) "Introduction to the Physics of the Earth's Interior", Cambridge University Press.</ref>
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| :<math>
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| (24) \qquad p_2 - p_1 = \frac{c_0^2\, \rho_1\, \rho_2\, (\rho_2-\rho_1)}{[\rho_2 - s(\rho_2 - \rho_1)]^2}
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| = \frac{c_0^2\,(v_1 - v_2)}{[v_1 - s(v_1-v_2)]^2} \,.
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| </math>
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| Alternative equations of state, such as the [[Mie–Gruneisen equation of state]] may also be used instead of the above equation.
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| The shock Hugoniot describes the locus of all possible [[thermodynamic state]]s a material can exist in behind a shock, projected onto a two dimensional state-state plane. It is therefore a set of equilibrium states and does not specifically represent the path through which a material undergoes transformation.
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| Weak shocks are [[isentropic]] and that the isentrope represents the path through which the material is loaded from the initial to final states by a compression wave with converging characteristics. In the case of weak shocks, the Hugoniot will therefore fall directly on the isentrope and can be used directly as the equivalent path. In the case of a strong shock we can no longer make that simplification directly. However, for engineering calculations, it is deemed that the isentrope is close enough to the Hugoniot that the same assumption can be made.
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| If the Hugoniot is approximately the loading path between states for an "equivalent" compression wave, then the jump conditions for the shock loading path can be determined by drawing a straight line between the initial and final states. This line is called the '''Rayleigh line''' and has the following equation:
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| :<math>
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| (25) \qquad p_2 - p_1 = u_s^2\left(\rho_1 - \frac{\rho_1^2}{\rho_2}\right)\,
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| </math>
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| === Hugoniot elastic limit ===
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| Most solid materials undergo [[plasticity (physics)|plastic]] deformations when subjected to strong shocks. The point on the shock Hugoniot at which a material transitions from a purely [[elasticity (physics)|elastic]] state to an elastic-plastic state is called the '''Hugoniot elastic limit''' (HEL) and the pressure at which this transition takes place is denoted ''p''<sub>HEL</sub>. Values of ''p''<sub>HEL</sub> can range from 0.2 GPa to 20 GPa. Above the HEL, the material loses much of its shear strength and starts behaving like a fluid.
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| ==See also==
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| * Calculate normal shock wave parameters for mixtures of imperfect gases. [http://web.ics.purdue.edu/~alexeenk/GDT/index.html Gas Dynamics Toolbox]
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| * [[Shock polar]]
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| * [[Mie–Gruneisen equation of state]]
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| * [[b:Engineering Acoustics|Engineering Acoustics Wikibook]]
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| ==References==
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| {{Reflist}}
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| {{Use dmy dates|date=September 2010}}
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| {{DEFAULTSORT:Rankine-Hugoniot Conditions}}
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| [[Category:Equations of fluid dynamics]]
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| [[Category:Scottish inventions]]
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| [[Category:Continuum mechanics]]
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