On converging shock waves of spherical and polyhedral form

376 D. W. Schwendeman
characteristic form. The third equation is already in characteristic form, while a linear
combination of the first and second equations in (3.7) gives
mF u (u) du
dx + G v(u) dv = 0, (3.10)
dx
which holds on characteristics with slope dy/dx = m, where m is a root of the
quadratic
m 2 F u (u) − m(F v (u) + G u (u)) + G v (u) = 0. (3.11)
We now eliminate m from equations (3.10) and (3.11), and use the definitions of F
and G in (3.8) to obtain the differential form
(u 2 + v 2 + w 2 )(du 2 + dv 2 ) = (1 + β 2 )(udu + vdv) 2 , β 2 = − MA′
A , (3.12)
which holds on the two families of characteristics. Again, it is convenient to use polar
coordinates, u = r cos ϑ, v = r sin ϑ, so that the differential form in (3.12) becomes
dϑ
dr = ± [ β 2 r 2 − w 2
r 2 (r 2 + w 2 )
] 1/2
. (3.13)
For the strong shock case, β 2 = n and (3.13) may be integrated (keeping in mind that
w is constant) to give
ϑ 2 − ϑ 1 = ±[P (r 2 , w) − P (r 1 , w)], (3.14)
for a shock-expansion transition from a state (r 1 , ϑ 1 , w) to (r 2 , ϑ 2 , w). The function
P (r, w) in (3.14) plays the role of a Prandtl–Meyer function, and is given by
P (r, w) =
√ ( n
2 ln r 2 + n − 1
2n w2 + √ pq )
− 1 ( p 2
n 2 arctan − q 2
2pq
where p 2 = nr 2 − w 2 and q 2 = r 2 + w 2 . The choice of sign in (3.14) reflects the two
families of shock-expansion solutions and can be made following the condition that
ϑ increases with increasing y for both families.
We now have the necessary formulas available to complete the solution of the
Riemann problem. The solution consists of constant states in u separated by shockshocks
or shock-expansions given by (3.9) and (3.14), respectively, with w = w l or w r ,
and a contact discontinuity at y = 0 across which w jumps from w l to w r and G and
u are continuous. A numerical method of iteration is used to determine the solution
and in particular the values of G and u. The iteration begins with values for each
constant state given by a linearization of the equations. Newton’s method is used to
update the values for u iteratively until the values for G and u on y = 0 converge
to within a specified tolerance (taken to be 10 −6 ). These values become ˜G k and ũ k in
(3.2) and (3.3), respectively.
The numerical method described in §§ 3.1 and 3.2, essentially a Godunov method,
is first-order accurate if the left and right states for the Riemann problem at each
face are taken from the discrete data at neighbouring grid cells alone. Higher-order
methods (for smooth regions of the solution) could be obtained by making corrections
to the left and right states based on discrete approximations for the gradient of u
(see Schwendeman 1999, for example). This is not done here because of the added
numerical cost, mainly, and because for many of the problems computed the solutions
contain regions of uniform (or nearly uniform) u separated by shock-shocks. We also
note that approximate Riemann solvers could be used but since the problems of
interest involve collisions of shock-shocks an exact Riemann solver is preferred.
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