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In fluid dynamics, Kerr–Dold vortex is an exact solution of Navier–Stokes equations, which represents steady periodic vortices superposed on the stagnation point flow (or extensional flow). The solution was discovered by Oliver S. Kerr and John W. Dold in 1994[1][2]. These steady solutions exist as a result of a balance between vortex stretching by the extensional flow and viscous dissipation, which are similar to Burgers vortex. These vortices were observed experimentally in a four-roll mill apparatus by Lagnado and L. Gary Leal[3].
Mathematical description

The stagnation point flow, which is already an exact solution of the Navier–Stokes equation is given by \( {\displaystyle \mathbf {U} =(0,-Ay,Az)} \), where A is the strain rate. To this flow, an additional periodic disturbance can be added such that the new velocity field can be written as

\( {\displaystyle \mathbf {u} ={\begin{bmatrix}0\\-Ay\\Az\end{bmatrix}}+{\begin{bmatrix}u(x,y)\\v(x,y)\\0\end{bmatrix}}} \)

where the disturbance \( {\displaystyle v(x,y)} v(x,y) \) are assumed to be periodic in the x {\displaystyle x} x direction with a fundamental wavenumber k . Kerr and Dold showed that such disturbances exist with finite amplitude, thus making the solution an exact to Navier–Stokes equations. Introducing a stream function \( \psi \) for the disturbance velocity components, the equations for disturbances in vorticity-streamfunction formulation can be shown to reduce to

\( {\displaystyle {\begin{aligned}\omega &=-\left({\frac {\partial ^{2}}{\partial x^{2}}}+{\frac {\partial ^{2}}{\partial y^{2}}}\right)\psi \\[6pt]{\frac {\partial \psi }{\partial y}}{\frac {\partial \omega }{\partial x}}-{\frac {\partial \psi }{\partial x}}{\frac {\partial \omega }{\partial y}}&-Ay{\frac {\partial \omega }{\partial y}}-A\omega =\nu \left({\frac {\partial ^{2}}{\partial x^{2}}}+{\frac {\partial ^{2}}{\partial y^{2}}}\right)\omega \end{aligned}}} \)

where ω {\displaystyle \omega } \omega is the disturbance vorticity. A single parameter

λ = A ν k 2 {\displaystyle \lambda ={\frac {A}{\nu k^{2}}}} {\displaystyle \lambda ={\frac {A}{\nu k^{2}}}}

can be obtained upon non-dimensionalization, which measures the strength of the converging flow to viscous dissipation. The solution will be assumed to be

\( {\displaystyle \psi =\sum _{k=-\infty }^{\infty }[a_{k}(y)+ib_{k}(y)]e^{-ikx}.} \)

It is easy to verify that \( {\displaystyle a_{k}=a_{-k},\,b_{k}=-b_{-k},\,a_{0}=b_{0}=b_{1}=0.} \) Upon substitution, an infinite sequence of differential equation will be obtained which are coupled non-linearly. To derive the following equations, Cauchy product rule will be used. The equations are[4][5]

\( {\displaystyle {\begin{aligned}&a_{k}''''+Aya_{k}'''+(A-2k^{2})a_{k}''-k^{2}Aya_{k}'-k^{2}Aa_{k}+k^{4}a_{k}\\[6pt]&{}+i\left[b_{k}''''+Ayb_{k}'''+(A-2k^{2})b_{k}''-k^{2}Ayb_{k}'-k^{2}Ab_{k}+k^{4}b_{k}\right]\\[6pt]={}&i\sum _{\ell =-\infty }^{\infty }\left\{\left(a_{k-\ell }'+ib_{k-\ell }'\right)\left[\ell a_{\ell }''-\ell ^{3}a_{\ell }+i(\ell b_{\ell }''-\ell ^{3}b_{\ell })\right]-(k-\ell )\left(a_{k-\ell }+ib_{k-\ell }\right)\left[a_{\ell }'''-\ell ^{2}a_{\ell }'+i(b_{\ell }'''-\ell ^{2}b_{\ell }')\right]\right\}.\end{aligned}}} \)

The boundary conditions

\( {\displaystyle a_{k}'(0)=b_{k}(0)=a_{k}(\infty )=b_{k}(\infty )=0} \)

and the corresponding symmetry condition is enough to solve the problem. It can be shown that non-trivial solution exist only when \( {\displaystyle \lambda >1.} \) On solving this equation numerically, it is verified that keeping first 7 to 8 terms suffice to produce accurate results[6]. The solution when \( \lambda =1 \) is \( {\displaystyle \psi =\cos x} \) was already discovered by Craik and Criminale in 1986[7].
References

Kerr, Oliver S., and J. W. Dold. "Periodic steady vortices in a stagnation-point flow." Journal of Fluid Mechanics 276 (1994): 307–325.
Drazin, P. G., & Riley, N. (2006). The Navier–Stokes equations: a classification of flows and exact solutions (No. 334). Cambridge University Press.
Lagnado, R. R., & Leal, L. I. (1990). Visualization of three-dimensional flow in a four-roll mill. Experiments in fluids, 9(1-2), 25-32.
Dold, J. W. (1997). Triple flames as agents for restructuring of diffusion flames. Advances in combustion science: In honor of Ya. B. Zel'dovich(A 97-24531 05-25), Reston, VA, American Institute of Aeronautics and Astronautics, Inc.(Progress in Astronautics and Aeronautics., 173, 61–72.
Kerr, O. S., & Dold, J. W. (1996). Flame propagation around stretched periodic vortices investigated using ray-tracing. Combustion science and technology, 118(1-3), 101–125.
Dold, J. W., Kerr, O. S., & Nikolova, I. P. (1995). Flame propagation through periodic vortices. Combustion and flame, 100(3), 359–366.
Craik, A. D. D., & Criminale, W. O. (1986). Evolution of wavelike disturbances in shear flows: a class of exact solutions of the Navier-Stokes equations. Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences, 406(1830), 13-26.

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