# Trkalian flow fields Pavel Jonáš Institute of Thermomechanics as cr, V v. I., Praha

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Colloquium FLUID DYNAMICS 2008

Institute of Thermomechanics AS CR, v.v.i., Prague, October 22 - 24, 2008

p.

Trkalian flow fields

Pavel Jonáš

Institute of Thermomechanics AS CR, v.v.i., Praha

The contribution of Professor Viktor Trkal to the fluid mechanics appearing from his paper from 1919 is the theme of this contribution. Trkal solution is briefly reproduced together with the outline how the main result became known as Trkal flow, Trkalian flow field etc. Several examples of the application of the Trkalian flows (sometimes known as ABC flows), extended up to present time follow in the full text of this contribution.

1. Introduction

The story of the Viktor Trkal Note on the hydrodynamics of viscous fluids [1] seems as very interesting in the epoch producing hundreds papers mostly after a short time forgotten. Professor Viktor Trkal, Czech physicist (1888-1956) published the paper [1] in Czech not long after the homecoming from Russia where he spent the wartime from 1915 till 1919, largely in the prisoner of war status. Trkal solution of the Navier-Stokes equations is describing the flow field belonging to an important domain of the fluid mechanics, to the fluid motions with nonzero vorticity. Such flows are embedding us in nature, exist in our bodies and frequently occur in technology. Later it has been recognised that Trkal solution of the Navier-Stokes equations forms an important particular class of Beltrami flows [6] however there is a little probability that Trkal was familiar with Beltrami study because Beltrami published his paper in Italian, international communications were difficult at that time and particularly during the wartime.

2. Beltrami flows

It is convenient introduce Beltrami flows before the interpretation of Trkal results. The equations of Navier and Stokes are usually written in the form



where is the velocity vector, are the operators of Hamilton and Laplace, is the resultant of external forces, P denotes the pressure and are the density and the kinematic viscosity. A bracket indicates the scalar product and similarly is the vector product of vectors . For the sake of simplification of derivations we suppose (if not explicitly cancelled) the constant property Newtonian incompressible - divergence free fluid and the conservative body forces with the potential



Usually, the second term on the left hand side of the equation  describing the convective derivation is useful to decompose and simultaneously the modified pressure p can be introduced as follows



Then the equation  simplifies to



This formulation of N-S equations emphasizes the significance of vorticity and the non-linearity of equations. Exact solutions of the N-S equations are rare but they are important because they solve significant problems and simultaneously they serve for testing of some mathematical or physical models. Regardless of the nonlinearity of , there exist exact solutions that are linearly superposable i.e. each of the distinct velocity vector fields and meet the solution of  as well as their sum if certain conditions would be accomplished. They must be valid following vector equations simultaneously







According to the vector analysis are valid following formulas

Hence equations ,  and  are met simultaneously if the relation is fulfilled



Thence two different solutions and are linearly superposable if they fulfil the relation



where is a scalar function of space-time coordinates. Similarly it follows that a self-superposable velocity field, i.e. , called the generalized Beltrami flow [6] must fulfil equation



The simplest self-superposable velocity fields satisfied the equation



Beltrami studied various flows of this kind and thence they are called Beltrami flows.

3. Trkal solution

Trkal [1] analysed consequences of different properties of the vector product on the flow field behaviour. The original paper [1] was written in Czech. Lakthakia [16] aroused English translation of the paper in 1994. So all details of Trkal solution are not necessary reproduce here. Trkal started from the equations  in component representation but assuming the compressibility of fluid (2nd viscosity) and the potential of body forces . Applying the operator rot (curl) to the N-S equations  we receive



Using relations from the vector analysis

we rearrange the previous equation to the vorticity equation in a common form



This equation relates to the equation (4) presented in [1]. The vector on the right hand side of the equation is perpendicular to the plane determined by vectors of velocity and vorticity. It plays a crucial role in fluid dynamics (vortex stretching, Coriolis force, Magnus effect etc.). Trkal [1] analysed properties of a flow field with vanishing vector . Then the components of the vorticity vector are subject to equations that are analogous to the heat convection equation (diffusion of vorticity)



Further the possibilities of the pressure field determination are discussed if the vector vanishes identically



Initially Trkal [1] assumes the scalar equals zero



then also the vorticity vanishes and the velocity potential exists. Inserting into the equation  and then performing the integration of the equation we receive similarly to Trkal [1] the formulae for the potential of body forces in compressible fluid flow



where T(t) is an arbitrary function of time.

Another possibility how achieve the zero value of the vector is the configuration with vortex lines coinciding with stream lines



This is the most important of the investigated events - a helical motion of fluid particles with a scalar function . Trkal has shown that then the equation  holds again and suggested the procedure of the pressure field or the potential of body forces calculation. However he stated that the necessary integrations could be performed only in some very special cases. From the relations  follows with the regard to vector analysis

This is possible to arrange in the form



Next the solution continues similarly to Trkal. Having in mind from the vector analysis



and that the difference of the substantial derivative and the local one equals the convective derivative, we find that in an incompressible fluid flow holds the relation



whence it is evident that the scalar c can be a function of time t only, if all components of the velocity vector are nonzero. Furthermore Trkal found the solution of the velocity vector field



This solution must meet the relation . Thus must hold



whence Trkal [1] derived the main result: “in a viscous incompressible fluid, unless at least one of the velocity components is identically equal to zero, c must be a constant



Then the velocity vector follows from  and  in the form



Next the boundary conditions and the limits of the validity are expressed and formulas describing the fields of the pressure and flow velocity are derived in the paper.

It is interesting that Ballabh [3] reached the result  twenty years after Viktor Trkal no doubt unaware on the Trkal paper [1] similarly as Trkal arrived at his solution no doubt unaware on Beltrami [6].

4. Story on Trkal flow

The flow defined by the formulas  and  became known as Trkal flow or Trkalian flow most likely since fifties of the 20th century owing to the Norwegian physicist Björgum [4]. However, the diffusion of Trkal result begun already in thirties of the 20th century when Dryden at al. [13 and 14] quoted the title of the paper [1] in the list of references attached to the chapter 4. But the author did not found a reference to it in the 4th chapter.

Björgum [4 and 5] pleads Berker [3] thesis from 1936 with cited Trkal [1]. According to the bibliography from Brdička and Trkal, Jr. [12], the name Trkal and titles Trkalian flow or field occur frequently in the above-mentioned publications and also in the monograph of Truesdell [22]. In the connection with Trkal results, the publications from the turn of 20th and 21st century refer mostly the Aris [2] monograph and the relevant chapters of the Berker [8] Handbuch der Physik and Flugge [15] Encyclopedie of physics. Aris does not quote Trkal directly but he refers to the Truesdell book. However Aris is repeatedly using the term Trkalian field for the vector fields where the curl of the vector is parallel to the vector and simultaneously directly proportional with the scalar of proportion independent of position.

The Lakhtakia paper [16] is the important milestone in the frequency of the Trkal citations in the world literature. Lakhtakia noticed the citation of Trkal from 1919 when reviewing the manuscript of Reed [18]. He recognized, from the Czech version, the significance of Trkal contribution in fluid mechanics, time-harmonic electromagnetism and astrophysics. Lakthakia [16] provoked to translate Trkal [1] in English and briefly contextualizes the vorticity dynamics, Beltrami fields and Trkal flows in his paper. Mechanical models utilising theory of vortex motions and ideas on vorticity are applied since the age of Helmholz, Kelvin, Maxwell etc. Similarly are still in use Beltrami fields. Further several examples of application of the class of the Beltrami flows that is known as Trkal flows (sometimes called ABC flows) are briefly introduced. The examples are meant as starting points for those who desire a deeper insight in the problem.

Tasso [20] has shown that the force free fields can be generalized to Trkal flows in magnetohydrodynamics if a special velocity field parallel to the magnetic field is introduced. He established an exponentiall decay of such flows if the viscosity and resistivity are constant. Next he derived a sufficient condition for nonlinear stability. Another example is the paper of Tasso and Throumoulopoulos [21] from 2003, dealing with cross-helicity and magnetized Trkal flows.

Another example is an extensive article from Mackay and Lakhtakia [17] on electromagnetics of moving at constant velocity of an isotropic chiral medium from the perspective of a co-moving observer published in 2006.

The contribution of Beltrami flows to theoretical study of turbulent flows and the Trkal contribution to exact solutions of N-S equations are appreciated by Changchun and Yongnian [19]. These authors commend the physical consequences of Trkal solution that found in Flugge [15].

Baldwin and Townsend [3] published study on the complex Trkalian fields and solutions to Euler´s equations in 1995. They distinguish between Trkalian fields, real vector fields that solve the equation  and the complex version of the Trkalian equation

where is a three component vector function with each component in the complex field which may be expressed in the form

with g real and F complex. Baldwin and Townsend point out the investigations of Trkalian (ABC-) flows in the context with problems of stability and chaos.

Many other examples of the utilisation and referring to the Viktor Trkal Sr. are available from last two decades.
5. Conclusion

The clever analysis of the Navier-Stokes equations brought Professor Viktor Trkal to the solution that became known after several decades. Next it was also modified and generalized to electromagnetism, magneto hydrodynamics, superconductivity and solar physics. Trkal flow, Trkal field are a name in the fluid mechanics.

## References

1. Trkal, V. (1919) Poznámka k hydrodynamice vazkých tekutin. Časopis pro pěstování matematiky a fysiky 48, 302-311.English translation: Trkal, V. (1994) A note on the hydrodynamics of viscous fluids (translated by I. Gregora) Czech. J. Phys. 44, 97-106.

2. Aris, R. (1962) Vectors, tensors and the basic eqs. of fluid mechanics. Prentice – Hall. Inc. 73.

3. Baldwin, P.R. and Townsend, G.M. (1995) Complex Trkalian fields and solutions to Euler’s equations for the ideal fluid. Archiv: Chao/Dyn/9502012v1, 1-29, Phys. Rev. E, 51 (3), 2059-2068.
4. Ballabh, R. (1940) Self superposale motions of the type  = .u etc.Proc. Benares Math. Soc. (N.S.) 2, 85-89.

5. Ballabh, R. (1950) On coincidence of vortex and stress lines in ideal liquids. Ganita 1, 1-4.

6. Beltrami, E. (1889) Considerazioni idrodinamiche. Rend. Inst. Lombardo Acad. Sci. Lett. 22, 122-131, english translation by G. Filipponi: Int. J. Fusion Energy 3 (1985) no. 3, 53-57.

7. Berker, A.R. (1936) PhD.-Thesis, Lille.

8. Berker, A.R. (1963) Handbuch der Physik. Springer – Verlag, Berlin, Vol. VIII/2, p. 161.

9. Bjǿrgum, O. (1951) On Beltrami vector fields and flows. Pt. I. A comparative study of some basic types of vector fields. Universitetet i Bergen Årbok., Naturvitenskapeling rekke Nr. 1.

10. Bjǿrgum, O. and Godal, T. (1952) On Beltrami vector fields and flows. Pt. II. The case when W is constant in space. Universitetet i Bergen Årbok., Naturvitenskapeling rekke Nr. 13.

11. Brdička, M. (1959) Mechanika kontinua. Nakladatelství ‚ČSAV, Praha.

12. Brdička, M. a Trkal, V. (2007) Profesor Viktor Trkal – Pouť moderní fyzikou. Academia, Praha.

13. Dryden, H.L., Murnaghan, E.D. and Bateman, H. (1932) Report of the Committee on Hydrodynamics. Bulletin of the National Research Council no. 84, 1-634.
14. Dryden, H.L., Murnaghan, F.D. and Bateman, H. (1956) Hydrodynamics. Dover Publications, INC. New York, N.Y., Unabridged and unaltered republication of [2].

15. Flugge, S. (1963) Encyclopedia of physics. Vol. VIII/2, Fluid dynamics II. New York, Springer-Verlag, p. 161.

16. Lakhtakia, A. (1994) Viktor Trkal, Beltrami fields and Trkalian flows. Czech. J. Phys. 44, 89-96.

17. Mackay, T.G. and Lakhtakia, A. (2006) On elektromagnetics of an isotropic chiral medium moving at constant velocity. Archiv: Physics/0606158v2, 1-22.

18. Reed, D. (1993) Archetypal vortex topology in nature. Speculat. Sci. Technol., accepted for publication.

19. Shi Changchun and Huang Yongnian (1991) Some properties of three-dimensional Beltrami flows. Acta Mechanika Sinica, Vol. 7, No. 4 (ISSS 0567-7718), 289-294.

20. Tasso, H. (1995) Trkal flows in magnetohydrodynamics. Phys. Plasmas 2 (5), 1789-1790.

21. Tasso, H. and Throumoulopoulos, G.N. (2003) Cross-helicity and magnetized Trkal flows. Physics of Plasmas, Vol. 10, No. 12, 4897-4898.

22. Truesdell, C. (1954) Kinetics of vorticity. Bloomington.