Externally illuminated single-sided wake pattern passing a hot-wire probe.

Typical instantaneous (phase-averaged) velocity vector field for the above smoke pattern.

Flow visualization of close wake behind a circular cylinder.

Combined with this experimental work, analytical and numerical tools have been developed so as to gain an insight into the basic motions of vortical and turbulent flow. The study of the topology and geometry of three-dimensional unsteady flow patterns will help formulate more realistic models of turbulence for the purposes of engineering calculations for a large range of practical problems. Producing the complete solutions of the governing equations of motion of a fluid is not possible for practical ranges of flow Reynolds numbers even with the world's largest computers. Hence there is a need for modelling so as to enable much simpler but approximate equations to be used. However, it has recently been possible to compute with the governing equations at low to moderate Reynolds numbers. This is known as Direct Numerical Simulation (DNS) and although the Reynolds numbers are still fairly low, "turbulence" has been successfully simulated to a sufficient extent to give trends and possible insights for the higher more practical Reynolds number ranges.

Isosurfaces of the discriminant of the velocity gradient tensor used to visualise focal structures in computation of homogeneous turbulence. The yellow surfaces represent flow regions with stable focus/stretching topology while the blue outlines of the isosurfaces show regions with unstable focus/contracting topology. 128^3 simulation with Taylor Reynolds number = 70.9.

Andrew Ooi simulated a vortex breakdown in a three-dimensional swrling jet using a code developed by Wolfgang Kollmann (MAE Dept., University of California, Davis).
Movie of swirling jet simulation (26M gif animation)

Other interests in the fluid mechanics group include the dynamics of vorticity, particularly the roll up of shear layers. The roll up of a shear layer at the face of a piston moving along a cylindrical wall is shown below from a flow visualisation experiment (from the work of Dr. J.J. Allen and Prof. M.S. Chong).

Left: Instantaneous streamline pattern induced by a piston in a cylinder.

Middle: Dye trace of vortex roll-up in front of circular piston.

Right:Instantaneous streamline pattern computed using point vortex model.

Streamline and vorticity field using particle tracking technique.

There are many projects being carried out by Emeritus Professor P.N.Joubert using unique facilities such as a rotating wind tunnel. The effects of extra strain rates on boundary layers is being performed in a specially designed tunnel and the study of particle transport properties of a vortex (tornado) attached to a solid boundary is in progress in a special vortex tank.

The following pictures are from the work of Dr. Trent Matter who investigated vortical flow past a sphere. The first part of this work consisted of the exploration of the behaviour of a concentrated vortex flow through a constant diameter pipe. The following pictures show the transient structure generated as the amount of swirl is increased. Trent also carried out and experimental and numerical investigaton of concentrated vortex flow past a sphere in a pipe.

The above pictures reveal a complicated transition process as the swirl intensity is increased. Increasing swirl resulted in a spiral vortex breakdown.

 

The above pictures show how the effect of swirl on the flow around a sphere. The flow is from top to bottom. The picture on the left is for zero swirl and the flow is attached on the upstream hemisphere and separated downstream of the point of maximum thickness. A separation bubble is formed on the downstream hemisphere and this is a region of very slow reversed axial flow. When the swirl is increased a separation bubble appears on the upstream hemisphere which is shown in the middle picture. As the swirl is further increased this upstream separation bubble grows in length and becomes increasingly unsteady and finally developed into the unsteady spiral structure as shown in the picture on the right.

K.A.M. Moinuddin, P.N. Joubert & M.S. Chong (2004) "Investigation of turbulence driven secondary motion on a streamwise external corner. Part 1. Experimental results." Accepted for publication by J. Fluid Mech.

T.W. Mattner, P.N. Joubert & M.S. Chong (2003) "Vortical flow. Part 2: Flow past a sphere in a constant diameter pipe." J. Fluid Mech., Vol. 481, 1-36.

M.B. Jones, N. Nishiwara, M.S. Chong & I. Marusic (2003) "Scaling of the turbulent boundary layer at high Reynolds numbers" In Proceeding of the IUTAM Symposium on Reynolds Number Scaling in Turbulent Flow (Ed. A.J. Smits), Kluwer Academic Publishers.

K.A.M. Moinuddin, P.N. Joubert, M.S. Chong & S. Hafez (2003) "Experimental investigation of turbulent boundary layer developing along a streamwise external corner (chine)." Experimental Thermal and Fluid Science, Vol. 27, Issue 5, 599-609

T.W. Mattner, P.N. Joubert & M.S. Chong (2002) "Vortical flow. Part 1: Flow in a constant diameter pipe." J. Fluid Mech., Vol. 463, 259-291.

K. Higgins, A. Ooi A. & M.S. Chong (2002) The structure of an unstable circular vortex in background straining flows. J. Fluid Mech., Vol. 462, 31-42.

A.E. Perry, I. Marusic & M.B. Jones (2002) "On the streamwise evolution of turbulent boundary layers in arbitrary pressure gradients." J. Fluid Mech., Vol. 461, pp.61-91.

M.B. Jones, I. Marusic & A.E. Perry (2001) "Evolution and structure of sink-flow turbulent boundary layers." J. Fluid Mech., Vol. 428, pp.1-27.

A.E. Perry & M.S. Chong (2000) Chapter 1: Interpretation of flow visualisation. in "Flow Visualisation: Techniques and Examples" (Eds. A. Smits and T.T. Lim), Imperial College Press.

J.J. Allen & M.S. Chong (2000) "Vortex formation in front of a cylinder moving through a cylinder." J. Fluid Mech., Vol. 416, pp.1-28.

T.B. Nickels & P.N. Joubert (1991) "The mean velocity profile of turbulent boundary layers with system rotation." J. Fluid Mech. Vol. 408, pp.323-345.

A. Ooi, M.S. Chong, J. Martin & J. Soria (1999) "A study of the evolution and characteristics of the invariants of the velocity-gradient tensor in isotropic turbulence." J. Fluid Mech., Vol. 381, pp.141-174.

M.S. Chong, J. Soria, A.E. Perry, J. Chacin, B.J. Cantwell & Y. Na (1998) "Turbulence structures of wall-bounded shear flows found using DNS data." J. Fluid Mech., Vol. 357, pp.225-247.

N.R. Panchapakesan, T.B. Nickels & P.N. Joubert (1991) "Lateral straining of turbulent boundary layers. Part 2. Streamline convergence." J. Fluid Mech. Vol. 349, pp.1-30.

A.E. Perry & I. Marusic (1995) "A wall-wake model for the turbulent structure of boundary layers. part 1. Extension of the attached eddy hypothesis." J. Fluid Mech., Vol. 298, pp.361-388.

S.G. Saddough & P.N. Joubert (1991) "Lateral straining of turbulent boundary layers. Part 1. Streamline divergence." J. Fluid Mech. Vol. 229, pp.173-204.

M.S. Chong, A.E. Perry & B.J. Cantwell (1990) "A general classification of three dimensional flow fields." Phys. Fluids A 2(5), pp.765-777

A.E. Perry & M.S. Chong (1987) "A description of eddying motions and flow patterns using critical-point concepts." Annual Review of Fluid Mechanics, Vol. 19, pp.125-155.

A.E. Perry & M.S. Chong (1986) "A series expansion study of the Navier-Stokes equations with applications to three-dimensional separation patterns." J. Fluid Mech., Vol.173, pp.207-223.

A.E. Perry, S. Henbest & M.S. Chong (1986) "A theoretical and experimental study of wall turbulence." J. Fluid Mech., Vol.165, pp.163-199.

A.E. Perry & M.S. Chong (1982) "On the mechanism of wall turbulence." J. Fluid Mech., Vol. 119, pp.173-217.

A.E. Perry, M.S. Chong & T.T. Lim (1982) "The vortex shedding process behind two-dimensional bluff bodies." J. Fluid Mech., Vol. 116, pp.77-90.

A.E. Perry, T.T. Lim & M.S. Chong (1980) "The instantaneous velocity fields of coherent structures in coflowing jets and wakes." J. Fluid Mech., Vol. 101, pp.33-51.

I.A. Hunt & P.N. Joubert (1979) "Effects of small streamline curvature on turbulent duct flow." J. Fluid Mech., Vol. 91, pp.633-659.

A.E. Perry & C.J. Abell (1975) "Scaling laws for pipe-flow turbulence." J. Fluid Mech., Vol. 67, pp.257-271.

A.E. Perry, W.H. Schofield & P.N. Joubert (1969) "Rough wall turbulent boundary layers." J. Fluid Mech., Vol. 37, pp.383-413.