Nils Tillmark

Experiments on transition in plane Couette flow

The first flow visualization experimental results of transition in plane Couette flow are reported. The Couette flow water channel was of an infinite-belt type with counter-moving walls. The belt and channel walls were transparent making it possible to visualize the flow pattern in the streamwise-spanwise plane by utilizing fluid-suspended reflective flakes. Transition was triggered by a high-amplitude pointwise disturbance. The transitional Reynolds number, i.e. the lowest Reynolds number for which turbulence can be sustained, was determined to be 360 ± 10, based on half-channel height and half the velocity difference between the walls. For Reynolds numbers above this value a large enough amplitude of the initial disturbance gave rise to a growing turbulent spot. Its shape and spreading rate was determined for Reynolds numbers up to 1000.

An investigation of turbulent plane Couette flow at low Reynolds numbers

The turbulent structure in plane Couette flow at low Reynolds numbers is studied using data obtained both from numerical simulation and physical experiments. It is shown that the near-wall turbulence structure is quite similar to what has earlier been found in plane Poiseuille flow; however, there are also some large differences especially regarding Reynolds stress production. The commonly held view that the maximum in Reynolds stress close to the wall in Poiseuille and boundary layer flows is due to the turbulence-generating events must be modified as plane Couette flow does not exhibit such a maximum, although the near-wall coherent structures are quite similar. For two-dimensional mean flow, turbulence production occurs only for the streamwise fluctuations, and the present study shows the importance of the pressure—strain redistribution in connection with the near-wall coherent events.

Flow regimes in a plane Couette flow with system rotation

Flow states in plane Couette flow in a spanwise rotating frame of reference have been mapped experimentally in the parameter space spanned by the Reynolds number and rotation rate. Depending on the ...

A study of swirling turbulent pipe and jet flows

Axially rotating turbulent pipe flow is an example in which the rotation strongly affects the turbulence, which then also influences the mean flow properties. For instance, in the fully developed flow as well, the fluid is not in solid body rotation due to the influence of the cross-stream Reynolds stress. The present paper reports new measurements from a rotating pipe flow and the streamwise mean velocity distribution is compared with recent scaling ideas of Oberlack [J. Fluid Mech. 379, 1 (1999)] and good agreement is found. A second part of the paper deals with the initial stages when the flow leaves the pipe and forms a swirling jet. The measurements in the jet show that at some distance downstream (approximately five jet diameters) the central part of the jet actually rotates in the opposite direction as compared to the rotation of the pipe. This effect is explained by the influence of the cross-stream Reynolds shear stress.

Experimental determination of self-similarity constant for converging cylindrical shocks

Guderley’s self-similarity solution r = r0(1 – t/t0)α for strong converging cylindrical shocks is investigated experimentally for three different gases with adiabatic exponents γ = 1.13; 1.40; and 1.66 and various values of the initial Mach number. Corresponding values of the similarity exponent α which determines the strength of shock convergence are obtained for each gas thus giving the variation of α with γ. Schlieren imaging with multiple exposure technique is used to track the propagation of a single shock front during convergence. The present experimental results are compared with previous experimental, numerical, and theoretical investigations.

Thermal radiation from a converging shock implosion

High energy concentration in gas is produced experimentally by focusing cylindrical shock waves in a specially constructed shock tube. The energy concentration is manifested by the formation of a hot gas core emitting light at the center of a test chamber at the instant of shock focus. Experimental and numerical investigations show that the shape of the shock wave close to the center of convergence has a large influence on the energy concentration level. Circular shocks are unstable and the resulting light emission varies greatly from run to run. Symmetry and stability of the converging shock are achieved by wing-shaped flow dividers mounted radially in the test chamber, forming the shock into a more stable polygonal shape. Photometric and spectroscopic analysis of the implosion light flash from a polygonal shock wave in argon is performed. A series of 60 ns time-resolved spectra spread over the 8 μs light flash shows the emission variation over the flash duration. Blackbody fits of the spectroscopic data...

Experiments On Rotating Plane Couette Flow

System rotation may drastically change the flow behavior both for laminar and turbulent shear flows clue to the effect of the Coriolis force. For plane Poiseuille flow the Coriolis force acts in such a way that in the part of the channel where the system rotation has opposite sign compared to the mean fiow vorticity the flow becomes destabilized whereas in the other part of the channel it becomes stabilized. Alfredsson & Persson (1989) showed that the instability takes the form of streamwise oriented roll cells and demonstrated excellent agreement between linear theory and experiments.

Energy concentration by spherical converging shocks generated in a shock tube

Spherical converging shock waves are produced in a conventional shock tube with a circular cross-section. Initially, plane shocks are transformed into the shape of a spherical cap by means of a smoothly convergent cross-section. The wall shape in the transformation section is designed to gradually change the form of the shock wave until it approaches a spherical shape. Thereafter, the shock enters a conical section where it converges towards the apex of the cone. Numerical calculations with the axisymmetric Euler equations show that the spherical form is only slightly dependent on the initial Mach number of the plane shock within the range 1.5 < MS < 5.5, and is preserved to a close vicinity of the focal point. The test gas is heated to very high temperatures as a result of shock convergence and emits a bright light pulse at the tip of the test section. The light radiation is collected by optical fibers mounted at the tip of the convergence chamber and investigated by photometric and spectroscopic measure...

ON THE SPREADING MECHANISMS OF A TURBULENT SPOT IN PLANE COUETTE FLOW

The spanwise growth of a turbulent spot in plane Couette flow is experimentally analysed by determining the flow field in the region between the spot and the undisturbed surrounding flow. Both the streamwise and spanwise velocity components were measured, and show that the spot forces fluid outward, giving rise to a spanwise velocity component. The resulting velocity profile has an inflectional character and the existence of spanwise travelling waves was verified. The wave velocity was determined and found to be in good agreement with numerical simulations as well as flow visualisation results, earlier reported in the literature.

Experimental observations of instabilities in rotating plane Couette flow

The transition from the two-dimensional (2D) longitudinal roll cell state to 3D flows in the rotating plane Couette system, predicted by the theoretical investigation [M. Nagata, J. Fluid Mech. 358, 357 (1998)], is examined experimentally. The streamwise and spanwise wave numbers of observed steady 3D flows seem to agree with those predicted by the theory when the rotation rate is relatively large. However, we observe unsteady 3D states in the region where the theory predicts stable steady 3D flows when the rotation rate is small.