Streak development and breakdown during bypass transition

The bypass transition process in a flat plate boundary layer exposed to strong free-stream disturbances plays an important role in many practical applications, for example gas turbines. This thesis presents the results of an experimental investigation of the streak development and breakdown process during bypass transition. The experimental set-up consists of a water channel in which a flat plate is positioned in the measuring section. The developing flat plate boundary layer is exposed to free-stream turbulence generated by a static turbulence grid. A natural breakdown process is present in the boundary layer, that means, no additional triggering is used to initiate an instability. This natural breakdown process is analyzed using a LIF visualization technique, Particle Image Velocimetry (PIV), a combined PIV-LIF technique and a combined stereoscopic PIV-LIF technique. Furthermore, all techniques are applied in combination with a camera traversing system, by which the cameras are translated in downstream direction with the main stream velocity. The bypass transition mechanism is characterized by the presence of long elongated structures in the boundary layer. These streaks form a spanwise alternating pattern of low and high streamwise velocity. They appear already in the first stages of the boundary layer and are highly stable. The average amplitude of the streaks increases linearly until it reaches its maximum value of A2 sttr (x)/U28 ˜ 0.14. The amplitude slightly decreases after it has reached its maximum value. The streaks obtain a constant wavelength of around 5.6d* 300. A study on the specific amplitude of streak pairs reveals that there is no relation between the amplitude value of a steak pair and the event that a streak becomes unstable, i.e. the amplitude of unstable streak pairs can be lower than the amplitude of stable streak pairs. Hence, the amplitude of a streak pair does not plays a decisive role in the breakdown process, which is in contrast to what was expected. The breakdown appears to be initiated by spanwise oscillations with a short wavelength. A detailed investigation of the oscillation indicates the presence of three types of secondary instabilities: a sinuous, varicose and a previously unclassified one, denoted as single branch instability. A distinction between these different types of instabilities is made on the basis of the wave-shape, the number of periods and the wavelength of the spanwise oscillation. The sinuous instability appears as an anti-symmetric spanwise oscillation in the boundary layer. The oscillation consists of around 3-6 periods and its wave-shape amplitude and wavelength are respectively 0.2 and 28d* 300. The instability arises in a streak configuration of alternating low and high speed streaks. In the configuration two low speed streaks are present at a small spanwise distance from each other. Characteristic for the sinuous streak configuration is the presence of perturbations in the formof short (in streamwise direction) patches of low speed fluid in the high speed streaks. In the vicinity of these discontinuities inclined (with respect to wall) tubes of streamwise vorticity arise in the high shear zone between the unstable low speed streak and adjoining high speed streaks. Since the tubes are inclined they manifest themselves as wall-normal vorticity in the horizontal plane. In the measurements two counter rotating tubes are present at one side of the unstable low speed streak. The most downstream tube is inclined towards the plate, while the upstream tube is inclined away from the plate. The measurement results indicate that the most downstream tube is located above the upstream tube in the region where both tubes overlap and so form a local dipole-like structure. Consequently, a spanwise motion is induced in the overlap region. Influenced by the mean shear the upstream tube is stretched while the downstream tube slides into itself, like a telescope. This gives a staggered pattern of long and short tubes around the unstable streak and through that the streak starts to oscillate in an anti-symmetric way. When the inclination angle of the downstream tube becomes too large it is ’pushed’ over after which the flow falls apart into smaller three-dimensional flow regions. The varicose instability expresses itself as a symmetric oscillation with a wavelength of 19d* 300 and an amplitude of 0.6. The oscillation possesses around 3 - 6 periods. The instability appears in a streak configuration in which a high speed streak frontally collides with a downstream low speed streak. At the collision point high speed fluid curls down sideways (symmetrically) into high speed streaks adjoining the low speed streak, generating two inclined (with respect to the wall as well as the x-axis) tubes of streamwise vorticity. The strong streak-streak interaction at the collision interface results downstream into two new tubes. These tubes form a ??-like structure. Due to the streak-streak interactions and the ??-like structure the unstable low speed streak narrows and with the streamwise evolution a patch of low speed fluid is separated from the initial low speed streak. This process repeats itself and consequently a ’train’ of ??-like structures (and thus patches) appears. Finally, the low speed streak starts to fall apart into smaller three-dimensional flow regions. The single branch instability appears in the boundary layer as an anti-symmetric oscillation. The wavelength of the oscillation equals 12d* 300 and the amplitude equals 0.35. The single branch instability arises in a streak configuration in which, likewise as in the varicose mode, a collision between a high speed streak and a downstream low speed streak occurs. Yet, in the single branch instability the center lines of both streaks are shifted with respect to each other in such a way that half of the high speed streak collides with half of the low speed streak. At the collision point high speed fluid rolls sideways over the collision interface into the low speed streak, by which an inclined tube of streamwise vorticity is generated. Due to this spiralmotion a local branch of low speed fluid arises. Themeasurement results indicate that under the influence of the streak-streak interactions at the branch new tubes appear in a staggered pattern around the branch which initiate an anti-symmetric oscillation of the branch. With the downstream evolution the branch is separated from the low speed streak after which the branch falls apart into smaller three-dimensional flow regions. The varicose as well as the single branch instability seem to arise due to a collision between a high speed streak and a downstream low speed streak. By direct numerical calculations de Lange and Brandt [11] analyzed the development of the frontal and ’half’ collision. The occuring flow structures in the numerical results and their evolution strongly correspond with the experimental structures and their development. This establishes that both instabilities result from the collision between streaks and the corresponding streakstreak interactions. The mechanism which generates streamwise vorticity in the sinuous instability remains unclear. Still, the results demonstrate that the vorticity is generated in the vicinity of the discontinuities and itmay be that the streak-streak interactions appearing at a discontinuity results in the spanwise perturbations which will generate streamwise vorticity via the streak transient growth mechanism of Schoppa and Hussain [50].