Masaharu Matsubara

Disturbance growth in boundary layers subjected to free-stream turbulence

This paper aims at a description of boundary-layer flow which is subjected to free-stream turbulence in the range from 1–6% and is based on both flow visualization results and extensive hot-wire measurements. Such flows develop streamwise elongated regions of high and low streamwise velocity which seem to lead to secondary instability and breakdown to turbulence. The initial growth of the streaky structures is found to be closely related to algebraic or transient growth theory. The data have been used to determine streamwise and spanwise scales of the streaky structures. Both the flow visualization and the hot-wire measurements show that close to the leading edge the spanwise scale is large as compared to the boundary-layer thickness, but further downstream the spanwise scale approaches the boundary-layer thickness. Wavenumber spectra in both the streamwise and the spanwise directions were calculated. A scaling for the streamwise structure of the disturbance was found, which allows us to collapse the spectra from different downstream positions. The scaling combines the facts that the streaky structures increase their streamwise length in the downstream direction which becomes proportional to the boundary-layer thickness and that the energy growth is algebraic, close to proportional to the downstream distance.

Transition induced by free-stream turbulence

Free-stream turbulence (FST) is perhaps the most important source inducing by-pass transition in boundary layer flows. The present study describes the initial energy growth of streamwise-oriented d ...

Stabilizing effect of surrounding gas flow on a plane liquid sheet

The stability of a plane liquid sheet is studied experimentally and theoretically, with an emphasis on the effect of the surrounding gas. Co-blowing with a gas velocity of the same order of magnitu ...

EXPERIMENTAL STUDY OF HEAT AND MOMENTUM TRANSFER IN ROTATING CHANNEL FLOW

The enhancement of momentum and heat transfer caused by stationary streamwise vortices due to a Coriolis instability in a rotating straight channel is examined. It is shown that the changes in skin friction and Nusselt number depend on changes in the spanwise averaged mean flow and temperature distributions. Hot wire anemometry was used to experimentally determine the streamwise velocity and temperature distributions in a cross stream plane 68 channel widths downstream of the inlet. A technique to accurately compensate the velocity readings for the varying temperature in the channel was developed. It is shown that the streamwise vortices give rise to disturbance profiles which are close to those obtained from linear theory for small rotation numbers and that in this region there is no enhancement of either the averaged momentum or heat transfer. However, even for a disturbance amplitude in the streamwise velocity of the order of 20%, the disturbances are close to linear (both the disturbance distribution ...

Laminar-Turbulent Transition at a High Level of a Free Stream Turbulence

Laminar-turbulent transition problem is one of the most significant and have not solved problem in fluid mechanics. The transition phenomenon investigation is important in both fundamental and application planes, in particular for controlling the boundary layer for drag reduction at vehicle elements, turbine blades etc.

Secondary instability in rotating channel flow

Experiments on rotating channel flow, where both the primary (induced by a Coriolis instability) and the secondary instability are triggered independently, are described, focusing on the development of a secondary instability consisting of high-frequency travelling waves and their subsequent breakdown. Detailed hot-wire velocity measurements of the secondary disturbance are made and the phase speed and growth rate for various frequencies are determined. It is shown that the frequency of highest growth rate is close to that which is observed for naturally developing flow. Some information on the later stages in the transition process is obtained from frequency spectra, which show that interaction between various modes gives rise to stochastic low-frequency disturbances, which may play an important role in the transition process. A theoretical model of the disturbance structure is described which is used to explain some of the measured results and also allows the determination of the disturbance cross-stream flow field from only streamwise velocity measurements.

Experimental investigation of relaminarizing and transitional channel flows

A hot-wire measurement was conducted in a planar channel flow that originated from a strongly disturbed flow in an entrance channel followed by an expansion channel used to reduce the Reynolds number (Re). From ceasing decrease of the streamwise velocity fluctuation energy and the linear extrapolation of the intermittency factor, the lower marginal Re, which is defined as the minimum Re for partial existence of sustainable turbulence, is estimated around 1400 based on the channel width and the bulk velocity. The upper marginal Re at which the intermittency factor reaches one is about 2600. The flow fields passing a turbulent patch were reconstructed with conditional sampling of the streamwise velocity data based on the time of laminar-turbulence interfaces and the reconstructed flow fields indicate a large-scale flow structure across laminar and turbulent parts. This large structure makes it possible for some regions to be at higher Re than the average, so that turbulence can partly survive. The moderate-...

Growth and breakdown of streaky structures in boundary layer transition induced by free stream turbulence

Freestream turbulence is maybe the most important source to force by-pass transition in boundary layer flows. The present study aims at describing the initial growth of streamwise oriented disturbances in the boundary layer and relate this growth to recent theories of non-modal growth of such disturbances, but also to lay the ground for a predication model for free stream turbulence induced transition. It is shown that the initial growth is linear and proportional to the free stream turbulence energy. The spanwise scale of the disturbance does not change in the downstream direction and is probably determined by the scale of the free stream turbulence.

Boundary Layer Transition at High Levels of Free Stream Turbulence

Transition to turbulence in laminar boundary layers subjected to high levels of free stream turbulence (FST) can still not be reliably predicted, despite its technical importance, e.g. in the case of boundary layers developing on gas turbine blades. In a series of experiments in the MTL-wind tunnel at KTH the influence of grid-generated FST on boundary layer transition has been studied, with FST-levels up to 6%. It was shown from both flow visualisation and hot-wire measurements that the boundary layer develops unsteady streaky structures with high and low streamwise velocity. This leads to large amplitude low frequency fluctuations inside the boundary layer although the mean flow is still close to the laminar profile. Breakdown to turbulence occurs through an instability of the streaks which leads to the formation of turbulent spots. Accurate physical modelling of these processes seems to be needed in order to obtain a reliable prediction method.Copyright

Experimental Study of Boundary Layer Transition Subjected to Weak Free Stream Turbulence

An experimental investigation of laminar turbulent transition in a flat plate boundary layer subjected to weak free stream turbulence has been done with focusing on transition scenarios. Flow visualization clearly indicated that a packet of Tollmien-Schlichting(T-S) wave emerges in the boundary layer and immediately breaks down to turbulence with deformation to j-shape structures. In a much weaker turbulence intensity case, the transition assumes more complication that a short streak breakdown coexists with the T-S wave breakdown in addition to interaction between the wave packets and streaky structures considerably elongated in the streamwise direction. There exist, at least, three scenarios of boundary layer transition due to free stream turbulent: wavy motion of the elongated streaky structure, the j-shape deformation of the T-S wave packet and the short streak breakdown to turbulence. In order to accomplish a proper prediction method for the boundary layer transition, further classification of boundary layer transition scenarios is needed.