Turbulent Spot

The transition from the laminar to the turbulent regime in linearly stable shear flows, for example, pipe and plane Couette flows, occurs abruptly with no precursor. The evolution of turbulent spots has been studied to better understand the dynamics of this transition and the onset of turbulence. These studies have mostly focused on pipe flows for which a finite lifetime of spots was proven. The same conclusion was drawn in the only available study performed in a Taylor Couette setup. Here, the spot lifetime is measured in a different size TC setup. It is shown that the lifetime is indeed finite and also very sensitive to boundary conditions, but not much to perturbation mechanisms. A scaling approach is provided which suggests in addition to the Reynolds number, the aspect and radius ratios are influential parameters on the lifetime. It is found that the spot size varies during its lifetime and increases with the Reynolds number that confirms the rise in turbulence proliferation by approaching the transitional point.

The transition from the laminar to the turbulent regime in linearly stable shear flows, for example, pipe and plane Couette flows, occurs abruptly with no precursor. The evolution of turbulent spots has been studied to better understand the dynamics of this transition and the onset of turbulence. These studies have mostly focused on pipe flows for which a finite lifetime of spots was proven. The same conclusion was drawn in the only available study performed in a Taylor Couette setup. Here, the spot lifetime is measured in a different size TC setup. It is shown that the lifetime is indeed finite and also very sensitive to boundary conditions, but not much to perturbation mechanisms. A scaling approach is provided which suggests in addition to the Reynolds number, the aspect and radius ratios are influential parameters on the lifetime. It is found that the spot size varies during its lifetime and increases with the Reynolds number that confirms the rise in turbulence proliferation by approaching the transitional point.

Evolution of the K-vortex and its transformation into the turbulent spot in a flat plate boundary layer are investigated experimentally. Extensive measurements visualizing the K-structure transformation into the turbulent spot on the smooth and ribbed surfaces of the flat plate are presented. The flow behavior in the course of spatial evolution of the K-structure and turbulent spot is discussed. Specific features of the downstream evolution of K-structure and turbulent spot on the smooth and ribbed surfaces are demonstrated, such as suppression by riblets of the K-vortex transformation into the turbulent spot, appearance of the coherent structures of K-vortex within ensemble-averaged turbulent spot, and oblique waves generation both by the K-vortex and turbulent spot.

The RANS code Fluent 6 TM is used to study the application of an array of oscillating sub-boundary layer vortex generators (SBVGs) to control flow separation in a diffuser with an opening angle of 10 degrees. Experimental data is available for the uncontrolled flow in the diffuser. The section of the duct upstream of the diffuser has a height H equal to 15 mm; its length and breadth are 101 and 41 H respectively; the diffuser has an expansion ratio of 4.7:1. Fully developed flow is achieved upstream of the diffuser. SBVGs with a trailing edge span of 2 mm (13.3% of the duct height) were considered. The array was oscillated in simple harmonic motion between 0 to 15 degrees at a range of frequency corresponding to the frequency of the largest eddies in the boundary layer. One SBVG passage was meshed; symmetry and periodic side-wall boundary conditions were used to model counter-rotating and co-rotating arrays, respectively. The preliminary results suggest that the oscillating SBVGs can be used suppressing the flow separation on the diffuser ramp.

Longitudinal and transverse components of the velocity were simultaneously measured for various vertical locations at 20 off-centreline positions in turbulent spots artificially generated in a zero-pressure-gradient laminar boundary layer. Global ensemble-averaged velocity diagrams and vertical vorticity contours were computed in similarity coordinates ξ = (x − x0)/(U∞(t−t0)) and ζ = z/(U∞(t−t0)) for different heights η = y/δ* above the plate. This global averaging technique inadequately describes the spot, which is not a single large vortex structure. A discriminative averaging technique was developed to construct a ‘statistically most-probable’ spot with sufficient resolution to include some of the largest substructures detected in visual studies. Four eddies were identified in vertical slices of the central region of the spot, while several rows appeared in the plan view. These features of the velocity and vorticity contours of the statistically most-probable spot exhibit similar...

Simultaneous measurements of the longitudinal velocity U, normal velocity V, and temperature T have been made in turbulent spots generated in a slightly heated laminar boundary layer. The measurements were made in the plane of symmetry of the spot at several streamwise stations downstream of the spot-producing spark. Ensemble averages of U, V and T, obtained relative to the leading edge of the spot, are presented in terms of disturbances relative to the unperturbed values in the laminar flow. Ensemble-averaged disturbances of U, V and T in the outer part of the spot are consistent with a picture of a relatively large vortical structure with a spanwise vorticity in the same sense as that of the laminar flow. A rather dominant feature of these distributions is the large negative disturbance in V and U near the leading edge in the outer part of the spot; associated with this disturbance is a positive perturbation in T. The terms contributing to the ensemble-averaged values of UV, UT an...

We report here on the results of a series of experiments carried out on a turbulent spot in a distorted duct to study the effects of a divergence with straight streamlines preceded by a short stretch of transverse streamline curvature, both in the absence of any pressure gradient. It is found that the distortion produces substantial asymmetry in the spot: the angles at which the spot cuts across the local streamlines are altered dramatically (in contradiction of a hypothesis commonly made in transition zone modelling), and the Tollmien–Schlichting waves that accompany the wing tips of the spot are much stronger on the outside of the bend than on the inside. However there is no strong effect on the internal structure of the spot and the eddies therein, or on such propagation characteristics as overall spread rate and the celerities of the leading and trailing edges. Both lateral streamline curvature and non-homogeneity of the laminar boundary layer into which the spot propagates are ...

The flow field associated with the passage of the turbulent spot in a 3-D duct with streamline divergence under zero pressure gradients was investigated and displayed as contour plots of the velocity perturbation in plan and elevation view of the spot. It suggests that, streamline divergence has no strong effect on the internal structure of the spot and eddies and their propagation in the downstream direction is similar to 2-D flows. The orthographic views are also shown. They are rather similar to contour plots but with different kind of presentation to show velocity excess and deficit as peaks and valleys respectively in the flow field while passage of the spot. This work was a part of detailed investigation of the structure of a distorted turbulent spot in a 3-D constant pressure flow.

The separation of unsteadiness (or intermittency) and turbulence is the key to improving understanding of the statistical behavior of the transitional boundary layer flow. In this study an attempt is made to review the different methods used for the detection of turbulent and non-turbulent interface in transitional flows. In this regard, methods for generating the intermittency function, conditional averaging and sampling are discussed. The salient features of each technique is also explained.

The separation of unsteadiness (or intermittency) and turbulence is the key to improving understanding of the statistical behavior of the transitional boundary layer flow. In this study an attempt is made to review the different methods used for the detection of turbulent and non-turbulent interface in transitional flows. In this regard, methods for generating the intermittency function, conditional averaging and sampling are discussed. The salient features of each technique is also explained.

An overview of laminar-turbulent transition zone modelling research at IISc is presented. The linear combination model based on the principle of combining mean laminar and turbulent profiles in proportions determined by the interminency has been found to provide an excellent representation of the transition zone in two-dimensional incompressible boundary-layer flows. Ongoing investigations in a three-dimensional constant pressure flow with lateral Streamline divergence indicate that many transitional characteristics are similar to those in two- dimensional constant pressure flows for modelling purposes.

Turbulent spots evolving in a laminar boundary layer on a nominally zero pressure gradient flat plate are investigated. The plate is towed through an 18 m water channel, using a carriage that rides on a continuously replenished oil film giving a vibrationless tow. Turbulent spots are initiated using a solenoid valve that ejects a small amount of fluid through a minute hole on the working surface. A novel visualization technique that utilizes fluorescent dye excited by a sheet of laser light is employed. Some new aspects of the growth and entrainment of turbulent spots, especially with regard to lateral growth, are inferred from the present experiments. To supplement the information on lateral spreading, a surbulent wedge created by placing a roughness element in the laminar boundary layer is also studied both visually and with probe measurements. The present results show that, in addition to entrainment, another mechanism is needed to explain the lateral growth characteristics of a ...

The separation of unsteadiness (or intermittency) and turbulence is the key to improving understanding of the statistical behavior of the transitional boundary layer flow. In this study an attempt is made to review the different methods used for the detection of turbulent and non-turbulent interface in transitional flows. In this regard, methods for generating the intermittency function, conditional averaging and sampling are discussed. The salient features of each technique is also explained.

In the self-preserving region of a slightly heated turbulent plane jet, conventional isocorrelation contours of velocity and temperature fluctuations support the existence of organized large-scale structures. Temperature fronts associated with these structures were visually detected using a spanwise rake of cold wires. This method of detection was then used to condition velocity and temperature fluctuations and products of these fluctuations. Ensemble-averaged velocity vectors, constructed in the plane of main shear, suggest a topology for the organized motion in which the temperature front is identified with the diverging separatrix connecting adjacent structures on the same side of the centreline. Coherent stresses and heat fluxes are particularly significant near the diverging separatrix. Contributions by the coherent and random motions to the averaged momentum and heat transports are generally of the same order of magnitude.

Origination and development of perturbations generated by a three-dimensional vibrating surface in the boundary layer on an airfoil with a zero sweep angle is experimentally studied. Surface vibrations were generated by a three-dimensional membrane. It is shown that high-amplitude vibrations of a three-dimensional surface lead to simultaneous formation of two types of perturbations in the boundary layer: quasi-stationary streamwise structures and wave packets accompanying them. The presence of regions with favourable and adverse pressure gradients does not exert a significant effect on evolution of streamwise structures but leads first to attenuation and then to amplification of wave packets.

Conditional averages of longitudinal, normal velocity and temperature fluctuations and of their products have been obtained in a slightly heated boundary layer with zero pressure gradient over a momentum-thickness Reynolds-number range 990 4 R m < 7100. These averages are based on the identification of coherent temperature fronts that extend across most of the layer. The average period between fronts is approximately independent of R m when Rm is greater than about 1500. The streamwise length scale of the fronts and the magnitude of velocity and temperature derivatives associated with the fronts scale on the thickness of the layer except for R, less than about 3000. This scaling is consistent with the Reynolds-number independence, for R m greater than about 3000, of both mean and turbulent velocity and temperature fields. Conditional averages are discussed in the context of Head & Bandyopadhyay's ( 1978) suggestion, based on smoke-flow visualization, that the boundary layer consists almost exclusively of hairpin eddies.

The characteristics of a turbulent spot propagating in a laminar boundary layer subjected to a self-similar adverse pressure gradient (defined by a Falkner-Skan parameter / 3 = -0.1) were investigated experimentally. It was observed that some small differences in the normalized shape of the undisturbed velocity profile caused by the pressure gradient had a major influence on the spreading rate of the spot at comparable Re,,. The rate of spread of the spot in the spanwise direction was affected most dramatically by the pressure gradient where the average angle at which the tips of the spots moved outward relative to the plane of symmetry was 21". It was noted that the strength and duration of the disturbance initiating the spots had an effect on their spanwise rate of spread. For example, a strong impulsive disturbance and a disturbance caused by a stationary three-dimensional roughness generated spots which spread at a much smaller rate. The interaction of the spot with the wave packet existing beyond its tip was enhanced by the adverse pressure gradient because the Reynolds number of the surrounding boundary layer was everywhere supercritical. Thus, the maximum linear amplification rate in this case is approximately four times larger than in Blasius flow. Some features of the breakdown and their relationship to the shape and the perturbation velocities in the spot are discussed. The normalized length of the calmed region relative to the length of the spot is enhanced by the adverse pressure gradient and by an increase in the intensity of the disturbance.

Origination and development of perturbations generated by a three-dimensional vibrating surface in the boundary layer on an airfoil with a zero sweep angle is experimentally studied. Surface vibrations were generated by a three-dimensional membrane. It is shown that high-amplitude vibrations of a three-dimensional surface lead to simultaneous formation of two types of perturbations in the boundary layer: quasi-stationary streamwise structures and wave packets accompanying them. The presence of regions with favourable and adverse pressure gradients does not exert a significant effect on evolution of streamwise structures but leads first to attenuation and then to amplification of wave packets.

We report here on the results of a series of experiments carried out on a turbulent spot in a distorted duct to study the effects of a divergence with straight streamlines preceded by a short stretch of transverse streamline curvature, both in the absence of any pressure gradient. It is found that the distortion produces substantial asymmetry in the spot: the angles at which the spot cuts across the local streamlines are altered dramatically (in contradiction of a hypothesis commonly made in transition zone modelling), and the Tollmien-Schlichting waves that accompany the wing tips of the spot are much stronger on the outside of the bend than on the inside. However there is no strong effect on the internal structure of the spot and the eddies therein, or on such propagation characteristics as overall spread rate and the celerities of the leading and trailing edges. Both lateral streamline curvature and non-homogeneity of the laminar boundary layer into which the spot propagates are shown to be strong factors responsible for the observed asymmetry. It is concluded that these factors produce chiefly a geometric distortion of the coherent structure in the spot, but do not otherwise affect its dynamics in any significant way.

Experi$ents have been carried out to investiqate the developrnent of an artificially generated spot in a 3-D distorted duct with diverging streamlines and zero streamwise pressure giradient. Results indicate that the overall spot propagation characterisli.cs like growth rate and celerity are not aftected by streamline divergrence even though it is of the order of the spot growth arrgle. However. the spot shape is distorted, and it does not propaqa!e at constant angle with the streamlines as in 2-D flows.

The flow field associated with the passage of the turbulent spot in a 3-D duct with streamline divergence under zero pressure gradients was investigated and displayed as contour plots of the velocity perturbation in plan and elevation view of the spot. It suggests that, streamline divergence has no strong effect on the internal structure of the spot and eddies and their propagation in the downstream direction is similar to 2-D flows. The orthographic views are also shown. They are rather similar to contour plots but with different kind of presentation to show velocity excess and deficit as peaks and valleys respectively in the flow field while passage of the spot. This work was a part of detailed investigation of the structure of a distorted turbulent spot in a 3-D constant pressure flow.

The separation of unsteadiness (or intermittency) and turbulence is the key to improving understanding of the statistical behavior of the transitional boundary layer flow. In this study an attempt is made to review the different methods used for the detection of turbulent and non-turbulent interface in transitional flows. In this regard, methods for generating the intermittency function, conditional averaging and sampling are discussed. The salient features of each technique is also explained.