Non-turbulent Zones Research Articles

Complex Effects of Free Stream Turbulence and Surface Roughness on the Transitional Intermittency

The comparison of transitional intermittency distributions in boundary layers on smooth and rough surface is presented. The conditional analysis of the instantaneous wall friction is applied on time records made in transitional boundary layers originating on smooth surface or surface covered by sand paper. Special procedures of measurement and evaluation are applied that allowed evaluations of turbulent and nonturbulent zone averages.

An investigation of the scales in transitional boundary layers under high free-stream turbulence conditions

The scales in a transitional boundary layer subject to high (initially 8%) free-stream turbulence and strong acceleration (K as high as 9×10−6) were investigated using wavelet spectral analysis and conditional sampling of experimental data. The boundary layer shows considerable evolution through transition, with a general shift from the lower frequencies induced by the free-stream unsteadiness to higher frequencies associated with near-wall-generated turbulence. Within the non-turbulent zone of the intermittent flow, there is considerable self-similarity in the spectra from the beginning of transition to the end, with the dominant frequencies in the boundary layer remaining constant at about the dominant frequency of the free-stream. The frequencies of the energy-containing scales in the turbulent zone change with streamwise location and are significantly higher than in the non-turbulent zone. When normalized on the local viscous length scale and velocity, however, the turbulent zone spectra also show good self-similarity throughout transition. Turbulence dissipation occurs almost exclusively in the turbulent zone. The velocity fluctuations associated with dissipation are isotropic, and their normalized spectra at upstream and downstream stations are nearly identical. The distinct differences between the turbulent and non-turbulent zones suggest the potential utility of intermittency based transition models in which these zones are treated separately. The self-similarity noted in both energy containing and dissipation scales in both zones suggests possibilities for simplifying the modeling for each zone.

Conditional Sampling in a Transitional Boundary Layer Under High Freestream Turbulence Conditions

Conditional sampling has been performed on data from a transitional boundary layer subject to high (initially 9%) freestream turbulence and strong (K=ν/U∞2dU∞/dx as high as 9×10−6) acceleration. Methods for separating the turbulent and nonturbulent zone data based on the instantaneous streamwise velocity and the turbulent shear stress were tested and found to agree. Mean velocity profiles were clearly different in the turbulent and nonturbulent zones, and skin friction coefficients were as much as 70% higher in the turbulent zone. The streamwise fluctuating velocity, in contrast, was only about 10% higher in the turbulent zone. Turbulent shear stress differed by an order of magnitude, and eddy viscosity was three to four times higher in the turbulent zone. Eddy transport in the nonturbulent zone was still significant, however, and the nonturbulent zone did not behave like a laminar boundary layer. Within each of the two zones there was considerable self-similarity from the beginning to the end of transition. This may prove useful for future modeling efforts.

Effects of Concave Curvature on Boundary Layer Transition Under High Freestream Turbulence Conditions

An experimental investigation has been carried out on a transitional boundary layer subject to high (initially 9%) freestream turbulence, strong acceleration (K=ν/Uw2dUw/dx as high as 9×10−6), and strong concave curvature (boundary layer thickness between 2% and 5% of the wall radius of curvature). Mean and fluctuating velocity as well as turbulent shear stress are documented and compared to results from equivalent cases on a flat wall and a wall with milder concave curvature. The data show that curvature does have a significant effect, moving the transition location upstream, increasing turbulent transport, and causing skin friction to rise by as much as 40%. Conditional sampling results are presented which show that the curvature effect is present in both the turbulent and nonturbulent zones of the transitional flow.

Effects of Low Reynolds Number on the Outer Layer Structure in a Smooth Wall Turbulent Boundary Layer. Re-examination of Statistical Quantities.

Re-examination of the low-Reynolds-number effect on the outer layer structure has been made in a zero pressure gradient, smooth wall turbulent boundary layer where the two-dimensionality of a flow field was corroborated using the momentum integral equation. The measurements associated with the higher-order moments and the conditional averages were made over a momentum thickness Reynolds number range 840&InE;Rθ&InE;6120. The magnitude of the triple products normalized with friction velocity and relative typical eddy scale Cx/δ are affected with a achievement level of the two-dimensionality of a flow field. For Rθ<2130, the absolute values of the triple products and the turbulent diffusion terms normalized with outer variables increase with Rθ. Intermittency factor is approximately independent of Rθ when Rθ is greater than 4410. Therefore, average turbulent and non-turbulent zone lengths are constant independent of Rθ.

Conserved scalar probability density functions in a turbulent jet diffusion flame

The first four moments of conserved scalar probability density functions (p.d.f.'s) measured by Raman scattering in an H2 turbulent jet diffusion flame are analysed and compared with those found by Pitts &amp; Kashiwagi (1984) in a non-reacting CH4 jet. The measurements are in good agreement, indicating that heat release and combustion have little effect on p.d.f. shapes. However, the measured p.d.f.'s are not qualitatively similar to the simple forms often assumed in combustion modelling. A three-zone model by Effelsberg &amp; Peters was used to separate the experimental p.d.f.'s into a delta function (non-turbulent zone), a Gaussian (turbulent zone) and the remainder (interface zone). The interface zone contributed as much as 90% of the total p.d.f. in both the H2 flame and the non-reacting CH4 jet. A physical interpretation for the existence of broad interface zones in reacting and non-reacting turbulent jet flows is suggested based upon large-scale structures.

Stress transport in the rotational and irrotational zones of turbulent shear flows

The transport equations for intermittency factor and conditioned moments are analyzed for turbulent shear flows with free boundaries. Conditions are established for molecular diffusion to dominate the progression of the turbulent zone. The terms related to the dynamics of the interface for mean velocity and apparent stresses in turbulent and nonturbulent zones are shown to be linked by a local relation, and the limit of large distance from the turbulent region for the nonturbulent zone stress equation is given.

A closure model for conditioned stress equations and its application to turbulent shear flows

A second-order closure model based on intermittency factor and conditioned moments is developed. The transport equations for the nonturbulent zone stresses are included in the model. The resulting model is then compared with measurements in several shear flows and satisfactory agreement between calculation and experiment is obtained.

The Structure of the Outer Intermittent Region of Turbulent Boundary Layers with Injection and Suction through a Slit

The structure of the outer intermittent region of non-equilibrium turbulent boundary layers disturbed by injection and suction through a slit is investigated experimentally. Conditional sampling and averaging are used to generate mean and fluctuating components for the turbulent and non-turbulent zones of fluid. Further, space-time correlation is measured. The main results are summarized as follows : The center of intermittency moves outward from the wall, and the width of the intermittent zone decreases by the effect of injection. In the case of suction, the reverse characteristics, compared with injection, are observed. There exists a circulatory flow inside the turbulent bulge. This result is consistent with that of an equilibrium turbulent boundary layer. On the basis of the experimental results, a conceptual explanation for the development of the boundary layer and wall shear stress is presented.

An experiment on the intermittent region of a corner turbulent boundary layer.

An experimental investigation of the zero pressure gradient turbulent boundary layer along a 90 degree streamwise corner is reported. Turbulent non-turbulent conditional sampling measurements in the outer intermittent region of the boundary layer as well as a comprehensive set of conventional measurements were made in a cross section at 1.7 m down stream from the leading edge. The results of a turbulent zone averaged secondary velocity field indicate a set of closed streamwise vortex patterns near the corner, as in the conventional measurements. A large difference was found in the outer part of the boundary layer, where the turbulent zone averaged secondary flow moves from the wall to the free stream direction. On the contrary, the non-turbulent zone averaged counterpart moves from the free stream to the wall and which includes almost no streamwise vorticity, indicating that the secondary flow of the second kind is generated only when the flow is turbulent.

The structure of the outer intermittent region of turbulent boundary layers with injection and suction through a slit.

The structure of the outer intermittent region of non-equilibrium turbulent boundary layers disturbed by injection and suction through a slit is investigated experimentally. conditional sampling and averaging are used to generate mean and fluctuating components for the turbulent and non-turbulent zones of fluid. Further, space-time correlation is measured. The main results are summarized as follows: The center of intermittency moves outward from the wall, and the width of the intermittent zone decreases by the effect of injection. In the case of suction, the reverse characteristics, compared with injection, are observed. There exists a circulatory flow inside the turbulent bulge. This result is consistent with that of an equilibrium turbulent boundary layer. On the basis of the experimental results, a conceptual explanation for the development of the boundary layer and wall shear stress is discussed.

Prediction of the intermittency factor for turbulent shear flows

Two closure models for the prediction of the intermittency factor are discussed and their performance in free shear flows is evaluated. The first model is based on a closed set of conditional moment equations containing mean velocities in turbulent and nonturbulent zones and selected second-order moments in the turbulent zone. The second model is centered around the closed equation for the probability density function of a passive scalar which is suitable for discrimination between zones. The results show that the first model gives better agreement with measurements.

Closure model for intermittent turbulent flows

A closure model for turbulent shear flows based exclusively on conditional moments and the intermittency factor is developed. The model contains the equations for intermittency factor, the turbulent zone and non-turbulent zone mean velocities and kinetic energy and dissipation rate for the turbulent zone. The model is applied to the prediction of plane jets and boundary layers and gives satisfactory results for intermittency factor and first order moments.

On conditioned averages for intermittent turbulent flows

A conditioning function is generally used to discriminate between the turbulent and non-turbulent zones in intermittent flows. Any fluid-mechanical property is conditioned by multiplying it by the intermittency function. A formalism is developed in which some integrals of the fluid-mechanical variables at the interface are important ingredients with a precise physical meaning. The present ideas may prove to be useful in turbulence dynamics and turbulent mixing of scalars.

Detection of the turbulent-nonturbulent interface in slightly heated turbulent shear flows

The detection of the interface between turbulent and nonturbulent zones in slightly heated turbulent shear flows on the basis of a temperature signal is discussed. Two detection parameters, a threshold level or gate δθg and a hold time τh, are used. A complete description of the detection process and of the procedure used to select nominal values for the detection param eters is presented. It is shown that the detection process is credible in the sense that the statistics of the intermittency function are nearly independent of the values of the detection parameters. Comparison of the intermittency function statistics with previous results is made.

Some turbulent/non-turbulent properties of the outer intermittent region of a boundary layer

A detailed study of the intermittency in the outer region of a flat-plate turbulent boundary layer has been carried out using digital sampling and processing techniques. Conditional averages are used to generate mean and fluctuating components for the turbulent and non-turbulent zones of fluid. More particularly, point averages of these variables, taken with reference to the instantaneous position of the turbulent/non-turbulent interface, have been made to show the distribution of various quantities through the turbulent front. The results indicate that significant differences exist at leading and trailing edges of the turbulent bursts and a more complete picture of the motion of an average large eddy is deduced.

Time Scales and Correlations in a Turbulent Boundary Layer

Space-time correlations with large streamwise separation were obtained in a turbulent boundary layer with a zero-pressure gradient. The auto and cross correlations of the velocities u and υ with streamwise spatial separation distances up to 20 boundary layer thicknesses revealed a difference in their structure and decay rate. Using conditional averaging techniques, the velocity product, uυ, was sampled in both the turbulent and nonturbulent zones. Further conditional sampling led to an average picture of the velocities in the interfacial bulges. Near the wall the space-time correlation results are consistent with the idea of retarded fluid being ejected outward from the wall region and influencing the intermittent region.

The two-dimensional mixing region

The two-dimensional incompressible mixing layer was investigated by using constant-temperature, linearized hot wire anemometers. The measurements were divided into three categories: (1) the conventional average measurements; (2) time-average measurements in the turbulent and the non-turbulent zones; (3) ensemble average measurements conditioned to a specific location of the interface. The turbulent energy balance was constructed twice, once using the conventional results and again using the turbulent zone results. Some differences emerged between the two sets of results. It appears that the mixing region can be divided into two regions, one on the high velocity side which resembles the outer part of a wake and the other on the low velocity side which resembles a jet. The binding turbulent–non-turbulent interfaces seem to move independently of each other. There is a strong connexion between the instantaneous location of the interface and the axial velocity profile. Indeed the well known exponential mean velocity profile never actually exists at any given instant. In spite of the complexity of the flow the simple concepts of eddy viscosity and eddy diffusivity appear to be valid within the turbulent zone.