Search

Numerical analyses of turbulent boundary layer parameters and skin-friction drag reduction on a flat plate under the effect of air micro-blowing with the use of the SST turbulence model are performed. The macroscale characteristics of a huge number of microjets are simulated by using a microporous wall model (MPWM) incorporated into ANSYS FLUENT by user-defined functions. Numerical results obtained within the Mach number range (Reynolds number ) confirm the experimental data of other researchers. Furthermore, a slight increase in the boundary layer thickness, displacement thickness, and momentum thickness, as well as a decrease in the velocity gradient and shear friction are well captured. In comparison to a simple flat plate, applying air micro-blowing reduces the skin-friction coefficient by 51% at the Mach number and blowing fraction of 0.008. Additionally, the skin-friction coefficient decreases as the blowing fraction and Mach number increase. [ABSTRACT FROM AUTHOR]

This is an analytical investigation of turbulent boundary layer flow over a flat plate in the range of Reynolds number between 10 6 and 10 9 . With the use of novel similarity variables, the governing partial differential equations are transformed to an ordinary differential equation with inconsistent coefficients and they are solved analytically via the optimal homotopy analysis method (OHAM). To achieve the convergence of analytical series solutions, residual errors have been calculated and minimized. In addition, the distribution of the velocity profile, friction coefficient and thickness of the boundary layer are calculated and compared with the pertained experimental results in the literature, where good agreements between them are observed. Based on the series solution, analytical expressions for the friction coefficient and the hydrodynamic boundary layer thicknesses were proposed and compared with previously existed expressions. It was shown that the proposed expressions are highly accurate for the high Reynolds number. [ABSTRACT FROM AUTHOR]

We further develop a simple counterflow body force-based approach to generate an equilibrium spatially developing turbulent boundary layer suitable for Direct Numerical Simulations ( DNS ) or Large Eddy Simulations ( LES ) of viscous–inviscid interactions. The force essentially induces a small separated region in an incoming specified laminar boundary layer. The resulting unstable shear layer then transitions and breaks down to yield the desired unsteady profile. The effects of wall thermal conditions are explored to demonstrate the capability of the method for both fixed wall and adiabatic wall conditions. We then describe an efficient method to select parameters that ensure transition by examining precursor signatures using generalized stability variables. These precursors are shown to be evident in a computational domain spanning only a small region around the trip and can also be detected using 2D simulations. Finally, the method is tested for different Mach numbers ranging from 1.7 to 2.9, with emphasis on flow field surveys, Reynolds stresses, and energy spectra. These results provide guidance on boundary conditions for desired boundary layer thickness at each Mach number. The consequences of using a much lower Reynolds number in computation relative to experiment are evident at the higher Mach number, where a self sustaining turbulent boundary layer is more difficult to obtain. [ABSTRACT FROM AUTHOR]

The characteristics of an incompressible turbulent boundary layer on a flat plate with air blown in though a finely perforated surface from an external confined flow through an input device, located on the "idle" side of the plate, have been investigated experimentally and numerically. A stable decrease in the local values of the coefficient of surface friction along the plate length that attains 85% at the end of the perforated portion is shown. The experimental and calculated data obtained point to the possibility of modeling, under earth conditions, the process of controlling a turbulent boundary layer with air injection by using the resources of an external confined flow. [ABSTRACT FROM AUTHOR]

A microelectromechanical systems-based microphone array on a chip has been developed and applied to aeroacoustic measurements. The array is designed to measure the fluctuating pressures present under a turbulent boundary layer (TBL). Each chip measures 1 cm2 and contains 64 individually addressable capacitively sensed microphones, with a center to center pitch of ~1.25 mm. Surface topology, including the packaging, is kept to less than 0.13 mm. Element-to-element sensitivity variation in the array is less than ±2.5 dB from least to most sensitive, and phase variation is less than ±6.5° (at 1 kHz). The microphone 3-dB bandwidth is 700 Hz to 200 kHz, and the microphones are linear to better than 0.3% at sound pressure levels up to 150-dB SPL. A unique switched architecture system electronics and packaging method are employed to reduce data acquisition channel count requirements, and to maintain a low surface roughness. The array has been applied to the measurement of single point turbulence spectra under a flat plate TBL in a flow duct at Mach numbers up to 0.6 and Reynolds numbers based on plate length of 107. [ABSTRACT FROM AUTHOR]

In the present paper, we discuss various approaches to measurement of velocity fields and velocity fluctuations in incompressible turbulent boundary layer by digital particle flow visualization methods. In particular, performing measurements in the vicinity of streamlined surface is considered. A description of the optical measuring system and methods to seed the flow with tracer particles is presented. Measured data for flow velocity fields and turbulent velocity pulsations in boundary layer on a flat impermeable plate are analyzed. Comparative data illustrating the efficiency of various algorithms for treating PTV images are presented. [ABSTRACT FROM AUTHOR]

Abstract: CFD has come to be used in the prediction of wind loads by the development of computer. Wind loads acting on buildings located in turbulent boundary layer are affected by the characteristic of approach flow strongly. Because inflow wind fluctuations affect the results of calculation with CFD, various kinds of methods to generate inflow wind fluctuations are suggested. On the other hand in wind tunnel experiment, PIV (Particle image velocimetry) technology has improved and been able to catch the high frequency turbulent fluctuations. We can acquire time–dependent–data of three-dimensional wind velocity components directly by stereo PIV. This report presents the result of LES calculation of turbulent boundary layer developed on flat floor for inflow generation using stereo PIV measurement results, and we discuss the applicability and the utility of this method. The simulated results show that the turbulent boundary layer is formed and reaches the equilibrium state at a relatively short distance from the starting point of computation. It can be thought that this method is useful for reproducing a turbulent boundary layer by LES. [Copyright &y& Elsevier]

Abstract: An experimental study was conducted in order to investigate the influence of upstream conditions with respect to near-wakes evolution by using a two-dimensional flat plate. A position of for the flat plate with a blunt trailing edge was settled upon as an upstream condition point. Three upstream conditions (i.e., natural laminar, bypass transitional, and turbulent boundary layers) were applied by tripping wires that were placed on the upper surface of the plate. The free-stream velocity was 6.0 m/s and the local Reynolds numbers based on momentum thickness at the upstream conditions point were 259, 303, and 331. The boundary layers and the near-wakes were measured by hot-wire anemometry. It was observed that for the symmetric near-wakes, the local similarity profiles of the mean defect velocity matched well with the traditional similarity profile in the center part, whereas some difference in the outer part occurred due to the different trailing edge shape. For the asymmetric upstream conditions, the center of the local similarity profiles moved to one side, and the amplitude of the local similarity profiles over the wakes edge varied according to the momentum thickness of the upstream conditions. Also, the collapse of the normalized Reynolds shear stress depended on turbulent quantities included in the upstream conditions, and these results showed that the evolution of the near-wakes is clearly affected by upstream conditions. [Copyright &y& Elsevier]

A numerical study of unsteady turbulent fluid flow over ERCOFTAC flat-plate T3A is represented. Four uniform flow rates are applied to an initially fully turbulent steady flow. The γ-SST turbulence model is adopted to determine the boundary layer development, analysis of turbulence structures, and the wall functions response in the presence of external acceleration. It is observed that imposing higher momentum rates affect the boundary layer thickness. Furthermore, enforced flow rate can change the shape of eddies and make them more stretched. Phase delay in turbulence propagation under this condition is the same as in previous studies. However, the quality and quantity of turbulence phase delay in two streamwise and wall-normal directions are investigated and compared with available experimental and numerical solutions. It is perceived that the amount of delay for each distinct acceleration, alongside the plate and at a specific distance from the wall, is almost the same. Whereas, in the normal direction, the delay is variable and emanates from the physical eddies shape. Moreover, in boundary layer, the outer region affeccted more severely than the near-wall region. • Presents the transient flow over the flat plate with changes in flow rate. • In platter external flows, the changes in flow rate directly affects the boundary layer thickness. • Flow experiences a semi-transitional behavior which emerges a kind of delay in accelerating flow. • More intense accelerations leads to higher vertical diffusion rate of turbulence perturbations at boundary layer region. [ABSTRACT FROM AUTHOR]

Due to the lack of data on hydraulic-jump dynamics in very large channels, the present paper describes the main characteristics of the velocity field and turbulence in a large rectangular channel with a width of 4 m. Although a hydraulic jump is always treated as a wave that is transversal to the channel wall, in the case of this study it has a trapezoidal front shape, first starting from a point at the sidewalls and then developing downstream in an oblique manner, finally giving rise to a trapezoidal shape. The oblique wave front may be regarded as a lateral shockwave that arises from a perturbation at a certain point of the lateral wall and travels obliquely toward the centreline of the channel. The experimental work was carried out at the Coastal Engineering Laboratory of the Water Engineering and Chemistry Department of the Technical University of Bari (Italy). In addition to the hydraulic jump formation, a large recirculating flow zone starts to develop from the separating point of the lateral shock wave and a separate boundary layer occurs. Intensive measurements of the streamwise and spanwise flow velocity components along one-half width of the channel were taken using a bidimensional Acoustic Doppler Velocimeter (ADV). The water surface elevation was obtained by means of an ultrasonic profiler. Velocity vectors, transversal velocity profiles, turbulence intensities and Reynolds shear stresses were all investigated. The experimental results of the separated boundary layer were compared with numerical predictions and related work presented in literature and showed good agreement. The transversal velocity profiles indicated the presence of adverse pressure gradient zones and the law of the wall appears to govern the region around the separated boundary layer. [ABSTRACT FROM AUTHOR]

In this work the authors describe the main characteristics of the velocity field of hydraulic jumps in a very large channel where lateral shockwaves occur. Experiments were carried out at the Coastal Engineering Laboratory of the Water Engineering and Chemistry Department of the Technical University of Bari (Italy). Extensive flow velocity measurements were investigated in order to have a clearer understanding of both hydraulic jump development and lateral shockwave formation in a very large channel. Eight experiments were performed in a 4m wide rectangular channel; the experiments differed in the inlet Froude number F 0 and the jump type. Seven tests were carried out with undular jumps and one with a roller jump. The flow velocity and the flow free surface measurements were taken using a two-dimensional Acoustic Doppler Velocimeter (ADV) and an ultrasonic profiler, respectively. The experimental results can be summarized as follow: (i) the formation of well developed lateral shockwaves similar to those of oblique jumps were observed; (ii) the comparison of the experimental and theoretical data shows that the classic shockwave theory is sufficiently confirmed in the analyzed range of Reynolds number, taking into account the experimental errors and the difference between the theoretical and experimental assumptions; (iii) the transversal flow velocity profiles in the recirculating zone show a good agreement with the numerical simulations presented in literature in the case of a separated turbulent boundary layer over a flat plate. This conclusion enables us to confirm the hypothesis that the lateral shockwaves in the channel are the result of a boundary layer which, as observed, forms on the channel sidewalls. [ABSTRACT FROM AUTHOR]

Abstract: A two-dimensional single phase computational fluid dynamics (CFD) model of microbubble-laden flow over a flat plate was used to assess the role of mixture density variation in microbubble drag reduction. The model consisted of Reynolds-averaged Navier–Stokes (RANS) transport equations, a standard k–ω turbulence model, and a convection–diffusion species transport model. Performance of the model was validated with available experimental data and numerical simulations of more advanced multiphase two-fluid model. A parametric study of the density ratio effect on the drag reduction was carried out. The study indicated that simple mixture density variation effect plays one of the major roles in the microbubble drag reduction phenomenon. Also, the influence of the density ratio (the ratio of density of injected gas to that of water) on gas volume fraction profiles was found to be minimal within studied parameter range. [Copyright &y& Elsevier]

• Turbulent boundary layer development under adverse pressure gradient is studied. • Effect of Reynolds number on the flow statistics is examined. • Enhanced amplitude modulation with increasing Re is observed. • The higher Re the higher convection velocity of small-scales. • Turbulent separation is postponed due to increasing amplitude modulation with Re. The effect of Reynolds number on turbulent boundary layer under adverse pressure gradient has been barely investigated in the available literature. So far, experimental or numerical research on that issue were performed either for low Re or for low range of Re to capture its clear effect on flow statistics. Hence, the purpose of the present paper is to examine whether the effect of Reynolds number can be observed when much higher Re values and its much wider range, than previously investigated, are considered. Herein, the momentum loss thickness based Reynolds number Re θ was changed from 4900 to 14600. Such a wide range of Re θ appeared to be sufficient to capture an evident footprint of Re θ on the flow. It was found that the separation point is shifted downstream with increasing Reynolds number due to increased convection velocity close to the wall. More detailed studies of the skewness factor and premultiplied energy spectra revealed that this phenomenon can be attributed to enhanced, with increasing Re , large- and small-scale interactions commonly known as the amplitude modulation. [ABSTRACT FROM AUTHOR]