Flat Plate Boundary Layer Research Articles

Wave-Front Measurements of a Supersonic Boundary Layer Using Laser Induced Breakdown

The aero-optical effect of a flat-plate adiabatic boundary layer has been measured using the light generated by a laser-induced breakdown spark. The measurements were performed in a blowdown wind tunnel at freestream Mach numbers of 3 and 4.38. The tests showed that the aero-optical effect of boundary layers with rms optical path difference as low as could be accurately measured using the laser-induced breakdown spark, including their deflection-angle spatial spectra. The results demonstrate that, using the laser-induced breakdown spark as a source of illumination, it is possible to make accurate measurements of low-amplitude aero-optical effects in a manner that is self-contained, nonintrusive, and suitable for a flight-test environment.

Development of streamwise counter-rotating vortices in flat plate boundary layer pre-set by leading edge patterns

Development of streamwise counter-rotating vortices induced by leading edge patterns with different pattern shape is investigated using hot-wire anemometry in the boundary layer of a flat plate. A triangular, sinusoidal and notched patterns with the same pattern wavelength λ of 15mm and the same pattern amplitude A of 7.5mm were examined for free-stream velocity of 3m/s. The results show a good agreement with earlier studies. The inflection point on the velocity profile downstream of the trough of the patterns at the beginning of the vortex formation indicates that the vortices non-linearly propagate downstream. An additional vortex structure was also observed between the troughs of the notched pattern.

Markovian properties of velocity increments in boundary layer turbulence

Markovian properties of the turbulent velocity increments in a flat plate boundary layer at Reθ of 19100 are investigated using hot-wire anemometry measurements of the streamwise velocity component in a wind tunnel. Increments of the longitudinal velocities at different wall-normal positions show that the flow exhibits Markovian properties when the separation between different scales, or the Markov–Einstein coherence length, is on the order of the Taylor microscale, λ. The results indicate that Markovian nature of turbulence evolves across the boundary layer showing certain characteristics pertaining to the distance to the wall. The connection between the Markovian properties of turbulent boundary layer and existence of the spectral gap is explored. Markovianity of the process is also discussed in relation to the nonlocal nonlinear versus local nonlinear transfer of energy, triadic interactions and dissipation.

Statistical evidence of anasymptotic geometric structure to the momentum transporting motions in turbulent boundary layers.

The turbulence contribution to the mean flow is reflected by the motions producing the Reynolds shear stress (〈-uv〉) and its gradient. Recent analyses of the mean dynamical equation, along with data, evidence that these motions asymptotically exhibit self-similar geometric properties. This study discerns additional properties associated with the uv signal, with an emphasis on the magnitudes and length scales of its negative contributions. The signals analysed derive from high-resolution multi-wire hot-wire sensor data acquired in flat-plate turbulent boundary layers. Space-filling properties of the present signals are shown to reinforce previous observations, while the skewness of uv suggests a connection between the size and magnitude of the negative excursions on the inertial domain. Here, the size and length scales of the negative uv motions are shown to increase with distance from the wall, whereas their occurrences decrease. A joint analysis of the signal magnitudes and their corresponding lengths reveals that the length scales that contribute most to 〈-uv〉 are distinctly larger than the average geometric size of the negative uv motions. Co-spectra of the streamwise and wall-normal velocities, however, are shown to exhibit invariance across the inertial region when their wavelengths are normalized by the width distribution, W(y), of the scaling layer hierarchy, which renders the mean momentum equation invariant on the inertial domain.This article is part of the themed issue 'Toward the development of high-fidelity models of wall turbulence at large Reynolds number'.

Influence Of Heavy Gas Blowing Into The Wall Layer Of Supersonic Boundary-Layer On Its Transition

The experimental investigation of the influence of the distributed blowing of heavy gas (sulfur hexafluoride of SF6) into the wall layer of supersonic flat-plate boundary layer (at free-stream Mach number M = 2) on the laminar-turbulent transition have been performed. For the first time experimentally it is shown that in case of such blowing there is a boundary-layer stabilization, and the laminar-turbulent transition is removed from the model leading edge.

Active and Natural Suction at Forward-Facing Steps for Delaying Laminar–Turbulent Transition

Two-dimensional, direct numerical simulations are used to study the impact of active and natural (passive) suction at forward-facing steps on laminar–turbulent transition. Suction is applied through a gap in front of a step that is located in a flat-plate boundary-layer flow without streamwise pressure gradient. A steady base flow is used with a Mach number of 0.6. Subsequently, Tollmien–Schlichting waves are introduced by suction and blowing at the wall, and their growth over the surface imperfection is evaluated by factors. The investigated step heights are in the range of one to two times the local displacement thickness of the smooth flat plate. Both sharp and rounded step corners are investigated. Cases with and without suction are compared with the smooth flat plate without suction based on factors according to the method. Thus, it is shown that suction is capable to compensate or even overcompensate the negative impact of different steps on laminar–turbulent transition. The work concludes with a configuration that allows natural (passive) suction in front of a forward-facing step. Here, a significant reduction of the factor compared to the smooth flat plate without suction is possible. Further estimations indicate that a net drag reduction may be possible as well.

Spanwise boundary layer modulations using finite discrete suction for transition delay

Discrete suction is deployed in a flat plate boundary layer to create spanwise mean velocity gradients (SVG) with the goal of delaying the onset of laminar-to-turbulent transition. It is shown that finite boundary layer suction through a set of holes in a spanwise oriented array in the flat plate is successful in setting up steady and robust streamwise streaks in the boundary layer. Today, the SVG method for transition control is known to attenuate the growth of Tollmien–Schlichting (TS) waves and delay the transition to turbulence. In this investigation, low-amplitude forced TS waves are attenuated with the implication of extending the laminar flow by at least 120% for a discrete suction of 0.8% of the free-stream velocity. The control technique is also tested successfully for natural transition, with a resulting transition delay of 30%.

Effect of Yaw Angle on the Flow Structure of Low Aspect Ratio Elliptical Cavities

AbstractThe flow regimes associated with a 2∶1 aspect ratio, elliptical planform cavity in a turbulent flat-plate boundary layer have been systematically examined for various depth/width ratios (0....

Hot-wire visualization of the evolution of localized wave packets in a supersonic flat-plate boundary layer

Hot-wire visualization of the evolution of localized wave packets in a supersonic flat-plate boundary layer

自由来流湍流与三维壁面局部粗糙诱导平板边界层不稳定T-S波的数值研究

The direct numerical simulation (DNS) method was adopted to study the physical problem of unstable Tollmien-Schlichting (T-S) waves in the flat-plate boundary layer under the interaction between free-stream turbulence and 3D localized wall roughness. The numerical results show that the spatial arrays of the wave packets composed of 2D and 3D T-S waves form in the flat-plate boundary layer, with the propagation speeds of the wave packets calculated. Then it is proved that the interaction between the free-stream turbulence and the 3D localized wall roughness makes the mechanism for excitation of unstable T-S waves in the flat-plate boundary layer. Subsequently, the relations between the initial 2D & 3D T-S wave amplitudes and the free-stream turbulence intensity, the roughness length, the roughness width and the roughness height, were built. The in-depth research of this problem is conducive to understanding of the hydrodynamic stability theory.

平板乱流境界層内部の Superstructure とバースト現象との関連性

平板乱流境界層内部の Superstructure とバースト現象との関連性

Transition due to streamwise streaks in a supersonic flat plate boundary layer

A possible bypass transition scenario in a Mach 3 flat plate boundary layer is proposed and investigated numerically.

Energy efficiency and performance limitations of linear adaptive control for transition delay

A reactive control technique with localised actuators and sensors is used to delay the transition to turbulence in a flat-plate boundary-layer flow. Through extensive direct numerical simulations, it is shown that an adaptive technique, which computes the control law on-line, is able to significantly reduce skin-friction drag in the presence of random three-dimensional perturbation fields with linear and weakly nonlinear behaviour. An energy budget analysis is performed in order to assess the net energy saving capabilities of the linear control approach. When considering a model of the dielectric-barrier-discharge (DBD) plasma actuator, the energy spent to create appropriate actuation force inside the boundary layer is of the same order as the energy gained from reducing skin-friction drag. With a model of an ideal actuator a net energy gain of three orders of magnitude can be achieved by efficiently damping small-amplitude disturbances upstream. The energy analysis in this study thus provides an upper limit for what we can expect in terms of drag-reduction efficiency for linear control of transition as a means for drag reduction.

A laminar kinetic energy transition model appropriate for hypersonic flow heat transfer

A laminar kinetic energy transition model appropriate for hypersonic flow heat transfer is proposed in this paper. Based on the laminar kinetic energy transition model at low speed, three rectifications are made to make it possess the ability to predict hypersonic flow transition. Firstly, turbulence length scale is revised to consider boundary layer information directly and reflect the kinematic wall effect correctly at hypersonic speed. Secondly, large-scale eddy viscosity is reconstructed to take hypersonic unstable modes into account to describe the procedure of flow instability. Additionally, for the sake of a better results of the full-turbulence simulation and to be compatible with the revised transition model, the eddy viscosity in sheer stress transport model is adopted to substitute for small-scale eddy viscosity. The model has been applied to a number of relevant test cases, including flat plate boundary layers and boundary layer transition over a blunt cone at different Reynolds numbers. The test cases demonstrate the capacity of the model to reproduce transition flow behavior with a reasonable degree of accuracy and reflect the effect of Reynolds number successfully. While it still needs further investigation to capture the overshoot of heat transfer and take more factors showing significant influence on transition into consideration.

Numerical Study of Nanosecond Pulsed Plasma Actuator in Laminar Flat Plate Boundary Layer

AbstractNanosecond (ns) pulsed dielectric barrier discharge (DBD) actuator in a laminar flat plate boundary layer is investigated numerically in an attempt to gain some new insights into the understanding of ns DBD actuation mechanism. Special emphasis is put on the examination, separation and comparison of behaviors of discharge induced micro shock wave and residual heat as well as on the investigation of response of external flow to the two effects. The shock wave is found to introduce highly transient, localized perturbation to the flow and be able to significantly alter the flow pattern shortly after its initiation. The main flow tends to quickly recover to close to its undisturbed state due to the transient nature of perturbation. However, with the shock decay and final disappearance, another perturbation source in the vicinity of discharge region, which contains contribution from both residual heat and shock, becomes increasingly pronounced and eventually develops into a perturbation wave train in the boundary layer. The perturbation is relatively weak and may not be a Tollmien-Schlichting (TS) wave and not trigger the laminar-turbulent transition of boundary layer. Instead, it is more likely to manipulate the flow stability to achieve the strong control authority of this kind of actuation in the case of flow separation control. In addition, a parametric study over the different electrical and hydrodynamic parameters is also conducted.

Drag reduction via wall‐normal oscillations in turbulent flat plate boundary layer flow at very high Reynolds number

AbstractThe effects of wall‐normal single point oscillations in turbulent boundary layers at very high Reynolds number are investigated by numerical simulation. The impact on the friction drag and on the turbulent structures is analyzed. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Instability and transition mechanisms induced by skewed roughness elements in a high-speed laminar boundary layer

The disturbance evolution in a Mach-4.8 zero-pressure-gradient flat-plate boundary-layer flow altered by discrete three-dimensional roughness elements is investigated including a laminar breakdown scenario. Direct numerical simulation (DNS), as well as the biglobal linear stability theory based on two-dimensional eigenfunctions in flow cross-sections, are applied. Roughness elements with high ratios of spanwise width to streamwise length are compared at varying height and skewing angles with respect to the oncoming flow. For an oblique roughness, the element’s height is varied between 27 % and 68 % of the undisturbed boundary-layer thickness. Compared to a symmetric roughness element an obliquely placed element generates a more pronounced low-speed streak in the roughness wake. The linear stability analysis reveals the occurrence of eigenmodes that can be associated with the first and second modes in the flat-plate flow. At identical roughness height, larger amplification is found for the eigenmodes of the oblique set-up. The results are confirmed by unsteady DNS showing very good agreement with stability theory; transient-growth behaviour in the near wake of the roughness is of minor importance. The comparison of the results gained for adiabatic wind-tunnel flow conditions with those for atmospheric-flight conditions with wall cooling reveals significant differences in the wake vortex system with subsequent impact on the stability properties of the flow. The hot-flow cases are less unstable at identical roughness Reynolds numbers. A variation of the wall cooling shows that the roughness-wake first- and second-mode behaviour is similar to that of the flat-plate flow: wall cooling stabilizes the first-mode and destabilizes the second-mode instabilities of the roughness wake.

Induced Velocity and Density Gradients due to Nanosecond Plasma Actuation

An experimental and numerical investigation is presented on the effects due to the energy deposition induced by a nanosecond pulsed dielectric barrier discharge plasma actuator on a flat-plate boundary layer. High magnification rate particle image velocimetry was used in a phase-locked arrangement to resolve the velocity field after actuation. The parameters investigated were the orientation of the actuator with respect to the flow direction and the energy input. The backcurrent shunt technique was employed to monitor the high-voltage pulse and to calculate the energy input. Results indicate that effects induced by this type of plasma actuator are strongly dependent on the orientation of the actuator with respect to the direction of the flow. When the covered high-voltage electrode is in the upstream position, a region of decelerated flow is observed in the vicinity of the actuator. On the other hand, when the covered high-voltage electrode is in downstream position, a region of accelerated flow is observed, highlighting the presence of a weak directional body force in addition to the thermal energy deposition typically associated with nanosecond plasma actuation. The thermal energy deposition is responsible for considerable density gradients, which are calculated using the compressible continuity equation on the measured velocity fields. Calculations show good qualitative agreement with experimental schlieren imaging. In general, nanosecond pulsed dielectric barrier discharge plasma actuators appear to induce a combination of thermal effects manifested as density/viscosity gradients and momentum effects manifested as weak directional body forces.

A high-performance finite-volume algorithm for solving partial differential equations governing compressible viscous flows on structured grids

This work focuses on the development of a high-performance fourth-order finite-volume method to solve the nonlinear partial differential equations governing the compressible Navier–Stokes equations on a Cartesian grid with adaptive mesh refinement. The novelty of the present study is to introduce the loop chaining concept to this complex fourth-order fluid dynamics algorithm for significant improvement in code performance on parallel machines. Specific operations involved in the algorithm include the finite-volume formulation of fourth-order spatial discretization stencils and optimal inter-loop parallelization strategies. Numerical fluxes of the Navier–Stokes equations comprise the hyperbolic (inviscid) and elliptic (viscous) ​components. The hyperbolic flux is evaluated using high-resolution Godunov’s method and the elliptic flux is based on fourth-order centered-difference methods everywhere in the computational domain. The use of centered-difference methods everywhere supports the idea of fusing modular codes to achieve high efficiency on modern computers. Temporal discretization is performed using the standard fourth-order ​Runge–Kutta method. The fourth-order accuracy of solution in space and time is verified with a transient Couette flow problem. The algorithm is applied to solve the Sod’s shock tube and the transient flat-plate boundary layer flow. The numerical predictions are validated by comparing to the analytical solutions. The performance of the baseline code is compared to that of the fused scheme which fuses modular codes via loop chaining concept and a significant improvement in execution time is observed.

Modulation of turbulence by dispersed solid particles in a spatially developing flat-plate boundary layer

Direct numerical simulations of particle-laden spatially developing turbulent boundary layers over a flat plate have been performed to investigate the effect of inertial particles on turbulence modulation, using the Eulerian–Lagrangian point-particle approach with two-way coupling. The particles are smaller than the Kolmogorov length scale of the dilute flow, and inter-particle collisions are not considered. The simulation results show that the addition of small solid particles increases the mean streamwise fluid velocity, which in turn leads to a reduction in the boundary layer integral parameters and an increase in the skin-friction drag. These effects become more pronounced as the particle Stokes number and mass loading increase. The streamwise turbulence intensity is slightly enhanced in the close vicinity of the wall but damped in the outer layer. In contrast, the Reynolds stress and the turbulence intensities in the wall-normal and spanwise directions are substantially attenuated across the entire boundary layer, and the levels of attenuation increase monotonically with both particle Stokes number and mass loading. The exchange of kinetic energy between particles and fluid indicates that particle–fluid interactions cause extra energy dissipation, which plays a crucial role in turbulence modulation.