Response of a Transitioning Hypersonic Boundary Layer to Repetitive Laser–Surface Interactions

The influence of the boundary layer (BL) thickness on the tonal noise generated by subsonic impinging jets is investigated. For that, initially laminar jets at Mach numbers$0.6$and$0.9$with BL thicknesses$0.05r_0$,$0.1r_0$and$0.2r_0$, where$r_0$is the pipe-nozzle radius, impinging on a plate at$6r_0$from the nozzle, are simulated. For Mach number$0.9$, acoustic tones due to feedback loops of axisymmetric nature between the nozzle and the plate are generated at frequencies that do not vary with the BL thickness. The two dominant tones are, however,$17$and$26\ \mathrm {dB}$stronger for the thickest BL compared with the thinnest one. For Mach number$0.6$, for the thinnest BL no acoustic peaks appear, as observed in the experiments of the literature, but narrow peaks resulting from axisymmetric feedback loops emerge for thicker BLs. Therefore, low subsonic impinging jets can be resonant for specific nozzle-exit conditions. The increase in tone amplitude for Mach number$0.9$, and the establishment of feedback loops for Mach number$0.6$with increasing BL thickness, are found to result from two changes in the jet flow. The first change is that the shear-layer laminar–turbulent transition occurs farther downstream for a thicker BL, leading to a greater predominance of the axisymmetric aerodynamic fluctuations near the plate. The second change is that the amplification of the flow fluctuations between the nozzle and the plate at the tone frequencies is stronger for thicker BLs.

Pulsed arc plasma excitation is characterized by strong local heating effect and wide disturbance range, and it has a broad application prospect in supersonic flow control. In this paper, by using electrical parameter measurement system and high speed schlieren technique, we study the electrical and flow field characteristics of pulsed arc plasma excitation under the condition of <i>Ma</i> = 3 incoming flow. The nano-particle planar laser scattering (NPLS) is used to investigate the flow structure of the supersonic flat boundary layer, and the transition characteristics of the boundary layer at different plasma excitation frequencies are studied. The experimental results show that in the flow field with <i>Ma</i> = 3 and the total incoming pressure <i>P</i><sub>0</sub> = 1 atm (1 atm = 1.01 × 10<sup>5</sup> Pa), the peak voltage of the pulsed arc plasma actuator discharge is 6 kV, the peak current is 70 A, the time scale of the discharge is about 300 ns, the single discharge energy is 70 mJ; the pulsed arc discharge will produce the precursor shock wave with higher velocity and the thermal deposition zone with higher temperature, which will exert continuously disturbance on the boundary layer. The pulsed arc plasma excitation with perturbation can promote the transition of supersonic plate boundary layer. Moreover, the high-frequency impact effect of pulsed discharge can promote the transition to occur ahead of time, and the higher the frequency, the better the effect is. As the excitation frequency increases, the transition position of the boundary layer of the supersonic flat plate moves forward, and the length of the transition area of the boundary layer becomes shorter as the excitation frequency increases. When the excitation frequency is 60 kHz, the length of transition zone is 0 and the thickness of turbulent boundary layer is 25 mm. When a high frequency is applied (<i>f</i> = 40, 60 kHz), the transition path of the boundary layer is that the shock wave generated by the plasma excitation triggers the unstable wave, and the development of unstable waves directly skips the linear growth stage, passes through the bypass and transitions into turbulent flow. The pulsed arc plasma excitation can be used to promote supersonic boundary layer transition.

This study uses numerical methods to investigate the influence of bow shape and model scale on the natural transition in bow boundary layers on underwater axisymmetric bodies with a superhydrophobic surface. The transition locations for eight bow shapes and four model scales are predicted by using the eN method, which is based on a linear stability theory. The results show that the superhydrophobic surface always stabilizes the boundary layer and delays the transition for different bow shapes and model scales. However, differences are observed in the basic laminar flow, boundary-layer stability, and transition location when different bow shapes are used. These differences are not prominent at the small model scale, but become significant at the medium and large model scales. As the model scale increases, the transition location in the boundary layer on the superhydrophobic surface moves upstream and then downstream, leading to a “dangerous” scale, at which the transition location is closest to the leading edge. Furthermore, this study provides the optimal bow shape with the latest transition location at each model scale, and the optimal bow shape with the longest transition delay distance.

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.

The relationship between free-stream turbulence and boundary layer behavior has been investigated using spectral measurements. The power spectral densities of turbulence quantities in transitional and fully turbulent boundary layers were computed and compared to the power spectra of the same quantities measured in the free stream. Comparisons were made using the “transfer function.” The transfer function is the ratio of two spectra at each frequency in the spectra. Comparisons were done in flows with low (0.6 percent) and high (8 percent) free-stream turbulence intensities. Evidence was gathered that suggests that relatively low-frequency, large-scale eddies in the free stream buffet the boundary layer, causing boundary layer unsteadiness at the same low frequencies. These fluctuations are present in both transitional and fully turbulent boundary layers. They are seen under both high and low free-stream turbulence conditions, although they are stronger in the high-turbulence case. Examination of the turbulent shear stress suggests that the low-frequency fluctuations enhance transport in the boundary layer but they are not so effective in promoting eddy transport as are turbulent eddies produced and residing within the boundary layer. In the fully-turbulent boundary layer, higher-frequency fluctuations are added to the low-frequency unsteadiness. These higher-frequency fluctuations, not seen in the transitional boundary layer, are associated with turbulence production in the boundary layer and appear not to be directly related to free-stream unsteadiness. [S0889-504X(00)00403-7]

Purpose – The purpose of this paper is to report a numerical solution for the problem of steady, two dimensional boundary layer buoyant flow on a vertical magnetized surface, when both the viscosity and thermal conductivity are assumed to be temperature-dependent. In this case, the motion is governed by a coupled set of three nonlinear partial differential equations, which are solved numerically by using the finite difference method (FDM) by introducing the primitive variable formulation. Calculations of the coupled equations are performed to investigate the effects of the different governing parameters on the profiles of velocity, temperature and the transverse component of magnetic field. The effects of the thermal conductivity variation parameter, viscosity variation parameter, magnetic Prandtl number Pmr, magnetic force parameter S, mixed convection parameter Ri and the Prandtl number Pr on the flow structure and heat transfer characteristics are also examined. Design/methodology/approach – FDM. Findings – It is noted that when the Prandtl number Pr is sufficiently large, i.e. Pr=100, the buoyancy force that driven the fluid motion is decreased that decrease the momentum boundary layer and there is no change in thermal boundary layer is noticed. It is also noted that due to slow motion of the fluid the magnetic current generates which increase the magnetic boundary layer thickness at the surface. It is observed that the momentum boundary layer thickness is increased, thermal and magnetic field boundary layers are decreased with the increase of thermal conductivity variation parameter =100. The maximum boundary layer thickness is increased for =100 and there is no change seen in the case of thermal boundary layer thickness but magnetic field boundary layer is deceased. The momentum boundary layer thickness shoot quickly for =40 but is very smooth for =50.There is no change is seen for the case of thermal boundary layer and very clear decay for =40 is noted. Originality/value – This work is original research work.

Experimental exercise has been conducted to validate the capability of a new test apparatus. The test stand has been designed and constructed at the laboratory of aerodynamics, University of Defence to carry out the experimental investigation of transitional flow prediction and development over flat plate. The test facility consists of a rectangular duct set on the suction side of air source apparatus. The working section is 2 m long with a cross section of 0.44 m in width and 0.25 m in height. The exercise is performed into two stages. In the first stage, the basic parameters such as freestream velocity, turbulence intensity and pressure gradient in streamwise direction were measured and manipulated to set- up acceptable values. Second stage of the exercise, the bottom wall of the test section was used as a flat plate model to conduct turbulent boundary-layer experiment. The characteristics of the boundary layer obtained by using the apparatus are represented by a qualitative and quantitative agreement with those predicted by boundary-layer theory for turbulent boundary layer while more improvements seems to be required to satisfy the rules of boundary layer stability experiments. The results are show a fair agreement for mean velocity profile, U� , boundary layer thickness, � , momentum thickness, , and skin friction coefficient, Cf

There are three types of flow structures known as laminar, transitional, and turbulent flows in the boundary layer adjacent to an object submerged in a stream For being able to obtain the most efficient numerical simulation of these flows, the equations must be linearized as much as possible. Based on the previous studies about the linearization of the equations of both the laminar flow and the turbulent flow, the work is now extended to the transitional flow. An interactive boundary condition is also employed and completely linearized so as to possess the potential of effectively simulating the separation flow in the transitional region. The set of linearized equations are then lined up to fit the Full Bordering Algorithm.

The influences of the forward-facing step (FFS) and backward-facing step (BFS) on the 7° half-angle conical boundary layer instability and transition are investigated at different angles-of-attack (AoAs) in a hypersonic quiet wind tunnel using the Nano-tracer-based Planar Laser Scattering techniques, Temperature-Sensitive Paints, and high-frequency pressure sensors. The results show that the FFS stabilizes the second mode instability but strongly destabilizes the crossflow instability. Conversely, the BFS destabilizes both the second mode and crossflow instabilities, yet its impact on crossflow instability is weaker compared to that of an FFS with an identical height. At a small AoA (AoA < 3° in this paper), the boundary layer transition is dominated by the second mode instability. For sharp cones, the transition is delayed on the windward side but promoted on the leeward side, resulting in a monotonically inclined transition front. In contrast, blunt cones exhibit localized depressions in the transition front on the leeward side. At large AoA, the transition process is dominated by the crossflow instability, resulting in heat flux stripes on the leeward side, making the transition front distribution more complex. Therefore, at small AoAs, the promotion effect of the BFS on the conical boundary layer transition is stronger than that of the FFS with the same height. Moreover, the FFS with a small height even exhibits a suppressive effect on the transition. However, as the AoA increases, low-frequency instability modes, such as crossflow instability, gradually become dominant. Consequently, the promoting effect of the FFS surpasses that of the BFS.

The present work focuses on the evolution of multi-scale characteristics and inter-scale interaction in a transitional curve-wall boundary layer, which was triggered by free-stream turbulence (FST), via time-resolved two-dimensional (2D) particle image velocimetry measurement. Using multi-component variational mode decomposition, 2D velocity fields are decomposed into three intrinsic mode functions (IMFs). The proper orthogonal decomposition was then applied to each IMF to reveal the corresponding statistical representative structures with various characteristic length scales. It is found that the first IMF primarily depicts low-frequency large-scale motions inside the boundary layer, which are characterized as the Klebanoff mode in the pre-transition region, as well as inclined shear layer and lift-up of low-speed streaky structures in the nonlinear stage. Meanwhile, the second IMF is characterized as mild-to-high-frequency small-scale motions, whose quick amplification leads to the secondary streak instability. Analysis of the amplitude and frequency modulation effects shows that the top-down and bottom-up scenarios alternately govern the inter-scale interaction in various transition stages. Before the transition onset, a reversal top-down scenario depicts that the small-scale fluctuations in FST positively modulate Klebanoff-mode streaky structures in the near-wall region, while in the nonlinear transition stage, the bottom-up modulation is characterized as positive amplitude and frequency modulation in the near-wall region, as well as negative ones in higher flow layer, the latter of which is attributed to the lift-up of near-wall streaky structures and their subsequent burst and breakdown.

In a linear cascade of low pressure turbine, LPT, blades, the position and strength of the vortices forming the endwall flows depends on the state of the inlet endwall boundary layer, i.e., whether it is laminar or turbulent. The latter will determine, amongst other effects, the location where the inlet boundary layer rolls up into a passage vortex, the amount of fluid that gets swept up by the passage vortex and the interaction with the pressure side separation bubble, de la Rosa Blanco et al. [1]. As a consequence, the mass-averaged stagnation pressure loss and therefore the design of a low-pressure turbine are influenced by the state of the inlet endwall boundary layer. The paragraph above highlights the importance of determining the state of the boundary layer along the endwalls if an understanding of the endwall flows in a LPT at realistic engine conditions is sought. The results presented in this paper are taken from hot film measurements performed on the endwalls of selected nozzle guide vanes from the fourth stage of the Affordable Near Term Low Emission, ANTLE, LPT rig. These results are compared with those from a low speed linear cascade of similar LPT blades. In the cold flow four-stage LPT rig, a transitional boundary layer has been found on the platforms upstream of the leading edge of the blades. The boundary layer is more turbulent nearer the leading edge of the blade and for higher Reynolds numbers. As for the passage, for both the cold flow four-stage rig and the low speed linear cascade, the new inlet boundary layer formed behind the pressure leg of the horseshoe vortex is a transitional boundary layer. The transition process progresses from the pressure to the suction surface of the passage in the direction of the secondary flow.

Particle fluid mechanics in shear flows, acoustic waves, and shock waves

The endwall flow features are heavily dependent on the incoming boundary layer. It was particularly important to increase understanding the effect of inlet boundary layer thickness on endwall secondary flow under unsteady conditions. In present study, the influences of incoming wakes and various boundary layer thickness on endwall secondary flow were studied in a typical high-lift low-pressure turbine cascade, numerical calculation and experiment measurement of seven-hole probe were adopted at Re = 25,000 (based on the inlet velocity and the axial chord). Upstream wakes were simulated through moving rods upstream of the cascade. Detailed analysis was focused on the mechanisms of periodic wake influencing on the endwall vortex structures under thick endwall boundary layer condition. Influences of two different endwall boundary layer thickness on endwall secondary vortices structures were also comparatively analyzed. Under steady condition without wake, although thick incoming boundary layer reduces the cross-passage pressure gradient near endwall, more low momentum fluid inside thick endwall boundary layer is drawn into secondary vortices, finally resulting in stronger the pressure side leg of the leading edge horseshoe vortex and passage vortex, compared to the results of thin boundary layer condition. Under unsteady condition with thick inlet boundary layer, the “negative jet” effect of incoming wakes delays intersection of pressure side leg and suction side leg of leading edge horseshoe vortex on blade suction surface. The time-averaged strength of passage vortex and counter vortex core decreases by about 32%, and the underturning and overturning of endwall secondary flow is suppressed. The instantaneous results also indicate the endwall secondary vortices are reduced periodically at the position of wakes passing.

The role of chemical boundary layers in regulating the thickness of continental and oceanic thermal boundary layers

The oblique shock impinging on the supersonic turbulent boundary layer leads to a mixing layer and the emergence of large-scale coherent structures within the interaction zone which leave significant velocity defect and turbulence amplification downstream. In the present study, we investigate the turbulence recovery in the post-shock region by exploiting direct numerical simulation data of the oblique-shock/turbulent boundary layer interaction flow at the incoming Mach number of $2.28$ and the shock angle of $33.2^\circ$ , with special attention paid to the contribution of the mixing layer and large-scale structures to flow dynamics. For that purpose, we propose to split the mean velocity, Reynolds stresses and spanwise spectra into a canonical portion that is constructed according to the statistics of canonical turbulent boundary layers, and a mixing-layer-induced portion. We found that the hidden mixing layer grows with the boundary layer thickness and that the induced mean shear and Reynolds stresses decay at different rates. The mean velocity recovers to the canonical profiles at a distance of 13 boundary layer thicknesses downstream where the mixing-layer-induced mean shear ceases to have strong impacts. The recovery of Reynolds stresses requires 10 boundary layer thicknesses in the near-wall region but a much longer streamwise extent in the outer region due to the slow decay of large-scale motions. These large-scale motions superpose on the near-wall turbulence, intensifying the turbulent fluctuations, yet having a trivial impact on the skin friction, for the contribution of the mixing-layer-induced mean shear and Reynolds shear stress are balanced by the advection term. We further establish a simple physical model capable of approximately predicting the streamwise evolution of mixing-layer-induced mean shear and turbulent kinetic energy. This model suggests that the complete recovery of turbulence in the outer region requires a streamwise extent of approximately 50 boundary layer thicknesses.