Disturbances In Boundary Layer Research Articles

Experimental study on thermal flow characteristics in square serpentine heat exchangers mounted with louver-type turbulators

The present study aims to propose innovative louver-type turbulators to enhance the heat transfer rate in three major ways, i.e. core flow disturbance, jet impingement, and extended heat transfer surface. These louvers are installed in the twin-pass square channel with a hydraulic diameter (DH) of 45.5mm and a fully developed inlet condition. Three parameters are examined to find out the optimal design, including the pitch ratio (Pi/DH=1, 2, 3, 4, and ∞), the number of slat per half louver (1≤Ns≤4), and Reynolds number (5000≤Re≤20000). Particle Image Velocimetry (PIV) and Infrared Thermometry (IT) are respectively employed to measure the detailed velocity maps and wall temperature distributions. With acquired Nusselt number (Nu) ratio, the pressure measurements are also performed to estimate the Fanning friction factor (f) and further evaluate the thermal performance factor (TPF). The results show that both Nu‾/Nu∞ and f¯/f∞ ratios rise with descending Pi/DH and ascending Ns under the present test conditions. Among all the tested cases, the case with Pi/DH=1 and Ns=4 provides the highest Nu‾/Nu∞, almost twice the value of smooth reference; nevertheless, it suffers from high f¯/f∞ penalty. It is also found that the TPF level is a relatively weak function of Pi/DH. The new finding is that there exists a critical slat number of Ns=3 above which the TPF value is a weak function of Ns. In contrast, below the critical Ns the TPF value increases with decreasing Ns. From the viewpoint of heat transfer enhancement, one could apply the louvered channel as a heat exchanger with small Pi/DH and large Ns. The boundary layer disturbance, on the other hand, is more cost-effective than core flow disturbance as a mechanism to augment heat transfer from the viewpoint of thermal performance.

The effect of temperature of a cone nose-tip on the spectra of disturbances in the hypersound boundary layer

The results of an experimental study of the influence of local heating/cooling of a blunted cone nose-tip on the development of perturbations in the boundary layer for Mach number M = 6 are presented. A comparative analysis of the pressure-pulsation spectra on the cone surface has been carried out. It is shown that heating/cooling in the boundary layer of an acute cone leads to destabilization/stabilization of the perturbations of the second mode. For blunted cones, the opposite effect has been observed: heating leads to weak stabilization of disturbances, and cooling leads to destabilization. This effect is enhanced by increasing the bluntness radius of the cone nose-tip.

Application of a novel conical strip insert to improve the efficacy of water–Ag nanofluid for utilization in thermal systems: A two-phase simulation

In this research, hydrothermal characteristics and energy efficiency of the water–Ag nanofluid in a circular tube equipped with twisted conical strip inserts are investigated through the two-phase Eulerian–Lagrangian approach. The novel inserts under study are a combination of twisted tape inserts and conical strip inserts. Both staggered and non-staggered alignments for the strip inserts are examined. The effects of different forces including drag force, thermophoretic force, Brownian force and lift force are considered. Application of the strip inserts intensifies flow mixing and disturbance in boundary layer and therefore, increases Nusselt number and friction factor compared with the plain tube. The non-staggered alignment disturbs the boundary layer with a greater intensity than the staggered alignment, such that a completely swirling flow is developed for this configuration. The results show that heat transfer for the non-staggered alignment is higher than that for the staggered alignment, while the merit characteristic is greater for the staggered alignment. Moreover, the temperature and velocity distributions are more uniform for the non-staggered alignment. Additionally, Nusselt number augments by increasing either concentration or Reynolds number, whereas friction factor decreases with Reynolds number increment and intensifies by increasing the concentration. The nanofluid has a greater merit than pure water for utilization in this geometry. Meanwhile, the non-staggered alignment causes a more intense nanoparticle migration compared with the staggered alignment.

Swept-wing boundary-layer transition at various external perturbations: Scenarios, criteria, and problems of prediction

This experimental work is devoted to a parametric study of characteristics of the laminar-turbulent transition occurred in a model of a crossflow-dominated swept-wing boundary layer. The experiments are performed in 39 different regimes with low and elevated freestream turbulence levels both in the absence and in the presence of steady freestream vortices of various spanwise scales. The measurements are performed at several values of freestream velocity for two types of distributed surface roughness. Transition scenarios and criteria of turbulence onset are investigated and the problem of transition prediction is examined. It is shown that in all studied cases, the turbulence onset starts with the appearance of local high-frequency secondary instability of the base flow perturbed by primary instability modes. It is found that the 30% level of the threshold zero-to-peak amplitude of combined boundary-layer disturbances (i.e., the steady ones plus unsteady ones) can be used in all studied cases as a simplified turbulence-onset criterion, which was found in previous experiments at other flow and disturbance conditions. A very important, often decisive role of extremely weak steady free-steam disturbances produced by turbulence generating grids is investigated. A generalized combined variable N-factor method of transition prediction is suggested.

Extension of e N method to general three-dimensional boundary layers

In order to extend the eN method to general three-dimensional boundary layers, the conservation law of the imaginary parts for the wave parameters with a fixed wave vector is deduced. The compatibility relationship (CR) and the general theory of ray tracing (RT), which have been extensively used in conservative systems, are applied to a general three-dimensional boundary layer belonging to non-conservative systems. Two kinds of eN methods, i.e., the eN-CR method and the eN-RT method, are established. Both the two kinds of methods can be used to predict the evolutions of the spanwise wavenumber and the amplitude of the disturbances in general three-dimensional boundary layers. The reliability of the proposed methods is verified and validated by performing a direct numerical simulation (DNS) in a hypersonic general three-dimensional boundary layer over an aircraft model. The results are also compared with those obtained by other eN methods, indicating that the proposed methods have great potential applications in improving the transition prediction accuracy in general three-dimensional boundary layers.

Dual-scale micro/nanostructures for high-efficiency water collection

We describe a simple method of synthesizing dual-scale dome-like micro/nanostructures on copper using temperature controlled surface oxidation. The produced surfaces show two conflicting attributes, large contact angles and large droplet departure radii. Together, these two attributes promote water collection from air. A range of 80%-100% collection rate enhancements was observed for copper surfaces with a number of different wettability characteristics. Theoretical results showed that large departure droplets introduce strong disturbances in the diffusion boundary layer, which may be responsible for decreasing the vapor mass transfer resistance and enhancing the rate of water collection through moisture condensation.

Nonlinear unsteady streaks engendered by the interaction of free-stream vorticity with a compressible boundary layer

The nonlinear response of a compressible boundary layer to unsteady free-stream vortical fluctuations of the convected-gust type is investigated theoretically and numerically. The free-stream Mach number is assumed to be of $O(1)$ and the effects of compressibility, including aerodynamic heating and heat transfer at the wall, are taken into account. Attention is focused on low-frequency perturbations, which induce strong streamwise-elongated components of the boundary-layer disturbances, known as streaks or Klebanoff modes. The amplitude of the disturbances is intense enough for nonlinear interactions to occur within the boundary layer. The generation and nonlinear evolution of the streaks, which acquire an $O(1)$ magnitude, are described on a self-consistent and first-principle basis using the mathematical framework of the nonlinear unsteady compressible boundary-region equations, which are derived herein for the first time. The free-stream flow is studied by including the boundary-layer displacement effect and the solution is matched asymptotically with the boundary-layer flow. The nonlinear interactions inside the boundary layer drive an unsteady two-dimensional flow of acoustic nature in the outer inviscid region through the displacement effect. A close analogy with the flow over a thin oscillating airfoil is exploited to find analytical solutions. This analogy has been widely employed to investigate steady flows over boundary layers, but is considered herein for the first time for unsteady boundary layers. In the subsonic regime the perturbation is felt from the plate in all directions, while at supersonic speeds the disturbance only propagates within the dihedron defined by the Mach line. Numerical computations are performed for carefully chosen parameters that characterize three practical applications: turbomachinery systems, supersonic flight conditions and wind tunnel experiments. The results show that nonlinearity plays a marked stabilizing role on the velocity and temperature streaks, and this is found to be the case for low-disturbance environments such as flight conditions. Increasing the free-stream Mach number inhibits the kinematic fluctuations but enhances the thermal streaks, relative to the free-stream velocity and temperature respectively, and the overall effect of nonlinearity becomes weaker. An abrupt deviation of the nonlinear solution from the linear one is observed in the case pertaining to a supersonic wind tunnel. Large-amplitude thermal streaks and the strong abrupt stabilizing effect of nonlinearity are two new features of supersonic flows. The present study provides an accurate signature of nonlinear streaks in compressible boundary layers, which is indispensable for the secondary instability analysis of unsteady streaky boundary-layer flows.

Excitation Of Localized Wave Packet In 3d Supersonic Boundary Layer

The results of experimental study of excitation of localized in time and space controlled disturbances (wave packets) in a supersonic swept-wing boundary layer are presented. The experiments were performed at Mach number M = 2 on the model of wing with a lenticular profile and a 40 degrees sweep angle of the leading edge at zero angle of attack. Wave packets were generated by a pulse electric discharge on the surface of the model. A structure of controlled wave packet was studied. It was found that the wave packet has an asymmetric shape. Comparison with the case of twodimensional boundary layer was done.

Optimally growing boundary layer disturbances in a convergent nozzle preceded by a circular pipe

We report the findings from a theoretical analysis of optimally growing disturbances in an initially turbulent boundary layer. The motivation behind this study originates from the desire to generate organized structures in an initially turbulent boundary layer via excitation by disturbances that are tailored to be preferentially amplified. Such optimally growing disturbances are of interest for implementation in an active flow control strategy that is investigated for effective jet noise control. Details of the optimal perturbation theory implemented in this study are discussed. The relevant stability equations are derived using both the standard decomposition and the triple decomposition. The chosen test case geometry contains a convergent nozzle, which generates a Mach 0.9 round jet, preceded by a circular pipe. Optimally growing disturbances are introduced at various stations within the circular pipe section to facilitate disturbance energy amplification upstream of the favorable pressure gradient zone within the convergent nozzle, which has a stabilizing effect on disturbance growth. Effects of temporal frequency, disturbance input and output plane locations as well as separation distance between output and input planes are investigated. The results indicate that optimally growing disturbances appear in the form of longitudinal counter-rotating vortex pairs, whose size can be on the order of several times the input plane mean boundary layer thickness. The azimuthal wavenumber, which represents the number of counter-rotating vortex pairs, is found to generally decrease with increasing separation distance. Compared to the standard decomposition, the triple decomposition analysis generally predicts relatively lower azimuthal wavenumbers and significantly reduced energy amplification ratios for the optimal disturbances.

On the development of controlled stationary and travelling disturbances in the supersonic boundary layer of a swept wing

Experimental data on the evolution of controlled stationary and travelling disturbances in a 3D supersonic boundary layer, over a 45° swept-wing at Mach number 2.0, is presented. Travelling artificial disturbances were introduced in the boundary layer by periodical glow discharge, at a frequency of 20 kHz. Stationary disturbances were acquired by setting the roughness elements on the surface of the model. Spatial-temporal and spectral-wave characteristics of the wave train at the frequency 20 kHz, in the linear region of development, were obtained. It was found that the periodic modulation of mean flow can lead to the stabilization of unstable travelling disturbances in the supersonic boundary layer of a swept wing. These experiments have investigated the viability of using roughness elements to control laminar-turbulent transition.

境界層に導入された点撹乱の下流発達に与えるレイノルズ数の影響

境界層に導入された点撹乱の下流発達に与えるレイノルズ数の影響

乱流境界層における人工的に抽出した乱れ構造の周波数依存について

乱流境界層における人工的に抽出した乱れ構造の周波数依存について

Linear and nonlinear development of controlled disturbances in the supersonic boundary layer on a swept wing at Mach 2.5

Experimental data on the linear and nonlinear wave train development in 3D supersonic boundary layer over a 45° swept-wing at Mach number 2.5 are presented. Travelling artificial disturbances were introduced in the boundary layer by periodical glow discharge at frequencies 10 and 20 kHz. The spatial-temporal and spectral-wave characteristics of the wave train of unstable disturbances in the linear region are obtained. It is shown that the additional peaks in β'-spectra arise for both subharmonic and fundamental frequencies. The experiments indicate the presence of subharmonic resonance mechanism in 3D boundary layer at Mach number 2.5.

Experimental investigation on heat transfer enhancement of a tube with coiled-wire inserts installed with a separation from the tube wall

An experimental investigation was conducted for the thermo-hydraulic performance of a circular tube with coiled-wire inserts which were installed with a small separation from the inner wall of the tube. The wire inserts had an equilateral triangular cross-sections with a constant side length of e=6mm and they were coiled with three different pitch-to-diameter ratios: P/D=1, P/D=2, P/D=3. A specific method was employed to coil the wires so that an edge of the triangle was oriented to face the incoming air flow. The coiled-wire inserts were installed with 1 and 2mm separation from the inner tube wall so that the heat transfer enhancement due to the laminar boundary layer disturbance could be investigated. Experiments were performed for Reynolds numbers from 3429 to 26,663. The coiled-wire inserts led to a significant increase in both the heat transfer rate and friction factor over the smooth tube based on coil pitches and clearance. The maximum thermal performance was observed around 1.82 for the P/D=1, s=1 type at a Reynolds number of 3429. In conclusion, the laminar boundary layer disturbance can be effectively enhanced by using these types of coiled-wire inserts.

Experimental study of the effect of a passive porous coating on disturbances in a hypersonic boundary layer 2. Effect of the porous coating location

Experimental study of the effect of a passive porous coating on disturbances in a hypersonic boundary layer 2. Effect of the porous coating location

Probing horizontal convection instability via perturbation of the forcing boundary layer using a synthetic jet

Horizontal convection driven by a linear temperature profile along the bottom of a two-dimensional rectangular enclosure is perturbed by a small tuned synthetic (zero-net-mass-flux/ZNMF) jet. The jet permits instability in the horizontal convection boundary layer to be investigated in a controlled manner. At a Prandtl number Pr = 6.14 and Rayleigh number Ra = 2.5 × 108, slightly below the natural onset of instability, the boundary layer is found to be convectively unstable, exhibiting a disturbance pattern consistent with a Rayleigh–Bénard mechanism. Advection of the boundary layer disturbance gives rise to unsteadiness in the vertical end-wall plume. Nusselt number is enhanced across a range of frequencies for all perturbation amplitudes, with the response dominated by two frequencies differing by approximately a factor of two: each invokes the periodic shedding of vorticity into the vertical end-wall plume at the higher frequency. This suggests that there exists a natural sensitivity in this flow to disturbances convecting in the upstream boundary layer. That is, a convective instability in the horizontal convection boundary layer serves as a disturbance amplifier, with the end-wall plume controlling the frequency of the resulting instability mode. The increase in Nusselt number achieved by a jet with peak speed of the same order as the horizontal convective velocity in the unperturbed flow is consistent with a greater than two-fold increase in Rayleigh number.

Boundary-layer disturbances subjected to free-stream turbulence and simulation on bypass transition

The phenomena associated with the entrainment of free-stream turbulence (FST) into boundary-layer flows are relevant for a number of subjects. It has been believed that the continuous spectra of the Orr-Sommerfeld (O-S)/Squire equations describe the entrainment process, and thus they are used to specify the inlet condition in simulation of bypass transition. However, Dong and Wu (Dong, M. and Wu, X. On continuous spectra of the Orr-Sommerfeld/Squire equations and entrainment of free-stream vortical disturbances. Journal of Fluid Mechanics, 732, 616–659 (2013)) pointed out that continuous spectra exhibit several non-physical features due to neglecting the non-parallelism. They further proposed a large-Reynolds-number asymptotic approach, and showed that the non-parallelism is a leading-order effect even for the short-wavelength disturbance, for which the response concentrates in the edge layer. In this paper, the asymptotic solution is verified numerically by studying its evolution in incompressible boundary layers. It is found that the numerical results can be accurately predicted by the asymptotic solution, implying that the latter is adequate for moderate Reynolds numbers. By introducing a series of such solutions as the inflow perturbations, the bypass transition is investigated via the direct numerical simulation (DNS). The transition processes, including the evolution of streaks, the amplification of secondary-instability modes, and the emergence of turbulent spots, agree with the experimental observations.

Linear development of controlled disturbances in the supersonic boundary layer on a swept wing at Mach 2

Experimental results are presented on the linear development of a wave train in a three-dimensional supersonic boundary layer on a swept wing with 45°-sweep angle at Mach number M = 2.0. Artificial disturbances in the boundary layer were excited by periodical glow discharge mainly at frequencies 10 and 20 kHz. The maximum of the controlled pulsation amplitude was localized in the boundary layer in normal to the swept-wing surface direction for spatial amplitude distributions as well as for the amplitude wave spectra. The asymmetry of the wave characteristics of the unstable disturbances in the boundary layer of a swept wing caused by the presence of crossflow is confirmed. The stability characteristics, including dispersion relations and amplification rates over a wide range of wave inclination angles, are obtained for disturbances with prescribed frequencies 10 and 20 kHz.

Entrainment of short-wavelength free-stream vortical disturbances in compressible and incompressible boundary layers

The fundamental difference between continuous modes of the Orr–Sommerfeld/Squire equations and the entrainment of free-stream vortical disturbances (FSVD) into the boundary layer has been investigated in a recent paper (Dong & Wu, J. Fluid Mech., vol. 732, 2013, pp. 616–659). It was shown there that the non-parallel-flow effect plays a leading-order role in the entrainment, and neglecting it at the outset, as is done in the continuous-mode formulation, leads to non-physical features of ‘Fourier entanglement’ and abnormal anisotropy. The analysis, which was for incompressible boundary layers and for FSVD with a characteristic wavelength of the order of the local boundary-layer thickness, is extended in this paper to compressible boundary layers and FSVD with even shorter wavelengths, which are comparable with the width of the so-called edge layer. Non-parallelism remains a leading-order effect in the present scaling, which turns out to be more general in that the equations and solutions in the previous paper are recovered in the appropriate limit. Appropriate asymptotic solutions in the main and edge layers are obtained to characterize the entrainment. It is found that when the Prandtl number $\mathit{Pr}<1$, free-stream vortical disturbances of relatively low frequency generate very strong temperature fluctuations within the edge layer, leading to formation of thermal streaks. A composite solution, uniformly valid across the entire boundary layer, is constructed, and it can be used in receptivity studies and as inlet conditions for direct numerical simulations of bypass transition. For compressible boundary layers, continuous spectra of the disturbance equations linearized about a parallel base flow exhibit entanglement between vortical and entropy modes, namely, a vortical mode necessarily induces an entropy disturbance in the free stream and vice versa, and this amounts to a further non-physical behaviour. High Reynolds number asymptotic analysis yields the relations between the amplitudes of entangled modes.

Growth and Characterization of Pencil-Like ZnO Nanowires in the Presence of a Disturbance in Boundary Layer

Pencil-like zinc oxide (ZnO) nanowire was synthesized on Si (111) substrate through a simple vapor phase method using a mixture of zinc oxide and graphite as the source material. The source inside a quartz tube created a Zn-rich vapor that facilitated the formation and growth of ZnO nanowires. Field emission scanning electron microscopic studies indicated that pencil-like ZnO nanowires had a size of the range from 50 to 150 nm in diameter and several microns in length. X-ray diffraction was used to investigate the crystal structure of ZnO nanowires. Raman scattering and photoluminescence were applied to characterize the optical properties of the pencils. The growth mechanism of the nanopencils was discussed based on the growth conditions.