摘要 :
The nonlinear interaction between Gortler vortices and two-dimensional Tollmien-Schlichting (TS) waves is studied with a spatial, nonparallel model based on the parabolized stability equations. The effect of the TS waves on the de...
展开
The nonlinear interaction between Gortler vortices and two-dimensional Tollmien-Schlichting (TS) waves is studied with a spatial, nonparallel model based on the parabolized stability equations. The effect of the TS waves on the development of the vortices is accounted for, showing that TS wave amplitudes of the same order of magnitude as the vortices result in significant nonlinear interaction. The range of governing parameters that has been studied so far is extended and the main effects of Gortler number, spanwise wave number, and initial amplitudes are identified. The study shows that the relative growth rates and initial amplitudes are the two most significant parameters. (C) 2000 American Institute of Physics. [S1070-6631(00)02905-6]. [References: 20]
收起
摘要 :
In this study, the spatial growth rates of supersonic streamwise vortices were investigated by inviscid linear stability analysis. The freestream Mach numbers were 2.5, 5.0, and 7.5. In previous measurements taken to define the st...
展开
In this study, the spatial growth rates of supersonic streamwise vortices were investigated by inviscid linear stability analysis. The freestream Mach numbers were 2.5, 5.0, and 7.5. In previous measurements taken to define the streamwise vortices, the stagnation temperature profile of supersonic flows is approximately uniform. This study found that the growth rate of vortices at the uniform stagnation temperature is smaller than that of isentropic vortices. The instability properties of the streamwise vortices can be explained by the ratio of the circulation to the axial velocity deficit, and also by the Mach number. Moreover, it is found that the compressibility effect, by which the instability reduces as the Mach number increases, is caused by the negative energy arising from the entropy gradient of supersonic vortices that accompanies the axial velocity deficit-like wake. From an energy perspective, the effect may reasonably be correlated with the large density perturbations in supersonic flows. This study also proposes a general convective Mach number for supersonic streamwise vortices. The normalized growth rates are shown to be a function of convective Mach number within the investigated range of ratio parameters.
收起
摘要 :
An experimental study was performed to analyze the stability of localized streaky structure in a Blasius boundary layer. An artificial streaky structure was created by using suction or blowing through a thin spanwise slot at the w...
展开
An experimental study was performed to analyze the stability of localized streaky structure in a Blasius boundary layer. An artificial streaky structure was created by using suction or blowing through a thin spanwise slot at the wall. The velocity gradient generated by the suction or blowing was controlled by a damper. The Reynolds number based on the displacement thickness delta(1) was Re-delta 1 = 280 at the slot. The behavior of the artificial streaky structure was scrutinized by damping the velocity gradient. It was found that the local streamwise and spanwise velocity gradients play a significant role in the formation of different types of instability. Artificial Tollmien - Schlichting ( T - S) wave packets were created by a loudspeaker to elucidate the interaction of the streaky structure with the T - S wave packets. The T - S wave packets imposed on the streaky structure become unstable when the frequency of the T S wave packets exceeds a certain critical frequency. The development of the T - S wave packets was investigated on the basis of the neutral stability curve.
收起
摘要 :
The present investigation deals with the boundary layer transition problem in relation to the fundamental aerodynamics of general airplanes. Focus has been especially placed on the mechanism of three-dimensional (3D) boundary laye...
展开
The present investigation deals with the boundary layer transition problem in relation to the fundamental aerodynamics of general airplanes. Focus has been especially placed on the mechanism of three-dimensional (3D) boundary layer transition and its control, which is meaningful not only for its fundamental interest but also important in relation to aero- and hydro-dynamic applications. In order to clarify the complicated transition mechanism, both flow visualizations and hot wire measurements are conducted in the transition region of general 3D boundary layer flows, namely, on spinning, yawed, and curved surfaces of the kind that often appear in surfaces of industrial fluid machinery. A unique attempt to delay such transition, applying an effective control method, is also reported. [References: 33]
收起
摘要 :
The fluid dynamics of microflows has recently commanded considerable attention because of their potential applications. Until now, with a few exceptions, most of the studies have been limited to low speed flows. This experimental ...
展开
The fluid dynamics of microflows has recently commanded considerable attention because of their potential applications. Until now, with a few exceptions, most of the studies have been limited to low speed flows. This experimental study examines supersonic microjets of 100-1,000 mu m in size with exit velocities in the range of 300-500 m/s. Such microjets are presently being used to actively control larger supersonic impinging jets, which occur in STOVL (short takeoff and vertical landing) aircraft, cavity flows, and flow separation. Flow properties of free as well as impinging supersonic microjets have been experimentally investigated over a range of geometric and flow parameters. The flowfield is visualized using a micro-schlieren system with a high magnification. These schlieren images clearly show the characteristic shock cell structure typically observed in larger supersonic jets. Quantitative measurements of the jet decay and spreading rates as well as shock cell spacing are obtained using micro-pitot probe surveys. In general, the mean flow features of free microjets are similar to larger supersonic jets operating at higher Reynolds numbers. However, some differences are also observed, most likely due to pronounced viscous effects associated with jets at these small scales. Limited studies of impinging microjets were also conducted. They reveal that, similar to the behavior of free microjets, the flow structure of impinging microjets strongly resembles that of larger supersonic impinging jets.
收起
摘要 :
Growth processes in supersonic streamwise vortices with linear unstable modes at Mach numbers 2.5 and 5.0 were numerically investigated using a weighted compact nonlinear scheme (WCNS) with three different accuracies. In the evolu...
展开
Growth processes in supersonic streamwise vortices with linear unstable modes at Mach numbers 2.5 and 5.0 were numerically investigated using a weighted compact nonlinear scheme (WCNS) with three different accuracies. In the evolution of the inviscid linear unstable mode m = -6 as a high-wavenumber mode, the growth rate and eigenfunction profiles of the mode numerically resolved were generally consistent with those obtained with the linear stability theory (LST) during the early transition stage, regardless of the computational accuracy. The numerical scheme used here can capture the growth of three-dimensional linear disturbance in structure. However, the numerical results obtained by nonlinear developments differed according to the accuracy. Among the different accuracies under the same grid resolution, the ninth-order accuracy scheme for the interpolation of primitive variables was able to capture small vortical structures at the downstream in the supersonic flow. In addition, the negative circulation generated and the total disturbance energy indicated that such high accuracy is effective in resolving the developed flow in supersonic vortices, even at moderate grid resolutions. (C) 2020 Elsevier Ltd. All rights reserved.
收起
摘要 :
In this paper we perform large eddy simulations of variable density mixing layers, which originate from initially laminar conditions. The aim of this work is to capture the salient flow physics present in the laboratory flow. This...
展开
In this paper we perform large eddy simulations of variable density mixing layers, which originate from initially laminar conditions. The aim of this work is to capture the salient flow physics present in the laboratory flow. This is achieved through varying the nature of the inflow condition, and assessing the vortex structure present in the flow. Two distinct inflow condition types are studied; the first is an idealised case obtained from a mean inflow velocity profile with superimposed pseudo-white-noise, and the second is obtained from an inflow generation technique. The inflow conditions generated have matching mean and root mean squared statistics. Validation of the simulations is achieved through grid dependency and subgrid-scale model testing. Regardless of the inflow condition type used, the change in growth rate of the mixing layer caused by the density ratio is captured. It is found that the spacing of the large-scale spanwise structure is a function of the density ratio of the flow. Detailed interrogation of the simulations shows that the streamwise vortex structure present in the mixing layer depends on the nature of the imposed inflow condition. Where white-noise fluctuations provide the inflow disturbances, a spatially-stationary streamwise structure is absent. Where the inflow generator is used, a spatially stationary streamwise structure is present, which appears as streaks in plan-view visualisations. The stationary streamwise structure evolves such that the ratio of streamwise structure wavelength to local vorticity thickness asymptotes to unity, independent of the density ratio. This value is in agreement with previous experimental studies. Recommendations are made on the requirements of inflow condition modelling for accurate mixing layer simulations.
收起
摘要 :
Flow and noise fields are explored for the concept of distributed propulsion. A model-scale experiment is performed with an 8:1 aspect ratio rectangular nozzle that is divided into six passages by five septa. The septa geometries ...
展开
Flow and noise fields are explored for the concept of distributed propulsion. A model-scale experiment is performed with an 8:1 aspect ratio rectangular nozzle that is divided into six passages by five septa. The septa geometries are created by placing plastic inserts within the nozzle. It is found that the noise radiation from the septa nozzle can be significantly lower than that from the baseline rectangular nozzle. The reduction of noise is inferred to be due to the introduction of streamwise vortices produced by secondary flow within each passage. Thus, the geometry of the internal passages of the septa nozzle can have a large influence. The flow evolution is profoundly affected by slight changes in the geometry. These conclusions are reached by experimental results of the flowfield aided by brief numerical simulations.
收起
摘要 :
Previous studies of the interaction between boundary layer streaks and Tollmien-Schlichting (TS) waves have shown puzzling effects. Streaks were shown to reduce the growth rate of primary TS waves and, thereby, to delay transition...
展开
Previous studies of the interaction between boundary layer streaks and Tollmien-Schlichting (TS) waves have shown puzzling effects. Streaks were shown to reduce the growth rate of primary TS waves and, thereby, to delay transition; however, they can also promote transition by inducing a secondary instability. The outcome of the interaction depends on the spanwise wavelength and intensity of the streaks as well as on the amplitude of the TS waves. A Floquet analysis of secondary instability is able to explain many of these features. The base state is periodic in two directions: it is an Ansatz composed of a saturated TS wave (periodic in x) and steady streaks (periodic in z). Secondary instability analysis is extended to account for the doubly periodic base flow. Growth rate computations show that, indeed, the streak can either enhance or diminish the overall stability of the boundary layer. The stabilizing effect is a reduction in the growth rate of the primary two-dimensional TS wave; the destabilizing effect is a secondary instability. Secondary instability falls into two categories, depending on the spanwise spacing of the streaks. The response of one category to perturbations is dominated by fundamental and subharmonic instability; the response of the other is a detuned instability. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3040302]
收起
摘要 :
We studied the stability of leeward streamwise vortices over a hypersonic inclined blunt cone under a flight condition by solving the twodimensional spatial eigenvalue problem (BiGlobal) and plane-marching parabolized stability eq...
展开
We studied the stability of leeward streamwise vortices over a hypersonic inclined blunt cone under a flight condition by solving the twodimensional spatial eigenvalue problem (BiGlobal) and plane-marching parabolized stability equations (PSE3D). The stability analyses were performed based on the laminar flow obtained by direct numerical simulation. Due to the azimuthal pressure gradient and large bluntness, a pair of large-scale inward vortices dominate in the vicinity of the leeward ray. Such vortical structures are different from previous studies, where outward vortices are most prominent. Two types of unstable modes are identified, namely, “inner mode” with low phase velocities and “outer mode” with high phase velocities. The inner modes are unstable in a wide frequency range compared to the outer modes, differing from previous studies where inner modes are deemed to lie in a much lower frequency range compared to the outer modes. Moreover, the inner modes, with supersonic phase velocities, are found to be able to radiate weak acoustics outside the vortices. Mack mode, belonging to the outer-mode instability, is most amplified that it is likely responsible for the breakdown of the vortices. BiGlobal and PSE3D results show good agreement regarding the downstream developments of the Mack mode and inner modes and yet exhibit remarkable discrepancies in tracing the evolution of the outer modes, highlighting the necessity of considering the non-parallel effects when studying the stability of streamwise vortices.
收起
原文获取