Excitation of shear layer instability at low Reynolds number via an unsteady inflow

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On the coupling between a supersonic boundary layer and a flexible surface

The coupling between a two-dimensional, supersonic, laminar boundary layer and a flexible surface is studied using direct numerical computations of the Navier-Stokes equations coupled with the plate equation. The flexible surface is forced to vibrate by plane acoustic waves at normal incidence emanated by a sound source located on the side of the flexible surface opposite to the boundary layer. The effect of the source excitation frequency on the surface vibration and boundary layer stability is analyzed. We find that, for frequencies near the fifth natural frequency of the surface or lower, large disturbances are introduced in the boundary layer which may alter its stability characteristics. The interaction between a stable two-dimensional disturbance of Tollmien-Schlichting (TS) type with the vibrating surface is also studied. We find that the disturbance level is higher over the vibrating flexible surface than that obtained when the surface is rigid, which indicates a strong coupling between flow and structure. However, in the absence of the sound source the disturbance level over the rigid and flexible surfaces are identical. This result is due to the high frequency of the TS disturbance which does not couple with the flexible surface

Numerical lnvestigation of Disturbance Environments in Low Pressure Turbines

Using a series of direct numerical simulations, the individual and cumulative effects of various disturbance environments existing in a low pressure turbine (LPT) are investigated. In particular, the effects of free-stream turbulence (FST), unsteady wakes, roughness and blade oscillations on the separation-induced transition on the suction surface of a low pressure turbine blade are analyzed. Two configurations are considered: (i) a flat plate subjected to streamwise pressure gradient representative of the suction surface of a low pressure turbine blade, (ii) a flat plate subjected to a convecting free-stream vortex of fixed strength and at a fixed height over the plate. The first configuration represents the ‘ultra high-lift’ blades for the next generation low pressure turbine. The local pressure gradient induced by the convecting vortex in the second configuration is representative of the adverse pressure gradient on the suction surface of a low pressure turbine blade. The results are validated against existing experimental or numerical data and it is demonstrated that the numerical framework has captured most of the phenomena to a reasonable level of accuracy. A kernel experiment for bypass transition is simulated for the vortex-induced instability. The effect of the convection speed and strength of the vortex are discussed and the paths of transition adopted are distinguished. At low disturbance levels, the transition to turbulence is primarily due to the breakdown of ‘Kelvin-Helmholtz’ roll up vortices. In the presence of aeroelastic blade oscillations, unsteady wakes, free-stream turbulence and roughness, transition takes the bypass route and the results show evidence of streamwise streaks. These streaks impart spanwise waviness to the separated shear layer and cause early destabilisation. The blade oscillation has an effect in reducing the separated region and hence, the profile loss, which is further accentuated in the presence of free-stream turbulence. A blade fluctuating at higher reduced frequency is found to be more effective in shrinking the separation region. Blade vibration is found to increase the level of pre-transitional fluctuations, without having a significant influence on the growth beyond separation. There is a cumulative effect in suppressing the separation region when blade oscillation and free-stream turbulence are studied in conjunction, although the additional effect of free-stream turbulence is marginal. A secondary separation bubble, noted in the unperturbed flow, is reduced in size with blade oscillation and further reduced in the presence of free-stream turbulence. The vortex-induced instability has been proposed to be a unit process of free-stream turbulence, the effect of which is studied in the presence of a discrete roughness element (similar in functionality to a trip wire). The roughness element triggers early transition by destabilizing the mean flow. Streaks are observed in the presence of the convecting and rotating cylinder, generating a vortex of fixed strength, and are enhanced by the presence of roughness in the pre-transitional zone. Enhanced spanwise waviness is noted with the roughness, leading to earlier breakdown to turbulence. The route of transition and the origin of three-dimensionality marked by the prominence of the vortex stretching is shown. An optimum range of convection speeds of the free-stream vortex is obtained and the maximum receptivity is noted at a speed of 0.386, which concurs with prior experiments on a periodic convecting vortex (Kendall, 1987). The unsteady wake has a direct effect on the velocity profile. A lag is noted between the wake passing and transition. While the wake convects at the local free-stream velocity, its impression in the boundary layer convects much slower, between 50% and 70% of the local free-stream velocity. Both unsteady wakes and blade oscillation promote near-wall mixing. The unsteady wakes and blade oscillations have a conjunctive effect on reducing the size of separation bubble. The secondary separation bubble observed in the unperturbed flow is reduced with the presence of wakes and is completely suppressed with the addition of the blade oscillation. Turbulent kinetic energy production increases with increasing perturbation levels, with the maximum effect seen for the combination of wakes and oscillation. A receptivity method based on disturbance enstrophy transport equation (DETE) is proposed. The disturbance enstrophy is evaluated as the difference between the instantaneous and mean enstrophies, and while these are positive definite quantities, the difference may not be so. This aspect of disturbance enstrophy has been used meaningfully to obtain new information about the flow instability, such as segmenting regions of flow instabilities into those dictated by positive growth rates of disturbance enstrophy, and those due to its negative counterpart. The positive growth rates are associated with large scale coherence in the free-stream whereas the negative growth rates are found to be arising in near-wall viscous structures (Sengupta et al., 2019a,b). The extension of this method to structure detection and comparison against existing vortex identification methods, indicates the ability of the DETE method to capture small-scale structures induced by the viscous term of the Navier- Stokes equation. DETE has been used for the two configurations operating with varying perturbation levels and valuable insight pertaining to the flow dynamics has been attained with respect to its budget terms. In particular, the role of vortex stretching in leading the flow to three-dimensionality is highlighted. The global and local spatio-temporal receptivity analysis of flows perturbed by plate oscillations, wakes and free-stream excitation yields a spatio-temporal wave front to be the causal mechanism for flow transition. This is essentially representative of the nonmodal part of the disturbance spectrum. While this flow instability has already been established for geophysical flows such as tsunami and other fluid dynamical problems such as for zero pressure gradient boundary layer formed over a semi-infinite flat plate excited from inside the shear layer, its role for pressure gradient dominated flows (such as in LPTs) is shown for the first time. The importance of nonlinearity and its dispersion effects is shown, specifically for flows excited at the free stream, even when the onset of disturbances follows a linear mechanism. The need for using a global, nonlinear, spatio-temporal setup is established in order to capture all pertinent flow physics.Cambridge India Ramanujan Scholarshi

Vortex shedding in a two-dimensional diffuser: theory and simulation of separation control by periodic mass injection

We develop a reduced-order model for large-scale unsteadiness (vortex shedding) in a two-dimensional diffuser and use the model to show how periodic mass injection near the separation point reduces stagnation pressure loss. The model estimates the characteristic frequency of vortex shedding and stagnation pressure loss by accounting for the accumulated circulation due to the vorticity flux into the separated region. The stagnation pressure loss consists of two parts: a steady part associated with the time-averaged static pressure distribution on the wall, and an unsteady part caused by vortex shedding. To validate the model, we perform numerical simulations of compressible unsteady laminar diffuser flows in two dimensions. The model and simulation show good agreement as we vary the Mach number and the area ratio of the diffuser. To investigate the effects of periodic mass injection near the separation point, we also perform simulations over a range of the injection frequencies. Periodic mass injection causes vortices to be pinched off with a smaller size as observed in experiments. Consequently, their convective velocity is increased, absorption of circulation from the wall is enhanced, and the reattached point is shifted upstream. Thus, in accordance with the model, the stagnation pressure loss, particularly the unsteady part, is substantially reduced even though the separation point is nearly unchanged. This study helps explain experimental results of separation control using unsteady mass injection in diffusers and on airfoils

Development and validation of a pressure based CFD methodology for acoustic wave propagation and damping

Combustion instabilities (thermo-acoustic pressure oscillations) have been recognised for some time as a problem limiting the development of low emissions (e.g., lean burn) gas turbine combustion systems, particularly for aviation propulsion applications. Recently, significant research efforts have been focused on acoustic damping for suppression of combustion instability. Most of this work has either been experimental or based on linear acoustic theory. The last 3-5 years has seen application of density based CFD methods to this problem, but no attempts to use pressure-based CFD methods which are much more commonly used in combustion predictions. The goal of the present work is therefore to develop a pressure-based CFD algorithm in order to predict accurately acoustic propagation and acoustic damping processes, as relevant to gas turbine combustors. The developed computational algorithm described in this thesis is based on the classical pressure-correction approach, which was modified to allow fluid density variation as a function of pressure in order to simulate acoustic phenomena, which are fundamentally compressible in nature. The fact that the overall flow Mach number of relevance was likely to be low ( mildly compressible flow) also influenced the chosen methodology. For accurate capture of acoustic wave propagation at minimum grid resolution and avoiding excessive numerical smearing/dispersion, a fifth order accurate Weighted Essentially Non-Oscillatory scheme (WENO) was introduced. Characteristic-based boundary conditions were incorporated to enable accurate representation of acoustic excitation (e.g. via a loudspeaker or siren) as well as enable precise evaluation of acoustic reflection and transmission coefficients. The new methodology was first validated against simple (1D and 2D) but well proven test cases for wave propagation and demonstrated low numerical diffusion/dispersion. The proper incorporation of Characteristic-based boundary conditions was validated by comparison against classical linear acoustic analysis of acoustic and entropy waves in quasi-1D variable area duct flows. The developed method was then applied to the prediction of experimental measurements of the acoustic absorption coefficient for a single round orifice flow. Excellent agreement with experimental data was obtained in both linear and non-linear regimes. Analysis of predicted flow fields both with and without bias flow showed that non-linear acoustic behavior occurred when flow reversal begins inside the orifice. Finally, the method was applied to study acoustic excitation of combustor external aerodynamics using a pre-diffuser/dump diffuser geometry previously studied experimentally at Loughborough University and showed the significance of boundary conditions and shear layer instability to produce a sustained pressure fluctuation in the external aerodynamic

Active Control of Coherent Structures in an Axisymmetric Jet

The primary objective of this work is to develop high-fidelity simulation model for jet noise control predictions and quantify the sound reduction when an external source frequency mode excitation is imposed on the jet flow. Whereas passive approaches using mixing devices, such as chevrons, have been shown to reduce low-frequency noise in jet engines, such approaches incur a performance penalty since they result in a reduced thrust. To avoid a performance penalty in reducing jet noise, the current work investigates a open-loop active noise control (ANC) system that utilizes a unsteady microjet actuator on the nozzle lip in the downstream direction to produce a desired effect on the jet flow-field dynamics thereby directly affecting the source source. In contrast to the passive approach, the proposed open-loop control design will utilize a local flow excitation device that can be turned off when not needed or adjusted according to the desired control signal. To make it feasible, the effectiveness of every forcing frequency mode has to be mapped for a certain jet velocity. This analysis considers an axisymmetric round jet at supersonic and subsonic speeds. Current studies are verified against previous low-order simulations conducted using Linearized Euler Equations (LEE), and compare qualitatively acheived noise reduction results against available experimental data. High-fidelity analysis, such as Detatched-Eddy Simulations (DES), was implemented using OpenFOAM, an open source CFD software. Results show that some excited frequency modes reduced the far-field jet noise by around 2 dB, supporting the use of unsteady microjet actuators as a jet noise reduction technology

Grtler Instability and Its Control via Surface Suction over an Axisymmetric Cone at Mach 6

The characteristics of Grtler instability over an axisymmetric cone with an aft concave section are studied via linear and nonlinear instability analysis and direct numerical simulations. Several options for the cone geometry have been investigated numerically, subject to a fixed forecone section and constraints on the maximum cone diameter, overall cone length, and minimum N-factor for the most amplified Grtler modes. Computations show that it is possible to design a cone with a peak N-factor of Nmax > 8 at the target Reynolds number of 12.110(exp 6) per meter, corresponding to the maximum quiet Reynolds number in the Boeing/AFOSR Mach-6 Quiet Tunnel at Purdue University. Direct numerical simulations show that an array of roughness elements corresponding to a peak roughness height of 0.1006 mm at the center can excite Grtler vortices that evolve into sufficiently strong streamwise streaks that may break down via high-frequency secondary instability. Thus, the selected axisymmetric configuration of interest should provide an acceptable baseline to investigate the feasibility of several aspects of laminar-flow control via boundary-layer suction. The apparatus that is being used for measurements in the Boeing/AFOSR Quiet Tunnel is described, along with some preliminary experimental results