Effect of de-correlating turbulence on the low frequency decay of jet-surface interaction noise in sub-sonic unheated air jets using a CFD-based approach

In this paper we extend the Rapid-distortion theory (RDT)-based model derived by Goldstein, Afsar & Leib (J. Fluid Mech., vol. 736, pp. 532-569, 2013) for the sound generated by the interaction of a large-aspect-ratio rectangular jet with the trailing edge of a flat plate to include a more realistic upstream turbulence spectrum that possess a de-correlation (i.e. negative dip) in its space-time structure and use results from three-dimensional Reynolds-Averaged Navier-Stokes (RANS) solutions to determine the mean flow, turbulent kinetic energy and turbulence length & time scales. Since the interaction noise dominates the low-frequency portion of the spectrum, we use an appropriate asymptotic approximation for the Rayleigh equation Green’s function, which enters the analysis, based on a two-dimensional mean flow representation for the jet. We use the model to predict jet-surface interaction noise for a range of subsonic acoustic Mach number jets, nozzle aspect ratios, streamwise and transverse trailing-edge locations and compare them with experimental data. The RANS meanflow computations are also compared with flow data for selected cases to assess their validity. We find that finite de-correlation in the turbulence spectrum increases the low-frequency algebraic decay (the low-frequency “roll-off”) of the acoustic spectrum with angular frequency to give a model that has a pure dipole frequency scaling. This gives better agreement with noise data compared to Goldstein et al. (2013) for Strouhal numbers less than the peak jet-surface interaction noise. For example, through sensitivity analysis we find that there is a difference of 10 dB at the lowest frequency for which data exists (relative to a model without de-correlation effects included) for the highest acoustic Mach number case. Secondly, our results for the planar flow theory provide a first estimate of the low-frequency amplification due to the jet-surface interaction for moderate aspect ratio nozzles when RANS meanflow quantities are used appropriately. This work will hopefully add to noise prediction efforts for aircraft configurations in which the exhaust systems are tightly integrated with the airframe

A general approach to modeling jet-turbulence interaction problems using rapid-distortion theory

In this talk we present recent developments in the Rapid-distortion theory of turbulence when applied to the prediction of sound radiated by non-homogeneous turbulence interacting with a trailing edge of a semi-infinite flat plate positioned parallel to the level curves of an otherwise arbitrary mean flow field. The latter problem has received much attention in Aero-acoustics research community and is a canonical representation of jet installation effects

An LES investigation of the 2D Tollmien-Schlichting wave instability in channel flow

Classical linear hydrodynamic stability analysis predicts the existence of an unstable 2D ‘Tollmien-Schlichting’ (T-S) wave, which may be excited in parallel boundary layer flows by mechanisms of receptivity. Of particular interest is the evolution of these disturbances beginning with the linear growth phase governed by the Orr-Sommerfeld equation, subsequent non-linearity with the development of 3D flow structures and the breakdown to turbulent flow. Understanding the mechanisms by which these unstable waves derive energy from the mean flow has significance with regard to the aim of maintaining laminar boundary layer flow and reducing the drag generated by aerodynamic surfaces. In this paper we solve the Orr-Sommerfeld eigenvalue problem numerically via a spectral method in MATLAB, approximating the solution via a truncated series of Lagrange polynomials. The stream function of the single unstable mode was found from the spectral solution from which the streamwise and wall-normal components of the perturbation’s velocity were derived

Improved predictions of trailing-edge noise using rapid-distortion theory and CFD data

We show the effect of correcting the acoustic spectrum formula for a high speed jet interacting with a trailing edge of a semi-infinite flat plate derived by Goldstein et al., (2017, J Fluid Mech. vol. 824, p. 477) using the Rapid-distortion theory (RDT). The correction involves using the next order term for the amplitude function in the WKBJ approximation for the scattered pressure. The high frequency roll-off of the acoustic spectrum predictions that we obtain using this correction are closer to experiment for low speed jets. In the talk we will discuss the role of using CFD data in the model

Investigating incipient separation for an aerofoil at an arbitrary orientation subjected to on oncoming gust

The purpose of this paper is to develop an efficient method for analysing the gust response of an aerofoil using CFD. The new method was then applied to investigate how varying the characteristics of a sinusoidal gust affects flow separation over a NACA0012 aerofoil. A two-dimensional periodic gust was implemented in ANSYS Fluent using the User-Defined Function tool. A specialised mesh was designed to allow for accurate modelling of a gust past a NACA0012 aerofoil. The features a rounded trailing edge and an O-grid structure with 164,000 cells. An incompressible SIMPLE pressure-based solver was used in ANSYS Fluent. The k-ωSST turbulence with low Reynold’s corrections was chosen due to its effectiveness in modelling pressure-induced flow separation. The velocity-inlet boundary was defined by the user-defined function velocity components with an angle of attack, α

Streaks in high-speed boundary layers : assessment via the full nonlinear boundary-region equations

Streamwise vortices and the associated streaks evolve in boundary layers over flat orconcave surfaces as a result of various disturbances initiated in the upstream or from the wall surface. Following the transient growth phase, the fully-developed vortex structures become susceptible to inviscid secondary instabilities resulting in early transition to turbulence via bursting processes. In the incompressible regime, a vast body of work hasbeen devoted to understand the initiation and development of these streaks, as well as the conditions under which they undergo secondary instabilities. For high-speed boundary layers, on the other hand, additional complications due to the compressibility and thermal effects arise, the level of contribution of which scales with the Mach number. In this paper, we study streaks in high-speed boundary layers via the numerical solution to the full nonlinear boundary region equations, which is the high Reynolds number asymptotic form of the Navier-Stokes equations, under the assumption that the streamwise wave numberof the disturbances is much smaller than the wave numbers associated with the crossflow directions, commensurate with long streamwise wavelength of the primary vortex disturbance. The effect of the spanwise separation of the vortices and the Mach number, which is varied between high-subsonic (M= 0.8) to low-hypersonic (M= 6) regimes, is quantified and discussed

Structure of turbulence prior to rapid-distortion

Rapid-distortion theory (RDT) uses linear analysis to study the interaction of turbulence with solid surfaces. It applies whenever the turbulence intensity is small and the length (or time) scale over which the interaction takes place is short compared to the length (or time) scale over which the turbulent eddies evolve. When both of these conditions are interpreted asymptotically, it implies that the upstream boundary condition that enters as an input to a scattering problem (given by the term,ωc, below) may be specified at an infinite distance from the surface discontinuity on the scale of the interaction but one that is still short on the scale of the turbulence evolution. In this paper we develop the mathematical theory for the leading edge interaction of turbulence by considering a canonical problem of a jet flow interacting with a semi-infinite infinitesimally thin flat plate positioned parallel to the level curves of the mean flow field. To fix ideas we consider a constant shear flow and derive a formula for the two-point velocity correlation function

Effects of cigarette smoking on erythrocyte sedimentation rate, platelet count, total and differential leucocyte counts in adult male smokers: Blood parameters effected by cigarette smoking in males

Smoking is one of the leading causes of death worldwide. Smokers have higher risk for coronary heart disease, atherosclerosis, acute myocardial infarction, hypertension, clotting disorders, inflammation, respiratory diseases, cancers, etc. A cigarette smoker is exposed to a number of harmful substances. In this study we hypothesized that smoking causes inflammatory reactions and induces hyperthrombic state in the body which may be reflected in erythrocyte sedimentation rate (ESR), total leucocyte count (TLC), differential leucocyte count (DLC) and platelet count values. The purpose of the study was to study the effects of cigarette smoking on erythrocyte sedimentation rate, total leucocyte count and platelet count in adult male smokers and to compare the results with non-smokers and to establish a relationship between the duration and quantity of smoking with the change in ESR, TLC, DLC and platelet count. A cross sectional study was conducted in the department of Pathology on 86 healthy male subjects (smokers=43 and non-smokers=43). ESR was estimated using Westergrens method. TLC, DLC and platelet counts were estimated using HORIBA Pentra ES60 autoanalyser. TLC and basophil counts were significantly higher in smokers than in non-smokers (p<0.05). The mean value of ESR was higher among smokers than non-smokers but it was statistically insignificant. Platelets counts showed no significant difference between smokers and non- smokers. No correlation was observed in various blood parameters and smoking (in pack years). We conclude that smoking initiates an inflammatory response as evidenced from raised TLC, monocyte and basophil counts

Short Story: “Villain’s Suicide”

As B.R. Ambedkar stated in Annihilation of Caste, day laborers only suffer grueling labor on subsistence wages owing to their terror of punishment given to those who question it. This story reminds readers that so-called ritual impurity or untouchability has never stopped non-privileged caste people from being violated through touch and sexual abuse. The agitation over the 2012 Delhi gang rape and the more recent revelations of #MeToo occur amidst a long history of low-income Dalit women and girls routinely facing rape and other forms of repeated sexual assault from powerful landowners whose word can stop or start their wages. This story allows us to listen to the conversations and internal monologue of two-day laborers, both of whom struggle on a non-living wage. Devamani waits for her abuse to end, hoping the next generation will not be treated as a spittoon for men’s bodily fluids. Meanwhile, her friend Suguna finds the only way possible to feed her extended family