Rajani Satti

Simulation of Flow Over a 3-Element Airfoil Using a Lattice-Boltzmann Method

A Lattice-Boltzmann Method (LBM) based very large eddy simulation (VLES) approach is applied to simulate the flow field around a generic three-element airfoil. LBM describes a fluid flow in terms of a discrete kinetic equation based on the particle density distribution function (the Lattice Boltzmann equation). The effects of turbulence are modeled through an effective particle-relaxation-time scale in the extended kinetic equations. In the present study, 3D time-dependent simulations were conducted to capture the instantaneous and mean flow fields. The computed results provided good predictions of the mean flow field, which include the pressure distributions along the elemental surfaces and the time averaged mean flow field inside the slat cove. Typical unsteady flow features that characterize the shear layer emanating from the slat cusp, slat trailing edge vortex shedding, convection and reattachment of vortical structures near the slat gap were also well predicted by the present simulations.

Unsteady Flow Analysis of a Multi-Element Airfoil Using Lattice Boltzmann Method

High-lift devices employed onmodern aircraft are significant contributors to overall airframe noise. In this paper, a lattice Boltzmannmethodwith a very large eddy simulation approach is applied to computationally investigate the aerodynamic and aeroacoustic behavior of the flow around a generic high-lift configuration (three-element airfoil) at low Mach number. Three-dimensional time-dependent nearly incompressible simulations were conducted at different angles of attack to capture the instantaneous andmeanflowfields around the airfoil, previously predicted by Navier–Stokes studies. The computed mean flow results showed good agreement with existing experimental and numerical data, which include the pressure distributions around the elemental surfaces and the time-averagedmean flowfield within the slat cove. As a major objective of the present study, the unsteady flow simulations were used to capture the slat cove unsteadiness, a source of both broadband and narrowband noise. In particular, the effect of angle of attack on the shear layer emanating from the slat cusp, slat trailing-edge vortex shedding, convection, and reattachment of vortical structures near the slat gap were explored by the present simulations. Consequently, the acoustic implications of such complex unsteady flow phenomenon within the slat cove were explained and discussed in detail.

Computational Analysis of Gravitational Effects in Low-Density Gas Jets

This study deals with the computational analysis of the near-field flow structure in an isothermal helium jet injected into quiescent ambient air environment. Laminar, axisymmetric, and unsteady flow conditions were considered for the analysis. The transport equations of helium mass fraction coupled with the conservation equations of mixture mass and momentum were solved using a staggered grid finite-volume method. Jet Richardson numbers encompassing both buoyant and inertial jet flow regimes were considered. Buoyancy effects were isolated by initiating computations in Earth gravity and subsequently, reducing the gravity to simulate microgravity conditions in the 2.2 s drop tower. Computed results concur with experimental observations, i.e., a self-excited buoyant jet with periodic flow oscillations in Earth gravity becomes steady in microgravity. In an inertial jet, the flow oscillations occur at the same frequency regardless of the buoyancy, although the oscillation amplitude decreases in microgravity.

Miniature rainbow schlieren deflectometry system for quantitative measurements in microjets and flames

Recent interest in small-scale flow devices has created the need for miniature instruments capable of measuring scalar flow properties with high spatial resolution. We present a miniature rainbow schlieren deflectometry system to nonintrusively obtain quantitative species concentration and temperature data across the whole field. The optical layout of the miniature system is similar to that of a macroscale system, although the field of view is smaller by an order of magnitude. Employing achromatic lenses and a CCD array together with a camera lens and extension tubes, we achieved spatial resolution down to 4 mum. Quantitative measurements required a careful evaluation of the optical components. The capability of the system is demonstrated by obtaining concentration measurements in a helium microjet (diameter, d=650 microm) and temperature and concentration measurements in a hydrogen jet diffusion flame from a microinjector (d=50 microm). Further, the flow field of underexpanded nitrogen jets is visualized to reveal details of the shock structures existing downstream of the jet exit.

Numerical Analysis of Flow Evolution in a Helium Jet Injected Into Ambient Air

A computational model to study the stability characteristics of an evolving buoyant helium gas jet in ambient air environment is presented. Numerical formulation incorporates a segregated approach to solve for the transport equations of helium mass fraction coupled with the conservation equations of mixture mass and momentum using a staggered grid method. The operating parameters correspond to the Reynolds number varying from 30 to 300 to demarcate the flow dynamics in oscillating and non-oscillating regimes. Computed velocity and concentration fields were used to analyze the flow structure in the evolving jet. For Re = 300 case, results showed that an instability mode that sets in during the evolution process in Earth gravity is absent in zero gravity, signifying the importance of buoyancy. Though buoyancy initiates the instability, below a certain jet exit velocity, diffusion dominates the entrainment process to make the jet non-oscillatory as observed for the Re = 30 case. Initiation of the instability was found to be dependent on the interaction of buoyancy and momentum forces along the jet shear layer.Copyright

Numerical Simulations of Buoyancy Effects in low Density Gas Jets

Several studies This paper deals with the computational analysis of buoyancy effects in the near field of an isothermal helium jet injected into quiescent ambient air environment. The transport equations of helium mass fraction coupled with the conservation equations of mixture mass and momentum were solved using a staggered grid finite volume method. Laminar, axisymmetric, unsteady flow conditions were considered for the analysis. An orthogonal system with non-uniform grids was used to capture the instability phenomena. Computations were performed for Earth gravity and during transition from Earth to different gravitational levels. The flow physics was described by simultaneous visualizations of velocity and concentration fields at Earth and microgravity conditions. Computed results were validated by comparing with experimental data substantiating that buoyancy induced global flow oscillations present in Earth gravity are absent in microgravity. The dependence of oscillation frequency and amplitude on gravitational forcing was presented to further quantify the buoyancy effects.

Unsteady Flow Predictions around Tandem Cylinders with Sub-Critical Spacing

In the quest to better understand landing gear noise sources, the complex unsteady flow around a simplified configuration of a Tandem Cylinder Arrangement is considered and investigated in the current study. Recent experiments from the Basic Aerodynamics Research Tunnel (BART) and Quiet Flow Facility (QFF) at NASA Langley Research Center have provided extensive aerodynamic and aeroacoustic measurements around Tandem Cylinders with Subcritical spacing, which can be used to validate Computational Fluid Dynamics codes. In this study, Lattice Boltzmann simulations with a very large eddy simulation (VLES) approach were performed around the tandem cylinder arrangement with sub-critical spacing using the commercially available CFD code, PowerFLOW 4.1. Timedependent, three-dimensional simulations with a limited span of six cylinder diameters were conducted to compute the mean and unsteady flow structure and validate computed data using experimental data. Further, an in-depth analysis of the associated aeroacoustics is planned for future work.

Computational Analysis of a Wingtip Vortex in the Near-Field using LBM-VLES Approach

In recent years, the near-field flow behavior of the tip vortex originating from airfoils has been widely studied to provide insight into the complex physics governing the growth and propagation of wingtip vortices. For this purpose, extensive experiments on a semi-span NACA0012 airfoil were recently conducted in the low speed wind tunnel at NASA Ames Research Center to provide data for benchmarking CFD codes. In the current study, a Lattice Boltzmann Method based very large eddy simulation (LBM-VLES) approach was used to simulate flow past a semi-span NACA0012 airfoil with rounded wingtip. Timedependent, three-dimensional simulations were conducted to compute the span wise distribution of mean pressure coefficient and the mean velocity field distributions around the tip vortex region. The computed results for the mean flow field, including surface pressure distribution, flow field velocity through the vortex core, and the location of the vortex core were found to agree well with experimental data. Further, an in-depth analysis of the unsteady flow field and the comparison and validation of computational results for the Reynolds stress tensor is planned for future work.

Unsteady Flow Computations and Noise Predictions on a Rod-Airfoil Using Lattice Boltzmann Method

Accurate prediction of unsteady flow phenomena and corresponding generation of tonal/broadband noise in turbomachinery applications is a challenging problem for existing numerical methods. In this study, a Lattice Boltzmann method (LBM) with a very large eddy simulation (VLES) approach is applied to computationally investigate the aerodynamic behavior of the flow around a generic Rod-Airfoil configuration, where both narrowband and broadband noise are generated during the interactions between the flow and the rodairfoil structure. Three-dimensional, time accurate, fully turbulent simulations are performed to capture the complex flow field in accordance with recent experiments conducted by Jacob et al. 1 As part of the benchmarking efforts, the mean and RMS flow fields, unsteady aerodynamics and acoustic far field results were compared with experiments. For the acoustic far field computation, a Ffowcs Williams Hawkings acoustics formulation was applied. Good agreement of the computed results with experimental data were obtained, which demonstrated the viability of the LBM-VLES/FWH coupling approach as a reliable tool for predictions of aerodynamics/aeroacoustics from complex flow fields.

Lattice Boltzmann Simulations of the Flow over a Hump with Flow Control

The wall-mounted hump test case was first presented as a benchmark problem for active flow control at the NASA sponsored workshop CFDVAL2004. For this case, both steady suction and oscillatory blowing-suction are used to control the separation and reattachment of the turbulent flow over the hump. In this paper the lattice Boltzmann method, coupled with a very large eddy simulation (VLES) turbulence model, is used to predict three cases: uncontrolled flow, controlled flow using steady suction, and controlled flow using oscillatory blowing-suction. As the lattice Boltzmann method is an inherently unsteady method it is uniquely suitable for predicting separated flows as well as flows with transient boundary conditions. We compare reattachment locations with experiments and previous CFD results. Profiles of velocity and turbulent kinetic energy in the recirculation zone and the recovery zone are also compared with experiments. Comparisons with PIV data at four phases in the oscillatory cycle are made to evaluate the accuracy of the predicted flow structures. Simulations show very good agreement with the experiments for the uncontrolled and oscillatory controlled cases, with the decrease in separation length achieved with oscillatory control accurately predicted by the lattice Boltzmann-VLES method. Simulations were found to be less accurate for the steady suction case; a reduction in recirculation length is predicted, although the reduction is smaller than observed in experiments.