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NASA NTRS2017

Aeroacoustic Simulations of a Nose Landing Gear with FUN3D: A Grid Refinement Study

A systematic grid refinement study is presented for numerical simulations of a partially-dressed, cavity-closed (PDCC) nose landing gear configuration that was tested in the University of Florida's open-jet acoustic facility known as the UFAFF. The unstructured-grid flow solver FUN3D is used to compute the unsteady flow field for this configuration. Mixed-element grids generated using the Pointwise (Registered Trademark) grid generation software are used for numerical simulations. Particular care is taken to ensure quality cells and proper resolution in critical areas of interest in an effort to minimize errors introduced by numerical artifacts. A set of grids was generated in this manner to create a family of uniformly refined grids. The finest grid was then modified to coarsen the wall-normal spacing to create a grid suitable for the wall-function implementation in FUN3D code. A hybrid Reynolds-averaged Navier-Stokes/large eddy simulation (RANS/LES) turbulence modeling approach is used for these simulations. Time-averaged and instantaneous solutions obtained on these grids are compared with the measured data. These CFD solutions are used as input to a FfowcsWilliams-Hawkings (FW-H) noise propagation code to compute the farfield noise levels. The agreement of the computed results with the experimental data improves as the grid is refined.

Vatsa, Veer N.↗

NASA NTRS2001

A Hybrid Numerical Method for Turbulent Mixing Layers

A hybrid method has been developed for simulations of compressible turbulent mixing layers. Such mixing layers dominate the flows in exhaust systems of modern day aircraft and also those of hypersonic vehicles currently under development. The method configurations in which a dominant structural feature provides an unsteady mechanism to drive the turbulent development in the mixing layer. The hybrid method uses a Reynolds-averaged Navier-Stokes (RANS) procedure to calculate wall bounded regions entering a mixing section, and a Large Eddy Simulation (LES) procedure to calculate the mixing dominated regions. A numerical technique was developed to enable the use of the hybrid RANS-LES method on stretched, non-Cartesian grids. Closure for the RANS equations was obtained using the Cebeci-Smith algebraic turbulence model in conjunction with the wall-function approach of Ota and Goldberg. The wall-function approach enabled a continuous computational grid from the RANS regions to the LES region. The LES equations were closed using the Smagorinsky subgrid scale model. The hybrid RANS-LES method is applied to a benchmark compressible mixing layer experiment. Preliminary two dimensional calculations are used to investigate the effects of axial grid density and boundary conditions. Vortex shedding from the base region of a splitter plate separating the upstream flows was observed to eventually transition to turbulence. The location of the transition, however, was much further downstream than indicated by experiments. Actual LES calculations, performed in three spatial directions, also indicated vortex shedding, but the transition to turbulence was found to occur much closer to the beginning of the mixing section. which is in agreement with experimental observations. These calculations demonstrated that LES simulations must be performed in three dimensions. Comparisons of time-averaged axial velocities and turbulence intensities indicated reasonable agreement with experimental data.

Georgiadis, Nicholas J.↗

NASA NTRS1971

Zero-gravity transient thermal mixing simulation

The experimental program described is an outgrowth of independent investigations into alternate redesign concepts for the Apollo SM cryogenic oxygen storage system. The experiments were continued, after the redesign was established, to provide physical insight into transient thermal mixing in zero-gravity and to aid in the characterization of the system performance in flight. Zero-gravity heat transfer and fluid mixing were simulated experimentally through an analogy between unsteady heat conduction and species diffusion. To further support numerical analyses of the cryogenic oxygen storage system, the experimental investigation was extended to include a cubical tank geometry, representative of existing numerical models. In general, the transient flow patterns in the cubical tank are far more complex than those of the spherical tank and the extent of fluid mixing is significantly greater but less repeatable.

Lands, J. F., Jr.↗

NASA NTRS1979

Linearized unsteady jet analysis

The introduction of a time dependency into a jet flow to change the rate at which it mixes with a coflowing stream or ambient condition is investigated. The advantages and disadvantages of the unsteady flow are discussed in terms of steady state mass and momentum transfer. A linear system which is not limited by frequency constraints and evolves through a simplification of the equations of motion is presented for the analysis of the unsteady flow field generated by the time dependent jet.

Viets, H.↗

NASA NTRS1991

The compressible mixing layer

The capability of turbulence modeling correctly to handle natural unsteadiness appearing in compressible turbulent flows is investigated. Physical aspects linked to the unsteadiness problem and the role of various flow parameters are analyzed. It is found that unsteady turbulent flows can be simulated by dividing these motions into an 'organized' part for which equations of motion are solved and a remaining 'incoherent' part represented by a turbulence model. Two-equation turbulence models and second-order turbulence models can yield reasonable results. For specific compressible unsteady turbulent flow, graphic presentations of different quantities may reveal complementary physical features. Strong compression zones are observed in rapid flow parts but shocklets do not yet occur.

Vandromme, Dany↗

NASA NTRS1987

Planar compressible reacting shear layer

Information on the planar compressible reacting shear layer is given in viewgraph form. topics covered include heat transfer in 3D flow regions, chemical reacting flows, an unsteady 2D compressible reacting code (MRVC2D), a plane mixing layer, and critical needs in shear layer physics.

Marek, C. J.↗

NASA NTRS1975

Numerical calculation of unsteady transonic potential flow over helicopter rotor blades

The small-disturbance potential equation appropriate to a helicopter in forward flight is derived. This equation is then solved for the flow over a nonlifting transonic rotor blade, using a completely implicit scheme that is an extension of the Murman-Cole mixed-difference technique. The flow in the tip region is most unsteady in the decelerating flow region, after the blade passes the psi = 90 deg azimuthal station. The unsteadiness appears to be caused by expansion and compression waves that move slowly upstream of the blade as the relative incident flow decelerates. The influence of aspect ratio, advance ratio, and Mach number on this process is discussed.

Caradonna, F. X.↗

NASA NTRS1994

Mixing and Noise Benefit Versus Thrust Penalty in Supersonic Jets Using Impingement Tones

This paper reports the results of an experimental investigation on the effect of impingement tones generated by obstacles of various geometries on the spreading of a supersonic jet flow. A rectangular supersonic jet was produced using a convergent-divergent nozzle that was operated near its design point (with shocks minimized). The immersion of obstacles in the flow produced an intense impingement tone which then propagated upstream (as feedback) to the jet lip and excited the antisymmetric hydrodynamic mode in the jet, thus setting up a resonant self-sustaining loop. The violent flapping motion of the jet due to excitation of the antisymmetric mode, combined with the unsteady wakes of the obstacles, produced large changes in jet mixing. It was possible to control the frequency and amplitude of the impingement tone excitation by varying the nozzle-to-obstacle distance and the obstacle immersion. By proper shaping of the obstacles it was possible to reduce the thrust penalty significantly.

Raman, Ganesh↗

NASA NTRS1992

Anomalous ion mixing within a Kelvin-Helmholtz vortex in a collisionless plasma

Anomalously fast ion mixing is observed in a hybrid code simulation (particle ions and a massless electron fluid) of the Kelvin-Helmholtz instability in a collisionless plasma. While the traditional view predicts that the ion mixing occurs on a time scale longer than the roll-up time T(v) of the Kelvin-Helmholtz vortex by a factor of (velocity shear scale length)/(ion Larmor radius), the simulation results show that the mixing occurs within a time scale comparable to T(v). It is shown that this fast mixing is due to the scattering of ions by unsteady electromagnetic fields.

Terasawa, T.↗

NASA NTRSDate not supplied

TPSAS-NF1676L-14454-DND

FUN3D Core Capabilities - Established as a research code in late 1980’s; now supports numerous internal and external efforts across the speed range - Solves 2D/3D steady and unsteady Euler and RANS equations on node-based mixed element grids for compressible and incompressible flows - General dynamic mesh capability: any combination of rigid / overset / morphing grids, including 6-DOF effects - Aeroelastic modeling using mode shapes, full FEM, CC, etc. - Constrained / multipoint adjoint-based design and mesh adaptation - Distributed development team using agile/extreme software practices including 24/7 regression and performance testing - Capabilities fully integrated, online documentation, training videos, tutorials

Eric J Nielsen↗

NASA NTRSDate not supplied

NASA LaRC Contribution to the High Angle Working Group of the Third Aeroelastic Prediction Workshop: BSCW Shock Buffet

FUN3D Core Capabilities - Established as a research code in late 1980s; now supports numerous internal and external efforts across the speed range - Solves 2D/3D steady and unsteady Euler and RANS equations on node-based mixed element grids for compressible and incompressible flows - General dynamic mesh capability: any combination of rigid / overset / morphing grids, including 6-DOF effects - Aeroelastic modeling using mode shapes, full FEM, etc. - Constrained / multipoint adjoint-based design and mesh adaptation - Distributed development team using agile/extreme software practices including 24/7 regression, performance testing - Capabilities fully integrated, online documentation, training videos, tutorials

Pawel Chwalowski↗

NASA NTRS1985

A first-order Green's function approach to supersonic oscillatory flow: A mixed analytic and numeric treatment

A frequency domain Green's Function Method for unsteady supersonic potential flow around complex aircraft configurations is presented. The focus is on the supersonic range wherein the linear potential flow assumption is valid. In this range the effects of the nonlinear terms in the unsteady supersonic compressible velocity potential equation are negligible and therefore these terms will be omitted. The Green's function method is employed in order to convert the potential flow differential equation into an integral one. This integral equation is then discretized, through standard finite element technique, to yield a linear algebraic system of equations relating the unknown potential to its prescribed co-normalwash (boundary condition) on the surface of the aircraft. The arbitrary complex aircraft configuration (e.g., finite-thickness wing, wing-body-tail) is discretized into hyperboloidal (twisted quadrilateral) panels. The potential and co-normalwash are assumed to vary linearly within each panel. The long range goal is to develop a comprehensive theory for unsteady supersonic potential aerodynamic which is capable of yielding accurate results even in the low supersonic (i.e., high transonic) range.

Freedman, M. I.↗

NASA NTRS1995

Review of the PDWA Concept for Combustion Enhancement in a Supersonic Air-Breathing Combustor Environment

This paper reviews the design of the Pulsed Detonation Wave Augmentor (PDWA) concept and the preliminary computational fluid dynamics studies that supported it. The PDWA relies on the rapid generation of detonation waves in a small tube, which are then injected into the supersonic stream of the main combustor. The blast waves thus generated are used to stimulate the mixing and combustion inside the main combustor. The mixing enhancement relies on various forms of the baroclinic interaction, where misaligned pressure and density gradients combine to produce vortical flow. By using unsteady shock waves, the concept also uses the Richtmyer-Meshkov effect to further increase the rate of mixing. By carefully designing the respective configurations of the combustor and the detonation tubes, one can also increase the penetration of the fuel into the supersonic air stream. The unsteady shocks produce lower stagnation pressure losses than steady shocks. Combustion enhancement can also be obtained through the transient shock-heating of the fuel-air interface, and the lowering of the ignition delay in these regions. The numerical simulations identify these processes, and show which configurations give the best results. Engineering considerations are also presented, and discuss the feasibility of the concept. Of primary importance are the enhancements in performance, the design simplicity, the minimization of the power, cost, and weight, and the methods to achieve very rapid cycling.

Canbier, Jean-Luc↗

NASA NTRS1993

The effect of unsteadiness on the time-mean thermal loads in a turbine stage

Two steady numerical analysis methods and one unsteady method are used to study the viscous three-dimensional flow in the middle stage of the Pratt & Whitney alternate design Space Shuttle Main Engine fuel turbine. The principal characteristic of this flow is that the secondary flows generated in the rotor blade reconfigure a radial inlet total temperature distortion into one with a pitchwise exit hot streak distortion. Secondary flows in the following vane redistribute the radial variation while unsteadiness causes a segregation of hot and cold flow from the hot streak within the vane. Such redistribution and segregation can lead to unexpected thermal loads and reduced durability. The physical phenomena and the ability of a steady analysis to capture them are investigated by performing a numerical experiment whereby the results of the two steady analysis methods are compared to the time-mean of the unsteady simulation. The flow physics related to the segregation and mixing of total temperature are discussed.

Kirtley, K. R.↗

NASA NTRS2006

PAB3D: Its History in the Use of Turbulence Models in the Simulation of Jet and Nozzle Flows

This is a review paper for PAB3D s history in the implementation of turbulence models for simulating jet and nozzle flows. We describe different turbulence models used in the simulation of subsonic and supersonic jet and nozzle flows. The time-averaged simulations use modified linear or nonlinear two-equation models to account for supersonic flow as well as high temperature mixing. Two multiscale-type turbulence models are used for unsteady flow simulations. These models require modifications to the Reynolds Averaged Navier-Stokes (RANS) equations. The first scheme is a hybrid RANS/LES model utilizing the two-equation (k-epsilon) model with a RANS/LES transition function, dependent on grid spacing and the computed turbulence length scale. The second scheme is a modified version of the partially averaged Navier-Stokes (PANS) formulation. All of these models are implemented in the three-dimensional Navier-Stokes code PAB3D. This paper discusses computational methods, code implementation, computed results for a wide range of nozzle configurations at various operating conditions, and comparisons with available experimental data. Very good agreement is shown between the numerical solutions and available experimental data over a wide range of operating conditions.

Abdol-Hamid, Khaled S.↗

NASA NTRS1980

Calculation of three-dimensional unsteady transonic flows past helicopter blades

A finite difference code for predicting the high speed flow over the advancing helicopter rotor is presented. The code solves the low frequency, transonic small disturbance equation and is suitable for modeling the effects of advancing blade unsteadiness on blades of nearly arbitrary planform. The method employs a quasi-conservative mixed differencing scheme and solves the resulting difference equations by an alternating direction scheme. Computed results showed good agreement with experimental blade pressure data and illustrate some of the effects of varying the rotor planform. The flow unsteadiness is shown to be an indispensible part of a transonic solution. Close to the tip at high advance ratio, cross flow effects can significantly affect the solution.

Chattot, J. J.↗

NASA NTRS1997

Unsteady Flow Field in a Multistage Axial Flow Compressor

The flow field in a multistage compressor is three-dimensional, unsteady, and turbulent with substantial viscous effects. Some of the specific phenomena that has eluded designers include the effects of rotor-stator and rotor-rotor interactions and the physics of mixing of velocity, pressure, temperature and velocity fields. An attempt was made, to resolve experimentally, the unsteady pressure and temperature fields downstream of the second stator of a multistage axial flow compressor which will provide information on rotor-stator interaction effects and the nature of the unsteadiness in an embedded stator of a three stage axial flow compressor. Detailed area traverse measurements using pneumatic five hole probe, thermocouple probe, semi-conductor total pressure probe (Kulite) and an aspirating probe downstream of the second stator were conducted at the peak efficiency operating condition. The unsteady data was then reduced through an ensemble averaging technique which splits the signal into deterministic and unresolved components. Auto and cross correlation techniques were used to correlate the deterministic total temperature and velocity components (acquired using a slanted hot-film probe at the same measurement locations) and the gradients, distributions and relative weights of each of the terms of the average passage equation were then determined. Based on these measurements it was observed that the stator wakes, hub leakage flow region, casing endwall suction surface corner region, and the casing endwall region away from the blade surfaces were the regions of highest losses in total pressure, lowest efficiency and highest levels of unresolved unsteadiness. The deterministic unsteadiness was found to be high in the hub and casing endwall regions as well as on the pressure side of the stator wake. The spectral distribution of hot-wire and kulite voltages shows that at least eight harmonics of all three rotor blade passing frequencies are present at this measurement location. In addition to the basic three rotor blade passing frequencies (R1, R2 and R3) and their harmonics, various difference frequencies such as (2R1 -R2) and (2R3-R2) and their harmonics are also observed. These difference frequencies are due to viscous and potential interactions between rotors 1, 2 and 3 which are sensed by both the total pressure and aspirating probes at this location. Significant changes occur to the stator exit flow features with passage of the rotor upstream of the stator. Because of higher convection speeds of the rotor wake on the suction surface of the downstream stator than on the pressure side, the chopped rotor wake was found to be arriving at different times on either side of the stator wake. As the rotor passes across the stator.

Suryavamshi, N.↗

NASA NTRS1972

Experimental investigation of recirculating cells in laminar coaxial jets.

Utilizing several unique means of introducing smoke into the flow field for careful visualization in addition to hot-wire techniques, experiments are performed in a specially designed facility producing laminar flows up to considerably high Reynolds numbers. Characteristics of the cells and the flow conditions that bring them about are documented by smoke photographs in the Reynolds number velocity ratio plane and the results are compared to previous analytical predictions. The cells are found to fall into three categories with different flow characteristics involving unsteadiness in position, and shear layer instabilities which result in higher mixing with the outer streams.-

Warpinski, N. R.↗