Results for “flow instability” — Page 43

NASA NTRS1988

The nonlinear development of Goertler vortices in growing boundary layers

The development of Goertler vortices in boundary layers over curved walls in the nonlinear regime is investigated. The growth of the boundary layer makes a parallel flow analysis impossible except in the high wavenumber regime so in general the instability equations must be integrated numerically. Here the spanwise dependence of the basic flow is described using Fourier series expansion while the normal and streamwise variations are taken into account using finite differences. The calculations suggest that a given disturbance imposed at some position along the wall will eventually reach a local equilibrium state essentially independent of the initial conditions. In fact, the equilibrium state reached is qualitatively similar to the large amplitude high wave-number solution described asymptotically by Hall (1982). In general, it is found that the nonlinear interactions are dominated by a mean field type of interaction between the mean flow and the fundamental. Thus, even though higher harmonics of the fundamental are necessarily generated, most of the disturbance energy is confined to the mean flow correction and the fundamental. A major result of the calculations is finding that the downstream velocity field develops a strongly inflectional character as the flow moves downstream. The latter result suggests that the major effect of Goertler vortices on boundary layers of practical importance might be to make them highly receptive to rapidly growing Rayleigh modes of instability.

Hall, Philip↗

NASA NTRS2017

Stereo Particle Image Velocimetry Measurements of Transition Downstream of a Backward-Facing Step in a Swept-Wing Boundary Layer

Stereo particle image velocimetry measurements were performed downstream of a backward-facing step in a stationary-cross flow dominated flow. The PIV measurements exhibit excellent quantitative and qualitative agreement with the previously acquired hotwire data. Instantaneous PIV snapshots reveal new information about the nature and cause of the \spikes" that occurred prior to breakdown in both the hotwire and PIV data. The PIV snapshots show that the events occur simultaneously across multiple stationary cross flow wavelengths, indicating that this is not simply a local event, but is likely caused by the 2D Tollmien-Schlichting instability that is introduced by the step. While the TS instability is a 2D instability, it is also modulated in the spanwise direction due to interactions with the stationary cross flow, as are the other unsteady disturbances present. Because of this modulation, the "spike" events cause an instantaneous increase of the spanwise modulation of the streamwise and spanwise velocity initially caused by the stationary cross flow. Breakdown appears to be caused by this instantaneous modulation, possibly due to a high-frequency secondary instability similar to a traveling-cross flow breakdown scenario. These results further illuminate the respective roles of the stationary cross flow and unsteady disturbances in transition downstream of a backward-facing step.

Eppink, Jenna L.↗

NASA NTRS2021

Transition Analysis for the CRM-NLF Wind Tunnel Configuration using Transport Equation Models and Linear Stability Correlations

Transition models based on auxiliary transport equations augmenting the Reynolds-averaged Navier-Stokes (RANS) framework rely upon transition correlations that were derived from a limited number of low-speed experiments. Furthermore, these models often account for only a subset of the relevant transition mechanisms and/or cannot accurately predict the sensitivity of those mechanisms to the changes in significant flow parameters. A preceding investigation had targeted the assessment of the transport-equation-based transition models in NASA's OVERFLOW 2.3b solver, namely, the amplification factor transport (AFT-2017b) equation model coupled with the Spalart-Allmaras RANS model and the Langtry-Menter transition models (LM2009 without crossflow effects and LM2015 including the modeling of crossflow transition) implemented with Menter’s shear-stress transport equation (SST2003) RANS model. Comparisons with recent measurements at transonic freestream conditions on the Common Research Model with Natural Laminar Flow (CRM-NLF) reinforced our earlier finding that all three of the above models significantly underpredict the reported extent of the laminar flow region over the entire span of the wing, regardless of the dominant instability mechanism(s) underlying the onset of the transition process. The underprediction of the laminar flow extent was attributed to the failure of the above models in accounting for the stabilizing effect of compressibility on the amplification of Tollmien-Schlichting instabilities. Based on previous linear stability studies related to compressibility effects, the present work proposes modifications to the two classes of transition models that reduce to the original form of each model at low subsonic speeds and do not require any nonlocal flow information or additional transport equation(s). The modifications are shown to significantly improve the predicted laminar extent of the flow and compare well against the data from the CRM-NLF experiment. Additionally, a previous assessment of transition prediction based on the dual, nonparallel N -factor method in conjunction with linear parabolized stability equations (PSE) is extended to additional angles of attack to provide the first comprehensive assessment of transition models based on nonparallel disturbance amplification over the CRM-NLF. In general, the transition criterion based on the dual, nonparallel N-factor method with N TS = N CF = 6 is reasonably successful at correlating with the measured transition fronts at R eMAC = 15 million for all angles of attack investigated herein and provides additional validation of the improved predictions from the compressibility-corrected transition models.

CFD modeling↗

NASA NTRS2022

Transition Analysis for the CRM-NLF Wind Tunnel Configuration using Transport Equation Models and Linear Stability Correlations

Transition models based on auxiliary transport equations augmenting the Reynolds-averaged Navier-Stokes (RANS) framework rely upon transition correlations that were derived from a limited number of low-speed experiments. Furthermore, these models often account for only a subset of the relevant transition mechanisms and/or cannot accurately predict the sensitivity of those mechanisms to the changes in significant flow parameters. A preceding investigation had targeted the assessment of the transport-equation-based transition models in NASA's OVERFLOW 2.3b solver, namely, the amplification factor transport (AFT-2017b) equation model coupled with the Spalart-Allmaras RANS model and the Langtry-Menter transition models (LM2009 without crossflow effects and LM2015 including the modeling of crossflow transition) implemented with Menter’s shear-stress transport equation (SST2003) RANS model. Comparisons with recent measurements at transonic freestream conditions on the Common Research Model with Natural Laminar Flow (CRM-NLF) reinforced our earlier finding that all three of the above models significantly underpredict the reported extent of the laminar flow region over the entire span of the wing, regardless of the dominant instability mechanism(s) underlying the onset of the transition process. The underprediction of the laminar flow extent was attributed to the failure of the above models in accounting for the stabilizing effect of compressibility on the amplification of Tollmien-Schlichting instabilities. Based on previous linear stability studies related to compressibility effects, the present work proposes modifications to the two classes of transition models that reduce to the original form of each model at low subsonic speeds and do not require any nonlocal flow information or additional transport equation(s). The modifications are shown to significantly improve the predicted laminar extent of the flow and compare well against the data from the CRM-NLF experiment. Additionally, a previous assessment of transition prediction based on the dual, nonparallel N -factor method in conjunction with linear parabolized stability equations (PSE) is extended to additional angles of attack to provide the first comprehensive assessment of transition models based on nonparallel disturbance amplification over the CRM-NLF. In general, the transition criterion based on the dual, nonparallel N-factor method with N TS = N CF = 6 is reasonably successful at correlating with the measured transition fronts at R eMAC = 15 million for all angles of attack investigated herein and provides additional validation of the improved predictions from the compressibility-corrected transition models.

CFD modeling↗

NASA NTRS1990

Vortex instabilities in 3D boundary layers: The relationship between Goertler and crossflow vortices

The inviscid and viscous stability problems are addressed for a boundary layer which can support both Goertler and crossflow vortices. The change in structure of Goertler vortices is found when the parameter representing the degree of three-dimensionality of the basic boundary layer flow under consideration is increased. It is shown that crossflow vortices emerge naturally as this parameter is increased and ultimately become the only possible vortex instability of the flow. It is shown conclusively that at sufficiently large values of the crossflow there are no unstable Goertler vortices present in a boundary layer which, in the zero crossflow case, is centrifugally unstable. The results suggest that in many practical applications Goertler vortices cannot be a cause of transition because they are destroyed by the 3-D nature of the basic state. In swept wing flows the Goertler mechanism is probably not present for typical angles of sweep of about 20 degrees. Some discussion of the receptivity problem for vortex instabilities in weakly 3-D boundary layers is given; it is shown that inviscid modes have a coupling coefficient marginally smaller than those of the fastest growing viscous modes discussed recently by Denier, Hall, and Seddougui (1990). However the fact that the growth rates of the inviscid modes are the largest in most situations means that they are probably the most likely source of transition.

Bassom, Andrew↗

NASA NTRS1987

Spacelab experiments on convection in a rotating spherical shell with radial gravity

Data from the geophysical fluid flow cell experiment on Spacelab 3 revealed the diverse forms of fluid motion that can occur in geometrically simple models of global convection with rotation and radial gravity. The following types of convection were observed: midlatitude waves interacting with the low-latitude columnar convection rolls or 'banana cells', spiral waves' near the poles when latitudinal heating gradients are present on the bounding surfaces, and 'triangular waves' coupling midlatitude and equatorial disturbances under similar differential heating. It is believed that concepts based on the Taylor-Proudman theorem have a central role in many flows realized in the rapidly rotating experiments when H is approximately 0; however, the resulting banana cells are subject to secondary instabilities that produce intricate time dependence and eventually turbulent flows in which only hints of the simpler patterns are recognizable.

Toomre, Juri↗

NASA NTRS1997

Drag Prediction and Transition in Hypersonic Flow

This paper discusses progress on issues such as instability studies, nose-bluntness and angle-of-attack effects, and leading-edge-contamination problems from theoretical, computational, and experimental points of view. Also included is a review of wind-tunnel and flight data, including high-Re flight transition data, the levels of noise in flight and in wind tunnels, and how noise levels can affect parametric trends. A review of work done on drag accounting and the role of viscous drag for hypersonic vehicles is also provided.

Reed, Helen L.↗

NASA NTRS1991

A parametric study of the leading edge attachment line for the F-16XL

A three-dimensional Navier-Stokes code is used to computationally simulate the flow about a modified F-16XL. Transition mechanisms (e.g. attachment line location and crossflow instability) near the swept wing leading edge are analyzed in detail. Flow visualization is used to study the influence of angle-of-attack on the aforementioned transition mechanisms. Validation of the code is accomplished by comparison of numerically generated surface pressures with that obtained by in-flight experiments.

Flores, Jolen↗

NASA NTRS1983

Unsteady Flow Effects in Combustor Systems

A wide variety of combustion problems, including combustion instabilities and turbulent diffusion flames, appear to involve the entrainment and deformation of laminar flames by large vortex structures in the flow field. First, some details of this process of laminar flame distortion are examined by considering the interactions of time-dependent diffusion flames with two dimensional vortices. Second, Some results on the modelling of the non-steady combustion in burners for aircraft gas turbines are given. The general aim of the work is to develop a one dimensional model applicable to the NASA-Lewis Non-Steady Combustion Rig.

Subbaiah, M. V.↗

NASA NTRSDate not supplied

Computational Assessment of Inlet Backflow Effects on Rotating Detonation Engine Performance and Operability

The performance impact of flow reversal at the inlet of an airbreathing rotating detonation engine (RDE) is investigated using 2 and 3-dimensional computational fluid dynamic (CFD) simulations. Flow reversal, or backflow, occurs in RDE inlets in the high-pressure region directly behind the rotating detonation front. This is also where most of the engine thrust or pressure gain is produced. The amount of backflow relative to throughflow depends on the inlet design. For the present work, a simple annular ‘slit’ design is used. The simulations are idealized in several ways, including that fuel and air are premixed, but prevented from reacting when within the inlet region. The results indicate that even with idealizations, the impact of inlet backflow on pressure gain can be substantial. The simulations also reveal an intriguing instability that develops in certain configurations. The mass flow rate into the RDE begins to oscillate at a regular frequency that is substantially less than the detonation rotational frequency. This is accompanied by oscillations in the detonation height. The oscillation amplitude grows over time until the detonation ultimately fails. Both the performance and instability results emphasize the need for carefully designed RDE inlets that provide low loss when flow is in the forward direction, but high resistance when the flow is reversed. Development of such high-diodicity inlets is critical to achieving pressure gain in airbreathing RDE’s.

Detonation↗

NASA NTRSDate not supplied

Computational Assessment of Inlet Backflow Effects on Rotating Detonation Engine Performance and Operability

The performance impact of flow reversal at the inlet of an airbreathing rotating detonation engine (RDE) is investigated using 2 and 3-dimensional computational fluid dynamic (CFD) simulations. Flow reversal, or backflow, occurs in RDE inlets in the high-pressure region directly behind the rotating detonation front. This is also where most of the engine thrust or pressure gain is produced. The amount of backflow relative to throughflow depends on the inlet design. For the present work, a simple annular ‘slit’ design is used. The simulations are idealized in several ways, including that fuel and air are premixed, but prevented from reacting when within the inlet region. The results indicate that even with idealizations, the impact of inlet backflow on pressure gain can be substantial. The simulations also reveal an intriguing instability that develops in certain configurations. The mass flow rate into the RDE begins to oscillate at a regular frequency that is substantially less than the detonation rotational frequency. This is accompanied by oscillations in the detonation height. The oscillation amplitude grows over time until the detonation ultimately fails. Both the performance and instability results emphasize the need for carefully designed RDE inlets that provide low loss when flow is in the forward direction, but high resistance when the flow is reversed. Development of such high-diodicity inlets is critical to achieving pressure gain in airbreathing RDE’s.

detonation↗

NASA NTRSDate not supplied

Acoustic Insights into Flow Condensation Mechanisms

Two-phase thermal management systems, with both boiling and condensation processes, offer great potential and heat transfer coefficients that are orders of magnitude higher than traditional single-phase systems. However, two-phase flows can suffer from a wide range of interfacial instabilities leading to significant thermal performance degradation. In this study, we aim to detect regime transitions and characterize dominating physical mechanisms of flow condensation, such as turbulent diffusion in annular liquid film and interfacial waves, using an integrated system of acoustic, modal, and optical sensing techniques and thermofluidic characterizations. A wideband acoustic emissions sensor and high-sensitivity accelerometer are utilized to capture acoustic and vibrational signatures that signal the onset of liquid film formation and interfacial waves during flow pattern transitions. Compared to optical imaging, wideband acoustic emission sensing allows for higher sampling rates to capture high-frequency interface oscillations critical to the flow regime transitions and works well even for condensation in opaque tubes. Acoustic features (e.g., amplitude, frequency, energy, duration) are correlated with thermofluidic processes (e.g., capillary flows, turbulent flows, boiling, condensation). By relating thermal performance metrics with these dynamic signatures in acoustic and modal regimes, we explore the ability to probe and monitor critical flow regime transitions and transport efficiency in flow condensation.

Acoustic Modal Regimes↗

NASA NTRS1987

A linear MHD instability analysis of solar mass ejections with gravitation

The linear MHD instability of a cylindrical plasma is used to investigate the origin of solar mass ejections, and the dispersion relation is solved numerically. The initial plasma-flow velocity is found to have a significant effect on the instability criteria and growth rate, and the instability growth-rate is shown to be larger in cases where plasma flow exists, relative to the static case. Results suggest that the plasma column may break into small pieces. Assuming a thin-tube approximation, gravity is found to have little effect on the instability of quasi-horizontal ejection, but to have considerable effect on the vertical ejection. In considering the gravitational force, an exact analytical solution is found for the vertical case, while asymptotic solutions are given for the horizontal and oblique cases.

Song, M. T.↗

NASA NTRS2015

Discrete-Roughness-Element-Enhanced Swept-Wing Natural Laminar Flow at High Reynolds Numbers

Nonlinear parabolized stability equations and secondary-instability analyses are used to provide a computational assessment of the potential use of the discrete-roughness-element technology for extending swept-wing natural laminar flow at chord Reynolds numbers relevant to transport aircraft. Computations performed for the boundary layer on a natural-laminar-flow airfoil with a leading-edge sweep angle of 34.6 deg, freestream Mach number of 0.75, and chord Reynolds numbers of 17 × 10(exp 6), 24 × 10(exp 6), and 30 × 10(exp 6) suggest that discrete roughness elements could delay laminar-turbulent transition by about 20% when transition is caused by stationary crossflow disturbances. Computations show that the introduction of small-wavelength stationary crossflow disturbances (i.e., discrete roughness element) also suppresses the growth of most amplified traveling crossflow disturbances.

Malik, Mujeeb↗

NASA NTRS2001

Fundamental Processes of Atomization in Fluid-Fluid Flows

This report outlines the major results of the grant "Fundamental Processes of Atomization in Fluid-Fluid Flows." These include: 1) the demonstration that atomization in liquid/liquid shear flow is driven by a viscous shear instability that triggers the formation of a long thin sheet; 2) discovery of a new mode of interfacial instability for oscillatory two-layer systems whereby a mode that originates within the less viscous liquid phase causes interfacial deformation as the oscillation proceeds; 3) the demonstration that rivulet formation from gravity front occurs because the local front shape specified by gravity and surface tension changes from a nose to a wedge geometry, thus triggering a large increase in viscous resistance; and 4) extension of the studies on nonlinear wave evolution on falling films and in stratified flow, particularly the evolution towards large-amplitude solitary waves that tend to generate drops.

McCready, M. J.↗

NASA NTRS1992

Electrostatic ion cyclotron velocity shear instability

A local electrostatic dispersion equation is derived for a shear flow perpendicular to an ambient magnetic field, which includes all kinetic effects and involves only one important parameter. The dispersion equation is cast in the form of Gordeyev integrals and is solved numerically. Numerical solutions indicate that an ion cyclotron instability is excited. The instability occurs roughly at multiples of the ion cyclotron frequency (modified by the shear), with the growth rate or the individual harmonics overlapping in the wavenumber. At large values of the shear parameter, the instability is confined to long wavelengths, but at smaller shear, a second distinct branch at shorter wavelengths also appears. The properties of the instability obtained are compared with those obtained in the nonlocal limit by Ganguli et al. (1985, 1988).

Lemons, D. S.↗

NASA NTRS2002

Investigation of Body Force Effects on Flow Boiling Critical Heat Flux

The bubble coalescence and interfacial instabilities that are important to modeling critical heat flux (CHF) in reduced-gravity systems can be sensitive to even minute body forces. Understanding these complex phenomena is vital to the design and safe implementation of two-phase thermal management loops proposed for space and planetary-based thermal systems. While reduced gravity conditions cannot be accurately simulated in 1g ground-based experiments, such experiments can help isolate the effects of the various forces (body force, surface tension force and inertia) which influence flow boiling CHF. In this project, the effects of the component of body force perpendicular to a heated wall were examined by conducting 1g flow boiling experiments at different orientations. FC-72 liquid was boiled along one wall of a transparent rectangular flow channel that permitted photographic study of the vapor-liquid interface at conditions approaching CHF. High-speed video imaging was employed to capture dominant CHF mechanisms. Six different CHF regimes were identified: Wavy Vapor Layer, Pool Boiling, Stratification, Vapor Counterflow, Vapor Stagnation, and Separated Concurrent Vapor Flow. CHF showed great sensitivity to orientation for flow velocities below 0.2 m/s, where very small CHF values where measured, especially with downflow and downward-facing heated wall orientations. High flow velocities dampened the effects of orientation considerably. Figure I shows representative images for the different CHF regimes. The Wavy Vapor Layer regime was dominant for all high velocities and most orientations, while all other regimes were encountered at low velocities, in the downflow and/or downward-facing heated wall orientations. The Interfacial Lift-off model was modified to predict the effects of orientation on CHF for the dominant Wavy Vapor Layer regime. The photographic study captured a fairly continuous wavy vapor layer travelling along the heated wall while permitting liquid contact only in wetting fronts, located in the troughs of the interfacial waves. CHF commenced when wetting fronts near the outlet were lifted off the wall. The Interfacial Lift-off model is shown to be an effective tool for predicting the effects of body force on CHF at high velocities.

Zhang, Hui↗

NASA NTRS2006

Laboratory Study of Magnetorotational Instability and Hydrodynamic Stability at Large Reynolds Numbers

Two plausible mechanisms have been proposed to explain rapid angular momentum transport during accretion processes in astrophysical disks: nonlinear hydrodynamic instabilities and magnetorotational instability (MRI). A laboratory experiment in a short Taylor-Couette flow geometry has been constructed in Princeton to study both mechanisms, with novel features for better controls of the boundary-driven secondary flows (Ekman circulation). Initial results on hydrodynamic stability have shown negligible angular momentum transport in Keplerian-like flows with Reynolds numbers approaching one million, casting strong doubt on the viability of nonlinear hydrodynamic instability as a source for accretion disk turbulence.

Ji, H.↗