Study of Vortex Shedding as Related to Self-excited Torsional Oscillations of an Airfoil
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WINGS, COMPLETE - WAKE↗
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Computation of Vortex Shedding and Radiated Sound for a Circular Cylinder
The Lighthill acoustic analogy approach combined with Reynolds-averaged Navier Stokes is used to predict the sound generated by unsteady viscous flow past a circular cylinder assuming a correlation length of ten cylinder diameters. The two- dimensional unsteady ow field is computed using two Navier-Stokes codes at a low Mach number over a range of Reynolds numbers from 100 to 5 million. Both laminar ow as well as turbulent ow with a variety of eddy viscosity turbulence models are employed. Mean drag and Strouhal number are examined, and trends similar to experiments are observed. Computing the noise within the Reynolds number regime where transition to turbulence occurs near the separation point is problematic: laminar flow exhibits chaotic behavior and turbulent ow exhibits strong dependence on the turbulence model employed. Comparisons of far-field noise with experiment at a Reynolds number of 90,000, therefore, vary significantly, depending on the turbulence model. At a high Reynolds number outside this regime, three different turbulence models yield self-consistent results.
Cox, Jared S.↗
Vortex Shedding Characteristics of the Wake of a Thin Flat Plate with a Circular Trailing Edge
The near and very near wake of a thin flat plate with a circular trailing edge are investigated with direct numerical simulations (DNS). Data obtained for two different Reynolds numbers (based on plate thickness, D) are the main focus of this study. The separating boundary layers are turbulent in both cases. An earlier investigation of one of the cases (Case F) showed shed vortices in the wake that were about 1.0 D to 4.0 D in spanwise length. Considerable variation in both the strength and frequency of these shed vortices was observed. One objective of the present investigation is to determine the important contributors to this variability in strength and frequency of shed vortices and their finite spanwise extent. Analysis of the data shows that streamwise vortices in the separating boundary layer play an important role in strengthening/weakening of the shed vortices and that high/low-speed streaks in the boundary layer are important contributors to variability in shedding frequency. Both these features of the boundary layer contribute to the finite extent of the vortices in the spanwise direction. The second plate DNS (Case G, with 40 percent of the plate thickness of Case F) shows that while shedding intensity is weaker than obtained in Case F, many of the wake features are similar to that of Case F. This is important in understanding the path to the wake of the thin plate with a sharp trailing edge where shedding is absent. Here we also test the efficacy of a functional relationship between the shedding frequency and the Reynolds numbers based on the boundary layer momentum thickness (Re (sub theta) and D (Re (sub D)); data for developing this behavioral model is from Cases F & G and five earlier DNSs of the flat plate wake.
Turbulent wakes↗
The Effect of Boundary Layer Character on Stochastic Rotor Blade Vortex Shedding Noise
This work illustrates the effect of a rotor blade’s boundary layer on the broadband LBL-VS self-noise emitted from an optimum hovering rotor through experimental and multi-fidelity computational studies. Blade surface roughness effects associated with different manufacturing techniques and the effect of adding a boundary layer trip were shown to decrease LBL-VS noise by upwards of 30 dB at the frequency of maximum emission with a slight penalty in aerodynamic performance when compared with smooth rotor blades. Low-fidelity 2-D viscous flow analysis verified the presence of laminar separation bubbles on the rotor blades which are responsible for LBL-VS noise. Three high-fidelity lattice-Boltzmann simulations were conducted with different wall-functions to predict the boundary layer character correspondent to their experimental counterpart and the resultant presence or absence of LBL-VS noise. Excellent aerodynamic and aeroacoustic agreement was seen between the lattice-Boltzmann simulations and the experimental data for the cases with surface roughness and the boundary layer trip, with the broadband noise results from the simulation with fully turbulent wall-functions diverging from the experimental results above 5 kHz. The transitional wall-function simulation which emulated the smooth experimental blades underpredicted thrust by 14% and broadband noise by a minimum of 10 dB with an accurately predicted broadband noise trend.
Christopher S Thurman↗
The Effect of Boundary Layer Character on Stochastic Rotor Blade Vortex Shedding Noise
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Christopher Thurman↗
Wakes of thin flat plates: Intermittency, cessation of vortex shedding and the emergence of an intermediate-wake instability
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Man Mohan Rai↗
Visualizing Time-Varying Phenomena In Numerical Simulations Of Unsteady Flows
Streamlines, contour lines, vector plots, and volume slices (cutting planes) are commonly used for flow visualization. These techniques are sometimes referred to as instantaneous flow visualization techniques because calculations are based on an instant of the flowfield in time. Although instantaneous flow visualization techniques are effective for depicting phenomena in steady flows,they sometimes do not adequately depict time-varying phenomena in unsteady flows. Streaklines and timelines are effective visualization techniques for depicting vortex shedding, vortex breakdown, and shock waves in unsteady flows. These techniques are examples of time-dependent flow visualization techniques, which are based on many instants of the flowfields in time. This paper describes the algorithms for computing streaklines and timelines. Using numerically simulated unsteady flows, streaklines and timelines are compared with streamlines, contour lines, and vector plots. It is shown that streaklines and timelines reveal vortex shedding and vortex breakdown more clearly than instantaneous flow visualization techniques.
Lane, David A.↗
Experimental and Computational Study of Tones Occurring with a Coaxial Nozzle
The source of audible tones occurring with a coaxial nozzle in a range of low Mach numbers is explored experimentally as well as computationally. The hardware is comprised of an inner and an outer nozzle, without a center-body, that are held together by a set of four struts. With increasing jet Mach number (M(sub j)), first a tone occurred at about 2550 Hz around M(sub j)=0.06. At higher values of M(sub j) a tone at 5200 Hz dominated the noise spectra. The corresponding nondimensional frequency, based on effective thickness of the inner nozzle lip and jet exit velocity, turned out to be about 0.2, a value characteristic of Karmann vortex shedding. Thus, vortex shedding from the inner nozzle lip could be linked to the tones. From a comparison of the acoustic wavelengths and the nozzle dimensions, it was inferred that the vortex shedding excited a one-quarter-wave resonance within the divergent section of either the inner nozzle or the outer nozzle. This led to the generation of the sharp tones.
nozzle↗
Aeroacoustics Noise Model of OpenFAST
The report describes theory and application of a newly released module of OpenFAST to simulate the aeroacoustic noise generated by the rotor of an arbitrary wind turbine. OpenFAST is a fully open-source publicly available wind turbine analysis tool actively developed at the National Renewable Energy Laboratory. The aeroacoustic module, which is also fully open-source and publicly available, is based on work performed over the past three decades. Frequency-based models for turbulent inflow, turbulent boundary layer – trailing edge, laminar boundary layer – vortex shedding, tip vortex, and trailing edge bluntness – vortex shedding noise mechanisms are included. A simple directivity model is also included. The appendices of this report describe in detail the inputs to the module and how to find them in the input files of OpenFAST. The noise models are exercised simulating the aeroacoustic noise emissions of the IEA Wind Task 37 land-based reference wind turbine. A code-to-code comparison between the implementation presented here and the implementation available at the Wind Energy Institute of the Technical University of Munich, Germany, is also presented.
17 WIND ENERGY↗
LES Analysis of Loss Anomaly in a Turbine Blade with Large Trailing Edge Radius
There is great interest in the usage of Ceramic Matrix Composites (CMC) as a turbine blade material. CMCs can tolerate higher temperatures than alloyed metallic blades, resulting in better thermal efficiency of gas turbine engines. However, depending on the manufacturing process of the CMC, blades may have a larger trailing edge thickness. The design space therefore needs to be updated due to the resulting flow physics. Recently, experimental results acquired at NASA Glenn Transonic Turbine Blade Cascade Rig showed that a loss measure generally increased with increasing trailing edge thickness. For one case however, the losses initially decreased with increasing Reynolds number but rose and peaked at Re ~1.24x10 6 and then subsequently dropped to the original pre-peak trend. A possible cause of this intriguing phenomenon was speculated to be transonic vortex shedding, which is the mechanism of vortex shedding promoted by reflected shed pressure waves at the trailing edge at relatively high Reynolds numbers and higher Mach numbers. A Reynolds-averaged Navier-Stokes (RANS) analysis or a low-resolution large eddy simulation (LES) does not reproduce this apparent anomaly and thus it is worth performing a wall-resolved LES (the total mesh count of ~290 million cells) to investigate the aerodynamics of the CMC blade in the context of transonic vortex shedding. Our numerical results at Re=1,246,350 show that the pressure waves generated by the vortex shedding in the wake travel upstream and significantly influence the transition and separation on the suction side thus enhancing the said vortex shedding in the wake. This feedback seems not to hold under a low-Re condition (Re=621,900). The Reynolds number dependence was also examined by numerical perturbation of the pressure waves in the wake and by examining how such perturbation attenuates or endures. It is confirmed that the perturbation of the pressure waves is quickly damped.
LES↗
An experimental study of airfoil-spoiler aerodynamics
The steady/unsteady flow field generated by a typical two dimensional airfoil with a statically deflected flap type spoiler was investigated. Subsonic wind tunnel tests were made over a range of parameters: spoiler deflection, angle of attack, and two Reynolds numbers; and comprehensive measurements of the mean and fluctuating surface pressures, velocities in the boundary layer, and velocities in the wake. Schlieren flow visualization of the near wake structure was performed. The mean lift, moment, and surface pressure characteristics are in agreement with previous investigations of spoiler aerodynamics. At large spoiler deflections, boundary layer character affects the static pressure distribution in the spoiler hingeline region; and, the wake mean velocity fields reveals a closed region of reversed flow aft of the spoiler. It is shown that the unsteady flow field characteristics are as follows: (1) the unsteady nature of the wake is characterized by vortex shedding; (2) the character of the vortex shedding changes with spoiler deflection; (3) the vortex shedding characteristics are in agreement with other bluff body investigations; and (4) the vortex shedding frequency component of the fluctuating surface pressure field is of appreciable magnitude at large spoiler deflections. The flow past an airfoil with deflected spoiler is a particular problem in bluff body aerodynamics is considered.
Mclachlan, B. G.↗
Analytical simulation of the far-field jet noise and the unsteady jet flow-field by a model of periodic shedding of vortex ring from the jet exit
The construction of a theoretical flow field due to shedding of vortex rings, the identification of the controlling parameters, and the determination of whether the theoretical model successfully simulated the unsteady pressure field near jet (and consequently the far field noise) was studied. The basic parameters contained in the analytic solutions were the epoch at which a vortex ring was shed near the jet exit and the eddy viscosity coefficient. These parameters were identified from the experimental data for the real-time pressure and from the spread of the mixing layer of the jet. Results of the theoretical analysis show good qualitative agreement with the experimental data.
Liu, C. H.↗
The modelling of symmetric airfoil vortex generators
An experimental study is conducted to determine the dependence of vortex generator geometry and impinging flow conditions on shed vortex circulation and crossplane peak vorticity for one type of vortex generator. The vortex generator is a symmetric airfoil having a NACA 0012 cross-sectional profile. The geometry and flow parameters varied include angle-of-attack alfa, chordlength c, span h, and Mach number M. The vortex generators are mounted either in isolation or in a symmetric counter-rotating array configuration on the inside surface of a straight pipe. The turbulent boundary layer thickness to pipe radius ratio is delta/R = 0. 17. Circulation and peak vorticity data are derived from crossplane velocity measurements conducted at or about 1 chord downstream of the vortex generator trailing edge. Shed vortex circulation is observed to be proportional to M, alfa, and h/delta. With these parameters held constant, circulation is observed to fall off in monotonic fashion with increasing airfoil aspect ratio AR. Shed vortex peak vorticity is also observed to be proportional to M, alfa, and h/delta. Unlike circulation, however, peak vorticity is observed to increase with increasing aspect ratio, reaching a peak value at AR approx. 2.0 before falling off.
Reichert, B. A.↗
LES simulations of a vacuum membrane distillation channel with geometric alterations
3D LES simulations were carried out to study the performance of a vacuum membrane distillation module. A wiggly wall profile with/without embedded stiffeners was considered to alleviate axial and radial temperature polarization, the cause of performance decrease in membrane distillation. Results of the flow field show the wiggles create unsteady vortex shedding inducing intense mixing in the channel. Vortex shedding intensity increases as the Reynolds number increases or stiffeners are added. 3D results show a 56% improvement in flux when moving from a flat sheet membrane to a wiggly membrane with stiffeners at a constant Reynolds number, corresponding to an improvement from 11.7 to 41.7 in the Nusselt number, showing that the alterations improved the flux performance by enhancing the heat transfer along the membrane surface and therefore alleviating temperature polarization, a critical bottleneck in membrane distillation systems. Further, a merit criterion was defined based on the Nusselt number and friction factor, and a 40% increase in merit was shown switching from a flat to a wiggly channel, while a 97% merit increase was seen going from flat to wiggly with stiffeners. A 42% enhancement in the flux was also seen moving from a straight channel to a wiggly channel at a higher Reynolds number which highlights the importance of strategically choosing the mass flow rate, as well as inducing flow separation and vortex shedding in the channel to promote mixing and dissipate the thermal boundary layer. The variation in results between the wiggly module with stiffeners and the wiggly module at high Reynolds numbers suggests enhancing mixing structures can be more impactful on flux for this geometry than increasing the flow rate to a turbulent/transitional regime. However, both are preferable for peak system flux performance. Furthermore, a 2D approximation was used to perform simulations on more extended channels to examine the length degradation. The modules with wiggly channels performed at the same flux level with a doubling of the length. On the other hand, the flat sheet modules experienced length degradation by temperature polarization and dropped in flux yield by around 13% of the short-channel value. This work illustrates that modeling the system and understanding how the performance decreases as the membrane surface area increases are critical for a larger module (scaling up from a lab to a prototype module) to maintain high flux performance.
42 ENGINEERING↗
An exploratory study of the vortex sheets shed from the leading edges of slender wings
Analysis of vortex sheets shed from leading edges of slender wings
SLENDER WING↗
A Numerical Study of Low-Reynolds-Number Separation Bubbles
The present study uses two dimensional numerical simulations to study unsteady low-Reynolds-number separation bubbles. The numerical study is in two parts: (1) a two dimensional time-accurate Navier-Stokes solver is used to simulate flows over the APEX airfoil, and (2) a numerical procedure is developed for localized simulations of transitional separation bubbles. The 2-D computations of flow over the APEX airfoil show that the flow is unsteady with periodic vortex shedding. A linear stability analysis of the separated flow shows that the vortex shedding is caused due to the instability of the separated flow. For transonic flows over the APEX airfoil the vortex shedding is additionally influenced by the presence of shocks. The flowfield has two characteristic time scales, one corresponding to the vortex shedding and another corresponding to the movement of the shocks. The two dimensional (2-D) airfoil simulations also showed the presence of nonlinear effects in the separated region. To better understand the characteristics of separation bubbles a numerical procedure has been developed for localized separation bubble calculations. This procedure is used to perform computations for a flat plate separation bubble test case. The separation bubble is induced by specifying a velocity gradient in the freestream. The growth of disturbances in the separation bubble is analyzed by introducing disturbances upstream of the separation bubble.
Tatineni, Mahidhar↗
Aeroelastic Ground Wind Loads Analysis Tool for Launch Vehicles
Launch vehicles are exposed to ground winds during rollout and on the launch pad that can induce static and dynamic loads. Of particular concern are the dynamic loads caused by vortex shedding from nearly-cylindrical structures. When the frequency of vortex shedding nears that of a lowly-damped structural mode, the dynamic loads can be more than an order of magnitude greater than mean drag loads. Accurately predicting vehicle response to vortex shedding during the design and analysis cycles is difficult and typically exceeds the practical capabilities of modern computational fluid dynamics codes. Therefore, mitigating the ground wind loads risk typically requires wind-tunnel tests of dynamically-scaled models that are time consuming and expensive to conduct. In recent years, NASA has developed a ground wind loads analysis tool for launch vehicles to fill this analytical capability gap in order to provide predictions for prelaunch static and dynamic loads. This paper includes a background of the ground wind loads problem and the current state-of-the-art. It then discusses the history and significance of the analysis tool and the methodology used to develop it. Finally, results of the analysis tool are compared to wind-tunnel and full-scale data of various geometries and Reynolds numbers.
Ivanco, Thomas G.↗
Flow Testing of Corrugated Metal Flexhoses to Evaluate Flow-Induced Vibration and Stiffness
Corrugated metal flexhoses are used to supply fluid routing where straight rigid pipes cannot meet the design requirements due to vibrations, thermal expansion, or motion. These types of hoses are found in a wide range of applications at KSC (Exploration Ground Systems Program in particular) with various fluid commodities such as fuel, oxidizers, coolant, and cryogenics. However, flexibility of design comes at a price: due to increased levels of turbulence generated by the hose geometry, the necessary supply pressure must increase to achieve the same flow rate. Furthermore, the convolutes of the flexhoses interact with the flow field to generate areas of flow separation which leads to a phenomenon known as vortex shedding. When the frequency of vortex shedding couples with the natural frequency of the hose, this can be detrimental and cause premature failure. There are a limited amount of software that can correctly model the coupled fluid structure interactions(FSI) between the solid and fluid physics. This project aims to develop a computer model that can predict Flow-Induced Vibration (FIV) on flexhoses in one of the atypical configurations found in the State-of-the-Art (SOTA) standard for hoses in an angulated state. Our research seeks to extend the literature and expand NASA’s FIV standard. The computational rigor and resources available in the Apollo Era did not allow for flow coupling between fluids and structure interfaces to evaluate FIV of the flexhose. Our team believes that modern techniques and methods of evaluation should be used to reevaluate and extend the database of FIV and stiffness properties of flexhoses to assess the risk of failure. The core performance criteria is a computer model that is able to predict the FIV frequency within 10%.
Jared F. Congiardo↗