Near-field Flow Research Articles

Noise Simulation around NACA0012 Wingtip using Large Eddy Simulation

A flow around NACA0012 wingtip is solved using a zonal LES/RANS hybrid method in order to understand the noise generation mechanism around the flap-edge. It is known from the previous studies that the flow around the blunt wingtip is similar to that of the flap-edge. Grid dependency studies are performed for both timeaveraged and unsteady components and the results are assessed by comparing with the experimental data. It became apparent that zonal LES/RANS hybrid results are more sensitive to the chordwise grid resolution, compared with that of RANS. Using the validated data, the noise generation mechanism around the wingtip is discussed. The near-field flow structures relevant to noise generation are obtained successfully.

The effects of initial perturbation to mixing-layer noise

The far-field noise radiated from mixing layers is determined by the near-field flow dynamics which is sensitive to the initial perturbation of instability introduced physically or numerically. This study focuses on the effects of the phase delay in two initial perturbations, one at the fundamental wave number and the other at its subharmonic both calculated from linear instability analysis, on the sound generation in mixing layers. When different phase delays φ1 changing from zero to 2π is applied on the fundamental mode, we observe different vortex merging processes (e.g. vortex pairing or tearing). The strong nonlinear interaction in the merging process generates most of the noise from mixing layers. There shows a pattern in a period of 2π for the response of far-field sound to the change of φ1. Similar effects on the dynamics and acoustics can be achieved by adding different phase delays φ2 to the subharmonic mode instead, however, the response repeats in a period of only π for φ2. The effects of the combination of different phase delays to other parameters, including the amplitude and wave number for each perturbations, are also investigated. All the results indicate a critical role of nonlinearity in the sound generation mechanism of mixing layers.

Vortex Shedding and its Nonlinear Acoustic Effect Occurring at a Slit

This paper presents a theoretical study of the nonlinear acoustic properties of a slit in a thin plate subjected to highintensity sound excitation. In the present model, the conventional discrete vortexmethod is employed to simulate the near-field two-dimensional unsteady flow in order to capture the mechanism of the sound–vortex interaction, which is in combination with a spanwise-averaged three-dimensional Green’s function method used to connect the nearfield flow quantities with the far-field sound pressure. This model is compared with an existing particle image velocimetry flow visualization and a direct numerical simulation model, showing good qualitative agreement. It is revealed that the oscillatory slitflow is dominated by a pair of spiral-like counter-rotating vorticesmoving away from the slit and eventually colliding into each other. Because of the vortical flow effect, increases in the sound pressure level result in significant increases in acoustic resistance and modest decreases in acoustic reactance. For the parametric range in this study, increases in the aspect ratio of the slit result in slight increases in acoustic resistance but unnegligible reductions in acoustic reactance. However, the influence of the aspect ratio on acoustic impedance tends to be less important when the sound pressure level exceeds a certain high value.

Energy-based acoustic measurement system for rocket noise

Accurate estimates of the vibroacoustic loading placed on space vehicles and payloads during launch require knowledge of rocket noise source properties and near-field acoustic energy flow characteristics. Without these data, structures may not be designed to handle the correct vibroacoustic loads, which can result in either an over-built, excessively massive structure or an under-designed vibration mitigation system that could result in damage to payloads. These measurements are difficult to perform because of the extreme nature of the acoustic and temperature environments near the rocket plume as well as the large physical size of the rocket noise source. With these design constraints in mind, a field-deployable data acquisition system and energy-based measurement probe have been developed to measure the magnitude, directivity, and spectral content of the rocket source. Initial measurements with various prototypes were conducted during a static test fire at ATK Space Systems Test Services in Promontory, Utah with limited results presented here. [Work sponsored by NASA John C. Stennis Space Center.]

A liquid crystal polymer membrane MEMS sensor for flow rate and flow direction sensing applications

The paper reports the design, fabrication and experimental results of a liquid crystal polymer (LCP) membrane-based pressure sensor for flow rate and flow direction sensing applications. Elaborate experimental testing results demonstrating the sensors' performance as an airflow sensor have been illustrated and validated with theory. MEMS sensors using LCP as a membrane structural material show higher sensitivity and reliability over silicon counterparts. The developed device is highly robust for harsh environment applications such as atmospheric wind flow monitoring and underwater flow sensing. A simple, low-cost and repeatable fabrication scheme has been developed employing low temperatures. The main features of the sensor developed in this work are a LCP membrane with integrated thin film gold piezoresistors deposited on it. The sensor developed demonstrates a good sensitivity of 3.695 mV (ms−1)−1, large operating range (0.1 to >10 ms−1) and good accuracy in measuring airflow with an average error of only 3.6% full-scale in comparison with theory. Various feasible applications of the developed sensor have been demonstrated with experimental results. The sensor was tested for two other applications—in clinical diagnosis for breath rate, breath velocity monitoring, and in underwater applications for object detection by sensing near-field spatial flow pressure.

Numerical simulation for prediction of aerodynamic noise characteristics on a HAWT of NREL phase VI

The purpose of this study is to numerically predict the characteristics of aerodynamic noise generated from rotating wind turbine blades according to wind speeds using commercial CFD code, FLUENT. The near-field flow around a HAWT of NREL Phase VI is simulated directly by LES, whereas the far-field aerodynamic noise for frequencies below 500 Hz is modeled using FW-H analogy. As there was no experimental noise data, we first compared aerodynamic noise analysis with experimental data. This result showed a difference of power outputs by 0.8% compared with the experimental one with 6.02 kW. Then the characteristics of aerodynamic noise were predicted at a specific location P1 according to IEC 61400-11 international standard. When the wind turbine blades rotate with time, tip-vortices occur at the tip of two blades and are generated periodically in a circle. These vortices in the vicinity of the blade tip cause intense aerodynamic noise due to the tip vortex-trailing edge interaction by local cross flows along the trailing edge. In a wind speed of 7m/s the sound intensity ratio of quadrupole to dipole at P1 location is about 21.1%, but as wind speed increases the sound intensity ratio increases up to 54.3% in the case of no-weighted correction. This means that there is a considerably close relation between the quadrupole noise by small and large scales and the increase of wind speeds. With the purpose of a rough prediction of sound power level, CFD results were compared with a simple model of previous researchers and showed a good agreement with one by Hagg of three other models.

IH-3VOF: A THREE-DIMENSIONAL NAVIER-STOKES MODEL FOR WAVE AND STRUCTURE INTERACTION

This paper describes the capability of a new model, called IH-3VOF to simulate wave-structure interaction problems using a three-dimensional approach. The model is able to deal with physical processes associated with wave interaction with porous structures. The model considers the VARANS equations, a volume-averaged version of the traditional RANS (Reynolds Averaged Navier-Stokes) equations. Turbulence is modeled using a k-ï¥ approach, not only at the clear fluid region (outside the porous media) but also inside the porous media. The model has been validated using laboratory data of free surface time evolution in a fish tank containing a porous dam. Numerical simulations were calibrated by adjusting the porous flow empirical coefficients for two granular material characteristics. Sensitivity analysis of porous parameters has also been performed. The model is proven to reproduce with a high degree of agreement the free surface evolution during the seeping process. Simulations of a three- dimensional porous dam breaking problem has been studied, showing the excellent performance of the model in reproducing fluid patterns around a porous structure. The model is powerful tool to examine the near-field flow characteristics around porous structures in three dimensional flow conditions.

Combined Experimental and Numerical Diagnostics for Near-field Flow around a Supersonic Flight Model

Combined Experimental and Numerical Diagnostics for Near-field Flow around a Supersonic Flight Model

Predicting near-field dam-break flow and impact force using a 3D model

A three-dimensional (3D) numerical model based on the unsteady Reynolds equations was used to simulate near-field dam-break flows and estimate the impact force on obstacles. The model employs a projection method to solve the governing equations and the method of volume of fluid (VOF) to capture the water surface movement. The model is first applied to simulate two physical model experiments of dam-break flows. Model-predicted pressure, water depth and velocity distributions are compared with laboratory measurements. For the second case, the 3D-VOF model predictions are also compared with predictions made by a two-dimensional model. The 3D-VOF model is then used to calculate the impact force of dam-break flow on a steady obstacle. A physical model experiment is set up to assist the numerical model study. The model-predicted impact force on the obstacle and the critical condition for it to move are compared with the measurements from the experiment.

Behaviour of adult Pacific lamprey in near-field flow and fishway design experiments

Abstract An experimental fishway was used to examine adult Pacific lamprey, Lampetra tridentata (Gairdner), behaviour in a series of attraction and passage-performance tests. Among all experiments, lamprey oriented to the fishway floor and walls and were attracted to both ambient and concentrated flow. When confronted with high-velocity areas (vertical-slot and submerged-orifice weirs), many lamprey failed to pass upstream. However, lamprey were able to find and take advantage of low-velocity refuges when they were provided. Lamprey climbed shallow- and steep-angled ramps when attraction cues were sufficient and other passage routes were restricted. The combined results demonstrated the passage challenges that fishways designed and operated for salmonids present to non-salmonid species. They also highlight the importance of evaluating trade-offs between fishway attraction and passage efficiency. The experiments were integrated with tagging studies and development of lamprey-specific passage structures, a research combination that provided an effective template for fishway performance evaluations.

Coral density and predation affect growth of a reef-building coral

The influence of predation on the growth of stony corals has gained increased attention, although the degree to which coral conspecific density can modify the effects of corallivores remains poorly studied. Here, a field experiment was used to quantify the independent and combined effects of coral colony density and coral predators on the skeletal growth of massive Porites. Predator exclusion increased coral growth by 20%. Increasing coral density increased growth by 30%. However, the effect of predators was independent of coral density. Possible alternative mechanisms for increased skeletal growth at higher colony density include changes in near-field flow, resulting in increases in photosynthetic activity, nutrient uptake, or the increased accessibility of coral mutualists.

Noise prediction of a subsonic turbulent round jet using the lattice-Boltzmann method

The lattice-Boltzmann method (LBM) was used to study the far-field noise generated from a Mach, M(j)=0.4, unheated turbulent axisymmetric jet. A commercial code based on the LBM kernel was used to simulate the turbulent flow exhausting from a pipe which is 10 jet radii in length. Near-field flow results such as jet centerline velocity decay rates and turbulence intensities were in agreement with experimental results and results from comparable LES studies. The predicted far field sound pressure levels were within 2 dB from published experimental results. Weak unphysical tones were present at high frequency in the computed radiated sound pressure spectra. These tones are believed to be due to spurious sound wave reflections at boundaries between regions of varying voxel resolution. These "VR tones" did not appear to bias the underlying broadband noise spectrum, and they did not affect the overall levels significantly. The LBM appears to be a viable approach, comparable in accuracy to large eddy simulations, for the problem considered. The main advantages of this approach over Navier-Stokes based finite difference schemes may be a reduced computational cost, ease of including the nozzle in the computational domain, and ease of investigating nozzles with complex shapes.

On the near-field flow structure and mode behaviors for the right-angle and sharp-edged orifice plane jet

The study of flow structures in a sharp-edged and right-angle orifice plane jet was investigated experimentally by means of hot-wire technique and smoke flow visualization. The results indicate that the vena contracta effect depends upon the Reynolds number as well as the geometric configuration of the orifice exit. The vena contracta effect in the sharp-edged orifice plane jet is much significant and to suppress the development of coherent structures. In addition, the sharp-edged orifice plane jet is considered more complexity since two competing initial instability frequencies were found at Re = 10,300 or above. Meanwhile, two types of instability modes exist in the orifice plane jet, i.e. varicose and sinuous mode. These instability modes were observed to depend upon the Reynolds number and geometric configuration of orifice exit as well.

On Turbulent Jets Issuing from Notched-Rectangular and Circular Orifice Plates

This paper reports an experimental investigation of the near-field flow characteristics of two air/air turbulent jets issuing respectively from notched-rectangular and circular orifice plates with identical opening areas or equivalent diameters (De). Planar particle image velocimetry (PIV) was used to measure the velocity field at the same Reynolds number, based on De, of Re = 72,000. Consistent with previous work on other noncircular jets, the present study finds that the notched jet has a higher rate of mixing than does the circular counterpart. In particular, this jet in the very near field transfers its momentum to the surroundings at a greater rate, evidenced by a notably shorter unmixed core and faster turbulence intensity growth. The higher rates of overall decay and spread of the notched jet are maintained over the entire measurement region and likely beyond. In addition, the phenomenon of axis switching is also found to occur in this jet.

High-order sonic boom modeling based on adaptive methods

This paper presents an accurate approach to simulate the sonic boom of supersonic aircrafts. The near-field flow is modeled by the conservative Euler equations and is solved using a vertex-centered finite volume approach on adapted unstructured tetrahedral meshes. A metric-based anisotropic mesh adaptation is considered to control the interpolation error in L p norm. Then, from the CFD solution, the pressure distribution under the aircraft is extracted and used to set up the initial conditions of the propagation algorithm in the far-field. The pressure distribution is propagated down to the ground in order to obtain the sonic boom signature using a ray tracing algorithm based upon the Thomas waveform parameter method. In this study, a sonic boom sensitivity analysis is carried out on several aircraft designs (low-drag and low-boom shapes).

Terahertz transmission properties of an individual slit in a thin metallic plate

We report on the terahertz transmission properties through a single slit in a thin metallic film. The properties are studied by comparing the transmissions of TE- and TM-polarized electromagnetic waves over a broad spectral range from the geometrical regime to the subwavelength limit. In the geometrical regime, the remarkable terahertz transmission due to guided modes is observed even without the contribution of surface waves. Whereas in the subwavelength limit, the surface charge oscillations associated with the TM-polarized guided mode give rise to strong transmission enhancement. The nature of the mechanisms for the terahertz transmission is elucidated using theoretical simulations of the near-field distributions and electromagnetic energy flow.

Time resolved investigations on flow field and quasi wall shear stress of an impingement configuration with pulsating jets by means of high speed PIV and a surface hot wire array

The effects of jet pulsation on flow field and quasi wall shear stress of an impingement configuration were investigated experimentally. The excitation Strouhal number and amplitude were varied as the most influential parameters. A line-array with three submerged air jets, and a confining plate were used. The flow field analysis by means of time resolved particle image velocimetry shows that the controlled excitation can considerably affect the near-field flow of an impinging jet array. These effects are visualized as organization of the coherent flow structures. Augmentation of the Kelvin–Helmholtz vortices in the jet shear layer depends on the Strouhal number and pulsation magnitude and can be associated with pairing of small scale vortices in the jet. A total maximum of vortex strength was observed when exciting with Sr = 0.82 and coincident high amplitudes. Time resolved interaction between impinging vortices and impingement plate boundary layer due to jet excitation was verified by using an array of 5 μm surface hot wires. Corresponding to the global flow field modification due to periodic jet pulsation, the impact of the vortex rings on the wall boundary layer is highly influenced by the above mentioned excitation parameters and reaches a maximum at Sr = 0.82.

A Digital Particle Image Velocimetry Study on Jets Issuing from Hybrid Inclined Nozzles

Digital particle image velocimetry was used to study hybrid inclined nozzles formed by combining flat- and inclined-sections, where the latter are designed based on the aspect-ratios (AR = 2 and 4) of half-ellipses. Results show that AR2 nozzle exhibits flow behaviour largely similar to inclined nozzles with inclined vortex roll-ups moving away from the nozzle centerline. In contrast, AR4 nozzle leads to significantly more intense near-field flow behaviour caused by the sharper junctions, which prevent similar movement of the vortex roll-ups. Streamwise vortices are also observed to form off the peaks of inclined-sections which produce wider jets-spreads along the inclined-sections due to associated lateral jet fluid ejection, though there is a limit to the jet-spread increment. Lastly, both nozzles produce higher turbulent stress levels over those of the conventional circular nozzle, and vortex roll-up vectoring leads to higher turbulent stresses for the AR2 nozzle along certain measurement planes.

Multiple-point statistical prediction on fracture networks at Yucca Mountain

In many underground nuclear waste repository systems, such as Yucca Mountain project, water flow rate and amount of water seepage into the waste emplacement drifts are mainly determined by hydrological properties of fracture network in the surrounding rock mass. Natural fracture network system is not easy to describe, especially with respect to its connectivity which is critically important for simulating the water flow field. In this paper, we introduced a new method for fracture network description and prediction, termed multi-point-statistics (MPS). The process of Multi-point Statistical method is to record multiple-point statistics concerning the connectivity patterns of fracture network from a known fracture map, and to reproduce multiple-scale training fracture patterns in a stochastic manner, implicitly and directly. It is applied to fracture data to study flow field behavior at Yucca Mountain waste repository system. First, MPS method is used to create fracture network with original fracture training image from Yucca Mountain dataset. After we adopt a harmonic and arithmetic average method to upscale the permeability to a coarse grid, THM simulation is carried out to study near-field water flow in surrounding rock of waste emplacement drifts. Our study shows that connectivity or pattern of fracture network can be grasped and reconstructed by Multi-Point-Statistical method. In theory, it will lead to better prediction of fracture system characteristics and flow behavior. Meanwhile, we can obtain variance from flow field, which gives us a way to quantify uncertainty of models even in complicated coupled THM simulation. It indicates that Multi-Point Statistics is a potential method to characterize and reconstruct natural fracture network in a fractured rock mass with advantages of quantifying connectivity of fracture system and its simulation uncertainty simultaneously.

Near-field flow structure of a confined wall jet on flat and concave rough walls

Experimental results are presented of the mean flow and turbulence characteristics in the near field of a plane wall jet issuing from a nozzle onto flat and concave walls consisting of fixed sand beds. This is a flow configuration of interest for sediment erosion, also referred to as scouring. The measurements were made with an acoustic profiler that gives access to the three components of the instantaneous velocities. For the flat-wall flow, it is shown that the outer-layer spatial growth rate and the maxima of the Reynolds stresses approach the values accepted for the far field of a wall jet at a downstream distance x/b0 ≈ 8. These maxima are only about half the values of a plane free jet. This reduction in Reynolds stresses is also observed in the shear-layer region, x/b0 < 6, where the Reynolds shear stress is about half the value of a free shear layer. At distances x/b0 > 11, the maximum Reynolds shear stress approaches the value of a plane free jet. This change in Reynolds stresses is related to the mean vertical velocity that is negative for x/b0 < 8 and positive further downstream. The evolution of the inner region of the wall jet is found to be in good agreement with a previous model that explicitly includes the roughness length.On the concave wall, the mean flow and the Reynolds stresses are drastically changed by the adverse pressure gradient and especially by the development of Görtler vortices. On the downslope side of the scour hole, the flow is nearly separating with the wall shear stress tending to zero, whereas on the upslope side, the wall-friction coefficient is increased by a factor of about two by Görtler vortices. These vortices extend well into the outer layer and, just above the wall, cause a substantial increase in Reynolds shear stress.