Spectra Of Pressure Fluctuations Research Articles

Characteristics of Scalar Frequency-Wave Spectrum of Wall Pressure Fluctuations at Gradient-Free Turbulent Boundary Layer

Characteristics of Scalar Frequency-Wave Spectrum of Wall Pressure Fluctuations at Gradient-Free Turbulent Boundary Layer

Measurements of Natural Turbulence During the BOLT II Flight Experiment

A Mach 6.0 flight experiment was performed to characterize the turbulent skin friction and heat flux associated with natural transition for vehicle-length Reynolds numbers up to 45 million. This boundary-layer turbulence flight, termed BOLT II, was the second in a series coordinated by the Air Force Office of Scientific Research. Surface heat flux, skin friction, and pressure fluctuation spectra were acquired to characterize the transition process. The test geometry used concave curvature and swept leading edges to introduce a boundary layer with stationary laminar vortex streaks, competing transition mechanisms, and complex early turbulence. The analyses also showed that the spatial evolution of turbulence varied with respect to the location of the vortex heating streaks. Prominent overshoots were observed in the early turbulence within the streak. Turbulence data was collected for Reynolds numbers ReL up to 45×106. A common Rex=12×106 was identified as the start of equilibrium turbulence for the data presented. Conjugate heat transfer simulations, both laminar and turbulent, agreed well with the experimental data, including the laminar leading edge. The Reynolds analogy ratios based on the curve fits to the data, including compressibility, were generally between 0.9 and 1.0. The observed variations were likely the result of the spatial separation of the sensors and different definitions of Stanton number normalization between flight and theory.

Transient numerical simulation to investigate pressure fluctuation spectrum and deficit in the near wake of a horizontal axis wind turbine

Wind energy becomes one of the promised solutions as alternatives of using fossil power energy in the upcoming decades. As pressure fluctuation and flow deficit in the near wake play a crucial role in predicting the turbine performance, the novelty in the current study focuses on a three dimensional (3 D) transient numerical simulation to investigate near wake characteristics of NREL phase IV wind turbine. Pressure fluctuation spectrum and wake deficit at four sections downstream the turbine are the main parameters studied as well as the flow streams and iso-surface characteristics. The mentioned parameters are studied at four sections: (0.25 D), (0.5 D), (0.75 D), and (1 D) of turbine diameter. The obtained results are compared with corresponding data from the NREL Phase IV experiments for validation purposes. Results show that the highest-pressure fluctuation spectrum would be at the nearest wake region, and, then it decreases periodically. Hub pressure spectrum density is much lower than that density for the turbine blade. The wake deficit profile shows that the deficit is developing with time at a rate equal to (0.115 rate per sec). In addition, it was found that the wake is recovered just (1 d) of the hub diameter while the blade wake deficit becomes the dominant after (0.75 D) of turbine diameter to cover the whole wake region. It was also found that the downstream distances should be extended more to resolve the whole wake recovery. The comparison with the tests measurements shows agreement between both the numerical and experimental works such that the error doesn't pass 4.3 %.

Physical consistency and invariance in machine learning of turbulent signals

This paper concerns an investigation of the invariance and consistency of deep learning of turbulent pressure fluctuations. The long-short-memory model is employed to predict wall pressure fluctuations across physical regimes featuring turbulence, shock–boundary layer interaction, and separation. The model's sensitivity to the data inputs is examined using different input data sets. Training the deep learning model based on the raw signals from different flow regions leads to large inaccuracies. It is shown that the data must be appropriately pre-processed before training for the deep learning model predictions to become consistent. Removing the mean and using the normalized fluctuating component of the signal, the deep learning predictions not only greatly improved in accuracy but, most importantly, converged and became consistent, provided that the signal sparsity remains within the inertial sub-range of the turbulence energy spectrum cascade. The power spectra of the surface pressure fluctuations reveal that the model provides high accuracy up to a certain frequency for the fully turbulent flow. The deep learning model's consistency is evidenced by being transferable across the various probe positions on the wall despite the significant differences in the turbulent flow properties in the training data set, i.e., signals obtained before, after, and inside the shock–boundary layer interaction regions. The model's prediction consistency and invariance to the turbulent signal training location(s) are promising for applying deep learning models to various turbulent flows.

Investigation of energy loss patterns and pressure fluctuation Spectrum for pump-turbine in the reverse pump mode

Investigation of energy loss patterns and pressure fluctuation Spectrum for pump-turbine in the reverse pump mode

Dynamics and acoustics of bubbly plumes

Transient bubbly plume dynamics including their effects on the free surface were studied numerically using a simple numerical model based on integral theory [1, 2] and Eulerian–Eulerian coupled large Eddy simulation (EE-LES) method. The EE-LES model was validated with the interfacial closures evaluated for drag, lift and virtual mass forces. The simulation data showed good agreement with both numerical predictions and experimental measurements in the literature. The spectrum of the surface pressure fluctuations and acoustical pressure induced by a bubbly plume were extracted from numerical simulations [3]. The effect of interaction of two plumes is discussed. [1] B.R. Morton, G.I. Taylor, J.S. Turner, Turbulent gravitational convection from maintained and instantaneous sources, Proc. R. Soc. Lond. A234, 1–23 (1956) [2] A. Skvortsov, T. DuBois, M. Jamriska, M. Kocan, Scaling laws for extremely strong thermals, Phys. Rev. Fluids 6, 053501 (2021) [3] S.J. Zhu, A. Ooi, R. Manasseh, A. Skvortsov, Prediction of gas holdup in partially aerated bubble columns using an EE-LES coupled model, J. Chem. Eng. Science 217, 115492 (2020)

On wall pressure fluctuations in conical shock wave/turbulent boundary layer interaction

The structure and the frequency spectra of wall pressure fluctuations beneath a planar turbulent boundary layer interacting with a conical shock wave at Mach number $M_\infty =2.05$ and Reynolds number $\textit {Re}_\theta \approx 630$ (based on the upstream boundary layer momentum thickness) are examined to elucidate the effects of pressure gradient and flow separation on the characteristics of the wall pressure fluctuations, by exploiting a direct numerical simulation database. Upstream of the interaction, in the zero pressure gradient region, wall pressure statistics compare well with canonical compressible boundary layers in terms of fluctuation intensities and frequency spectra. Across the main interaction zone (APG1), the root-mean-square of wall pressure fluctuations becomes very large (corresponding to approximately 173.3 dB), with maximum increase approximately 12.7 dB from the incoming level. In the second adverse pressure gradient zone (APG2), the root-mean-square of wall pressure fluctuations attains a second peak (corresponding to $164.7$ dB), with an increase of 8.4 dB from the upstream level. Both the APG1 and APG2 regions feature a substantial fraction of flow reversal events, which are, however, scattered and interspersed with regions of attached flow. The wall pressure power spectral density exhibits a broadband and energetic low-frequency component associated with the global unsteadiness of the separation bubble/conical shock system. Analysis of the two-point correlations and wavenumber/frequency spectra of wall pressure fluctuations further suggests that the typical eddies become more elongated along the spanwise direction, as the flow in the separated region tends to escape the centreline, and the convection velocity is significantly reduced.

Estimation of wavenumber–frequency spectra of wall pressure due to turbulent flow over a flat plate using large-eddy simulation

The wavenumber–frequency spectra (WFS) of turbulent wall pressure fluctuations are used to design underwater acoustic systems, and a method is presented to determine them for flat plate flow using large-eddy simulations (LES). First, the Reynolds averaged Navier–Stokes (RANS) analysis is carried out for flow over the plate using k–ω shear-stress transport model. Flow parameters are extracted from the output of this analysis and used to impose boundary conditions for large-eddy simulations using a smaller domain with finer mesh. Mesh convergence studies are done to establish the adequacy of the proposed meshing scheme for estimating the turbulent boundary layer wall pressures. The stream-wise velocity profiles, turbulence intensities, and power spectra obtained using LES are compared with other computational and experimental results. The time history of fluctuating pressure on the wall at various stream-wise locations is used to estimate the WFS. Estimations are made of the convective ridge as well as sub-convective and low wavenumber portions of the spectrum. The computations are performed at momentum thickness-based Reynolds numbers (Rθ) of 5761, 7988, and 7709. The effect of downstream distance on the WFS is studied by computing it at three downstream locations and shown that the downstream distance has little effect on the WFS once the flow has become fully turbulent. The use of a desktop workstation for the estimation of WFS has not been reported earlier. The results show that the WFS of turbulent pressure due to flow over more complex geometries can be estimated using a similar method.

Numerical study of the natural transition of water boundary layers over heated/cooled flat plates

The natural transition of water boundary layers on heated/cooled flat plates is investigated using numerical methods. The influences of wall temperature, oncoming flow velocity, and onco ming flow temperature on the transition are studied. Laminar basic flows are obtained by solving the Blasius equations, while water temperature is taken into consideration. Linear stability analyses of the basic flows are performed, and the transition positions are predicted using the eN method. The spectra of wall pressure fluctuations in the laminar region are also predicted. The results show that the heated wall produces a fuller velocity profile and a thinner boundary layer for the laminar flow and that a cooled wall causes the opposite effects. Consequently, the heated wall delays transition, while the cooled wall advances transition. As wall temperature increases, the effect of transition delay becomes stronger until the maximal delay effect occurs at an optimal wall temperature. Wall temperatures higher than the optimal value weaken the transition delay effect. This phenomenon of the heated wall is called transition reversal. A heated wall suppresses the wall pressure fluctuation, and a cooled wall has the opposite effect. The oncoming flow velocity has little influence on the non-dimensional transition position, while the oncoming flow temperature has a strong influence on it. At the same wall temperature, a higher oncoming flow temperature leads to a thicker boundary layer, stronger instability, and an earlier transition. A lower oncoming flow temperature leads to stronger sensitivity of the transition to the wall temperature.

Mean-field model of synchronization for open-loop, swirl controlled thermoacoustic system.

Open-loop control is known to be an effective strategy for controlling self-excited periodic oscillations, known as thermoacoustic instability, in turbulent combustors. Here, we present experimental observations and a synchronization model for the suppression of thermoacoustic instability achieved by rotating the otherwise static swirler in a lab-scale turbulent combustor. Starting with the state of thermoacoustic instability in the combustor, we find that a progressive increase in the swirler rotation rate leads to a transition from the state of limit cycle oscillations to the low-amplitude aperiodic oscillations through a state of intermittency. To model such a transition while also quantifying the underlying synchronization characteristics, we extend the model of Dutta et al. [Phys. Rev. E 99, 032215 (2019)] by introducing a feedback between the ensemble of phase oscillators and the acoustic. The coupling strength in the model is determined by considering the effect of the acoustic and swirl frequencies. The link between the model and experimental results is quantitatively established by implementing an optimization algorithm for model parameter estimation. We show that the model is capable of replicating the bifurcation characteristics, nonlinear features of time series, probability density function, and amplitude spectrum of acoustic pressure and heat release rate fluctuations at various dynamical states observed during the transition to the state of suppression. Most importantly, we discuss the flame dynamics and demonstrate that the model without any spatial inputs qualitatively captures the characteristics of the spatiotemporal synchronization between the local heat release rate fluctuations and the acoustic pressure that underpins a transition to the state of suppression. As a result, the model emerges as a powerful tool for explaining and controlling instabilities in thermoacoustic and other extended fluid dynamical systems, where spatiotemporal interactions lead to rich dynamical phenomena.

Influence of Reynolds number on the natural transition of boundary layers over underwater axisymmetric bodies

The influence of the Reynolds number on the natural transition of boundary layers over underwater axisymmetric bodies is studied using numerical approaches. This is a fundamental problem in fluid mechanics and is of great significance in practical engineering problems. The transition locations are predicted over diameter Reynolds numbers ranging from 1.79 × 105 to 2.32 × 108 for eight different forebody shapes. The transition onsets are predicted using the semi-empirical eN method based on the linear stability theory (LST), and the wall pressure fluctuation spectra are estimated. The effects of the forebody shapes and the Reynolds numbers on the transition location are studied. At the same Reynolds number, the forebody shape has a great influence on transition. As the Reynolds number increases, the changes in the dimensionless transition location are qualitatively similar for different forebody shapes. The dimensionless transition location shifts closer to the leading edge as the Reynolds number increases and is more sensitive at lower Reynolds numbers. However, the quantitative changes in transition location for different forebody shapes are distinctly different. Consequently, the sequential order of the transition locations for the eight forebody shapes is not fixed but changes dramatically with increasing Reynolds number. This irregularity in the sequential order of the transition locations is called the “Reynolds number effect.” Finally, the fundamental causes of this effect are analyzed.

The Effect of the Step Height on the Wall Pressure Fluctuations near Its Side Edge in the Turbulent Boundary Layer

Wall pressure fluctuations in a turbulent flow are a source of noise and vibrations in elastic structures immersed in a flow. This paper presents the results of an experimental study on the effect produced by the height of a step on the spatiotemporal structure of wall pressure fluctuations in the vicinity of its side edge in the turbulent boundary layer. Measurements were performed in a subsonic low-noise wind tunnel of the Moscow Complex of the Zhukovsky Central Aerohydrodynamic Institute. The height of a step was varied from 3 to 17% of the incident-boundary-layer thickness. It has been shown that the area of the most intensive pressure fluctuations is located near the frontal side corner of the step. The characteristic Strouhal number determining the spectra of pressure fluctuations behind the leading edge of the step was established. An essential effect of the step height on the spatiotemporal structure of the pressure field in the vicinity of the side edge was shown. The obtained results evidence the existence of a strong correlation with the field of pressure fluctuations in the incident turbulent boundary layer in the case of steps with a small height.

Vibration Characteristics of a Tubular Turbine Prototype at Different Heads with Considering Free Surface and Water Gravity

Tubular turbines are widely used in low water head and tidal power development due to their straight flow path, simple structure, and wide efficient area. However, the severe vibration during actual operation greatly affects the safe operation of the tubular turbine. This study performs a numerical calculation of the tubular turbine, which meets the actual machine conditions considering the free surface and water gravity; compares and analyzes the flow characteristics and pressure fluctuation spectrum characteristics in the tubular turbine under different water heads; and verifies the comparison with the actual machine test results to explore the vibration characteristics and vibration mechanism of the tubular turbine. Research results show that a large pressure difference is observed between the top and bottom of the runner chamber, and the runner needs to experience large periodic pressure fluctuations during rotation due to the combined effect of hydrostatic pressure and hydrodynamic pressure. Under different water heads, obvious flow turbulence and high turbulent kinetic energy areas are observed in the runner and draft tube due to the influence of the shape of the blade wake vortex. The vibration in the tubular turbine is mainly concentrated in the runner and draft tube and influenced by the water gravity and the runner structure of the transverse cantilever beam. The amplitude of pressure fluctuation is the largest when the frequency inside the runner is the blade passing frequency at each water head, so the maximum vibration position is located at the runner. The research results serve as a guide for the design and operation of the horizontal tubular turbine.

Trailing edge noise reduction using bio-inspired finlets

Inspired by the silent flight of the owl, the current work focuses on the reduction of trailing edge noise using 2D finlets. Overset-Large Eddy Simulations are carried out of two configurations, (i) the baseline configuration consisting of a NACA0012 airfoil and (ii) an identical configuration, however, now with finlets installed near the trailing edge. The considered chord based Reynolds number Rec equals 4.2×105 for a non-zero angle of attack α of 4°. The resulting turbulence statistics in the boundary layer and the power spectral density (PSD) spectrum of the surface pressure fluctuations show good agreement with the experimental data. Detailed analysis of the unsteady flow data revealed various noise reduction mechanisms for the here considered case. Firstly, finlets provide a ‘lifting-up’ effect for the most energetic eddies in the boundary layer, which prolongs downstream of the finlets. As a result, the edge scattering is significantly weakened. Secondly, the finlets mounted on the airfoil provide an additional wetted area to the flow which in-turn has a dissipating effect on the surface pressure fluctuations, thereby again weakening the edge scattering phenomena. A reduction in the flow velocity is observed for the flow exiting the finlets channel and towards the trailing edge, which also has a favourable effect in reducing the trailing-edge noise. Finally, a marginal breakdown of the span-wise coherent length scale for frequencies between 2.5 kHz to 9 kHz is also observed.

Parameters of surface standing waves in fluidized bed apparat with different diameter

Regarding medium- and low-temperature processes of heterogeneous heterophase interaction in the units for cleaning from pollutants and trapping CO2, the analogy of the behavior of standing surface waves of an ideal liquid with oscillations of the surface of a fluidized bed is considered. Waves on the surface of a fluidized bed are formed when gas bubbles leave it and are one of the elements of the mechanism of self-sustained fluctuations of the entire mass of the bed. The frequency modes of the surface waves correspond to the main modes in the spectra of pressure fluctuations in the bed and depend on its geometry. The fluctuations of a bubble three-dimensional fluidized bed in apparatuses of different sizes are numerically simulated in a high-performance parallel computing mode using the Ansys Fluent program. Comparison of analytical calculations for fluid oscillations in a vessel with the results of numerical simulation of a fluidized bed and experimental data on the frequency of the emergence of bubbles and modes of surface waves is carried out.

Influence of swirling flow parameters on frequency response of a simplified draft-tube in presence of cavitation

This paper focuses on investigating the influence of swirling flow parameters on frequency response of a simplified draft-tube under different cavitation numbers by using experimental approaches. The experiments are conducted for three swirl configurations featuring different swirl intensities conforming with the flow field in the draft-tube during high-load operation of a hydro-turbine. For this purpose, three stationary-blades swirl generators are used to generate axisymmetric swirling flows in a Venturi-tube with a diffuser resembling the draft-tube cone of a hydraulic turbine; and for each swirl configuration, the experiments are conducted over a wide range of cavitation numbers and flowrate values. Experiments include transient pressure measurements at different locations and high-speed visualization of cavitating flow in the Venturi. The results of transient pressure measurements are analyzed to identify the frequency spectrum of pressure fluctuations in the draft-tube. The first natural frequency of the draft-tube is identified for a wide range of operating conditions; and several correlations are established for its relationship with the flow parameters (cavitation number and swirl number). It is observed that the operating flowrate does not affect the natural frequency of the draft-tube. An unstable mode of cavitating flow is highlighted and a transition from stable to unstable modes is identified. Investigating the transition mechanism from the stable to the unstable modes of cavity led to identifying an excitation force for the onset of cavitation instability in the draft-tube. High-speed visualization of cavitation in the draft-tube is performed to capture the mean cavity volume and its fluctuations in time domain. The 1st natural frequency of the draft-tube is further predicted by analyzing the fluctuations of cavity volume. Furthermore, a 1D hydroacoustic model is also used to predict the local wave speed and the 1st natural frequency of the draft-tube. The obtained values of natural frequency from cavity visualization and 1D modelling are compared with the results of pressure measurements at the end.

Boundary-Layer Instabilities over a Cone–Cylinder–Flare Model at Mach 6

Computations are performed to investigate the convective and global boundary-layer instabilities over a sharp cone–cylinder–flare model at zero-degree angle of attack. The model geometry and the flow conditions are selected to match the experiments conducted in the Boeing/AFOSR Mach 6 Quiet Tunnel at Purdue University. The geometry consists of a nominally sharp 5 deg half-angle cone, followed by a cylindrical segment, and then a 10 deg flare. Additionally, flare half angles equal to 8 and 12 deg and nosetip radii equal to 0.1, 1, and 5 mm are studied. An axisymmetric separation bubble is generated as a result of the laminar shock–boundary-layer interaction in the cylinder–flare region. The comparison of the laminar flow solution and the schlieren images shows a remarkable agreement between the respective locations of both the boundary-layer edge and the reattachment shock. The linear amplification of first and second Mack mode instabilities that begin to amplify in the cone region are computed with a combination of the parabolized stability equations and the harmonic linearized Navier–Stokes equations. The predicted frequency spectra of the surface pressure fluctuations associated with both planar and oblique instability waves capture the distinct lobes within the disturbance amplification spectra measured with PCB® and Kulite sensors. To our knowledge, this represents the first successful comparison between convective instability analysis and measured surface pressure fluctuations for a hypersonic configuration with a separation bubble. Finally, the global instability analysis shows that the laminar flow becomes supercritical for flare half angles larger than 8 deg.

Experimental Study of Hydrodynamic Pressures Acting on a Submerged Gate

Vortical flow formed by a submerged hydraulic jump may produce significant hydrodynamic lift and drag pressures on a gate beneath the hydraulic jump. In this study, an experimental setup equipped with multi-pressure sensors was used to measure fluctuating impact pressures on the submerged gate for different flow conditions characterized by the inlet Froude number and submergence factor (S). Time-averaged and instantaneous pressure coefficients are evaluated based on simultaneous measurements of wall pressures at multiple locations, including those at the lip and downstream face of the gate. In particular, instantaneous lift pressure coefficients are observed to be independent of the submergence ratio for S > 0.6. It is found that low Froude number flows produce high surface pressure fluctuations and the dominant frequency of pressure fluctuations shifts to higher frequency as the Froude number increases. Pressure measurements for the free hydraulic jump suggest that the power spectra of lift pressure fluctuations are devoid of any significant energy level, whereas the resulting power spectra of the submerged flow exhibits significant energy level in the frequency range of 14-19 Hz. The proposed measurement system can be used for the in situ identification of hydrodynamic pressures acting on the gates in irrigation canals.

Supersonic Freestream Density Fluctuations from Focused Laser Differential Interferometry and Pitot-Probe Measurements

Focused laser differential interferometry (FLDI) is used to make freestream density fluctuation measurements in the von Kármán Gas Dynamics Facility (VKF) Wind Tunnel D at Arnold Engineering Development Complex. A comparison is made between pitot-probe and FLDI measurements after converting both to freestream pressure fluctuation spectra. A modification of Stainback and Wagner’s theory, incorporating recent numerical results from Chaudhry et al., is used to interpret the pitot data, while a new inversion algorithm is applied to the FLDI data. These methods of interpretation make the assumption that the tunnel freestream disturbance environment is dominated by the acoustic Kovásznay mode, in line with the theoretical and experimental evidence. Close agreement is found between the two sets of spectra, showing that accurate quantitative data can be obtained with FLDI, and the instrument can be used to extend spectra beyond the pitot bandwidth.

Influence of Roughness Trips on Near- and Far-Field Trailing-Edge Noise

Roughness trips are widely applied in wind-tunnel testing to hasten the laminar–turbulent boundary-layer transition. It is well known that they influence the boundary-layer transition and its development. However, their influence on aeroacoustic quantities (in particular, on the airfoil trailing-edge noise) is not yet fully understood. This paper discusses the relationship between the trip height and the boundary-layer development and radiated trailing-edge noise. Wind-tunnel experiments were performed with zigzag strips and grit elements placed on the surface of a NACA 0012 airfoil. The roughness trip heights ranged from 17 to 214% of the undisturbed boundary-layer thickness at the trip location. The chord-based Reynolds number varied from to . This paper shows that trip heights within 50 to 105% of have little influence on the near field and the trailing-edge noise. However, trips with increase the spectra of velocity and pressure fluctuations in the low-frequency range up to 2.5 and 3 dB, respectively, resulting in an increase of the trailing-edge radiated noise up to 4 dB in a similar frequency range. In summary, this paper shows the importance of carefully defining the roughness trip used to trigger a laminar–turbulent transition in aeroacoustic and aerodynamic measurements.