Unsteady Pressure Field Research Articles

Physics of Dynamic Stall Vortex During Pitching Oscillation of Dynamic Airfoil

Dynamic stall, which has a significant effect on the aerodynamic performances of dynamic airfoils, is closely related to the physics of the dynamic stall vortex (DSV). The physics of the DSV on the NACA 0012 airfoil was experimentally studied using unsteady pressure measurements with high time accuracy. The experimental Reynolds number was Re = 1.5 × 106, and the reduced frequency was k = 0.069. The propagation of the unsteady pressure field during the dynamic stall process was analyzed in detail. The motion characteristics of the DSV were examined, including its near-wall development characteristics and near-wall evolution velocity. Moreover, the frequency characteristics of the near-wall DSV were studied using wavelet analysis combined with proper orthogonal decomposition (POD) technology. In addition, the effects of the mean angle of attack (AoA) and the amplitude on the DSV motion and frequency characteristics were examined in detail. The effects of the mean AoA on the near-wall DSV strength and the propagation velocity were linear, while the effects of amplitude were nonlinear. The mean AoA and amplitude had a significant influence on the frequency of the leading-edge vortex (LEV) at the initial stage of the DSV development (x/c = 0.10–0.20). This work allows the DSV physics to be understood more deeply.Graphic abstract

Influence of the elbow shape on the unsteady pressure field in decelerated swirling flows

The induced flow instabilities are developed in the draft tube cone of the hydraulic turbines leading to the pressure fluctuations with negative impact on the mechanical components lifetime. Usually, two components (rotating and plunging) are discriminated in the unsteady pressure field induced by the swirling flow in the draft tube cone. The rotating component is associated with the precession motion of the flow instabilities (e.g. vortex rope) while the plunging part corresponds to the interaction of the swirling flow with the elbow. Extensive experimental investigations were carried out on Timisoara Swirl Generator test case in order to examine the influence of the elbow shape on the unsteady pressure field. Two elbow shapes (90° S shape elbow and 90° sharp heel) are selected to investigate their influence on the unsteady pressure field. Several geometrical configurations with pipe extensions placed between the cone and the elbow are considered for each elbow shape to determine the response of the hydraulic circuit. Two unsteady pressure signals located in opposite positions are measured on four levels placed along the cone. The Fourier spectra are determined for discriminated time series identifying both rotating and plunging components. Then, the ratio between the frequencies of the plunging and the rotating components is obtained for both elbow shapes in relation to the pipe length placed between the cone and the elbow. It is clear that the shape of the elbow and the distance between the cone and the elbow lead to a significant influence on the response of the hydraulic circuit.

Experimental investigation of swirl number influence on spiral vortex structure dynamics

The hydraulic turbines are recently forced to operate far away from the optimal conditions in order to balance fluctuations in electricity generation. In case of Francis, pump and propeller turbines, using only single control component of guide vanes, it means that in regions where the high residual swirl enters the draft tube, the flow is decelerated and convenient conditions for the vortex rope development are created. Such flow conditions are considered to be the triggering mechanism for occurrence of different forms of vortex structures in the Francis turbine draft tube, e.g. spiral or straight vortex rope at part load or full load respectively. Independently on the vortex rope shape the unsteady pressure fields develop producing periodic stress on turbine components and possibly resulting in noise, blade cracks, runner lift, power swing, etc. To study and mimic such flow conditions, a simplified device of vortex generator apparatus is employed. Thanks to its design, the vortex generator enables to change the ratio between fluxes of axial momentum and tangential moment of momentum of generated swirl. Then, the behavior of vortex structure changes in a similar way as the flow rate variation in the draft tube of Francis turbine. For above mentioned reasons the unsteady cavitating spiral vortex is experimentally studied using both high speed video record and particle image velocimetry (PIV). The main focus is on change of vortex dynamics regarding to the swirl number variation. The proper orthogonal decomposition (POD) together with the classical fast Fourier transformation (FFT) are employed to extract dominant modes and frequencies from experimental data.

DGEN Aeropropulsion Research Turbofan Core/Combustor-Noise Measurements—Experiment and Modal Structure at Core-Nozzle Exit1

Abstract Data from a recent core/combustor-noise source-diagnostic test (SDT) utilizing a small turbofan engine are analyzed. The campaign continued the exploration begun in a baseline test, but with more extensive acoustic instrumentation. Both tests were aimed at developing a better understanding of propulsion-noise sources and their impact on the farfield noise signature, in order to enable improved turbofan noise-prediction methods and noise-mitigation techniques. Simultaneous high-data-rate acoustic measurements (93 channels in total) were obtained using a circumferential sensor array at the core-nozzle exit in conjunction with sideline and farfield microphone arrays for several relevant engine operational points. Measurements were repeated for different circumferential and sideline array configurations, as well as for redundancy. The unsteady pressure field at the core-nozzle exit is documented in detail. Previous work suggested that the ±1 azimuthal duct mode could be cut on at this location, which would have implications for combustor-noise modeling and prediction. The modal decomposition of the combustor noise at the core-nozzle exit verifies this observation. Select farfield sound pressure level (SPL) spectra are also presented.

Influence of the Reshaped Elbow on the Unsteady Pressure Field in a Simplified Geometry of the Draft Tube

The paper focuses on the influence of the reshaped elbow geometry of the draft tube on the unsteady pressure field under different operating conditions. The experimental investigations are conducted considering two simplified geometrical configurations of the draft tubes with sharp heel elbow and modified/reshaped elbow, respectively. The discriminated power spectra (rotating and plunging components) of the acquired pressure signals on the wall are determined on five levels for seven operating conditions to quantify the influence of the reshaped elbow. The influence of the reshaped elbow on the fundamental frequencies of both rotating and plunging components and on the amplitude of the rotating component is negligible. In contrast, the equivalent amplitude associated with the root mean square of the plunging power spectrum that propagates along the hydraulic passage is mitigated up to 25% by the reshaped geometry of the elbow. The equivalent amplitude on the narrow band around the fundamental frequency of the plunging component is diminished with 40–50% by reshaping the elbow geometry of the simplified draft tube.

Characteristic unsteady pressure field on a civil aircraft wing related to the onset of transonic buffet

Unsteady pressure fields on a civil aircraft wing near the onset of transonic buffet have been investigated experimentally to identify flow unsteadiness that relates to the buffet onset determined by global criteria. An 80%-scale NASA Common Research Model was tested in the JAXA 2 m × 2 m Transonic Wind Tunnel at a Mach number of 0.85 and a chord Reynolds number of 2.27 × 106. The angle of attack was varied in small increments around the buffet onset angle determined by global criteria based on the lift curve and wing-root strain-gauge data. Unsteady pressure fields over the wing were measured using unsteady pressure-sensitive paint (PSP) with temperature-effect correction by temperature-sensitive paint (TSP). Characteristic pressure fluctuations, known as “buffet cells”, were observed under the off-design conditions at a bump Strouhal number of 0.2–0.5. The PSP results showed that the buffet cells arise at the mid-span wing at η ≈ 0.45, where a strong shock wave causes an initial boundary-layer separation. Phase shift distributions indicated that a pressure perturbation propagates from the inboard wing toward the outboard wing. The convection velocity and spanwise wavelength were approximately 0.5U∞ and 1.3cMAC, respectively. The angle of attack at which buffet cells first appear was found to be approximately equal to the buffet onset determined by the global criteria, indicating that the occurrence of the buffet cells is deeply related to the buffet onset for the present wing geometry.

МОДЕЛЮВАННЯ РОЗДРІБНЕННЯ ГАЗОВОЇ БУЛЬБАШКИ НА ОСНОВІ АНАЛОГІЇ З ХИТНОЮ ПРУЖИНОЮ

Extinguishing fires in gas-liquid mixtures in vertical tanks containing flammable liquids should be accompanied by constant crushing of gas bubbles in these mixtures. The mechanical method makes it possible to crush the bubbles due to acoustic pressure waves generated by impacts on the metal membrane. A more progressive method of crushing involves the action of an acoustic wave directly on the bubbles. This allows you to intervene in the process of grinding the bubbles by changing the pressure frequency, which affects the quality of the gas-liquid mixture. In the works of Petrov A. and his students proposed a resonant model of grinding a gas bubble in a liquid in an unsteady pressure field. Resonant fragmentation of a bubble in a liquid occurs due to the transfer of energy between the radial and deformation modes of vibrations. An interesting effect is observed - with a relatively small amplitude of the alternating pressure of the acoustic wave in the liquid, a sufficiently large amplitude of deformation vibrations develops - due to which the bubble is crushed. A feature of research is the use for this mechanical analogue - a swinging spring (swinging spring). That is, a variety of a pendulum consisting of a point load attached to a weightless spring. The second end of the spring is fixed motionless. The pendulum oscillations of the spring in the vertical plane are studied, provided that its axis are straightforward. The feasibility of choosing such an analogue is explained by the need to study the dynamic system "grinding of the bubble" when nonlinearly coupled vibrational components exchange energy with each other. Indeed, in the case of a bubble, an energy exchange occurs between the radial and deformation modes of vibrations. In this paper, this phenomenon is investigated using the mathematical apparatus of a swinging spring, which illustrates the energy exchange between pendulum and spring oscillations. Keywords: swing spring, Lagrange mechanics, generalized coordinates, load path.

Multiscale Filtering of Compressible Wave Propagation in Complex Geometry through a Wavelet-Based Approach in the Framework of Pressurized Water Reactors Depressurization Transient Analysis

The proposed research takes place in the framework of the analysis of the mechanical consequences of accidental scenarios for pressurized water reactors (PWR). It is particularly dedicated to the effects of the propagation of a transverse rarefaction wave through the assemblies of the nuclear core, consecutive to a pipe break in the primary circuit of the reactor. This paper focuses on the representation, with a reduced number of well-chosen variables, of a pressure wave propagating through a highly congested medium composed of rod bundles, with the primary objective of accurately evaluating the resulting pressure forces exerted on the rods. To achieve this goal, a description of the fluid domain as a homogenized or porous medium is introduced, yielding the need for a new filtering technique to be applied to the fluid fields. A new homogenized and multiscale representation of the fluid variables, based on continuous wavelet transform (CWT), is thus proposed. The capabilities of CWT to accurately approximate a reference representative unsteady pressure field, corresponding to a wave propagation at microscale, is assessed. The proposed technique is applied to a pressure field obtained numerically at local scale. The number of variables that shall be kept at macroscale to have a meaningful representation of the pressure field is fully evaluated through the comparison of the fluid force applied to the microstructure.

Instantaneous pressure determination from unsteady velocity fields using adjoint-based sequential data assimilation

A sequential data assimilation (DA) method is developed for pressure determination of turbulent velocity fields measured by particle image velocimetry (PIV), based on the unsteady adjoint formulation. A forcing term F, which is optimized using the adjoint system, is added to the primary Navier–Stokes (N–S) equations to drive the assimilated flow toward the observations at each time step. Compared with the conventional unsteady adjoint method, which requires the forward integration of the primary system and the backward integration of the adjoint system, the present approach integrates the primary-adjoint system all the way forward, discarding the requirement of data storage at every time step, being less computationally resource-consuming, and saving space. The pressure determination method of integration from eight paths [J. O. Dabiri et al., “An algorithm to estimate unsteady and quasi-steady pressure fields from velocity field measurements,” J. Exp. Biol. 217, 331 (2014)] is also evaluated for comparison. Using synthetic PIV data of a turbulent jet as the observational data, the present DA method is able to determine the instantaneous pressure field precisely using the three-dimensional velocity fields, regardless of the observational noise. For the two-dimensional three-component (3C) or two-component (2C) velocity fields, which are not sufficient for pressure determination by the integration method due to the lack of off-plane derivatives, the present DA method is able to reproduce pressure fields whose statistics agree reasonably well with those of the referential results. The 3C and 2C velocity fields yield quite similar results, indicating the possibility of pressure determination from only planar-PIV measurements in turbulent flows. The tomography PIV measurements are also used as observational data, and a clear pressure pattern is obtained with the present DA method.

Experimental study of swirling flow from conical diffusers using the water jet control method

The actual requirements of the energy market enforce the hydraulic turbine to operate far from the best efficiency point. When hydraulic turbines operate at partial discharge, downstream of the runner (in the conical diffuser), the decelerated swirling flow becomes highly unstable. In these conditions a spiral vortex breakdown occurs, also known in engineering literature as the precessing vortex rope. The flow unsteadiness produced by the vortex rope results in severe pressure fluctuations that hinder the turbine operation or may cause accidents. We propose the water jet method for decelerated swirling flow with vortex rope from conical diffuser to mitigate the unsteadiness. The method involves injecting water at the inlet of the conical diffuser. Initial experimental investigations of the unsteady pressure field at conical diffuser wall reveal that the water injection method mitigates the pressure pulsations associated to the precessing vortex rope. In this paper, we investigate experimental measurements of the unsteady velocity field in a conical diffuser using LDA (Laser Doppler Anemometry). The main goal of the paper is to show how different water-jet discharge rates change the velocity field and the characteristics of the vortex rope in the wake.

Spatiotemporal Characterization and Suppression Mechanism of Supersonic Inlet Buzz with Proper Orthogonal Decomposition Method

The buzz phenomenon of a typical supersonic inlet is analyzed using the unsteady Reynolds Average Navier-Stokes (RANS) simulation and proper orthogonal decomposition (POD) method. The dominant flow patterns and characteristics of the buzzed flow are obtained by decoupling the computed pressure field into spatial and temporal sub-parts based on the POD method. The supersonic inlet buzz phenomenon could be approximated as a product of decoupled temporal and spatial terms, and the one-dimensional (1D) mathematical model is therefore proposed. The standard deviations of the unsteady pressure fields from both the numerical simulation and the model prediction are compared. The limited discrepancy can be observed, and the good agreement validates the credibility of the proposed 1D model. The numerical simulation and the 1D model prediction are presented to explore the unsteady-jet control with a small perturbation. The results of the 1D model and the numerical simulation achieve good agreements with each other in terms of the overall trend. Finally, POD modal energy is employed to analyze the buzz suppression mechanism. When the jet frequency is identical to the dominant frequency of the buzz and the jet phase is opposite to the oscillation phase of the captured mass flow, the buzz suppression could be more efficient. The buzz suppression mechanism could be explained in two aspects. For one thing, the complex flow structure is suppressed and the first average modal energy in the inlet is increased. For another, the energy redistribution among each POD mode is achieved and the flow stability is gradually enhanced.

Effects of Heaving Motion on the Aerodynamic Performance of a Double-Element Wing in Ground Effect

The broad implication of the paper is to elucidate the significance of the dynamic heaving motion in the aerodynamic performance of multi-element wings, currently considered as a promising aspect for the improvement of the aerodynamic correlation between CFD, wind tunnel and track testing in race car applications. The relationship between the varying aerodynamic forces, the vortex shedding, and the unsteady pressure field of a heaving double-element wing is investigated for a range of mean ride heights, frequencies, and amplitudes, using a two-dimensional (2D) unsteady Reynolds-averaged Navier-Stokes (URANS) approach and an overset mesh method for modelling the moving wing. The analysis of the results shows that at high frequencies, i.e., k ≥ 5:94 and amplitudes a/c ≥ 0:05 the interaction of the shear vorticity between the two elements results in the generation of cohering leading and trailing edge vortices on the flap, associated to the rapid variation of thrust and downforce enhancement. Both the occurrence and intensity of these vortices are dependent upon the frequency, amplitude, and mean ride height of the heaving wing. The addition of the flap significantly alters the frequency of the shed vortices in the wake and maintains the generation of downforce for longer time in ground proximity. The comparison with the static wing provides evidence that the dynamic motion of a race car wing can be beneficial in terms of performance, or detrimental in terms of aerodynamic correlation.

Identification of coherent flow structures and experimental analysis of the hydroacoustic emission of a hubless propeller

The progress in drive technology and the increasing needs for compact ship propulsion systems has brought a new class of hubless propellers into the field of operation. So far, no detailed analyses have been done concerning the unsteady flow structures which are important for the hydroacoustic emission. The objective of this paper is to analyze the unsteady flow topology and the related hydroacoustic emission of a hubless propeller with numerical and experimental methods. In order to capture the unsteady wake flow structures, a stress-blended eddy simulation (SBES) has been done. Experimental investigations in a circulating water channel provided the validation data in terms of integral quantities like thrust, wall pressure fluctuation and the hydroacoustic noise emission. The wake flow can be characterized by a multi-helix vortex system which develops from the blade root and tip region as well as from the trailing edge. The transition length of the helixes is larger compared to hub propellers in conjunction with an accelerated dissipation. The application of the proper orthogonal decomposition (POD) onto the unsteady wall pressure fields obtained from the SBES simulation was capable to assign different flow structures to the measured hydroacoustic emissions.

Modes of Base Pressure Fluctuations: Shape, Nature, and Origin

An experimental study was carried out to examine the unsteady pressure field on the base of a cylindrical afterbody at freestream Mach numbers ranging from high subsonic to low supersonic speeds. The objective was to gain insights into spatial modes of base pressure fluctuations by identifying their shape and nature, and the mechanism of their origin using spectral proper orthogonal decomposition. It is observed that 90–95% of the cumulative energy content resides within the first three modes at all Strouhal numbers for all freestream Mach numbers. Further, at subsonic Mach numbers and for Strouhal numbers , the shape of the modes indicates that their nature is symmetric, and the mechanism for the origin of the dominant mode is the pulsing action of the recirculation bubble. However, for Strouhal numbers , the shape of the modes is asymmetric, indicating that their nature is antisymmetric, and the mechanism associated with the dominant mode is the radial displacement of the recirculation bubble coupled with the motion of structures within the recirculation bubble. At supersonic Mach numbers and for all Strouhal numbers, the shape of the modes indicates that they are symmetric, and the dominant mechanism is the pulsing action of the recirculation bubble.

Growth of a gas bubble in a perfused tissue in an unsteady pressure field with source or sink.

In the context of decompression sickness, this paper presents analytical formulae and explanations for growth of a gas bubble in blood and other tissues in an unsteady diffusion field with a source or a sink. The formulae are valid for variable (through decompression) and constant (concerning diving stops/at sea level) ambient pressure. Under a linear decompression regime for ambient pressure, the gas bubble growth is proportional to ascent rate, tissue diffusivity and initial tissue tension and inversely proportional to surface tension, initial ambient pressure and the strength of the source/sink parameter [Formula: see text] which gives the conditions for bubble growth. We find that the growth process is noticeably affected by changing k-values within a specified range, with no significant effect on the value of the bubble radius when k is outside this range. We discuss the effect of the presence of multiple bubbles, and of repetitive diving. Of the three available models for bubble growth, the predicted time to completion is longest in the model by Srinivasan et al. (J Appl Physiol 86:732-741, 1999), where the bubble grows in a steady diffusion field, but shortest in the model we describe for k-values closest to the boundaries of the interval [Formula: see text]. This is because our model considers the effect of the presence of a source, increasing the bubble growth rate and not taken into account in our previous (2010) model predicting an intermediate timeframe for bubble growth. We believe our new model provides a more accurate and widely applicable description of bubble growth in decompression sickness than previous versions.

Turbulence Amplitude Amplification in an Externally Forced, Subsonic Turbulent Boundary Layer

Experimental studies of the changes in turbulence characteristics inside a boundary layer due to external forcing were performed using hot-wire anemometry. The forcing was created by a periodically forced shear layer that was external to a compressible subsonic turbulent boundary layer. The convecting coherent structures in the shear layer create a concomitant unsteady pressure and velocity field and provide an external disturbance for the turbulent boundary layer on the wall of the tunnel close to the forced shear layer. Both the pressure and velocity fluctuations inside the boundary layer were simultaneously measured along with the forcing signal, and a phase-locked analysis was performed. Regions of amplified turbulence inside the boundary layer were observed. Near the wall, the region of amplified turbulence was slightly upstream or lagging of the external forcing and away from the wall it was downstream or leading the forcing signal. Analysis of the convective speeds in the region of amplified turbulence supported the existence of the critical layer inside the wake region of the boundary layer, and the critical layer is believed to be responsible for the amplified levels of the turbulence in the wake region. Various modulation and amplification correlation coefficients were computed and analyzed, and the results also indicated the presence of the critical layer. Examination of the phase-locked turbulence revealed similarities between the turbulence amplitude amplification results due to these externally forced experiments and modulation response of an internally forced, subsonic boundary layer in the literature.

Numerical Investigation of the Clocking Effect Between Inducer and Impeller on Pressure Pulsations in a Liquid Rocket Engine Oxygen Turbopump

The clocking positions between the inducer and the impeller have a certain impact on the performance of the high-speed centrifugal pump, which however, is often ignored by designers. In the present study, three-dimensional numerical simulation based on detached eddy simulation method is adopted to evaluate the influence of this clocking effect on unsteady pressure pulsations in a full-scale liquid rocket engine oxygen turbopump. A new omega vortex identification method is introduced to clarify the internal correlation between unsteady flow structures and pressure pulsations and to shed comprehensive light on the formation mechanism of this clocking effect. Results show that the clocking effect has little influence on the unsteady pressure field in inducer passages while it significantly affects the rotor–stator interaction (RSI) effect leading to the alteration of the pressure spectra in RSI region, diffuser and volute diffuser pipe. The components at the inducer blade passing frequency in the pressure spectra are remarkably suppressed and the total pressure pulsation energy in these regions is decreased by an average of 13.94%, 12.94%, and 34.65%, respectively, when the inducer blade trailing edges are located in the middle of two adjacent impeller blades. The vortex analysis in the specific region reveals that the pressure pulsations in RSI region and the downstream regions are closely associated with the unsteady vortex shedding from the diffuser blades and the formation of the clocking effect is precisely due to different processes of the periodic vortex shedding from the diffuser blade pressure surfaces.

Unsteadiness control of laminar junction flows on pressure fluctuations

Smoke-wire flow visualization is conducted carefully in a laminar junction to explore the physical behavior of laminar junction flows. The two-dimensional (2D) velocity fields in the 30° plane of a laminar junction flow are acquired by a time-resolved particle image velocimetry (PIV) system at a frame rate of 1 kHz, based on which the unsteady fluctuating pressure fields can be calculated by the multi-path integration method proposed in the literature (GAND, F., DECK, S., BRUNET,V., and SAGAUT, P. Flow dynamics past a simplified wing body junction. Physics of Fluids, 22(11), 115111 (2010)). A novel control strategy is utilized to attenuate the unsteadiness of the horseshoe vortices of the laminar junction flow, and the consequent effect on pressure fields is analyzed.

Unsteady pressure estimation and compensation capabilities of the hybrid simulation combining PIV and DNS

The hybrid unsteady-flow simulation, which has been developed to assimilate the Navier–Stokes simulation into a time-resolved particle image velocimetry (PIV) field by synchronizing the frame rate with the direct-numerical-simulation time step, can simultaneously produce an unsteady pressure field. The hybrid process typically introduces a ‘patch function’, which transplants the PIV fields inside the camera window into a larger computational domain. This study evaluates the pressure fields produced by the hybrid simulation inside the patch function (i.e. estimation) and outside of it (i.e. compensation). We take time-resolved planar PIV data of a separated flow past the NACA 0012 airfoil at an angle of attack of 30° in a water tunnel as an example. First, an unsteady pressure field produced from the hybrid simulation is compared with other existing pressure-estimation methods in two dimensions over the entire domain of the camera window. The results of the two-dimensional (2D) hybrid simulation are found to agree best with those of the sequential-integration method with much lower noise levels in time and space. Next, the domain of the forcing is reduced by truncating the patch function in the hybrid simulation, and the pressure fields in the missing parts of the patch function are compared with those solved using the full patch function. When at least the regions in which 2D vortical structures collapse are included in the patch function downstream, the rest of the pressure fields can be compensated for relatively well. In contrast, the forces acting on the airfoil can be estimated accurately only if it is enclosed by the camera window.

Understanding the unsteady pressure field inside combustion chambers of compression-ignited engines using a computational fluid dynamics approach

In this article, a numerical methodology for assessing combustion noise in compression ignition engines is described with the specific purpose of analysing the unsteady pressure field inside the combustion chamber. The numerical results show consistent agreement with experimental measurements in both the time and frequency domains. Nonetheless, an exhaustive analysis of the calculation convergence is needed to guarantee an independent solution. These results contribute to the understanding of in-cylinder unsteady processes, especially of those related to combustion chamber resonances, and their effects on the radiated noise levels. The method was applied to different combustion system configurations by modifying the spray angle of the injector, evidencing that controlling the ignition location through this design parameter, it is possible to decrease the combustion noise by minimizing the resonance contribution. Important efficiency losses were, however, observed due to the injector/bowl matching worsening which compromises the performance and emissions levels.