Cavity Oscillations Research Articles

Study of the Jet Output Characteristics under Multi−Source Coupling

The challenges posed by elevated rock hardness, deficient drillability, excessive friction torque, and significant underpressure in extended−reach horizontal wells are the primary factors that contribute to low ROP (Rate of Penetration) and limited horizontal reach during the drilling operation. Reducing drag and friction is one of the primary methods of addressing the aforementioned challenges. To augment the pulse output characteristics of the oscillating jet and bolster the energy of the hydrodynamic impact load, we developed and designed a multi−source impact oscillation speed−increasing tool coupled with blade rotation disturbance and multi−order oscillation cavity self−excitation. We utilized fluid dynamics software to model and conduct numerical analysis on the multi−source pulsed jet generator. Furthermore, we constructed a prototype and subjected it to testing. This paper examines the impact of dimensionless structural parameters on the pressure output characteristics of the multi−source pulse−jet generator. Specifically, we used three dimensionless quantities (cavity length ratios, cavity diameter ratios, and inner wall collision angle ratios) to study this effect. The findings indicate that the multi−source impact oscillation speed−increasing tool is capable of augmenting the pulse oscillation amplitude, and the frequency of pulse oscillation can be adjusted within the range of 5 Hz to 15 Hz. During the study, we determined that the optimal oscillation output characteristics can be achieved when the cavity diameter ratio is 0.8, the cavity length ratio is 1.0, and the inner wall collision angle ratio is 1.5. These findings present a novel approach for the development of downhole hydraulic impact oscillation speed−increasing tools.

Interaction of shock train with cavity shear layer in a scramjet isolator

The interaction between the self-excited shock train flow and the cavity shear layer in a scramjet isolator is investigated numerically using detached-eddy simulations. The effect of changing the position of the shock train by controlling the back pressure ratio and the effect of changing the cavity front wall angle are analyzed using unsteady statistics and modal analysis. The propagation mechanism of the pressure disturbance was investigated by spatiotemporal cross-correlation coefficient analysis. In the present numerical study, a constant isolator section with a cavity front wall (θ = 90° and 60°) was considered, followed by a diffuser section simulated at Mach number 2.2 with three different back pressure ratios (pb/p∞ = 0.7, 5.0, and 6.0). The change in back pressure provides three different conditions (i.e., no shock train, shock train ends before the leading edge of the cavity, and shock train present above the cavity). To understand the unsteady dynamics of the interaction of the shear layer with the shock train, the spatiotemporal trajectory of the wall pressure and the centerline pressure distribution, the spatiotemporal cross-correlation coefficient, and the modal analysis by dynamic mode decomposition are obtained. The results show that the low-frequency shock train oscillation dominates the self-sustained cavity oscillation. The spatiotemporal cross-correlation between the wall surface and the center of the cavity bottom wall indicates the propagation of local disturbances originating from the separated boundary layer caused by the shock and the recirculation zone in the corners of the cavity. Dynamic mode decomposition analysis shows the shear layer at the leading edge of the cavity and the downstream propagation of large eddies from the cavity. It also shows the pairing of coherent structures between the shock train and the recirculation zone of the cavity.

Large Eddy Simulation Study on the Relationship Between Large-Scale Turbulence Vortices and Unsteady Cavitation Around a Hydrofoil

Abstract We conducted a large eddy simulation (LES) of cavitating turbulent flow around a Clark-Y11.7% hydrofoil at an angle of attack of 8 deg. We focused on the improvement of the reproducibility of hydrofoil performance under cavitating conditions when considering the unsteadiness of the flow field sufficiently by LES. A homogeneous cavitation model was adopted. The subgrid-scale model for LES was the one-equation dynamic model. The unsteady phenomena such as cavity oscillation and cloud cavity shedding accompanied by the reentrant jet were simulated. Large-scale spanwise vortices were also captured, which occurred intermittently from the onset of the sheet cavity. Then, the flow inside the sheet cavity was de-accelerated and the streamlines went around the sheet cavity. Streamline curvature formed a separation bubble behind the sheet cavity accompanied by a low-pressure region. Thus, the sheet cavity length was sufficient and the reproducibility of the pressure distribution around the hydrofoil and lift coefficient were improved.

Oscillatory characteristics of cavities in supersonic flow

We examine the flow structures, acoustic emissions, and oscillation types for supersonic flow past a baseline cavity and cavity with a bump. To study these, time-resolved shadowgraph images and unsteady pressure measurements are used. According to the results, the baseline cavity oscillates at various dominant frequencies. By adding a bump inside the cavity, many of its dominant oscillations are inactivated which are also corroborated by cross-correlation. Using time-resolved images, we can show the differences in wave propagation mechanisms between the two cavities. Through pressure spectra, it is demonstrated that the baseline cavity displays both fluid-dynamic and fluid-resonant types of oscillations, while the cavity with bump undergoes only the fluid-resonant type. The Rossiter prediction for the baseline cavity is very accurate but greatly deviates when applied to the cavity with a bump. As well, the wavelet analysis reveals that only the baseline cavity undergoes mode-switching phenomena. Additionally, the shear layer large-scale structures influencing the cavity oscillations are identified by using proper orthogonal decomposition (POD) and spectral proper orthogonal decomposition (SPOD) techniques. Furthermore, an analogy is established between POD and SPOD mode shapes along with the shadowgraph images for dominant modes of cavity oscillation. Finally, the results demonstrate that these modal decomposition techniques complement each other.

Broadband, Continuous-Wave, Mid-Infrared Generation Based on TFBG Spectral Modulation

We demonstrated a tilted fiber Bragg grating (TFBG)-based modulated amplified spontaneous emission (ASE) source, followed by the broadband, continuous-wave (CW) mid-infrared laser generated by intracavity difference-frequency generation (DFG). By flattening the spectrum of the ASE source during its power amplification and injecting another 1018 nm fiber laser in the optical parametric oscillation(OPO) cavity, nonlinear frequency conversion was achieved in the nonlinear crystal. By tuning the polarization period of the crystal, a three-peaked mid-infrared laser with a full width at half maximum (FWHM) of 130 nm and a four-peaked mid-infrared laser with an FWHM of 180 nm was generated.

A single-degree-of-freedom mathematical model for energy harvesting from cavity flow oscillations

Flow-induced vibrations have proven to be a valuable power source for wireless sensors and MEMS devices. In this study, the energy harvesting aspects of an open cavity flow is explored and modelled as a single-degree-of-freedom system. Experiments with flow over a cavity of length-to-depth ratio of 3, with an incoming velocity of 30 m/s was taken as a case study to validate the model. It was observed in experiments that three distinct flow oscillation frequencies of significant amplitudes, corresponding to the different modes of oscillation were generated. To harvest this oscillatory power, a piezoelectric cantilever beam, mounted on the aft wall was subjected to unsteady pressure forces. To harvest efficiently, the natural frequency of the cantilever beam was tuned to lock in with the flow oscillation frequencies. The frequencies of cavity oscillations agreed well with Rossiter’s model in the experiments. A single-degree-of-freedom system assuming lumped parameters was used for modelling. The forcing term was described using a periodic function that uses empirical values for amplitude. The model was able to predict the overall trend and values of the average power and RMS voltage generated across a resistor load with reasonable accuracy. Using the model, the effects of cavity length, incoming velocity and resonant frequency were also studied.

Synergistic effect of hydrodynamic cavitation characteristics of self-excited oscillation cavity for degradation of dye wastewater

Dye wastewater in production process of the textile industry will cause serious environmental problems. Hydrodynamic cavitation includes many structures, such as a venturi tube, orifice, and pulsed jet. Self-excited oscillation cavitation is a kind of pulsed jet. In this paper, hydrodynamic cavitation is realized by self-excited oscillation cavitation. In this research, to study the hydrodynamic cavitation (HC) characteristics of the self-excited oscillation cavity and the degradation effect of dye wastewater, several experimental studies were carried out by selecting methylene blue as the colorant pollutant. During the experiment, the effects of experimental operating condition (including system pressure and initial solution concentration) on the degradation of methylene blue were studied, and its optimal operating conditions were obtained. And then the effects of the combination method of HC and oxidizing additives (hydrogen peroxide and ozone) on methylene blue degradation were analyzed. The results show that under the optimal system pressure of 1.4 MPa and solution concentration of 5.6 mg/L, the maximum extent of degradation is 24.90%, and the degradation rate is 2.3 × 10−3 min−1. The degradation effects of the combination methods of HC + Hydrogen peroxide and HC + Ozone can be greatly improved compared to only HC. The extent of methylene blue degradation of HC + Hydrogen peroxide is 83.70%, the degradation rate is 14.9 × 10−3 min−1, and the synergy index is 3.63; the extent of degradation of HC + Ozone is 92.62%, the degradation rate is 37.7 × 10−3 min−1, and the synergy index is 5.80. The lowest cost of the HC + Ozone combination method is 9.797 USD/m3, thereby the comparison of cavitation yield and energy consumption cost shows that HC + Ozone is the preferential combination treatment method. This study will not only play a significantly guiding role on the optimization of process parameters of SEOC for AOPs to obtain the preferable cavitation effect, but also provide support for the development of hybrid AOPs for wastewater treatment in the direction of large-scale application.

Дослідження характеристик резонансної системи на основі прямокутного хвилеводу з двома діафрагмами

Microwave methods for the study and control of the dielectric properties of substances using regular and irregular rectangular metal waveguides have many applications and are of great practical interest. The dielectric properties of materials are assessed based on the determination of the reflection and transmission coefficients in a resonant system. This article considers resonant phenomena of electromagnetic oscillations in a prismatic cavity based on a length of a rectangular waveguide with two diaphragms and with a dielectric specimen in the form of a cylinder centrally positioned between them. The goal of this work is to evaluate the resonant properties of a prismatic cavity based on a length of a rectangular waveguide with two diaphragms as a function of the geometrical dimensions of the rectangular diaphragm windows, the distance between the diaphragms, and the dimensions and dielectric properties of the specimen located at the center of the waveguide length. By computer simulation, the resonant properties of the system were determined as a function of the dielectric constant and diameter of the specimen and the geometrical dimensions of the diaphragm windows. It was found that for each value of the specimen diameter there exists a maximum allowable value of the dielectric constant at which the resonant properties of the system are kept in a selected operating frequency range. The effect of the height of the diaphragm windows on the resonant frequency of a prismatic cavity and the reflection and transmission coefficients was studied, and the behavior of the intrinsic Q factor was assessed qualitatively. A minimum possible diaphragm window height was found such that both the required coupling between the rectangular cavity and the external microwave line is provided and the maximum value of the loaded Q factor in the absence of a dielectric specimen is kept. The results of this work may be used in the design of high-frequency sensors for controlling the dielectric properties of materials in various industries.

Numerical investigation on the inhibition mechanisms of unsteady cavitating flow around stepped hydrofoils

In this work, the influence of obstacles on the control of unsteady cavitating flows is investigated numerically within the framework of homogeneous flow. The predicted Strouhal number of the cavity shedding around the smooth hydrofoil is close to the experimental value, indicating the accuracy of the numerical simulation. Four installation sites are examined, including 10%, 20%, 30%, and 40% of the chord length from the leading edge. The instantaneous and time-averaged hydrodynamics in conjunction with the unsteady cavity behaviors are discussed in detail. The results indicate that all cases display similar flow patterns within one shedding cycle, including the development of the attached sheet cavity, sheet/cloud transformation, and cloud cavity collapse. The hydraulic lift is governed by the partial cavity oscillation but could be significantly modified by the trailing wake vortices, especially after installing the obstacle. The recommended optimal position of the obstacle is X/L=0.4, and the corresponding pressure fluctuations and lift-to-drag ratio achieve the best performance. However, the mean vapor volume is significantly reduced when the obstacle is located at X/L=0.3, indicating the minimum risk of cavitation erosion. Installing the obstacle at different locations along the suction surface will provide different effects on oscillation based on the analyses of the Lagrangian coherent structures. The corresponding primary frequency varies in a narrow range between 106.67 Hz ∼ 121.33 Hz. Proper orthogonal decomposition (POD) analyses of the vapor fraction reveal the contribution of flow morphology to the partial cavity oscillation. Installing the obstacle could significantly limit the size of each pattern, especially for X/L=0.4.

Numerical Analysis and Experimental Investigation of Cavitating Flows Considering Thermal and Compressibility Effects

This article deals with the numerical simulation of unsteady cavitating flow around hydrofoils, supported by experimental research realized in a cavitation tunnel situated in the Centre of Hydraulic Research. Two straight NACA hydrofoils (NACA0020 and NACA2412) were employed. The comprehensive unsteady CFD analysis was based on scale-resolving simulations (hereinafter SRS) with the aim of capturing correctly the interactions between the cavitation structures and re-entrant flow as well as the compressibility and thermal effects of cavitation. The static pressure fluctuations during the cavity oscillation cycles and the evaporation and condensation processes are discussed in detail. To predict correctly the high-pressure peaks during the bubble cloud collapses and the pressure pulse propagation speed, the real properties of water and the mixture total energy conservation equation were considered. In addition, the estimated content of undissolved air was taken into account. The numerical simulations were validated by means of already published experiments or compared with experiments conducted by the authors, and with good agreement.

Characteristics of Oscillation in Cavity of Helmholtz Nozzle Generating Self-excited Pulsed Waterjet

Cavity flow oscillations in the axisymmetric cavity are critical to the operating efficiency of self-excited pulsed waterjets, which are widely employed in many practical applications. In this study, the behaviors of a turbulent flow in axisymmetric cavities causing cavity flow oscillations are investigated based on wall pressure characteristics. Experiments are performed using four Helmholtz nozzles with varying length-to-radius ratios at flow velocities of 20–80 m/s. Three orders of hydrodynamic modes in axisymmetric cavity are obtained through the spectral analysis of wall pressure. Based on the experimental results, the empirical coefficient of Rossiter’s formula is modified, and the values of the parameter phase lag and the ratio of convection velocity to free stream velocity are obtained as 0.061 and 0.511, respectively. In addition, the spectral peak with a relatively constant frequency shows that the flow-acoustic resonance is excited significantly. A modified model is introduced based on the fluidic networks to predict the lock-on frequency. The results obtained can provide a basis for the structural optimization of the nozzle to improve the performance of self-excited pulsed waterjets.

Experimental investigation on concrete rock breaking performance of self-excited oscillation pulsed waterjet

To obtain the influence of structural parameters, system pressure, standoff distance, rock characteristics on rock breaking volume and specific energy consumption, the rock breaking experiment using self-excited oscillation pulsed waterjet (SOPW) was carried out. The relationship among rock initial crushing pressure, fracturing pressure with rock compressive strength was investigated, and the change trend between rock breaking morphology and standoff distance was also observed. In addition, the order and optimal parameter combination of influential factors on rock breaking performance using SOPW was obtained through an orthogonal test. The research results show that the rock breaking performance under the impact of SOPW is better than that of continuous water jet in the initial stage, and the difference of rock breaking volume under the action of two-type jet is getting larger as the time going on. Moreover, the initial crushing pressure and fracturing pressure increases with the increase of the compressive strength, and the initial crushing pressure and the fracturing pressure is predicted as 54.4 MPa and 125.8 MPa, respectively, when the rock compressive strength is 80 MPa. Furthermore, the primary and secondary order of various factors on rock breaking performance is system pressure, standoff distance, oscillation cavity length, section cone angle, and the optimal combination is 40 MPa, 10 mm, 70 mm, and 140° for the system pressure, standoff distance, oscillation cavity length and section cone angle, respectively.

Study on Self-Excited Oscillation Suppression of Supersonic Inlet Based on Parallel Cavity

Aiming at the problem of self-excited oscillation in a supersonic inlet, the oscillation suppression of parallel cavities in a shock system is studied. Based on the shock dynamic model, the theoretical calculation model of parallel cavity under dynamic shock is established, and the effects of cavity volume and oscillation frequency on shock oscillation flow field parameters are analyzed. On this basis, an integrated numerical model including cavity and inlet and outflow fields is established, and the effects of cavity on the inlet flow field parameter distribution and parameter oscillation are compared by using unsteady numerical calculation algorithm. The theoretical calculation results show that the parallel cavity can reduce the amplitude of flow field pressure oscillation, and increasing the cavity volume is beneficial to suppress parameter oscillation. The unsteady numerical calculation of three groups of working conditions shows that the cavity changes the amplitude of parameter oscillation, and the high amplitude frequency point also decreases compared to the model without cavity. Through the alternating change of pressure between the channel and cavity during the movement of the shock wave, the cavity gas filling and overflow dampen the shock wave forward and pressure change of the mainstream, so as to suppress the self-excited oscillation.

Oscillation of Cavitating Vortices in Draft Tubes of a Simplified Model Turbine and a Model Pump–Turbine

The self-oscillation of the cavitating vortices is one of the dangerous phenomena of hydraulic turbine operation near full-load conditions. This work is an attempt to generalize data and expand insight on the phenomenon of self-excited oscillations by comparing the experimental results obtained on a simplified turbine and scaled-down pump–turbine models. In both cases, a series of high-speed imaging was carried out, which made it possible to study these phenomena with high temporal resolution. The high-speed imaging data was subjected to additional processing such as binarization, cropping, and scaling. For a simplified turbine model, the volume of the vapor cavity was calculated based on the assumption of the axial symmetry of the cavity, after which fast Fourier transform (FFT) analysis was carried out. A proper orthogonal decomposition (POD) analysis was also performed to examine individual modes in the original digital imaging data. For the pump–turbine, visualization data on the cavitation cavity oscillations were supplemented by pressure measurements in the draft tube cone to determine the frequency characteristics. Based on obtained experimental data, an improved one-dimensional model describing the oscillations of the cavitation cavity arising behind the hydraulic turbine runner is proposed.

Dynamic Interactions of Multiple Wall-Mounted Flexible Plates in a Laminar Boundary Layer

The interactions between large arrays of wall-mounted flexible plates and oncoming laminar boundary-layer flows are studied numerically by using the immersed boundary method. The influences of bending rigidity, mass ratio and gap distance between adjacent plates on the dynamic behaviors are explored. With the variation of control parameters, five distinct dynamic modes, namely, static reconfiguration, sectional waving, regular waving, upright oscillation and cavity oscillation, are identified. The frequency lock-in phenomenon and various types of flow instability associated with different dynamic modes are discussed. The findings of this study indicate that the coherent motions of the arrays are governed by a coupled mechanism in which the frequency of flow instability is locked onto the structural natural frequency.

Experiments on flow past cavity and the application of modal decomposition techniques

In this work, Mach 1.8 flow over a shallow open cavity is studied through time-resolved schlieren images as well as unsteady pressure measurements respectively. Additionally, the cavity shear layer dynamics are explored using modal decomposition techniques namely; proper orthogonal decomposition (POD) and spectral proper orthogonal decomposition (SPOD). The results display that the cavity undergoes a self-sustaining oscillation with a number of modes/tones that are closely related to the modified Rossiter relation. Depending on the cavity oscillation, the time-resolved images reveal various complex flow features. Through modal analysis, the shear layer structures which are responsible for the cavity oscillation are identified. It is understood that POD requires several hundreds of modes to capture 67.3% of the total intensity for resembling the cavity flow field, while SPOD shows 65.7% of the total intensity is available in the 1st mode to reconstruct the cavity flow field. Further, modal analyses have demonstrated that cavity oscillatory frequencies are very similar to unsteady pressure measurement and the modified Rossiter relation. We confirmed that, both these techniques can be successfully applied to cavity flows and are complementary.

Oscillations of Air Cavity Under Hull of Accelerating Boat

Abstract Substantial reduction of ship drag is achievable with air cavities arranged on the hull bottom. However, the air cavities can become unstable, particularly in unsteady environments such as sea waves. Oscillations of cavities can reach high amplitudes, especially near-resonance conditions, and this may lead to the cavity collapse and degradation of drag reduction. In the present study, a lumped-element model is applied for the analysis of dynamics of the air cavity under a boat hull moving in the presence of sea waves. When the hull accelerates from a low initial speed to a high operational speed, the forcing frequency can pass through the cavity resonance condition. Numerical modeling demonstrates that cavities on slowly accelerating hulls may exhibit dangerously high amplitudes. On the other hand, when the hull acceleration is sufficiently fast, then a cavity will not have enough time to develop large-amplitude oscillations. It is found that higher damping and faster acceleration can limit magnitudes of the cavity oscillations, preventing the air cavity from disintegration and enabling the hull to attain high speeds by keeping the drag-reducing air cavity intact.

Numerical Study on Flow-Induced Noise of Deflector Jet Servo Valve Based on LES/Lighthill Hybrid Method

The flow-induced noise occurring in the prestage of the deflector jet servo valve (DJV) significantly affects the performance of DJV, which is a critical issue for electrohydraulic servo valves. To capture the root causes of flow-induced noise, DJV is numerically studied using computational aeroacoustics (CAA) methods in this paper. The acoustic field is investigated with Lighthill’s acoustic analogy based on the pulsation data from a large eddy simulation (LES). The flow-induced noise generation mechanism in DJV is revealed by analyzing the sound pressure level and vorticity distribution at different moments. Moreover, a kind of oscillation cavity model based on the Helmholtz oscillator is built, which reveals that the flow-induced noise is caused by self-excited oscillation and vortex. In addition, the result shows that continuous broadband dipole noise with medium- and high-frequency components of 1100 Hz and 2501 Hz dominates the DJV flow field. In order to avoid unexpected resonance, the natural frequency of the prestage should not coincide with the frequency of the flow oscillation frequency. Thus, this work can benefit the design and optimization of the prestage structure.

Multi-mode Jaynes–Cummings model results for the collapse and the revival of the quantum Rabi oscillations in a lossy resonant cavity

We have numerically obtained theoretical results for the collapse and the revival of the quantum Rabi oscillations for low average number of coherent photons injected on a two-level system in a lossy resonant cavity. We have adopted the multimode Jaynes–Cummings model for the same and especially treated the “Ohmic” loss to the walls of the cavity, the leakage from the cavity, and the loss due to the spontaneous emission through the open surface of the cavity. We have compared our results with the experimental data obtained by Brune et al. [Phys. Rev. Lett. 76 (1996) 1800] in this regard.

Flow Transition Analysis of Multiple Jets in Narrow Parallel Walls using POD and DMD

We experimentally and numerically study the cavity oscillation of a compound jet in narrow parallel plates. Particle Image Velocimetry (PIV) obtains the time series velocity distribution with four jets. To perform the mode decomposition of the flow field, Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) are applied to the PIV data. The POD analysis reveals that the cavity oscillation involves two hierarchical structures: one corresponds to the cavity-scale oscillating of the jet, and the other to the advecting multiple smaller vortices. The DMD analysis classifies the oscillation modes into three categories: (i) oscillation mode on the container scale, (ii) pulsating mode on the container scale, and (iii) oscillation modes of individual jets. The results suggest that the superposition of these oscillation modes at the high flow rate is observed as cavity oscillation.