Total Pressure Fluctuations Research Articles

Phenomenological Aerodynamic Analysis of an Ejector Exposed to Large Inlet Pulsations

To enable the coupling of the unsteady, high-enthalpy flow exiting a Rotating Detonation Combustor (RDC) with a subsonic turbine, an ejector may be installed between the combustor and turbine. In the present work, a conventional ejector is investigated numerically using a Large-Eddy Simulation. Firstly, a stationary operating point is analyzed where sufficient agreement could be obtained with experimental data. Secondly, a new operating point is proposed where a sinusoidal acoustic pulsation is imposed at the primary ejector inlet. The pulsation generates a periodic fluctuation between sub- and supersonic velocity regimes at the primary nozzle exit, which is representative of local RDC exhaust velocities. In this first step, the exhaust gas temperature, flow angle, and gas composition are not considered. At this operating condition, a secondary normal shock appears periodically at the end of the constant-area mixing chamber and retracts upstream as a pressure wave. The shear layer, the boundary layer, the secondary shock, and the diffuser-induced separation are identified as kinetic energy loss sources within the ejector. The amplitude of total pressure fluctuations is reduced by 85% throughout the device, whereas the frequency is retained. The dampening characteristics are considered favorable for gas turbine integration.

Comprehensive Aerothermal Investigation of Turbine Blade Multicavity Squealer Tip Using a Novel Methodology With Uncertainty Quantification

Abstract Ascertaining the uncertainty in the aerothermal performance of blade tips is crucial, as it represents the most delicate component of modern gas turbines. In this research article, a novel and efficient approach is proposed for quantifying uncertainties in aerothermal performance using a combination of universal kriging, polynomial chaos expansions, and Smolyak sparse grid technology. This method was applied to investigate the aerothermal performance of a high-pressure gas turbine rotor blade tip with high-dimensional robustness. The outcomes of the uncertainty quantification calculation reveal that the downstream total pressure loss coefficient and leakage flowrate increase under normal-speed (subsonic) and high-speed (transonic) conditions. The key uncertainty input that affects the aerodynamic performance of normal-speed and high-speed squealer tip is inlet total pressure fluctuation, with a variance index on the leakage flowrate of normal-speed and high-speed squealer tip of up to 73.92 and 83.85%, respectively. The study suggests that it is more important to control the operating conditions fluctuation than the cavity depth machining accuracy for aerodynamic performance robustness, which applies to both normal-speed and high-speed squealer tips. In line with the aerodynamic performance, the heat flux of normal-speed and high-speed squealer tip increases during operation. Notably, the sensitivity of high-speed squealer tip aerodynamic performance to operating condition fluctuations increases compared to the normal-speed squealer tip, necessitating active intervention for fluctuations in operating conditions at a higher cost for the high-speed squealer tip. The sensitivity analysis results indicate that the inlet total temperature fluctuation is the key parameter that controls the normal-speed and high-speed squealer tip heat flux uncertainty. Finally, it is worth noting that while the use of ribs can effectively enhance the robustness of blade tip heat transfer performance, the heat flux near the root of the ribs fluctuates significantly, which may further increase the thermal fatigue tendency in this region during actual operation.

Characterization of Disturbance Resonance in Postshock of Blunt Body in Hypersonic Flow

A pitot probe is commonly used for the disturbance quantification of a hypersonic freestream, but the pitot results cannot reflect the actual disturbance amplitudes due to the turbulence/shock-wave interactions as well as the disturbance resonances. In this work, we characterize the disturbance resonance in the postshock zone of a blunt body using both experimental and theoretical approaches. A one-dimensional interference model with four traveling sound waves is proposed to simulate the resonance phenomenon due to the reflection between the detached shock and the wall of the pitot probe. Subsequently, the total pressure fluctuation on the surface and the density fluctuation in the space along the centerline are investigated using a pressure sensor and a noninvasive focused laser differential interferometer. Both the experimental results and theoretical analysis show the resonance mechanism on the surface is dominated by the interference between the initial diverging sound wave and the shock-reflected sound wave. In contrast, the stationary wave formed by the initial diverging sound wave and the body-reflected wave plays a leading role in the space. Finally, an equation is proposed to approximate the destructive resonant frequency in the space.

Numerical study on the evolution characteristics of phase interface of subsonic and sonic steam jets in transverse subcooled flow

The cooling process of high-temperature steam ejecting into subcooled liquid widely exists in industrial production, with high mass and heat transfer efficiency. This paper establishes an unsteady numerical simulation method for jet condensation based on the two-fluid model and the thermal phase change model, focusing on the phase interface evolution and pressure fluctuation characteristics of steam jets in the crossflow of subcooled water under different mass fluxes. With the increase of mass flux, the ability of the jet plume to resist water flow is enhanced; thus, the heat and mass transfer at the two-phase interface becomes more stable; the length of the plume increases, and the range of oscillation of the plume length in the axial direction and the radial direction decreases. The pressure vortex that generated by the two-phase region and the subcooled water is more stable with reduced total pressure fluctuations. Under subsonic conditions, bubbles are shed on the leeward side, and the ranges of axial oscillation, radial oscillation and total pressure oscillation are 1.57de–3.41de, 0.21de–0.37de, and 0.17de–0.32de, respectively. The steam plume becomes progressively longer in the axial direction as the steam velocity increases from subsonic to sonic and supersonic speeds, and the range of interfacial oscillations and total pressure fluctuations along the radial direction are further reduced. Under the same working condition, with the increase of axial distance, the momentum of the steam jet decreases, the ability to resist the impact of water flow decreases, and the windward side is greatly affected; the axial and radial amplitude fluctuations of the pressure increases, with the amplitude on the windward side being significantly larger than that on the leeward side, and the maximum difference between the fluctuations on both sides increases continuously. The momentum change of the two-phase interface is intense, which is the main factor of pressure fluctuation. The correlation coefficients of pressure oscillation with radial and axial interface oscillations are 0.9110 and 0.7477, respectively.

Numerical investigation of unsteady pressure pulsation characteristics in an ultra-low specific-speed centrifugal pump as a turbine

The unsteady flow characteristics of pump as hydraulic turbine play a vital role in its safe and stable operation, while the ultra-low specific-speed centrifugal pump may face more stability problems due to the limitations of its flow conditions under the turbine working condition. Therefore, in this study, the unsteady characteristics of an ultra-low specific-speed centrifugal pump under turbine conditions are studied using a numerical simulation method, and the numerical simulation is verified using an experimental method. Based on the hydraulic losses of each flow passage component, the energy characteristics of pump as turbine (PAT) are established, and the distribution pattern of total pressure fluctuation in the turbine is studied. The results show that the rotor–stator interaction between the impeller and the tongue makes the hydraulic performance and the internal flow field change periodically. The pressure fluctuation intensities at the tongue, blade inlet edge, and balance hole are large, and the total pressure fluctuation in the three areas is intense in space and time. The internal flow characteristics at typical blade positions show that the secondary flow phenomena such as separation flow and wake flow near the tongue make the pressure gradient larger, which is an important influence mode of the rotor–stator interaction. This study provides a reference and guidance for the unsteady study of low specific-speed PAT.

Experimental and numerical studies of the effects of the contraction ratios on the swirling flow characteristics of the model combustor outlet in lean gas turbines

Large Eddy Simulation (LES) is utilized in this work to study isothermal turbulent swirling flow characteristics for a lean partially premixed gas turbine model combustor. The goal is to investigate the influence of contraction ratio (CR) variation on the vortex breakdown phenomena features and the turbulence structure in these combustors. Particle image velocimetry (2D-PIV) measurements were carried out on a single-case model combustor using (air-air) phase to perform the mean flow behavior and the validation process under atmospheric conditions with CR equal to 0 (largest diameter, Dinlet = Doutlet), the equivalence ratio (φ) of 0.3, swirl number of (Sn) = 0.66 and Reynolds number of (Re) = 42,000. While five test cases with various CRs were applied in LES under the same conditions using (air-CH4) phase to predict the vortical structure and the precession vortex core (PVC). The CRs ranged as follows: 0, 50 %, 60 %, 70 % and 80 % (smallest diameter). Experimental and LES results show that there was a reasonable agreement between the PIV and LES results with maximum deviations of 2.8 % and 4.65 % for the normalized mean axial and radial velocities, respectively. As the CR was increased, the pressure drop and the mean axial velocity increased linearly at the contraction part. In addition, the downstream stagnation point and the size of the central toroidal recirculation zone (CTRZ) stabilized when CR = 60 %, 70 %, and 80 %. The swirl level was further enhanced by 40 % when CR = 60 % and 80 %. The presence of CR successfully suppressed the pressure and axial velocity fluctuations generated in the shear layer zones by 35 %. LES data was analyzed by Fast Fourier Transformation (FFT) and Proper Orthogonal Decomposition (POD) to investigate the PVC dynamics. The FFT showed four dominant frequencies as a result of the appearance of the PVC and vortex breakdown oscillation. The frequency of PVC (fpvc) decreased linearly with an increase in the CR where the fpvc was 20.8 Hz when CR = 0 and became 12.5 Hz at CR = 80 %. In contrast, the other frequencies were still constant even with increased CR at 187 Hz, 562 Hz, and 979 Hz. The POD analysis showed the first mode was more energetic and had 40 % of the total pressure fluctuations.

Simulation study on pressure fluctuation characteristics of a high-pressure common rail system

Pressure fluctuations in the high-pressure common rail systems have a significant impact on the fuel injection and engine performance. In this study, the pressure fluctuation in a high-pressure common rail system was investigated using simulations. The fluctuation can be caused by hydraulic structures and control factors. The degree of influence of those two causes was evaluated and discussed in this work. First, the hydraulic and control factors that dominate the models of injection systems were established. Then, the mechanism of pressure fluctuation on high-pressure common rail systems was highlighted. A dimensionless parameter was defined to quantitatively describe the effects of control factors and hydraulic structures on pressure fluctuation. Finally, the developed model was used to study the effects of injection conditions and structural parameters on pressure fluctuation. The results show that the injection frequency has a limited effect on the system pressure fluctuation. In addition, the increase in the injection energizing time and the injection pressure will increase the hydraulic pressure fluctuations of the system, thus increasing the total pressure fluctuations of the system. Moreover, the increase in the volume by increasing the diameter was found to decrease the pressure fluctuations more effectively compared to the impact of the length, but there is a marginal diminishing effect. The results demonstrated that the effect of a 20 bar signal error and a 160 mm common rail length is almost the same on decreasing pressure fluctuations. It can be concluded that the improvements in the electronic part of the high-pressure common rail system have more potential than the mechanical part.

Modal Decomposition of the Precessing Vortex Core in a Hydro Turbine Model

We report on the experimental study of a precessing vortex core (PVC) in an air model of a Francis turbine. The focus is placed on the modal decomposition of the PVC that occurs in the draft tube of the model turbine for a range of operation conditions. The turbulent flow fluctuations in the draft tube are assessed using stereo particle image velocimetry (PIV) measurements. Proper orthogonal decomposition (POD) is applied to the antisymmetric and symmetric components of the velocity fields to distinguish the dynamics of the azimuthal instabilities. The pressure pulsations induced by the PVC are measured by four pressure sensors mounted on the wall of the hydro turbine draft tube. Spatial Fourier decomposition is applied to the signals of the pressure sensors to identify the contributions of azimuthal modes, m=1 and m=2, to the total pressure fluctuations. The analysis based on velocity and pressure data shows similar results regarding the identification of the PVC. The contribution of the m=2 mode to the overall turbulent kinetic energy is significant for the part load regimes, where the flow rates are twice as low as at the best efficiency point (BEP). It is also shown that this mode is not the higher harmonic of the PVC, suggesting that it is driven by a different instability. Finally, we show a linear fit of the saturation amplitudes of the m=1 and m=2 oscillations to determine the critical bifurcation points of these modes. This yields critical swirl numbers of Scr=0.47 and 0.61, respectively. The fact that the PVC dynamics in hydro turbines are driven by two individual instabilities is relevant for the development of tailored active flow control of the PVC.

On the wavenumber–frequency spectrum of the wall pressure fluctuations in turbulent channel flow

Direct numerical simulations (DNS) of turbulent channel flows up to${Re}_{\tau} \approx 1000$are conducted to investigate the three-dimensional (consisting of streamwise wavenumber, spanwise wavenumber and frequency) spectrum of wall pressure fluctuations. To develop a predictive model of the wavenumber–frequency spectrum from the wavenumber spectrum, the time decorrelation mechanisms of wall pressure fluctuations are investigated. It is discovered that the energy-containing part of the wavenumber–frequency spectrum of wall pressure fluctuations can be well predicted using a similar random sweeping model for streamwise velocity fluctuations. To refine the investigation, we further decompose the spectrum of the total wall pressure fluctuations into the autospectra of rapid and slow pressure fluctuations, and the cross-spectrum between them. We focus on evaluating the assumption applied in many predictive models, that is, the magnitude of the cross-spectrum is negligibly small. The present DNS shows that neglecting the cross-spectrum causes a maximum error up to 4.7 dB in the subconvective region for all Reynolds numbers under test. Our analyses indicate that the approximation of neglecting the cross-spectrum needs to be applied carefully in the investigations of acoustics at low Mach numbers, in which the subconvective components of wall pressure fluctuations make important contributions to the radiated acoustic power.

Separation of convective and acoustic pressure fluctuations on the front side window of DrivAer model based on pellicular mode decomposition

When a car is driven at high speed, the total pressure fluctuations on the side window include convective pressure fluctuation and acoustic pressure fluctuation, which have different generation mechanisms, transmission characteristics and efficiency. In order to understand the aerodynamic noise transmission mechanisms through the side window and calculate the aerodynamic noise accurately inside the car, it is necessary to separate these two kinds of pressure fluctuations. Based on the existing full-size DrivAer model, the pellicular mode decomposition theory proposed in recent years was investigated. With this approach, the two pressure fluctuations from a compressible CFD calculation acting on the side window were separated successfully. Through comparison with the pressure data obtained with the improved wavenumber decomposition approach, the reliability and accuracy of the pellicular mode decomposition approach for solving the acoustic pressure fluctuation was validated. Moreover, in comparison with wavenumber decomposition, the pellicular mode decomposition can be applied easily to any surface with arbitrary shape, even the surface curved in 3D. Further advantage of this approach is the ability to reconstruct the pressure field of convective and acoustic components individually at any frequency, which can help to understand the characteristics of these two pressure components. Disadvantage is however the handling of a large number of numerically computed pellicular modes, which is limited in principle by the implementation of finite element method. As a consequence, a reliable convective pressure fluctuation is difficult to be acquired directly with this approach.

Relativistic, axisymmetric, viscous, radiation hydrodynamic simulations of geometrically thin discs. II. Disc variability

ABSTRACT An analysis of two-dimensional viscous, radiation hydrodynamic numerical simulations of thin α-discs around a stellar mass black hole reveals multiple robust, coherent oscillations. Our disc models are initialized on both the gas- and radiation-pressure-dominated branches of the thermal equilibrium curve, with mass accretion rates between $\dot{M} = 0.01 L_\mathrm{Edd}/c^2$ and $10\, L_\mathrm{Edd}/c^2$. In the initially radiation-pressure-dominated disc, we confirm the presence of global inertial–acoustic oscillations of frequency slightly above the maximum radial epicyclic one. In the gas-pressure-dominated Schwarzschild-metric models, we find a velocity oscillation occurring at the maximum value of the radial epicyclic frequency, $3.5\times 10^{-3}\, t_\mathrm{g}^{-1}$, which is most likely a trapped fundamental g-mode. For the Kerr-metric, gas-pressure-dominated disc with dimensionless black hole spin parameter a* = 0.5, the mode frequency is well below the epicyclic frequency maximum, thus confirming that this oscillation is a trapped g-mode. Additionally, the total pressure fluctuations in the discs strongly suggest standing-wave p-modes with frequencies below the apparent g-mode frequency, some trapped in the inner disc close to the innermost stable circular orbit (ISCO), others present in the middle/outer parts of the disc. The strongest oscillations occur at the breathing oscillation frequency and are present in all the numerical models we report here, as are weaker velocity oscillations at the vertical epicyclic frequencies. The vertical oscillations show a 3:2 frequency ratio with oscillations occurring approximately at the radial epicyclic frequency, which could be of astrophysical importance in systems with observed twin peak, high-frequency quasi-periodic oscillations.

Experimental Investigation of the Nozzle Roughness Effect on the Flow Parameters in the Mixing Layer of an Axisymmetric Subsonic High-Velocity Jet

The paper presents the results of an experimental investigation of the mixing layer parameters in the initial regions of transonic jets issuing from convergent nozzles with different internal roughness at the same Mach number in the jet core. It is shown that the mixing layer characteristics, namely, the radial profiles of the measured total pressure and the r.m.s. total pressure fluctuations, are self-similar in nature at distances greater than 1.5 of the nozzle exit diameter from the exit. The possibility of applicating nozzles with an increased degree of the internal surface roughness in investigating high-velocity jet flows is shown.

Controlled stationary/travelling cross-flow mode interaction in a Mach 6.0 boundary layer

Experiments were performed to further investigate a quadratic interaction between stationary and travelling cross-flow modes that was revealed in the previous experiment by Corke et al. (J. Fluid Mech., vol. 856, 2018, pp. 822–849) in the boundary layer on a sharp right-circular cone at an angle of attack at Mach 6.0. As with the previous experiment, passive discrete roughness was applied near the cone tip, just upstream of Branch I of the linear stability neutral curve for stationary cross-flow modes. The passive roughness consisted of indentations (dimples) that were evenly spaced azimuthally to excite a specific azimuthal wavenumber. A plasma actuator was located just downstream of the discrete roughness array. This was designed to produce an azimuthally uniform unsteady disturbance with a frequency that was at the centre of the band of most amplified travelling cross-flow modes. Measurements consisted of off-wall azimuthal profiles of mean and fluctuating total pressure at different axial locations. Spectra of total-pressure fluctuations verified the receptivity of the boundary layer to the unsteady excitation. This affected the azimuthal and streamwise development of the stationary cross-flow modes, with a general effect to move the transition location upstream by approximately 16 %. The quadratic interaction between the stationary and travelling cross-flow modes was further enhanced by the excitation of the travelling cross-flow mode. This was particularly evident by an enlarged band of azimuthal wavenumbers over which a significant triple phase locking existed. The significantly enlarged range of wavenumber sum and difference interactions offered a mechanism for rapid spectral broadening that could account for the hastened transition by the addition of unsteady disturbances.

Centrifugálkompresszor-karakterisztika regressziója a teljes működési tartományban nem lineáris matematikai modellhez

Gázturbinás berendezésekben – legyenek azok hajtóművek vagy ipari alkalmazásban tengelyteljesítményt leadó eszközök – a centrifugális kompresszorok széles körben elterjedtek. Jellemzőjük az egy fokozatban elérhető jelentős nyomásviszony, valamint a relatíve széles tömegáram-tartományban való stabil üzemelés. Ez utóbbi ellenére kis szállítás esetén – az axiális kompresszorokhoz hasonlóan – különféle instabil jelenségeket mutatnak (forgó leválás, pompázs), amelyek nyomás- és tömegáramlengések formájában jelentkeznek. Ezek elkerülése elsődleges fontosságú a berendezés és a kapcsolódó rendszerek megóvása érdekében, mert a lengésekben tárolt energia akár a lapátok kiszakításához is elegendő. A kompresszor matematikai modellezése kiemelkedő jelentőségű abból a szempontból, hogy egyrészt a különböző üzemállapotokban előre jelezhetővé válik a berendezés viselkedése, továbbá azon tartományokban, ahol az instabilitások fellépése várható, ott egy aktív beavatkozást biztosító rendszer a megfelelő reakciót kiváltva elháríthatja a szabályozás nélküli esetben bekövetkező rendellenes működést. Cikkünk célja, hogy egy centrifugális kompresszor teljes üzemi tartományában történő mérése alapján polinomos regresszióval közelítse a berendezés tömegáram-nyomásviszony, valamint tömegáram-hatásfok jelleggörbéit. Az eljárással megalkotott összefüggés a teljes üzemmódtartományt lefedi, előnyeit egy összehasonlítás mutatja be a korábban alkalmazott lineáris (például átszámított fordulatszám és dimenziótlan tömegáram szerint bilineáris) módszerek eredményeivel szemben. A kompresszor matematikai modelljét dinamikus szimulációnak vetettük alá, amelyet MATLAB Simulink környezetben hajtottunk végre, és az összehasonlítás érdekében tranziens mérések eredményeit használtuk fel. Az így megalkotott dinamikus modell pedig korszerű, aktív pompázsszabályozó rendszerek fejlesztését teszi lehetővé a későbbiekben.

Numerical simulation of inlet buzz

Comprehensive numerical analyses are conducted to simulate and capture “Buzz” phenomenon in a supersonic mixed compression air inlet. The buzz is an unsteady self-sustained feature that occurs in supersonic inlets, especially when operating in their subcritical condition. In such a situation, the shock waves oscillate along the inlet and cause mass flow fluctuations inside the inlet that will deteriorate the engine performance significantly. An axisymmetric unsteady numerical simulation was used to solve the Navier–Stokes equations combined with the URANS SST k–ω turbulence model. The simulations for two different free-stream Mach numbers of M∞=2.0 and 2.2 and at two specific subcritical conditions of the inlet operation, where the buzz was experimentally detected, were performed. The numerical solution was able to accurately capture the buzz and its oscillation which is seen to be periodic for this specific inlet. The results show that a large separation region on the compression ramp blocks the duct entry and causes conical and lambda shocks, which are formed on the compression ramp, to move upstream and creates the self-sustained oscillation. The predicted buzz frequencies are in a good agreement with the experimental frequencies calculated based on the surface pressure data with a discrepancy of less than 2%. Further, the peak and trough of both total and static pressure fluctuations and, as a result, the amplitude of buzz all are accurately predicted. The inlet performance curve during the buzz cycle indicates that both TPR and MFR values fluctuate about 70% and 60% of their maximum possible values, respectively. In addition, the influence of the Hysteresis-Effect on the inlet operation and its performance curve is beheld clearly.

Unsteady Characteristics of a Swept-Shock/Boundary-Layer Interaction at Mach 2

An unswept fin is placed in Mach 2 flow at an angle () of 15 deg, creating a fully separated interaction of strength, . The time-averaged behavior of the interaction is characterized by mean surface pressure measurements inside the inception zone and in the quasi-conical region. The interaction dynamics are explored through unsteady surface pressure measurements, where the highest unsteadiness is observed near the intermittent separation and underneath the open separation bubble. Further insights into the unsteadiness are sought by examining the contributions to the total pressure fluctuations in three spectral regimes: low-frequency (), mid-frequency (), and high-frequency (). Mid-frequency fluctuations dominate the current 3-D SBLI, in contrast to 2-D interactions where low-frequency disturbances are shown to be prominent. Finally, the interaction is visualized using high-frame-rate conical shadowgraphy (24,000 fps), where a shock identification scheme is used. The PDF of the separation shock locations revealed that it travels the farthest compared with the other two shock features. It is inferred that the smaller extent of the -shock is more probable, which is further confirmed by conditionally sampling the shock slope based on the movement of the separation shock.

Characterization of Periodic Incoming Wakes in a Low-Pressure Turbine Cascade Test Section by Means of a Fast-Response Single Sensor Virtual Three-Hole Probe

This paper aims at evaluating the characteristics of the wakes periodically shed by the rotating bars of a spoked-wheel type wake generator installed upstream of a high-speed low Reynolds linear low-pressure turbine blade cascade. Due to the very high bar passing frequency obtained with the rotating wake generator (fbar = 2.4−5.6 kHz), a fast-response pressure probe equipped with a single 350 mbar absolute Kulite sensor has been used. In order to measure the inlet flow angle fluctuations, an angular aerodynamic calibration of the probe allowed the use of the virtual three-hole mode; additionally, yielding yaw corrected periodic total pressure, static pressure and Mach number fluctuations. The results are presented for four bar passing frequencies (fbar = 2.4/3.2/4.6/5.6 kHz), each tested at three isentropic inlet Mach numbers M1,is = 0.26/0.34/0.41 and for Reynolds numbers varying between Re1,is = 40,000 and 58,000, thus covering a wide range of engine representative flow coefficients (ϕ = 0.44−1.60). The measured wake characteristics show fairly good agreement with the theory of fixed cylinders in a cross-flow and the evaluated total pressure losses and flow angle variations generated by the rotating bars show fairly good agreement with theoretical results obtained from a control volume analysis.

Test Facility for High-Speed Probe Calibration

A new test facility was built up as a part of a closed-loop transonic wind tunnel in VZLU´s High-speed Aerodynamics Department. The wind tunnel is driven by a twelve stage radial compressor and Mach and Reynolds numbers can be changed by the compressor speed and by the total pressure in the wind tunnel loop by a set of vacuum pumps, respectively. The facility consists of an axisymmetric subsonic nozzle with an exit diameter de = 100 mm. The subsonic nozzle is designed for regimes up to M = 1 at the nozzle outlet. At the nozzle inlet there is a set of a honeycomb and screens to ensure the flow stream laminar at the outlet of the nozzle. The subsonic nozzle can be supplemented with a transonic slotted nozzle or a supersonic rigid nozzle for transonic and supersonic outlet Mach numbers. The probe is fixed in a probe manipulator situated downstream of the nozzle and it ensures a set of two perpendicular angles in a wide range (±90°). The outlet flow field was measured through in several axial distances downstream the subsonic nozzle outlet. The total pressure and static pressure was measured in the centreline and the total pressure distribution in the vertical and horizontal plane was measured as well. Total pressure fluctuations in the nozzle centreline were detected by a FRAP probe. From the initial flow measurement in a wide range of Mach numbers the best location for probe calibration was chosen. The flow field was found to be suitable for probe calibration.

Letter: The effects of streamwise system rotation on pressure fluctuations in a turbulent channel flow

In this letter, we report the modulating effects of streamwise system rotation on both the amplitude and the wavenumber of pressure fluctuations in a plane channel flow. The analysis of the pressure field is conducted based on a set of comprehensive direct numerical simulation data of six rotation numbers. It is observed that high pressure fluctuation regions collocate with the Taylor–Görtler-like (TGL) vortex cores. By decomposing the pressure field into rotation-induced and convection-induced parts, it is observed that the rotation-induced part dominates the total pressure fluctuations and facilitates the growth of TGL vortices. Furthermore, through a spectral analysis, it is discovered that the system rotation acts as a “linear amplifier,” which converts high-wavenumber low-amplitude streamwise vorticity fluctuations into low-wavenumber high-amplitude pressure fluctuations.

Bubble convection and bubbly flow turbulent time and length scales in two-dimensional plunging jets

Air entrainment and air-water mixing by a flow impingement are enhanced by the turbulent shear layer and associated instabilities in the receiving waterbody. This paper presents an experimental study aiming at a quantitative description of bubble-turbulence interplay in two-dimensional supported plunging water jets. In addition to the basic air-water flow properties, the turbulence intensity in the highly-aerated plunging pool was estimated based on void fraction and total pressure fluctuation measurements, while the turbulent time and length scales were recorded systematically. The coupling of bubble convection and formation of macroscopic turbulent structures was characterised in terms of bubble clustering behaviours and turbulent length and time scales of the bubbly flow eddy structures. The effects of jet impact velocity were investigated for a fixed jet length. These advanced data analyses were applied to plunging jet two-phase flow for the first time. The results would provide new benchmark data for numerical modelling of intense air-water flow at a higher level than the basic two-phase flow dynamic properties. A discussion was developed at the end on the turbulent length scales and the Schmidt number in the bubbly flow regions of horizontal hydraulic jump and vertical supported plunging jet.