Time-averaged Pressure Research Articles

Investigating effect of pulsed flow on hydrodynamics of gas-solid fluidized bed using two-fluid model simulation and experiment

Gas-solid flow behavior in a pulsed fluidized bed was studied experimentally and numerically. Eulerian-Eulerian two-fluid model approach in conjunction with the kinetic theory of granular flow was used for simulation of dense gas-solid flow. The effect of pulsating flow with frequency range of 1 to 10Hz for particle sizes of Geldart B and A/B group with variation of particle density was investigated. Model simulation results were compared reasonably well with researcher's findings and the experimental data. Time-averaged local pressure drop and solid volume fraction were obtained and the effect of various pulsation frequencies on the bed expansion ratio, solid volume fraction, voidage and solid axial velocity was discussed. By increasing the pulsation frequency up to 10Hz, results show reduction in the bubble size, bed expansion ratio and pressure fluctuation. Time-averaged axial velocity and voidage have a flatter profile with increasing pulsation frequency.

Reducing the pressure drag of a D-shaped bluff body using linear feedback control

The pressure drag of blunt bluff bodies is highly relevant in many practical applications, including to the aerodynamic drag of road vehicles. This paper presents theory revealing that a mean drag reduction can be achieved by manipulating wake flow fluctuations. A linear feedback control strategy then exploits this idea, targeting attenuation of the spatially integrated base (back face) pressure fluctuations. Large-eddy simulations of the flow over a D-shaped blunt bluff body are used as a test-bed for this control strategy. The flow response to synthetic jet actuation is characterised using system identification, and controller design is via shaping of the frequency response to achieve fluctuation attenuation. The designed controller successfully attenuates integrated base pressure fluctuations, increasing the time-averaged pressure on the body base by 38%. The effect on the flow field is to push the roll-up of vortices further downstream and increase the extent of the recirculation bubble. This control approach uses only body-mounted sensing/actuation and input–output model identification, meaning that it could be applied experimentally.

Burst-Mode Frequency Effects of Dielectric Barrier Discharge Plasma Actuator for Separation Control

The various separation control mechanisms of burst-mode actuation with a dielectric barrier discharge plasma actuator were experimentally investigated in this study. The control of the separated flow around a NACA 0015 airfoil at a Reynolds number of was investigated using a plasma actuator mounted at a distance from the leading edge of 5% of the chord length. A parametric study on the nondimensionalized burst frequency was conducted at three poststall angles of attack and various input voltages using time-averaged pressure measurements and time-resolved particle imaging velocimetry (PIV) results. The measurement results of the trailing edge pressure, which was selected as the index of separation control, indicate that the optimal burst frequency varies with the angle of attack. Several flow fields are discussed in detail in this paper, and two flow control mechanisms were observed: the use of a large-scale vortex and the promotion of turbulent transition. With regard to the first mechanism, the phase-locked PIV results indicate that a vortex structure, the size of which increases with decreasing burst frequency in the experimental range, is shed from the shear layer for each burst actuation. With regard to the second mechanism, time-averaged pressure and PIV measurements reveal that a burst frequency of is able to promote turbulent transition. Among these two mechanisms, at higher angles of attack, the use of a large-scale vortex structure provides better separation control, whereas near the stall angle, the promotion of the turbulent transition provides better separation control.

Time-averaged second-order pressure and velocity measurements in a pressurized oscillating flow prime mover

Nonlinear phenomena in oscillating flow devices cause the appearance of a relatively minor secondary flow known as acoustic streaming, which is superimposed on the primary oscillating flow. Knowledge of control parameters, such as the time-averaged second-order velocity and pressure, would elucidate the non-linear phenomena responsible for this part of the decrease in the system’s energetic efficiency. This paper focuses on the characterization of a travelling wave oscillating flow engine by measuring the time-averaged secondorder pressure and velocity. Laser Doppler velocimetry technique was used to measure the time-averaged second-order velocity. As streaming is a second-order phenomenon, its measurement requires specific settings especially in a pressurized device. Difficulties in obtaining the proper settings are highlighted in this study. The experiments were performed for mean pressures varying from 10 bars to 22 bars. Non-linear effect does not constantly increase with pressure.

Three-Dimensional Numerical Simulation of a Rotating Detonation Engine: Effects of the Throat of a Converging-Diverging Nozzle on Engine Performance

ABSTRACTThe effect of a converging-diverging nozzle on the performance of a rotating detonation engine was estimated using three-dimensional numerical simulations with a detailed chemical reaction model. The nozzle in this article is composed of a long constant cross section, a short converging section, and a diverging section. The expansion area ratios of nozzles A and B in this article are 2.0 and 1.89, respectively. The periodic exhaust oscillation due to rotating detonation is considerably reduced by nozzle A. The variation of the time-averaged pressure is approximately 0.0048 MPa and the ratio to the time-averaged value is 1.3%. For nozzle B, the variation in pressure is approximately 0.018 MPa, and the ratio to the time-averaged value is 5%. The specific impulse of nozzle A increases by approximately 11 s (3.8%) compared with the value of nozzle B.

Effect of a volute on the unsteady flow in the vane diffuser of a centrifugal compressor

A volute is the only circumferential asymmetric component in a centrifugal compressor, and thus, it should account for the circumferential asymmetry of the flow in a vane diffuser. This study performs a transient numerical analysis to investigate the effect of a volute on the flow in the vane diffuser of a centrifugal compressor under three operating conditions (near-stall, middle, and high mass flow). We compare numerical and experimental performance of the compressor, including polytropic efficiency, total pressure ratio, and unsteady pressure on a diffuser vane. The numerical scheme is proven valid owing to the fact that the numerical and experimental results considerably agree well with each other. Under middle and high mass flow conditions, the time-averaged static pressure recovery and the total pressure loss coefficients for all the diffuser passages indicate that the performance of the passages near and upstream of the volute tongue is affected negatively by the volute, whereas that of the passages downstream of the volute tongue is less affected. Under near-stall condition, the performance of all the passages is disturbed, and the diffuser passage marked as DP 3 demonstrates the worst performance. Investigation on the time-averaged aerodynamic forces, loading, and pressure on the vanes yields results that are consistent with those of the investigation on the performance of the passages. The harmonics with 0.5 fb and fb, which are included in the unsteady loading and pressure on the pressure and suction sides of the vanes, are dominant, where fb is the impeller main and splitter blades passing frequency. Their amplitude values increase as mass flow deviates from the middle mass flow condition. Under middle and high mass flow conditions, the harmonic with 0.5 fb is affected more negatively because of the larger amplitude on the vanes near and upstream of the volute tongue than those downstream, whereas the harmonic with fb is less affected by the volute. Under the near-stall condition, the transient vorticity fields along with the harmonics of 0.5 fb and fb are investigated to evaluate the performance of the diffuser passages. DP 3, which is located at approximately 90° downstream of the volute tongue, suffers the strongest flow deterioration and is inferred to stall first. Further researches for designing more matching diffuser/volute combination will be performed by referring this study.

Large-scale volumetric pressure from tomographic PTV with HFSB tracers

The instantaneous volumetric pressure in the near-wake of a truncated cylinder is measured by use of tomographic particle tracking velocimetry (PTV) using helium-filled soap bubbles (HFSB) as tracers. The measurement volume is several orders of magnitude larger than that reported in tomographic experiments dealing with pressure from particle image velocimetry (PIV). The near-wake of a truncated cylinder installed on a flat plate (Re D = 3.5 × 104) features both wall-bounded turbulence and large-scale unsteady flow separation. The instantaneous pressure is calculated from the time-resolved 3D velocity distribution by invoking the momentum equation. The experiments are conducted simultaneously with surface pressure measurements intended for validation of the technique. The study shows that time-averaged pressure and root-mean-squared pressure fluctuations can be accurately measured both in the fluid domain and at the solid surface by large-scale tomographic PTV with HFSB as tracers, with significant reduction in manufacturing complexity for the wind-tunnel model and circumventing the need to install pressure taps or transducers. The measurement over a large volume eases the extension toward the free-stream regime, providing a reliable boundary condition for the solution of the Poisson equation for pressure. The work demonstrates, in the case of the flow past a truncated cylinder, the use of HFSB tracer particles for pressure measurement in air flows in a measurement volume that is two orders of magnitude larger than that of conventional tomographic PIV.

Combustion oscillation study in a kerosene fueled rocket-based combined-cycle engine combustor

This study reports the combustion oscillation features in a three-dimensional (3D) rocket-based combined-cycle (RBCC) engine combustor under flight Mach number (Mflight) 3.0 conditions both experimentally and numerically. Experiment is performed on a direct-connect ground test facility, which measures the wall pressure along the flow-path. High-speed imaging of the flame luminosity and schlieren is carried out at exit of the primary rocket. Compressible reactive large eddy simulation (LES) with reduced chemical kinetics of a surrogate model for kerosene is performed to further understand the combustion oscillation mechanisms in the combustor. LES results are validated with experimental data by the time-averaged and root mean square (RMS) pressure values, and show acceptable agreement. Effects of the primary rocket jet on pressure oscillation in the combustor are analyzed. Relation of the high speed rocket jet oscillation, which is thought to among the most probable sources of combustion oscillation, with the RBCC combustor is recognized. Results reveal that the unsteady over-expanded rocket jet has significant impacts on the combustion oscillation feature of the RBCC combustor, which is different from a thermo-acoustics type oscillation. The rocket jet/air inflow physical interactions under different rocket jet expansion degrees are experimentally studied.

An Enhanced Physically Based Scour Model for Considering Jet Air Entrainment

Abstract Based on systematic experiments on the influence of air entrainment on rock block stability in plunge pools impacted by high-velocity jets, this study presents adaptations of a physically based scour model. The modifications regarding jet aeration are implemented in the Comprehensive Scour Model (CSM), allowing it to reproduce the physical-mechanical processes involved in scour formation concerning the three phases; namely, water, rock, and air. The enhanced method considers the reduction of momentum of an aerated jet as well as the decrease of energy dissipation in the jet diffusive shear layer, both resulting from the entrainment of air bubbles. Block ejection from the rock mass depends on a combination of the aerated time-averaged pressure coefficient and the modified maximum dynamic impulsion coefficient, which was found to be a constant value of 0.2 for high-velocity jets in deep pools. The modified model is applied to the case of the observed scour hole at the Kariba Dam, with good agreement.

Large eddy simulation of unsteady shedding behavior in cavitating flows with time-average validation

Cavitation always leads to complex gas–liquid interactions and turbulence structures with multi-scale eddies and vortices. It usually involves cavity growth, break-off and collapse processes; posing great challenges in modeling. This paper focuses on modeling instantaneous cavitating flows using the large eddy simulation (LES) and validating the predictions against experimental data using the time-average method. The volume of fluid (VOF) model was adopted to describe phase equations and coupled with the Schnerr–Sauer cavitation model for describing the evaporation-condensation mass transfer. Simulations were performed to predict the unsteady cavitating flows of both the cylinder and Clark-Y hydrofoil configurations. Firstly, the mechanisms of cavity shedding, vapor cloud forming and collapsing were well revealed. The time-averaged pressure distribution and cavity length around the cylinder were in good agreement with experimental data. Moreover, the periodic cavity shedding and pressure fluctuation around the Clark-Y hydrofoil were also predicted. Different cavity patterns were clearly identified in a typical cycle, and the effect of cavity pattern on hydrodynamic forces was investigated. The computational results of cavity patterns, velocity profiles, drag and lift coefficients were compared with experimental results and good agreements were obtained. The present work provides a valid numerical modeling framework for various cavitating flows.

Time-averaged and transient pressure drop for flow boiling with saturated inlet conditions

This study explores flow boiling pressure drop of FC-72 in a rectangular channel subjected to single-side and double-sided heating for vertical upflow, vertical downflow, and horizontal flow with positive inlet quality. Analysis of temporal records of pressure transducer signals is used to assess the influences of orientation, mass velocity, inlet quality, heat flux, and single-sided versus double-sided heating on magnitude of pressure drop oscillations, while fast Fourier transforms of the same records are used to capture dominant frequencies of oscillations. Time-averaged pressure drop results are also presented, with trends focusing on the competing influences of body force and flow inertia, and particular attention paid to the impact of vapor content at the test section inlet and the rate of vapor generation within the test section on pressure drop. Several popular pressure drop correlations are evaluated against the present pressure drop database. Predictions are presented for subsets of the database corresponding to low and high ranges of inlet quality and mass velocity. The correlations are ranked based on mean absolute error, overall data trends, and data spread. While most show general success in capturing the data trends, they do so with varying degrees of accuracy.

Transport Mechanism of Hot Streaks and Wakes in a Turbine Cascade

This paper aims to understand the combined effects of upstream hot streaks and wakes on the blade-to-blade flowfields in a turbine cascade. The computational-fluid-dynamics method was first validated with the unsteady experimental results of the low-speed T106 cascade and then used to study more cases. To simulate the incoming hot streak, a parabolic temperature distribution was applied at the inlet of the computational domain. The incoming wake was simulated using a bar. Four cases with different relative phases between the hot streak and wake were investigated. In different cases, the hot streak and the wake interacted differently, which resulted in a different flowfield in the downstream blade passage. The distributions of the perturbation velocity magnitude and the perturbation velocity vectors were used to explain different convection of the hot streak and the wake. The effects of the transport mechanism on the aerothermal performance of the cascade were discussed. It was found that the time-averaged blade surface pressure distribution and heat flux are similar for the cases studied. However, the fluctuation of the blade surface pressure distribution and heat flux differs significantly as a result of the different transport mechanisms.

Multi-Objective Aeroacoustic Design Exploration of Launch-Pad Flame Deflector Using Large-Eddy Simulation

This study explores possible design of an aeroacoustic flame deflector for a rocket launch pad. The design objectives are 1) minimization of the overall sound pressure level near the payload fairing, 2) minimization of the time-averaged maximum pressure on the flame-deflector surface, and 3) minimization of the shape difference from a flat plate inclined at 45 deg. The acoustic wave characteristics associated with deflector shapes are identified by large-eddy simulations. To overcome difficulties of required computational time, the following are adopted: 1) a high-order scheme that reduces the computational cost of large-eddy simulations, 2) a multi-objective evolutionary algorithm for efficient parallelization, and 3) large-scale parallelization on the Japanese supercomputer “K”. Total computational time for optimization is approximately 350 h with 6500 processors of the K computer. The analysis of nondominated (Pareto-optimal) solutions reveals a tradeoff relation and correlation among the objective functions. In the result, there appears a well-balanced solution that significantly reduces the overall sound pressure level. The shape difference is relatively minor, with a small bump located somewhat upstream of the impinging region. The result suggests that the local angle of the inclined deflector near the impinging region plays an important role for the reduction of overall sound pressure level near the rocket fairing.

Flow Separation and Turbulence in Jet Pumps for Thermoacoustic Applications

The effect of flow separation and turbulence on the performance of a jet pump in oscillatory flows is investigated. A jet pump is a static device whose shape induces asymmetric hydrodynamic end effects when placed in an oscillatory flow. This will result in a time-averaged pressure drop which can be used to suppress acoustic streaming in closed-loop thermoacoustic devices. An experimental setup is used to measure the time-averaged pressure drop as well as the acoustic power dissipation across two different jet pump geometries in a pure oscillatory flow. The results are compared against published numerical results where flow separation was found to have a negative effect on the jet pump performance in a laminar flow. Using hot-wire anemometry the onset of flow separation is determined experimentally and the applicability of a critical Reynolds number for oscillatory pipe flows is confirmed for jet pump applications. It is found that turbulence can lead to a reduction of flow separation and hence, to an improvement in jet pump performance compared to laminar oscillatory flows.

On the performance and flow characteristics of jet pumps with multiple orifices.

The design of compact thermoacoustic devices requires compact jet pump geometries, which can be realized by employing jet pumps with multiple orifices. The oscillatory flow through the orifice(s) of a jet pump generates asymmetric hydrodynamic end effects, which result in a time-averaged pressure drop that can counteract Gedeon streaming in traveling wave thermoacoustic devices. In this study, the performance of jet pumps having 1-16 orifices is characterized experimentally in terms of the time-averaged pressure drop and acoustic power dissipation. Upon increasing the number of orifices, a significant decay in the jet pump performance is observed. Further analysis shows a relation between this performance decay and the diameter of the individual holes. Possible causes of this phenomenon are discussed. Flow visualization is used to study the differences in vortex ring interaction from adjacent jet pump orifices. The mutual orifice spacing is varied and the corresponding jet pump performance is measured. The orifice spacing is shown to have less effect on the jet pump performance compared to increasing the number of orifices.

Large-Eddy Simulation of High-Frequency Combustion Instability in a Single-Element Atmospheric Combustor

Large-eddy simulation is performed to simulate high-frequency combustion instability in a single-element atmospheric combustor. Simulations are conducted for corresponding combustion-instability experiments, and the self-excited combustion instability is successfully captured. The first tangential mode of the combustion chamber is excited in the large-eddy simulation, and the amplitude and frequency of the pressure fluctuations are consistent with the experimental observations. The first tangential mode in the large-eddy simulation was observed at 1 kHz, and the peak-to-peak amplitude was approximately 4% of time-averaged pressure. The higher-order modes were also observed in the large-eddy simulation at frequencies ranging from 2 to 4 kHz, although those amplitudes were approximately one-fourth of the first tangential mode. The coupling mechanism between the flame and acoustic mode is explored based on the large-eddy-simulation results. The periodic ignition of the unburnt mixture exhibits lifted combustion in a pulsating motion, and the frequency is similar to the first-tangential-mode frequency. This unsteady pulsating-flame behavior is caused by the coupling between the fuel injection and the first-tangential-mode oscillation. The Rayleigh index indicates that a primary driving factor for the instability is the acoustically coupled pulsating-flame motion. The present results demonstrate that large-eddy simulation can accurately capture the unsteady heat release and its coupling with pressure oscillations. The large-eddy-simulation results clarify details of flame structures that are not completely understood from the experimental measurements. These results are also valuable for understanding the coupling mechanism of the flame and acoustic mode in a combustion chamber.

CFD modelling of unsteady electro-osmotic permeate flux enhancement in membrane systems

Electro-osmotic flow (EOF) perturbations induced near the membrane surface are a promising approach to increase wall shear, which in turn has the potential to slow the onset of fouling for nanofiltration and reverse osmosis processes. As such, it is important to understand the mechanisms that increase wall shear and mass transfer resulting from time-varying EOF perturbations. In this paper, a Computational Fluid Dynamics (CFD) model is used to simulate spatially-uniform unsteady EOF with permeation inside a 2D unobstructed empty membrane channel. Time-averaged permeate flux is used to measure productivity and maximum shear rate is used as a proxy measure for fouling reduction/prevention. The dependencies of solute concentration amplitude and maximum wall stress on slip velocity amplitude are shown to be linear, both in terms of homogeneity and additivity. This implies that time-averaged hydrodynamics and mass transfer do not vary significantly regardless of changes in the frequency and amplitude of the slip velocity, because the effect is cancelled within the time oscillation period. Nevertheless, there are still advantages for this type of perturbation, as larger slip velocity frequency and amplitude increase the maximum wall stress with a negligible change in time-averaged pressure drop, which may have advantages for limiting concentration polarisation and fouling.

Simulation and experimental validation of droplet dynamics in microchannels of PEM fuel cells

In this study, dynamics of droplets in the channels of proton exchange membrane fuel cells with straight and serpentine flow-fields was investigated. Tapered and filleted channels were suggested for the straight and serpentine flow-fields respectively in order to improve water removal in channels. Surface tension and wall adhesion forces were applied by using the volume of fluid method. The hydrophilic walls and hydrophobic gas diffusion layer were considered. The mechanism of droplets movement with different diameters was studied by using the Weber and capillary numbers in simple and tapered straight channels. It was illustrated that the flooding was reduced in tapered channel due to increase of water removal rate, and available reaction sites improved subsequently. In addition, film flow was formed in the tapered channel more than the simple channel, so pressure fluctuation was decreased in the tapered channel. Moreover, the water coverage ratio of hydrophilic tapered surface was more than the simple channel, which enhanced water removal from the channel. The filleted serpentine channel was introduced to improve water removal from the simple serpentine channel. It was shown by observation of the unsteady and time-averaged two-phase pressure drop that in the filleted serpentine channels, the two-phase pressure drop was far less than the simple serpentine channel, and also the accumulation of water droplets in the elbows was less leading to lower pressure fluctuation. The numerical simulation results were validated by experiments.

Large-Eddy Simulation of Turbulent Flow Around a Finite-Height Wall-Mounted Square Cylinder Within a Thin Boundary Layer

In this paper, large-eddy simulation (LES) has been performed to investigate the turbulent wake behind a wall-mounted square cylinder. The flow features a relatively high cylinder aspect ratio of 4, a Reynolds number of 12,000 (based on the free-stream velocity and cylinder side length), and a thin developing boundary layer with a thickness of only 18 % of the obstacle height. These characteristics of the flow impose challenges to LES for accurate simulation of large energetic eddies induced by the cylinder and their intense interactions with the thin developing boundary layer. The coherent flow structures around the cylinder have been studied based on the instantaneous and time-averaged resolved velocity and pressure fields. The local kinetic energy transfer between the resolved and subgrid scales and the streamwise evolution of the subgrid-scale viscosity have been investigated to provide physical insights into the subgrid-scale dynamics. The LES predictions of the flow statistics have been validated against a set of recently reported wind-tunnel experimental data.

AVT-183 diamond wing flow field characteristics Part 1: Varying leading-edge roughness and the effects on flow separation onset

Experimental investigations on a 53° leading-edge sweep diamond wing configuration with rounded leading-edge contour are presented. The analyses pertain to recent work that was conducted within the NATO Science and Technology Organization (STO) task group AVT-183 (Applied Vehicle Technology panel). The results were obtained in a low-speed wind tunnel facility and include aerodynamic forces and moments, and time-averaged surface pressures. Special emphasis is placed on the effects of different surface roughness, which was applied in the experimental analyses at the wing leading edge to fix turbulent boundary-layer characteristics. The results show that the flow separation onset and the emerging leading-edge vortex are very sensitive to the roughness height and any leading-edge contour modification. Compared to the free transition case, both reasonably-tripped and over-tripped cases are obtained, which is discussed in detail. Moreover, measurement repeatability issues are reviewed from a short-term and a long-term perspective. As an outcome, one specific leading-edge roughness is selected for the target flow conditions, which provided the baseline for subsequent flow field investigations and general CFD validation within the task group AVT-183.