Downstream Velocity Research Articles

Impact of the Acoustic Forcing Level on the Transfer Matrix of a Turbulent Swirling Combustor with and Without Flame

Thermoacoustic instabilities are a major issue for industrial and domestic burners. One possible framework to study these instabilities is to represent the system by a network of Dimensionless Acoustic Transfer Matrices (DATM) that link pressure and velocity fluctuations upstream and downstream each element of the network. In this article, the DATM coefficients of a turbulent swirling combustor are determined for a thermoacoustically stable configuration using harmonic acoustic forcing. Since the dynamics of the whole system is controlled by nonlinearities, the impact of the forcing level needs to be considered. The four DATM coefficients are thus measured for reactive operating conditions (premixed flame) and cold flow conditions for increasing acoustic excitation levels. The velocity level is controlled by a hot wire located inside the injector, in a region with a laminar top-hat velocity profile. The upstream and downstream specific acoustic impedances are also measured. Results for the acoustic response under cold flow conditions are first presented. In this case, the DATM coefficients are found to be independent of the forcing level except for the modulus of the coefficients linking the downstream velocity fluctuations to the upstream pressure and velocity fluctuations. This behavior is linked to the nonlinear response of the injector but is not entirely captured by the acoustic network model developed in this work. For reactive operating conditions, measurements indicate that all DATM coefficients depend on the forcing level to a certain extent. The Flame Describing Function, linking heat release rate fluctuations to velocity fluctuations, is used to reconstruct the transfer matrix through an acoustic network model. This network model accurately predicts the trend of the measured coefficients but the impact of the forcing level is not reproduced. Saturation for reactive operating conditions is shown to be not only related to the nonlinear flame response but also to the nonlinear injector dynamics. Finally, a data-driven reconstruction of the FDF using the acoustic network model along with the hot wire and microphone measurements is performed. This data-driven acoustic reconstruction is subsequently compared with the FDF determined with an optical technique.

Flow interactions with an aquatic macrophyte: a field study using stereoscopic particle image velocimetry

This paper reports the morphology of a natural patch of Ranunculus penicillatus and presents high-resolution measurements of flow velocities in its wake using a stereoscopic PIV field measurement system. The patch was 3.80 m long, 1.24 m wide and caused substantial changes to downstream mean velocities and turbulence. Vertical profiles of streamwise mean velocity were not logarithmic and flow was redirected under the positively buoyant canopy, enhancing vertical turbulent mixing in the wake and generating a large region where the velocity covariance was positive. Turbulent kinetic energy was enhanced downstream from the patch lateral shear layer, but not at the centre of the wake. Spectra downstream from the patch showed that turbulence was neither dominated by fine-scale nor large-scale structures, likely due to the low energy of the flow conditions and lack of a developed vortex street within the measurement domain. Sedimentation was observed at the upstream end of the patch, but not underneath the floating canopy. The methods and results of this work will be useful for planning other in situ studies. Also, the reported data on macrophyte geometry and biometrics will assist with the design of more realistic replicas for use in laboratory studies.

Optimisation of low energy cooling through phase variation between adjacent piezoelectric fan blades

Piezoelectric fans are highly efficient and low energy air-movers. The technology may be applied to reduce the power requirements of thermal management systems in a range of power electronics. The understanding of piezoelectric fan arrays, in which multiple blades are beneficially coupled, is of paramount importance for the research field. In the present research, a numerical model consisting of two piezoelectric fan blades is developed and validated against previous experimental work. The model is used to investigate the effect of phase variation, ϕ, between two adjacent piezoelectric fan blades in face-to-face orientation, operating within a channel. When ϕ≠0°, 180°, an asymmetrical flow domain is observed. Considering an incremental increase of phase variation from ϕ=0°, the skewness of the downstream velocity profile is increased heavily when ϕ⩽60°, whilst the magnitude of the average downstream velocity is only enhanced when ϕ⩾75°. The generated skew is attributed to the dominance of one vortex over another between the adjacent blade faces during their opening period. At peak skewness (ϕ=75°), the vortex behind the phase leading blade is 101% larger than its counterpart. Evenly matched vortices between the two blades are optimal for generating peak mass transfer. This is observed when the blades are in counter-phase, ϕ=180°. Conclusions in the present study build upon and are in good agreement with those previously published.

Study on reduction in pressure losses in pipe bends using guide vanes

The pipe bends are common elements in pipeline network of mechanical systems. The losses in pipes and bend are generally considered as insignificant and unavoidable. The recent trends in achieving a higher order of performance in machines led to optimization by reduction in minor energy losses. The usage of components with the higher surface finish is the main methodology adopted for minimizing frictional loss. In bends, especially in the case of turbulent flows, the losses are significant when comparing the frictional pipe losses. The viscous interaction between fluid layers is more in bends due to the presence of flow separation secondary swirling motion. The reduction in these interactions can be achieved by installing turning vane (guide vanes) inside the pipe bend. The current literature available lacks detailing of many included flow physics in bends and vaned bends. The present work focuses on flow characteristics on a 90° curved bend. Incompressible isothermal RANS solutions were performed using turbulence treatment with the k − ω SST model. The initial validation study is carried out with available experimental results. The bend downstream velocity distribution and bend wall pressure distribution are carried out. Further, the computational methodology is extended to bend with different vane configurations and for different Reynolds numbers. The velocity, wall pressure, swirl strength, turbulent kinetic energy, etc. are reported for downstream locations. The comparison of pressure loss through different cases and improvements in flow through bend due to the inclusion of vanes are discussed in this work.

Kink solutions of the generalized, super-diffusive Burgers equation with memory

Analytical and numerical solutions of the shockwave type to a one–dimensional, generalized Burgers equation with a nonlinear constitutive law that accounts for stress relaxation via a material derivative are reported as functions of the parameters that appear in the equation and the boundary conditions. It is shown that the existence of kink solutions depends on the nonlinear convection and diffusion terms, the viscosity, the relaxation time, and the upstream and downstream boundary conditions. It is also shown that the slope of the kink’s profile decreases as the viscosity coefficient, the upstream–to–downstream velocity ratio and the relaxation time are increased, and the coefficient of the nonlinear convection term is decreased. The slope of the kink’s profile is not an odd function of the coordinate in a reference system moving at the kink’s speed, but it is nearly independent of the relaxation times for small values of this parameter. The kink’s width is found to decrease and increase as the exponent of the nonlinear convection term is decreased and increased from unity, respectively. The kink’s downstream and upstream widths increase and decrease, respectively, as the exponent of the diffusion term is increased, for exponents of the nonlinear convection term greater than unity. It is also shown that the differences between the shock wave profiles corresponding to a material frame–indifferent constitutive equation and those corresponding to Maxwell’s stress relaxation model are at most six percent, but depend strongly on the relaxation time and the exponents of the nonlinear convection and diffusion terms. The local dissipation rate is also determined as a function of the coefficients and powers of the nonlinearities and the relaxation time.

Geometric optimisation of piezoelectric fan arrays for low energy cooling

A numerical model of a typical piezoelectric fan blade is derived and validated against experimental data. Numerical error is found to be 5.4% and 9.8% using two data comparison methods. The model is used to explore the variation of pitch as a function of amplitude, A, for a confined two-blade piezoelectric fan array in face-to-face orientation, with the blades oscillating both in-phase and counter-phase. It has been reported that in-phase oscillation is optimal for generating maximum downstream velocity and flow rate in unconfined conditions, due at least in part to the beneficial coupling between the adjacent blades that leads to an increased oscillation amplitude. The present model demonstrates that confinement has a significant detrimental effect on in-phase oscillation. Even at low pitch, counter-phase oscillation produces enhanced downstream air velocities and flow rates. Downstream air velocity from counter-phase oscillation can be maximally enhanced, relative to that generated from a single blade, by 17.7% at P=8A. Flow rate enhancement at the same pitch is found to be 18.6%. By comparison, in-phase oscillation at the same pitch outputs 23.9% and 24.8% reductions in peak downstream air velocity and flow rate, relative to that generated from a single blade. This optimal pitch, equivalent to those reported in the literature, suggests that counter-phase oscillation is less affected by confinement. The optimal pitch for generating bulk airflow from counter-phase oscillation is large, P>16A, due to the small but significant downstream velocity across the span between adjacent blades. However, by considering design in a confined space, counter-phase pitch should be minimised to maximise the bulk airflow generated from a certain cross-sectional area within a channel flow application. Quantitative values are found to deviate to a small degree as other geometric and operational parameters are varied, but the established relationships are maintained.

A freely moving body in a boundary layer: Nonlinear separated-flow effects

In the boundary layer of fluid on a moving solid surface (such as on a moving vehicle e.g. ships) a thin body is translating parallel to the surface. The streamwise movement of the body relative to the solid surface is substantial, with the upstream or downstream velocity being comparable with the free-stream velocity. The dynamic fluid/body interaction here incorporates the effects of incident vorticity, nonlinear and separating flow. Modelling, analysis and reduced computation explore the all-important influences of body shape, moment of inertia and centre-of-mass location. These influences and the initial conditions affect whether nonlinear oscillations of the fluid and body motions continue or fully nonlinear focussing occurs after a finite time.

BEPU Method Applied to CFD Simulation of Mixing Flows

Prediction of highly turbulent flows using the computational fluid dynamics (CFD) tools is not an easy task. Besides the uncertainty in the choice of turbulence model parameters, the physical properties of the fluid and experimental boundary conditions also can be largely affected by uncertainties. The objective of the study is uncertainty quantification of CFD simulation to obtain figures of merits, downstream velocity, and turbulence kinetic energy. The water-mixing experiment in the GEneric MIxing Experiment (GEMIX) facility performed at Paul Scherrer Institute is used as a benchmark case. The NEPTUNE_CFD code that solves Reynolds-averaged Navier Stokes equations with k-eps turbulence model has been used to perform a series of simulations. For uncertainty quantification with the Monte Carlo method the Optimal Statistical Estimator (OSE) was used for response surface (RS) generation from the set of CFD calculations. The results of the uncertainty analysis show that OSE is a very suitable method for RS generation, which is then used in uncertainty analysis using the Monte Carlo method to determine the 5% lower limit and 95% upper limit with 95% confidence level. In this way, the impact of some sources of uncertainty is evaluated. Also, OSE can reproduce the CFD simulation with high accuracy at the CFD calculation points, even in the case when only 5 out of 40 calculation points are used for RS generation. The results further suggest that it is very important to perform accurate reference calculation and select appropriate ranges of variation for uncertain input parameters.

A 3D Study of an Air-Core Vortex Using HSPIV and Flow Visualization

A free-surface vortex is a mass of water rotating around an axis perpendicular to the free surface. It can occur when withdrawing water from reservoirs or rivers at hydropower intakes with low submergence. Existing vortex models provide general information about the symmetric vortex structure. The vortex structure occurring in an approach flow at the critical submergence condition is examined in detail. In the laboratory, a steady air-core vortex over a bottom intake was created in a wide recirculating flume, in which the water depth, mean velocity of the approach flow and intake discharge could be adjusted. Flow visualization shows that the approach flow results in a non-symmetrical velocity distribution in the vortex throughout the flow depth. The detailed set of high-speed particle image velocimetry data in a series of horizontal and vertical planes was used to observe the formation and evolution of the three-dimensional flow structure of the strong air-core vortex and determine the origin of the vortex. Analysis of these data revealed a complex three-dimensional vortex structure due to the approach flow interacting with the air-core vortex forming a secondary vortex originating at the mixing zone upstream of the vortex, identified by a zero downstream velocity component, and feeding into the upstream side of the intake.

Traveling waves for conservation laws with nonlocal flux for traffic flow on rough roads

We consider two scalar conservation laws with non-local flux functions, describing traffic flow on roads with rough conditions. In the first model, the velocity of the car depends on an averaged downstream density, while in the second model one considers an averaged downstream velocity. The road condition is piecewise constant with a jump at $x=0$. We study stationary traveling wave profiles cross $x=0$, for all possible cases. We show that, depending on the case, there could exit infinitely many profiles, a unique profile, or no profiles at all. Furthermore, some of the profiles are time asymptotic solutions for the Cauchy problem of the conservation laws under mild assumption on the initial data, while other profiles are unstable.

Size-dependent enrichment of leukocytes from undiluted whole blood using shear-induced diffusion.

Little work has been done in microfluidics with separation of cells directly from whole blood, and the handful of microfluidic systems reported the literature offer only limited throughput. Yet high throughput is highly desirable to avoid degradation of samples, which can result in loss of information critical to disease diagnosis or monitoring. In this work, we investigated particle migration dynamics in whole blood flow at a single-particle level and subsequently successfully demonstrated the preferential enrichment of white blood cells (WBCs) in unprocessed whole blood flows flanking a buffer flow. Our in-depth investigation reveals a counter-intuitive, size-based migration of cells in whole blood flow and their tendency to accumulate in the regions near flow interfaces, which is employed for inherent enrichment of WBCs. More importantly, we found the strong size-dependent migration in blood flow stemming from the differentiated downstream velocity of particles, which inversely scales with particle size. Our new insights improve understanding of this counterintuitive microfluidics field, offering guidance for new device design to directly handle whole blood and to expand the applications to meet the real-world need for ultra-fast cell separation.

Flow fluctuation induced by coaxial plasma device at atmospheric pressure

The flow structure and velocity fluctuation generated downstream from coaxial geometry dielectric-barrier-discharges are investigated at atmospheric pressure. The discharges are characterized using in-situ electrical measurements and optical diagnostics. Both streamers and glow-like discharges are detected in each alternating-current cycle. The flow structure is temporally and spatially resolved using tracer particles, and vortices are visualized in planar velocity distribution patterns. The flow upstream of the discharge is laminar; however, we discover that the spectrum of downstream fluctuation velocity exhibits a nearly Kolmogorov −5/3 slope, which is a typical feature for high Reynolds number turbulent flows. Based on the electron density measured through a line-ratio method, the dimensionless electrical body force derived from the Navier-Stokes equation is estimated to illustrate the generation of Reynolds stress. It is found that although less than 0.1% of the discharge power is converted into the fluctuation kinetic energy, the electrical body force rather than Joule heating plays a dominant role in flow fluctuation.

Particle transport mechanics and induced seismic noise in steep flume experiments with accelerometer‐embedded tracers

AbstractRecent advances in fluvial seismology have provided solid observational and theoretical evidence that near‐river seismic ground motion may be used to monitor and quantify coarse sediment transport. However, inversions of sediment transport rates from seismic observations have not been fully tested against independent measurements, and thus have unknown but potentially large uncertainties. In the present study, we provide the first robust test of existing theory by conducting dedicated sediment transport experiments in a flume laboratory under fully turbulent and rough flow conditions. We monitor grain‐scale physics with the use of ‘smart rocks’ that consist of accelerometers embedded into manufactured rocks, and we quantitatively link bedload mechanics and seismic observations under various prescribed flow and sediment transport conditions. From our grain‐scale observations, we find that bedload grain hop times are widely distributed, with impacts being on average much more frequent than predicted by existing saltation models. Impact velocities are observed to be a linear function of average downstream cobble velocities, and both velocities show a bed‐slope dependency that is not represented in existing saltation models. Incorporating these effects in an improved bedload‐induced seismic noise model allows sediment flux to be inverted from seismic noise within a factor of two uncertainty. This result holds over nearly two orders of magnitude of prescribed sediment fluxes with different sediment sizes and channel‐bed slopes, and particle–particle collisions observed at the highest investigated rates are found to have negligible effect on the generated seismic power. These results support the applicability of the seismic‐inversion framework to mountain rivers, although further experiments remain to be conducted at sediment transport near transport capacity. © 2018 John Wiley & Sons, Ltd.

A Numerical Investigation of Flow and Heat Transfer of Laminar Multiple Slot Jets Impinging on Multiple Protruding Heat Sources

The flow and heat transfer behavior of laminar incompressible slot jets impingement cooling of an array of heated surfaces in a channel have been investigated numerically. The computations are done for a variety of values of slot jets Reynolds number, channel height and distance between two heated blocks. The influences of these geometrical and physical parameters are predicted. The results, streamline contour, velocity profile, isothermal contour, local Nusselt number, and average Nusselt number are compared and documented. The first and second recirculation cells size are gradually increased, and the highest heat transfer rate is attained when Reynolds number increased. However, the heat transfer rates are decreased when channel height increased. The peak local Nusselt number value is noticed at stagnation point of the first block by first jet, and the second peak local Nusselt number value is observed at fourth block by second jet. The distances between two blocks play a significant role in the downstream velocity which leads to create the strong recirculation cells in between the two heated blocks when the distance between the two blocks increased.

Bubble image velocimetry with a field-deployable acoustic camera

This study explores performing bubble image velocimetry (BIV) analysis on images acquired from an acoustic camera (AC) to obtain measurements of the flow in the wake of a circular cylinder at two flowrates. The AC observes the returns from air micro-bubbles suspended in the flow which are used as tracer particles in PIV-style analysis. The size distribution of the micro-bubbles is measured with an acoustic bubble spectrometer (ABS). The AC BIV measurements of the wake agree well with the comparison measurements made by cross-stream arrays of acoustic Doppler velocimeters (ADVs) for the lower of the two measured flowrates. The AC BIV measurements of cross-stream profiles of the mean downstream velocities and the profiles of characteristics of the velocity fluctuations, such as the standard deviation of the downstream and cross-stream velocity fluctuations, the covariance of the horizontal velocity fluctuations, and the kinetic energy of the horizontal velocity fluctuations, all matched the ADV measurements well. However, the measurement technique is sensitive to bubble size distribution, as the higher flowrate AC BIV measurements struggled to replicate the ADV measurements, particularly the measurements of the velocity fluctuations. The higher flowrate experiment is revealed to have an order of magnitude more bubbles with size exceeding 15 present in the flow than the lower flowrate experiment. The increased bubble density caused increased noise in the AC images and a ‘flickering’ effect, with bubbles further away from the camera being intermittently obscured by those closer to the camera. The ability to make PIV-style measurements with an AC has ramifications for flow measurements in the field. ACs operate in turbid environments, do not require a laser and extensive optics, and do not require calibration with a grid plate to convert pixel space to physical space.

The need for prediction in feedback control of a mixing layer

We present the first application of a Smith predictor in the model-based feedback control of a convectively unstable flow. The two-dimensional incompressible mixing layer is chosen as a plant and integrated with a finite-element Navier–Stokes solver. The flow is actuated by a modulation of the inflow mimicking a pulsed jet actuator. The state is monitored by a downstream velocity sensor. The control goal is to command the fluctuation level at the sensor location. As an additional challenge, the incoming shear-flow profile is perturbed by the small noise level. The starting point of this study is a standard low-order control without prediction. A Smith predictor is included to compensate for the convective delay from actuation to sensing. The closed-loop results with a Smith predictor are distinctly better than without it. The design of the linear robust controllers is performed with an open-loop control. We conjecture that predictors may augment feedback control for a large class of convectively unstable flows with significant time-delay from actuation to sensing.

Study on the Lateral Velocity and Vortex in a Spacer Grid

The spacer grid is one of the most important parts of a fuel assembly for enhancing thermal-hydraulic performance. The mixing vane is a critical component of the spacer grid (or mixing vane grid) to produce lateral flow, turbulence, vortex, and other mixing mechanisms downstream of the grid. The design and arrangement of mixing vanes could influence the flow condition downstream, especially the lateral flow and vortex surrounding the rod bundle, which directly affect not only the hydraulics and heat transfer but also the critical heat flux. Understanding fundamental mixing characteristics is of great importance for fuel assembly design. Instead of using complicated geometries, this paper studies the lateral flow velocity and vortex in a simple 2 × 1 channel with a spacer grid in order to obtain the behavior of a single mixing vane and its effect on the downstream lateral velocity and vortex. Then, the effect of mixing vane arrangements on pressure drop is studied by researching the lateral velocity and vortex in the 2 × 1 channel with two mixing vanes. Several indexes that could reflect the mixing characteristics, such as lateral velocity and vortex, are introduced to assess the performance of the mixing vanes. Because of the presence of walls in both the experiment and the simulation, the lateral flow will reflect upon the impact of the obstruction walls or rods, which would cause vortex rotation. Rotation time and flow pattern are also studied in this paper. Qualitative and quantitative analyses are carried out to obtain the lateral velocity, the vortex angle, and its period of rotation. This work may be applied to the design and optimization of mixing vanes.

In-plume acceleration of leading-edge ions from a pulsed plasma thruster

Pulsed plasma thrusters are a form of electric propulsion for spacecraft. Their research has undergone recent growth due to an increasing interest in small satellites. However, their plasma plumes are transient and challenging to study due to the extremely short discharge time that lasts in the order of microseconds. The plasma plume is accelerated by a combination of electrothermal and electromagnetic forces. With electromagnetic acceleration, the ions are typically thought to be only accelerated within the thruster by the Lorentz force. Estimated downstream exhaust velocities have ranged widely from 5 to 60 km s−1, with values on the lower end corresponding to electrothermal acceleration. However, we show here that the fastest leading-edge ions continue to accelerate in the plume even in the absence of electromagnetic forces. Pulsed plasma thruster plumes are composed of electrons and a mixture of ion species with different ionization levels. Due to this quasi-neutrality of the plasma plume with electrons featuring much larger velocities than ions, we suggest that the acceleration might be ambipolar in nature. The leading-edge ions eventually reach velocities in excess of 100 km s−1, far greater than values typically associated with electrothermal acceleration. This indicates that care must be taken in the accurate measurement of the ion velocity in the plume. The ion plume may also be more energetic than previously believed. Furthermore, we also propose that simple extrapolation can be used to determine the actual exit plane ion velocity; this value can be used as accurate input data for future simulations.

The importance of riparian plant orientation in river flow: implications for flow structures and drag

In a series of high resolution numerical modelling experiments, we incorporated submerged riparian plants into a computational fluid dynamics (CFD) model used to predict flow structures and drag in river flow. Individual plant point clouds were captured using terrestrial laser scanning (TLS) and geometric characteristics quantified. In the first experiment, flow is modelled around three different plant specimens of the same species (Prunus laurocerasus). In the second experiment, the orientation of another specimen is incrementally rotated to modify the flow-facing structure when foliated and defoliated. Each plant introduces a unique disturbance pattern to the normalized downstream velocity field, resulting in spatially heterogeneous and irregularly shaped velocity profiles. The results question the extent to which generalized velocity profiles can be quantified for morphologically complex plants. Incremental changes in plant orientation introduce gradual changes to the downstream velocity field and cause a substantial range in the quantified drag response. Form drag forces are up to an order of magnitude greater for foliated plants compared to defoliated plants, although the mean drag coefficient for defoliated plants is higher (1.52 defoliated; 1.03 foliated). Variation in the drag coefficients is greatest when the plant is defoliated (up to ∼210% variation when defoliated, ∼80% when foliated).

Three‐Dimensional Magnetohydrodynamic Simulations of the Solar Wind Interaction With a Hyperfast‐Rotating Uranus

AbstractWe present magnetohydrodynamic simulations of a fast‐rotating planetary magnetosphere reminiscent of the planet Uranus at solstice, that is, with the spin axis pointing to the Sun. We impose a 10 times faster rotation than for Uranus, in order to emphasize the effects of rotation on the magnetospheric tail without the need of an excessively large simulation domain while keeping the qualitative aspects of a supersonic magnetized solar wind interacting with a fast‐rotating magnetosphere. We find that a complex helical Alfvénic structure propagates downstream at a velocity exceeding the plasma velocity in the magnetosheath. Similarly, the reconnection regions, which mediate the interaction of the planetary magnetic field and the interplanetary magnetic field, do also form a helical structure with the same downstream velocity but a 2 times larger pitch. We speculate that the magnetic field of the helical structure connected to the interplanetary magnetic field asymptotically reduces the phase velocity of the helical structure toward the tailward velocity in the magnetosheath. For our simulations we use the MPI‐AMRVAC code which we enhanced with a time‐dependent background magnetic field in the splitting of the magnetic field.