Streamwise Momentum Flux Research Articles

Alleviation of self-support in dual synthetic jet and its self-similarity of streamwise momentum flux

A dual synthetic jet (DSJ) actuator has two chambers sharing the same wall equipped with a single oscillatory diaphragm. The novel structure doubles the function of synthetic jet (SJ) actuator and brings it merits that a traditional synthetic jet actuator does not have like resolving the problems of pressure loading and energy inefficiency. However, self-support, which is unique to DSJ, may result in weak jets or even no jets. In order to effectively solve the self-support problem of DSJ and further improve its performance, this paper proposes a novel DSJ exit structure with a penetrating channel. The experimental and numerical investigations prove that the jet penetration depth and streamwise momentum flux of DSJ are increased by nearly 25%–50%. Furthermore, through the analysis of a large number of simulation data, it is found that the streamwise momentum flux distributions of DSJ under random driving frequency and driving amplitude can be expressed by a unified mathematical fitting formula, that is, the streamwise momentum flux has self-similarity. This criterion can efficiently and quickly calculate the SJ/DSJ driving parameters required for effectively controlling the flow field.

Supporting data for: Numerical Study of Near-Surface Jet in the Atmospheric Surface Layer over Oceanic Fronts

Simulation data used for figures in Numerical study of near-surface jet in the atmospheric surface layer over oceanic fronts including the mean velocity profiles, the turbulent variance profiles, the vortex structures, the streamwise momentum flux, the surface shear stress, and the local gradient Richardson number, etc. The data was generated and written by our in-house codes. Its format matches the requirement of the software Tecplot, and hence users can open the data using Tecplot.

Turbulent Heat and Momentum Exchange in Nocturnal Drainage Flow Through a Sloped Vineyard

High-frequency measurements are available at five heights within and above a row-gap trellised vineyard located on a 7$$^{\circ }$$ slope in the Southern Okanagan Valley, British Columbia, Canada. During a 3-week campaign in July 2016, approximately 17% of the nocturnal conditions exhibit drainage flow along the local slope. Drainage conditions are characterized by temperature inversions beginning around $$z/h_c = 0.39$$, where z is the height above ground level (a.g.l.) and $$h_c$$ is the canopy height (2.3 m a.g.l.), and near-surface lapses. Changes in the sign of the streamwise momentum flux suggest the presence of a jet maximum around $$z/h_c = 1.65$$, while a weak inflection point is observed near the canopy top, suggesting dynamical influences from both the drainage layer and canopy layer on the turbulent flow field. The largest observed fluxes in both the streamwise momentum flux and the turbulent sensible heat flux are near the top of the canopy, consistent with the location of the inflection point. Calculated two-point length scales from along-slope distributed temperature measurements reveal that turbulent structures are smallest near the canopy top. Conditional sampling of the three-dimensional velocity components and temperature indicate that a large fraction of canopy-layer transport is driven by canopy top turbulence, with sweeps dominating over ejections, particularly at $$z/h_c = 0.65$$.

Wake structures behind a rotor with superhydrophobic-coated blades at low Reynolds number

We experimentally investigate the flow structures generated by a rotor with the superhydrophobic coating applied on the blade surface in static water. Considered Reynolds number based on the rotating velocity and blade tip to tip distance is 96 000, and spray coating of hydrophobic nanoparticles is used to produce a superhydrophobic surface. We focus on the changes in both instantaneous and time-/ensemble-averaged flows measured with a stereoscopic particle image velocimetry. The vortical structures behind a rotor is characterized by the periodic shedding of hub and tip vortices, whose interactions induce a cone-shaped low-speed region where higher velocities are induced over it. These are closely connected to the spatial distribution of velocity fluctuation. With superhydrophobic surface, the organized formation of vortical structures is disturbed due to the slip on the blade surface, that is, the accumulation of vorticity on the surface is delayed or not strong. Thus the conical region shrinks toward the rotation axis, and the vortex strength is reduced. As a result, about 20% reduction in the turbulent kinetic energy is achieved in the wake, followed by smaller decrease (∼6%) in the streamwise momentum flux. Also, it is found that superhydrophobic surface on the pressure side is more effective, in terms of turbulence reduction. This is the first study to investigate the effect of superhydrophobic surface on the flow around a rotating body, and we think the results will be useful to extend the application of superhydrophobic surface.

An evaluation of the dissimilarity in heat and momentum transport through quadrant analysis for an unstable atmospheric surface layer flow

To elucidate the role of each fluid motion in the transport of momentum and heat fluxes in an unstable atmospheric surface layer (ASL) flow from single point measurements on a micrometeorological tower, we develop a novel method based on quadrant analysis where the contour maps of the turbulent statistics (fluxes, temperature variances and triple order moments between vertical velocity and temperature) are plotted on the quadrant planes between streamwise (u)–vertical (w) velocities, and vertical velocity (w)–temperature (T). We find that the dissimilarities in the heat and momentum transport with atmospheric stability are closely linked to the non-Gaussian nature of the joint probability density function (JPDF) between w and T. To highlight the changes in the fluid motions which cause this dissimilarity, we plot the contour maps of the third order moments between w and T on the u − w quadrant plane, and also of the streamwise momentum flux conditioned on every quadrant of u − w plane onto the T − w plane, referred to as octant analysis. The results indicate that in a highly-convective ASL, the cold downdrafts interspersed with strong ejections of hot fluid, carry a significant amount of both down-gradient and counter-gradient momentum flux, thus making the momentum transport inefficient. However, in a near-neutral ASL, the heat and momentum both are carried by the ejection and sweep quadrants of u − w quadrant plane, which indicates the temperature fluctuations are highly correlated with the high-speed and low-speed streaks commonly found in pure shear flows in the laboratory experiments.

Three-dimensional inspiratory flow in a double bifurcation airway model

The flow in an idealized airway model is investigated for the steady inhalation case. The geometry consists of a symmetric planar double bifurcation that reflects the anatomical proportions of the human bronchial tree, and a wide range of physiologically relevant Reynolds numbers (Re = 100–5000) is considered. Using magnetic resonance velocimetry, we analyze the three-dimensional fields of velocity and vorticity, along with flow descriptors that characterize the longitudinal and lateral dispersion. In agreement with previous studies, the symmetry of the flow partitioning is broken even at the lower Reynolds numbers, and at the second bifurcation, the fluid favors the medial branches over the lateral ones. This trend reaches a plateau around Re = 2000, above which the turbulent inflow results in smoothed mean velocity gradients. This also reduces the streamwise momentum flux, which is a measure of the longitudinal dispersion by the mean flow. The classic Dean-type counter-rotating vortices are observed in the first-generation daughter branches as a result of the local curvature. In the granddaughter branches, however, the secondary flows are determined by the local curvature only for the lower flow regimes (Re ≤ 250), in which case the classic Dean mechanism prevails. At higher flow regimes, the field is instead dominated by streamwise vortices extending from the daughter into the medial granddaughter branches, where they rotate in the opposite direction with respect to Dean vortices. Circulation and secondary flow intensity show a similar trend as the momentum flux, increasing with Reynolds number up to Re = 2000 and then dropping due to turbulent dissipation of vorticity. The streamwise vortices interact both with each other and with the airway walls, and for Re > 500 they can become stronger in the medial granddaughter than in the upstream daughter branches. With respect to realistic airway models, the idealized geometry produces weaker secondary flows, suggesting that realistic anatomical features may generate more lateral dispersion than canonical symmetric models.

A Kinematic Description of the Key Flow Characteristics in an Array of Finite-Height Rotating Cylinders

Experimental data are presented for large arrays of rotating, finite-height cylinders to study the dependence of the three-dimensional (3D) mean flows on the geometric and rotational configurations of the array. Two geometric configurations, each with two rotational configurations, were examined at a nominal Reynolds number of 600 and nominal tip-speed ratios of 0, 2, and 4. It was found that the rotation of the cylinders drives the formation of streamwise and transverse flow patterns between cylinders and that net time–space averaged transverse and vertical flows exist within the developed flow region of the array. This net vertical mean flow provides an additional mechanism for the exchange of momentum between the flow within the array and the flow above it, independent from the turbulent exchange mechanisms which are also observed to increase by almost a factor of three in a rotating array. As an array of rotating cylinders may provide insight into the flow kinematics of an array of vertical axis wind turbines (VAWTs), this planform momentum flux (both mean and turbulent) is of particular interest, as it has the potential to increase the energy resource available to turbines far downstream of the leading edge of the array. In the present study, the streamwise momentum flux into the array could be increased for the rotating-element arrays by up to a factor of 5.7 compared to the stationary-element arrays, while the streamwise flow frontally averaged over the elements could be increased by up to a factor of four in the rotating-element arrays compared to stationary-element arrays.

Flow Management of a Plane Turbulent Wall Jet by a Streamwise Vortex Pair with Periodic Variation in Strength and Radius

Flow management of a plane turbulent wall jet has been studied experimentally by utilizing a streamwise vortex pair with periodic variation in strength and radius. The magnitudes of the streamwise vorticity and secondary current induced by the vortex pair are weakened by the periodic variation. The integrated streamwise momentum flux is increased by the introduction of the streamwise vortex pair which consists of two counter-rotating longitudinal vortices into the shear layer. Despite the weakening of the vorticity and secondary current caused by the periodic variation, the increasing increment rate of the momentum flux is slightly increased due to the periodic variation. In an analysis with triple velocity decomposition and phase averaging, it is evident that the periodic fluctuating velocity component contributes significantly to the production of the Reynolds shear stress.

Flow Management of a Plane Turbulent Wall Jet by a Streamwise Vortex Pair with Periodic Variation in Strength and Radius

Flow management of a plane turbulent wall jet has been studied experimentally by utilizing a streamwise vortex pair with periodic variation in strength and radius. The magnitudes of the streamwise vorticity and secondary current induced by the vortex pair are weakened by the periodic variation. The integrated streamwise momentum flux is increased by the introduction of the streamwise vortex pair which consists of two counter-rotating longitudinal vortices into the shear layer. Despite the weakening of the vorticity and secondary current caused by the periodic variation, the increment rate of the momentum flux is slightly increased due to the periodic variation. In an analysis with triple velocity decomposition and phase averaging, it is evident that the periodic fluctuating velocity component contributes significantly to the production of the Reynolds shear stress.

Large-eddy simulation of a bi-periodic turbulent flow with effusion

Large-eddy simulations of a generic turbulent flow with discrete effusion are reported. The computational domain is periodic in both streamwise and spanwise directions and contains both the injection and the suction sides. The blowing ratio is close to 1.2 while the Reynolds number in the aperture is of order 2600. The numerical results for this fully developed bi-periodic turbulent flow with effusion are compared to available experimental data from a large-scale spatially evolving isothermal configuration. It is shown that many features are shared by the two flow configurations. The main difference is related to the mean streamwise velocity profile, which is more flat for the bi-periodic situation where the cumulative effect of an infinite number of upstream jets is accounted for. The necessity of considering both sides of the plate is also established by analysing the vortical structure of the flow and some differences with the classical jet-in-crossflow case are highlighted. Finally, the numerical results are analysed in terms of wall modelling for full-coverage film cooling. For the operating point considered, it is demonstrated that the streamwise momentum flux is dominated by non-viscous effects, although the area where only the viscous shear stress contributes is very large given the small porosity value (4%).

Applicability of the Planar Fit Technique in Estimating Surface Fluxes over Flat Terrain using Eddy Covariance

The planar fit technique is a coordinate rotation scheme widely used for tower-based eddy covariance measurements. Applicability of this technique was examined with respect to the geometry of the mean stream surface. Analyses based on a 3-year eddy-covariance dataset obtained over flat terrain of a paddy field revealed that the mean streamline was upward at almost all wind directions possibly because of flow distortion by the instruments. Fitting a single plane to the concave stream surface led to miscalculations in the offset and tilt angle of the mean streamline. Consequently, the half-hourly mean vertical wind velocity and streamwise momentum flux demonstrated inconsistent distributions in planar fit coordinates. In contrast, they were consistent under a modified planar fit technique, the sectorial planar fit, in which the mean stream plane was determined at each wind sector with a center angle of 30°. The planar fit technique is not applicable when the mean stream surface is not approximated by a plane. In such cases, the sectorial planar fit can be a practical alternative to the ordinary planar fit.

Momentum Flux in Plane, Parallel Jets

The evolution of the streamwise momentum flux for two turbulent, plane, parallel jets discharging through slots in a direction normal to a wall was studied both numerically and experimentally. The numerical results, obtained by solving the Reynolds-averaged Navier-Stokes equations employing a standard k−ε turbulence model, predicted to within experimental error measured integrals of the momentum flux downstream of the merge point for jet spacing S/d=5. Integration of the streamwise component of the Reynolds-averaged Navier-Stokes equations over a control volume results in an integral constant that was evaluated numerically for jet spacings S/d=3, 5, 7, 9, and 11, and for different levels of turbulence kinetic energy and dissipation rate at the jet inlet boundaries. Results revealed that the integral constant is decreased as the jet spacing increases, and is also decreased as jet entrainment rates are increased due to higher levels of inlet turbulence kinetic energy, or alternatively, decreased levels of dissipation rate. Streamwise distance to the merge point was also found to decrease for increased levels of turbulence kinetic energy or decreased levels of dissipation rate at the jet inlet.

Management of a Stronger Wall Jet by a Streamwise Vortex with Periodic Perturbation. Mean Flow Properties and Evolution of a Streamwise Vortex.

Effect of periodic perturbation was investigated experimentally on the management of a stronger wall jet with a streamwise vortex. A half-delta wing located on the wall generates the streamwise vorticity and the periodic perturbation is added by varying in angle of attack of the wing. Measurements of mean velocity, secondary velocity vectors, and streamwise vorticity were examined to see effect of the periodic perturbation on the evolution process of the wall jet and streamwise vortex. The periodic perturbation accelerates development of vortex radius in spanwise direction over the initial stage and increases relative strength of spanwise secondary velocity vector to the transverse velocity vector. The streamwise momentum flux is increased by interaction between streamwise vortex and wall jet. However, the periodic perturbation reduces increasing rate of the momentum flux.

Structure of a Stronger Wall Jet Managed by a Pair of Streamwise Vortex. Behavior of Streamwise Vortex and Distortion of Mean Velocity Field.

Interaction process between a turbulent plane wall jet and a pair of streamwise vortex was investigated experimentally. Two kinds of vortex pair, i.e., common-flow up and common-flow down generated with half-delta wings mounted on the wall, were introduced into a self-preserving stronger wall jet. Measurements on the mean velocity and mean streamwise vorticity were made to examine deformation and relaxation process of the mean flow field as well as evolution of the streamwise vortex. As a strategy in the present experiment, the vortex center goes into the outer layer in the case of common-flow up and stays in the inner layer in the case of common-flow down. Consequently, streamwise vortex pair interacts strongly with the large-scale eddy governing the entire wall jet. The following remarks were recognized for the common-flow up in comparison with results of both cases ; 1) growth of vortex radius and decay of streamwise vorticity are greater, 2) recovery of the mean velocity field is much slower. The evaluation of streamwise momentum flux confirms that it is possible to manage turbulent wall jet with streamwise vortex.

Management of a Stronger Wall Jet by a Pair of Streamwise Vortices. Reynolds Stress Tensor and Production Tenns.

Reynolds stress tensor for a complex three-dimensional flow field was investigated experimentally in a stronger wall jet managed by two typical kinds of streamwise vortex pair, namely, common -flow up and common-flow down cases. In the turbulent shear flow distorted by secondary flow and strong vorticity, six components of Reynolds stress tensor and rate of strain tensor were obtained with hot-wire measurement. The Reynolds shear stresses -and - are reasonably explained with occurrence of corresponding rate of strain-Sxy ∼-(=U/=y)/2and -Sxz ∼-(=U/=z)/2, respectively. The production terms and its global measure integrated in a cross-stream plane show that most of turbulent kinetic energy is produced by - - =U/=y as well known in many types of two -dimensional flow. In spite of influence by other velocity gradient components, the term - -=U/=y plays a dominant role on production of Reynolds shear stress component - through the whole shear layer. It is seen that mean velocity gradient =U/=y is still the most important distortion in such three-dimensional flow field.Difference in increasing rate of streamwise momentum flux between common -flow up and down cases can be related to difference in modification of the mean velocity gradient =U/=y.

縦渦による壁面噴流の制御

The effect of a streamwise vortex on the development of a wall jet has been investigated experimentally. A streamwise vortex created with a half delta wing mounted on the wall, which has the same height as inner layer thickness, is introduced into a self-preserving wall jet. The mean flow field and its deformation due to secondary flow induced by the vortex are reported in detail. Evolution of the streamwise vortex itself, decay of vorticity, and path and growth of vortex radius are measured and compared with corresponding results obtained in turbulent boundary layers. An increased streamwise momentum flux due to the vortex shows that control with a vortex generator, which is a well known technique for boundary layers, is successful for control of the wall jet.

A model for the onset of breakdown in an axisymmetric compressible vortex

A simple inviscid model to predict the onset of breakdown in an axisymmetric vortex is proposed. Three problems are considered: the shock-induced breakdown of a compressible vortex, the breakdown of a free compressible vortex, and the breakdown of a free incompressible vortex. The same physical reasoning is used in all three problems to predict the onset of breakdown. It is hypothesized that breakdown is the result of the competing effects of adverse pressure rise and streamwise momentum flux at the vortex centerline. Breakdown is assumed to occur if the pressure rise exceeds the axial momentum flux. A formula with no adjustable constants is derived for the critical swirl number in all three problems. The dependence of the critical swirl number on parameters such as upstream Mach number, excess/deficit in centerline axial velocity, and shock oblique angle is explored. The predictions for the onset of shock-induced breakdown and free incompressible breakdown are compared to experiment and computation, and good agreement is observed. Finally, a new breakdown map is proposed. It is suggested that the adverse pressure rise at the vortex axis be plotted against the axial momentum flux to determine the onset of breakdown. The proposed map allows the simultaneous comparison of data from flows ranging from incompressible breakdown to breakdown induced by a shock wave.

Upstream influence on the near field of a plane turbulent jet

The effects of the mean and turbulence characteristics of the upstream (initial) boundary layer on the evolution of the flow in the near field of a plane jet have been experimentally investigated for four initial conditions. Rates of jet widening and centerline mean velocity decay as well as the kinematic and geometric virtual origins show evidence of systematic dependence on initial conditions. The growth rate of longitudinal turbulence intensity, and the mass flux are higher when the initial boundary layer is laminar than when turbulent. Immediately downstream of the exit, the nondimensional entrainment rates for the laminar initial boundary layer cases reach peak values which are about twice the delayed peak values for the fully turbulent initial boundary layer cases. Within the first 40 slit widths, increases in total average streamwise momentum flux range from 20% to 56%, the larger increases occurring for the laminar initial boundary layers; about 10% of each increase is due to the turbulence field. While such increases violate the traditionally accepted momentum flux invariance, they are consistent with the negative mean static pressure data.