Local Streamline Research Articles

Aerodynamic Characteristics inside the Rhino-Sinonasal Cavity after Functional Endoscopic Sinus Surgery

The aim of this study was to evaluate effects of functional endoscopic sinus surgery (FESS) on transient nasal aerodynamic flow patterns using computational fluid dynamics (CFD) simulations. A three-dimensional model of the nasal cavity was constructed from CT scans of a patient with FESS interventions on the right side of the nasal cavity. CFD simulations were then performed for unsteady aerodynamic flow modeling inside the nasal cavity as well as the sinuses. Comparisons of the local velocity magnitude and streamline distributions inside the left and right nasal cavity and maxillary sinus regions were presented. Because of the FESS procedures in the right nasal cavity, existences and distributions of local circulations (vortexes) were found to be significantly different for the same nasal airflow rate but at different acceleration, deceleration, or quiet phases in the maxillary sinus region on the FESS side. Because of inertia effects, local internal airflow with circulation existences was continuous throughout the whole respiration cycle. With a larger peak inspiration flow rate, the airflow intensity inside the enlarged maxillary sinus increased significantly. Possible outcomes on functional performances of the nose were also examined and discussed. Surgical enlargements of natural ostium of the maxillary sinus will change the aerodynamic patterns inside the main nasal cavity and maxillary sinus regions, which may affect normal nasal physiological functions. Local inertia effects play more important roles for the internal nasal airflow pattern changes and thus such conventional FESS procedures should be carefully planned.

Effects of an axisymmetric contraction on a turbulent pipe flow

AbstractThe flow in an axisymmetric contraction fitted to a fully developed pipe flow is experimentally and numerically studied. The reduction in turbulence intensity in the core region of the flow is discussed on the basis of the budgets for the various turbulent stresses as they develop downstream. The contraction generates a corresponding increase in energy in the near-wall region, where the sources for energy production are quite different and of opposite sign compared to the core region, where these effects are caused primarily by vortex stretching. The vortices in the pipe become aligned with the flow as the stretching develops through the contraction. Vortices which originally have a spanwise component in the pipe are stretched into pairs of counter-rotating vortices which become disconnected and aligned with the mean flow. The structures originating in the pipe which are inclined at an angle with respect to the wall are rotated towards the local mean streamlines. In the very near-wall region and the central part of the contraction the flow tends towards two-component turbulence, but these structures are different. The streamwise and azimuthal stresses are dominant in the near-wall region, while the lateral components dominate in the central part of the flow. The two regions are separated by a rather thin region where the flow is almost isotropic.

Lattice Boltzmann simulation of natural convection heat transfer in nanofluids

In this study, lattice Boltzmann method is applied to investigate the natural convection flows utilizing nanofluids in a square cavity. The fluid in the cavity is a water-based nanofluid containing Al 2O 3 or Cu nanoparticles. The study has been carried out for the Rayleigh number 10 3–10 6 and the solid volume fraction 0–0.05. The effective thermal conductivity and viscosity of nanofluid are calculated by Chon and Brinkman models, respectively. The effects of solid volume fraction of nanofluids on hydrodynamic and thermal characteristics are investigated and discussed. The average and local Nusselt numbers, streamlines, temperature contours and vertical component of velocity for different values of solid volume fraction and Rayleigh number are illustrated. The results indicate that by increasing solid volume fraction, the average Nusselt number increases for both nanofluids. It is found that the effects of solid volume fraction for Cu are stronger than Al 2O 3.

Flow streamline based Navier–Stokes Characteristic Boundary Conditions: Modeling for transverse and corner outflows

Some limitations of three dimensional Navier–Stokes Characteristic Boundary Conditions (3D-NSCBC) are discussed for flows traveling in a direction that is oblique to the boundary. To limit errors generated at boundaries with flows having any arbitrary direction, it is proposed to organize the wave decomposition in a coordinate system that is attached to the local flow streamline crossing the boundary, because some modeled expressions are not frame independent. Compared to previous 3D-NSCBC, the modified strategy accounting for oblique waves is found to improve the outflow treatment for transverse outgoing vortices, up to vortices crossing an outflow corner. The method is also applied to an expanding laminar flame.

A field study of mean and turbulent flow characteristics upwind, over and downwind of barchan dunes

ABSTRACTField‐measured patterns of mean velocity and turbulent airflow are reported for isolated barchan dunes. Turbulence was sampled using a high frequency sonic anemometer, deriving near‐surface Reynolds shear and normal stresses. Measurements upwind of and over a crest‐brink separated barchan indicated that shear stress was sustained despite a velocity reduction at the dune toe. The mapped streamline angles and enhanced turbulent intensities suggest the effects of positive streamline curvature are responsible for this maintenance of shear stress. This field evidence supports an existing model for dune morphodynamics based on wind tunnel turbulence measurements.Downwind, the effect of different dune profiles on flow re‐attachment and recovery was apparent. With transverse incident flow, a re‐attachment length between 2·3 and 5·0h (h is dune brink height) existed for a crest‐brink separated dune and 6·5 to 8·6h for a crest‐brink coincident dune. The lee side shear layer produced elevated turbulent stresses immediately downwind of both dunes, and a decrease in turbulence with distance characterized flow recovery. Recovery of mean velocity for the crest‐brink separated dune occurred over a distance 6·5h shorter than the crest‐brink coincident form.As the application of sonic anemometers in aeolian geomorphology is relatively new, there is debate concerning the suitability of processing their data in relation to dune surface and streamline angle. This paper demonstrates the effect on Reynolds stresses of mathematically correcting data to the local streamline over varying dune slope. Where the streamline angle was closely related to the surface (windward slope), time‐averaged shear stress agreed best with previous wind tunnel findings when data were rotated along streamlines. In the close lee, however, the angle of downwardly projected (separated) flow was not aligned with the flat ground surface. Here, shear stress appeared to be underestimated by streamline correction, and corrected shear stress values were less than half of those uncorrected. Copyright © 2011 John Wiley & Sons, Ltd.

Improved Iteration Algorithm for Nonlinear Vortex Lattice Method

VORTEX lattice method (VLM) based on inviscid theory is widely used in practice as it is very effective for aerodynamic design. As conventional VLM is not suitable for aerodynamic analysis of low-aspect-ratio wings or wings at a high angle of attack, nonlinear VLM (NLVLM) was introduced in the 1970s [1–3]. Since then, NLVLMhas been adopted frequently for aerodynamic analysis of low-aspect-ratio wings, delta wings, slender bodies, etc. (as can be found in [4–12]). In NLVLM, vortex segments of a free vortex of a horseshoe vortex system are allowed to move to be aligned with local streamlines, and this inevitably requires an iterative numerical procedure. Convergence property of the iteration algorithm employed in the calculation is therefore a very practical concern [6,8,9]. In this work, we propose a modified iteration algorithm, which has been found to be more efficient and stable than the conventional method often adopted for NLVLM.

Mathematical modelling of complex heat transfer in a rectangular enclosure

Numerical simulation of convective-radiative heat transfer in an enclosure with a heat source in the presence of heat-conducting walls of the finite thickness was carried out. The distributions of both local (streamlines, temperature fields) and integral (mean Nusselt numbers at typical interfaces) characteristics describing specific features of the investigated process in a real range of the variation of determining parameters were obtained. The radiation influence scales at thermal modes formation were determined. The effect of transient factor on the fields development of both hydrodynamic and thermodynamic characteristics was analysed. Correlation ratios for determining the mean Nusselt number at solid-gas interfaces were obtained depending on the Grashof number.

Streamline upwind finite element method using 6-node triangular element with adaptive remeshing technique for convective-diffusion problems

A streamline upwind finite element method using 6-node triangular element is presented. The method is applied to the convection term of the governing transport equation directly along local streamlines. Several convective-diffusion examples are used to evaluate efficiency of the method. Results show that the method is monotonic and does not produce any oscillation. In addition, an adaptive meshing technique is combined with the method to further increase accuracy of the solution, and at the same time, to minimize computational time and computer memory requirement.

Active Flow Control at Low Angles of Attack: Stingray Unmanned Aerial Vehicle

Active flow control using fluidic actuators, via arrays of synthetic jet actuators, was used to provide control power for the Stingray unmanned aerial vehicle in the longitudinal (pitch) and lateral (roll) directions at low angles of attack. Using this technique, the pitch and roll moments were altered such that the effect is similar to that of a deflection of conventional control effectors. The control effectiveness of the synthetic jets on the aerodynamic performance of the Stingray unmanned aerial vehicle was investigated experimentally in a wind tunnel. Global flow measurements were conducted, where the moments and forces on the vehicle were measured using a six-component sting balance. The effect of the actuation was also examined on the surface static pressure at two spanwise locations. In addition, a particle image velocimetry technique was used to quantify the flowfield over the model, both the global flowfield as well as the localized interaction domain near the synthetic jet orifice. The synthetic jets were able to alter the local streamlines and displace the boundary layer through the formation of a small quasi-steady interaction region on the suction surface of the Stingray unmanned aerial vehicle's wing. Phase-locked particle image velocimetry data were acquired to provide insight into the growth, propagation, and decay of the synthetic jets impulse and their interaction with the crossflow. Furthermore, the changes induced on the moments and forces can be proportionally controlled by either changing the momentum coefficient or by driving the synthetic jets with a pulse modulation waveform. This can lead the way for future development of closed-loop control models.

Streamline topology near non-simple degenerate critical points in two-dimensional flow with symmetry about an axis

The local flow patterns and their bifurcations associated with non-simple degenerate critical points appearing away from boundaries are investigated under the symmetric condition about a straight line in two-dimensional incompressible flow. These flow patterns are determined via a bifurcation analysis of polynomial expansions of the streamfunction in the proximity of the degenerate critical points. The normal form transformation is used in order to construct a simple streamfunction family, which classifies all possible local streamline topologies for given order of degeneracy (degeneracies of order three and four are considered). The relation between local and global flow patterns is exemplified by a cavity flow.

Determining the contribution of vertical advection to the net ecosystem exchange at Hyytiälä forest, Finland

In nighttime the importance of advection processes to full carbon balance estimated by micrometeorological methods is pronounced. The vertical advection needs the determination of mean vertical velocity which can be obtained from planar fitting, which is the determination of mean local streamline coordinates based on the statistics of the wind field measured over long periods. We tested the utilization of planar-fitting based vertical advection using long-term eddy covariance and CO2 concentration gradient data at SMEAR II field station (Hyytiålå, Southern Finland). The vertical-advection corrected carbon balance, without any friction velocity filtering, agrees very well with those obtained by filtering and gap-filling procedure, and those by chambers and ecosystem model. Although no direct measurements for horizontal advection is available, the results indicate minor significance of horizontal advection in the studied cases.

Physical and logistical considerations of using ultrasonic anemometers in aeolian sediment transport research

Recently, ultrasonic anemometers (UAs) have become available for precise, high-frequency measurement of three-dimensional velocity and turbulence properties. Except for a few wind tunnel and computational fluid dynamics (CFD) simulations, advances in aeolian sediment transport and bedform research have been limited to field studies using instrumentation that is either incapable of measuring turbulence (e.g., cup anemometers) or unable to withstand sediment-laden airflow (e.g., hotfilms). In contrast, extensive progress has occurred in fluvial research where turbulence instrumentation has been available for some time. This paper provides a pragmatic discussion on using UAs in aeolian research. Recent advances using this technology are reviewed and key physical and logistical considerations for measuring airflow properties and near-surface shear stress using UAs over complex terrain are discussed. Physical considerations include limitations of applying boundary layer theory to flow over natural surfaces such as non-logarithmic velocity profiles resulting from roughness- and topographically induced effects and the inability of instrumentation to measure within the thin constant-stress region. These constraints hinder accurate shear velocity ( u *), shear stress and sand transport estimation. UAs allow measurement of turbulent Reynolds stress (RS) that, in theory, should equal profile-derived shear stress. Discrepancies often exist between these quantities however due to three-dimensional (spanwise) flow components and rapid distortion effects (i.e., unbalanced production and dissipation of turbulence) common in flow over complex terrain. While the RS approach yields information on turbulent contributions to near-surface stress generation, little evidence exists showing that RS is a better measure of forces responsible for sediment transport. Consequently, predictive equations for sediment transport using RS do not exist. There is also a need to identify the role of micro-turbulent events (e.g., burst–sweep cycles) and macro-turbulent structures (e.g., separation cells, shear layers) in aeolian dynamics in field settings to validate recent wind tunnel and CFD simulations. A conundrum exists regarding whether velocity data should be rotated to correct for potential sensor misalignment effects. In unsteady, non-uniform flow over complex terrain, streamline angles vary spatially and temporally with height and location. Thus, determination of, and correction to, true streamline coordinates is difficult. Caution should be exercised with correction methods that remove implicit vertical velocity trends as this may preclude detection of geomorphically important flow behaviour (e.g., lift at a dune crest) and may complicate interpretations of RS. Instruments should be aligned with the underlying surface and flow visualization should be used to adjust sensor measurement planes as close as possible to local streamlines. Logistical considerations include sensor design advantages and operational limitations, data communication formats and field deployment strategies—each can affect measurement accuracy and are easily overlooked. Sampling range, frequency and period are also important as they limit the range of velocities and scales of turbulence that can be characterized. Ultrasonic anemometers offer a new sampling resolution to measure turbulent airflow properties in field settings. With proper considerations of their limitations, UAs may allow researchers to close the gap between fluvial research and develop more robust models of aeolian processes and morphodynamics.

Comparing different methods of bed shear stress estimates in simple and complex flow fields

AbstractBed shear stress is a fundamental variable in river studies to link flow conditions to sediment transport. It is, however, difficult to estimate this variable accurately, particularly in complex flow fields. This study compares shear stress estimated from the log profile, drag, Reynolds and turbulent kinetic energy (TKE) approaches in a laboratory flume in a simple boundary layer, over plexiglas and over sand, and in a complex flow field around deflectors. Results show that in a simple boundary layer, the log profile estimate is always the highest. Over plexiglas, the TKE estimate was the second largest with a value 30 per cent less than the log estimate. However, over sand, the TKE estimate did not show the expected increase in shear stress. In a simple boundary layer, the Reynolds shear stress seems the most appropriate method, particularly the extrapolated value at the bed obtained from a turbulent profile. In a complex flow field around deflectors, the TKE method provided the best estimate of shear stress as it is not affected by local streamline variations and it takes into account the increased streamwise turbulent fluctuations close to the deflectors. It is suggested that when single‐point measurements are used to estimate shear stress, the instrument should be positioned close to 0·1 of the flow depth, which corresponds to the peak value height in profiles of Reynolds and TKE shear stress. Copyright © 2004 John Wiley & Sons, Ltd.

Modeling Electrophotographic Developer Flow with a Viscous Fluid Flow Model

In laser printers, the developing station forms toner images on the photoconductor drum surface. The primary concern for formation of stable toner images is obtaining smooth developer flow. Accordingly, observation of the developer flow at a free surface in an actual developing station has been carried out. For better flow optimization, developer flow details such as local flow velocity vectors and streamlines, not only at the free surface but also in the developer flow, should be clarified. A simulation method for optimizing the flow was examined. In the simulation method, a large calculation domain in the developing station is required. Therefore, viscous fluid analysis is employed to minimize the calculation load. Viscosity should be defined in terms of the developer particle flow. To do this, viscosity measurements were carried out. It was found that the developer flow should be non-Newtonian, whereby viscosity is proportional to the reciprocal of its shear rate. This means that shear stress in the developer flow should optimally be constant. This property was used to simulate the developer particle flow behavior successfully, even in three-dimensional models. Flow details in the developing station, on both large and small scales, could be observed.

Numerical simulation of orientation distribution function of cylindrical particle suspensions

The orientation distribution function of cylindrical particle suspensions was deduced and numerically simulated, and an application was taken in a wedge-shaped flow field. The relationship between the orientation distribution function and particle orientation angles was obtained. The results show that comparing with the most probable angle distribution which comes to being in short time, the distribution of the steady state doesn't vary much in range; the main difference is the anti-clockwise rotation in the right and upper field, that is, particles rotate more at the points where the velocity gradients are larger. The most probable orientations are close to the direction of local streamlines. In the direction of streamlines, with poleradius decreasing, the most probable angles increase, but the angles between their orientations and the local streamlines decrease.

SYNTHETIC JETS

▪ Abstract The evolution of a synthetic (zero-net mass flux) jet and the flow mechanisms of its interaction with a cross flow are reviewed. An isolated synthetic jet is produced by the interactions of a train of vortices that are typically formed by alternating momentary ejection and suction of fluid across an orifice such that the net mass flux is zero. A unique feature of these jets is that they are formed entirely from the working fluid of the flow system in which they are deployed and, thus, can transfer linear momentum to the flow system without net mass injection across the flow boundary. Synthetic jets can be produced over a broad range of length and timescale, and their unique attributes make them attractive fluidic actuators for a number of flow control applications. The interaction of synthetic jets with an external cross flow over the surface in which they are mounted can displace the local streamlines and induce an apparent or virtual change in the shape of the surface, thereby effecting flow changes on length scales that are one to two orders of magnitude larger than the characteristic scale of the jets. This control approach emphasizes an actuation frequency that is high enough so that the interaction domain between the actuator and the cross flow is virtually invariant on the global timescale of the flow, and therefore, global effects such as changes in aerodynamic forces are effectively decoupled from the operating frequency of the actuators.

Effects of transverse helium injection on hypersonic boundary layers

Results are presented on the control of a zero-pressure gradient Mach 8 boundary layer using low-momentum helium injection through a transverse slot. The boundary layer structure was visualized in three orthogonal planes using planar filtered Rayleigh scattering. Sequential images of the boundary layer were obtained using a new MHz imaging system and employed to construct three-dimensional representations of the instantaneous boundary layer behavior. The results indicate that low levels of helium injection into transitional and turbulent boundary layers can have a substantial effect on the boundary layer structure. Longitudinal structures develop downstream of the slot and persist for long times. They appear to be streamwise vortices, and it is speculated that they are formed near the injection slot due to local streamline concave curvature. The results may be important for aero-optic applications.

Long-Term Carbon Dioxide Fluxes from a Very Tall Tower in a Northern Forest: Flux Measurement Methodology

Methodology for determining fluxes of CO2 and H2O vapor with the eddy-covariance method using data from instruments on a 447-m tower in the forest of northern Wisconsin is addressed. The primary goal of this study is the validation of the methods used to determine the net ecosystem exchange of CO2. Two-day least squares fits coupled with 30-day running averages limit calibration error of infrared gas analyzers for CO2 and H2O signals to ≈2%–3%. Sonic anemometers are aligned with local streamlines by fitting a sine function to tilt and wind direction averages, and fitting a third-order polynomial to the residual. Lag times are determined by selecting the peak in lagged covariance with an error of ≈1.5%–2% for CO2 and ≈1% for H2O vapor. Theory and a spectral fit method allow determination of the underestimation in CO2 flux (<5% daytime, <12% nighttime) and H2O vapor flux (<21%), which is due to spectral degradation induced by long air-sampling tubes. Scale analysis finds 0.5-h flux averaging periods are sufficient to measure all flux scales at 30-m height, but 1 h is necessary at higher levels, and random errors in the flux measurements due to limited sampling of atmospheric turbulence are fairly large (≈15%–20% for CO2 and ≈20%–40% for H2O vapor at lower levels for a 1-h period).

Modeling Electrophotographic Developer Flow with a Viscous Fluid Flow Model

In laser printers, the developing station forms toner images on the photoconductor drum surface. The primary concern for formation of stable toner images is obtaining smooth developer flow. Then, observation of the developer flow at a free surface in an actual developing station has been done. For better flow optimization, developer flow details such as local flow velocity vectors and streamlines, not only at the free surface but also in the developer flow, should be clarified. A simulation method for optimizing the flow was examined.In the simulation method, a large calculation domain in the developing station is required. Therefore, viscous fluid analysis to minimize the calculation load is employed. Viscosity should be defined as fitting the developer particle flow. To do this, viscosity measurements were carried out. It was found that the developer flow should be non-Newtonian fluid, in which viscosity is proportional to the reciprocal of its shear rate. This means that shear stress in the developer flow should be constant.This property was used to simulate the developer particle flow behavior successfully, even in three-dimensional models. Flow details, on both large and small scales in the developing station, could be observed.

Streamwise Curvature Effect on the Incompressible Turbulent Mean Velocity Over Curved Surfaces

A curvature law of the wall, which determines the mean velocity profile, is analytically derived for near-wall turbulent flows to include strong curved-channel wall curvature effects through a perturbation analysis. The new law allows improved analysis of such flows, and it provides the basis for improved wall function boundary conditions for their computation (CFD), even for strong curvature cases. The improved law is based on the algebraic eddy viscosity and curvature-corrected mixing length concepts, the latter of which is a linear function of the gradient Richardson number. To include the complete Richardson number effects, the local streamline curvature effects in the gradient Richardson number are kept. To overcome the mathematical difficulty of keeping all of these local streamline curvature terms, an innovative nonconstant-parameter perturbation solution technique is successfully applied. [S0098-2202(00)00903-2]