Peak Tangential Velocity Research Articles

Eulerian–Lagrangian fluid dynamics study on the effects of inlet radial angle on the performance and flow pattern of multi-inlet cyclone separator

This paper presents a study on the influence of inlet radial angle in a multi-inlet cyclone separator using Eulerian-Lagrangian (EL) fluid dynamics approach. The investigation focuses on the behavior of air and solid particles within the cyclone. Air circulation was simulated at velocities ranging from 2 to 6 m/s, with inlet radial angles of 15°, 32°, 45°, 61°, and 72°. To model the turbulent flow, the Reynolds stress model (RSM) and renormalization group k-model (RNG) were employed, while the particle trajectories were simulated using the discrete phase model (DPM) in ANSYS Fluent. The purpose of this study was to determine how the inlet radial angle impacts various parameters: collection efficiency, pressure drop, cut-off diameter, axial velocity, and tangential velocity. The numerical data obtained from the simulations were validated and found to be consistent with the existing literature. The results revealed interesting findings. In the case of an inlet velocity of 6 m/s, the highest pressure drop of 84.5 Pa was observed for the 15° inlet radial angle, while the lowest pressure drop of 77.42 Pa was observed for the 32° inlet radial angle. It was found that the collection efficiency varied according to the radial angle of the inlet, with values of 89.00%, 91.70%, 92.50%, 94.77%, and 94.83% for 32°, 45°, 15°, 72°, and 61°, respectively. Additionally, at an inlet velocity of 2 m/s and a radial angle of 72°, 61°, 15°, 45°, and 32°, the corresponding cut-off sizes were determined to be 1.17 μm, 1.19 μm, 1.25 μm, 1.33 μm, and 1.38 μm, respectively. At an inlet velocity of 6 m/s, the peak tangential velocities were found to be approximately 1.48, 1.44, 1.44, 1.39, and 1.34 times the inlet velocity for the 15°, 72°, 61°, 45°, and 32° radial angles, respectively, within the axial location range of z = 1.5D to z = 3.4D. The peak axial velocity values for the 32°, 72°, 45°, 15°, and 61° radial angles, were reported as 0.85, 0.89, 0.92, 0.94, and 0.96 times the inlet velocity respectively.

Effect of sieve plate packing on bubble size distribution in a cyclonic flotation column

The application of packed technology effectively reduces the height of flotation column and improves the treatment capacity. The effect of packed sieve plate on bubble size and velocity in flotation column affect the flotation recovery. Based on high-speed dynamic measurement and particle image velocimetry (PIV) measurement, the velocity and size of bubbles in open and packed flotation column were compared first and then the effect of different packing method was studied. With sieve plate packing, the peak tangential velocity of bubble decreased by 92%; the number of bubbles within the range of 1–2.5 mm increased from 62.79% to 83.34% and the mean bubble diameter decrease from 2.14 mm to 1.82 mm. Sieve plate packed weakens the influence of rotation flow, reduces the bubble size and makes the distribution is more uniform in flotation column, which is conducive to the collision and adhesion of particles and bubbles. The effect study of opening hole size, thickness and packed position of sieve plate on bubble size distribution shows that the small opening hole size of sieve plate has a strong segmentation effect on bubbles, and can reduce the bubble size; Thick sieve plate will increase bubble coalescence and increase bubble size; The bubbles have a good distribution with sieve plate packed at a height of 0.5 times the diameter of the column. This study can provide guidance for the design of packed method in flotation column.

Numerical investigation of shear flow structure induced by guided vane in a liquid-liquid cyclone reactor

This work described the shear flow structure induced by a guided vane in the turbulent mixing layer of a liquid-liquid cyclone reactor. And the results show that the tangential velocity is much greater than radial velocity at the same axial section, which means the tangential velocity gives greater impact on the breakup of dispersed phase jet and droplet deformation. In addition, the variations of tangential velocity along radial, circumferential and axial directions were investigated. It was concluded that the tangential velocity increased firstly and then decreased with the radial position approaching the axis. The position of the peak tangential velocity along radial direction in the same axial position barely changed with the change of radian. But the peak tangential velocity value changed along the circumferential and axial directions, which attenuated in the form of power function along axial direction. Moreover, the tangential velocity showed a fluctuation along the circumferential direction. And the fluctuation of tangential velocity at the radial position of peak tangential velocity is more obvious. Besides, the amplitude of fluctuation gradually decreased with the axial section getting closer to the separation chamber. And a stable flow area was found at the axial section of z = 70~0 mm.

Investigation of The Decay of a Single vortex with PIV Technique

The formation and development of a single vortex is investigated in a wind tunnel. The wake from a lifting rectangular wing based on CLARK-Y profile of 11.7% is studied up to 80 chords downstream. Digital particle image velocimetry is used to measure the velocity field in the cross stream plane from which is the vortex is measured. Two different experimental test were carried out in two different conditions of the tunnel, without and with grid. From the first condition, the vortex is observed to be quite stable and long-lived. A correlation for the peak tangential velocity u θ , max and the vortex core r c has been compared with the Lamb-Oseen model, and it gives a good agreement. The circulation Γ is observed to be proportional to the free-stream U ∞ (range 8 − 20 m/s) and the angle of attack α (range 1−12 deg). The vorticity ω, however, diffuses over time within the growth of the vortex core r c . This growth can be approached by the Squire model. Apparently, the vortex characteristics become weaker when the experiments are performed under grid turbulence condition. Due to the influence of the grid turbulence, some experiments give inconsistent result. This makes difficult to draw conclusions. However, still some interesting information is found in this case.

Coupling Mechanism of Rotating Casing Effect and Impeller Structure of Roto-Jet Pump

An increase in internal pressure of a Roto-Jet pump is due to combined action of its impeller and rotating casing. Internal pressure of the pump chamber was determined according to outlet pressure of the impeller, and the influence of the rotary casing effect was ignored. To study the combined action mechanism of the rotating casing effect and impeller structure on the Roto-Jet pump, we used the open test bed of the Roto-Jet pump and four model pumps with impellers of different structures as research objects. We also conducted a comprehensive experimental study on the coupling mechanism between the rotating casing effect and impeller structure. Numerical calculation was performed to avoid the assumption of isotropic eddy viscosity, Reynolds stress linear pressure-strain model is selected, and the numerical calculation results are compared with the experimental results to verify its credibility. The results show that the rotating casing effect has multiple functions to reduce the friction loss of the disc, improve the pressure distribution inside the rotating casing, and increase the pump head. All scheme, pressure, and velocity fluctuations occur in the upstream and wake regions of the collecting pipe and the energy loss is concentrated in the upstream region. The difference in velocity distribution inside the collecting pipe is small and negligible. As long as the impeller and rotating casing continue to rotate synchronously, the liquid shear velocity at the same coordinate position of each scheme remains unchanged, and the liquid rotation angular velocity in the rotating casing is approximately 75% of the rotating casing which conforms to the rigid motion law. In the same scheme, the coefficient of uneven velocity inside the rotating casing gradually increases along the radial direction. The closer to the axis, the faster is the decrease of the peak tangential velocity and the velocity tends to be uniform. The size, shape, and position of the vortex core inside the rotating casing change constantly with various schemes. The distribution of vortex cores varies under each scheme. The front cover and rotating casing have a serious effect on the vortex core. The extremely poor analysis of the test results shows that the performance of the Roto-Jet pump is better when the closed impeller is rotated in synchronisation with the rotating casing. The advantages and disadvantages of each blade type can be determined according to the situation. The research results can exhibit the influence mechanism of the Roto-Jet pump shell effect. Selection of an impeller structure provides a reference.

Near Field of a Vortex Generated by Chevron-Tipped Flat Plates

Flat plates with chevron tips of variable depths () were designed and their effect on the tip vortex was studied at downstream distances of to at a Reynolds number of 67,000. The test plates were mounted at an angle of attack of 5 deg and the tip vortex was measured using a constant temperature anemometer. The chevron plates formed tip vortices with lower peak tangential velocities and larger core radii as compared to the flat plate (with the exception of the , which had a smaller core radius than the flat plate). It was also found that the tip vortices formed over plates with deeper chevrons exhibited turbulent cores, as opposed to the flat plate and shallow chevron plates. Vortex wandering, an inherent meandering of the tip vortex, was found to be present for every test plate. It results in an overprediction of the core radius and underprediction of the peak tangential velocities. To ensure that the lower peak tangential velocities and larger core radii of the tip vortices formed over chevron tip plates were due to the change in geometry and not a consequence of vortex wandering, Devenport’s correction was applied to the shallow chevron plates; and it was found that after correcting for wandering, the tip vortices formed over chevron tips still had lower peak tangential velocities and larger core radii (with the exception of the plate) as compared to the flat plate.

Experimental Research on Wingtip Vortices using a Half Deltawing at the Tips

The counter rotating wing tip vortices produced by the aircraft continues to be a big concern for the aviation industry and the aircraft manufacturers due to its hazardous effects on the flight safety and aircraft efficiency. The strength of the vortices poses severe problems to the aircraft operations. Manufacturers developed various wingtip devices to alleviate this problem, but still it is not fully understood and solved. In this thesis, the effectiveness of using a half delta wing at the tips is investigated. The flow field over a low aspect ratio NACA 0015 wing fitted with a slender sharp half delta wing with a leading edge sweep angle 700 at a Reynolds number 1.87 ×105 is investigated. Particle image velocimetry is used to quantify the vortex structure and force balance measurements are used to calculate the aerodynamic data of the wing. The peak vorticity, peak tangential velocity are decreased due to the addition of half delta wing. The over-all radius of the wingtip vortex increased showing a diffused vortex due to the addition of the half delta wing. The core circulation is decreased leading to a lower strength vortex. Though the tip device increased the drag, it increases the aerodynamic efficiency through the improvement in L/D

Experiment on Wingtip Vortices using a Half Deltawing at the Tips

The counter rotating wing tip vortices produced by the aircraft continues to be a big concern for the aviation industry and the aircraft manufacturers due to its hazardous effects on the flight safety and aircraft efficiency. The strength of the vortices poses severe problems to the aircraft operations. Manufacturers developed various wingtip devices to alleviate this problem, but still it is not fully understood and solved. In this thesis, the effectiveness of using a half delta wing at the tips is investigated. The flow field over a low aspect ratio NACA 0015 wing fitted with a slender sharp half delta wing with a leading edge sweep angle 700 at a Reynolds number 1.87 ×105 is investigated. Particle image velocimetry is used to quantify the vortex structure and force balance measurements are used to calculate the aerodynamic data of the wing. The peak vorticity, peak tangential velocity are decreased due to the addition of half delta wing. The over-all radius of the wingtip vortex increased showing a diffused vortex due to the addition of the half delta wing. The core circulation is decreased leading to a lower strength vortex. Though the tip device increased the drag, it increases the aerodynamic efficiency through the improvement in L/D.

Right and left ventricular interaction in pulmonary hypertension: Insight from velocity vector imaging.

To evaluate whether global peak systolic strain (PSS) and peak systolic strain rate (PSSR) derived from velocity vector imaging (VVI) allow early recognition of regional and global right ventricular (RV) dysfunction and the impact of this on left ventricular (LV) function in patients with pulmonary hypertension (PHT). RV function is an important determinant of prognosis in patients with heart failure, pulmonary hypertension, heart transplant, and congenital heart diseases. However, evaluation of the right ventricle is often limited by its complex geometry and inadequate visualization of RV free wall. Furthermore, the impact of RV dysfunction on the LV function is not well elucidated. Ninety-nine participants, 35 control patients with normal RV systolic pressure (RVSP) (<30mm Hg) and 64 patients with PHT (25 with mild-to-moderate increase in RVSP [≥36 and <60mm Hg] and 39 with severe increase in RVSP [≥60mm Hg]), underwent comprehensive echo-Doppler assessment and velocity vector imaging (VVI) for strain rate analysis. RV regional peak systolic and diastolic tangential velocity, strain, and strain rate were obtained from the basal, mid and apical segments of the RV free wall and interventricular septum (IVS) from apical 4-chamber view at end-expiration. Similar data were obtained from eighteen LV segments from apical 4-chamber, 2-chamber, and long-axis views. Peak systolic strain, strain rate, and tangential velocity at all segments in the RV free wall and IVS were decreased compared to controls in patients with PHT (P<0.001). Significant correlation (r>0.60; P<0.001) was noted between RVSP and systolic and diastolic strain and strain rate at basal segment in IVS and global RV function. Peak early diastolic strain rate at all segments was also decreased in PHT patients compared with control patients (P<0.01). Furthermore, RV systolic and diastolic strain and strain rate were lower in group 2 with mild-to-moderate hypertension while the conventional echo parameters were normal. Except for IVS segments, other LV segments had no statistical differences in systolic and diastolic velocity, strain, and strain rate compared to controls. However, they were lower than the published normal range. Strain and strain rate derived from VVI demonstrates early recognition of systolic and diastolic RV dysfunction in patients with PHT compared to controls. PHT is associated with global and regional RV systolic and diastolic dysfunction. Systolic and diastolic strain and strain rate from LV was lower compared to controls but were not statistically significant. This may indicate subclinical LV dysfunction in these patients, suggesting that conventional LV function parameters may not be sensitive to recognize subclinical LV dysfunction.

A semi-empirical model for streamwise vortex intensification

Vortex intensification plays an important role in a wide range of flows of engineering interest. One scenario of interest is when a streamwise vortex passes through the contracting streamtube of an aircraft intake. There is, however, limited experimental data of flows of this type to reveal the dominant flow physics and to guide the development of vortex models. To this end, the evolution of wing-tip vortices inside a range of streamtube contractions has been measured using stereoscopic particle image velocimetry. A semi-empirical model has been applied to provide new insight on the role of vorticity diffusion during the intensification process. The analysis demonstrates that for mild flow contractions, vorticity diffusion has a negligible influence due to the low rates of diffusion in the vortex flow prior to intensification and the short convective times associated with the streamtube contraction. As the contraction levels increase, there is a substantial increase in the rates of diffusion which is driven by the greater levels of vorticity in the vortex core. A new semi-empirical relationship, as a function of the local streamtube contraction levels and vortex Reynolds number, has been developed. The model comprises a simple correction to vortex filament theory and provides a significant improvement in the estimation of vortex characteristics in contracting flows. For the range of contractions investigated, errors in the estimation of vortex core radius, peak tangential velocity and vorticity are reduced by an order of magnitude. The model can be applied to estimate the change in vortex characteristics for a range of flows with intense axial strain, such as contracting intake streamtubes and swirling flows in turbomachinery.

Comparison of different approaches tracking a wing-tip vortex

This paper compares the performance of different grid based and grid free modelling approaches to predict the tip vortex evolution in both near and far wing wake fields. The grid based methods cover different turbulence modelling approaches, adaptive mesh refinement and the adaptive vorticity confinement (VC) method using the OpenFOAM code. Computational vortex method (CVM) coupled with the OpenFOAM simulation of the near field is utilised to properly predict the tip vortex behaviour in the far field. All simulation results are compared to results of the wind tunnel experiments conducted by Devenport et al. (1996). The comparison is based on the analysis of the vortex core parameters: the core size, the peak tangential velocity and the axial velocity deficit. Additionally, the results are compared with another numerical study by Wells (2009, 2010). It turns out that turbulence modelling plays an important role since simple one and two-equation models overpredict the turbulence intensity in the vortex core resulting in its fast decay. The potential of the adaptive VC method depends on the underlying turbulence model. Grid free vortex method shows a good potential to improve the simulation accuracy.

Multi-objective optimization of a Stairmand cyclone separator using response surface methodology and computational fluid dynamics

The performance of a cyclone is generally assessed using the pressure drop and the collection efficiency of the cyclone. In the present paper, a multi-objective optimization of a classic Stairmand cyclone separator is executed using the response surface methodology (RSM), combined with computational fluid dynamics (CFD) techniques for minimizing the pressure drop and maximizing the collection efficiency. Ten cyclone geometrical factors are considered in this work. Three of them are studied using the RSM according to the results of the preceding screening experiments. The second-order response surface models for each response are successfully carried out using the central composite design (CCD) in the RSM. The desirability function approach is used to optimize the geometrical factors of the cyclone. In comparison to the reference model, the optimal cyclone model decreases the pressure drop by 20.70% and the cut-off size by 75.38%. The accuracies of the response surface models are confirmed and the correctness of the optimal cyclone model is also validated using CFD; the results indicate excellent performance and reliability of the response surface model and the RSM optimization result. According to the analyses of the obtained flow fields, there are several reasons why decreases in the cut-off size with a lower pressure drop are found in the optimal model. The primary reasons for improvements are optimized values of relevant geometrical factors, the production of a distinct, undisturbed downward flow, an increase in the tangential velocity at the near wall region, and a decrease in the peak tangential velocity.

Earlier and greater hand pre-shaping in the elderly: a study based on kinematic analysis of reaching movements to grasp objects.

Elderly people are less able to manipulate objects skilfully than young adults. Although previous studies have examined age-related deterioration of hand movements with a focus on the phase after grasping objects, the changes in the reaching phase have not been studied thus far. We aimed to examine whether changes in hand shape patterns during the reaching phase of grasping movements differ between young adults and the elderly. Ten healthy elderly adults and 10 healthy young adults were examined using the Simple Test for Evaluating Hand Functions and kinetic analysis of hand pre-shaping reach-to-grasp tasks. The results were then compared between the two groups. For kinetic analysis, we measured the time of peak tangential velocity of the wrist and the inter-fingertip distance (the distance between the tips of the thumb and index finger) at different time points. The results showed that the elderly group's performance on the Simple Test for Evaluating Hand Functions was significantly lower than that of the young adult group, irrespective of whether the dominant or non-dominant hand was used, indicating deterioration of hand movement in the elderly. The peak tangential velocity of the wrist in either hand appeared significantly earlier in the elderly group than in the young adult group. The elderly group also showed larger inter-fingertip distances with arch-like fingertip trajectories compared to the young adult group for all object sizes. To perform accurate prehension, elderly people have an earlier peak tangential velocity point than young adults. This allows for a longer adjustment time for reaching and grasping movements and for reducing errors in object prehension by opening the hand and fingers wider. Elderly individuals gradually modify their strategy based on previous successes and failures during daily living to compensate for their decline in dexterity and operational capabilities.

An experimental investigation of wing-tip vortex decay in turbulence

Particle image velocimetry measurements were conducted for a wing-tip vortex decaying in free-stream turbulence. The vortex exhibited stochastic collapse with free-stream turbulence present, with the breakdown initiating earlier for higher levels of turbulence. An increased rate of decay of the vortex tangential velocity was also observed, increasing with increasing free-stream turbulence. The decay of the vortex tangential velocity without the free-stream turbulence was well represented by viscous diffusion, resulting in an increase in the core radius and decrease in the peak tangential velocity. With the addition of free-stream turbulence, the rate of decay of the peak tangential velocity of the vortex increases, whereas the rate of increase of core radius remains unchanged. The circulation of the vortex decayed in time when immersed in free-stream turbulence, whereas it remained approximately constant when free-stream turbulence was not present. This decay in circulation was found to be almost entirely due to a decrease in circulation of the vortex core, caused by the relative decrease in the peak tangential velocity without a corresponding increase in the core radius. The scaling of the radial profiles of velocity and circulation was also examined, and it was found that, regardless of the free-stream condition, the core was scaled by the peak tangential velocity and core radius. The region outside the core did not scale with these quantities, and an alternative scaling for circulation is proposed which results in improved collapse of the profiles.

Near wake development of a wing tip vortex under the effect of synthetic jet actuation

An experimental investigation on the effectiveness of synthetic jet actuation in controlling a wingtip vortex of a rectangular, square-tipped NACA 0012 wing at a chord Reynolds number Rec=8×104 is reported. Spanwise pressure measurements on the suction surface of the wing showed that forcing the wingtip vortex at a frequency in the range of frequencies of the long and short-wave instabilities resulted in decreased pressure coefficients as compared to natural vortex measurements. Two control configurations were then considered for hotwire wake measurements, namely, a “least effective” configuration with momentum coefficient Cμ=0.004 and actuation frequency F+=0.71, and a “most effective” configuration with Cμ=0.04 and F+=0.213. Measurements at x/c=1 showed that, under the latter case, the vortex was stretched into an ellipsoid shape with a 50% average decrease in the peak tangential velocity, and 30% broadening of the effective vortex core radius. Further downstream, the vortex seemed to regain its universal shape but with reduced strength. The results suggested that the lower frequency control configuration allowed the synthetic jet to travel larger distances into the vortex bringing turbulent structures within its core resulting in increased mixing and subsequently decreased vortex strength.

25 A ZEBRAFISH MODEL OF MYELODYSPLASTIC SYNDROME PRODUCED THROUGH TET2 GENOMIC EDITING

Misovortices frequently occur near the coastline of the Japan Sea during wintertime cold air outbreaks, generally developing over the sea and moving inland. To clarify the behavior of misovortices during landfall, temporal changes in the intensity and tilt of 12 misovortices over the coastal region of the Japan Sea were investigated during the winters of 2010/11 and 2011/12 using an X-band Doppler radar. For 11 vortices whose diameters were more than twice the effective radar beamwidth, the temporal change in the peak tangential velocity at lower levels (averaged below 400 m AGL) was analyzed. It was found that 8 out of the 11 vortices decreased after progressing between 0 and 6 km inland. For the remaining three vortices, the patterns of Doppler velocity couplet became unclear between 0 and 5 km inland, suggesting that these vortices also decayed soon after landfall. For four of the vortices, for which the analysis of the temporal evolution of tilt with height was made possible by several successive volume scans, the forward tilt with height increased after landfall. This study showed that modification to both the intensity and tilt with height of misovortices occurred after landfall.

Modification of misovortices during landfall in the Japan Sea coastal region

Misovortices frequently occur near the coastline of the Japan Sea during wintertime cold air outbreaks, generally developing over the sea and moving inland. To clarify the behavior of misovortices during landfall, temporal changes in the intensity and tilt of 12 misovortices over the coastal region of the Japan Sea were investigated during the winters of 2010/11 and 2011/12 using an X-band Doppler radar. For 11 vortices whose diameters were more than twice the effective radar beamwidth, the temporal change in the peak tangential velocity at lower levels (averaged below 400m AGL) was analyzed. It was found that 8 out of the 11 vortices decreased after progressing between 0 and 6km inland. For the remaining three vortices, the patterns of Doppler velocity couplet became unclear between 0 and 5km inland, suggesting that these vortices also decayed soon after landfall. For four of the vortices, for which the analysis of the temporal evolution of tilt with height was made possible by several successive volume scans, the forward tilt with height increased after landfall. This study showed that modification to both the intensity and tilt with height of misovortices occurred after landfall.

Universality and scaling phenomenology of small-scale turbulence in wall-bounded flows

The Reynolds number scaling of flow topology in the eigenframe of the strain-rate tensor is investigated for wall-bounded flows, which is motivated by earlier works showing that such topologies appear to be qualitatively universal across turbulent flows. The databases used in the current study are from direct numerical simulations (DNS) of fully developed turbulent channel flow (TCF) up to friction Reynolds number Reτ ≈ 1500, and a spatially developing, zero-pressure-gradient turbulent boundary layer (TBL) up to Reθ ≈ 4300 (Reτ ≈ 1400). It is found that for TCF and TBL at different Reynolds numbers, the averaged flow patterns in the local strain-rate eigenframe appear the same consisting of a pair of co-rotating vortices embedded in a finite-size shear layer. It is found that the core of the shear layer associated with the intense vorticity region scales on the Kolmogorov length scale, while the overall height of the shear layer and the distance between the vortices scale well with the Taylor micro scale. Moreover, the Taylor micro scale collapses the height of the shear layer in the direction of the vorticity stretching. The outer region of the averaged flow patterns approximately scales with the macro scale, which indicates that the flow patterns outside of the shear layer mainly are determined by large scales. The strength of the shear layer in terms of the peak tangential velocity appears to scale with a mixture of the Kolmogorov velocity and root-mean-square of the streamwise velocity scaling. A quantitative universality in the reported shear layers is observed across both wall-bounded flows for locations above the buffer region.

The interaction of a vortex ring with a sloped sediment layer: Critical criteria for incipient grain motion

Experiments were performed to analyse the interaction between a vortex ring and a sloped sediment layer. Attention focussed on interactions under “critical” conditions, in which sediment motion was only just induced by the ring's flow field. Both hydraulically smooth and hydraulically rough bedforms were analysed, using near-spherical monodisperse sediments with relative densities of 1.2 and 2.5 and mean diameters (dp) ranging between 80 and 1087 μm. Measurements of the vortex-ring flow field were obtained, during the interaction, using two-dimensional particle imaging velocimetry. The threshold conditions for incipient sediment motion were analysed in terms of the critical Shields parameter (Nc), defined in terms of the peak tangential velocity measured adjacent to the bed surface. Bed-slope effects were investigated by tilting the sediment layer at various angles between the horizontal and the repose limit for the sediment. In all cases, the propagation axis of the vortex ring was aligned normal to the bed surface. The measured values of Nc were compared with a force-balance model based on the conditions for incipient grain motion on a sloping bed. For hydraulically smooth bedforms, where the bed roughness is small compared to the boundary-layer depth, the model was derived to account for how viscous stresses affect the drag and lift forces acting on the near surface sediment. For hydraulically rough bedforms, where this viscous-damping effect is not present, the model assumes the drag and lift forces scale with the square of the near-bed (inviscid) velocity scale. In both cases, the model predicts that bedforms become more mobile as the bed slope is increased. However, the damping effect of the viscous sublayer acts as a stabilizing influence for hydraulically smooth bedforms, to reduce the rate at which the bed mobility increases with bed slope. The measured values of Nc were in agreement with the trends predicted by this model, and exhibit a transition in behaviour between the smooth-bed and rough-bed cases when dp/δs ≈ 20 (where δs is the viscous-sublayer length scale).

Study of the Reverse Delta Wing

Particle image velocimetry was used in a low-speed wind tunnel to investigate the vortex structures of a slender reverse delta wing at various angles of attack and roll. This work investigates the characteristics of the vortices generated downstream in planes perpendicular to the freestream direction and their dependence on angles of attack and roll at a chord-based Reynolds number of Rec= 382000 The peak tangential velocities at�angle of attack higher than 5 deg show a trend similar to a delta wing. A six-component balance was used to obtain the aerodynamic coefficients for a reverse delta wing as well as a delta wing for comparison. A simulation of the streamlines, velocity vectors, and surface pressure contours was carried out using computational fluid dynamics software to show the characteristics of the flow over a reverse delta wing.