Reattachment Distance Research Articles

Effect of the incident wind angle in a frigate with Coanda active flow control

The aerodynamic environment around frigates can be complex. This can cause helicopter pilots operating on frigates to face risky situations during the recovery maneuver. In this paper, the introduction of active flow control using Coanda effect has been tested to improve flow characteristics above a frigate flight deck. Wind tunnel tests with Particle Image Velocimetry (PIV) have been performed in order to analyze the effectiveness of introducing Coanda effect at the hangar at four different wind over deck conditions. Applying Coanda effect on the upper part of the frigate hangar the re-attachment distance, and the recirculation areas are reduced. When the Coanda effect is applied also on the sides, the flow recirculation can be completely eliminated, especially for WOD angles different from 0°. The profiles of velocities at the height where the helicopter rotor operates are also corrected, making them uniform and similar to the free stream velocity. In exchange, there is an increase in turbulence intensity close to the hangar, without affecting the rest of the flight deck. The results have shown that Coanda effect clearly improves the aerodynamics over the flight deck where the helicopter operates, and it could help to increase the safety of helicopter operations around frigates.

Wind Pressure Characteristics Based on the Rise–Span Ratio of Spherical Domes with Openings on the Roof

Wind loads are a primary concern in dome roof structures with openings such as retractable dome roofs. This is because the openings can cause damage to the cladding owing to high internal pressure. In this study, the wind pressure characteristics of a dome with an opening that varied based on the opening, rise–span ratio, and height span were examined by comparing the results from wind tunnel tests with those from previous studies. The negative pressure dominated the internal pressure of the roof in all regions and was not significantly affected by changes in the rise–span and height–span ratios. The reattachment distance of the windward region increased as the rise–span ratio increased, increasing the negative net pressure and decreasing the positive net pressure owing to a relatively large vortex. The roof inclination angle of the leeward region decreased as the rise–span ratio decreased, resulting in a decrease in the negative net pressure and an increase in the positive net pressure owing to a relatively small vortex. Based on the experimental results, a peak net pressure coefficient for cladding design was proposed for an open dome roof with a rise–span ratio of 0.05.

Numerical Investigation of Laminar Impinging Jet Cooling of a Protruded Heat Source

Abstract Thermofluid dynamics of an unconfined steady two-dimensional laminar jet impinging on an isothermal protruded heater is numerically studied for low jet inlet Reynolds number (Re) between 50 and 250. Results are shown for a range of impingement distances (h/W) between 1 and 10 for Prandtl numbers (Pr) 0.71 and 7.56. The volumetric entrainment increases with increasing h/w and decreasing Re. The reattachment distance of the wall jet appears to increase with Re and shows discernible deviation from the backward-facing step flow prediction for Re>150. Correlations are presented for average heater surface and sidewall Nusselt numbers as functions of Re and h/w for Pr=0.71 and Pr=7.56. In an overall convection dominant heat transfer, a relatively warmer and diffusion-dominated recirculation zone is identified adjacent to the sidewall with a low Nusselt number, which enhances significantly at Pr=7.56 when Re is increased above 100. At a low impingement distance, integrated kinetic energy flux shows greater magnitude in the impingement region but with a higher rate of decay. The integrated heat flux is greatly influenced by Re, and the effect is more pronounced at Pr=0.71. Self-similar behavior is observed for the velocity and heat flux profiles throughout the length in the developed region and for the temperature distribution over the heater surface. Both high Re and high h/w seem to adversely affect the self-similar behavior owing to a slower wall jet development.

Flow Characteristics of Two-Dimensional Supersonic Under-Expanded Coanda-Reattached Jet

When there is a wall near the jet, it deflects and flows while being attached to the wall owing to the Coanda effect. The flow characteristics of the incompressible and two-imensional (2D) Coanda-reattached jets have been considerably explained. However, 2D supersonic under-expanded jets, reattached to side walls, have not been sufficiently investigated. These jets are used in gas-atomization to produce fine metal powder particles of several micrometers to several tens micrometers. In this case, the supersonic under-expanded jets are issued from an annular nozzle, which is set around a vertically installed circular nozzle for molten metal. The jet flow at the center cross-section of the annular jet resembles a 2D Coanda-reattached jet that deflects and attaches on the central axis. In this study, the flow characteristics of a supersonic under-expanded Coanda air jet from a 2D nozzle that reattaches to an offset side wall are elucidated through experiment and numerical analysis. For numerical analysis, we show how much it can express experimental results. The effects of supply pressure P0 on the flow characteristics such as the flow pattern, size of shock cell, reattachment distance, and velocity and pressure distributions, etc. are examined. The flow pattern was visualized by Schlieren method and the velocity distribution was measured using a Pitot tube. These results will be also useful in understanding the flow characteristics of a gas-atomization annular nozzle approximately.

A numerical investigation on the unidirectional unsteady oscillatory wind flow over a fixed isolated downsized barchan dune.

Turbulent wind patterns over a two-dimensional isolated downsized barchan dune under the influence of sinusoidal inflow with different amplitudes and periods are simulated. The evolution rules of wind velocity over time at different positions are revealed. The flow reattachment distance and turbulence intensity distribution are also compared. The results show that wind velocities at different positions of the dune whose similar evolvement process of going from short-term fast adjustment transition to long-term stable sinusoidal fluctuation, can be reasonably estimated by the present simulation. It is found that, for the leeward toe of a dune with complex reversed flow, the balance position value of the sinusoidal wind velocity fluctuation is no longer close to the value of the steady wind velocity but shows a velocity deviation of about 0.40m/s. The flow reattachment distances under different unsteady inflows ultimately show a sinusoidal fluctuation with time, and their values are all larger than that of the steady flow. These synthetically predict that the unsteady flow has a stronger shaping effect on the leeward side of the dune body by enhancing sand transport. In addition, the predicted distribution comparison between unsteady wind velocity and turbulence intensity indicates that the unsteady wind velocity has a dominant effect on the turbulence intensity.

Wind tunnel simulation of an opencut tunnel airflow field along the Linhe-Ceke Railway, China

Abstract Opencut tunnels are linear arch constructions that are being used as new structures to prevent sand encroachment on railway lines. The large size and shape of the contained structure affect the wind field around the structure in a manner different to how traditional constructions prevent sand accumulation. The present study identified the characteristics of wind velocity and flow fields in opencut tunnels in a wind-tunnel simulation. Results show that airflow having different wind angles (15°–90°) had different effects on the sand-accumulation-prevention characteristics of the opencut tunnel. The inflection point of a rapid change in wind speed was 6H (where H is the vertical height of the model) from the windward side of the opencut tunnel, and the position of an area of strong wind on the upper part of the opencut tunnel gradually moved to the windward side with an increase in the wind angle. This trend was not affected by a change in the indicated wind speed. The reattachment distance on the leeward side decreased with an increase in the wind angle, being 1H at 75°–90°, 2H at 30°–60° and 6H at 15° or less. The protection effect was best for a large angle of the main wind direction (exceeding 75°). Observed characteristics of the wind field show that the opencut tunnel is effective at all wind angles in preventing and controlling railway sand flow hazards and can thus ensure smooth railway operation.

Hysteresis of the Coanda Effect

For scientist, the Coanda effect has been an object of interest for a long time. All the time, some new applications of it are found although it has been more than a hundred years since Henri Coanda got a patent that was critical for that issue. Apart from aviation, it is more and more often used in ventilation systems to control the manner of air division and the design nozzles and ventilators. It is surprising, however, that a good command of that phenomenon and a need to apply it in different solutions did not entail a significant increase of the interest in the Coanda effect hysteresis, although it was mentioned for the first time by Newman in 1961. This article presents results of experimental measurements for a two-dimensional incompressible plane jet by an inclined plate. The hysteresis has been observed as a different jet behavior (a free jet or a jet attached to a flat plate) depending on the direction in which the plate deflection angle changes. The observed hysteresis area, defined by critical values for the αca attachment and αcd detachment angles, spanned from 8 deg to 14 deg. Its dependency on the Reynolds number has also been examined for Re ranging from 3500 to 26,500. Considering the Coanda effect hysteresis, a pressure distribution on the plate and the xR reattachment distance has been examined. The distribution of forces on a plate has been identified, which has facilitated a graphical mirroring of the Coanda effect hysteresis loop.

Numerical characterization of three-dimensional bluff body shear layer behaviour

Three-dimensional bluff body aerodynamics are pertinent across a broad range of engineering disciplines. In three-dimensional bluff body flows, shear layer behaviour has a primary influence on the surface pressure distributions and, therefore, the integrated forces and moments. There currently exists a significant gap in understanding of the flow around canonical three-dimensional bluff bodies such as rectangular prisms and short circular cylinders. High-fidelity numerical experiments using a hybrid turbulence closure that resolves large eddies in separated wakes close this gap and provide new insights into the unsteady behaviour of these bodies. A time-averaging technique that captures the mean shear layer behaviours in these unsteady turbulent flows is developed, and empirical characterizations are developed for important quantities, including the shear layer reattachment distance, the separation bubble pressure, the maximum reattachment pressure, and the stagnation point location. Many of these quantities are found to exhibit a universal behaviour that varies only with the incidence angle and face shape (flat or curved) when an appropriate normalization is applied.

Afterbody Effects on Axisymmetric Base Flows

The influence of an afterbody on an axisymmetric wake flow is investigated with two-component particle image velocimetry at a Reynolds number of based on the model diameter. The reattachment distance is found to vary from two to four step heights as the afterbody diameter is increased. Measurements performed on equivalent planar backward-facing step geometries reveal that the reattachment length scales similarly to a planar backward-facing step when the afterbody to main-body diameter ratio approaches unity. The distribution of turbulent normal and shear stresses reveals a progressive inhibition of the radial fluctuations for increasing afterbody diameter, whereas a proper orthogonal decomposition analysis highlights a concurrent weakening of the shear-layer flapping mode. For , the frequency spectrum of the time coefficient of the first proper orthogonal decomposition mode appears associated with the wake flapping motion and peaks in the range of . This frequency is associated with the recirculating flow unsteadiness observed in the azimuthal plane of the wake. The configuration without afterbody features an additional contribution at , which is associated with vortex shedding. The current results reveal the dependence of low-frequency unsteadiness upon changes in the afterbody diameter.

Numerical Analysis of Flow Field around a Surface Mounted Square-Based Pyramid

Results of numerical analysis using Finite Volume Techniques of flow around a pyramidal structure have been reported in this paper. The conservation equations of mass and momentum have been solved in an unstructured grid by employing two equation based k-e model. The results of numerical experiments have been validated with existing results of experiments available in the literature. The apex angle and attack angle of the pyramid has been optimized for maximum reattachment distance. Similarly the apex angle and attack angle of the pyramid is also optimized for maximum turbulent intensity. From the present computation of velocity profile it is found that the back flow intensity is more at the rear side of the pyramid.

Turbulent flow over a backward-facing step. Part 1. Effects of anti-cyclonic system rotation

The effects of rotation on turbulent flow with separation and reattachment are investigated by means of direct numerical simulations. The backward-facing step configuration is rotated about a spanwise axis such that the sudden expansion of the channel is on the pressure side. The upstream flow is a fully developed plane Poiseuille flow subjected to orthogonal-mode rotation, which subsequently detaches from the step corner and eventually reattaches further downstream. The size of the resulting separation bubble with recirculating flow diminishes monotonically with increasing rotation rates and the reattachment distance is reduced from about 7 to 3 step heights. This is ascribed to the augmentation of the cross-stream turbulence intensity in theanti-cyclonicshear layer formed between the bulk flow and the recirculating eddy due to the destabilizing influence of the Coriolis force. The spanwise-oriented vortex cells or roller eddies found in non-rotating shear layers were disrupted by the enhanced turbulence. The flow along the planar wall is subjected to an adverse pressure gradient induced by the sudden expansion. The stabilizing influence of the system rotation in thiscyclonicshear layer tends to damp the turbulence, the flow becomes susceptible to flow separation, and a substantial cyclonic recirculation bubble is observed at the highest rotation rates. The resulting meandering of the bulk flow is associated with interactions between the anti-cyclonic shear layer at the stepped side and the cyclonic shear flow along the planar surface. These give rise to enhanced turbulence levels at the cyclonic side in spite of the otherwise stabilizing influence of the Coriolis force. Exceptionally high velocity fluctuations in the spanwise direction are observed in the vicinity of flow reattachment behind the step and ascribed to longitudinal Taylor–Görtler-like roll cells which extend into the backflow region. These roll cells arise from a centrifugal instability mechanism associated with the convex streamline curvature in the reattachment zone.

An experimental investigation of the recirculation zone formed downstream of a forward facing step

An experimental investigation of the recirculation zone formed downstream of a forward facing step immersed in a turbulent boundary layer has been undertaken using particle image velocimetry. Bluff body flow is observed with the fixed separation point located at the leading edge of the step. The recirculation region dimensions are characterised over a range of Reynolds numbers (1400–19 000), with Re h based on the step height and the free stream velocity. Turbulent perturbations are produced in the free shear layer which develops between the recirculating flow close to the step and the free stream flow. Contour maps of amplification factor, streamwise perturbation velocity and Reynolds stresses are constructed, providing insight into optimal placement of structures within such topographical features. The mechanisms affecting the reattachment distance, namely the turbulent mixing within the boundary layer and the velocity deficit in the boundary layer, are discussed.

Effects of free-stream turbulence on a transitional separated–reattached flow over a flat plate with a sharp leading edge

The transitional separated–reattached flow on a flat plate with a blunt leading edge under 2% free-stream turbulence (FST) is numerically simulated using the Large-eddy simulation (LES) approach. The Reynolds number based on the free-stream velocity and the plate thickness is 6500. A dynamic subgrid-scale model is employed and the LES results compare well with the available experimental data. It is well known that FST enhances shear-layer entrainment rates, reduces the mean reattachment distance, and causes early transition to turbulence leading to an early breakdown of the separated boundary layer. Many experimental studies have shown that different vortex shedding frequencies exist, specially the so called low-frequency flapping when there is a separation bubble but its mechanism is still not completely understood. The previous study by us without free-stream turbulence (NFST) did not show the existence of such a low-frequency flapping of the shear layer and it is not clear what the effects of FST will have on these shedding modes. Detailed analysis of the LES data has been presented in the present paper and the low-frequency flapping has not been detected in the current study.

Studies on Design Optimization of Coronary Stents

The stent design itself seems to be one of the factors responsible for restenosis. As a remedy, the present work attempts to perform a design optimization of coronary stents from a hemodynamic point of view. For the purpose, we have applied the principles of modern exploration of design space restricting ourselves to two-dimensional considerations. Width, thickness, and spacing of the struts of the stent formed the design variables. The objectives chosen for optimization were the vorticity generated, length of recirculation zone, and the reattachment distance in between the struts. Both semicircular and rectangular cross sections of stents were included. Starting with the range of design variables, sample stent cases were generated using Latin hypercube sampling. Objective functions were calculated for each of these by computing the two-dimensional flow using software FLUENT under the assumption of a steady, Newtonian flow considering a model stent with three struts. This was followed by Kriging to construct a response surface, which gives the relationship between the objectives and the design variables. The procedure gave nondominated fronts, which consist of optimized designs. Stents with minimum vorticity, with minimum recirculation distance, and the ones with maximum reattachment length in between struts were generated. The procedure is capable of producing the optimum set of design variables to achieve the prescribed objectives.

A wind tunnel simulation of the effects of stoss slope on the lee airflow pattern over a two-dimensional transverse dune

[1] Secondary airflow plays an important role in dune formation and development. The lee airflow pattern over transverse dunes is important in determining the shape, alignment, and spacing of dunes and is influenced significantly by the lee slope angle. In this paper we present the results of scaled wind tunnel simulations of the effects of stoss slope on the mean lee airflow patterns of transverse dunes. The leeward velocity field was measured nonintrusively using particle image velocimetry to assess the airflow pattern. We found that the flow separated over the dunes with a leeward angle close to the slip face angle of natural transverse dunes and was insensitive to stoss angle. Separation cells were characterized by a back eddy. Reattachment distance, height, area, and aspect ratio were used to characterize the separation cell. The first three parameters increased with increasing stoss angle for angles less than 15° but remained constant or decreased slightly with angles greater than 15°. On the basis of the leeward velocity profiles, six flow regions corresponding to those in previous models were recognized. These regions are initially bounded by kinks but gradually transition downwind and merge to recover a uniform velocity profile after a significant distance. We suggest that the separation cell is most developed when the angle of stoss slope reaches 15°, but the significance of this angle in the evolution of transverse dunes needs further exploration.

Stochastic estimation of a separated-flow field using wall-pressure-array measurements

Concurrent, surface-pressure and planar, particle image velocimetry (PIV) measurements were obtained in the separating/reattaching flow region downstream of an axisymmetric, backward-facing step at a Reynolds number of 8081, based on step height. The surface-pressure and PIV measurements were used to investigate the evolution of coherent structures in the flow field by employing proper orthogonal decomposition (POD) and multipoint, linear, stochastic estimation (mLSE) analysis techniques. POD was used to determine the dominant modes in the pressure signature, while mLSE was used to estimate the dominant flow structures above the wall from the wall-pressure POD modes over a series of time steps. It was found that a large-scale, coherent structure develops in place (i.e., temporally) at approximately half the reattachment distance. Once this structure reaches a height equivalent to the step, it sheds and accelerates downstream. This growth in place, and then shedding, resembles the evolution of the flow structure in the wake of bluff bodies. Such a “wake mode” has been observed in numerical-simulation studies of long cavities and backward-facing steps, where flow two dimensionality is controllable. The present study shows for the first time evidence for the existence of a wake mode in an experimental study of a backward-facing step. This is believed to relate to the quality of the two dimensionality (i.e., axisymmetry) of the test geometry and the ability to track the temporal evolution of structural features through mLSE.

Mean velocity field around prismatic bodies in tandem arrangement

This experimental investigation reports the wind tunnel measurements of the mean velocity fields around (a) an isolated square prismatic body with height to width ratio ( h/b) of 3 and (b) the same body in tandem with an identical interfering body. In the case of the studies involving interference, the ratios of the centre to centre distance to the body width ( G/b) are 4 and 8. The mean velocity components are measured using a Conrad six-hole probe. The approach flow is atmospheric boundary layer with the velocity profile satisfying 1 7 th power law. It is observed that the effect of interference is considerably large when G/b is small. Due to interference (i) the re-attachment distance shortens and (ii) the maximum values of transverse and vertical velocities are small. The data should be useful for validating computational codes.

Experimental Study on the Flow behind a Swept Backward-Facing Step (1st Report, Mean Flow Field in a Whole Region)

In the present study, the flow over a swept backward-facing step, at a Reynolds number (based on the step height and the free stream velocity) of 8 000 for the swept angle of 30 degrees, was investigated experimentally. Measurements were made for the mean flow field such as the surface flow direction, forward flow fraction, and mean velocity components. The flow field was examined in the regions of separation, reattachment, and following recovery to the two-dimensional flow. Lateral flows induced by the swept step significantly contribute to the variation of mean velocity profiles in the separated region. Reattachment distance of the flow over the swept step becomes about 10% shorter than that of the two-dimensional one. The mean velocity profiles collapse reasonably well on the Johnston's triangular model in the recovery region, and the profiles near the wall also collapse on the three-dimensional logarithmic law proposed by Johnston.

Heat transfer and flow behind a step in high enthalpy superorbital flow

AbstractHeat transfer levels have been investigated behind a rearward-facing step in a superorbital expansion tube. The heat transfer was measured along a flat plate and behind 2 and 3mm steps with the same length to step height ratio. Results were obtained with air as the test gas at speeds of 6·76kms-1and 9·60kms-1corresponding to stagnation enthalpies of 26MJ/kg and 48MJ/kg respectively. A laminar boundary layer was established on the flat plate and measured heat transfer levels were consistent with classical empirical correlations. In the case of flow behind a step, the measurements showed a gradual rise in heat transfer from the rear of the step to a plateau several step heights downstream for both flow conditions. Reattachment distance was estimated to be approximately 1·6 step heights downstream of the 2mm step at the low enthalpy condition through the use of flow visualisation.

Flow around a Permeable Rectangular Plate Standing Vertically on the Flat Wall. 2nd Report. Effects of the Aspect and the Oen-Area Ratios.

The flow around a permeable rectangular plate with the aspect ratio w/h and the open-area ratio β, standing vertically against a flat Wall on which a turbulent boundary layer is developing, was investigated experimentally. The surface pressure distribution on the flat wall and the flow visualization by both smoke-wire and oil-film techniques clarified the following. The circulation region behind the permeable rectangular plate was moved downstream with increase of both the open area ratio and the width of the plate, and disappeared for a large open-area ratio. The critical values of the open-area ratio in which there is a circulation region behind the present plates are between 0.325 and 0.485, and become large with increasing aspect ratio. The reattachment distance on the x axis behind the plate can be effectively estimated by both w/h and β. It seems that the wall pressure distributions on the x axis normalized by both the reattachment distance and the maximum suction pressure in the circulation region become similar for the range of 0.5≤w/h≤4 and βl0.196.