Reattachment Point Research Articles

Effect of the Inlet Boundary Conditions on the Flow over Complex Terrain Using Large Eddy Simulation

For large eddy simulation, it is critical to choose the suitable turbulent inlet boundary condition as it significantly affects the calculated flow field. In this paper, the effect of different inlet boundary conditions, including random method (RAND), Lund method, and divergence-free synthetic eddies method (DFSEM), on the flow in a channel with a hump are investigated through large-eddy simulation. The simulation results are further compared with experimental data. It has been found that turbulence is nearly fully developed in the case based on the Lund method, not fully developed in the case based on DFSEM, and not developed in the case based on the RAND method. In the flow region before the hump, mean velocity profiles in the case applying the Lund method gradually fit the law of the wall as the main flow moves towards the hump, but the simulation results based on the RAND and DFSEM methods cannot fit the wall function. In the flow region after the hump, cases applying Lund and DFSEM methods could relative precisely predict the size of turbulent bubble and turbulent statistics profiles. Meanwhile, the case based on the RAND method cannot capture the positions of flow separation and re-attachment point and overestimates the turbulent bubble size. From this research, it could be found that different turbulent inflow generation methods have a manifested impact on the flow separation and re-attachment after the hump. If the coherent turbulence is maintained in the approach flow, even though turbulent intensity is not large enough, the simulation can still predict the flow separation and turbulent bubble size relative precisely.

Investigation of Incompressible ROUCH-WALL boundary Layer Flows.(Dept.M)

COLES [1] suggested to represent the velocity distribution in turbulent boundary layers with adverse pressure gradient over smooth surface by a linear combination of two universal functions.one is the well known law of the wall, the other the law of the wake which is characterised by the profile at a point of separation or reattachment. The aims of this investigation are to modify Coles velocity distribution to perform the calculations for turbulent boundary layers over smooth and rough surfaces and also to study analytically the various boundary layer parameters and surface roughness function derived from the modified velocity distribution and experimental results with flows over rough surfaces given in [10].

ψ‐v Computation of steady‐state conjugate heat transfer in backward‐facing step flow

AbstractIn this paper, we study steady‐state conjugate heat transfer over a backward‐facing step flow using a combination of a compact finite difference scheme for the ‐ form of the Navier–Stokes equations and a higher‐order compact scheme for the temperature equations on nonuniform grids. We investigate the effect of Reynolds number (), conductivity ratio (), Prandtl number (), and slab thickness () on the heat transfer characteristics. Isotherms remain clustered near the reattachment point in the fluid, while the temperature in the solid decreases vertically, with the minima at the reattachment point. Heat transfer rate (HTR) increases with Re, the maximum at the reattachment point. The HTR increases with till after, which it becomes invariant as . Isotherms at the inlet become more disorderly with increasing Pr, and progressively clustered near the interface, indicating an increase in HTR, while the temperature in the solid region decreases with Pr. Increasing b decreases the HTR. In addition to obtaining an excellent match with results previously reported in the literature, we offer more comprehensive and previously unreported insights on flow physics.

Flow around a diamond-section cylinder at low Reynolds numbers

The steady separated flow past a diamond cylinder at low Reynolds numbers, Re, is associated with diverse separation topologies not resolved for a circular or square cylinder. The present study, conducted for Re≤150, also uncovers three unique separation topologies for the time-averaged flow. In this regard, the most striking observation is the formation of a small sub-wake around the base of the cylinder at certain Re between 80 and 90. While two of these structures were previously captured by some recent studies, these studies did not investigate their origin or kinematics. In the present study, conducted via stabilized finite-element computations in two-dimensions, these wake topologies are analyzed in detail. For secondary separation, the pressure at the reattachment point on the cylinder rear surface exceeds the pressure at the corresponding separation point located upstream. In a similar manner, for primary separation, the pressure at the wake stagnation point(s) surpasses the one at the separation points. Via direct steady and unsteady computations, the value of the critical Re indicating the onset of vortex-shedding is found to be 41, approximately. At this Re, the flow quantities, such as wake length, drag, and pressure coefficients, obtained from the steady and unsteady computations are found to diverge. The presentation of wake length and drag as a function of base suction establishes that the drag and wake length share an inverse relationship.

Study of vibration modes and strain distribution of a flow energy harvester in the wake region of different bluff bodies

This paper aims to analyze mode shapes and corresponding strain distribution in a two-dimensional plane structure exhibiting flow-induced vibration in the wake field of four different upstream bluff bodies such as cylinder, square, triangle, and D-shape. This research is important from the point of view of flow-induced piezoelectric energy harvesting; wherein, induced strain in the structure is directly related to the amount of energy generated. Mainly, all investigations are carried out at low Reynolds number ( Re = 200); however, to widen the scope of the work, other Reynolds numbers are also considered ( Re = 300, 500, and 750 ). The results obtained indicated that the plane structure vibrates in different mode shapes under different wake fields. The square section dominantly gives rise to the fundamental mode vibration, whereas, cylinder, D-shape, and triangular bluff body induce vibration in a mix of fundamental and higher bending modes. Analysis of the corresponding flow regime shows that the position of reattachment point of the downstream shear layer plays an important role in the realization of different vibration modes. The strain distribution under different cases revealed that the wake of a cylindrical bluff body produces highest peak strain, and D-shape bluff body results in highest cumulative strain. From the aspect of energy harvesting, a quantitative comparison of strain-induced and per second charge generation have indicated that for an equivalent flow condition, the D-shape will produce higher energy per unit time than the cases of a square, cylinder, and triangular cross-sections.

Implementation and characterization of a physiologically relevant flow waveform in a 3D microfluidic model of the blood–brain barrier

Previous in vitro studies interrogating the endothelial response to physiologically relevant flow regimes require specialized pumps to deliver time-dependent waveforms that imitate in vivo blood flow. The aim of this study is to create a low-cost and broadly adaptable approach to mimic physiological flow, and then use this system to characterize the effect of flow separation on velocity and shear stress profiles in a three-dimensional (3D) topology. The flow apparatus incorporates a programmable linear actuator that superposes oscillations on a constant mean flow driven by a peristaltic pump to emulate flow in the carotid artery. The flow is perfused through a 3D in vitro model of the blood-brain barrier designed to induce separated flow. Experimental flow patterns measured by microparticle image velocimetry and modeled by computational fluid dynamics reveal periodic changes in the instantaneous shear stress along the channel wall. Moreover, the time-dependent flow causes periodic flow separation zones, resulting in variable reattachment points during the cycle. The effects of these complex flow regimes are assessed by evaluating the integrity of the in vitro blood-brain barrier model. Permeability assays and immunostaining for proteins associated with tight junctions reveal barrier breakdown in the region of disturbed flow. In conclusion, the flow system described here creates complex, physiologically relevant flow profiles that provide deeper insight into the fluid dynamics of separated flow and pave the way for future studies interrogating the cellular response to complex flow regimes.

A semi-implicit discrepancy model of Reynolds stress in a higher-order tensor basis framework for Reynolds-averaged Navier–Stokes simulations

With the rapid development of artificial intelligence, machine learning algorithms are becoming more widely applied in the modification of turbulence models. In this paper, with the aim of improving the prediction accuracy of the Reynolds-averaged Navier–Stokes (RANS) model, a semi-implicit treatment of Reynolds stress anisotropy discrepancy model is developed using a higher-order tensor basis. A deep neural network is constructed and trained based on this discrepancy model. The trained model parameters are embedded in a computational fluid dynamics solver to modify the original RANS model. Modification computations are performed for two cases: one interpolation and one extrapolation of different Reynolds numbers. For these two cases, the ability of the modified model to capture anisotropic features has been improved. Moreover, when compared with the mean velocity of large eddy simulations (LES), the root mean square error of the modified model is significantly lower than the original RANS model. Meanwhile, the modified model can better simulate flow field separation and fluctuation in the shear layer and has better prediction accuracy for the reattachment point and the mean velocity profile compared with the original RANS model. In addition, the modified model also improves the prediction accuracy for the mean pressure coefficient and mean friction coefficient of the underlying wall surface. The previously trained model is also directly performed for the modification computation of the two massive separation periodic hill flows. It is shown that the results simulated by the modified model and LES approach are more consistent in both trend and magnitude than the original RANS model and LES approach.

A Comparative Study on Aerodynamic Drag Reduction of a Blunt Nose Body using Aerospike and Aerodisk – Numerical Approach

A spike attached to a hemispherical body drastically changes its flow field and influences aerodynamic drag in supersonic and hypersonic flow. It is, therefore a potential candidate for drag reduction of a future high-speed vehicle. The effect of spike nose configuration and angle of attack on the reduction of drag is studied. The studies show that the aerodisk is superior to the aerospike. The aerodisk of appropriate length, diameter and nose configuration may have the capability for the drag reduction. Design criteria is studied and the aerodisk attached to the blunt nose body are designed using CATIA. Numerical solutions are obtained using a commercial CFD software ANSYS-Fluent. Flow observations on the blunt body with a conical, hemispherical aerodisk and flat-faced aerodisk are carried out at Mach number of 2. The flow fields show different flow features between the conical spike, the hemispherical aerodisk and the flat-faced aerodisk. The effects of the aerospike and aerodisk on the lift and drag coefficient are studied. Finally, it is found that A forward-facing spike attached to a hemispherical body alters significantly the structure of the flow field and serves to reduce drag by the formation of a recirculation region around the stagnation point of the blunt body. The flow field, immediately behind the aerodisk shows a complex flow field due to back-disk geometry as compared to the conical spike. To take advantage of the forward-facing spike for more efficient drag reduction, the reattachment point of the shear layer on the body should be moved backward by choosing the optimal spike length with suitable geometrical configuration of the nose.

Low-frequency oscillation characteristics of flow for NACA66 hydrofoil under critical stall condition

Hydrofoil is the core unit of energy conversion for axial-flow pumps and hydro-energy turbines. The dynamic characteristics of hydrofoil significantly affect the operation stability of pumps and turbines, especially under the critical stall condition. In this paper, the unsteady flow field of NACA66 hydrofoil under critical stall condition at Reynolds number 0.75 × 10E6 is investigated by IDDES method. The low-frequency oscillations (LFO) under critical stall condition characterized by two typical frequencies with Strouhal number (St) 0.031 and 0.065 are reported in pressure pulsation and lift coefficient respectively. It is found that the merging phenomenon of separation vortices are the main reason of the wave peak of LFO with St 0.031, while the phenomenon of generation and shedding of separation vortices exist at the moment corresponding to the wave trough. The reason of the LFO of lift coefficient with St 0.065 is found to be the long-short switching of streamwise coverage of the leading edge separation vortex. The change of streamwise coverage is corresponding to the movement of the reattachment point of the separation area. The revelation of LFOs flow mechanism in hydrofoil under critical stall can provide reference for its unsteady flow control and applications.

Effect of Ice accretion on the aerodynamic characteristics of wind turbine blades

Cold regions with high air density and wind speed attract wind energy producers across the globe exhibiting its potential for wind exploitation. However, exposure of wind turbine blades to such cold conditions bring about devastating impacts like aerodynamic degradation, production loss and blade failures etc. A series of wind tunnel tests were performed to investigate the effect of icing on the aerodynamic properties of wind turbine blades. A baseline clean wing configuration along with four different ice accretion geometries were considered in this study. Aerodynamic force coefficients were obtained from the surface pressure measurements made over the test model using MPS4264 Simultaneous pressure scanner. 3D printed Ice templates featuring different ice geometries based on Icing Research Tunnel data is utilized. Aerodynamic characteristics of both the clean wing configuration and Ice accreted geometries were analysed over a wide range of angles of attack (α) ranging from 0° to 24° with an increment of 3° for three different Reynolds number in the order of 105. Results show a decrease in aerodynamic characteristics of the iced aerofoil when compared against the baseline clean wing configuration. The key flow field features such as point of separation, reattachment and formation of Laminar Separation Bubble (LSB) for different icing geometries and its influence on the aerodynamic characteristics are addressed. Additionally, attempts were made to understand the influence of Reynolds number on the iced-aerofoil aerodynamics.

Transitional pulsatile flows with stenosis in a two-dimensional channel

Although blood flows are mostly laminar, transition to turbulence and flow separations are observed at curved vessels, bifurcations, or constrictions. It is known that wall-shear stress plays an important role in the development of atherosclerosis as well as in arteriovenous grafts. In order to help understand the behavior of flow separation and transition to turbulence in post-stenotic blood flows, an experimental study of transitional pulsatile flow with stenosis was carried out using time-resolved particle image velocimetry and a microelectromechanical systems wall-shear stress sensor at the mean Reynolds number of 1750 with the Womersley number of 6.15. At the start of the pulsatile cycle, a strong shear layer develops from the tip of the stenosis, increasing the flow separation region. The flow at the throat of the stenosis is always laminar due to acceleration, which quickly becomes turbulent through a shear-layer instability under a strong adverse pressure gradient. At the same time, a recirculation region appears over the wall opposite to the stenosis, moving downstream in sync with the movement of the reattachment point. These flow behaviors observed in a two-dimensional channel flow are very similar to the results obtained previously in a pipe flow. We also found that the behavior in a pulsating channel flow during the acceleration phase of both 25% and 50% stenosis cases is similar to that of the steady flow, including the location and size of post-stenotic flow separation regions. This is because the peak Reynolds number of the pulsatile flow is similar to that of the steady flow that is investigated. The transition to turbulence is more dominant for the 50% stenosis as compared to the 25% stenosis, as the wavelet spectra show a greater broadening of turbulence energy. With an increase in stenosis to 75%, the accelerating flow is directed toward the opposite wall, creating a wall jet. The shear layer from the stenosis bifurcates as a result of this, one moving with the flow separation region toward the upper wall and the other with the wall jet toward the bottom wall. Low wall-shear stress fluctuations are found at two post-stenotic locations in the channel flow – one immediately downstream of the stenosis over the top wall (stenosis side) inside the flow separation region, and the other in the recirculation region on the bottom wall (opposite side of the stenosis).

Flow structures of a precessing jet in an axisymmetric chamber

This work investigates the flow structures of a precessing jet in an axisymmetric chamber with expansion ratio $$D/d=5$$ and length-to-diameter ratio $$L/D=2.75$$ at Reynolds number $${\mathrm{Re}}_{d}=2.4\times 10$$ 4 by use of planar particle image velocimetry. Time-averaged and statistical results indicate that the precessing jet flow is symmetric, on average, in the streamwise plane. Large velocity fluctuations and vorticity exist in the inner and outer shear layers. In addition, vorticity is found in the boundary layer of the chamber wall, due to backflow and confinement. Subsequent correlative analysis indicates that coherent structures are mainly distributed in the shear layer and decay with its development. The results also indicate that precession occurs in the region $$x/d>5$$ and may lead to changes in the structure and position of the shear layer. Further spectral analysis shows that a low-frequency structure with $$\mathrm{St}\approx 7.4\times 10$$ −3, which can be interpreted as precession, exists in the flow field and that the corresponding dominant frequency decreases as the fluid flows downstream. Finally, proper orthogonal decomposition (POD) analysis reveals the most energetic mode representing the flow structure of precession, which has the highest proportion of the total downstream energy. The precession induces an alternating flow, switching between outflow from one side of the chamber and inflow from another side. In addition, four typical instances of flow structures of precession with different intensities or phases are presented through low-order reconstruction of the specified POD modes. These cases show that due to the instability of the reattachment point, the mainstream oscillates up and down in the region near the nozzle exit and twists back and forth in the region near the chamber exit, with changes of scale and position of flow structures on the measurement plane, exposing complex behavior of the instantaneous flow field of precession.

Erosion-Corrosion of AISI 302 Stainless Steel Sudden Expansion Pipes in High Salt Wastewater: Effect of Fluid Flow on Different Positions of a Sudden Expansion Pipe

Erosion-corrosion (E-C) is common in chemical industries. Sudden expansion pipes (SEP) are one of the flow components which may experience severe erosion rates. Computational fluid dynamic modeling (CFD) and flow-through experiments were used to study E-C for a SEP of AISI 302 stainless steel. A typical geometry for SEP was investigated: 20 mm diameter inlet pipe, 40 mm diameter outlet pipe, with an inlet flow rate of 0.1 m/s, and 10 wt% concentration of SiO2. CFD simulation results showed that turbulence energy and wall shear were highest at 5 mm and fluid axial velocity was lowest at 5 mm from the inlet/outlet SEP connection point. E-C test results showed that the most severe E-C occurred between 5 mm and 7 mm downstream of the SEP. At deeper lengths into SEP, the corrosion rate decreased and remained constant. The results indicated that increasing wall shear and turbulence energy increased the mechanical effects of particles on SEP and hence increased the E-C rates of the reattachment point. This work provides a means of understanding E-C behavior and predicting erosion damage of SEP.

Heat and Fluid Flow Characteristics of Nanofluid in A Channel Baffled Opposite to The Heated Wall

In this paper, a nanofluid-based solar collector duct equipped with baffles is examined numerically. Baffles are located on the back plate to guide nanofluid flow toward absorber plate for heat transfer enhancement purposes. Cu-water nanofluid with fixed flow rate and concentration in the baffled duct are investigated for thermohydraulic mechanisms. Baffles with different inclination angles, heights and pitches are considered in this study. Numerical simulations are performed using Ansys fluent software with verified results compared to those of an experiment in the literature. The results show that the baffle angle 60° causes the lowest thermohydraulic performance. Because in the angle range of 30 to 60° the heat transfer is less variable while the pressure loss increases sharply. At the baffle pitch of 40 mm, there is no reattachment point at the non-heated surface. At the angle of 90°, three eddies are formed around a baffle. The slope linear regression analysis yields that baffle height has the strongest effects on thermohydraulic performance followed by baffle pitch and baffle angle. Nanofluid pressure loss respectively increases with baffle height and baffle angle at the rate of 0.463675 and 0.0049607 while absorber plate temperature respectively decreases with the baffle height and baffle angle at the rate of -0.176746 and -0.001377. Flow patterns and isotherms of all the cases examined are presented and analyzed in this study.

Hypersonic Oscillating Shock-Wave/Boundary-Layer Interaction on a Flat Plate

This work discusses the design, measurement, and simulation of an oscillating shock-wave/boundary-layer interaction on a flat plate at Mach 5.8 and . The shock generator is free to pitch and oscillates with a frequency of 42 Hz, resulting in a shock that varies in intensity and impingement point, with a maximum flow-deflection angle of approximately 10 deg. Transition appears to take place downstream of the separated region for both static (with a fixed flow-deflection angle) and dynamic experiments; however, heat-flux values are typically between laminar and turbulent solutions, thus suggesting that a complete transition to a fully turbulent boundary layer is delayed because of the favorable pressure gradient induced by the impinging expansion wave originating from trailing edge of the shock generator. Peak pressure is typically overpredicted by laminar simulations for large deflection angles. Starting from the reattachment point, heat-flux measurements show that the boundary layer gradually deviates from the laminar solution towards a fully turbulent boundary layer. Vortices are observed in the reattachment region, and their distribution is solely a function of the boundary-layer properties at the separation point. Transient effects induced by the shock motion result in a maximum bubble length variation of 30%. For the static cases, the separated region amplified disturbances with a frequency of approximately 200 Hz. In the dynamic experiment, harmonics induced by the pseudosinusoidal motion of the shock generator were measured everywhere on the plate.

Effects of corner rounding on aerothermodynamic properties in rarefied hypersonic flows over an open cavity

A direct simulation Monte Carlo (DSMC) method is utilized to numerically investigate rarefied hypersonic flows over a group of two-dimensional rectangular cavities with rounded separation/reattachment corner. The effects of the rounded separation/reattachment corner on flow characteristics are discussed. The sensitivities of aerodynamic surface quantities are hence assessed comprehensively. Flows over the cavity configurations specified in the study hold the “open-cavity” characteristics and do not show any unsteadiness. The rounded separation corner does not significantly alter the flow patterns, while the rounded reattachment corner has a remarkable effect. Rounding the separation and reattachment corners can weaken the baroclinic effect in the separation and reattachment regions, respectively. For the rounded separation corner, it is easier for external gas molecules to enter the cavity and hence promote the momentum and energy exchanges with gas molecules inside the cavity. The fluid density inside the cavity, peak density at the reattachment point, pressure on and heat fluxes to the cavity floor are augmented, compared to those in the rectangular cavity case. However, the rounded reattachment corner may enhance the escape of gas molecules from the cavity and weaken the exchanges of momentum and energy. This leads to a significant decrease in the gas density inside the cavity, peak gas density at the reattachment point, pressure on and heat fluxes to the cavity floor. It is concluded that the approach of rounding the reattachment corner can achieve the most dramatic drop in the peak pressure and heat flux at the reattachment point. The research is guidable to the aerodynamic design of local configuration of hypersonic vehicles.

Flow Topology around Low Reynolds Number Airfoils with Vortex Shedding

In depth studies on the vortex formation process as well as the periodic behavior of the reattachment profile have been mainly sparked by broad discussions on the flow topology. While the topology methodology is quite well established, the majority of airfoil types have still not been covered. In this study, six low Reynolds number airfoils were considered to investigate the separation bubble structure, vortex shedding and reattachment point. This was done by means of computational fluid dynamics simulations involving the airflow passing several Eppler and Selig models. The method was validated against an established mathematical relationship to indicate that both simulations and analysis technique were reliable. It was observed that the phenomena under investigation directly affect the airfoils aerodynamic performance. In particular, vortex shedding and big reattachment length contribute to the minimum lift. Moreover, high number of vortices in the separation bubble as well as secondary separation bubble result in maximum drag. The results which correspond to low Reynolds number airfoils are applicable in micro aerial vehicles field.

State observer data assimilation for RANS with time-averaged 3D-PIV data

State observer techniques are investigated for the assimilation of three-dimensional velocity measurements into computational fluid dynamics simulations based on Reynolds-averaged Navier–Stokes (RANS) equations. The method relies on a forcing term, or observer, in the momentum equation, stemming from the difference between the computed velocity and the reference value, obtained by measurements or high-fidelity simulations. Two different approaches for the forcing term are considered: proportional and integral-proportional. This technique is demonstrated considering an experimental database that describes the time-average three-dimensional flow behind a generic car-mirror model. The velocity field is obtained by means of Robotic Volumetric PIV measurements. The effects of the different forcing terms and the spatial density of the measurement input to the numerical simulation are studied. The state observer approach forces locally the solution to comply with the reference value and the extent of the region modified by the forcing input is discussed. The velocity distribution and flow topology obtained with data assimilation are compared with attention to the object wake and the reattachment point where the largest discrepancy is observed between the different approaches. The results show that the integral term is more effective than the proportional one in reducing the mismatch between simulation and the reference data, with increasing benefits when the density of forced points, or forcing density, is increased.

Flow boiling heat transfer and associated bubble behaviors over backward- and forward-facing steps

The influence of backward-facing step and forward-facing step on flow boiling heat transfer performance was experimentally studied. The experiments were carried out with expansion ratio of 2, 2.5 and 3, and channel Reynolds number of 3048 to 6097. The heated surface was immobilized at the downstream next to the step. FC-72 was used as the working fluid with the inlet subcooling of 35 K. The bubble behaviors were analyzed by the high-speed camera. The results reveal that the backward-facing step geometry has a negative effect on the heat transfer performance compared with the flat channel. The effect of forward-facing step geometry is related with the dimension of the recirculation regions: the heat transfer performance is enhanced if the reattachment point falls on the heated surface, and is deteriorated if the recirculation region is larger than the length of the heated surface. The bubble behaviors were analyzed to illustrate the dimension of the recirculation region and the mechanism of flow boiling heat transfer over a step.

Potential of Micro-Vortex Generators in Enhancing the Quality of Flow in a Hypersonic Inlet-Isolator

The rush to be the first to demonstrate a practical hypersonic cruise missile has never been more frantic among the world’s superpowers, especially since China and India have also announced their own programme. The main hurdle for safe hypersonic flight is the severe shock wave-boundary layer interaction (SWBLI) that could induce flow separation. The separation could lead to inlet unstart and also structural damage at the flow re-attachment point. The simplest method to control these phenomena is by using passive flow control devices such as micro-vortex generator (MVG). The MVG is typically sized in the range of sub-boundary layer and the vortex generated can induce an early transition to turbulence thus avoiding or reducing the impact of flow separation. Many studies have been published with regard to MVG, but most were done in low supersonic speed and not in the hypersonic flow regime. In the current study, the MVG array was placed strategically at various locations on a hypersonic inlet-isolator representative geometry. The MVG has been proven to be very effective in eliminating or reducing the size of flow separation thus reducing the associated peak pressure at the re-attachment point.