Delay Flow Separation Research Articles

TVC effects on flow separation on slender cylinders

Occurrence of flow separation within the boundary-layer is primarily attributed to the growth of adverse pres-sure gradient within the boundary-layer. Such a separating boundary-layer can, however, be made to stay attached to the bounding surface due to various remedies where the surface transverse curvature (TVC) is one of them while the body contour is another. Assistive role of the surface transverse curvature is utilized in delaying the boundary-layer separation in a retarded flow past the slender bodies by assuming different body configurations. Axially-symmetric bodies of revolution of different shapes having a varying cross section in x, following the power-law form, have been considered in this study. It is shown that the point of separation is strongly dependent upon the surface curvature parameter k and the body-shape index n, which in fact control the flow separation. This fact ensures that an increase in the surface curvature parameter k and the body-shape index n leads to an increase in the wall shear stress which consequently delays the flow separation. Significant delay in flow separation is observed on cylinders having sufficiently large transverse curvature. Percent increase in the separation length corresponding to various values of the curvature parameter and the body-shape index has been calculated and reported in the form of Tables.

Effect of fin shape on the thermal performance of nanofluid-cooled micro pin-fin heat sinks

The study presents the combined effects of using nanofluid and varying fin cross-sectional shape on the heat transfer characteristics of a micro pin-fin heat sink by employing discrete phase model (DPM). Three fins configurations of the square, circular and hexagon cross-section with constant fin diameter and height have been analyzed for the inline arrangement of 17 × 34 fins. Aqueous nanofluid containing spherical shaped particle dispersions of TiO2 has been simulated for the particle concentration and size of 4.31 vol% and 30 nm respectively. Constant heat flux (192 W) boundary condition at the base of heat sink has been considered for the range of Reynolds number 250 ≤ Re ≤ 550. The influence of fin shape on the thermal efficiency of the heat sink has been analyzed by evaluating heat sink base temperature, Nusselt number, convective heat transfer coefficient distribution and temperature contours along the surface of the heat sink. Additionally the velocity streamlines and contours have also displayed to elaborate the fluid flow attributes. Results demonstrate that under identical flow conditions, the nanofluid cooled circular fins displayed most efficient thermal performance followed by the hexagon and square fins. While the water cooled square fins depicted lowest heat transfers characteristics. The best thermal performance of the circular fins is the response of the delayed flow separation along the smooth surface of fins and the subsequent uniform flow distribution along the whole sink. For all the cases, upstream fin rows played a primary contribution in flow distribution and hence thermal characteristics of the heat sink.

Parameter optimization of a double-deflector rectangular cambered otter board: Numerical simulation study

An otter board is an important device that provides a desired horizontal opening of a trawl net. A high lift-to-drag ratio is required for an otter board to maintain fishing efficiency. This study optimized the parameters of a double-deflector rectangular cambered otter board based on the maximum lift coefficient and lift-to-drag ratio and studied the hydrodynamic performance of the optimal otter board. Computational fluid dynamics (CFD) analysis was used (verified by a flume tank experiment) in the optimization process. Simulation results showed that the aspect ratios and deflectors had a significant influence on the hydrodynamic performance of an otter board. The optimal otter board had a better performance when the aspect ratio was 1.4 and the cambered ratios of the deflectors were 10%. Its maximum lift-to-drag ratio was 4.835, which was 1.10 times that of the initial otter board. The streamlines around the otter board showed that the structure of the deflectors can delay flow separation. The range of the wing-tip vortex increased with the angle of attack until stall occurred at which time the vortex began to break down.

Experimental study of square riblets effects on delta wing using smoke visualization and force measurement

Riblets are effective approach to improve the aerodynamic performance of low speed wing configurations. For a delta type wing, it is important to alleviate the flow features such as large rollup and tip vortices in low Reynolds number. In this paper, for first time, the effect of the square riblets on delta wings is studied. In the first step, flow visualization is accomplished to find out the effects of riblets on the flow pattern. For this purpose, two slender delta wings with and without riblet are examined in a vertical wind tunnel in which smoke stream lines are passed over the delta wings to visualize the flow pattern. The visualization tests are performed at five angles of attack; i.e., 0, 10, 20, 30 and 35 degrees. The results of flow visualization indicate that considerable improvements are obtained because of delaying flow separation at high attack angles. In the second step, the aerodynamic performance of the two delta wings are quantitatively evaluated by measuring the lift and drag forces using two load cells in another wind tunnel. The force tests are performed at attack angles of 5–65 degrees with 5-degree step sizes. The measured forces show that the riblets decreases the drag coefficient for all attack angles and increase the lift to drag ratio more than 12 percent for 5–20 degrees attack angles.

Effect of square dimples on the temperature distribution and heat transfer coefficient of an NACA0012 airfoil

ABSTRACTThe thermal boundary layer is the barrier for convective heat transfer, since the temperature within the thermal boundary layer is lower than the free stream temperature; therefore, it is preferable to maintain a small thermal boundary layer thickness (δt). Among the various categories of dimples like square, rectangle and triangle we considered square dimples for the upper surface of the NACA0012 wing throughout the span. The dimples increase the coefficient of lift due to delayed flow separation. The skin friction coefficient improvement reduces the δh which in turn reduces the (δt), therefore the rate of heat transfer is increased and surface temperature distribution becomes smooth. Dimples also act as fins, reducing the upper surface temperature to lower than the lower surface, which in turn increases the lift particularly in low-speed and micro-scale airfoils [Kim, Jongmin, Zvi Rusak, and Nikhil Koratkar. 2003. ‘Small-Scale Airfoil Aerodynamic Efficiency Improvement by Surface Temperature and Heat Transfer.’ AIAA Journal 41 (11): 2105–2113].

Computational Analysis on Application of Tubercles for the Performance Improvement of Horizontal Axis Wind Turbine

In this paper, we aim to delay the onset of stall phenomenon of the blades by applying leading edge modifications to the turbine blade. In nature, the control over the phenomenon of stall is observed in case of Humpback Whales. For these marine creatures, their flippers are having certain projections called tubercles which help it delay flow separation while performing tight underwater maneuvers while preying .These tubercles acts like a passive vortex generator at the tip of blade [2]. So we selected an optimum design from literatures to study their effects when applied to wind turbine blade airfoils. A widely used wind turbine airfoil developed by Delft University in Netherlands was chosen for this project. CFD analysis was used to evaluate and compare the aerodynamic characteristics like lift and drag coefficients of the tubercle modified and the baseline airfoil. The aerodynamic characteristics were analyzed at various angles of attack (AOA) of the airfoil and the study was mainly focused at the performance on the stall angles of the airfoil. After the numerical analysis of the tubercle modified wind turbine airfoil, it was found that the coefficient of lift was increased and coefficient of drag was reduced in the stalling angles effectively delaying the stall. Also it was observed that the tubercle modification does not have any detrimental effects on the aerodynamic performance in the pre-stall regions.

Flow Characteristics around Circular Cylinders with a Normal Slit

Alterations of boundary layer separation along the upper-rear surface of a baseline and slit cylinder and the formation of a vortex in the near-wake are investigated by particle image velocimetry (PIV) at Reynolds number 1000. The slit ratio (S/D) is 0.3. The phase-lock flow structures are referred to the time-dependent volume flux at the slit exit and are achieved by the modified phase-averaged technique. The alterations and the evolution of boundary-layer flow along the upper-rear surface are demonstrated by the phase-lock flow structures. It is found that the alternate blowing and suction at the slit exit serves as a perturbation to the boundary layer near the shoulder of the slit cylinder leading to a significant delay of flow separation and the flow reattachment of boundary-layer flow along the upper-rear surface of the cylinder. After perturbation, the vortex street behind a slit cylinder is more organized and stronger than that behind a baseline cylinder at Reynolds number 1000.

Numerical simulation of vortex generators on a winglet control surface

The objective of the present paper is improving the effectiveness of a winglet control surface at deflections greater than ≈5° through passive vortex generators (VG) placed on the control surface. Without deflection, the VGs are aligned with the flow direction and thus have almost no impact. The conceptual idea is that a control surface deflection changes the flow direction in such a way that the VGs become active and delay flow separation from the trailing edge. To decide on an appropriate initial Vortex Generator (VG) configuration the aerodynamic characteristics of the winglet are investigated first with Reynolds averaged Navier–Stokes (RANS) simulations. The functional principle of the concept is then demonstrated. A parametric study of different VG dimensions investigates the effectiveness for a range of flow conditions and deflection angles.

Numerical Investigation of Three-dimensional Separation Control on a High-speed Compressor Stator Vane with Tailored Synthetic Jet

Abstract A numerical study on the performance of synthetic jet for flow separation control on a high-speed compressor stator vane is performed. Four control schemes including full-span and part-span configurations are investigated at both design and off-design conditions. Results indicate that both full-span and part-span schemes could effectively delay flow separation and reduce total pressure loss for the compressor stator vane, the adaptability of the flow control under off-design conditions is also validated. Within the investigated incidence range, the full-span configuration is able to gain the most significant performance improvement, by which a maximum loss reduction of 23.8 % is obtained at i=2 deg. The part-span configuration could reorganize the vortex structures more efficiently and cut off the interaction between the ring-like vortex and the passage vortex, thus improving its performance in the corner region. In terms of flow control efficiency, the part-span configurations turn out to be more superior, where the highest control efficiency of 614.0 % is achieved at i=0 deg with the total height of the actuator being 40 %H. The flow control efficiency for all the schemes is higher than 100 % within the whole operating range, demonstrating a promising prospect for the application of synthetic jet in axial compressors.

Modeling of wind turbine vortex generators in considering the inter-effects between arrays

Vortex generators (VGs) are commonly placed on wind turbine blades to delay flow separation in the boundary layer. VGs can be parametrically modeled in computational fluid dynamics for effective and efficient simulations of wind blade flow fields. Many researchers have studied the vortex circulation created by VGs and created various parametric models used with the Navier-Stokes equations, but most of them are based on a single winglet of VGs and do not include the inter-effects between the winglets. This paper proposes a parameterized VG array model based on counter-rotating VGs, which properly takes into account the inter-effects between winglets. Two cases, i.e., a plate with a pair of VGs and a DU-W2–250 blade segment with five pairs of VGs, are investigated to validate this model; the array type parametric model is in closer agreement with experimental data than traditional models. Compared to the solid VG model, the array type model has similar streamlines and surface pressure coefficients on the suction surface. The array type VG model can effectively reduce the number of grid points and yield highly accurate predictions of wind turbine blade aerodynamic characteristics.

A model of flow separation controlled by dielectric barrier discharge

Flow separation, asan aerodynamic phenomenon, occurs in specific conditions. The conditions are studied in a wind tunnel on different airfoils. The phenomenon can be delayed or suppressed by exerting an external momentum to the flow. Dielectric barrier discharge actuators arranged in a row of 8 and perpendicular to the flow direction can delay flow separation by exerting the momentum. In this study, a mathematical model is developed to predict a parameter, which is utilized to represent flow separation on an NACA0012 airfoil. The model is based on the neuro-fuzzy method applied to experimental datasets. The neuro model is trained in different flow conditions and the parameter is measured by pressure sensors.

Separation Control on a Bridge Box Girder Using a Bypass Passive Jet Flow

In the present study, a bypass passive jet flow control method was proposed to mitigate unsteady wind loads and to manipulate the flow field around a single box girder of a bridge. With a geometric ratio of 1:125, the single box girder model was determined using the cross-section of the Great Belt East Bridge. During the experiments, one test model without control was adopted, while five different test models with different suction/jet configurations were employed to analyze the effects of the control method and to reveal the underlying mechanism of different control schemes. The incoming wind speed was fixed to 12 m/s and the wind attack angles were changed from −20° to 20°, resulting in a corresponding Reynolds number of Re = 0.28 × 105–0.74 × 105 based on the different attack angles. A six-component force balance, a set of digital sensor array (DSA) pressure transducers, and a particle image velocimetry (PIV) system was used to measure the aerodynamic forces, pressure distributions, and flow fields around the test models to evaluate the control effectiveness of different control cases. Detailed flow structures are presented and discussed for two test cases when the angles of attack are +15° and −20°. The effects of control on the aerodynamic forces were first investigated to determine and select the best one out of five control cases. The pressure distributions on the surface of the test model without control and the best control case were then compared to evaluate the control effectiveness of the pressure gradient and the fluctuating pressure coefficients. The flow fields around the test models demonstrate that the bypass passive jet flow control could decrease vortex strength, delay flow separation, and change recirculation region and size. The results of the aerodynamic forces, pressure distributions, and flow fields indicate that the bypass passive jet flow control method results in effective control.

Prediction of Turbulent Flow Around a Square Cylinder With Rounded Corners

Predictions are reported of the two-dimensional turbulent flow around a square cylinder with rounded corners at high Reynolds numbers. The effects of rounded corners have proved difficult to predict with conventional turbulence closures, and hence, the adoption in this study of a two-equation closure that has been specifically adapted to account for the interactions between the organized mean-flow motions due to vortex shedding and the random motions due to turbulence. The computations were performed using openfoam and were validated against the data from flows past cylinders with sharp corners. For the case of rounded corners, only the modified turbulence closure succeeded in capturing the consequences of the delayed flow separation manifested mainly in the reduction of the magnitude of the lift and drag forces relative to the sharp-edged case. These and other results presented here argue in favor of the use of the computationally more efficient unsteady Reynolds-averaged Navier-Stokes approach to this important class of flows provided that the effects of vortex shedding are properly accounted for in the turbulence closure.

Parametric study of low-profile vortex generators

Vortex generators (VGs) are flow control devices employed to avoid or delay flow separation. In most cases, the VG is calculated with the same height as the boundary layer (BL) thickness at the device position. However, these so-called conventional VGs may produce overload drag in some applications. The low-profile VGs can decrease the residual drag linked to this kind of passive flow control actuators. The main goal of the present work is to investigate the trajectory and size of the primary vortex produced by low-profile VGs on a flat plate with a height to length ratio of ½ and a vane incident angle of 18.5°. Hence, numerical simulations have been performed using Navier–Stokes equations at Re = 1350 based on the local boundary layer thickness where the VG was placed. Additionally, a prediction model has been developed to describe the progression of the vortex size behind the passive vanes.

Numerical simulation of a vertical axis wind turbine airfoil experiencing dynamic stall at high Reynolds numbers

Abstract Multi-megawatt floating vertical axis wind turbines (VAWTs) are a promising solution to exploit offshore wind energy resources in deep water sites. At this large-scale, the VAWT’s blades will operate at high Reynolds numbers and encounter dynamic stall at low tip-speed ratios. In particular, the selection of an accurate turbulence modeling approach is still a challenging undertaking in the prediction of transient blade forces during this complex unsteady event. In the present paper, the performance of Unsteady Reynolds-Averaged Navier–Stokes (U-RANS) and Detached Eddy Simulation (DES) modeling methods are compared in simulating the aerodynamics of an isolated NACA0018 airfoil experiencing Darrieus pitching motion. The U-RANS turbulence models employed were the Spalart–Allmaras (S-A) model and the k − ω SST model. Investigations were conducted to ensure satisfactory independency of the solution for both spatial and temporal discretisations, respectively. A quantitative assessment identified the S-A model as the most applicable for a VAWT design study as it showed the most desirable compromise between model fidelity and computational requirement. A qualitative analysis revealed that the thick VAWT airfoil creates a dynamic stall vortex topology highly concentrated at the trailing edge region. Finally, increasing the Reynolds number showed to be beneficial to the airfoil’s aerodynamic performance as a higher maximum tangential coefficient is attained, owing to the delay in flow separation to much higher angles of attack.

On the separation mechanics of accelerating spheres

The instantaneous drag forces and wake mechanics of an accelerating sphere have been investigated experimentally. Drag forces are first compared to the circular flat plate, which is characterized by stable and Reynolds-number independent vortex-ring formation during accelerations from rest. For the sphere, vortex-ring formation is shown to be greatly suppressed by the time-dependent movement of the separation line during start-up towards the steady-state position, which induces strong vortex-body interactions. Next, inviscid theory is used to predict the state of the pressure-gradient field around a sphere during accelerations from a non-zero initial velocity. The azimuthal point of separation extracted from experimental data for the subcritical cases is found to be strongly correlated with the start of the adverse pressure gradient predicted by theory. For the supercritical cases, the point of separation is unaffected by the imposed accelerations and remains at the steady-state position. The results suggest that accelerations can only be exploited to delay flow separation at Reynolds numbers with steady separation points near the apex, where the tangential accelerations are the largest.

The influence of blade roughness on the performance of a vertical axis tidal turbine

This paper reports the findings of an experimental study investigating the influence of blade roughness on the performance of a vertical axis tidal turbine. Due to their design, vertical axis turbines undergo periods of stall, i.e. flow separation from the blade, during each revolution. It is hypothesised that roughening turbine blades delays flow separation (in analogy to flows over rough bluff bodies) and hence diminishes turbine stall which in turn should result in an increase in turbine performance. Laboratory experiments were undertaken in Cardiff University’s hydraulics laboratory, testing vertical axis turbines with rotors comprising smooth and rough blades. Three different blade surface roughnesses were tested, with the results showing a significant reduction in performance when the turbine is operating at high chord Reynolds numbers and with rough blades. In addition, the combined effect of blade roughness and rotor solidity as well as blade roughness and number-of-blades on the performance of vertical axis turbines are analysed. It is shown that solidity and number-of-blades appear to be similarly influential than blade roughness.

Experimental study of laminar and turbulent boundary layer separation control of shark skin

The Shortfin Mako shark (Isurus oxyrinchus) is a fast swimmer and has incredible turning agility, and has flexible scales known to bristle up to 50° in the flank regions. It is purported that this bristling capability of the scales may result in a unique pass flow control method to control flow separation and reduce drag. It appears that the scales have evolved to be only actuated when the flow over the body is reversed; thereby inducing a method of inhibiting flow reversal close to the surface. In addition, bristled scales form cavities which could induce boundary layer mixing and further assist in delaying flow separation. To substantiate the hypothesis, samples of skin from the flank region of the mako have been tested in a water tunnel facility under various strengths of adverse pressure gradient (APG). Laminar and turbulent separation over the skin was studied experimentally using time-resolved digital particle image velocimetry, where the APG was generated and varied using a rotating cylinder. Shark skin results were compared with that of a smooth plate data for a given amount of APG. Both the instantaneous and time-averaged results reveal that shark skin is capable of controlling laminar as well as turbulent separation. Under laminar conditions, the shark skin also induces an early transition to turbulence and reduces the degree of laminar separation. For turbulent separation, the presence of the shark skin reduces the amount of backflow and size of the separation region. Under both flow conditions, the shark skin also delayed the point of separation as compared to a smooth wall.

Numerical study of the flow structures in flat plate and the wall-mounted hump induced by the unsteady DBD plasma

In this work, the dielectric-barrier-discharge plasma actuator was employed to study the flow structures induced by the plasma actuator over a flat plate and a wall-mounted hump. A phenomenological dielectric-barrier-discharge plasma model which regarded the plasma effect as the body force was implemented into the Navier–Stokes equations solved by the method of large eddy simulations. The results show that a series of vortex pairs, which indicated dipole formation and periodicity distribution were generated in the boundary layer when the plasma was applied to the flow over a flat plane. They would enhance the energy exchanged between the near wall region and the free stream. Besides, their spatial trajectories are deeply affected by the actuation strength. When the actuator was engaged in the flow over a wall-mounted hump, the vortex pairs were also produced, which was able to delay flow separation as well as to promote flow reattachment and reduce the generation of a vortex, achieving the goal of reducing dissipation and decreasing flow resistance.

Nanosecond-Pulsed Dielectric Barrier Discharge Plasma Actuator for Airflow Control Along an NACA0015 Airfoil at High Reynolds Number

The goal of the present experimental study is to investigate the ability of a surface nanosecond pulsed dielectric barrier discharge plasma actuator, located at the leading edge of the airfoil, to delay flow separation along the suction side of an NACA0015 airfoil inclined at 13.5°, using a particle image velocimetry system. The free-stream velocity is equal to U <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> = 40 m/s, giving a chord-based Reynolds number of R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ec</sub> = 1.33×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> . At such conditions, the baseline flow separation occurs at about 50% of chord. On the one hand, the results show that the separation location moves progressively toward the trailing edge when the high-voltage frequency Fpulse is increased, up to 85% of chord at 1630 Hz (reduced frequency F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pulse</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> = 10.6), which is due to the increase of the injected electrical power with F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pulse</sub> (from 8 mW/cm at 40 Hz to 290 mW/cm at 1630 Hz). On the other hand, the use of a burst-modulated actuation (with a constant electrical power consumption equal to 145 mW/cm) does not produce the same behavior. At the lowest and highest frequencies (16 and 408 Hz, corresponding to the reduced frequencies of 0.1 and 2.65, respectively), the separation is delayed to about 60% of chord, while the most effective actuation is observed at F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">burst</sub> = 80 Hz (F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">burst</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> = 0.53) with a separation delaying to 83% of chord.