Separated Shear Flow Research Articles

Large eddy simulation of fuel-air mixing process in a convergent-divergent duct spray under non-vaporizing conditions

The spray mixing process can be improved via straight (ST) duct fuel injection, but there is a risk of potential heat transfer loss due to prolonged spray tip penetration (STP) and spray impingement. We proposed a convergent-divergent (CD) duct spray to produce an acceptable STP along with improved spray air entrainment and spray cone angle (SCA), which is validated in optical spray experiments. How the CD duct enhances the fuel–air mixing and why the spray left–right swing phenomenon occurs near the outlet of the CD duct is still unknown. This study focuses on the spray mixing process inside CD and ST ducts using large eddy simulations. Results showed that the larger vortex cluster is formed in the divergent outlet with the large diameter of 4.5 mm and 6 mm, named CD4.5 and CD6 ducts, which leads to the unstable fluctuations of the separated shear flow and a large number of vortex shedding from the inner wall of the CD duct outlet to promote the radial fuel–air mixing with the left–right swing phenomenon, which further enlarges the SCA and appropriately shortens the STP with reduced spray energy. Compare to free spray, there are two turbulence kinetic energy (TKE) peaks in the spray axial centerline, indicating that duct spray brings extra disturbance to spray and is conducive to promoting the fuel–air mixing process. The strongest TKE region appears near the outlet of CD4.5 and CD6 duct sprays that are consistent with the left–right spray swing motion phenomenon.

Flow over a forward-facing step with a flexible membrane at its leading edge

The effects of a flapping membrane at the leading edge of a forward-facing step (FFS) on the flow over and pressure fluctuation on it were experimentally investigated. The FFS was immersed in the oncoming boundary layer (δ/h = 0.4, where δ and h are the oncoming boundary layer thickness and step height, respectively) and the Reynolds number, based on the step height and oncoming flow velocity (U∞) was 3.4×104. Both the length and width of the membrane were identical to h, the height of the FFS. Two-dimensional particle image velocimetry (PIV) and smoke-wire flow visualization techniques were used to investigate the flow over the FFS. Pressure scanners have also been used to study the pressure distribution on an FFS. It was found that the membrane flapped periodically under the effects of the separated shear flow from the leading edge of the FFS, which significantly suppressed the separation bubble on the FFS and prompts an early recovery of the time-averaged pressure. The phase-averaged results revealed that the flapping membrane generated alternating vortices over the FFS. These flapping-induced vortices (FIVs) enhance the Reynolds stresses over the FFS, increasing momentum transport in the wall-normal direction. Moreover, these FIVs synchronize with the pressure fluctuation on the FFS.

Numerical study of flow separation control over a circular hump using synthetic jet actuators

This study deals with the wall resolved Unsteady Reynolds-Averaged Navier–Stokes (URANS) simulation of boundary layer flow separation over a circular hump model and its active control. An array of Synthetic Jet Actuators (SJAs) is implemented in the hump model to introduce a train of vortex rings into the boundary layer to control flow separation. The OpenFOAM solver is used to numerically simulate and analyze the fluid flow using the k–ω shear stress transport model. Hot wire anemometry and particle image velocimetry measurements are carried out to evaluate the accuracy of the URANS technique as well as the effectiveness of SJAs by comparing numerical predictions to experimental data. The time-averaged results are in a good agreement with experimental results and demonstrate a successful application of SJAs to delay the flow separation by the interactions of vortical structures with the separated shear flow. The three-dimensional simulation also reveals that near wall coherent flow structures (streamwise and spanwise vortices) are responsible for the wall shear stress components. The results can be used to better understand the performance of SJAs and to further improve future actuator configurations.

Experimental study of self-sustained spanwise streaks and turbulent mixing in separated shear flow

The self-sustained streaks in turbulent backward-facing step flow are experimentally studied at a step height Reynolds number of 1.0 × 103. As one of the featured dynamic behaviors, evolution of the spanwise streaks plays an important role in the large-scale, aperiodic and vertical flapping motions of the separated/reattaching shear layer. By time-resolved particle image velocimetry, we measure the velocity vector fields in the streamwise-spanwise and streamwise-vertical planes downstream of a two-dimensional backward-facing step. The two cross-sectional velocity fields show that the shear layer remains two-dimensional along the span in the initial half of the shear layer, and then three-dimensional structures start to evolve in the latter half, leading to an unsteady reattaching shear layer and a shifting reattachment point on the downstream wall. By analyzing the finite-time Lyapunov exponent in the spanwise field of view, we find that low-speed fluid elements are vertically entrained upwards into the shear layer. As a result, the entrained fluid elements produce transient and localized low-speed fluid patches in the streamwise-spanwise plane, which move and decay downstream with strong straining and stretching motions along their boundaries. Furthermore, the large-scale streak-type and flap-type flows are clearly extracted and reconstructed by the energy-containing proper orthogonal decomposition modes.

Periodic hill flow simulations with a parameterized cumulant lattice Boltzmann method

AbstractThe article is concerned with the assessment of a cumulant lattice Boltzmann method in wall‐bounded, separated turbulent shear flows. The approach is of interest for its resolution‐spanning success in turbulent channel flows without using a specific turbulence treatment. The assessment focuses upon the flow over a periodic hill, which offers a rich basis of numerical and experimental data, for Reynolds numbers within . The analysis involves the mean flow field, second moments and their invariants, as well as spectral data obtained for a wide range of resolutions with . With the emphasis on a recently published parameterized cumulant collision operator, the universality of the value assigned to a stability preserving regularization parameter is assessed. Reported results guide resolution‐dependent optimized values and indicate a required minimum resolution of . Analog to the findings for attached turbulent shear flows, the approach appears to adequately resolve complex turbulent flows without the need for ad hoc modeling for a range of scale resolving resolutions.

Experimental investigation on active control of flow around a finite-length square cylinder using dual synthetic jet

This paper presents an active flow control method for flow around a finite-length square cylinder using dual synthetic jet (DSJ). A piezoelectric dual synthetic jet actuator (DSJA) was installed at the leading edge of the free end for generating the DSJ. The amplitude and frequency of DSJ can be adjusted through the driving voltage and driving frequency. In our research, the pressure measurements and flow visualization are conducted to test the control effectiveness of dual synthetic jet. According to the aerodynamic results, there is a correlation between the control efficiency and the amplitude and frequency of DSJ, which are determined by jet momentum coefficient and driving frequency. After the application of the free-end DSJ with moderate amplitude and frequency, the maximum reduction of total mean drag coefficients, fluctuating drag coefficients and fluctuating lift coefficients respectively reached 4.3%, 18.0% and 30.8%. Moreover, the smoke-wire and PIV results indicate that there is a remarkable suppression of the free-end separated shear flow and a prominent increase in the TKE in the area near the free end. All results point to that the aerodynamic forces and wakes of a surface-mounted finite-length square cylinder can be effectively controlled using dual synthetic jet at free end.

Eulerian and Lagrangian analysis of coherent structures in separated shear flow by time-resolved particle image velocimetry

We investigate the turbulent shear flow that separates from a two-dimensional backward-facing step. We aim to analyze the unsteady separated and reattaching shear flow in both the Eulerian and Lagrangian frameworks in order to provide complementary insight into the self-sustaining coherent structures and Lagrangian transport of the entrainment process. The Reynolds number is Reh = 1.0 × 103, based on the incoming free-stream velocity and step height. The separated and reattaching shear flow as well as the recirculation region beneath is measured by time-resolved planar particle image velocimetry. As a result, time sequences of velocity vector fields in a horizontal–vertical plane in the center of the step model are obtained. In the Eulerian approach, a set of temporally orthogonal dynamic modes are extracted, and each one represents a single-frequency vortex pattern that neutrally evolves in time. The self-sustaining coherent structures are represented by reduced-order reconstruction of the identified high-amplitude dynamic modes, showing the basic unsteady flapping motion of the shear layer and the vortex rolling-up, pairing, and shedding processes superimposed on it. On the other hand, trajectories of passive fluid tracers depict the Lagrangian fluid transport by the entrainment process in the separated shear flow and identify the time-dependent vortex rolling-up process as well as complex vortex interactions. The contours of the finite-time Lyapunov exponent reveal the unsteady Lagrangian coherent structures that significantly shape the vortex patterns and contribute substantial parts to the fluid entrainment in the shear flow.

Influence of upstream disturbances on the primary and secondary instabilities in a supersonic separated flow over a backward-facing step

The development of primary and secondary instabilities is investigated numerically for a supersonic transitional flow over a backward-facing step at Ma = 1.7 and Reδ0=13 718. Oblique Tollmien–Schlichting (T–S) waves with properties according to linear stability theory (LST) are introduced at the domain inlet with zero, low, or high amplitude (cases ZA, LA, and HA). A well-resolved large eddy simulation (LES) is carried out for the three cases to characterize the transition process from laminar to turbulent flow. The results for the HA case show a rapid transition due to the high initial disturbance level such that the non-linear interactions already occur upstream of the step, before the Kelvin–Helmholtz (K–H) instability could get involved. In contrast, cases ZA and LA share a similar transition road map in which transition occurs in the separated shear flow behind the step. Case LA is analyzed in detail based on the results from LST and LES to scrutinize the evolution of T–S, K–H, and secondary instabilities, as well as their interactions. Upstream of the step, the linear growth of the oblique T–S waves is the prevailing instability. Both T–S and K–H modes act as the primary mode within a short distance behind the step and undergo a quasi-linear growth with a weak coupling. Upon pairing of the large K–H vortices, subharmonic waves are produced, and secondary instabilities begin to dominate the transition. Simultaneously, the growth of T–S waves is retarded by the slow resonance between subharmonic K–H and secondary instabilities. The vortex breakdown and reattachment downstream further contribute to the development of the turbulent boundary layer.

Aerodynamics of a scale model of a high-speed train on a streamlined deck in cross winds

The cross wind aerodynamics of a high-speed train (HST) on a streamlined deck was investigated using sectional models in a wind tunnel. Aerodynamics of the HST-only model was also measured as a benchmark under similar conditions. The experimental results indicate that when the HST is on the downstream track of the deck it experiences significant changes in its aerodynamics depending on the angle of attack (α) of the oncoming flow. With an increase in α beyond zero, the mean drag and mean lift forces on HST begin to decrease or increase rapidly. This observation is attributed to the variation in the flow regime around HST from subcritical to critical although the approaching flow Reynolds number (based on the free-stream oncoming flow velocity U∞ and the height of train d) remains unchanged. The separated shear flow from the leading edge of the bridge impinges on the upstream shoulder of the HST on the downstream track at a critical orientation of the approach flow angle. The resulting accelerated flow in the shear layer, characterized by higher turbulence intensity, prompts the flow regime transition near the upstream shoulder of the HST without a change in the inflow Reynolds number. This change in the pressure field over the upstream shoulder of HST when it is on the downstream track on the bridge bears strong similarity to the changes experienced by circular cross-sections or cross-sections with rounded corners with a change in the inflow Reynolds number.

Investigations on the accurate prediction of supersonic shear layers for detached eddy simulation

One of the current problems of the detached eddy simulation (DES) and delayed DES (DDES) methods is the inaccuracy in prediction of the separated shear layer flows, which mainly reflects in the delayed development of the shear layer. To correctly simulate the separated shear flow, various modified definitions of the DDES length scale are investigated and the improved definition of the subgrid eddy viscosity is introduced to deal with the delayed development problem for the first time. In this paper, the shear layer without the upstream boundary layer is studied in order to assess the subgrid model of the DDES in the simulation of shear layers. Large eddy simulation (LES) and experiment results are introduced to test the performance of different definitions of the DDES parameters. According to the results, both the definitions of the length scale and subgrid eddy viscosity coefficient should be improved to get the best prediction. DDES with the LES length scale and vorticity-correlated length scale as well as the modified definition of the subgrid eddy viscosity coefficient predict a more accurate development of the shear layer than DDES with the original definitions, which provides a possible solution to the delay development problem of the shear layer. Furthermore, the LES mode of DDES performs considerably differently from the Smagorinsky-LES, which shows the former subgrid model is essentially different from that of Smagorinsky-LES. Theoretical analyses are made in order to find out the quantitative difference of the subgrid eddy viscosity between the LES mode of DDES and Smagorinsky-LES.

Bio-inspired cab-roof fairing of heavy vehicles for enhancing drag reduction and driving stability

Cab-roof fairing (CRF) has been developed to reduce the drag exerted on the forebody, thereby promoting fuel saving and environmental pollution control. Conventional CRFs with a 2D streamlined curvature have limitations in enhancing the driving stability and in reducing the drag at the forebody. In this study, new CRFs were designed with flow-guiding structures by mimicking the external forebody shape of sea lions. To evaluate the drag reduction effects of the proposed bio-inspired CRFs, three different types of CRFs (i.e., basic, bio-inspired, and advanced bio-inspired) were installed at scaled-down vehicle models of 15-ton (1:8) and 5-ton (1:6) heavy vehicles. The advanced bio-inspired CRF considerably reduced the drag coefficient of the 15-ton and 5-ton models by ∼20% and 22.4%, respectively. The side force was also reduced by up to 8% and 9% for the 15-ton and 5-ton models with the advanced bio-inspired CRF at a yaw angle of β = 3°, respectively. The flow characteristics around the forebody of the 15-ton model (1:15) with and without CRFs were analyzed by particle image velocimetry to elucidate the drag-reduction mechanism of the proposed CRFs. The bio-inspired CRFs significantly reduced the regions of separated shear flow and turbulent kinetic energy level on the side surfaces of the vehicle models. The findings provide useful information for improving the design of new forebody devices to reduce the drag and enhance the driving stability of heavy vehicles.

Experimental Investigation of Separated Shear Flow under Subharmonic Perturbations over a Backward-Facing Step

Subharmonic-perturbed shear flow downstream of a two-dimensional backward-facing step was experimentally investigated. The Reynolds number was Reh = 2.0 ×104, based on free-stream velocity and step height. Planar 2D-2C particle image velocimetry was employed to measure the separating and reattaching flow in the horizontal-vertical plane in the center position. The subharmonic perturbations were generated by an oscillating flap which was implemented over the step edge and driven by periodic Ampere force. The subharmonic frequency was 55 Hz as the half of the fundamental frequency of the turbulent shear layer. As a result of the subharmonic perturbations, the size of recirculation region behind the backward-facing step is reduced and the time-averaged reattachment length is 31.0% shorter than that of the natural flow. The evolution of vortices, including vortex roll-up, growth and breakdown process, is analyzed by using phase-averaging, cross-correlation function and proper orthogonal decomposition. It is found that Reynolds shear stress is considerably increased in which the vortices roll up and then break down further downstream. In particular, rapid growth of vortices based on the “step mode” occurs at approximate half of the recirculation region, caused by in interaction between the shear layer and the recirculation region. Furthermore, the coherent structures, which are represented by a phase-correlated POD mode pair, are reconstructed in phases in order to show regular patterns of the subharmonic-perturbed coherent structures.

Comparison of signal-based and POD-based phase-averaged Reynolds stress in a perturbed separated shear flow

Comparison of signal-based and POD-based phase-averaged Reynolds stress in a perturbed separated shear flow

Structure and dynamics of edge flames in the near wake of unequal merging shear flows

We examine in this study the structure and dynamic properties of an edge flame formed in the near-wake of two initially separated shear flows, one containing fuel and the other oxidiser. A comprehensive study is carried out within the diffusive-thermal framework where the flow field, computed a-priori, is used for the determination of the combustion field. Our focus is on the effects of three controlling parameters: the Damköhler number controlling the overall flow rate, the oxidiser-to-fuel strain rate ratio of the supply streams that determines the extent of oxidiser entrainment towards the mixing zone, and the Lewis number, assumed equal for the fuel and oxidiser, that depends on the mixture composition. Response curves, representing the edge flame standoff distance as a function of Damköhler number, exhibit two distinct shapes: C-shaped and U-shaped curves characterising the response of low and high Lewis number flames, respectively. Stability considerations show that the upper solution branch of the C-shaped response curve is unstable and hence corresponds to physically unrealistic states, but due to heat conduction toward the cold plate the lower solution branch is always stable. The states forming this solution branch correspond to flame attachment, where the edge flame remains practically attached to the tip of the plate until it is blown off by the flow when the velocity exceeds a critical value. The U-shaped response, on the other hand, consists of equilibrium states that are globally stable. Thus, high Lewis number flames can be always stabilised near the splitter plate, with the edge held stationary or undergoing a back and forth motion, or lifted and stabilised downstream by the flow. Insight into the distinct stabilisation characteristics, exhibited by the different Lewis number cases, is given by examining the relationship between the local flow velocity and the edge propagation speed.

Turbulent separated shear flow control by surface plasma actuator: experimental optimization by genetic algorithm approach

The potential benefits of active flow control are no more debated. Among many others applications, flow control provides an effective mean for manipulating turbulent separated flows. Here, a nonthermal surface plasma discharge (dielectric barrier discharge) is installed at the step corner of a backward-facing step (U 0 = 15 m/s, Re h = 30,000, Re θ = 1650). Wall pressure sensors are used to estimate the reattaching location downstream of the step (objective function #1) and also to measure the wall pressure fluctuation coefficients (objective function #2). An autonomous multi-variable optimization by genetic algorithm is implemented in an experiment for optimizing simultaneously the voltage amplitude, the burst frequency and the duty cycle of the high-voltage signal producing the surface plasma discharge. The single-objective optimization problems concern alternatively the minimization of the objective function #1 and the maximization of the objective function #2. The present paper demonstrates that when coupled with the plasma actuator and the wall pressure sensors, the genetic algorithm can find the optimum forcing conditions in only a few generations. At the end of the iterative search process, the minimum reattaching position is achieved by forcing the flow at the shear layer mode where a large spreading rate is obtained by increasing the periodicity of the vortex street and by enhancing the vortex pairing process. The objective function #2 is maximized for an actuation at half the shear layer mode. In this specific forcing mode, time-resolved PIV shows that the vortex pairing is reduced and that the strong fluctuations of the wall pressure coefficients result from the periodic passages of flow structures whose size corresponds to the height of the step model.

Flow-Excited Acoustic Resonance Excitation Mechanism, Design Guidelines, and Counter Measures

The excitation mechanism of acoustic resonances has long been recognized, but the industry continues to be plagued by its undesirable consequences, manifested in severe vibration and noise problems in a wide range of industrial applications. This paper focuses on the nature of the excitation mechanism of acoustic resonances in piping systems containing impinging shear flows, such as flow over shallow and deep cavities. Since this feedback mechanism is caused by the coupling between acoustic resonators and shear flow instabilities, attention is focused first on the nature of various types of acoustic resonance modes and then on the aeroacoustic sound sources, which result from the interaction of the inherently unstable shear flow with the sound field generated by the resonant acoustic modes. Various flow-sound interaction patterns are discussed, in which the resonant sound field can be predominantly parallel or normal to the mean flow direction and the acoustic wavelength can be an order of magnitude longer than the length scale of the separated shear flow or as short as the cavity length scale. Since the state of knowledge in this field has been recently reviewed by Tonon et al. (2011, “Aeroacoustics of Pipe Systems With Closed Branches”, Int. J. Aeroacoust., 10(2), pp. 201–276), this article focuses on the more practical aspects of the phenomenon, including various flow-sound interaction patterns and the resulting aeroacoustic sources, which are relevant to industrial applications. A general design guide proposal and practical means to alleviate the excitation mechanism are also presented. These are demonstrated by two examples of recent industrial case histories dealing with acoustic fatigue failure of the steam dryer in a boiling water reactor (BWR) due to acoustic resonance in the main steam piping and acoustic resonances in the roll posts of the Short Take-Off and Vertical Lift Joint Strike Fighter (JSF).

Turbulent shear flow downstream of a sphere with and without an o-ring located over a plane boundary

Flow-structure interaction of separated shear flow from the sphere and a flat plate was investigated by using dye visualization and the particle image velocimetry technique. Later, a passive control method was applied with 2mm o- ring located on the sphere surface at 55 o from front stagnation point. The experiments were carried out in open water channel for Reynolds number value of Re=5000. Flow characteristics have been examined in terms of the 2-D instantaneous and time-averaged velocity vectors, patterns of vorticity, streamlines, rms of velocity fluctuations and Reynolds stress variations and discussed from the point of flow physics, vortex formation, lengths of large-scale Karman Vortex Streets and Kelvin-Helmholtz vortices depending on the sphere locations over the flat plate. It is demonstrated that the gap flow occurring between the sphere bottom point and the flat plate surface has very high scouring effect until h/d=0.25 and then unsymmetrical flow structure of the wake region keeps up to h/D=1.0 for smooth sphere. For the sphere with o-ring, the wake flow structure becomes symmetrical at smaller gap ratios and reattachment point on the flat plate surface occurs earlier. Moreover, o-ring on the sphere diminishes peak magnitudes of the flow characteristics and thus it is expected that the flow-induced forces will be lessened both on the sphere and flat plate surface. Vortex formation lengths and maximum value occurring points become closer locations to the rear surface of the sphere with o-ring.

Visualization and laser measurements on the flow field and sand movement on sand dunes with porous fences

The installation of windbreak sand fences around sand dunes is one of the most promising methods to suppress windblown sand movement. In the study reported in this paper, we investigated the influence and validity of a small fence mounted on a model sand dune, in order to understand the fence’s suppression mechanism on the sand movement. The flow field around the dune and the process of sand-dune erosion were measured using LDV, PIV, and laser-sheet visualization techniques. A non-porous fence was found to suppress sand movements in its upstream area, but to enhance erosion downstream of the fence. This intensive erosion was caused by separated shear flow from the leading edge of the fence. In this study, four levels of porosity rate of the fence were tested. The fence-porosity dependences of the turbulent flow field and the erosion were discussed. The shapes of eroded sand dunes were found to depend on the porosity rate. The relationship between the sand-dune erosion and the flow field around the dune was illustrated with schematic diagrams. We concluded that the most desirable fence porosity should be 30% in order to avoid dune erosion if installed at a middle height on the stoss surface of a dune. This porosity provides a mean velocity reduction with avoiding a separated flow, although the flow bleeding through the porous fence is accompanied by grid turbulence and induces serious erosion in a narrow space behind the fence. Furthermore, we confirmed that the empirical correlation of the critical friction velocity can be applied to sand movements influenced by a fence.

수직벽 전방에서의 흡입/토출을 이용한 후류제어 연구

The effect of periodic blowing and suction of upstream flow on the separated shear flow behind the vertical fence was experimentally investigated. The fence was submerged in the turbulent shear flow and DPIV method was used to measure the instantaneous velocity fields around the fence. Periodic blowing and suction flow was precisely generated by the syringe pump. Spanwise nozzle made 2D planar periodic jet flow in front of the fence and the effect of frequency and maximum jet velocity was studied. From the results, the reattachment length can be reduced by 60% of uncontrolled fence case under the control.

303 HPFモデルによるはく離せん断流れのLES(OS5-1 乱流現象の実験と数値シミュレーション1(計測・計算手法),OS5 乱流現象の実験と数値シミュレーション)

303 HPFモデルによるはく離せん断流れのLES(OS5-1 乱流現象の実験と数値シミュレーション1(計測・計算手法),OS5 乱流現象の実験と数値シミュレーション)