Flow Unsteadiness Research Articles

Analyses for the Second Aeroelastic Prediction Workshop Using the EZNSS Code

The paper presents analyses that were performed with the EZNSS flow solver for the second Aeroelastic Prediction Workshop. The reference test cases for the Aeroelastic Prediction Workshop are based on two wind-tunnel experiments of the Benchmark Supercritical Wing, including a flutter test and forced excitation tests. Three cases are addressed at different transonic flow conditions: two cases at lower Mach numbers of 0.7 and 0.74 and 3 and 0 deg angles of attack, respectively; and one more physically complex case at Mach 0.85 and a 5 deg angle of attack. The cases are analyzed with the EZNSS code, using several computational setups and turbulence models. The simulations are able to predict relatively accurately the flutter response and the response to prescribed motion at the lower Mach number. The higher-Mach-number case, which involves a strong shock, separated flow behind the shock, and some flow unsteadiness, is more challenging. In the static analysis, different turbulence models yield different upper-surface shock positions, and none of the models are able to capture accurately the pressure recovery behind the shock. However, the unsteady aerodynamic response to prescribed pitch motion is simulated with good correlation to the wind-tunnel data. The paper also presents flutter predictions based on unsteady aerodynamic reduced-order modeling, thus validating and assessing the efficiency of the reduced-order modeling and the flutter prediction methodology.

The failure of delayed detached eddy simulation for flow control over a hump using synthetic jet actuator

The performance of three different numerical methods for predicting the large separated flow over a hump model and its control using steady suction and techniques is investigated. Unsteady Reynolds Averaged Navier-Stokes (URANS), Detached Eddy Simulation (DES) and Delayed Detached Eddy Simulation (DDES) approaches are developed and applied to determine the proficiency of these approaches in predicting such degree of separation and assess their applicability to capture the turbulent flow structures in the separated flowfield. Active flow control techniques are applied by means of two-dimensional slot located at x/c -0.65 along the model spanwise with an opening of 0.00187c, directly upstream on the concave surface. The novelty of the present work is that the dynamic grid technique is formulated to model the repeated motion of the synthetic jet membrane instead of using the oscillating boundary conditions at the jet exit. Dynamic grid is found to be useful in providing more insight into the physical mechanism that drive the interface between the plenum flow and separated boundary layer flow. The numerical results obtained from the baseline and controlled cases are validated against the measurements and compared with published numerical results in terms of mean flow quantities. Despite the fact that URANS and DES methods provide good agreement with the measurements for the uncontrolled and steady suction controlled cases, only DES showed the flow unsteadiness, however the DDES method failed to predict the mean flow quantities or the turbulent flow features. The dynamic grid technique successfully simulated the cavity flow, showed up the existence of two counter rotating vortices inside the plenum move up and down during the control cycle and finally energized the boundary layer in the separated flow region.

Suppressing unsteady flow in arterio-venous fistulae

Arterio-Venous Fistulae (AVF) are regarded as the “gold standard” method of vascular access for patients with end-stage renal disease who require haemodialysis. However, a large proportion of AVF do not mature, and hence fail, as a result of various pathologies such as Intimal Hyperplasia (IH). Unphysiological flow patterns, including high-frequency flow unsteadiness, associated with the unnatural and often complex geometries of AVF are believed to be implicated in the development of IH. In the present study, we employ a Mesh Adaptive Direct Search optimisation framework, computational fluid dynamics simulations, and a new cost function to design a novel non-planar AVF configuration that can suppress high-frequency unsteady flow. A prototype device for holding an AVF in the optimal configuration is then fabricated, and proof-of-concept is demonstrated in a porcine model. Results constitute the first use of numerical optimisation to design a device for suppressing potentially pathological high-frequency flow unsteadiness in AVF.

Pulsating flow past a tube bundle

Visualization of the pulsating cross-flow past the in-line and staggered tube bundles has been performed. The frequency and amplitude of forced flow pulsations and the tube pitch in the bundle varied in the experiments. The main attention was focused on the flow pattern in the near wake of the third-row tube. The most indicative regimes of flow past a tube in a bundle have been revealed depending on forced flow unsteadiness parameters. The obtained data have been generalized in the flow maps in the space of dimensionless frequency (Strouhal number, St) and relative pulsation amplitude, β, individually for the in-line and staggered tube arrangement. Three most indicative regimes of pulsating flow past the tubes in a bundle have been singled out in each flow map.

Commencement and Development Processes of Flow Unsteadiness at Tip Clearance Region of a Low Speed Axial Compressor Rotor Blade Row

Commencement and development processes of unsteadiness, caused by blade row tip leakage flow in a low speed axial compressor, are investigated and results are presented in this paper. Analyses are based on results obtained through numerical simulation of unsteady three dimensional viscous flows. Discretization of the Navier-Stokes’s equations has been carried out based on upwind second-order scheme and k-ω-SST turbulence modeling was used for estimation of eddy viscosity.Three different circumstances, including design point and two near stall conditions are considered for investigation and discussion. Tip leakage flow frequency spectrums were examined through surveying instantaneous static pressure signals imposed on the blades surfaces. Focusing on time dependent flow structure results signified existence of some pressure peaks at near stall conditions. These regions, which are created as a result of interaction between main inflow and tip leakage flow, lead to occurrence of self-induced unsteadiness. However, at design condition, flow is more affected by the main inflow instead of the tip leakage flow. By occurrence of self-induced unsteadiness, which occurs at near stall condition, tip leakage vortex flow starts to fluctuate at a frequency about the blade passing frequency. Further decrease in the flow rate up to a specified value showed no significant variations in the leakage flow frequency, but, on the other hand, magnified amplitudes of this unsteadiness.

Flow Unsteadiness and Stability Characteristics of Low-Re Flow Past an Inclined Triangular Cylinder

The present study investigates the two-dimensional flow past an inclined triangular cylinder at Re = 100. Numerical simulation is performed to explore the effect of cylinder inclination on the aerodynamic quantities, unsteady flow patterns, time-averaged flow characteristics, and flow unsteadiness. We also provide the first global linear stability analysis and sensitivity analysis on the targeted physical problem for the potential application of flow control. The objective of this work is to quantitatively identify the effect of cylinder inclination on the characteristic quantities and unsteady flow patterns, with emphasis on the flow unsteadiness and instability. Numerical results reveal that the flow unsteadiness is generally more pronounced for the base-facing-like cylinders (α → 60 deg) where separation occurs at the front corners. The inclined cylinder reduces the velocity deficiency in the near-wake, and the reduction in far-wake is the most notable for the α = 30 deg cylinder. The transverse distributions of several quantities are shifted toward the negative y-direction, such as the maximum velocity deficiency and maximum/minimum velocity fluctuation. Finally, the global stability and sensitivity analysis show that the spatial structures of perturbed velocities are quite similar for α ≤ 30 deg and the temporal growth rate of perturbation is sensitive to the near-wake flow, while for α ≥ 40 deg there are remarkable transverse expansion and streamwise elongation of the perturbed velocities, and the growth rate is sensitive to the far-wake flow.

The Influence of Sniffing on Airflow and Odorant Deposition in the Canine Nasal Cavity.

Nasal airflow plays a critical role in olfaction by transporting odorant from the environment to the olfactory epithelium, where chemical detection occurs. Most studies of olfaction neglect the unsteadiness of sniffing and assume that nasal airflow and odorant transport are "quasi-steady," wherein reality most mammals "sniff." Here, we perform computational fluid dynamics simulations of airflow and odorant deposition in an anatomically accurate model of the coyote (Canis latrans) nasal cavity during quiet breathing, a notional quasi-steady sniff, and unsteady sniffing to: quantify the influence of unsteady sniffing, assess the validity of the quasi-steady assumption, and investigate the functional advantages of sniffing compared to breathing. Our results reveal that flow unsteadiness during sniffing does not appreciably influence qualitative (gross airflow and odorant deposition patterns) or quantitative (time-averaged olfactory flow rate and odorant uptake) measures of olfactory function. A quasi-steady approximation is, therefore, justified for simulating time-averaged olfactory function in the canine nose. Simulations of sniffing versus quiet breathing demonstrate that sniffing delivers about 2.5 times more air to the olfactory recess and results in 2.5-3 times more uptake of highly- and moderately-soluble odorants in the sensory region per unit time, suggesting one reason why dogs actively sniff. Simulations also reveal significantly different deposition patterns in the olfactory region during inspiration for different odorants, and that during expiration there is little retronasal odorant deposition in the sensory region. These results significantly improve our understanding of canine olfaction, and have several practical implications regarding computer simulation of olfactory function.

Study of the flow unsteadiness in the human airway using large eddy simulation

Large-eddy simulation is used in a lung geometry to study flow in the bronchial tree. After becoming unsteady at a constriction in the oropharynx, the flow turns chaotic, exhibiting fluctuations with broadband spectra even at the most distal simulated airways with Reynolds numbers as low as 300.

Ability of URANS approach in prediction of unsteady turbulent flows in an unbaffled stirred tank

Three-dimensional, unsteady Navier-Stokes equations are numerically solved to investigate the turbulent flows in a stirred vessel. The computational domain consists of an unbaffled, cylindrical vessel with a pitched-blade turbine impeller. An Eulerian-Eulerian multiphase flow model is applied to determine the shape of the free-surface vortex core. This numerical method is validated by comparing its results with laser Doppler velocimetry measurements in terms of velocity distribution and turbulence kinetic energy profiles at different positions. In the present study, URANS approach with a hybrid zonal turbulence model, k−ω SST and SST-SAS, is used to predict the unsteady pressure and velocity fluctuations within the vessel. Pressure and inward-outward radial velocity waves are generated by the impeller rotation and are captured in the time/space domain close to the impeller trailing edge. The pressure and velocity spectra are computed to characterize the blade passing frequency as the main source of unsteadiness in the turbulent flow within the vessel. The results indicate that the current URANS approach with a proper turbulence model and well-resolved grids can be used as a predictive tool for the flow field and large turbulence scales in the stirred tanks.

Numerical Investigation for the Impact of Single Groove on the Stall Margin Improvement and the Unsteadiness of Tip Leakage Flow in a Counter-Rotating Axial Flow Compressor

A low-speed counter-rotating axial flow compressor (CRAC) with single circumferential grooved casing treatment (CT) was investigated numerically. Both steady and time-accurate numerical calculations were performed to study the effects of the single grooved CTs over the rear rotor on the stability enhancement and the unsteadiness of tip leakage flow (TLF) in the CRAC. Parametric studies indicate that the best position of the single groove should be located near about 20% axial tip chord in terms of the stall margin improvement (SMI). The coincidence of the effective CT locations and the high fluctuating region on blade pressure surface in the smooth wall case shows that the unsteadiness of TLF plays an important role in the stall inception process. Frequency analysis for the static pressure signals near the blade tip shows that both the disappearance of the low frequency components and the suppression of unsteady TLF are beneficial to the SMI. Detailed observation of the flow structures illustrates that the action of the grooves on the different parts of TLF is responsible for the difference of SMI in the CTs. It is more effective to improve the flow stability by controlling the critical TLF released from near the mid-chord.

Computing the sensitivity of drag and lift in flow past a circular cylinder: Time-stepping versus self-consistent analysis

We provide in-depth scrutiny of two methods making use of adjoint-based gradients to compute the sensitivity of drag in the two-dimensional, periodic flow past a circular cylinder (Re≲189): first, the time-stepping analysis used in Meliga et al. [Phys. Fluids 26, 104101 (2014)] that relies on classical Navier-Stokes modeling and determines the sensitivity to any generic control force from time-dependent adjoint equations marched backwards in time; and, second, a self-consistent approach building on the model of Mantic-Lugo et al. [Phys. Rev. Lett. 113, 084501 (2014)] to compute semilinear approximations of the sensitivity to the mean and fluctuating components of the force. Both approaches are applied to open-loop control by a small secondary cylinder and allow identifying the sensitive regions without knowledge of the controlled states. The theoretical predictions obtained by time-stepping analysis reproduce well the results obtained by direct numerical simulation of the two-cylinder system. So do the predictions obtained by self-consistent analysis, which corroborates the relevance of the approach as a guideline for efficient and systematic control design in the attempt to reduce drag, even though the Reynolds number is not close to the instability threshold and the oscillation amplitude is not small. This is because, unlike simpler approaches relying on linear stability analysis to predict the main features of the flow unsteadiness, the semilinear framework encompasses rigorously the effect of the control on the mean flow, as well as on the finite-amplitude fluctuation that feeds back nonlinearly onto the mean flow via the formation of Reynolds stresses. Such results are especially promising as the self-consistent approach determines the sensitivity from time-independent equations that can be solved iteratively, which makes it generally less computationally demanding. We ultimately discuss the extent to which relevant information can be gained from a hybrid modeling computing self-consistent sensitivities from the postprocessing of DNS data. Application to alternative control objectives such as increasing the lift and alleviating the fluctuating drag and lift is also discussed.

Vortex-induced vibrations of a cylinder in planar shear flow

The system composed of a circular cylinder, either fixed or elastically mounted, and immersed in a current linearly sheared in the cross-flow direction, is investigated via numerical simulations. The impact of the shear and associated symmetry breaking are explored over wide ranges of values of the shear parameter (non-dimensional inflow velocity gradient, $\unicode[STIX]{x1D6FD}\in [0,0.4]$) and reduced velocity (inverse of the non-dimensional natural frequency of the oscillator, $U^{\ast }\in [2,14]$), at Reynolds number $Re=100$; $\unicode[STIX]{x1D6FD}$, $U^{\ast }$ and $Re$ are based on the inflow velocity at the centre of the body and on its diameter. In the absence of large-amplitude vibrations and in the fixed body case, three successive regimes are identified. Two unsteady flow regimes develop for $\unicode[STIX]{x1D6FD}\in [0,0.2]$ (regime L) and $\unicode[STIX]{x1D6FD}\in [0.2,0.3]$ (regime H). They differ by the relative influence of the shear, which is found to be limited in regime L. In contrast, the shear leads to a major reconfiguration of the wake (e.g. asymmetric pattern, lower vortex shedding frequency, synchronized oscillation of the saddle point) and a substantial alteration of the fluid forcing in regime H. A steady flow regime (S), characterized by a triangular wake pattern, is uncovered for $\unicode[STIX]{x1D6FD}>0.3$. Free vibrations of large amplitudes arise in a region of the parameter space that encompasses the entire range of $\unicode[STIX]{x1D6FD}$ and a range of $U^{\ast }$ that widens as $\unicode[STIX]{x1D6FD}$ increases; therefore vibrations appear beyond the limit of steady flow in the fixed body case ($\unicode[STIX]{x1D6FD}=0.3$). Three distinct regimes of the flow–structure system are encountered in this region. In all regimes, body motion and flow unsteadiness are synchronized (lock-in condition). For $\unicode[STIX]{x1D6FD}\in [0,0.2]$, in regime VL, the system behaviour remains close to that observed in uniform current. The main impact of the shear concerns the amplification of the in-line response and the transition from figure-eight to ellipsoidal orbits. For $\unicode[STIX]{x1D6FD}\in [0.2,0.4]$, the system exhibits two well-defined regimes: VH1 and VH2 in the lower and higher ranges of $U^{\ast }$, respectively. Even if the wake patterns, close to the asymmetric pattern observed in regime H, are comparable in both regimes, the properties of the vibrations and fluid forces clearly depart. The responses differ by their spectral contents, i.e. sinusoidal versus multi-harmonic, and their amplitudes are much larger in regime VH1, where the in-line responses reach $2$ diameters ($0.03$ diameters in uniform flow) and the cross-flow responses $1.3$ diameters. Aperiodic, intermittent oscillations are found to occur in the transition region between regimes VH1 and VH2; it appears that wake–body synchronization persists in this case.

Numerical investigation on unsteady refraction of planar shock wave at air/SF$_6$ gas interface

The unsteady refractions of a planar shock wave at a concave (convex) air/SF$_6$ interface are numerically studied. The finite difference method is utilized to solve the compressible multi-component Euler equations. Pressure oscillation near the interface due to the difference of the specific heat ratio is successfully overcome using Double-Flux algorithm. High temporal and spatial resolutions are achieved through combining the Double-Fluid algorithm with 5th WENO and 3rd Runge-Kutta.Different from pseudo-steady refraction, the continuous change of incident angle when a planar shock wave moves at a concave or convex interface will result in the transitions between different refraction wave patterns. It turns out that when the incident angle changes from 90$^\circ$ to 0$^\circ$ during the shock propagating along the concave interface, the refraction patterns of concave-forwards refraction (CFR), regular refraction with reflected shock (RRR) and transitioned regular refraction with reflected shock (TRRR) are observed in sequence. Compared with pseudo-steady refraction, the Mach type irregular refraction (MRR) is not observed while a new refraction pattern of TRRR occurs, which is possibly caused by hypothesis effect of the flow. With the incident angle increasing from 0$^\circ$ to 90$^\circ$ when the shock moves along the convex interface, only two refraction types of RRR and MRR are captured, and CFR, which occurs under pseudo-steady condition, does not occur. However, at the late stage, the Mach stem becomes curved and resembles the incident shock wave near the interface in CFR. Besides, the critical angle when RRR transits into MRR is delayed while the range of MRR is extended. From this work, it is shown that the flow unsteadiness has a significant influence on the refraction wave pattern.

Impact of hydropeaking on downstream fish habitat at the Goesan Dam in Korea

AbstractThe impact of hydropeaking on downstream fish habitat was investigated. The study area is a 2.3‐km long reach of the Dal River, located downstream from the Goesan Dam, Korea. Such short‐term effects of hydropeaking as flow unsteadiness, down‐ramping rate, transport of benthic macro‐invertebrate, and changes in composite suitability index (CSI) and weighted usable area (WUA) were examined. To assess the change in CSI and WUA, physical habitat simulations were carried out. The 2D unsteady flow model and the habitat suitability index model were used for hydraulic and habitat simulations, respectively. Zacco platypus was selected as a target species of the study area. Hydraulic simulations showed that flow depth changes rapidly with time at the rising and falling stages of discharge due to hydropeaking, and this impact propagates with time. The down‐ramping rate was found to decrease gradually in the downstream direction, implying that the fish can be stranded in the 1.5‐km upstream reach. In addition, simulation results indicate that the transport of macro‐invertebrate may occur in the riffle, and the hydropeaking flows increase slightly the area of possible transport of macro‐invertebrate compared with the normal flow. However, catastrophic drift will not occur due to hydropeaking flows. The results of physical habitat simulations revealed that the hydropeaking flows decrease the CSI, although the river morphology affects the CSI to some extent. Regarding the WUA, the overall habitat quality in the entire study reach was found to decrease due to hydropeaking.

Heat Transfer Enhancements from Elastic Turbulence Using Sucrose-Based Polymer Solutions

The influences of elastic turbulence on convective heat transfer, within a C-shaped passage of a viscous disk pump, are experimentally determined using viscoelastic fluids and Boger fluids, which are constant-viscosity Newtonian solvents. Different concentrations of polyacrylamide in 65% sucrose solutions are used, along with solutions with 65% sucrose only, as different magnitudes of shear stress and strain rate are imposed upon the flowfield. Transition and development of elastic turbulence are characterized, along with convective heat transfer enhancements. The resulting increased levels of mixing, transport, and diffusion from elastic turbulence give convective heat transfer coefficient enhancements that are as large as 240%, relative to the Newtonian, Boger fluids at the same shear rate, rotation speed, flow passage height, and flow temperature. Variations of spectra of static temperature fluctuations and mean-square magnitudes of fluctuating static temperature provide evidence of increased flow irregularities and unsteadiness (relative to Boger solution flows), which result from elastic turbulence-induced polymer twisting, convolutions, and interactions. Such observed enhancements are shown to not be due to increased viscosity values alone, nor to centrifugal instabilities associated with streamline curvature, because Dean numbers are not large enough to allow development of the associated centrifugal-instability induced secondary flows.

Numerical investigation of flow unsteadiness and heat transfer on suction surface of rotating airfoils within a gas turbine cascade

The effects of the periodical turbulence and pressure fluctuation on suction surface heat transfer over airfoils of a row of rotor blades with a certain type have been investigated numerically in this paper. The calculation is performed using ν2¯−f model with the numerical results of pressure fluctuation and heat transfer performance over 4 sample points being analyzed and compared with existing experimental data. It shows that the static pressure change has significant impact on heat transfer performance of the fore suction surface, especially in the active region of the shock waves formed from the trailing edge of upstream nuzzles. While, for the rear suction surface, the flow turbulence contributes more to the heat transfer change over the surface, due to the reduced pressure oscillation through this region. Phase shifted phenomenon across the surface can be observed for both pressure and heat transfer parameters, which should be a result of turbulence migration and wake passing across the airfoil.

High-order discontinuous Galerkin method for applications to multicomponent and chemically reacting flows

This article focuses on the development of a discontinuous Galerkin (DG) method for simulations of multicomponent and chemically reacting flows. Compared to aerodynamic flow applications, in which DG methods have been successfully employed, DG simulations of chemically reacting flows introduce challenges that arise from flow unsteadiness, combustion, heat release, compressibility effects, shocks, and variations in thermodynamic properties. To address these challenges, algorithms are developed, including an entropy-bounded DG method, an entropy-residual shock indicator, and a new formulation of artificial viscosity. The performance and capabilities of the resulting DG method are demonstrated in several relevant applications, including shock/bubble interaction, turbulent combustion, and detonation. It is concluded that the developed DG method shows promising performance in application to multicomponent reacting flows. The paper concludes with a discussion of further research needs to enable the application of DG methods to more complex reacting flows.

Unsteady flow structures in the tip region for a centrifugal compressor impeller before rotating stall

To get an insight into the occurrence and the mechanism of flow unsteadiness in the tip region of centrifugal compressor impellers, the flow in Krain’s impeller is investigated by using both steady and unsteady RANS solver techniques. It is found that the flow unsteadiness on the pressure side is much stronger than that on the suction side. The periodical frequency of the unsteady flow is around half of the blade passing frequency. The originating mechanism of the flow unsteadiness is illustrated with the time-dependent tip leakage flow and blade loading at the tip region. Due to the blockage caused by the joint effects of broken-downed tip leakage vortex, separated fluids and tip leakage flow at downstream, a low pressure region is formed on the pressure side, consequently the blade loadings is altered. In turn, the changed blade loadings will alter the intensity of tip leakage flow. Such alternative behavior finally results in the periodic process. By comparing the calculated flow field in the cases of single-passage and four-passage models, it is confirmed that the investigated flow unsteadiness is confined in each single passage, as no phase differences are found in the model of four passages. This is different from the situation in axial compressor when the rotating instability is encountered. The flow unsteadiness only occurs at the working conditions with small mass flow rates, and the oscillation intensity will be enhanced with the decrease of mass flow rate. When the mass flow rate is too small, the flow unsteadiness in a single passage may trigger rotating stall, as the disturbance propagates in the circumferential direction.

Efficient suction control of unsteadiness of turbulent wing-plate junction flows

A wing-body junction flow of a navigating underwater vehicle is considered to be a crucial source of the flow radiating acoustic noise, which attracts much research interest. In this paper, wing-plate junction flows are experimentally investigated in a low-speed wind tunnel by smoke-wire flow visualizations and time-resolved PIV measurements. To reveal the physical behavior of such flows, smoke-wire flow visualizations are conducted for a laminar wing-plate junction. A novel control strategy is proposed, to accurately locate the suction openings where the streamline is about to roll up to form a vortex in the turbulent junction flows. The control effect is discussed in perspectives of both the time-averaged and instantaneous flow fields.

Technical Note: Monitoring of unsteady open channel flows using the continuous slope-area method

Abstract. The advent of low-cost pressure transducers capable of directly measuring water surface elevation enables continuous measurements of dynamic water surface slopes. This opens up a new possibility of dynamically monitoring unsteady flows (i.e., hysteresis) during the course of flood wave propagation. Hysteresis in this context refers to a looped stage–discharge rating caused by unsteadiness of flows. Hysteresis is monitored in this study using a continuous slope area (CSA) method, which uses Manning's equation to calculate unsteady discharges based on continuously measured water surface slopes. In the rising stage, water surface slopes become steeper than a steady water surface slope, resulting in higher discharges than steady-based discharges, while the trends are reversed in the falling stage. The CSA method applied to Clear Creek near Oxford (Iowa, USA) estimates the maximum differences of peak discharges by 30–40 %, while it shows sound agreements for a low to medium range of discharges against USGS steady-based records. The primary cause of these differences is the use of a single channel bed slope in deriving Manning's roughness coefficients. The use of a single channel bed slope (conceptually equal to the water surface slopes at every stage in uniform flow conditions) causes substantial errors in estimating the channel roughness, specifically at high stages, because non-uniformities of natural channels result in varying (non-uniform) steady water surface slopes at each stage. While the CSA method is promising for dynamically tracking unsteady water surface slopes and flows in natural streams, more studies are still needed to increase the accuracy of the CSA method in future research.