Large Secondary Flow Research Articles

Effect of straight riblets of the underlying surface on wall bounded flow drag

Direct numerical simulations of thin, straight riblets were conducted to investigate the physical mechanisms involved when the inertial effects within the ribs become important. The main parameter that we assess here is the ribs’ height to spacing ratio (a=h/s). In all cases, ‘k’-type behaviour was observed despite the absence of the pressure drag component. This trend also refers to the destruction of the near-wall cycles and was indicated by the shortening of Reynolds stress structures, mimicking the flows over rough walls. Closely-packed ribs with large h/s have a tendency to generate Kelvin–Helmholtz (KH) like structures at the tips, inducing large pressure fluctuations. Conversely, sparser ribs with low h/s trigger the formation of large mean secondary flows that suppress the pressure fluctuations. We further split the drag contribution of these distinct flow features by decomposing the roughness function into the dispersive and the Reynolds-stress terms. This analysis concludes that the fully-rough (logarithmic) scaling within the drag-increasing regime is transitional, and as the Reynolds number increases, deviates to a more gradual power-law scaling due to the drag increasing mechanism of streamwise aligned secondary motions.

Effects of corrugation cross section shape on thermal-hydraulic performance in outward corrugated tube

The corrugation cross-section shape is an important parameter that affects the thermal-hydraulic performance of corrugated tubes. In this study, the three-dimensional models of corrugated tubes are established to study the influence of corrugation cross-section shapes on the thermal-hydraulic performance of corrugated tubes in the range of 10,000–40,000 Reynolds numbers. The influence mechanism of corrugation cross-section shapes on thermal-hydraulic performance is analyzed in terms of the distributions of flow field, turbulence characteristics and temperature, etc. The research results point out that the corrugated tube with a smooth corrugation inlet cross-section shape and a large enough secondary flow zones can provide better thermal-hydraulic performance. Conversely, the steeper corrugation outlet cross-section shape is beneficial to induce a stronger impact between the fluid and the corrugation wall, thereby improving the thermal-hydraulic performance of the corrugated tube. Besides, the maximum THP = 1.41 is obtained at Re = 40,000 for Case 3 with R1 = 24.8 mm and R2 = 1.5 mm.

The Influence of Fan Root Flow on the Aerodynamic of a Low-Pressure Compressor Transition Duct

Abstract To reduce fuel-burn and emissions, there is a drive toward higher bypass ratio and smaller high-pressure ratio core engines. This makes the design of the ducts connecting compressor spools more challenging as the higher radius change increases aerodynamic loading. This is exacerbated at inlet to the engine core by fan root flow which is characterized by a hub-low-pressure profile and large secondary flow structures. Additionally, shorter, lighter nacelles mean that the intake may not provide a uniform inlet flow when the aircraft is at an angle of attack or subject to cross winds. Such inlet distortion can further degrade the flow entering the engine. A combination of experiments and computational fluid dynamics (CFD) has been used to examine the effects on the aerodynamics of an engine section splitter (ESS) and transition duct designed to feed the low-pressure spool of a high bypass ratio turbofan. A test facility incorporating a 1½ stage axial compressor was used to compare system performance for a flat rotor exit profile to one with a hub deficient flow. Validated Reynolds averaged Navier–Stokes (RANS) CFD was then used to further investigate the effects of increased inlet boundary layer thickness and bulk swirl distortion at rotor inlet. These changes were seen to have a surprisingly small effect on the flow at the duct exit. However, increased secondary flows were observed which degraded the performance of the ESS and significantly increased loss. Nevertheless, the enhanced mixing delayed separation in the duct suggesting that overall the design was reasonably robust albeit with increased system loss.

In vitro 4D Flow MRI evaluation of aortic valve replacements reveals disturbed flow distal to biological but not to mechanical valves.

Aortic hemodynamics influence the integrity of the vessel wall and cardiac afterload. The aim of this study was to compare hemodynamics distal to biological (BV) and mechanical aortic valve (MV) replacements by in vitro 4D Flow MRI excluding confounding factors of in-vivo testing potentially influencing hemodynamics. Two BV (Perimount MagnaEase [Carpentier-Edwards], Trifecta [Abbott]) and two MV (On-X [CryoLife], prototype trileaflet valve) were scanned in a flexible aortic phantom at 3T using a recommended 4D Flow MR sequence. A triphasic aortic flow profile with blood-mimicking fluid was established. Using GTFlow (Gyrotools), area and velocity of the ejection jet were measured. Presence and extent of sinus vortices and secondary flow patterns were graded on a 0 to 3 scale. A narrow, accelerated central ejection jet (Area = 27 ± 7% of vessel area, Velocity = 166 ± 13 cm/s; measured at sinotubular junction) was observed in BV as compared to MV (Area = 53 ± 13%, Velocity = 109 ± 21 cm/s). As opposed to MV, the jet distal to BV impacted the outer curvature of the ascending aorta and resulted in large secondary flow patterns (BV: n = 4, grades 3, 3, 2, 1; MV: n = 1, grade 1). Sinus vortices only formed distal to MV. Although physiologically configured, they were larger than normal (grade 3). In contrast to mechanical valves, biological valve replacements induced accelerated and increased flow patterns deviating from physiological ones. While it remains speculative whether this increases the risk of aneurysm formation through wall shear stress changes, findings are contrasted by almost no secondary flow patterns and typical, near-physiological sinus vortex formation distal to mechanical valves.

A comparison of numerical investigations on the flow and heat transfer characteristics in the rotor-stator cavity

This paper presents an application of conjugate CFD and fully coupled FE/CFD to investigate the flow and heat transfer characteristics in the rotor-stator disc cavity. The results are compared with that predicted by the steady-state flow analysis and experimental data. The commercial solvers of System Coupling module integrated in ANSYS Workbench and CFX 15.0 coupled with standard k-ε turbulence model are adopted. The distributions of pressure, radial velocity, temperature and heat transfer coefficient in the rotor-stator disc cavity are discussed in detail. The results obtained in this study indicate that in the case of low secondary air flow, the predicted pressure from fully coupled FE/CFD shows a good agreement with the experimental data while the other two numerical methods provide a slight deviation from the experiment. With large secondary air flow, the pressure distributions predicted numerically are well matched with the measured values. The steady-state flow and conjugate CFD could predict well the thermal boundary near the rotor surface while the fully coupled FE/CFD overpredicts the thickness of thermal boundary. The fully coupled FE/CFD predicts well the heat transfer coefficients with the experimental data.

On the turbulent heat transfer in a square duct subjected to spanwise system rotation

In this paper, direct numerical simulation is performed to study the effects of the Coriolis force on turbulent heat transfer within a square duct subjected to spanwise system rotation. In order to investigate the influence of system rotation on the heat transfer process, a wide range of rotation numbers have been tested. In response to the system rotation, mean secondary flows appear as large streamwise counter-rotating vortices, which interact intensely with the four boundary layers of the square duct and have a significant impact on the statistics of the coupled velocity and temperature fields. It is observed that at a sufficiently high rotation number, heat convection approaches a complete laminarization state near the top and side walls. The influence of large organized secondary flows on the transport of the turbulent scalar energy of the temperature field has been analyzed in both physical and spectral spaces. The effects of system rotation on the turbulent heat fluxes have been examined through a budget analysis of their transport equations. The role of near-wall streamwise-elongated turbulence structures on heat transfer is studied using a linear stochastic estimation approach. It is observed that the near-wall ejection events are amplified by the system rotation, resulting in a strong negatively-valued vertical turbulent heat flux in the near-wall region.

A Recollection with Respect to Flow Induced Crystallization in Polymer Melt Processing

Abstract A review is given of the work in the field of flow induced crystallization in polymer melts, as has been carried out during the last twenty five years mostly in Linz. A concise discussion of some results of a recent monograph is given. In this connection polymer melt rheology provides insights into the differences of information, as provided by shear and extensional flow. These differences show up with increased degrees of deformation. One finds four important pictures in this report: An electron micrograph of the crystalline structure in a cross-section perpendicular to the previous flow direction. No branching is found of the tracks, which are lines of intersection with the plane of the cross-section. A ring-shaped sample of polypropylene is shown, which fits into a newly developed apparatus. Because of the large radius secondary flow is avoided. A picture, which was obtained already a long time ago, shows that the relaxation of flow induced structures occurs with polypropylene only at temperatures between 210 and 190°C. Below the latter temperature no relaxation of crystalline structures occurs. The sharp boundary between a highly oriented zone near the duct wall and the spherulitic structure in the interior of samples, which have been extruded through ducts of rectangular cross-section, can readily be explained.

Analysis of the coherent and turbulent stresses of a numerically simulated rough wall pipe

A turbulent rough wall flow in a pipe is simulated using direct numerical simulation (DNS) where the roughness elements consist of explicitly gridded three-dimensional sinusoids. Two groups of simulations were conducted where the roughness semi-amplitude h+ and the roughness wavelength λ+ are systematically varied. The triple decomposition is applied to the velocity to separate the coherent and turbulent components. The coherent or dispersive component arises due to the roughness and depends on the topological features of the surface. The turbulent stress on the other hand, scales with the friction Reynolds number. For the case with the largest roughness wavelength, large secondary flows are observed which are similar to that of duct flows. The occurrence of these large secondary flows is due to the spanwise heterogeneity of the roughness which has a spacing approximately equal to the boundary layer thickness δ.

Rapid mixing by turbulent-like electrokinetic microflow

Rapid mixing has been achieved in a pressure-driven microflow which is forced under AC electric field. In our previous investigations, the mixing on large scale at the centerline of microchannel has been studied. Here, we show that the evaluation on mixing effect by flow visualization could be misleading, if its temporal resolution is not sufficiently high. By using single-point laser induced fluorescence (LIF) method, the mixing on both large and small scales can be investigated with high spatiotemporal resolution. It is found fast mixing is not only achieved at the centerline, but also in the majority of cross-sectional area, even near the bottom wall region. This is resulted from the large scale secondary flow due to unbalanced AC electroosmotic flow near the bottom wall. The temporal −5/3 spectrum of concentration measured with LIF in our previous investigations is also supported by the spatial spectrum of concentration. The physical process of mixing on small scales is further investigated by the flatness of concentration gradients.

Numerical study on aerodynamic noise performances of axial spacing in a contra-rotating axial fan

In order to study the effect of axial spacing on behaviors of aerodynamic performance and aerodynamic noises in a contra-rotating fan, the steady/unsteady Reynolds-averaged Navier-Stokes equations are solved by the numerical method in conjunction with a SST turbulence model, and the effects of axial spacing on performance and aerodynamic characteristics are investigated. Furthermore, BEM is adopted to compute the radiation noise of the contra-rotating fan caused by unsteady pressure fluctuations. The results show that axial spacing is an important factor which can affect the aerodynamic performance of contra-rotating fan. As a whole, the effect of axial spacing on the blade loading of Rotor 2 is significantly greater than that of Rotor 1. For Rotor 2, the smaller axial spacing leads to the large secondary flow loss, and the larger axial spacing leads to the strong mixing loss. With the increase of axial spacing, the radiation noise at the characteristic frequency decreases, but showed different changing degrees. With consideration of the aerodynamic performance and aerodynamic noises of the contra-rotating fan, the optimal comprehensive performance appears at the axial spacing of 0.5 chord.

Optical properties of metal oxynitride thin films grown with atmospheric plasma deposition

Thin films of tantalum oxynitride (TaOxNy) and titanium oxynitride (TiOxNy) are deposited using atmospheric plasma deposition and a suite of optical properties are reported. Tantalum and titanium ethoxide are introduced into the afterglow of a radio-frequency capacitively coupled plasma, facilitating the growth of oxynitride films on silicon and polycarbonate at temperatures below 180 °C. The plasma power and nitrogen flow within the plasma are varied between 60 and 120 W and between 0.1 and 0.3 LPM respectively. We use spectroscopic ellipsometry to show that the optical properties of the metal oxynitride films grown in this study are comparable to those synthesized with sol–gel methods. Measurement of both the extinction coefficient and the transmission on polycarbonate substrates indicates good transparency in the visible wavelengths of light. Additionally, the refractive index increases when increasing the number of reactive nitrogen species within the discharge. We use x-ray photoelectron spectroscopy to correlate the higher indexes observed at large secondary gas flows to the presence of metal oxynitride bonding. Single layer anti-reflection coatings are deposited on silicon, with a five-fold and seven-fold reduction in reflection for TaOxNy and TiOxNy coatings, respectively. In total, we have found that the modulation of nitrogen concentration within the plasma discharge results in good control over optical constants. In addition, we observe similarities between films deposited with atmospheric plasma and those reported for sol–gel, indicating an alternative processing route where solution chemistries are currently applied.

Direct numerical simulation of turbulent flow in a spanwise rotating square duct at high rotation numbers

In this paper, direct numerical simulations have been performed to study the effects of Coriolis force on the turbulent flow field confined within a square duct subjected to spanwise system rotations at high rotation numbers. In response to the system rotation, secondary flows appear as large streamwise counter-rotating vortices, which interact intensely with the four boundary layers and have a significant impact on flow statistics, velocity spectra and coherent structures. It is observed that at sufficiently high rotation numbers, a Taylor–Proudman region appears and complete laminarization is almost reached near the top and side walls. The influence of large organized secondary flows on the production rate and re-distribution of turbulent kinetic energy has been investigated through a spectral analysis. It is observed that the Coriolis force dominates the transport of Reynolds stresses and turbulent kinetic energy, and forces the spectra of streamwise and vertical velocities to synchronize within a wide range of scales.

Influences of Design Parameters on a Double Serpentine Convergent Nozzle

Serpentine nozzles are supplied in stealth bombers and unmanned aerial vehicles (UAVs) to evidently suppress the infrared radiation signatures (IRSs) emitted by engine exhausts. It is commonly known that excessive geometric parameters are included in the double serpentine nozzle design and, as a result, the aim of this paper is to study the influences of the design parameters on the performance of double serpentine nozzle. To this end, the design method of the serpentine nozzle was concisely introduced, and the qualifications to completely shield turbine were given. Simulations using six different turbulence models were conducted and compared to the experimental data in order to determine the suitable turbulence model for serpentine duct simulations. Then, the effects of geometric design parameters at the first serpentine paragraph exit (the dimensionless width of W1/D, area of A1/Ain, and offset distance of ΔY1/L1) on the flowfield, and the performance of double serpentine nozzle was investigated numerically. The validation study shows that the simulations with shear-stress transport (SST) κ–ω turbulence model adopted can accurately predict the flux rate, the axial thrust, and the static pressure of the experimental nozzle, and therefore, SST κ–ω turbulence model is the most suitable turbulence model in the selected six turbulence models to be used for the simulation of the double serpentine nozzles. The numerical results show that friction loss increases with the increment of W1/D due to the increased wetted perimeter, but small value of W1/D would lead to large secondary flow loss; even the shock loss appears because of the steep curvature of the second turning. Small area of the first serpentine duct A1/Ain induces high flow velocity in the first duct, which corresponds to large friction loss. Steep offset distance of the first serpentine duct ΔY1/L1 induces high local losses. As the geometric design parameters of the double serpentine nozzle interact mutually with the qualifications to completely shield the turbine, the range of design parameters should be synthetically chosen during the design progress. Thus, the width of the first serpentine duct W1/D is recommended to be from 1.0 to 1.3. The area of the first serpentine duct A1/Ain should be as large as possible, and the offset distance of the first serpentine ΔY1/L1 should be small in the permission range of the design parameters.

Non-Newtonian perspectives on pulsatile blood-analog flows in a 180° curved artery model

Complex, unsteady fluid flow phenomena in the arteries arise due to the pulsations of the heart that intermittently pumps the blood to the extremities of the body. The many different flow waveform variations observed throughout the arterial network are a result of this process and a function of the vessel properties. Large scale secondary flow structures are generated throughout the aortic arch and larger branches of the arteries. An experimental 180° curved artery test section with physiological inflow conditions was used to validate the computational methods implemented in this study. Good agreement of the secondary flow structures is obtained between experimental and numerical studies of a Newtonian blood-analog fluid under steady-state and pulsatile, carotid artery flow rate waveforms. Multiple vortical structures, some of opposite rotational sense to Dean vortices, similar to Lyne-type vortices, were observed to form during the systolic portion of the pulse. Computational tools were used to assess the effect of blood-analog fluid rheology (i.e., Newtonian versus non-Newtonian). It is demonstrated that non-Newtonian, blood-analog fluid rheology results in shear layer instabilities that alter the formation of vortical structures during the systolic deceleration and onwards during diastole. Additional vortices not observed in the Newtonian cases appear at the inside and outside of the bend at various times during the pulsation. The influence of blood-analog shear-thinning viscosity decreases mean pressure losses in contrast to the Newtonian blood analog fluid.

Turbulence suppression by suspended sediment within a geophysical flow

Experiments are performed in a mixing box to evaluate the effect of suspended sediment on turbulence generated by an oscillating grid. Quartz-density sand of varying sizes and concentrations is used, and particle image velocimetry is employed to quantify only the fluid phase. Results show that (1) while a relatively large secondary flow field is present in the box, turbulence is a maximum near the grid and it decreases systematically toward the water surface; (2) relatively high concentrations of fine sediment can markedly alter this secondary flow field and significantly decrease both the time-mean and turbulent kinetic energy within the flow, yet these same sediment concentrations have little effect on the integral time and length scales derived for each velocity component; and (3) the overall turbulence suppression observed can be related to the transfer of energy from the fluid to the sediment and the maintenance of a suspended sediment load rather than commonly employed turbulence modulation criteria. These experimental data demonstrate unequivocally that the presence of a suspended sediment load can significantly reduce overall turbulent kinetic energy, and these results should be applicable to a range of sediment-laden geophysical flows.

A Low Pressure Turbine at Extreme Off-Design Operation

In a cooperative project between the Institute of Aircraft Propulsion Systems and MTU Aero Engines GmbH, a two-stage low pressure turbine with integrated 3D airfoil and endwall contouring is tested. The experimental data taken in the altitude test-facility study the effect of high incidence in off-design operation. Steady measurements are covering a wide range of Reynolds numbers between 40,000 and 180,000. The results are compared with steady multistage CFD predictions with a focus on the stator rows. A first unsteady simulation is taken into account as well. The CFD simulations include leakage flow paths with disk cavities modeled. Compared to design operation the extreme off-design high-incidence conditions lead to a different flow-field Reynolds number sensitivity. Airfoil lift data reveals changing incidence with Reynolds number of the second stage. Increased leading edge loading of the second vane indicates a strong cross channel pressure gradient in the second stage leading to larger secondary flow regions and a more three-dimensional flow-field. Global characteristics and area traverse data of the second vane are discussed. The unsteady CFD approach indicates improvement in the numerical prediction of the predominating flow-field.

Using a pressure controlled vortex design method to control secondary flow losses in a turbine stage

A turbine design method based on pressure controlled vortex design (PCVD) is presented to design a small-size turbine stage. Contrary to the conventional controlled vortex design (CVD) method, the main objective of PCVD is to control the axial velocity and radial pressure in the stator–rotor gap. Through controlling axial velocity, the PCVD establishes a direct tie to meridional stream surface. Thus stream surface variation is induced, resulting in a large secondary flow vortex covering the full blade passage in the respective stator and rotor. This secondary flow vortex could be dedicated to control the secondary flow mitigation and migration. Through radial pressure, the PCVD is also associated with the macroscopic driving force of fluid motion. So the better benefit of CVD can be achieved. The core concept behind PCVD is to mainly control the spanwise pressure gradient by altering profile loading at various spanwise locations. Therefore not only the local profile lift is affected, but also the resulting throat widths, stage reaction degree, and massflow rate are altered or redistributed respectively. With the PCVD method, the global stage efficiency is increased successfully while the mass flow rate keeps constant. Additionally there is no endwall shape optimization, stacking optimization, or pitch/chord variations, concentrating solely on varying blade profile deflections and stagger.

High speed Deans switch for low duty cycle comprehensive two-dimensional gas chromatography

A new high-speed valve-based modulator has been designed and tested for use in comprehensive two-dimensional gas chromatography (GC×GC). The modulator is a Deans switch constructed from two micro-volume fittings and a solenoid valve. Modulator performance was characterized over a wide range of device settings including the magnitude of the switching flow, the gap between the tips of the primary and secondary column, the primary column flow rate, and the carrier gas identity. Under optimized conditions, the modulator was found to be capable of generating narrow pulses (<50ms) of primary effluent with a 2mL/min secondary column flow. This capability will ultimately allow the modulator to be used with GC×GC separations employing a wide range of detectors and secondary column geometries. The main disadvantage of this modulator is that it employs a low sampling duty cycle, and thus it produces separations with sensitivities that are lower than those produced with thermal modulators or differential flow modulators. The efficacy of the new high-speed Deans switch modulator was demonstrated through the GC×GC separation of a hydrocarbon standard and gasoline. Precise quantitation of individual components was possible provided the modulation ratio was kept greater than 2.0.

Boiling flow of R141b in vertical and inclined Serpentine Tubes

A series of visual experiments are conducted to study the two-phase flow of refrigerant R141b in vertical and declined serpentine tubes of inner diameter 6mm at inlet velocities of 0.1–0.2ms−1 and wall heat fluxes of 4687–22064Wm−2. Under certain conditions, stagnant vapor slugs of length longer than the straight section are formed in the descent sections. A VOF (volume of fluid) model with phase change is developed to simulate the flow and heat transfer using the commercial CFD software Fluent. The simulated flow patterns agree well with the experimental observations. The formation of a stagnant slug is discussed based on the simulation results. It is found that a liquid film is formed on one side of the stagnant slug in the vertical layout, constituting a smooth flow path for each phase; while the film is twisted in the inclined layout because of intensive interactions between the two phases, causing a larger secondary flow and more efficient heat transfer. Flow in the descent straight sections turns more difficult in the vertical layout due to a stronger resistance of buoyancy imposed by the stagnant slugs, resulting in a larger pressure drop relative to that in the inclined layout.

Wall Shear Stress Measurements on a Highly Loaded Compressor Cascade

This paper presents wall shear stress measurements obtained with a new type of wall-mounted probe based on the thermal electrical principle. The sensor consists of three single surface hot wires arranged in a δ configuration. This allows for measuring wall shear stress magnitude and direction simultaneously. Each probe has to be calibrated in a flat plate experiment for a number of wall shear values and flow directions before applying it to the relevant flow situation. To assess the full potential of the newly designed sensors, they were applied to a low-speed, large-scale cascade test section equipped with highly loaded compressor blades. The high blade loading in conjunction with a small blade aspect ratio results in a strongly three-dimensional flow field with large secondary flow structures and flow separation. Furthermore, laminar separation bubbles can be observed on the blade surface. The wall shear stress distribution allows for resolving these existing flow structures and provides detailed insight into the flow on the blade’s surface. The additionally measured flow direction reveals further details of the flow field. Parallel to the experiments, RANS simulations were conducted using the commercial flow solver CFX to compare the simulated results with the measured values.