Reverse Flow Region Research Articles

ANALYTICAL STUDY ON FULLY DEVELOPED MIXED CONVECTION COUETTE FLOW IN A VERTICAL CHANNEL WITH VISCOUS DISSIPATION EFFECT

The study of mixed convection flow in a vertical channel with viscous dissipation has been examined. To solve the governing energy and momentum equations, the Homotopy perturbation method was employed. Graphs were generated to analyze the impact of the governing flow parameters. Numerical values for skin friction, rate of heat transfer, and mass flux were estimated. The study revealed that an increase in mixed convection expands the reverse flow region and raises the critical value of mixed convection that leads to flow reversal. Additionally, both fluid temperature and velocity rise with increased viscous dissipation, as higher viscous dissipative heat elevates temperature, subsequently increasing the buoyancy force.

Noise analysis of variable-speed rotor technology on the coaxial lift-offset helicopter

The impact of variable-speed rotor technology on the aerodynamic and aeroacoustic characteristics of a coaxial lift-offset helicopter was investigated using a numerical analysis approach. The free wake vortex lattice method was utilized to predict aerodynamic performance, while the Ffowcs-Williams Hawkings acoustic analogy was employed to evaluate the noise impact. The Harrington coaxial rotor was used as the baseline configuration. Results show that reducing the rotational speed, in combination with lift-offset, leads to a decrease in overall power required during forward flight by lowering both collective and cyclic pitch. This is primarily due to a reduction in profile power across the speed range. However, excessive rotor deceleration can lead to an expanded reverse flow region, increasing induced power at high forward speeds. From an acoustics perspective, the reduced rotor speed mitigated loading noise radiated toward the ground, especially in the region 40-60 degrees below the rotor plane, achieving a maximum 2.6 dB reduction at the evaluated observer location. This study indicates that the technology can offer advantages for both performance and noise in forward flight conditions.

Triglobal resolvent-analysis-based control of separated flows around low-aspect-ratio wings

We perform direct numerical simulations of actively controlled laminar separated wakes around low-aspect-ratio wings with two primary goals: (i) reducing the size of the separation bubble and (ii) attenuating the wing tip vortex. Instead of preventing separation, we modify the three-dimensional (3-D) dynamics to exploit wake vortices for aerodynamic enhancements. A direct wake modification is considered using optimal harmonic forcing modes from triglobal resolvent analysis. For this study, we consider wings at angles of attack of $14^\circ$ and $22^\circ$ , taper ratios $0.27$ and $1$ , and leading edge sweep angles of $0^\circ$ and $30^\circ$ , at a mean-chord-based Reynolds number of $600$ . The wakes behind these wings exhibit 3-D reversed-flow bubble and large-scale vortical structures. For tapered swept wings, the diversity of wake vortices increases substantially, posing a challenge for flow control. To achieve the first control objective for an untapered unswept wing, root-based actuation at the shedding frequency is introduced to reduce the reversed-flow bubble size by taking advantage of the wake vortices to significantly enhance the aerodynamic performance of the wing. For both untapered and tapered swept wings, root-based actuation modifies the stalled flow, reduces the reversed-flow region and enhances aerodynamic performance by increasing the root contribution to lift. For the goal of controlling the tip vortex, we demonstrate the effectiveness of actuation with high-frequency perturbations near the tip. This study shows how insights from resolvent analysis for unsteady actuation can enable global modification of 3-D separated wakes and achieve improved aerodynamics of wings.

Investigating three-dimensional vortex evolution in centrifugal pump under rotating stall conditions using tomographic particle image velocimetry

A rotating stall in centrifugal pumps commonly occurs under off-design operations, which is a detrimental phenomenon leading to flow instabilities, pressure fluctuations, and reduced performance. A time-resolved non-intrusive three-dimensional (3D) flow visualization method is developed for investigating complex vortex structures in centrifugal pumps based on Omega vortex identification and tomographic particle image velocimetry (tomo-PIV). A special-made centrifugal pump prototype was developed with acrylic glass allowing for optical access. This method enables both qualitative and quantitative analysis of high spatiotemporal resolution on flow behaviors and dynamics under various stall conditions. The ultra-high sampling frequency realized over 40 time-consecutive observations per revolution under 0.2 Qd, 0.4 Qd, 0.6 Qd, and 0.8 Qd. It captures the instantaneous evolution of vortex structures that undergoes a growth–breakup transition within 7–9 ms. The rotating stall mechanism is revealed experimentally from the evolution of the vortex structure. Our analysis shows the tomo-PIV's additional velocity component aids in understanding the 3D characteristics of the stall. A substantial region of reverse flow in the z-axis direction is observed under 0.2 Qd. Vortex structures are more prone to blockage at the impeller inlet, exacerbating the stall phenomenon. As the flow rate increases, the velocity distributions across different layers exhibit a laminar characteristic with a more uniform profile. The vortex structures extend radially and migrate toward the outlet. The evolutions of the stall vortex, wake vortex, and inlet vortex share the same dominant frequency components (4.75fn and 5.25fn), but the flow rate affects the proportion of different frequency components.

Multiscale study of reactive transport and multiphase heat transfer processes in catalyst layers of proton exchange membrane fuel cells

Improving the performance of proton exchange membrane fuel cells (PEMFCs) requires deep understanding of the reactive transport processes inside the catalyst layers (CLs). In this study, a particle-overlapping model is developed for accurately describing the hierarchical structures and oxygen reactive transport processes in CLs. The analytical solutions derived from this model indicate that carbon particle overlap increases ionomer thickness, reduces specific surface areas of ionomer and carbon, and further intensifies the local oxygen transport resistance (Rother). The relationship between Rother and roughness factor predicted by the model in the range of 800-1600 s m-1 agrees well with the experiments. Then, a multiscale model is developed by coupling the particle-overlapping model with cell-scale models, which is validated by comparing with the polarization curves and local current density distribution obtained in experiments. The relative error of local current density distribution is below 15% in the ohmic polarization region. Finally, the multiscale model is employed to explore effects of CL structural parameters including Pt loading, I/C, ionomer coverage and carbon particle radius on the cell performance as well as the phase-change-induced (PCI) flow and capillary-driven (CD) flow in CL. The result demonstrates that the CL structural parameters have significant effects on the cell performance as well as the PCI and CD flows. Optimizing the CL structure can increase the current density and further enhance the heat-pipe effect within the CL, leading to overall higher PCI and CD rates. The maximum increase of PCI and CD rates can exceed 145%. Besides, the enhanced heat-pipe effect causes the reverse flow regions of PCI and CD near the CL/PEM interface, which can occupy about 30% of the CL. The multiscale model significantly contributes to a deep understanding of reactive transport and multiphase heat transfer processes inside PEMFCs.

Aerodynamic performance and wind-induced response of carbon fiber-reinforced polymer (CFRP) cables

To study the aerodynamic performance and wind-induced response of carbon fiber-reinforced polymer (CFRP) cables, CFRP cable was designed by replacing a steel cable in a tied arch bridge based on stiffness, strength and area equivalent criteria, respectively. The aerodynamic performance and wind-induced response of CFRP cable and steel cable were studied and compared by computational fluid dynamics (CFD) model. Based on the computational results, optimal cable replacement criterion was proposed for CFRP cable to replace steel cable. In addition, surface modification was conducted by engrooving vertical symmetric (VS), vertical asymmetric (VA) and helical symmetric (HS) V-shaped grooves to improve the aerodynamic performance and wind-induced response of CFRP cable. Results showed that CFRP cables exhibited inferior aerodynamic performance and wind-induced response in most cases. However, CFRP cable based on stiffness equivalent criterion exhibited better aerodynamic performance and wind-induced vibration properties compared to the other two cable replacement criteria, thus is regarded as the optimal substitute for steel cable. In addition, HS grooves generated symmetric disturbances and caused approximately equivalent boundary layer separation delays uniformly and continuously along the cable length, thus exhibiting better effect in decreasing the reverse flow region, the maximum negative flow velocity, the vortex shedding frequency and the wind-induced vibration amplitude of CFRP cable. Hence, the stiffness equivalent criterion combined with surface modification with HS V-shaped grooves was proposed to replace steel cable with CFRP cable. This study can provide insights into the aerodynamic performance and wind-induced response of CFRP cable and instructions for cable replacement practice.

Validation of a RANS Turbulence Model for a S833 Wind Turbine Airfoil With a Trailing Edge Flap Using Oil Visualization and Pressure Taps

Abstract The aerodynamics of a small wind turbine blade was captured using a γ–Reθ k–ω shear stress transport transitional turbulence model tuned with production limiter coefficients at a Reynolds number of 1.70×105. The computational fluid dynamics simulations were validated against wind tunnel experiments that included airfoil pressure tap measurements and surface oil flow visualization (SOFV) to capture the flow field. The uniqueness of this blade included a trailing edge flap that was 20% of the chord controlled using a servomotor. The test matrix included angles of attack (AOA) between 1 deg and 7 deg with flap angles of 10 deg in the upward and downward position. Two locations were always observed on the airfoil: a leading edge region of high shear and a midsection of flow separation. Within the flow separation section, two distinct regions existed: a complete detachment of flow from the airfoil surface creating a stagnation region which was followed by a reverse flow region. A third location of flow reattachment near the trailing edge was observed for all cases excluding a downward angled trailing edge flap. The utilization of the flap resulted in changes to the size of the separation zone and the movement of the separation zone along the chord. The numerical skin friction coefficient, oil residue profiles from the SOFV, and pressure tap measurements all showed onset of separation locations on the chord within 10%. The computational fluid dynamics model also predicted the coefficient of pressure across the chord of the airfoil within 10% in comparison to the experimental measurements.

Control of Reverse Flow over Cantilevered Swept Blades Using Passive Camber Morphing

High-speed rotorcraft experience a region of reversed flow over the retreating blade at high advance ratios. This reverse flow region results in multiple unfavorable effects, including negative lift, increased drag, and a large pitching moment impulse. The effect of the sweep angle on a rotor blade in reverse flow was explored, and it was demonstrated that the introduction of a trailing edge reflex camber could mitigate the adverse effects seen in this regime. A cantilevered, finite-span NACA 63-218 blade was examined at a Reynolds number Rec=2.38×105 both with and without a 10° trailing edge reflex camber. Three blades were tested: one with 20° forward sweep, one with 20° backward sweep, and one with no sweep. The experiments consisted of load measurements across a range of angles of attack, as well as volumetric flowfield measurements using stereoscopic particle image velocimetry on the two swept models at 10° angle of attack in reverse flow. The flowfields exhibited a highly three-dimensional separation bubble that existed on the blades in both backward and forward sweep conditions. Cambering of the trailing edge led to almost complete flow attachment on the backward swept model and a significant reduction in separation over the forward swept model. Cambering also led to a large reduction in drag in the entire reverse flow regime in both swept and unswept conditions. A large reduction in pitching moment and a smaller reduction in negative lift were also observed at moderate angles of attack.

Modelling the effect of roughness density on turbulent forced convection

By examining a systematic set of direct numerical simulations, we develop a model which captures the effect of roughness density on global and local heat transfer in forced convection. The surfaces considered are zero-skewed three-dimensional sinusoidal rough walls with solidities, $\varLambda$ (defined as the frontal area divided by the total plan area), ranging from low $\varLambda = 0.09$ , medium $\varLambda = 0.18$ to high $\varLambda = 0.36$ . For each solidity, we vary the roughness height characterised by the roughness Reynolds number, $k^+$ , from transitionally rough to fully rough conditions. The findings indicate that, as the fully rough regime is approached, there is a pronounced breakdown in the analogy between heat and momentum transfer, whereby the velocity roughness function $\Delta U^+$ continues to increase and the temperature roughness function $\Delta \varTheta ^+$ attains a peak with increasing $k^+$ . This breakdown occurs at higher sand-grain roughness Reynolds numbers ( $k_s^+$ ) with increasing solidity. Locally, we find that the heat transfer can be meaningfully partitioned into two categories: exposed, high-shear regions experiencing higher heat transfer obeying a local Reynolds analogy and sheltered, reversed-flow regions experiencing lower and spatially uniform heat transfer. The relative contribution of these distinct mechanisms to the global heat transfer depends on the fraction of the total surface area covered by these regions, which ultimately depends on $\varLambda$ . These insights enable us to develop a model for the rough-wall heat-transfer coefficient, ${C_{h,k}(k^+, \varLambda, Pr)}$ , where $Pr$ is the molecular Prandtl number, that assumes different heat-transfer laws in exposed and sheltered regions. We show that the exposed–sheltered surface-area fractions can be modelled through simple ray tracing that is solely dependent on the surface topography and a prescribed sheltering angle. Model predictions compare well when applied to heat-transfer data of traverse ribs from the literature.

Shock-wave/turbulent boundary-layer interaction with a flexible panel

The dynamic coupling between a Mach 2.0 shock-wave/turbulent boundary-layer interaction (STBLI) and a flexible panel is investigated. Wall-resolved large-eddy simulations are performed for a baseline interaction over a flat-rigid wall, a coupled interaction with a flexible panel, and a third interaction over a rigid surface that is shaped according to the mean panel deflection of the coupled case. Results show that the flexible panel exhibits self-sustained oscillatory behavior over a broad frequency range, confirming the strong and complex fluid–structure interaction (FSI). The first three bending modes of the panel oscillation are found to contribute most to the unsteady panel response, at frequencies in close agreement with natural frequencies of the mean deformed panel rather than those for the unloaded flat panel. This highlights the importance of the mean panel deformation and the corresponding stiffening in the FSI dynamics. The time-averaged flow shows an enlarged reverse-flow region in the presence of mean surface deformations. The separation-shock unsteadiness is enhanced due to the panel motion, leading to higher wall-pressure fluctuations in the coupled interaction. Spectral analysis of the separation-shock location and bubble-volume signals shows that the STBLI flow strongly couples with the first bending mode of the panel oscillation. This is further confirmed by dynamic mode decomposition of the flow and displacement data, which reveals variations in the reverse-flow region that follow the panel bending motion and appear to drive the separation-shock unsteadiness. Low-frequency modes that are not associated with the fluid–structure coupling, in turn, are qualitatively similar to those obtained for the rigid-wall interactions, indicating that the characteristic low-frequency unsteadiness of STBLI coexists with the dynamics emerging from the fluid–structure coupling. Based on the present results, unsteady FSIs involving STBLIs and flexible panels are likely to accentuate rather than mitigate the undesirable features of STBLIs.

A Surge/Stall-Capable Dynamic Performance Simulation Methodology for a Turbojet Engine

Abstract A lumped-parameter dynamic performance model for a single-spool turbojet engine is presented in this paper. This model can handle pre and poststall transients under forward and reverse-flow conditions. The inter-component volume technique is employed instead of the standard matching technique to be able to handle high-frequency transients and reverse-flow conditions. Inspired by Greitzer's lumped-parameter surge model, momentum (duct) and volume elements are placed within the flow path to handle surge dynamics. Compressor and turbine maps are extended to low-flow and reverse-flow regions using a combination of the guidelines presented by Kurzke, the cubic axisymmetric characteristics of Moore and Greitzer, and a quadratic function guess for in-stall characteristics. Combustor efficiency, stability limits, and delay are taken from the literature. Poststall behavior of the model is validated using the data available in the literature for a Rolls-Royce Viper engine. A good match is observed with a correct prediction of poststall behaviors, which transition from surge after locked stall to multiple surge cycles around 80% speed and multiple surge cycles to surge after flameout around 95% speed. The effects of different modeling choices and modeling parameters on the obtained results are discussed. The produced model can be calibrated for a specific engine with surge tests, and it can be used for hard-to-test scenarios like surge after shaft breakage. Different surge/stall-causing events, such as fuel spiking, in-bleeding, and shaft breakage, are simulated to see the capabilities of the model.

Large eddy simulations and experiments on low-Reynolds-number laminar separation control for low-pressure turbine blades with oblong-dimpled surface

Modern high-performance low-pressure turbines (LPT) usually encounter flow problems at low Reynolds numbers, especially for high-altitude UAVs under cruise conditions. Flow separation may occur on the blade suction side, seriously degrading the turbine's aerodynamic efficiency. The present study investigates a passive laminar flow separation control method based on the oblong dimple surface structures for their more potent flow acceleration abilities. Linear cascade tests and large eddy simulations are conducted for a typical LPT blade profile, T106A, at the Reynolds numbers of 25,000, 50,000, and 100,000 based on the inlet velocity and axial chord length. Results show that applying the oblong dimples can apparently reduce the reverse flow region and improve the aerodynamic performance at low Reynolds numbers. The experimental results achieve a maximum total pressure loss reduction of 34.7% at a low Reynolds number of 25,000. The numerical simulations show the local flow acceleration after the oblong dimples and demonstrate two working modes. At low Reynolds numbers, the oblong dimples accelerate the fluid above, generating local high-speed flows, and the uneven velocity distributions in the spanwise direction feed the deformation and fragmentation of the downstream separating vortex. While at high Reynolds numbers, the oblong dimples act as vortex generators, leading to flow transition on the blade suction surface.

Microramp wake impinging on canonical shock/boundary-layer interaction

We analyze the influence of microramp vortex generators (mVGs) on a canonical oblique shock wave/turbulent boundary-layer interaction (SBLI) in terms of mean flow field and unsteady dynamics. The flow configuration of our wall-resolved large-eddy simulations (LES) reproduces the experiment of Bo et al. [“Experimental investigation of the micro-ramp based shock wave and turbulent boundary-layer interaction control,” Phys. Fluids 24, 055110 (2012)]: a rake of microramps is inserted upstream of the SBLI, protruding by 0.476δ in a turbulent boundary layer (TBL) at free-stream Mach number M = 2.7 and corresponding to a Reynolds number based on the displacement thickness of Reθ=3600. The long integration time of 1672 Lsep/U∞ allows an accurate characterization of the low-frequency dynamics of the SBLI under the influence of the microramps. With respect to the reference SBLI without control devices, the mean flow field shows a new spatial organization of the recirculation bubble due to the mVGs' wake. The alternating high and low-speed zones in the near-wall region of the incoming TBL, induced by the counter-rotating streamwise vortices generated by the mVGs, trigger spanwise corrugations of the separation and reattachment lines and locally alter the reverse flow region. Tornado-like vortices are found in the vicinity of these zones, yielding a new fluid collection mechanism of the reverse flow region. These vortices redirect the fluid coming from regions outside of the wake in the incoming TBL to three key spanwise exit locations located in between the mVGs and at their centerline. Interestingly, power spectral densities of wall-pressure probes show a damping of the low-frequency dynamics of the reflected shock foot for spanwise stations aligned with the mVGs' wake, whereas this activity appears to be reinforced in the planes located in between the mVGs. However, we found no evidence of unsteady forcing linked to the high-frequency shedding of the coherent structures developing in the wake of the microramps. Dynamic mode decomposition highlights a significant change in the low-frequency dynamics, mostly affecting the mass budget of the recirculation bubble. The breathing of the recirculation zone that occurs at StL=0.1 for the SBLI without control devices (with StL=fLsep/U∞) appears to shift toward a lower frequency of StL=0.05. Remembering that the reflected shock foot motion is related to frequencies in the range StL=[0.03−0.05], the SBLI with upstream mVGs seems to highlight a synchronization of this motion with the breathing of the separation bubble.

Implementation of coherence imaging spectroscopy for ion flow measurements in the Compact Toroidal Hybrid experiment

A new coherence imaging spectroscopy (CIS) system has been characterized and validated for measurements of impurity ion velocity in the Compact Toroidal Hybrid (CTH) experiment. CIS is an interferometric technique providing very high spatial resolution of line-integrated ion flows; however, these measurements are highly sensitive to environmental factors such as temperature fluctuations. A compact interferometer design housed in a thermally regulated environment yields CIS measurement variations due to thermal fluctuations of 0.25 km/s. CIS measurements of He+ flows in the outer regions of CTH are benchmarked using two optical grating spectrometers. The deviation between the measurements is typically less than 2 km/s for multiple lines of sight which is well within the uncertainties of the diagnostics. Additional experiments demonstrate a moderate robustness of ion flows in the outer region of CTH to changing the vacuum transform, direction of the magnetic field, and plasma current. In contrast, regions of complete flow reversal are observed when a bias probe is inserted into the plasma edge.

Low-Order Modeling and Sensor-Based Prediction of Stalled Airfoils at Moderate Reynolds Number

We present analysis from planar time-resolved particle image velocimetry fields in the streamwise surface-normal plane of turbulent flow surrounding NACA 0012 and NACA 65-410 airfoils at focusing on stall phenomena at intermediate to high angles of attack . Dominant flow frequencies, identified from the lift spectra obtained from a load cell mounted to the foils, highlight the presence of bluff-body shedding [ of , where is the frequency, the airfoil chord, and the freestream velocity] for all cases and prominent low frequencies [ of ] for cases in transient stall (TS). A data-driven modeling framework via the proper orthogonal decomposition (POD) reveals that the low frequencies are associated to flow separation and reattachment driven by underlying expansion and contraction normal to the suction surface. Further, the ability to predict the low-order features from (pseudo) pressure probes at the leading, midchord, and trailing edges for both TS and deep stall (DS) cases is quantified using linear stochastic estimation (LSE). The framework pinpoints the centroid of the region of reverse flow with error on the order of 5 and 20% for DS and TS regimes, respectively. Notably, it is found that LSE coefficients governing the low-order POD correlations do not strongly depend on the airfoil geometry. This is demonstrated by the comparative performance of training the LSE using the probes of the NACA 0012 cases to predict the NACA 65-410 velocity fields and vice versa. This work demonstrates the insight afforded by the POD on the low-order features of turbulent stalled airfoils as well as the similarity of such features across geometries toward predicting flow features for potentially any airfoil geometry without the need to retrain an LSE library.

Numerical Investigation on the Ventilated Supercavity around a Body under Free Surface Effect

Reducing vessel resistance by using ventilated cavities has been a highly researched topic in the marine industry. There is limited literature on ventilated supercavities near the free surface, which indicates that their dynamic behavior is more complex than conventional ventilated cavities due to the effect of the free surface. This paper employs numerical simulations to study the dynamic behavior of the ventilated supercavity, taking into account the effect of the free surface. Numerical simulations can predict gas leakage behaviors, cavity geometry, and internal flow structures. The influence of the free surface shortens the length of the ventilated cavity and increases the diameter. The presence of the free surface mainly changes the vertical velocity distribution between the free surface and the cavity. The results show that there are two typical gas leakage mechanisms under different immersion depths: twin-vortex tube leakage mode and re-entrant jet leakage mode. The internal flow field of ventilated supercavity is classified into three regions: the internal boundary layer, the ventilation influence region, and the reverse flow region. As the distance between the free surface and the ventilated supercavity decreases, the ventilated supercavity is affected by both the free surface effect and the gravity effect.

Investigation of Three-Dimensional Shock Control Bumps for Transonic Buffet Alleviation

This experimental study investigates the use of shock control bumps (SCBs) for controlling transonic buffet. Three-dimensional SCBs have been applied on the suction side of an OAT15A supercritical airfoil with the experiments conducted in the transonic–supersonic wind tunnel of Delft University of Technology at fully developed buffet conditions (, and ). The effectiveness of the SCBs for different spanwise array spacings (ranging from 20 to ) was verified using two optical techniques: schlieren visualization and particle image velocimetry. Both techniques confirmed the potential of controlling buffet using such devices, resulting in a reduction of the flow unsteadiness in terms of both shock oscillation and pulsation of the separated area. A dedicated particle image velocimetry investigation in a spanwise–chordwise measurement plane was conducted in order to characterize the effect of the spatial distribution of the bumps, focusing on the interaction of the shock-wave structures along the span. The configuration with a spacing of was demonstrated to be the most efficient in reducing the transonic buffet oscillations and was able to reduce the reverse flow region size as compared to the clean configuration.

On the evolution of supercavity geometry and unsteady behavior of supercavitation flow during the development of a twin-vortex supercavity

To investigate the evolution of supercavity geometry and the unsteady characteristics of the supercavitation flow from the initial generation to full development. A three-dimensional numerical model with the inhomogeneous multiphase model and SST k−ω turbulence model was established. The numerical model was validated by experimental results. The ventilated supercavitation flow was performed under the different gas entrainment coefficients and Froude numbers. The results show that the initial supercavity closure mode is the re-entrant jet, then the closure mode changes to the twin-vortex induced by the gravitational effect. For the fully developed twin-vortex supercavity, a flow structure with gas recirculation exists inside the supercavity, which includes the reverse flow region and downstream flow region. Compared to the supercavity with the twin-vortex closure mode, the increment in the supercavity dimension with the re-entrant jet closure mode increases drastically, and the oscillation characteristic of internal pressure increases, leading to the significant supercavity deformation and gas shedding. The difference of the supercavity dimension is revealed by analyzing the gas leakage behavior. Further, a theoretical model describing the strength of re-entrant jet is established to reveal the transition mechanism of the supercavity closure mode and analyze the unsteady flow characteristics during the supercavity development.

CFD analysis on the performance of a coaxial rotor with lift offset at high advance ratios

The aerodynamic performance of an isolated coaxial rotor in forward flight is analyzed by a high-fidelity computational fluid dynamics (CFD) approach. The analysis focuses on the high-speed forward flight with an advance ratio of 0.5 or higher, which is the ratio of the forward speed to the rotor tip speed. The effect of the degree of the rolling moment on the rotor thrust, called lift offset, is studied in detail. The coaxial rotor model is a pair of contrarotating rotors, each rotor consisting of two untwisted blades with a radius of 1.016 m. The pitch angle of the blades is controlled by both collective and cyclic as in a conventional single main-rotor helicopter. CFD analysis is performed using a flow solver based on the compressible Navier-Stokes equations with a Reynolds-averaged turbulence model. Laminar/turbulent transition in the boundary layer is taken into account in the calculation. The rotor trim for target forces and moments is achieved using a gradient-based delta-form blade pitch angle adjusting technique in conjunction with CFD analysis. The reliability of the calculations is confirmed by comparison with published wind tunnel experiments and two comprehensive analyses. Applying the lift offset improves the lift-to-effective drag ratio (lift-drag ratio) and reduces thrust fluctuations. However, in the case where the advance ratio exceeds 0.6, the lift-drag ratio drops significantly even if the lift offset is 0.3. The thrust fluctuation also increases with such a high advance ratio. Detailed analysis reveals that the degradation of aerodynamic performance and vibratory aerodynamic loads is closely related to the pitch angle control to compensate for the reduction in thrust on the retreating side due to the increased reverse flow region. It is effective to reduce the collective and longitudinal cyclic pitch angles for the improvement of the aerodynamic performance of coaxial rotors with an appropriate lift offset.

Experimental Study on Dynamic Stall of Airfoil in Rotor Reverse Flow Region

The large reverse flow region of the rotor with a high advance ratio is an important factor that affects the performance improvement of helicopters. This paper examines the dynamic stall of the airfoil in the reverse flow region of the rotor to develop a set of dynamic pressure measurement systems for the airfoil in the reverse low region. The pitch oscillation experiment of the NACA0018 airfoil in and out of the rotor reverse flow region has been carried out, and the influence of the airfoil motion parameters on dynamic stalls has been studied. The results show that the airfoil is more likely to stall in the reverse flow region under static conditions and that stall is invariant to the Reynolds number. After a stall occurs, the lift coefficient decreases more slowly with the increase of the angle of attack (about 10% of that in the forward flow state). Under the dynamic environment, in a pitching cycle, the quarter chord moment coefficient of the airfoil alternates between positive and negative frequently during reverse-blowing. The pitching oscillation in the reverse flow region is more sensitive to the changes in airfoil parameters than that under normal flow, and there is a marked difference. The alternating moment of the airfoil in the reverse flow region is significant: the greater the pitching amplitude, the greater the alternating moment. In the reverse-blowing state, with the increase in reduced frequency, the dynamic stall angle of attack increases more significantly.