Unsteady Loads Research Articles

Two-Stage RANS-DDES Model and Results of Computations of Unsteady Loads on the Reentry Capsule and Engine Compartment of a Manned Spacecraft in the Course of Their Emergency Separation

Two-Stage RANS-DDES Model and Results of Computations of Unsteady Loads on the Reentry Capsule and Engine Compartment of a Manned Spacecraft in the Course of Their Emergency Separation

Feedback control of limit cycle oscillations and transonic buzz, using the nonlinear transonic small disturbance aerodynamics

In this paper, a systematic method to suppress transonic buzz with feedback is presented. A trailing edge control surface in the form of part-span flap was used only to modify and control the unsteady aerodynamic loading on the wing. The flap rotation was used to provide feedback, which consisted of a weighted linear combination of the amplitudes of the principal modes of the structure, referred to as the control law. A linear, optimal feedback control law, that is synthesized systematically based on pseudo-spectral time domain analysis, may be used in principle, to assess its capacity to actively suppress the buzz in the transonic flow domain by using a servo-controlled control surface to modify the unsteady, nonlinear aerodynamic loads on the wing. Thus, it is essential that a set of feasible control laws are first constructed. In this paper, this is done by applying the doublet-lattice method. Restrictions, such as near-zero structural damping in the flap mode, were imposed on the aeroelastic model to facilitate the occurrence of transonic buzz. The feasible set of control laws were then assessed using the nonlinear transonic small disturbance theory and an optimum control is selected to suppress the buzz. The essential differences of the behaviour of the closed-loop system in nonlinear transonic flow, when compared to the applications of linear optimal control in linear potential flow, are presented and discussed.

On the Incipient Indicial Lift of Thin Wings in Subsonic Flow: Acoustic Wave Theory with Unsteady Three-Dimensional Effects

Enhanced approximate expressions for the incipient indicial lift of thin wings in subsonic potential flow are presented in this study, featuring explicit analytical corrections for the unsteady downwash. Lifting-line and acoustic-wave theories form the basis of the method, within an effective synthesis of the governing physics, which grants a consistent generalised framework and unifies previous works. The unsteady flow perturbation consists of a step-change in angle of attack or a vertical sharp-edged gust. The proposed model is successfully evaluated against numerical results in the literature for the initial airload development of elliptical and rectangular wings with a symmetric aerofoil, considering several aspect ratios and Mach numbers. While nonlinear downwash and compressibility terms demonstrate marginal (especially for the case of a travelling gust), both linear and nonlinear geometrical effects from a significant taper ratio, sweep angle or curved leading-edge are found to be more important than linear downwash corrections (which are crucial for the circulation growth at later times instead, along with linear compressibility corrections). The present formulae may then be used as a rigorous reduced-order model for validating higher-fidelity tools and complex simulations in industrial practice, as well as for estimating parametric sensitivities of unsteady aerodynamic loads within the preliminary design of aircraft wings in the subsonic regime.

Low Reynolds number stall flutter of a linear cascade with large amplitudes at low reduced frequencies: Unsteady loads and flow features

In this paper, we report on an experimental investigation of stall flutter of a linear cascade undergoing large amplitude heave oscillations (≈ 10% of chord) at low reduced frequencies (k<0.1) and at low Reynolds number (Re≈50,000). Three blade incidence cases are studied representing light, moderate, and deep stall conditions of the cascade. Simultaneous measurements of energy transfer to the blade using a load cell, and flow field measurements using Particle Image Velocimetry (PIV) are carried out while the blades in the cascade are allowed to perform large amplitude heaving oscillations at the prescribed reduced frequency (k) and Inter Blade Phase angle (σ). The resulting normalized energy transfer values (CE¯) indicate that the cascade becomes unstable at low k values for a range of σ values with the range of σ and k within the unstable region increasing as the cascade is pushed deeper into stall. The energy transfer variation with k and σ, for a given blade incidence, are observed to fit the empirical model CE¯=a(k)+b(k)sin(σ+c(k)) with the terms a and c corresponding to quadratic and linear functions of k for all the incidence cases. At very low k values (k∼0.03), the quadratic term is small, and the energy transfer curves are observed to be broadly consistent with the trends reported by Corral and Vega (2016a), with the terms a(k) and c(k) being linear and b(k) being a constant. As the cascade is pushed gradually deeper into stall, the term b(k), which represents the influence of adjacent blades, is observed to go from being nearly a constant value with k for light stall, to a quadratic function of k at deep stall. More importantly, the deviation of b from being a constant is observed to occur at lower k values as the cascade goes deeper into stall. These results indicate that the additional quadratic terms need to be considered in the classical flutter based linear models for the case with deep stall and large oscillation amplitudes. The flow field measurements using PIV indicate the presence of a completely separated shear layer that is alternating between a state of being attached and detached to the blade surface during the oscillation cycle. The results show the shear layer phase (ϕs) with respect to the blade displacement to be close to the phase of the unsteady force (ϕ), thus helping to link the flutter behavior with the unsteady shear layer dynamics for stall flutter of a cascade at low Re.

Large-eddy simulation of a ducted propeller in crashback

Large-eddy simulation (LES) using an unstructured overset numerical method is performed to study the flow around a ducted marine propeller for the highly unsteady off-design condition called crashback. Known as one of the most challenging propeller states to analyse, the propeller rotates in the reverse direction to yield negative thrust while the vehicle is still in forward motion. The LES results for the marine propeller David Taylor Model Basin 4381 with a neutrally loaded duct are validated against experiments, showing good agreement. The simulations are performed at Reynolds number of 561 000 and an advance ratio $J=-0.82$ . The flow field around the different components (duct, rotor blades and stator blades) and their impact on the unsteady loading are examined. The side-force coefficient $K_S$ is mostly generated from the duct surface, consistent with experiments. The majority of the thrust and torque coefficients $K_T$ and $K_Q$ arise from the rotor blades. A prominent contribution to $K_Q$ is also produced from the stator blades. Tip-leakage flow between the rotor blade tips and duct surface is shown to play a major role in the local unsteady loads on the rotor blades and duct. The physical mechanisms responsible for the overall unsteady loads and large side-force production are identified as globally, the vortex ring and locally, leading-edge separation as well as tip-leakage flow which forms blade-local recirculation zones.

COMPUTATIONAL STUDIES OF AEROELASTICITY OF AIRCRAFT ENGINE TURBINE BLADE

Transient blade loading limits the lifetime of aircraft engine turbine blades. Thus, accurate prediction of the unsteady aerodynamic loading and coupled fluid-structure interactions would improve the life the lifetime of the turbine blades. This study investigates the flutter instability of an axial turbine blade under unsteady aerodynamic loading. The viscous Navier-Stokes equations with the SST-kω turbulence model are employed. The results show that the flutter phenomenon causes unsteady oscillations of the aerodynamic coefficients lift and drag.

SNNKIQ IMPROVING DIESEL ENGINE DIAGNOSTICS RELIABILITY AT UNSTEADY LOAD

The issues of rational use of energy in the operation of machine-tractor units by reducing downtime, including hidden ones, are very relevant. An increase in the energy efficiency of machine-tractor units can be achieved by diagnosing diesel engines according to functional parameters that are used as the power plant of unit. The difference in engine loading parameters during diagnosis and operation can be taken into account by correction factors for the tolerance of diagnostic parameters, which are determined for each brand of engine. (Research purpose) The research purpose is developing the methodology for determining correction coefficients for the tolerances of diagnostic parameters, taking into account the distribution of input effects. (Materials and methods) The values of the functional parameters of tractors in real operation significantly differ from the indicators in bench conditions. The probabilistic, functional transformation of random variables and computational methods were applied. The article proposes calculating the mathematical expectations of the functional parameters of the engines using the functional transformation of random variables and probabilistic characteristics of the input effect to simplify the process. (Results and discussion) Without taking into account the correction coefficients, the error in determining the deviations of functional parameters can be significant. Deviations of functional parameters from the values of the typical characteristic are maximal for the arcsin function and minimal for the Gauss law. The methodology for determining the correction coefficients for the tolerances of functional parameters was substantiated using the example of the D-240 engine. (Conclusions) The article presents quantitative characteristics of correction coefficients for the transition from operational tolerance to diagnostic. For most technological operations, diagnosing diesel engines with an unsteady load will reduce the error in monitoring the permissible values of functional parameters by 10-24 percent.

Parametric Reduced-Order Modeling of Aeroelastic Systems

In this paper, two approaches for modeling parameter-dependent unsteady aerodynamic loads for control design purposes are presented. The approaches are based on parametric Loewner frameworks, namely, transfer matrix interpolation and global basis. Combined with postprocessing techniques, these frameworks generate highly accurate, reduced-order state-space models of aerodynamic loads. The approaches are computationally efficient since they can model the entire flight envelope while requiring only a single input set of aerodynamic transfer matrix data. Additionally, constant numerical settings can be used across wide ranges of the parameter values without loss of accuracy. The proposed approaches are applied to state-space modeling of a two-dimensional aeroelastic system. The evaluated accuracy and dynamic behavior of the model show excellent agreement with the reference frequency domain solutions for both approaches.

Coupling analysis of transient aerodynamic and dynamic response of articulated heavy vehicles under crosswinds

Vehicle aerodynamics and dynamics in gusty crosswind conditions are of increasing significance for the lateral stability of heavy ground vehicles, especially articulated heavy vehicles (AHVs). The unsteady aerodynamic loads acting on AHVs can greatly exceed loads of a single-vehicle unit; these may deteriorate the lateral stability and lead to a loss of handling control. In this study, the time characteristics of aerodynamic loads and dynamic response of a tractor semi-trailer were considered, based on simulating the relative motions of these two components to reproduce actual scenarios of AHVs in crosswinds. A dynamic fully coupled method was developed and adopted to realize a real-time data exchange of flow fields and multi-bodies. Two multi-body systems (for non-articulated heavy vehicles and AHVs, respectively) were created to study the influences of the relative motions on the aerodynamic performance and lateral stability of the vehicles. The Reynolds-averaged Navier–Stokes (RANS) method and renormalization group (RNG) k−ε equation were adopted to account for the turbulence. A wind tunnel experiment was conducted to validate the numerical method. The results show that AHVs are more sensitive to the crosswind, with significant differences in the magnitudes and directions of the aerodynamic forces, moments, dynamic yaw angle, and lateral displacement. Three different wind types were considered (step, linear, and sinusoidal). The step crosswind produces the largest average lateral force and yawing moment, resulting in the largest lateral displacement and yaw angle. The largest hitch angle is found for linear gusts, presenting the highest safety risks in regard to jackknifing and trailer swings.

Comparative Study of Machine Learning Modeling for Unsteady Aerodynamics

Modern fighters are designed to fly at high angle of attacks reaching 90 deg as part of their routine maneuvers. These maneuvers generate complex nonlinear and unsteady aerodynamic loading. In this study, different aerodynamic prediction tools are investigated to achieve a model which is highly accurate, less computational, and provides a stable prediction of associated unsteady aerodynamics that results from high angle of attack maneuvers. These prediction tools include Artificial Neural Networks (ANN) model, Adaptive Neuro Fuzzy Logic Inference System (ANFIS), Fourier model, and Polynomial Classifier Networks (PCN). The main aim of the prediction model is to estimate the pitch moment and the normal force data obtained from forced tests of unsteady delta-winged aircrafts performing high angles of attack maneuvers. The investigation includes three delta wing models with 1, 1.5, and 2 aspect ratios with four determined variables: change rate in angle of attack (0 to 90 deg), non-dimensional pitch rate (0 to .06), and angle of attack. Following a comprehensive analysis of the proposed identification methods, it was found that the newly proposed model of PCN showed the least error in modeling and prediction results. Based on prediction capabilities, it is seen that polynomial networks modeling outperformed ANFIS and ANN for the present nonlinear problem.

A Calculation Model for Temperature Responses of Active Cooling Lattice Sandwich Structures for Thermal Protection

Aimed at the active thermal protection with lattice sandwich structures, an unsteady heat transfer theoretical model coupling facesheet and core heat conduction with coolant convection in the sandwich structure, was established. The model equations were discretized with the finite volume method and solved iteratively in MATLAB; the constriction thermal resistance between the facesheet and the lattice struts was considered for the first time in the model, and the approximate analytical solution of the constriction thermal resistance was obtained with the variable separation method; based on the unit-cell model and periodic boundary conditions, heat transfer coefficients <i>h</i>_b and <i>h</i>_fin required by the model were first obtained through numerical simulation. Finally, a case study with a multi-cell structure was carried out to compare the numerical and theoretical results, and the influence of the constriction thermal resistance on the prediction accuracy was discussed. The results show that, the theoretical model can accurately predict the temperature variations of the sandwich structure and the internal fluid, and the maximum deviation between theory and simulation is less than 1%. As the external heat flux increases, the error of theoretical prediction rises with the constriction thermal resistance ignored. Compared with the numerical simulation, the theoretical model can significantly reduce the calculation time and save calculation resources, thus it is especially suitable for active cooling lattice sandwich structures subjected to complex, non-uniform and unsteady thermal loads.

A vortex sheet based analytical model of the curled wake behind yawed wind turbines

Motivated by the need for compact descriptions of the evolution of non-classical wakes behind yawed wind turbines, we develop an analytical model to predict the shape of curled wakes. Interest in such modelling arises due to the potential of wake steering as a strategy for mitigating power reduction and unsteady loading of downstream turbines in wind farms. We first estimate the distribution of the shed vorticity at the wake edge due to both yaw offset and rotating blades. By considering the wake edge as an ideally thin vortex sheet, we describe its evolution in time moving with the flow. Vortex sheet equations are solved using a power series expansion method, and an approximate solution for the wake shape is obtained. The vortex sheet time evolution is then mapped into a spatial evolution by using a convection velocity. Apart from the wake shape, the lateral deflection of the wake including ground effects is modelled. Our results show that there exists a universal solution for the shape of curled wakes if suitable dimensionless variables are employed. For the case of turbulent boundary layer inflow, the decay of vortex sheet circulation due to turbulent diffusion is included. Finally, we modify the Gaussian wake model by incorporating the predicted shape and deflection of the curled wake, so that we can calculate the wake profiles behind yawed turbines. Model predictions are validated against large-eddy simulations and laboratory experiments for turbines with various operating conditions.

Cavitation erosion behavior of hydraulic concrete under high-speed flow

PurposeCavitation erosion has always been a common technical problem in a hydraulic discharging structure. This paper aims to investigate the cavitation erosion behavior of hydraulic concrete under high-speed flow.Design/methodology/approachA high-speed and high-pressure venturi cavitation erosion generator was used to simulate the strong cavitation. The characteristics of hydrodynamic loads of cavitation bubble collapse zone, the failure characteristics and the erosion development process of concrete were investigated. The main influencing factors of cavitation erosion were discussed.FindingsThe collapse of the cavitation bubble group produced a high frequency, continuous and unsteady pulse load on the wall of concrete, which was more likely to cause fatigue failure of concrete materials. The cavitation action position and the main frequency of impact load were greatly affected by the downstream pressure. A power exponential relationship between cavitation load, cavitation erosion and flow speed was observed. With the increase of concrete strength, the degree of damage of cavitation erosion was approximately linearly reduced.Originality/valueAfter cavitation erosion, a skeleton structure was formed by the accumulation of granular particles, and the relatively independent bulk structure of the surface differed from the flake structure formed after abrasion.

Computational study of flow incidence effects on the aeroacoustics of low blade-tip Mach number propellers

This paper presents a computational study of flow incidence effects on the aeroacoustics of a propeller operating at low blade-tip Mach numbers. The numerical flow solution is obtained by using the Lattice-Boltzmann/Very Large Eddy Simulation method, while far-field noise is computed through the Ffowcs-Williams & Hawkings' acoustic analogy applied on the propeller surface. The presence of an angular inflow leads to: (i) the radiation of tonal loading noise along the propeller axis; (ii) the increment/reduction of the sound pressure level in the region from/to which the propeller is tilted away/towards. However, contrarily to propellers operating at high blade-tip Mach numbers, the noise directivity change is found to be governed only by the rise of periodic unsteady loadings, with the modulation of the strength of the noise sources on the blade, associated to the periodic variation of the observer-source relative Mach number (in the blade reference frame), being negligible. Finally, thickness noise and turbulent boundary-layer trailing-edge noise did not show a significant directivity variation due to the propeller yaw angle change.

State-Space Modeling of Viscous Unsteady Aerodynamic Loads

In this paper, we summarize our recently developed viscous unsteady theory, which couples potential flow with the triple-deck boundary-layer theory. This approach provides a viscous extension of potential-flow unsteady aerodynamics. As such, a Reynolds-number-dependent transfer function is determined for unsteady lift. We then use the Wiener–Hammerstein structure to develop a finite-dimensional approximation of such an infinite-dimensional theory, presenting it in a state-space model. This novel nonlinear state-space model of viscous unsteady aerodynamic loads is expected to serve aerodynamicists better than the classical Theodorsen’s model because it captures viscous effects (that is, Reynolds number dependence) as well as nonlinearity and additional lag in the lift dynamics; it also allows simulation of arbitrary time-varying airfoil motions (not necessarily harmonic). Moreover, being in a state-space form makes it quite convenient for simulation and coupling with structural dynamics to perform aeroelasticity, flight dynamics analysis, and control design. We then develop a linearization of such a model, which enables analytical results. Subsequently, we derive an analytical representation of the viscous lift frequency response function: an explicit function of both the frequency and Reynolds number. We also develop a state-space model of the linearized response. We finally simulate the nonlinear and linear models to a nonharmonic small-amplitude pitching maneuver at a Reynolds number of 100,000 and compare the resulting lift and pitching moment with those obtained from potential flow; this is in reference to relatively higher-fidelity computations of the unsteady Reynolds-averaged Navier–Stokes equations.

Behaviour of a Cylindrical Reinforced Carbon Fibre Shell Under Impact Load

This work is devoted to numerical analysis of the behaviour of reinforced cylindrical shell made of a polymer composite material (PCM) under the action of an unsteady shock load, taking into account interlayer defects of elliptical shape, as well as evaluate the strength of composite package and the development of delaminations.

Combined effect of passive vortex generators and leading-edge roughness on dynamic stall of the wind turbine airfoil

Dynamic stall causes the highly unsteady and nonlinear aerodynamic loads on wind turbines. Recently, dynamic stall with passive vortex generators (VGs) and leading-edge roughness (LER) has received considerable attention, but independently. Therefore, this paper presents a careful investigation into dynamic stall of the NREL S809 airfoil with both VGs and LER to reveal their combined effect. Fully-resolved URANS simulations are conducted to identify the unsteady flow characteristics, and equivalent sand-grain roughness is used to model LER. LER significantly increases the turbulence kinetic energy (TKE) and suction loss at the leading edge, thereby causing the earlier onsets of separated flow and dynamic stall. Increasing roughness height generally reduces linear lift-curve slope, increases aerodynamic hysteresis and makes the separated flow hard to be reattached. VGs increase near-wall TKE via streamwise vortices and effectively suppress the separated flow. Dynamic stall is therefore significantly delayed by VGs with higher maximum lift, and aerodynamic hysteresis is also greatly reduced with the flow reattachment accelerated. Interestingly, serious LER may cause strong vortical disturbances and change the dynamic stall behaviors from light stall into deep stall. Double-row VGs are also found better than single-row VGs in improving the aerodynamic performance of roughened airfoil. These findings imply that VGs effectively control dynamic stall and diminish the adverse LER effect. This study should advance the control of unsteady loads on wind turbines suffering from harsh environmental conditions.

On the mechanisms of jet-installation noise reduction with flow-permeable trailing edges

This paper investigates the mechanisms by which flow-permeable materials provide noise reduction in an installed jet configuration. Numerical simulations are carried out with a flat plate placed in the near field of a single-stream subsonic jet (Ma=0.3 and Ma=0.5). The trailing-edge region of the plate is replaced by three different permeable structures, with different properties: a metal foam, a perforated plate and a diamond-shaped structure. Due to its complex geometry, the metal foam is modeled with an equivalent region governed by Darcy’s law. The perforated plate has the highest resistivity among all investigated configurations, whereas the metal foam and diamond inserts have similar properties. Far-field spectra show significant noise reduction when the solid trailing edge is replaced with the permeable materials, with a maximum decrease of 12 dB at St=0.12, for Ma=0.5. Beamforming results show that the dominant acoustic source is located at the solid–permeable junction for the metal foam and diamond structures, whereas the perforated one has the source positioned near the trailing edge, similarly to the solid case. A breakdown of the far-field noise generated by the plate is also performed, where the contributions from the solid and permeable sections are computed separately. The former has distinct regions in the noise spectrum, which are dominated either by surface pressure fluctuations or trailing-edge scattering. However, for the permeable region, the results point to a significant mitigation of the noise due to scattering, which is no longer the dominant mechanism in any frequency range. This is confirmed by lower values of spanwise coherence, computed from the surface pressure, for the permeable trailing edge, compared to the solid case. Therefore, the clear dominant mechanism in the permeable region of the plate is the unsteady loading to pressure wave impingement. This is verified for all investigated configurations so that even with a low permeability structure, significant noise reduction can still be achieved.

Numerical simulation of lowering operations from the coupling between the Composite-Rigid-Body Algorithm and the weak-scatterer approach

A consistent frame for the numerical simulation of lowering operations is investigated in this paper from a new wave-structure coupling. The mechanical modeling is based on the Composite-Rigid-Body Algorithm, which is able to simulate the nonlinear dynamics of multibody systems. The hydrodynamic model is based on the weak-scatterer approach, which allows the computation of unsteady hydrodynamic loads without being limited by the classical hypotheses of the linear potential flow theory. The coupling of these two models leads to the numerical simulation of articulated multibody systems with large relative motions in waves. The coupling equation is derived in this paper.This new numerical modeling is compared to the classical linear potential flow theory in the case of a lowering operation with a payload in the water. The impact of the lowering velocity is studied. Results show that this new model matches the classical approach for small lowering velocities but as soon as nonlinearities arise, differences between the two models appear.

Development of a Canister Module for PCM Coupled Heat Pipe in Spacecraft Thermal Management

This paper investigates the performance of a canister module filled with phase change material (PCM) that is thermally coupled to an axially grooved heat pipe and designed for thermal management of unsteady or cyclic heat loads in spacecraft applications. The PCM canister stores heat from the electronic devices during brief transient peaks and slowly releases this heat to the heat pipe during idle cycle time. A rectangular prism canister module is manufactured and filled with eicosane based on the geometrical parameters obtained from a thermal network model. The performance of this canister is evaluated experimentally, and the thermal network model is validated. Detailed theoretical and experimental evaluation of heat balance for the PCM canister and the heat pipe is carried out. All the required input parameters, including the heat capacity of the heat pipe, for the thermal network model, are theoretically estimated and experimentally evaluated. In general, the difference between predicted and experimental temperature values is observed to be within 3°C, except at the ends. PCM coupled heat pipes result in significant enhancement of the operating time and reduction in the peak temperature. This would be useful in spacecraft applications to reduce the size (and hence the mass) of the thermal radiator.