Instantaneous Aerodynamic Load Calculations in Rotating Airfoils From Time Resolved PIV Measurements at Low Reynolds Number

2013 ◽  
Author(s):  
Arturo Villegas ◽  
F. Javier Diez
Keyword(s):  
Full Range ◽  
Temporal Variations ◽  
Momentum Equation ◽  
Aerodynamic Load ◽  
Time Resolved ◽  
Unsteady Forces ◽  
Aerodynamic Loads ◽  
Rotating Blade ◽  
Theoretical Predictions ◽  
Stationary Reference Frame

The time-resolved evolution of the instantaneous pressure fields and aerodynamic loads are obtained for rotating airfoils. This method allows evaluating the fluctuations in the instantaneous aerodynamic loads which cannot be evaluated with averaging methods. It also has the ability of capturing high temporal variations such as vortex shedding in the wake of the rotating blade. Briefly, this method obtains the velocity field from time-resolved particle image velocimetry TR-PIV. This is used to calculate the pressure field around the turbine from the Poisson pressure equation. Then, the forces are obtained using the integral momentum equation in a stationary reference frame. These experimental aerodynamic loads are compared to theoretical predictions from the Blade Element Momentum theory (BEM). Accurately determining instantaneous forces in turbines is needed for safety and understanding of their full range of operation. The standard deviation of the instantaneous forces establishes the limits of the forces expected on the turbine. The uncertainty in the measurements is calculated. The method presented may be used to measure unsteady forces in rotating airfoils, providing useful information not just for computational studies, but also for aerodynamics, material and structural optimization and safety purposes.

scholarly journals Modal Analysis of Pressures on a Full Scale Spinnaker

2016 ◽  
Author(s):  
Julien Deparday ◽  
Patrick Bot ◽  
Fréderic Hauville ◽  
Benoit Augier ◽  
Marc Rabaud ◽  
...  
Keyword(s):  
Pressure Fluctuations ◽  
Aerodynamic Load ◽  
Propulsive Force ◽  
Time Resolved ◽  
Aerodynamic Loads ◽  
Pressure Distributions ◽  
Measured Time ◽  
Spatio Temporal ◽  
Highly Correlated ◽  
Proper Orthogonal

While sailing offwind, the trimmer typically adjusts the downwind sail "on the verge of luffing", letting occasionally the luff of the sail flapping. Due to the unsteadiness of the spinnaker itself, maintaining the luff on the verge of luffing needs continual adjustments. The propulsive force generated by the offwind sail depends on this trimming and is highly fluctuating. During a flapping sequence, the aerodynamic load can fluctuate by 50% of the average load. On a J/80 class yacht, we simultaneously measured time resolved pressures on the spinnaker, aerodynamic loads, boat and wind data. Significant spatio-temporal patterns are detected in the pressure distribution. In this paper we present averages and main fluctuations of pressure distributions and of load coefficients for different apparent wind angles as well as a refined analysis of pressure fluctuations, using the Proper Orthogonal Decomposition (POD) method. POD shows that pressure fluctuations due to luffing of the spinnaker can be well represented by only one proper mode related to a unique spatial pressure pattern and a dynamic behavior evolving with the Apparent Wind Angles. The time evolution of this proper mode is highly correlated with load fluctuations. Moreover, POD can be employed to filter the measured pressures more efficiently than basic filters. The reconstruction using the first few modes allows to restrict to the most energetic part of the signal and remove insignificant variations and noises. This might be helpful for comparison with other measurements and numerical simulations.

Experimental Analysis of a NACA 0021 Airfoil Under Dynamic Angle of Attack Variation and Low Reynolds Numbers

2018 ◽  
Vol 141 (3) ◽  
Author(s):  
D. Holst ◽  
B. Church ◽  
F. Wegner ◽  
G. Pechlivanoglou ◽  
C. N. Nayeri ◽  
...  
Keyword(s):  
Design Process ◽  
Surface Pressure ◽  
Wind Turbines ◽  
Full Range ◽  
Vertical Axis ◽  
Reynolds Numbers ◽  
Time Resolved ◽  
Pressure Distributions ◽  
Pressure Development ◽  
Flow Phenomena

The wind industry needs reliable and accurate airfoil polars to properly predict wind turbine performance, especially during the initial design phase. Medium- and low-fidelity simulations directly depend on the accuracy of the airfoil data and even more so if, e.g., dynamic effects are modeled. This becomes crucial if the blades of a turbine operate under stalled conditions for a significant part of the turbine's lifetime. In addition, the design process of vertical axis wind turbines needs data across the full range of angles of attack between 0 and 180 deg. Lift, drag, and surface pressure distributions of a NACA 0021 airfoil equipped with surface pressure taps were investigated based on time-resolved pressure measurements. The present study discusses full range static polars and several dynamic sinusoidal pitching configurations covering two Reynolds numbers Re = 140k and 180k, and different incidence ranges: near stall, poststall, and deep stall. Various bistable flow phenomena are discussed based on high frequency measurements revealing large lift-fluctuations in the post and deep stall regime that exceed the maximum lift of the static polars and are not captured by averaged measurements. Detailed surface pressure distributions are discussed to provide further insight into the flow conditions and pressure development during dynamic motion. The experimental data provided within the present paper are dedicated to the scientific community for calibration and reference purposes, which in the future may lead to higher accuracy in performance predictions during the design process of wind turbines.

Application of the Rotating Water Table to Nozzle Wake Excitation in Low Pressure Turbines

1985 ◽  
Author(s):  
Shun Chen
Keyword(s):  
Water Table ◽  
Low Pressure ◽  
Test Facility ◽  
Test Results ◽  
Unsteady Forces ◽  
Low Pressure Turbine ◽  
Rotating Blade ◽  
Stator Vane ◽  
Two Dimensional Flow ◽  
Hydraulic Analogy

The hydraulic analogy was employed in a rotating water table for simulating the compressible two dimensional flow in a low pressure turbine stage. Both steady and unsteady forces were measured directly on a rotating blade in a blade row rotating concentrically with a row of stator vanes. With proper modeling of the simulation, it is shown that the rotating water table can yield results that agree favorably with the analytical predictions and turbine test results. Using this test facility, the effects of axial spacing between rotor and stator rows on the nozzle wake excitation have been investigated for two different stator vane profiles. The water table test results correlate qualitatively with the turbine test data. The cancellation of nozzle passing frequency excitation by off-setting nozzle pitch was demonstrated in the water table and the results compared with both the analytical predictions and the laboratory turbine test results.

Molecular Movements and Dynamics in Solutions Studied by Fluorescence Depolarization Measurement

1993 ◽  
Vol 58 (2) ◽  
pp. 213-233 ◽  
Author(s):  
Zuzana Limpouchová ◽  
Karel Procházka ◽  
Vlastimil Fidler ◽  
Jiří Dvořák ◽  
Bohumil Bednář
Keyword(s):  
Energy Transfer ◽  
Rotational Diffusion ◽  
Experimental Studies ◽  
Theoretical Models ◽  
Reorientational Motion ◽  
Fluorescence Depolarization ◽  
Time Resolved ◽  
Langevin Model ◽  
Theoretical Predictions ◽  
Media Systems

Theories allowing interpretation of the results of time-resolved polarization spectrofluorimetry in solutions are reviewed and their applicability under various conditions is discussed. For the reorientation of rigid molecules in an isotropic medium, the most frequently employed models are presented, such as rotational diffusion model, the Fokker-Planck-Langevin model, etc. Systems with internal rotation, systems in anisotropic media, systems with a complex electron relaxation and systems with energy transfer are discussed as examples of more complex systems. A special attention is devoted to the polarization fluorimetry of probes bound to/or sorbed at polymer and biopolymer chains. The review focuses on theoretical models of reorientational motion for interpretation of fluorescence anisotropy decays. Experimental studies and computer simulations are discussed only when it is necessary for comparison with theoretical predictions. Complicated models for simultaneous reorientational motion and energy transfer, solvent relaxation, etc., although very important for many applications, exceed the scope of this review and are mentioned only very briefly.

Surge Cycle of Turbochargers: Simulation and Comparison to Experiments

2004 ◽  
Author(s):  
M. B. Schmitz ◽  
G. Fitzky
Keyword(s):  
Static Pressure ◽  
Test Facility ◽  
Test Rig ◽  
Time Resolved ◽  
Operation Conditions ◽  
Operation Modes ◽  
Second Mode ◽  
Theoretical Predictions ◽  
Compressor Map ◽  
Work Done

The turbocharger test facility can be operated in two different modes. In the first mode, the turbocharger turbine is driven by an external blower and a combustor. The compressor blows off through the chimney. In the second mode the turbocharger is operated similar to a gas turbine: the turbine drives the turbocharger compressor which pressurizes the combustion chamber. This study is focused on the surge cycle of the turbocharger for both operation modes of the test facility. The turbocharger has to withstand surge for all pressure ratios it is designed for without damaging the test rig. Especially for small compressors large plenum volumes can cause such damages. The dynamical model of the system is developed based on the work done by Greitzer [5], [6] and has been extended to the special requirements of the turbocharger test rig. For the components of the turbocharger a quasi-steady behavior with respect to the time scales of the surge cycle is assumed. Consequently, the experimentally obtained steady state characteristics for both compressor and turbine are applied to the model. In order to describe the compressor behavior for backflow conditions, the compressor map is extended for negative mass flow. The theoretical model is calibrated on experimental data. Thereafter the model is used to predict the surge cycle for different operation conditions. For the two operation modes, the blow-off and the recirculation operation, the time resolved values of static pressure and speed oscillation were recorded and compared to the theoretical predictions.

Photoacoustic Depth-Resolved Analysis of Tissue Models

1997 ◽  
Vol 51 (1) ◽  
pp. 51-57 ◽  
Author(s):  
A. Beenen ◽  
G. Spanner ◽  
R. Niessner
Keyword(s):  
Imaging Techniques ◽  
Depth Resolution ◽  
Skin Model ◽  
Layer Depth ◽  
Time Resolved ◽  
Complementary Approach ◽  
Fiber Optical ◽  
Artificial Tissue ◽  
Theoretical Predictions ◽  
Tissue Models

Pulsed photoacoustic laser spectroscopy was used for depth-resolved analysis of artificial tissue models. The technique was applied to investigate the spatial resolution capabilities of a fiber-optical-coupled photoacoustic sensor head. The time-resolved measurements confirmed the theoretical predictions of a depth resolution of 0.1 mm. In an adapted skin model, a strongly absorbing target could be detected up to a layer depth of 14 mm. At a layer depth of 5 mm, a lateral resolution of 3.5 mm was achieved. Because of the depth-resolving capability, this method is well suited as a complementary approach for two-dimensional imaging techniques.

scholarly journals Small scale spatial variability of bare-ice albedo at Jamtalferner, Austria

2020 ◽  
Author(s):  
Lea Hartl ◽  
Lucia Felbauer ◽  
Gabriele Schwaizer ◽  
Andrea Fischer
Keyword(s):  
Satellite Data ◽  
Full Range ◽  
Temporal Variations ◽  
Small Scale ◽  
Landsat 8 ◽  
Dry Ice ◽  
In Situ Data ◽  
Depth Analysis ◽  
The Difference

Abstract. As Alpine glaciers recede, they are quickly becoming snow free in summer and, accordingly, spatial and temporal variations in ice albedo increasingly affect the melt regime. To accurately model future developments, such as deglaciation patterns, it is important to understand the processes governing broadband and spectral albedo at a local scale. However, little in situ data of ice albedo exits. As a contribution to this knowledge gap, we present spectral reflectance data from 325 to 1075 nm collected along several profile lines in the ablation zone of Jamtalferner, Austria. Measurements were timed to closely coincide with a Sentinel 2 and Landsat 8 overpass and are compared to the respective ground reflectance products. The brightest spectra have a maximum reflectance of up to 0.7 and consist of clean, dry ice. In contrast, reflectance does not exceed 0.2 at dark spectra where liquid water and/or fine grained debris are present. Spectra can roughly be grouped into dry ice, wet ice, and dirt/rocks, although transitions between types are fluid. Neither satellite captures the full range of in situ reflectance values. The difference between ground and satellite data is not uniform across satellite bands, between Landsat and Sentinel, and to some extent between ice surface types (underestimation of reflectance for bright surfaces, overestimation for dark surfaces). We wish to highlight the need for further, systematic measurements of in situ spectral albedo, its variability in time and space, and in- depth analysis of time-synchronous satellite data.

Experimental Analysis of a NACA 0021 Airfoil Under Dynamic Angle of Attack Variation and Low Reynolds Numbers

2018 ◽  
Author(s):  
D. Holst ◽  
B. Church ◽  
F. Wegner ◽  
G. Pechlivanoglou ◽  
C. N. Nayeri ◽  
...  
Keyword(s):  
Design Process ◽  
Surface Pressure ◽  
Wind Turbines ◽  
Full Range ◽  
Vertical Axis ◽  
Reynolds Numbers ◽  
Time Resolved ◽  
Pressure Distributions ◽  
Pressure Development ◽  
Flow Phenomena

The wind industry needs reliable and accurate airfoil polars to properly predict wind turbine performance, especially during the initial design phase. Medium- and low-fidelity simulations directly depend on the accuracy of the airfoil data and even more so if e.g. dynamic effects are modeled. This becomes crucial if the blades of a turbine operate under stalled conditions for a significant part of the turbine’s lifetime. In addition, the design process of vertical axis wind turbines (VAWTs) needs data across the full range of angles of attack between 0 and 180 deg. Lift, drag and surface pressure distributions of a NACA 0021 airfoil equipped with surface pressure taps were investigated based on time-resolved pressure measurements. The present study discusses full range static polars and several dynamic sinusoidal pitching configurations covering two Reynolds numbers Re = 140k and 180 k, and different incidence ranges: near stall, post stall and deep stall. Various bi-stable flow phenomena are discussed based on high frequency measurements revealing large lift-fluctuations in the post and deep stall regime that exceed the maximum lift of the static polars and are not captured by averaged measurements. Detailed surface pressure distributions are discussed to provide further insight into the flow conditions and pressure development during dynamic motion. The experimental data provided within the present paper is dedicated to the scientific community for calibration and reference purposes, which in the future may lead to higher accuracy in performance predictions during the design process of wind turbines.

scholarly journals Hunting Stability of High-Speed Railway Vehicles Under Steady Aerodynamic Loads

2018 ◽  
Vol 18 (07) ◽  
pp. 1850093 ◽  
Author(s):  
Xiao-Hui Zeng ◽  
Jiang Lai ◽  
Han Wu
Keyword(s):  
Stability Analysis ◽  
High Speed ◽  
Critical Speed ◽  
Railway Vehicle ◽  
Aerodynamic Load ◽  
High Speed Railway ◽  
Aerodynamic Loads ◽  
Railway Vehicles ◽  
Pitch Moment ◽  
Hunting Stability

With the rising speed of high-speed trains, the aerodynamic loads become more significant and their influences on the hunting stability of railway vehicles deserve to be considered. Such an effect cannot be properly considered by the conventional model of hunting stability analysis. To this end, the linear hunting stability of high-speed railway vehicles running on tangent tracks is studied. A model considering the steady aerodynamic loads due to the joint action of the airflow facing the moving train and the crosswind, is proposed for the hunting stability analysis of a railway vehicle with 17 degrees of freedom (DOF). The key factors considered include: variations of the wheel–rail normal forces, creep coefficients, gravitational stiffness and angular stiffness due to the actions of the aerodynamic load, which affects the characteristics of hunting stability. Using the computer program developed, numerical calculations were carried out for studying the behavior of the linear hunting stability of vehicles under steady aerodynamic loads. The results show that the aerodynamic loads have an obvious effect on the linear critical speeds and instability modes. The linear critical speed decreases monotonously as the crosswind velocity increases, and the influences of pitch moment and lift force on the linear critical speed are larger than the other components of the aerodynamic loads.

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